Assessing the Impact of Curing on Chloride Penetration ......2.2 Chloride Ion Permeability Based on Total Charge Passed (ASTM C1202,2019). . . . . .10 3.1 Concrete 1 Mixture Proportions

Assessing the Impact of Curing on Chloride Penetration Resistance ofthe Concrete Cover Zone

by

Majed A. Karam

A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science

Graduate Department of Civil and Mineral EngineeringUniversity of Toronto

© Copyright 2020 by Majed A. Karam

Abstract

Assessing the Impact of Curing on Chloride Penetration Resistance of the Concrete Cover Zone

Majed A. Karam

Master of Applied Science

Graduate Department of Civil and Mineral Engineering

University of Toronto

2020

There is a lack of rapid test methods for accurately assessing the impact of curing on the differential hy-

dration through the depth of the concrete cover as well as the impact on chloride penetration resistance.

In the absence of adequate performance assessment tools, prescriptive curing specifications have been

adopted for concretes exposed to chlorides and other exposures, such as in CSA A23.1-19. There is a

desire to switch to a performance specification for curing, particularly by the precast concrete industry,

that could be used to assess the impact of using of accelerated heat curing methods widely used to obtain

high early-strength gain and maturity.

Two methods are presented and evaluated in this thesis. The first involves profiling the initial rate

of absorption of a sodium chloride solution. The second makes use of embedded arrays of electrodes to

map the formation factor with depth, enabling a multi-mechanistic approach to the problem.

i

Acknowledgements

I would like to acknowledge my supervisor, Dr R.D. Hooton, for his constant constructive feedback and

would like to thank him for the invaluable knowledge and inspiration he has shared with me. I am

grateful for the people who have helped me by being involved in valuable discussions or by providing

laboratory aid, mainly Mrs O. Perebatova, Dr M. Nokken, Ms. S. Foster, Dr R. Masoudi, Mrs H.

Schell, Mr S. Narneni, Mr S. Mantelli, Dr K. Peterson, Mr S. Gao, Ms. S. Bi., and Mr T. Xu. The

financial contributions of the Canadian Precast/Precast Concrete Institute and that of Mitacs, through

their Accelerate program, are acknowledged. My time at the University of Toronto was also partially

supported by the Queen Elizabeth II award and the Doherty family donation.

ii

Contents

1 Introduction 1

2 Concrete Permeability, Curing, and Need for a Performance Test 3

2.1 Concrete Permeability and Penetration Mechanisms . . . . . . . . . . . . . . . . . . . . . 3

2.2 Curing Methods, Specifications, and Impact on Durability . . . . . . . . . . . . . . . . . . 5

2.3 Prescriptive vs. Performance Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Fluid and Ionic Transport Evaluation Methods . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 Durability Performance Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.6 Case of Canadian Precast Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Sorptivity Profiling 16

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3 Previously Collected Absorption Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4 Experimental Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.5 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4 Formation Factor Characterization 47

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.2 Review of Electrode Embedment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.3 Electrode Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.4 Accounting for the Differential Hydration Kinetics . . . . . . . . . . . . . . . . . . . . . . 53

4.5 Proof of Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5 Conclusions and Recommendations 59

5.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

References 66

A Cementitious and Aggregates Properties 67

B Chapter 3 Data 68

iii

C Absorption Profiling Test Method 72

D Chapter 4 Data 78

E Data Processing for In-situ QC 80

F AC Signal Frequency for Impedance Measurements 81

iv

List of Tables

2.1 Allowable Curing Regimes Defined in CSA A23.1-19 . . . . . . . . . . . . . . . . . . . . . 6

2.2 Chloride Ion Permeability Based on Total Charge Passed (ASTM C1202, 2019) . . . . . . 10

3.1 Concrete 1 Mixture Proportions (expressed in kg/m3 except for admixtures, expressed in

mL per 100 kg cementitious material) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Concrete 2 Mixture Proportions (expressed in kg/m3 except for admixtures, expressed in

mL per 100 kg cementitious material) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.3 Integral Initial Rate of Absorption - Concrete 1 . . . . . . . . . . . . . . . . . . . . . . . . 34

3.4 Integral Charge Passed - Concrete 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.5 Integral Electrical Resistivity - Concrete 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.6 Ratios of Initial Rates of Absorption with respect to MC - Concrete 2 . . . . . . . . . . . 36

3.7 Confidence on Differentiation Between AC and MC - Round 2 . . . . . . . . . . . . . . . . 37


3.9 Integral Bulk Electrical Resistivity - Concrete 2 . . . . . . . . . . . . . . . . . . . . . . . . 40

3.10 Integral Total Charge Passed - Concrete 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.11 Confidence on Differentiation Between AC and MC - Round 3 . . . . . . . . . . . . . . . . 42

3.12 Initial Rate of Absorption with respect to MC - Concrete 3 . . . . . . . . . . . . . . . . . 42


3.14 Integral Bulk Electrical Resistivity - Concrete 3 . . . . . . . . . . . . . . . . . . . . . . . . 45

3.15 Integral Total Charge Passed - Concrete 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.1 Mixture Proportions (expressed in kg/m3) . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

C.1 Measurement Schedule and Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

v

List of Figures

1.1 Early-age Moisture Loss and Effect on Porosity and Penetration Resistance . . . . . . . . 2

2.1 Simplified Service Life Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Generic Temperature Stages of Accelerated Moist Curing Regimes . . . . . . . . . . . . . 7

2.3 Performance versus Prescriptive Specifications . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Absorption Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2 Evolution of Initial Absorption as a Function of Depth (adapted from Hooton et al. (1993)) 19

3.3 Graphic Representation of the Development Objective . . . . . . . . . . . . . . . . . . . . 20

3.4 Sorptivity Measurement Setup (Adapted from ASTM C1585 (2013)) . . . . . . . . . . . . 22

3.5 Correlation Between Smax,0.98 and tmax,0.98 (raw data from Dadic (2018)) . . . . . . . . . 23

3.6 Concrete 1 Temperature History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26



3.9 Example of an Absorption Evolution Plot (Concrete 2, Regime AC) . . . . . . . . . . . . 30

3.10 Depth Evolution of the Initial Rate of Absorption - Concrete 1 . . . . . . . . . . . . . . . 31

3.11 Depth Evolution of the Chloride Penetration Front - Concrete 1 . . . . . . . . . . . . . . . 32

3.12 Depth Evolution of the Bulk Electrical Resistivity - Concrete 1. The abscissa corresponds

to one of the specimen test planes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.13 Depth Evolution of the Total Charge Passed - Concrete 1. The abscissa corresponds to

the test surface in contact with the NaCl solution . . . . . . . . . . . . . . . . . . . . . . . 33

3.14 Compressive Strength Gain - Concrete 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33





3.18 Depth Evolution of the Total Charge Passed - Concrete 2.The abscissa corresponds to the

test surface in contact with the NaCl solution . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.19 Compressive Strength Gain - Concrete 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.20 Colorimetric Chloride Penetration Front - Concrete 2 . . . . . . . . . . . . . . . . . . . . . 39



3.23 Example of Highly Variable Colorimetric Chloride Front . . . . . . . . . . . . . . . . . . . 41

vi



3.25 Depth Evolution of the Total Charge Passed - Concrete 3. The abscissa corresponds to

the test surface in contact with the NaCl solution . . . . . . . . . . . . . . . . . . . . . . . 43

3.26 Compressive Strength - Concrete 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.1 Graphic Representation of the Mechanistic Evaluation Framework . . . . . . . . . . . . . 47

4.2 Multi-Electrode Cylinder Electrodes Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.3 Cumulative Distribution of Resistivity Measurements for Different Electrode Layouts . . . 52

4.4 Illustration of Powers-Brownyard Model (w/c=0.45) . . . . . . . . . . . . . . . . . . . . . 54

4.5 Concrete Mold and Sensor Arrays Used for the Proof of Concept Tests. Note: one of the

sensor arrays has not yet been fully inserted. . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.6 Depth-dependent Electrical Resistivity of Concrete Specimens Fitted with Embedded

Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

C.1 Cored Slab for Test Specimen Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

C.2 Sorptivity Experimental Setup (adapted from ASTM C1585 (ASTM, 2013)) . . . . . . . . 76

vii

Chapter 1

Introduction

The efficiency, durability, and reliability of reinforced concrete structures highly depend on the quality

of the concrete materials, mixture design, placement, and curing, which ultimately define its fluid pen-

etration resistance. Most deterioration mechanisms affecting reinforced concrete are dependant on the

resistance to fluid ingress (Basheer et al., 2001). The parameters defining the mass and ionic penetration

resistance of concrete include: materials selection, cements, supplementary cementitious materials, mix-

ture design, such as the water-to-binder ratio, aggregate particle packing, proportioning; and placement,

finishing, and curing. Due to interactions between a concrete member and the surrounding environ-

ment during setting and early-age curing, the concrete properties do not develop uniformly throughout

the concrete’s volume, with high variability in the member’s boundary region (cover). This leads to a

non-uniform fluid and ionic penetration resistance of the concrete cover due to porosity and tortuosity

gradients. When considering the potential for reinforcement corrosion, one could argue that the pene-

tration resistance of the concrete cover is the most important parameter to consider and optimize for

extended service life. In the case of concrete structures in a marine environment or exposed to de-icing

salts, the ingress of chloride ions strongly dictates the service life of the structure. The concrete cover

acts as the corrosion protection of the steel reinforcement. In this case, the chloride penetration resis-

tance of the cover concrete has to be high, and quantified for acceptance, quality assurance, and service

life prediction purposes. Figure 1.1 shows the described concept. First, desiccation occurs during setting

and early-age curing. The desiccation is more pronounced closer to the member’s boundaries, leading

to a non-uniform microstructure with high porosity. Finally, when in service conditions, the concrete

cover, acting as the interface between the reinforcement and the aggressive agents, is penetrated by

chloride ions through multiple mechanisms. The mechanisms governing this phenomenon are described

in Chapter 2 of this thesis. The direct cost associated with corrosion of infrastructure (bridges, airports,

harbours, pipelines, drinking water, etc.), due to corrosion prevention, control, and repair, exceeds $ 220

billion, in the United States alone every year Angst (2018). In Canada, the Canadian Infrastructure

Report Card (Gonthier, 2016) indicated that 26% of bridges were in fair, poor, or very poor condition

mainly due to corrosion of concrete steel reinforcement or structural steel. From a resource allocation

optimization viewpoint, as well as maintenance and rehabilitation scheduling, it is valuable to model

the service life of structures. The latter has been of increasing interest in the past couple of decades

(Boddy et al. (1999); Hooton et al. (2002); and E. Bentz (2003)). Modelling the rate of deterioration

associated with chloride ingress is highly complex and still contains a large number of uncertainties in

1

Chapter 1. Introduction 2

its fundamental mechanisms, such as the size effect of the structure, the critical chloride concentration

threshold at the reinforcement, and the implication of cracks, pre- and post-corrosion initiation (Angst,

2018). However, mass and ionic transport mechanisms in concrete have been extensively studied and

are now relatively well understood. Those of interest to this study are briefly introduced in Chapter 2.

H2O loss differentialporosity

Cl-

multi-mechanisticingress

Figure 1.1: Early-age Moisture Loss and Effect on Porosity and Penetration Resistance

Estimating the penetration resistance of concrete to chlorides is possible using certain widely used lab-

oratory tests such as those discussed in Chapter 2, as well as using the results in modeling deterioration

processes. However, current accelerated test methods are usually not sensitive enough to detect the

effect of curing on the chloride penetration resistance through the depth of the cover concrete, limiting

their use by transportation agencies for curing performance evaluation. For this reason, curing specifi-

cations tend to be prescriptive. For instance, Canadian Standards Association (CSA) A23.1 prescribes

an extended wet curing period of “7 days at 10 °C and for the time necessary to attain 70% of the

specified strength” for a class C-XL exposure (CSA A23.1, 2019). Prescriptive specifications may be

adequate but they prevent the use of alternative methods that could achieve the same level of perfor-

mance. Performance specifications, set test limits based on industry-accepted test methods and allow

clarification of responsibilities and innovation in the construction industry (Hooton & Bickley, 2012).

Thus, performance specifications supported by a mechanistic approach to the problem are desired, and

the goal of this research. The effect of curing practices on the chloride penetration resistance of the con-

crete cover is evaluated by profiling the initial rate of a sodium chloride solution in the cover region, as

demonstrated in Chapter 3. The suggested method is a modified version of the ASTM C1585 “Standard

Test Method for Measurement of Rate of Absorption of Water by Hydraulic-Cement Concretes” (ASTM

C1585, 2013) initially suggested by Hall (1989), and modified for detecting curing effects by Hooton et al.

(1993), Hooton (2015), and Dadic (2018). A considerable challenge is defining a decision-making frame-

work based on the performance metrics obtained, this issue is discussed further in this thesis. Chapter 4

introduces the concept of a rapid multi-mechanistic assessment of curing in the concrete cover using the

formation factor by the means of electrodes embedded in the concrete in its fresh state, enabling quality

assurance, and eventually quality control and curing optimization using real-time performance data.

Chapter 2

Concrete Permeability, Curing, and

Need for a Performance Test

This Chapter introduces fluid and ionic penetration in the context of their role in defining the durability

performance of a concrete structure. A definition of concrete “permeability” and its characteristics are

presented, as well as the main mass and ionic transport mechanisms of interest. Different curing methods

and the role of curing in defining the durability performance are discussed along with an overview of

prescriptive requirements, and existing performance evaluation methods. The use of corresponding

performance metrics in service life modelling is also discussed along with its limitations. Finally, specific

issues relevant to the Canadian precast concrete industry are presented.

2.1 Concrete Permeability and Penetration Mechanisms

The ultimate goal in understanding the mechanisms involved in concrete deterioration and transport

processes is to develop the ability to predict the service life of reinforced concrete structures and opti-

mizing it based on resource allocation and desired use. The focus of this study lies in quantifying the

resistance of the concrete cover to the ingress of chloride ions, a complex problem influenced by the

following (Tang et al., 2012):

• The exposure conditions of a structure in service are not constant spatially nor temporally; in

fact the concentration of chloride ions is variable and a function of factor including the state of

submersion or degree of saturation of the concrete (for example: tidal zones or seasonal change in

relative humidity, RH), and the amount of de-icing salts at the surface.

• The properties of the hardened concrete vary spatially throughout the concrete structure, but also

evolve as a function of time. In addition to concrete mixture design and proportioning, these

properties are also a function of placement, finishing, and curing.

• The mechanisms of chloride penetration are not confined to one transport process, and may be a

combination of many such as absorption and diffusion, as well as interactions with existing ions

and solids in the pore solution. In addition, variations in temperature, precipitation, or sunshine

introduce variations that should be taken into account when predicting the service life of the

reinforced concrete structure.

3

Chapter 2. Concrete Permeability, Curing, and Need for a Performance Test 4

A simplified service life model accounting for the deterioration of reinforced concrete due to the corrosion

of the reinforcing steel was initially proposed by Tuutti (1982) and is composed of two primary phases:

an initiation phase where the chloride ions or carbonation front advance in the cover zone, and a damage

propagation phase. Figure 2.1 shows a simplistic schematic representation of the temporal model. The

complexity of damage initiation and propagation corresponding to reinforcement corrosion is much higher

than that depicted by Tuutti, as outlined by Angst (2018), and is partially due to the stability of the

anodic sites, the precipitation of corrosion products, the self-healing of cracks in the concrete affecting

the corrosion kinetics, and the structure’s size effect. The focus of this study lies in the quantification of

the concrete resistance to chloride ions ingress throughout the cover thickness, which is the main factor

affecting the duration of the initiation phase.

Initiation Propagation

Maximum Allowable Deterioration

Time

Degree of Damage

0

Figure 2.1: Simplified Service Life Model

The main mass transport mechanisms involved in the ingress of chloride ions in the concrete pore

structure are listed below, their rates and relative importance are a function of the pore size distribution

and continuity of the pore system as well as physical and chemical interactions of the chloride ions with

the solid (Claisse, 2014):

• Ionic diffusion: movement of a substance under a concentration gradient, from an area of high

concentration to an area of low concentration. Ionic diffusion does not require fluid flow. An-

alytical formulations, such as Fick’s 2nd law, depict the spatial and temporal evolution of ionic

concentration under the assumption of full saturation of the concrete pore system.Ionic diffusion:

movement of a substance under a concentration gradient, from an area of high concentration to an

area of low concentration.

• Migration: movement of a charged substance under the action of an electrical field requires a

potential difference. The potential difference can be due to the presence of pitting corrosion on

the reinforcing steel for example.

• Advection: fluid flow under a pressure gradient. The pressure differential can be applied externally

(e.g. hydrostatic pressure differential) or caused internally due to unsaturated capillaries leading

to suction. Capillary suction constitutes the main fluid ingress mechanism in partially saturated

concrete (Nokken & Hooton, 2004).

In addition to the complex interactions of mass transport in concrete, quantifying the progress of chloride

ions is more challenging given that only part of the ions initially penetrating the concrete pore structure

end up arriving to depassivate the reinforcing steel due to the chloride binding capacity of cement paste

hydrates. Bound chlorides are considered to be harmless unless they are released or desorbed due to

the progress of other mechanisms such as carbonation or sulphate ingress (Tang et al., 2012). Some of


the original descriptive and quantitative work investigating fluid flow in concrete was done by Powers

et al., published in a series of bulletins of the Portland Cement Association. Powers et al. (1959) rec-

ommends “eliminat[ing] continuous capillaries from the paste” in order to produce a durable concrete,

which needs to have the lowest permeability possible. It was suggested, and validated experimentally,

that the continuity of the capillary system is a function of the water-to-cement ratio. The capillary

system is made discontinuous through the production of hydrates blocking the pores. This statement

was later validated by Nokken & Hooton (2004), who elaborated on obtaining discontinuity with the in-

clusion of supplementary cementitious materials, and the effect of time on permeability and porosity. In

addition, Powers et al. noted the importance of curing and formulated that the length of curing required

to achieve a certain quality level depends on the water-to-cement ratio and the temperature of curing.

There has been a considerable amount of effort to determine pore structure parameters characterizing

mass transport in concrete. The intrinsic permeability, as defined by Darcy’s law, is of special interest

as it is widely used in soil and rock mechanics. Nokken & Hooton (2008) performed an extensive review

of methods aiming at determining the intrinsic permeability of concrete using the Katz-Thompson rela-

tionship and electrical conductivity measurements (Katz & Thompson, 1986) rather than determining

the permeability experimentally, which is an intensive process exhibiting high variability in its outcome.

It was concluded that the relationship does not necessarily hold for concrete and displays high variability

due to the need to measure the pore solution electrical conductivity and critical pore diameter using

mercury intrusion porosimetry.

2.2 Curing Methods, Specifications, and Impact on Durability

The Portland Cement Association (Kosmatka et al., 2003) defines curing as “the maintenance of a satis-

factory moisture content and temperature in concrete for a period of time immediately following placing

and finishing so that the desired properties may develop”. In essence, it consists of satisfying conditions,

both at depth and near the surface, for optimal hydration depending on the desired hardened concrete

properties and construction processes. These properties are essentially the concrete strength and its

resistance to penetration of aggressive agents; which are both a function of the total volume of pores. As

hydration and pozzolanic reactions advance, the hydrates fill the pores, reducing their volume and thus

increasing the strength. The evolution of the concrete’s permeability and other transport properties

have a higher degree of complexity than that of the strength development. In fact, it is possible for

two porous bodies to have similar porosity but different permeability values (Neville & Brooks, 1987).

In addition, permeability development is more sensitive to the quality of curing compared to strength

development, specially in the near-surface region (Hooton et al., 2002).

There are different ways to provide favorable conditions for the development of concrete properties;

which are a function of the intended service life and use, type of the structure, and resource and time

allocation. Kosmatka et al. (2003) classified curing methods in three categories as follows:

• Methods aimed at preserving the mixing water in the concrete during the early hardening period.

These include ponding or immersion, spraying or fogging, and saturated wet coverings. In addition,

the supplied curing water is beneficial in cooling the concrete through evaporation, which is valuable

in hot weather.


• Methods aimed at minimizing the loss of mixing water from the concrete. These include covering

the concrete with impervious paper or plastic sheets, or by applying membrane-forming curing

compounds.

• Methods aimed at accelerating strength gain by supplying heat and additional moisture to the

concrete. This is usually accomplished with live steam, heating coils, or electrically heated forms

or pads.

The choice of curing method is also a function of the concrete’s hydration stage. The ACI “Guide to

External Curing of Concrete” (2016) sequences a full curing process of unformed faces into three stages

where the first two are protective measures, and the third stage is for additional curing:

1. Initial curing: implemented between placement and final finishing of the concrete to reduce mois-

ture loss from the surface.

2. Intermediate curing: implemented when final finishing is completed but before the concrete has

reached final set. During this period, evaporation may need to be reduced, but the concrete may

not yet be able to tolerate the direct application of water or the mechanical damage resulting from

the application of fabric or plastic coverings.

3. Final curing: implemented after final finishing and after the concrete has reached final set.

The specifications for curing regimes are typically prescriptive. CSA A23.1 (2019) defines three allowable

curing regimes in Table 19 of the standard, reproduced in Table 2.1. Reinforced concrete exposed to

chlorides and not completely submerged at all time (exposure classes C-XL and C-1), such as bridge

decks, girders or parking decks exposed to de-icing chemicals, need to be conditioned in accordance to

curing types 2 or 3, depending on the level of cement replacement by supplementary cementitious mate-

rials. It is also specified that curing of precast elements shall be in accordance with CSA A23.4 (2016),

as discussed later. Note that the compressive strength is quantified, and is thus a performance metric

in evaluating the curing. However, performance evaluation of the effect of curing on fluid penetration

resistance is missing, and a prescriptive approach is adopted.

Table 2.1: Allowable Curing Regimes Defined in CSA A23.1-19

Curing Type Name Description

1 Basic curing 3 d at ≥ 10 °C or for the time necessary to attain 40% of

the specified strength.

2 Additional curing 7 d total at ≥ 10 °C and for the time necessary to attain

70% of the specified strength.

3 Extended wet curing A wet-curing period of 7 d at ≥ 10 °C and for the time nec-

essary to attain 70% of the specified strength. The curing

types allowed are ponding, continuous sprinkling, absorp-

tive mat, or fabric kept continuously wet.

The concrete maturity method, standardized as ASTM C1704 (2019), allows for the prediction of the

compressive strength evolution based on temperature history and age. It is often used on construction

projects in order to ensure levels of structural capacity and optimize the construction schedule. The


maturity, which is related to the compressive strength of a given concrete, is proportional to temperature.

The relationship needs to be calibrated for each concrete mixture and is based on the assumption that

sufficient moisture is present for hydration. Elevated temperature curing methods were developed and

heavily used in the precast concrete industry, enhancing productivity and cutting costs. Figure 2.2

depicts a typical accelerated moist curing cycle comprising of four periods. CSA A23.4 (2016) specifies

a minimum relative humidity of 95 %, as well as temperature and temperature gradient limits.

Time

Temperature

Preset Ramp Hold Cool

Figure 2.2: Generic Temperature Stages of Accelerated Moist Curing Regimes

2.3 Prescriptive vs. Performance Specifications

The service life of a concrete structure or a component of a structure can be seen as a serviceability-

defined reliability function where the concrete quality is a function of service conditions and time. Given

that costs and resources must be optimized over the life-cycle of the structure, and that an increased

level of concrete quality often corresponds to an increased initial cost (material selection, mixture de-

sign, curing, and finishing) but lower life-cycle cost, the quality of the concrete product is adapted as a

function of the intended use and service life. The demand for quality is usually quantified and explicit,

for instance a specified desired service life span. On the other hand, defining the level of quality and

quantifying it is more challenging. The understanding of fundamental mechanisms defining the quality

of the concrete material and the potential deterioration mechanisms is a first step towards estimating

the quality of a product. The use of test methods that evaluate specific performance metrics is a step

further towards quantifying the service life performance of the product.

As shown in Figure 2.3, prescriptive specifications rely on the control of a subset of certain parame-

ters (denoted as p1 to pN) that are believed to influence the quality of the product. However, the rest

of the system is treated as a black box due to either a lack of understanding of the elemental interac-

tions or for resource allocation considerations. On the other hand, performance-based specifications rely

on “the assessment of relevant material properties of a specific concrete through experiments, analytical

modelling, numerical modelling, or experience in order to predict the concretes resistance against deterio-

ration for a certain period under certain environmental exposure conditions” (Beushausen & Fernandez,

2016). Thus, the development of test methods for evaluating performance metrics is an essential step


towards upgrading prescriptive specifications to performance specifications. Switching to performance

considerations is advantageous at different levels including clarifying contract responsibilities, allow-

ing for innovation in construction, service-life modelling, and damage prediction leading to optimum

maintenance and repair scheduling and resource allocation (Hooton & Bickley, 2012).

p1

p2

p3

pN

Product Quality Quantification and Evaluation

Figure 2.3: Performance versus Prescriptive Specifications

Clause 4.1.2 of CSA A23.1 (2019) allows alternative methods for specifying concrete, as long as the

equivalent performance is proven. More specific to accelerated curing, clause 23.2.3.9 of CSA A23.4

(2016) allows for the adoption of curing procedures differing from the specified CSA A23.1 prescriptions

if all performance requirements are met or exceeded. Test results showing an equivalent or superior

durability performance to that corresponding to the prescriptive specifications should be provided by

the contractor and supplier to the owner. Hindering the adoption of performance specifications is the

lack of suitable evaluation test methods as well as the lack of specification limits, as outlined by Hooton

& Bickley (2012). Chapter 3 elaborates on a proposed test method aiming at providing performance

metrics for the acceptance of curing regimes in terms of the resistance of the concrete cover to chloride

ion ingress.

2.4 Fluid and Ionic Transport Evaluation Methods

The following describes some of the common test methods that have been used to estimate the perme-

ability of concrete and to evaluate the effect of curing.

2.4.1 Salt Ponding and Bulk Diffusion Tests

The salt ponding test, AASHTO T259 “Resistance of Concrete to Chloride Ion Penetration” was de-

veloped in order to determine the “resistance of concrete specimens to the penetration of chloride ions”

(AASHTO T259-02, 2006). Concrete slabs, wet cured for 14 days, then subjected to air drying at a

50% relative humidity for a period of 14 days, are exposed to a sodium chloride solution one a single

face for a period of 90 days. Given that the slab is initially not fully saturated, and the bottom face of

the slab is left open to drying, absorption and wicking action will occur. The extent of these actions is

also a variable and cannot be quantified given that concretes of different qualities dry at different rates

and the moisture gradient at day zero of the test is not quantified. At the end of the 90-day exposure


period, a total acid-soluble chloride profile is built based on ionic concentration values determined at

different depths from the exposure surface. As outlined in the standard test method itself, the described

procedure is not intended to quantify the performance of the concrete exposed to chlorides. In addition,

chloride binding by the aluminate phases is not accounted for, given that the total soluble chloride con-

centration is determined. Thus, this test is not suitable for durability assessment of the concrete cover,

as discussed by Stanish et al. (1996). In order to overcome some of the complexities arising from sample

conditioning, the bulk diffusion test was developed and standardized by NT 443 (1995) and by ASTM

C1556 (2016). In the bulk diffusion test, the concrete sample is exposed on one face and submerged

in an NaCl solution after being cured in limewater, thus eliminating the initial sorption effect arising

from unsaturated capillaries. While this test provides a better depiction of the ionic diffusion into the

concrete pore system, it requires a long period of time of exposure, specially for high quality concretes,

which does not serve the purpose of a quality control test, but rather that of an acceptance test provided

that enough time is available before the start of the project. The exposure period, which can be of up to

90 days in some cases (Stanish et al., 1996), can affect the interpretation of the results. In fact, the long

exposure period allows additional curing of the concrete, modifying its microstructure and transport

properties and thus affecting the computed diffusion coefficients. The latter represents an evolution of

diffusion coefficients over the exposure period and is hard to interpret (Nokken et al., 2006).

2.4.2 Rapid Chloride Permeability Test

The rapid chloride permeability test (RCPT) is a common name used to describe ASTM C1202, “Elec-

trical Indication of Concretes Ability to Resist Chloride Ion Penetration” (ASTM C1202, 2019). The

name can be misleading since it is a measure of rate of movement of ions, and not a direct measure of

penetrability (permeability). In addition, it is misleading due to the nature of ions penetrating; in fact;

ions other than chlorides could and do affect the measurement. It monitors “the amount of electrical

current passed through 50 mm thick slices of 100 mm nominal diameter cores or cylinders during a 6 h

period, under a constant potential difference of 60 V DC” (ASTM C1202, 2019). One of the exposed

faces of the concrete specimen is in contact with a 3% by mass NaCl solution and connected to the

negative terminal of a power supply unit through a stainless-steel electrode. The other face of the con-

crete specimen is in contact with a 0.3 M NaOH solution and connected to the positive terminal of

the power unit through another stainless-steel electrode. The current is monitored using an ammeter

or a combination of a voltmeter and a resistor of known resistance, depending on the data acquisition

equipment available. The current value is recorded at the start of the test, and at least at every 30 min

elapsed for a total duration of 6 hours. Note that the test temperature must be maintained between

20 °C and 25 °C. At the end of the test, the total charge passed Q (coulombs, C) can be computed as

follows:

Q = 900(I0 + 2I1 + ...+ 2I330 + I360) (2.1)

where:

Q the total charge passed during the 6-h period (C);

I0 the current intensity at the beginning of the test (A);

It the current intensity at the time t (A); and

t the time elapsed from the start of the test (min)

The value of the total charge Q is used as an estimate of the chloride penetration resistance of the


concrete and as a mixture acceptance quality control tool. For instance, the CSA A23.1-19 standard

specifies limits of total passed charged determined in accordance with CSA A23.2-23C (similar to ASTM

C1202) for concretes of different exposure classes. Class C-XL and A-XL concretes should have a total

charged passed lower than 1000 C by 90 days of age (CSA A23.1, 2019). The standard itself suggests a

scale of permeability as a function of the total charge passed, as shown in Table 2.2.

Table 2.2: Chloride Ion Permeability Based on Total Charge Passed (ASTM C1202, 2019)

Charge Passed (C) Chloride Ion Permeability

> 4,000 High

2,000 - 4,000 Moderate

1,000 - 2,000 Low

100 - 1,000 Very Low

< 100 Negligible

Two main sources of bias are identified when using the rapid chloride permeability test to evaluate the

durability performance of the concrete cover as follows:

• The total charge passed corresponds to all ionic movements: the ASTM standard cites certain

points affecting the results such as the use of surface treatment and the use of calcium nitrite

and some other chemical admixtures. In addition, the use supplementary cementitious materials,

which affect the composition of the pore solution affect the computed total charge passed. The

nature and concentration of the pore solution ions have a considerable effect on the charge passed,

as outlined by Pilvar et al. (2015). This concern is especially important when evaluating the

durability performance of concretes exposed to chloride ions given that they are most commonly

produced with blended cements containing relatively high level of supplementary cementitious

material replacement levels and corrosion inhibiting admixtures.

• The thickness of the sample reduces the test sensitivity to curing quality: the test specimens used

are 50 mm thick. Thus, the total charge passed correlates to the permeability of the full 50 mm

thick depth of concrete, which is of the same order as the cover depth. The computed charge value

is therefore an average of the permeability gradient, which is affected by curing quality. Thus, the

RCPT does not allow for a quantification of the effect of curing. On the other hand, reducing the

sample thickness is not a viable solution since it induces erroneous results due to the effect of the

interfacial transition zone. Thus, it is a trade-off between the accuracy of the test and the precision

and the RCPT presents issues for both of these parameters.

2.4.3 Chloride Migration Test

Ionic migration describes the ionic flux simultaneously caused by both an ionic concentration gradient and

an electric field. However, the effect of the electrical field tends to dominate. Standard test procedures

such as Nordtest NT 492 (NT 492, 1999) have been widely adopted with the following functioning

principle: an external electrical potential is applied axially across a concrete specimen, forcing ingress

of chloride ions from a sodium chloride solution. The specimen lies such that one face is exposed to

the sodium chloride solution, and the other to a sodium hydroxide solution acting as the anolyte. After

a fixed period with applied potential difference, the specimen is split open and sprayed with a silver


nitrate solution, enabling a colorimetric determination of the chloride penetration front. This enables

the determination of a non-steady state apparent migration coefficient. In order to quantify the effect

of curing, as a modification to the test, Hooton et al. (2002) suggested performing the test in way that

the colorimetric determination of the chloride front is done on a face orthogonal to the curing face,

providing a curing-affected chloride migration profile. Limitations related to this test method are more

complicated sample preparation and testing, as well as the interference of the coarse aggregates with

the colorimetric measurements. In fact, the split surfaces are rough and include relatively high levels of

variability of the chloride migration front, which reduces the accuracy and precision of the test method

(Hooton, 2015).

2.4.4 Sorptivity Methods

Fundamental concepts underlying sorptivity methods are found in Chapter 3. Sorptivity methods aim at

characterizing the rate of capillary suction of a liquid in unsaturated concrete pores. Different method-

ologies have been developed throughout the years, leading to a standard test method, ASTM C1585

(ASTM C1585, 2013). Original developments reported by Senbetta & Scholer (1984) suggested evaluat-

ing the effectiveness of curing compounds on the permeability of the near-surface concrete by profiling

the “absorptivity. The sample used were 10 mm thick disks, with one face exposed to water for a period

of 60 s. The method was standardized by ASTM C1151 (ASTM C1151, 1991) but later withdrawn due

to lack of use. The accuracy of the method is questionable given the sample preparation procedure and

the small thickness of the sample, which might cause the interfacial transition zone at the aggregates

to interfere with the results. Aiming at obtaining performance metrics for quality assurance measures,

DeSouza et al. (1998) developed an in-situ sorptivity device. Reported limitations include the unpre-

dictable moisture content in a structure which has a significant effect on the rate of capillary suction

(Nokken & Hooton, 2002). In order to overcome this limitation, DeSouza et al. (1997) suggested devel-

oping calibration curves for the moisture content of the field concrete to the rate of absorption of the

same concrete determined in controlled laboratory conditions. The authors did use of the test apparatus

for quality assurance purposes in a precast plant where the moisture condition for all samples, after a

given time period, was almost identical.

2.4.5 Electrical Resistivity Methods

The measurement of electrical properties of cementitious systems has gained popularity in the past few

decades. Many studies, such that of Tumidajski et al. (1996), and Gudimettla & Crawford (2014) have

investigated the use of electrical resistivity to characterize concrete transport properties in many ways

including external measurements (bulk resistivity, surface resistivity), and electrode embedment tech-

niques. The present is an overview of testing principles and the correlation between test results and

durability performance.

The composite nature of concrete makes it such that a bulk volume of concrete has three distinct

phases, each with its own electrical properties. The three phases are the vapour phase (air), the fluid

phase (pore solution), and the solid phase (aggregates and cementitious paste). The electrical conduc-

tivity that occurs in a bulk volume of concrete is mainly attributed to the fluid phase, given that the

conductivity of the vapour and solid phases are 11 and 17 orders of magnitudes lower than that of the


fluid phase respectively (Spragg et al., 2012). The electrical resistivity of a bulk concrete volume can be

expressed as shown in Equation 2.2, adapted from Spragg et al. (2013):

ρbulk = ρsolFf(S)f(Ttest)f(leach)f(microstructure) (2.2)

where:

ρbulk the bulk electrical resistivity;

ρsol the electrical resistivity of the pore solution;

F the formation factor;

f(S) a function describing the effect of the degree of saturation;

f(Ttest) a function describing the effect of testing temperature;

f(leach) a function describing the effect of leaching; and

f(microstructure) a microstructure function

The formation factor F is an intrinsic property of the porous medium. It is a function of the mix-

ture design and proportions, degree of hydration, and sample history. It is the parameter of interest

when evaluating the permeability of concrete given that it can be related to the concrete’s transport

properties. That is related to the porous medium’s porosity and pore structure connectivity in accor-

dance to Archie’s law (Archie, 1941), and corrected for different pore solution electrical resistivities.

Snyder (2001) has found experimental validation of the Nernst-Einstein relationship relating the forma-

tion factor to ionic diffusion, as shown in Equation 2.3, which corresponds to a fully saturated porous

system at a given testing temperature.

F =ρbulkρsol

=D0

D=

1

Φβ(2.3)

where:

D0 the self-diffusion coefficient of a certain ion, for instance Cl-;

D the diffusion coefficient of the same ion in concrete;

Φ the porosity of the cementitious system; and

β the pore structure connectivity

The bulk resistivity is determined from the electrical resistance. It is defined as the value of electri-

cal resistance for unit dimensions. The resistance (R) is determined by different means, of which the two

most popular are the surface resistivity (Wenner array) and the bulk resistivity. For all cases Equation

2.4 applies. High frequency AC signals (order of 10 kHz) are used in order to minimize the polarization

effect at the electrodes. In fact, the relationship between impedance and resistance is non-linear and

non-bijective and is a function of the concrete microstructure and the interface concrete-electrode (Layssi

et al., 2015). Commercial devices based on this principle are available and are becoming widely used.

ρ = Rk = Zcos(ϕ)k (2.4)


where:

R the electrical resistance;

k the shape factor, defined differently for every measurement configuration;

Z the electrical impedance; and

ϕ the phase angle between the input and output signals

Given the increasing and popular use of electrical resistivity as a way to characterize concrete ma-

terials, Spragg et al. (2012) conducted an inter-laboratory study, including 13 testing laboratories, to

evaluate the precision and bias of the uniaxial bulk electrical resistivity measurement made on sealed

cured concrete cylinders. A coefficient of variation of between 3 % and 4 % for within-laboratory mea-

surement was determined at all test ages. The multi-laboratory coefficient of variation was between 8

% and 13 %, depending on the test age. Gudimettla & Crawford (2014) also evaluated the sensitivity

of uniaxial and surface resistivity measurement over a range of concrete specimens used in different

construction projects. It was concluded that the inter-lab coefficient of variation of the tests are 12 %

and 13 % respectively for uniaxial and surface measurements, which is less than the 18 % of the rapid

chloride permeability test (ASTM C1202, 2019).

2.5 Durability Performance Prediction

As the RILEM definition of performance-based specifications suggests (Beushausen & Fernandez, 2016),

the goal in specifying minimum performance metrics is to predict the durability performance of a given

concrete and its resistance to specific deterioration mechanisms. The deterioration mechanisms associ-

ated with chloride-induced corrosion is comprised of two interacting parts, as outlined by Tuutti (1982):

the damage initiation, consisting of mass and ionic transport of chlorides to the reinforcement, and the

damage propagation, involving electro-chemical reactions at the steel-concrete interface (SCI). Each of

the phases presents its own challenges, briefly outlined below, based on Angst (2019):

• The interaction of mass and ionic transport processes in the concrete cover and their sensitivity to

the fluctuation in environmental conditions such as temperature, precipitation, relative humidity,

and occurrence of splash due to traffic.

• The definition of the failure of the system’s durability performance can be variable. For instance,

when computing the probability of damage, would a probability of failure correspond to a certain

percentage of the reinforcing bars starting to corrode, or that a percentage of the surface of a

structure starts corroding.

• The definition of the parameter(s) controlling the dynamics of the damage mechanisms. In fact,

defining a critical chloride content at SCI is common practice, however, the experimentally obtained

values scatter over a large range, ranging from virtually 0 to 3% chloride by mass of cement. In

addition, no general trends can be identified with respect to mixture design and proportioning

parameters.

• The role of structural elements in the design, such as the size of the reinforced concrete member

and the size of the reinforcing steel bars.

The role of the elements of the structural design, such as the size of the reinforced concrete member

and the size of the reinforcing steel bars. Thus, correlating performance metrics from test results to an


absolute durability performance is very challenging. A comparative approach is more realistic; however,

it does not satisfy the ultimate goal of performance prediction based on performance metrics. The

focus of this study, as mentioned earlier, lies in the transport rates of aggressive agents, chloride ions

specifically, rather than the damage propagation and its mechanisms.

2.6 Case of Canadian Precast Concrete

Clause 23.2.3.9 of the “Precast Concrete - Materials and Construction” standard (CSA A23.4, 2016)

requires that the curing of precast concrete members be in accordance with the specified regimes de-

scribed in Table 19 of CSA A23.1-19 (reproduced in Table 2.1 of this thesis) unless equivalent or superior

performance is proven for all metrics of interest. In the case of C-1 and C-XL exposure class concretes,

the standard requires a minimum curing period of 7 days at a minimum temperature of 10 °C which

corresponds to considerable resources (time, energy, and special storage space) that are needed to be

invested by the precast concrete producers. A common curing method used in precast concrete produc-

tion is accelerated moist curing at temperatures as high as 60 °C. It provides early strength gain in a

considerably shorter amount of time compared to ambient temperature moist curing. Other forms of

accelerated curing are also used, such as the use of high early-strength cements, which cause an increase

in temperature through accelerated highly exothermic hydration set of reactions.

Concerns have risen regarding the effect of accelerated moist curing on the microstructural development

of portland cement concrete and its transport properties due to the production of a coarser microstruc-

ture, as described by Kjellsen et al. (1990), Kjellsen (1996), and Bu et al. (2014). However, the use of

supplementary cementitious materials, such as silica fume and blast furnace slag, mitigate the negative

effects of high temperature curing, as demonstrated by Detwiler et al. (1994), and Hooton & Tither-

ington (2004). Given that C-1 and C-XL concretes need to satisfy certain maximum coulomb values

determined by RCPT (CSA A23.1, 2019), and that these values are usually not achievable without the

inclusion of supplementary cementitious materials, most precast concrete is now designed for chloride ex-

posure contains SCMs. For these reasons, the Canadian Precast/Prestressed Concrete Institute (CPCI)

believes that their product meets the required performance levels without additional moist curing. A

study conducted by the National Research Council of Canada (Makar, 2014), for the CPCI, consisted of a

round-robin evaluation of concrete samples produced in different Canadian precast plants and subjected

to different curing regimes recording the compressive strength and charge passed corresponding to the

rapid chloride permeability test (RCPT). The samples used for RCPT were obtained from core segments

and correspond to a 50 mm thickness starting 10 mm in from the slab’s boundary. It was concluded

that all samples tested satisfied the minimum compressive strength and maximum charge value limits

of Clause 4.1.1.1.3 in CSA A23.1 (CSA A23.1, 2019). In addition, when compared to samples from the

same concrete subjected to a Type 3 curing regime (Table 2.1), most of the accelerated cured sample

subjected to air curing after cooling were of equivalent chloride permeability performance, at a confidence

level of 95 %, and when tested at 56 days of age. The only significant difference observed in the chloride

permeability performance was caused by the inclusion of samples subjected to variant accelerated curing

methods, which suggested the possibility of following standard accelerated curing practices for better

quality prediction.


Given the multi-mechanistic nature of chloride ingress, relying on RCPT results performed on sam-

ples extracted from a certain depth underneath the member’s boundary does not necessarily guarantee

a satisfactory performance level. In fact, the near-surface concrete can be unsaturated, with a fluc-

tuating degree of saturation, causing mass transport through capillary absorption. The depth of the

zone controlled by capillary absorption is variable, and is a function of the concrete quality, and the

micro-climatic conditions in specific regions of the structure, as experimentally validated by Hudec et al.

(1986) on in-situ structures. In addition, given the thickness of the test samples, it can be challenging

to quantify the effect that the curing regime has on the near-surface permeability, as well as the depth

of that effect. For these reasons, a more representative test method, or set of test methods, is needed

along with a comprehensive decision-making framework. The work presented in Chapter 3 provides an

approach for satisfying the need for better performance metrics.

Chapter 3

Sorptivity Profiling

This test procedure was suggested by Hooton (2015) when evaluating the effective difference on transport

properties of different curing regimes for the Canadian Precast/Prestressed Concrete Institute. It is a

modified version of the ASTM C1585-13 “Standard Test Method for Measurement of Rate of Absorption

of Water by Hydraulic-Cement Concretes” (ASTM C1585, 2013), modified to determine the initial

rate of sodium chloride solution absorption as a function of depth from the surface of interest. This

Chapter builds upon work previously performed at the University of Toronto (Dadic, 2018) and aims

at offering a comprehensive description of the suggested test method, as well as experimentally derived

information that is key to implement the test method. Suggestions are made regarding the definition of

a decision-making framework for the determination of curing adequacy from a performance specification

perspective.

3.1 Introduction

Hydraulic diffusivity arising from capillary suction in an unsaturated porous media is, as mentioned

in Chapter 2, commonly encountered in concrete structures. The pressure gradient in the concrete

capillaries cause a Darcian fluid flow in the material, as expressed in Equation 3.1 where K and Fc are

a function of the moisture content θ (Hall, 1989).

q = K(θ)Fc(θ) (3.1)

where:

q the vector flow velocity;

K(θ) the hydraulic conductivity; and

Fc(θ) the capillary force, which equals the gradient of capillary potential (Ψ); Fc = −∇Ψ(θ)

In the case of one dimensional flow, which is of interest in this study, and assuming isotropic conductivity

K(θ), Equation 3.1 can be written as:

q = −D(θ)dθ

dx(3.2)

16

Chapter 3. Sorptivity Profiling 17

where:

D(θ) the hydraulic diffusivity, expressed as D(θ) = dΨdθ ; and

x the dimension of fluid flow

The concrete property that is of interest is the diffusivity coefficient, D. Note that this variable refers

to capillary, and not molecular diffusivity. Kelham (1988) formulated the mechanism of uni-directional

capillary suction in an unsaturated concrete volume and stated, with analytical and numerical support,

that the capillary suction due to a capillary pressure is orders of magnitude larger than the hydrostatic

pressure in most practical cases. Unsaturated fluid flow into an initially dry porous solid can be charac-

terized using sorptivity, as proven by Hall (1989) and Kelham (1988). Determining sorptivity is relatively

easy and leads to characterizing the capillary suction at a certain moisture content with a single test

metric. A common experimental metric used to characterize capillary flow in unsaturated concrete is

the absorption rate (S), which is related to absorption, and is defined in Equation 3.3, adopted by the

ASTM C1585 standard:

I = St12 +A (3.3)

where:

I the cumulative water absorption, per unit area of the inflow surface;

t the elapsed time;

A a residual obtained by linear fitting of i and t12 , corresponding to the filling of open surface

porosity on the inflow and adjacent surfaces;

i the absorption, it = ∆mt

a∗ρ ;

∆mt the change in mass of the specimen at time t;

a the area of the exposed surface; and

ρ the density of the absorbed fluid

The cumulative water absorption evolves in a bi-linear fashion (Figure 3.1) as a function of the square

root of time, thus defining two distinct values of S: the initial and secondary absorption rates. The

initial absorption rate corresponds to the filling of unsaturated capillary pores, and is thus dependant

on the quality of curing, whereas the secondary absorption rate corresponds to the hydraulic diffusivity

into the entrained and entrapped air voids (Li et al., 2012). From a durability point of view, where the

goal is to characterize the ingress of aggressive agents (such as chloride ions) in unsaturated concrete,

initial sorptivity is of interest since it depicts the rate of transport associated with damage initiation as

suggested by (Tuutti, 1982).

Initial Absorption Secondary Absorption

√Time

Absorption

0

Figure 3.1: Absorption Behavior

In addition, Kelham (1988) demonstrated the relationship between the depth of absorbed water and


the capillary pressure (Equation 3.4); which in turn allows the formulation of the rate of absorption

in function of the capillary pressure, which is constant throughout the concrete volume under pressure

equilibrium conditions. The equilibrium conditions are satisfied for values of exposure time corresponding

to a penetration depth of at least 10% of the concrete volume (Kelham, 1988). The capillary pressure is

formulated based on the Kelvin-Laplace equation (Equation 3.5).

x(t) =

√2KPcapϕµ

t (3.4)

where:

x(t) the fluid penetration depth at time t;

K the intrinsic permeability;

Pcap the capillary pressure;

ϕ the porosity of the concrete; and

µ the dynamic viscosity of the absorbed fluid

Pcap =RTln(RH)

Vm(3.5)

where:

R the universal gas constant;

T the absolute temperature;

RH the relative humidity of the concrete pore system; and

Vm the molar volume of the pore solution

Numerous studies have shown the efficacy of sorptivity for characterizing cementitious systems, such

as that of Parrott (1992), Martys & Ferraris (1997), and Dias (2000). Sorptivity is affected by varying

water to binder ratio, mixture design, curing, and maturity/age. Thus, one could quantify the effect of

curing on fluid penetration resistance by evaluating sorptivity. The assumption of isotropic conductiv-

ity does not always hold, specially in the case of poorly cured cover concrete. Due to the differential

hydration kinetics arising from the differential moisture and temperature conditions in the cover region,

the permeability of the concrete is not uniform. In order to quantify this effect, Hooton et al. (1993)

suggested evaluating the initial sorptivity at different depths from the member’s boundary, which corre-

sponds to the surface of ingress of aggressive agents. Core samples extracted from slabs were used in their

study, corresponding to depths of 0, 5, 10, 15, 20, 25, 30, 50, and 75 mm from the cast surface. Figure

3.2 shows the initial rate of absorption profile obtained for a ternary blend 0.43 w/c concrete subjected

to 3 different curing regimes. The test was proven to be highly sensitive to curing quality. The value

of the initial sorptivity spans an order of magnitude for the different curing regimes. In addition, the

depth of the member’s boundary effect can be quantified, and it is inversely proportional to the moisture

state during curing and the length of the curing period. Two parameters can be used to characterize the

effect of curing; first the gradient of the initial rate of absorption, which is greater as the desiccation,

caused by the low humidity curing environment, is higher. The second is the absolute value of the initial

absorption rate, even after the gradient is null. For instance, for the air cured specimen, the differential

effect of curing acts over the first 25 mm from the surface, however the effect of poor curing is observed

when comparing the initial rate of absorption at higher values of depths to that corresponding to the


fog cured concrete. The advantages of this test include its simplicity and rapidity. In addition, it is

highly sensitive to curing imperfections, particularly when compared to tests such as the rapid chloride

permeability where the output is a function of the average chloride penetration resistance of the test

50-mm thick test specimen. It is worth noting that the sorptivity value at a formed or finished surface

might not be consistent with the sorptivity at different depths even with a good quality curing and that

could be due to the poor particle packing at the member’s boundaries, or concrete skin, as described by

Kreijger (1984).

0 10 20 30 40 50 60 70 800

50

100

150

200

250

Depth from Surface (mm)

Init

ial

Ab

sorp

tion

Rat

e(∗

10−

6mm/min

1/2)

fog cured at 27 °C3 days of fog cured at 27 °C then air

air cured at 50 % RH at 23°C

Figure 3.2: Evolution of Initial Absorption as a Function of Depth (adapted from Hooton et al. (1993))

Given that the curing has to satisfy the prescribed requirements of Clause 7.7 (CSA A23.1, 2019), one

could suggest quantifying the performance obtained applying the prescribed curing and comparing it to

the performance of a certain curing regime of interest for a given concrete. The quantification has to be

such that the relationship between performance and test results should correspond to a unique range of

performance metrics, and the test should provide enough information on the quality of the curing. Figure

3.3 describes this concept. Defining a performance baseline for comparison is challenging given that it

requires defining a curing temperature for which the lowest performance is reached while conforming to

the relevant specifications. The minimum curing temperature allowed by CSA A23.1-19 is 10 °C, and can

go up to a temperature such that the internal temperature does not exceed the limits in clause 7.6.3.2.4,

while maintaining a temperature gradient smaller or equal to 20 °C (clause 7.6.3.2.5.1). In addition,

maximum internal temperatures allowed are inconsistent between the CSA A23.1-19 and CSA A23.4-16

standards. Given that the CSA A23.1-19 standard allows for the adoption of alternative curing regimes

if equivalent performance is demonstrated, and considering sorptivity as a representative performance

metric, accepting a curing regime applied to a given concrete mixture comes down to evaluating the


following:

H0 : Sd,regime ≤ Sd, prescribed

HA : Sd,regime > Sd, prescribed

where:

H0 the null hypothesis for which the curing regime would be acceptable;

HA the alternative hypothesis for which the curing regime would not be acceptable;

Sd,regime the initial rate of absorption at a depth d corresponding to the curing regime tested

(mm/min1/2); and

Sd, prescribed the initial rate of absorption at the depth d corresponding to the prescribed curing

regime (mm/min1/2)

The experimental determination of Sd,regime and Sd, prescribed is described later in this Chapter. These

performance metrics can be replaced or complemented by others from a variety of test methods. Differ-

ent frameworks for assessing the curing quality are explored in Section 3.5. The significance level of the

hypothesis testing can be defined by the decision maker. For example, a transportation agency could

allow acceptance of concrete not subject to the required specified curing regime if the initial sorptivity,

experimentally derived, satisfies H0 at a confidence level of 90%.

Experience

Prescriptive

SpecificationPerformance Metric

Performance

SpecificationMechanistic Approach

Quantification

Figure 3.3: Graphic Representation of the Development Objective

An alternative to the comparative approach would be defining a maximum value of the initial rate of

absorption for which the cured concrete is of acceptable durability performance. This approach would be

similar to that adopted when specifying a maximum value of the total charge passed determined by the

rapid chloride permeability test as a function of the exposure class and intended service life (CSA A23.1,

2019). This approach is preferred over a comparative approach given that it solely relies on performance

metrics and their correlation with service life. However, defining a maximum value of the initial rate of

absorption can be challenging. Both options are discussed in this thesis.

3.2 Test Procedure

The present procedure is based on the “Standard Test Method for Measurement of Rate of Absorption

of Water by Hydraulic-Cement Concretes”, ASTM C1585 (2013), with a set of modifications as follows:

1. Use of a 3.0% by mass NaCl solution as the absorbed solution.

2. Record the test specimen’s mass at time intervals of 0, 1, 5, 10, 20, 30, 60, 120, 180, 240, 300, and

360 minutes of exposure to the solution.


3. Once the measurements are done, split the specimen along its diameter cylindrical axis and spray

the two fracture surfaces with a 0.1 N solution of AgNO3 and determine the average chloride ion

penetration depth colorimetrically.

In addition to these modifications, the test specimens were conditioned in a different way. Given that the

rate of capillary absorption and the total mass of water absorbed, are a function of the initial moisture

content of the absorbing medium, the latter needs to be consistent over all test specimens in order to

have a comparative basis. In addition, the initial degree of saturation of the test specimen affects the

evolution of the measured absorption with time. Zhutovsky & Hooton (2019) evaluated the effect of 2

different drying procedures on the water absorption behavior of mortar. The first consists of placing the

test specimens for 3 days in a 50 °C and 80 % relative humidity chamber, after which the specimens are

placed in a sealed container for at least 15 days at room temperature to allow for moisture redistribution

as per ASTM C1585 (2013). The second consists of drying the specimens at 60 °C until constant mass

is reached (corresponding to a daily mass change of less than 0.2 %). The first observation noted by

the authors was the effect on the temporal evolution of absorption. The transition between initial and

secondary absorption is sharper with a higher coefficient of determination for the each of the two linear

trends for the specimens dried at 60 °C. In addition, a better correlation between sorptivity and water-

to-cementitious material; level of slag replacement; chloride migration coefficient (as per Nordtest NT

492); total charge passed as per ASTM C1202); electrical conductivity (as per ASTM C1760); threshold

pore diameter; and capillary porosity was observed. Thus, drying at 60 °C until a constant mass seems

to be a more appropriate method and is adopted in this study. Once the constant mass is achieved, the

test specimens are placed in sealed containers and allowed to cool at room temperature. Containers are

7 L in volume and contained 6 specimens each. Other sample conditioning methods have been explored

in different studies, including Hooton et al. (1993) and Castro et al. (2011). The former authors, whose

absorption profiles are presented earlier in this Chapter, submerged test specimens in isopropyl alcohol

for seven days, followed by drying in a vacuum oven at 50 °C until constant mass was obtained (defined

by a daily mass change equal to or lesser than 0.2 %). Castro et al. (2011) compared four sample

preparation procedures: oven drying at 105 °C, placing in environmental chambers at 23 °C and 50 %,

65 %, and 80 % relative humidity respectively until constant mass is reached. The constant mass was,

once again, defined as a daily mass change equal to or lesser than 0.2 %, and took up to 14 months

of conditioning to be achieved for samples placed in the 50 % RH chamber. The initial rate of water

absorption defined by the cumulative absorption over the initial 6 hours after contact with water was

found to be up to ten times larger for samples conditioned in the 50 % RH chamber when compared to

similar samples stored at 80 % RH (Castro et al., 2011). This difference is due to the contribution of

the gel pores, when the degree of saturation drops considerably below RH levels of 80 % (based on the

Kelvin-Laplace relationship). As the degree of saturation of gel pores is reduced, the moisture gradient

between the pore system and the absorbed fluid is increased, and thus the rate of suction is increased. In

addition, after conditioning in the different environments, similar samples were subjected to the ASTM

C1585 (2013) conditioning procedure. The absorption behavior, and initial rate of absorption, were con-

siderably different across the four set of samples. This suggests that the conditioning procedure in the

ASTM C1585 standard does not eliminate the “moisture history” (Castro et al., 2011). For this reason,

the samples in this study are obtained from concrete slabs that are conditioned in the same environment

over a considerable time period after the initial curing period and before being conditioned for testing.


The test specimens are obtained from slab cores, as described in ASTM C1585 (2013). The exper-

imental setup is shown in Figure 3.4. All surfaces, except for the exposed surface, are sealed with

electric tape (radial), or a plastic sheet (planar). The specimens lie on a grid support, allowing exposure

to the solution. Adjacent meshes of the grid are spaced 11 mm apart center to center, and are 1 mm wide.

The depth values at which initial rate of absorption is determined can vary based on the goal of the

investigation. The effect of curing is variable throughout the depth of the concrete volume, and can be

highly variable near the surface (Hooton et al., 1993). The quality of curing affects the initial sorptivity

values in the member’s boundary but also the variability of that parameter as a function of depth, as

discussed previously. Both the depth of the differential curing effect and the numerical values of the

initial rate of absorption need to be evaluated. Thus, a smaller spacing value between test depths is

desired near the member’s boundary.

In order to be implemented and used as a quality assurance tool, the test method would need to be

evaluated over a large set of samples in order to determine its coefficient of variation, to allow calculation

of the precision of the test (ASTM C670, 2015). ASTM C1585 reports a coefficient of variation of 6 % for

the absorption values recorded by a single operator in a single laboratory. It is believed that the changes

in procedures do not affect the sensitivity of the test, except for the change in the drying procedure,

which should provide more precise results. An evaluation of the sensitivity of the test method, and a

comparison to that of the ASTM C1202 and C1876 tests is presented later in this Chapter.

Figure 3.4: Sorptivity Measurement Setup (Adapted from ASTM C1585 (2013))

3.3 Previously Collected Absorption Profiles

In her thesis, Dadic (2018) compiled the experimental data obtained from performing the modified

absorption test procedure described earlier on a set of 2 concrete mixtures of relatively high quality

(conforming to the CSA A23.1-19 C-1 and C-XL exposure classes requirements). The slabs made from

these mixtures were subjected to accelerated heat curing at early age, and then conditioned in different

environment including submersion in a saturated calcium hydroxide solution, and air curing. The test

specimens were placed in a 50 °C oven for 3 days and then placed in a sealed container for 4 days at 50

°C, allowing for internal moisture redistribution, as described by DeSouza et al. (1997). This method


allows for quicker sample preparation and has demonstrated equivalent results to the sample preparation

procedure specified in ASTM C1585 (2013) (DeSouza et al., 1997). The following is an analysis of the

experimental data collected by Dadic in order to define the data collection time frame of the modified

absorption test and estimate its effect on interpreting the test outcomes.

The ASTM C1585-13 standard defines the initial rate of absorption as described in Equation 3.3 for

time values ranging from 1 minute to 6 hours. The slope has to be such that the coefficient of de-

termination (R2) is equal to or larger than 0.98 (ASTM C1585, 2013). The majority of the data sets

obtained by Dadic do not satisfy this requirement. In order to assess the effect of the time range used for

computing the rate of absorption, the maximum value of time for which the coefficient of determination

is equal to or larger than 0.98 was computed for all absorption experiments and denoted as tmax,0.98.

Based on tmax,0.98, a value for the so-called initial rate of absorption was computed, noted as Smax,0.98,

and is shown as a function of tmax,0.98 in Figure 3.5. A general trend can be observed: concrete samples

with relatively high initial rate of absorption values, defined by a coefficient of determination of 0.98

or more, tend to have lower initial absorption capacity. In other terms, the absorption of concretes of

lower quality seem to deviate from an initial linear initial rate of absorption at earlier test times when

compared to concretes of better quality. The values of tmax,0.98 span the interval [36 ; 400] minutes with

a mean value of 214.8 minutes and a standard deviation of 115.0 minutes. The deviation from linear

temporal evolution of absorption could be partially of fully explained by the sample conditioning, as

mentioned earlier and experimentally demonstrated by Zhutovsky & Hooton (2019).

0 50 100 150 200 250 300 350 400

2.5

3.5

4.5

5.5

tmax,0.98(min)

Smax,0.9

8(∗

10−

2mm/m

in1/2)

Figure 3.5: Correlation Between Smax,0.98 and tmax,0.98 (raw data from Dadic (2018))

3.4 Experimental Program

The experimental investigation has two objectives:

1. Determining whether the test method is sensitive enough to detect sorptivity differences caused


by different curing regimes as well as sorptivity gradients caused by differential curing effects.

2. Evaluating the quality of the test results in assessing the adequacy of curing by quantifying the

confidence level of the inference on curing quality, and by comparing it to that of the test results

from the rapid chloride permeability and bulk electrical resistivity tests.

Note that all tests presented here are performed as a function of a distance to the formed surface of a

concrete element. This decision was made since the precast products exposed to chlorides are mainly

bridge girders, where the formed surfaces are exposed to chlorides (splashing, misting, drainage from

deck, etc.) and finished surfaces are typically covered by structural elements such as decks, offering

protection from chlorides.

3.4.1 Round 1

A concrete mixture proportioned as as shown in Table 3.1, was used in this phase of the experimental

program.

Table 3.1: Concrete 1 Mixture Proportions (expressed in kg/m3 except for admixtures, expressed in mLper 100 kg cementitious material)

Coarse Aggregate 960

Fine Aggregate 850

Cement (GU) 284

Blast Furnace Slag 71

Water 142

Water-reducing Admixture 275 mL/100 kg

Air-entraining Admixture 150 mL/100 kg

For each of the curing regimes, the following were cast:

• Absorption tests: Two 450 mm x 290 mm rectangular, 100 mm thick slabs. Slabs were cored in

accordance to ASTM C42 (2018), obtaining 8 - 100 mm diameter cores from each slab. The cores

were then cut into 50 mm high specimens using a water-cooled diamond saw. The cut specimens

were such that one of their test surface lied 0 mm, 2 mm, 8 mm, 14 mm, 20 mm, and 50 mm away

from the formed surface of the slab. The 0 mm and 50 mm specimens were obtained from the

same original 100 mm high core, saving time and material. Note that this implies that the 50 mm

cut surface was effectively at 53 mm from the formed surface. Three replicates were obtained for

each depth from the formed surface. The specimens were used for absorption testing, as described

in Section 3.3, such that the surface exposed to the NaCl solution is the closest to the slab formed

surface. Sample conditioning started at 28 days of age, and constant mass was achieved after 7

to 8 days in a 60 °C oven. Once constant mass was reached, samples were placed in a sealed

container, allowing for cooling and moisture re-distribution, for one day. Circumferential surfaces

of each specimen were sealed with vinyl electrical tape and the non-exposed planar surface covered

with cling film. At the end of the 6-hour test, samples were split open diametrically and each of

the two fracture surfaces was sprayed with a 0.1 N AgNO3 solution. The depth of penetration of

the chloride ions was determined colorimetrically, given that silver-coloured AgCl precipitates in


the presence of Cl- and AgNO3 turns brown. The penetration depth was determined at 10 mm

intervals along the diameter of the split core.

• Electrical tests: Similarly to the above, two replicates for each depth from the formed surface were

obtained for rapid chloride permeability (RCPT) and bulk electrical resistivity (BR) testing. At

28 days of age, samples were placed in a desiccator under vacuum for 3 hours. De-aired water was

then introduced to the desiccator until full submersion of the concrete specimens. The vacuum was

maintained in the desiccator using a pump for an additional hour, after which the pressure was

released and the specimens were kept submerged for another 18 hours. At the end of the submersion

period, the surfaces of the samples were wiped with a wet cloth and the electrical impedance of

each specimen was measured. 2-mm thick sponges were placed between each electrode and the

corresponding specimen surface. The sponges were saturated with a simulated pore solution, as

required in ASTM C1876 (2019). Each sample was then tested in accordance with the 6-hour

ASTM C1202 (2019) (RCPT) after sealing all circumferential surfaces with electrical tape. At the

end of each test, the total charge passed was obtained, and normalized for the sample’s dimensions.

• Compressive strength tests: Six 100 mm x 200 mm cylinders. Cylinders were used for compressive

strength measurements at 3, 7, and 28 days in accordance with ASTM C39 (2018).

The following curing regimes were applied to each set of concrete samples:

• Regime AC23: Casting and air curing at 23 °C and 50% relative humidity until 7 days of age, after

which the samples are placed in a 10 °C chamber with a relative humidity of 80 %.

• Regime MC10: Preconditioning the mixing material at 10 °C and casting at room temperature.

Cast samples are then covered by plastic sheets to minimize moisture loss and kept at room

temperature until 18 hours of age. Samples are then demolded and placed in sealed containers in

the 10 °C chamber, suspended on top of a layer of water to keep the air within the container humid

until 7 days of age. The samples are then removed from the sealed containers and placed back in

the chamber until 28 days of age.

• Regime MC23: Casting at room temperature and covering the fresh samples with a plastic sheet

to minimize moisture loss until 1 day of age, after which the samples are placed in a 23 °C 100 %

relative humidity moist chamber until 7 days of age, then placed in the 10 °C chamber to 28 days.

• Regime HC10: Casting at room temperature then placed in a programmed environmental chamber

at 95 % relative humidity and temperature cycle as shown in Figure 2.2, with a preset period at 30

°C for 4 hours, a linear temperature increase to 50 °C during a 1.5 h period. The peak temperature

was maintained for a period of 7.5 hours, after which the temperature was linearly decrease down

to 25 °C in a period of 2 hours. At the end of the cycle, the samples are stored in the 10 °Cchamber until 28 days of age.

A thermocouple was placed at the center of a slab for each set of concrete samples subjected to a given

curing regime. The slump and fresh air content were measured in accordance with ASTM C143 (2015)

and ASTM C231 (2017) respectively.

Given the large volume of concrete samples, and for logistical reasons, the test samples were cast as

follows:


1. Four slabs and six cylinders subjected to MC10 used for absorption testing, RCPT, BR, and

compressive strength measurements.

2. Six slabs, such that one pair of slabs was subjected to one of the AC23, MC23, and HC10 regimes,

used to perform the RCP and BR measurements.

3. Six slabs and eighteen cylinders, equally divided into three groups, each subjected to one of the

AC23, MC23, and AC10 regimes, used for absorption and compressive strength testing.

0 6 12 18 24 30 36 42 480

10

30

50

Time (h)

Tem

per

ature

(°C

)

AC23MC10MC23HC10

Figure 3.6: Concrete 1 Temperature History

Concrete slabs were cored between 14 and 15 days of age, while the cores were cut and/or end ground

between 21 and 23 days of age. During the time from the specimens were cored and cut until they

were conditioned for testing, all surfaces of the specimens were exposed to air, causing desiccation. The

experimental results might have been affected by this desiccation. This effect was accounted for during

Rounds 2 and 3 of the experimental programs. In fact, after coring the slabs between the age of 14 and

15 days, all cores has the radial and cast surfaces sealed with cling film to minimize desiccation, leaving

only the formed surface exposed to air. In addition, cutting the test specimens was done closer to the

conditioning age (28 ± 2 days).

3.4.2 Round 2

Round 2 of the experimental investigation was designed based on the results of the first set of tests of

Round 1, later presented in Section 3.5 of this chapter. These experiments were based on that of Round

1, with the following modifications:

1. Given that the effect of curing conditions on the chloride penetration resistance of the region

near the formed surface is to be evaluated, any interference from other parameters need to be

avoided. Moisture and heat flow from the finished surface, as well as other formed surfaces might


be affecting the hydration and properties development in the near-formed surface region. For this

reason, moisture and heat barriers were placed on the finished and side-formed surfaces of every

slab. This was done using plastic sheeting for moisture and 25 mm thick Dow Styrofoam insulation

panels. These were placed right after casting and finishing, and re-placed after demolding (24 hours

after casting).

2. The additional curing at 10 °C and 80 % RH in Round 1 might have favored better microstructural

development (discussed later in this Chapter). The environmental conditions between 7 days and

28 days, or date of delivery to site, are variable and affect the test results. These are a function of

the precast plant storing area, weather, etc. Ambient air curing at room temperature was adopted

in this set of experiments, as described later.

The concrete mixture used in this set of experiments was proportioned as shown in Table 3.2. The

following curing regimes were applied to each set of concrete samples:

• Regime AC: The concrete samples were placed in the laboratory at 23 °C and 50 % RH from

finishing until 28 days of age.

• Regime MC: The concrete samples were placed in a moist chamber at 23 °C and 100 % RH right

after demolding and until 7 days of age. Samples were then stored in the laboratory at 23 °C and

50 % RH until 28 days of age.

• Regime HC/AC: Samples were placed in an environmental chamber and subjected to an accelerated

heat curing cycle at 95 % RH. The preset period is of 3 hours at 30 °C, after which the temperature

was linearly increased up to 60 °C for 2 hours. The peak temperature is maintained for 12 hours,

and linearly reduced to 25 °C at a rate of 15 °C/h. At the end of the cycle, samples were demolded,

removed, and placed in the laboratory at 23 °C and 50 % RH until 28 days of age.

• Regime HC/MC: Samples were placed in the environmental chamber similarly to the HC/AC

regime. At the end of the 18.5-hour cycle, samples were removed, demolded, and placed in the

moist chamber at 23 °C and 100 % RH until 7 days of age. At the end of the moist curing period,

samples were placed in the laboratory at 23 °C and 50 % RH until 28 days of age.

Table 3.2: Concrete 2 Mixture Proportions (expressed in kg/m3 except for admixtures, expressed in mLper 100 kg cementitious material)

Coarse Aggregate 1060

Fine Aggregate 835

Silica Fume blended Cement (GUb8SF) 300

Blast Furnace Slag 100

Water 160

Water-reducing Admixture 250 mL/100 kg

Air-entraining admixture 140 mL/100 kg

Temperature histories were obtained from embedded thermocouples in the slabs (Figure 3.7). Note that

the temperatures have cooled to ambient and are stable beyond 2 days of age, so are not shown on the

plot. Given the large volume of concrete samples, and for logistical reasons, the test samples were cast

as follows:


1. Eight slabs; with two slabs subjected to one of either the AC, MC, HC/AC, and HC/MC regimes,

used to perform absorption measurements.

2. Eight slabs; with two subjected to one of AC, MC, HC/AC, and HC/MC regimes, used to perform

RCPT and BR measurements.

3. Twenty-four cylinders; each six subjected to one of either the AC, MC, HC/AC, and HC/MC

regimes, used to perform compressive strength measurements.

0 6 12 18 24 30 36 42 480

20

40

60

Time (h)

Tem

per

ature

(°C

)

ACMC

HC/AC

HC/MC


3.4.3 Round 3

For this round of experiments, concrete samples were obtained from a precast concrete production

plant in Ontario. Given that the mixture design is proprietary, not all information is disclosed. The

cementitious materials used were general use cement (Type 1) and granulated ground blast furnace slag,

with a water-to-cementing material ratio of 0.37. Air-entraining and water-reducing admixtures were

also used. The aggregate used was a crushed stone with maximum particle diameter of 15 mm. The

cast was divided into 4 equal groups, subjected to the following curing regimes:

• Regime AC: The concrete samples were cast and covered with plastic sheeting and a Styrofoam

panel and placed in the production plant at 18 °C until demolding at 20 hours after casting. Once

demolded, the samples were placed with bottom formed surface exposed to air in a van at 20 °Cand 60 % RH for 2 hours until transported to the University of Toronto. Once at the laboratories,

the samples were placed in a 50 % RH and 23 °C environment until 28 days of age.

• Regime MC: The concrete samples were cast and covered with plastic sheeting and a Styrofoam

panel and placed in the production plant at 18 °C until demolding at 20 hours after casting. Once

demolded, the samples were placed with bottom formed surface covered with a saturated burlap

until transported to the Concrete Materials Laboratories at the University of Toronto. Once at


the laboratories, the samples were placed in a moist chamber at 23 °C and 100 % RH right until

7 days of age. Samples were then stored in the laboratory at 23 °C and 50 % RH until 28 days of

age.

• Regime HC/AC: Samples were cast and introduced into the production plant kiln with a relative

humidity of 100 %. The temperature profile was such that the preset period is of 3 hours at 37 °C,

after which the temperature is linearly increased at a rate of 3.75 °C/h until it reaches 50 °C. The

peak temperature was held for 10 hours and then reduced at a constant rate of 15 °C/h down to

20 °C. At the end of the cooling period, the samples were retrieved from the kiln, demolded and

subjected to the treatment described in AC.

• Regime HC/MC: Samples were cast and introduced to the kiln with the temperature profile de-

scribed in HC/AC. At the end of the curing cycle, samples were demolded, and the treatment

described in MC is applied to all samples.

The temperature history for each curing regime are shown in Figure 3.8. All samples were obtained

simultaneously from a single cast at the production plant. Similarly to Round 2, the cast surfaces were

covered with plastic sheeting and a 2.5 cm thick Styrofoam panel in order to simulate a semi-infinite

volume and isolate the curing effects to the formed surfaces.

0 6 12 18 24 30 36 42 480

20

40

60

Time (h)

Tem

per

ature

(°C

)

ACMC

HC/AC

HC/MC


3.5 Results and Discussion

The following presents the results of the experimental determination of the initial rate of absorption for

each of the 3 concrete mixtures subjected to the different curing regimes. Similar plots are produced for

each of the depth of chloride front determined colorimetrically, the total charge passed determined in

accordance with ASTM C1202, and the bulk electrical resistivity values. Note that the plots show the

average values along with error bars, on teh profile plots, corresponding to the standard deviation.


One of the objectives of the experimental work performed was to determine certain precision metrics.

The average coefficient of variation of the initial rate of absorption, determined on 3 test specimens at

a time, is 5.1 % over all the tests performed. The changes in conditioning and testing procedure did not

alter the repeatability and precision of the test results. They did, however, provide more representative

results, as demonstrated by Zhutovsky & Hooton (2019). In addition, all initial rate of absorption values

were determined with a coefficient of linear determination, R2, of the relationship between the square

root of time and absorption values equal to or larger than 0.98, as required by ASTM C1585 (2013).

This implies that in the time domain of 1 minute to 6 hours, which was considered in the regression,

the behavior was linear corresponding to the initial rate of absorption. Figure 3.9 shows the linear

absorption behavior within the 6-hour exposure.

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

0 mm2 mm8 mm14 mm20 mm50 mm

Figure 3.9: Example of an Absorption Evolution Plot (Concrete 2, Regime AC)

A variety of decision-making frameworks can be considered for the evaluation of the suitability of a

given curing regime. These would serve in designing performance specifications for concrete curing.

Two specification types can be defined. First, proving equivalent performance of a given curing regime

to the prescribed regime. This would require testing the specific concrete to be exposed to chlorides

subjected to both curing regimes and comparing the test results. The second type would be an absolute

performance specification where a threshold for a given performance metric is prescribed. This is adopted

in the CSA A23.1-19 standard for chloride exposed concrete (C-1 and C-XL exposure classes) where

maximum charge passed during the 6-hours RCPT by 91 days of age (CSA A23.1, 2019). In both cases,

the test metric(s) used need to be accurate and reliable. This Chapter evaluates a variety of potential

test metrics as well as their reliability based on the data obtained from the experimental program. The

following are the performance metrics evaluated in this Chapter:

1. Initial rate of absorption at each test depth;

2. Adjusted total charge passed at each depth;


3. Bulk electrical resistivity at each depth;

4. Integrated initial rate of absorption over the entire 50-mm depth domain;

5. Integrated adjusted total charge passed over the entire 50-mm depth domain; and

6. Integrated bulk electrical resistivity over the entire 50-mm depth domain.

Compressive strength, although routinely used for quality assurance, is not considered given that it is well

established that it is not a representative index of mass and ionic penetration resistance. In addition, the

chloride penetration depth, determined colorimetrically, is not considered as a decision-making metric

in this study due to the inherent variability in the corresponding results and the impractical aspect of

conducting the measurements.

3.5.1 Round 1 Test Results

Figures 3.10, 3.11, 3.12, 3.13, and 3.14 summarize the outcomes of the tests performed on the samples

subjected to the different curing regimes. The initial rate of absorption profiles are not uniform, which

was originally expected. The variability is partially, and considerably, due to the moisture loss from the

finished surface of the slab, which was not protected. This can be clearly observed on the plot showing

the evolution of the total charge passed. The samples subjected to AC23 have a charge value at 50 mm

2-times larger than that at 20 mm from the formed surface. The order of curing regimes in function

of the corresponding initial rate of absorption is not consistent throughout the sample depth, nor is it

when compared to the other test metrics. The most consistent test metric throughout the depth from

formed surface is the chloride penetration depth, determined colorimetrically. It is also believed that

the additional curing at 10 °C and 80 % RH for 21 days after the initial 7 days may have period induced

significant changes to the property development.

0 10 20 30 40 50

8

10

12

14


Init

ial

Rat

eof

Ab

sorp

tion

*10

−2

(mm/m

in1/2)

AC23MC10MC23HC10

Figure 3.10: Depth Evolution of the Initial Rate of Absorption - Concrete 1


0 10 20 30 40 500

5

10

15

20


Ch

lori

de

Fro

nt

(mm

)

AC23MC10MC23HC10

Figure 3.11: Depth Evolution of the Chloride Penetration Front - Concrete 1

0 10 20 30 40 500

20

40

60

80


Bu

lkE

lect

rica

lR

esis

tivit

y(Ohm.m

)

AC23MC10MC23HC10

Figure 3.12: Depth Evolution of the Bulk Electrical Resistivity - Concrete 1. The abscissa correspondsto one of the specimen test planes


0 10 20 30 40 50

2,000

3,000

4,000

5,000

6,000

7,000


Ad

just

edT

otal

Ch

arg

eP

asse

d(C

)

AC23MC10MC23HC10

Figure 3.13: Depth Evolution of the Total Charge Passed - Concrete 1. The abscissa corresponds to thetest surface in contact with the NaCl solution

Day 3 Day 7 Day 280

5

10

15

20

25

30

Com

pre

ssiv

eS

tren

gth

(MPa)

AC23 MC10 MC23 HC10

Figure 3.14: Compressive Strength Gain - Concrete 1

Given that no significant trend was observed, performance metrics 4, 5, and 6 as defined previously,

were quantified for each curing regime. Based on each of the metric, a ranking of the curing regime was

suggested. Lower values of integral initial rate of absorption and charge passed were associated with

better performance, whereas higher values of integral electrical resistivity were associated with better

performance. Tables 3.3, 3.4, and 3.4, show the outcome of each suggested performance metric. In

addition to the absolute and relative value of the metric, the confidence level (as a percentage) on the

difference between two successively ranked regimes for each of the four metrics is shown in the tables.


Inconsistencies in ranking are also observed here. However, HC10 is ranked least favourable regime in 2

out of the 3 cases shown; mainly the integral charge passed, and electrical resistivity. Note, however, that

these results are not representative given the inconsistencies observed and the lack of proper conditioning

during curing of the slabs.

Table 3.3: Integral Initial Rate of Absorption - Concrete 1

Ranking MC10 < HC10 < AC23 < MC23

Sin,integral (10−3mm3.min−1/2) 5.457 < 5.484 < 5.560 < 6.176

Normalized w.r.t best 1 < 1.005 < 1.019 < 1.132

Conf. level on difference (%) 54.14 ; 61.22 ; 94.14

Table 3.4: Integral Charge Passed - Concrete 1

Ranking MC23 < MC10 < AC23 < HC10

Qintegral (kC.mm) 138 < 177 < 211 < 216



Table 3.5: Integral Electrical Resistivity - Concrete 1

Ranking MC10 > MC23 > AC23 > HC10

ρintegral(kOhm.m.mm) 3.61 < 3.52 < 3.34 < 2.88

Normalized w.r.t best 1 > 0.973 > 0.925 > 0.796


Note that these results are not reliable due to the errors in sample preparation; mainly emerging from

not covering the cast surface, as explained earlier in Section 3.4.



0 10 20 30 40 50

2

3

4

5

6


Init

ial

Rat

eof

Ab

sorp

tion

*10

−2

(mm/m

in1/2)

ACMC

HC/AC

HC/MC


0 10 20 30 40 500

2

4

6

8

10

12

14

16


Ch

lori

de

Fro

nt

(mm

)

ACMC

HC/AC

HC/MC


Considering the initial rate of absorption evolution plotted in Figure 3.15, the curing-affected zone can

be estimated. It is defined as the distance from the formed surface after which the profiled metric is

relatively constant. For the heat cured and moist samples, this depth is around 20 mm from the formed

surface; whereas for the air cured samples, it is beyond 20 mm but cannot be determined since the last

measurement performed is at 50 mm. However, this value is below 50 mm for the air-cured samples


since the value of initial rate of absorption for MC and AC is equal at 50 mm from the formed surface.

This validates the choice of test domain, 0 mm to 50 mm, which includes any variation in properties

that needed to be detected.

Unlike the profiles obtained from Concrete 1, the initial rate of absorption profiles for Concrete 2 are

nearly consistently decreasing with test depth and a clear hierarchy can be defined. As expected, the

air cured samples present the highest absorption values, given that desiccation during setting and cur-

ing is the most pronounced on these samples. At the other extreme of the test output domain, are

the initial rate of absorption values corresponding to the HC/MC regime. The output metrics range

from 3.332∗10−2 to 6.797∗10−2mm/min1/2, which spread corresponds to 105 % of the minimum value.

Given the extent of the range of values, the initial rate of absorption is sensitive to variation due to curing.

Given that the baseline considered in this set of experiments is the CSA A23.1-19 prescribed moist

curing (regime MC), the initial rate of absorption, considered as the performance metric, is compared

for every alternate curing regime with respect to MC. The ratio of the initial rate of absorption of each

alternate regime to that of MC for each test depth is shown in Table 3.6. A ratio value equal to or

smaller than 1 would correspond to a curing quality as good as or better than that obtained with the

prescribed curing. Considering the case of accelerated heat curing where the additional moist curing

after heat treatment is to be evaluated, one could evaluate the ratio corresponding to HC/AC in Table

3.6. In parenthesis is the value of the confidence level for each comparison, obtained from a one-tail

student-t test. Based on these results, and considering the initial rate of absorption as the only perfor-

mance metric, one could conclude, with high confidence, that HC/AC is as good or even better than

MC for the concrete tested.

Table 3.6: Ratios of Initial Rates of Absorption with respect to MC - Concrete 2

Depth (mm) Air Cured Heat Cured + Air Heat Cured + Moist

0 1.21 0.79 (99.1%) 0.74

2 1.24 0.95 (78.3%) 0.86

8 1.25 0.72 (99.7%) 0.69

14 1.17 0.87 (95.4%) 0.69

20 1.21 0.76 (99.1%) 0.73

50 0.99 0.73 (99.9%) 0.73

One can be certain of a difference of the chloride penetration resistance between the air cured (AC)

and moist cured concretes (MC). Thus, the comparison between AC and MC can be used to assess the

reliability of each test method used to assess the fluid penetration resistance. In other terms, quantifying

the confidence on differentiation between the 2 curing regimes using each of the initial rate of absorption,

the total charge passed by RCPT, and the bulk electrical resistivity. This is done using a single tail

student-t test on the observation of each set of measurements, summarized in Table 3.7.


Table 3.7: Confidence on Differentiation Between AC and MC - Round 2

Depth (mm) Absorption RCPT BR

0 99.5% 55.8% 90.6%

2 95.8% 81.8% 17.0%

8 95.8% 81.4% 86.9%

14 97.5% 75.6% 43.2%

20 99.3% 97.4% 88.4%

50 61.2% 98.4% 24.9%

The bulk electrical resistivity and charge passed, shown in Figures 3.17 and 3.18 respectively, do not

provide as consistent profiles as did the initial rate of absorption. All charge values are considerably

below the 1000 C charge limit for C-XL concrete prescribed in CSA A23.1-19 (CSA A23.1, 2019). The

charge values for heat cured samples (HC/AC and HC/MC) are negligible and fall on the low range

of the test output (Figure 3.18). As shown in Table 3.7, the ASTM C1202 test was not capable of

detecting any significant difference between the air-cured (AC) and moist-cured samples (MC). That is

mainly due to the thickness of the test specimen (50 mm), which extends beyond the curing-affected

zone detected by absorption profiling, as described earlier. The same limitation is observed in the output

of the bulk electrical resistivity tests shown in Figure 3.17. In addition, there is a variation in electrical

properties, with a drop in electrical resistivity, and corresponding increase in charge passed for the 50

mm specimens. This change is due to the fact that the corresponding test specimens lie in the top 50

mm of the 100-mm thick slab thickness, with one face corresponding to the cast surface. It is more

sensitive to self-desiccation since it was not in contact with moisture during curing to allow additional

hydration. In addition, the finishing practice affects the properties of the tested specimens.

0 10 20 30 40 50

300

350

400

450

500

550


Bu

lkE

lect

rica

lR

esis

tivit

y(Ohm.m

)

ACMC

HC/AC

HC/MC



0 10 20 30 40 50300

400

500

600

700


Ad

just

edT

ota

lC

harg

eP

asse

d(C

)

ACMC

HC/AC

HC/MC

Figure 3.18: Depth Evolution of the Total Charge Passed - Concrete 2.The abscissa corresponds to thetest surface in contact with the NaCl solution

Day 3 Day 7 Day 280

10

20

30

40

Com

pre

ssiv

eS

tren

gth

(MPa)

AC MC HC/AC HC/MC

Figure 3.19: Compressive Strength Gain - Concrete 2

The average chloride ion penetration front determined colorimetrically, shown in Figure 3.16, correlates

well with the initial rate of absorption, as expected. However, the depth values are highly variable, with

a coefficient of variation up to 20 %. This relatively high variability is expected given that the capillary

absorption is not uniform across the volume of a concrete test specimen given its composite nature

and the interfering role of aggregates and interfacial transition zones. Figure 3.20 shows images of the

colorimetric profile determined for the formed surface specimens for each curing regime tested. McCarter

et al. (1992) demonstrated that the chloride penetration front lags behind the water penetration front


in chloride solution absorption experiments. The difference was attributed to the adsorption of chloride

ions by the hydrated cement paste, reducing the chloride ionic concentration in the absorbed fluid below

the colorimetric detection limit (McCarter et al., 1992).

(a) AC (b) MC

(c) HC/AC (d) HC/MC

Figure 3.20: Colorimetric Chloride Penetration Front - Concrete 2

Finally, Figure 3.19 shows the compressive strength development of the concrete subjected to the different

curing regime. As expected, the strength gain for heat cured concrete is larger at first (day 3) due to the

early maturity gain. However, the ultimate strength gain can be compromised, as described by many,

including Gallucci et al. (2013). Compressive strength testing is solely used for quality assurance in

this study since it is well established that it is not a good indicator of chloride penetration resistance,

particularly considering the focus on differential hydration in the near-surface depth. Tables 3.8, 3.9,

3.10 show the integral values over the full test domain (0 to 50 mm) for each test metric considered.

The highest spread in value, between the best and least performing curing regime, is observed for the

integral initial rate of absorption, which increases the confidence in this performance metric. In addition,

the confidence level of the differences between HC/AC and MC is very high (99.99 %). The integral

values are sufficient to conclude on the quality of a curing regime given that the integral total charge

passed has a wide spread and a high level of confidence between MC and AC. On the other hand, as

described earlier, the RCPT is deemed to be insensitive to curing effects, particularly as the majority of

the domain tested using this decision metric is beyond the curing affected zone.


Ranking HC/MC < HC/AC < MC < AC

Sin,integral (mm2) 1.77 < 1.88 < 2.43 < 2.78




Table 3.9: Integral Bulk Electrical Resistivity - Concrete 2

Ranking HC/MC > HC/AC > MC > AC

ρintegral (kOhm.m.mm) 23.5 < 22.3 < 17.5 < 16.5



Table 3.10: Integral Total Charge Passed - Concrete 2

Ranking HC/AC < HC/MC < MC < AC

Qintegral (kC.mm) 21.1 < 22.6 < 27.7 < 32.6



Concluding this round, absorption profiling is deemed to be highly sensitive to differential curing in the

near-surface zone. Heat curing followed by air curing performed as good, and even better, than the

prescribed 7-day moist curing for the concrete tested. The depth of the curing affected zone is less than

50 mm, making RCPT and resistivity testing unable to accurately assess curing performance.


0 10 20 30 40 50

2

3

4

5

6


Init

ial

Rat

eof

Ab

sorp

tion

*10

−2

(mm/m

in1/2)

ACMC

HC/AC

HC/MC



0 10 20 30 40 500

2

4

6

8

10

12

14

16


Ch

lori

de

Fro

nt

(mm

)

ACMC

HC/AC

HC/MC


Figure 3.23: Example of Highly Variable Colorimetric Chloride Front

Once again, looking at Figure 3.21, profiling the initial rate of absorption provides a good representation

of the differential hydration, and fluid penetration resistance, caused by the different curing regimes. A

similar trend is observed in the test results of Round 2, such that there is a strong gradient in the rate

of capillary absorption in the region close to the formed surface, after which the rate is effectively con-

stant, corresponding to practically uniform hydration. Accelerated moist curing followed by air curing

(HC/AC) did not provide performance equivalent to the prescribed 7-day moist curing (MC) for this

concrete. The best performing curing regime is accelerated moist curing followed by moist curing up

to 7 days of age (HC/MC), as expected. The superiority in performance, corresponding to a relatively

low initial rate of absorption, is attributed to the rapid gain in maturity at early age followed by moist

curing to further advance the degree of hydration. Note that all profiles, including moist-curing (MC

and HC/MC), present a gradient in the first few millimeters from the formed surface. This is due to the

loss of moisture to the exterior between the end of the curing (day 7) to the start of the conditioning

(day 28). A similar trend is observed on Figure 3.22, where the depth of chloride penetration was de-

termined colorimetrically. The standard deviation is not shown on the plot due to the high variability

in the data. In fact, aggregate interferences, as well as the non-uniform penetration front, caused a

wide spread in the depth values, as also noted by McCarter et al. (1992). Figure 3.23 shows examples


of chloride colorimetric profiles with high variability. One could hypothesize that the variability might

be totally or partially due to improper sealing of the sides of the cores, causing drying in the radial

direction. However, this would imply deeper penetration at the extremities of the fractured specimen,

which is not the case.

In order to further validate the sensitivity of the initial rate of absorption to the quality of curing,

a comparison of the curing regimes AC and MC is done. Once again, it is assumed that there should be

significant differences between these curing regimes, and the goal is to assess the outcome of their com-

parison using the three performance metrics evaluated; the initial rate of absorption (modified ASTM

C1585), the uniaxial bulk electrical resistivity (ASTM C1876), and the total charge passed (ASTM

C1202). Table 3.11 summarizes the outcome of a 1-tailed student-t test.

Table 3.11: Confidence on Differentiation Between AC and MC - Round 3

Depth (mm) Absorption RCPT BR

0 99.5% 79.4% 87.3%

2 99.8% 59.5% 42.0%

8 96.3% 55.9% 61.3%

14 99.3% 67.2% 92.9%

20 98.1% 96.7% 72.0%

50 55.0% 89.2% 87.6%

Based on Table 3.11, the ability of the initial rate of absorption at detecting curing differences is superior

to that of the total charge passed and electrical resistivity. For depth values between 0 mm and 20 mm,

where the effect of curing is the highest, the confidence on the difference is at least 96.3 %, as compared

to as low as 55.9 % and 42.0 % for the total charge passed and electrical resistivity respectively. Note

that any value lower than 50 % would correspond to a comparison where it is concluded that the air-

cured concrete performed better than the moist-cured concrete.

Table 3.12: Initial Rate of Absorption with respect to MC - Concrete 3

Depth (mm) Air Cured Heat Cured + Air Heat Cured + Moist

0 1.21 1.11 (16.4%) 0.83

2 1.33 1.07 (12.2%) 0.89

8 1.28 1.04 (53.4%) 0.93

14 1.16 1.13 (15.2%) 0.98

20 1.20 1.06 (32.4%) 0.88

50 1.00 1.02 (32.2%) 0.90

Considering the 7-day moist curing as a baseline, Table 3.12 can be used for comparison of the other test

curing regimes, using the initial rate of absorption as the comparative metric. The heat curing followed

by air curing (HC/AC) did not perform as well as moist curing for 7 days (MC), given that the ratio

of their respective initial rate of absorption for every test depth is larger than 1. The percentage value


in parenthesis corresponds to the confidence level that HC/AC is as good as MC (outcome of a 2-tailed

student-t test). These values are very low, meaning we are confident that HC/AC is underperforming

with respect to MC, however not significantly since the ratio is not excessively larger than 1.

0 10 20 30 40 500

50

100

150

200


Bu

lkE

lect

rica

lR

esis

tivit

y(Ohm.m

)

ACMC

HC/AC

HC/MC


0 10 20 30 40 50300

600

900

1,200

1,500


Ad

just

edT

otal

Ch

arge

Pas

sed

(C)

ACMC

HC/AC

HC/MC

Figure 3.25: Depth Evolution of the Total Charge Passed - Concrete 3. The abscissa corresponds to thetest surface in contact with the NaCl solution

Figures 3.24 and 3.25 show the profiles of the uniaxial bulk electrical resistivity and total charge passed

respectively. Although the former has lower data variability, none of these two performance metrics


provide a sense of the differential hydration in the near-surface region, as quantified in Table 3.11. The

profiles are effectively constant throughout the depth domain. All tested concrete complies with the

maximum allowable charge passed as per ASTM C1202, even the air cured concrete, as shown in Figure

3.25. As described in the Concrete 2 Section, this inability to differentiate is mainly due to the nature

of the test, where output is a function of the full thickness of the specimen (50 mm), which is usually

beyond the depth of the curing-affected zone. This same depth can be estimated by looking at the initial

rate of absorption profiles in Figure 3.21. Given that AC, MC, ad HC/AC are effectively the same at

50 mm, it is inferred that the depth of the curing effect is smaller than 50 mm from the formed surface.

The heat curing followed by moist curing (HC/MC) has a lower value of rate of absorption at 50 mm

due to an overall higher maturity and finer pore structure.

Figure 3.26 shows the compressive strength gain of Concrete 3 subjected to the different curing regimes.

Note that no data is available for the 3-day compressive strength. Compressive strength is used for

mechanical quality assurance purposes but cannot be used to evaluate the durability performance.

Day 3 Day 7 Day 280

10

20

30

40

50

60

Com

pre

ssiv

eS

tren

gth

(MPa)

AC MC HC/AC HC/MC

Figure 3.26: Compressive Strength - Concrete 3

In an attempt at defining a unique performance metric, the integral values, defined earlier, are also used

here. Tables 3.13, 3.14, and 3.15 summarize the comparison between the curing regimes with respect to

the integral initial rate of absorption, electrical resistivity, and charge passed respectively. Once again,

the inadequacy of the electrical methods at detecting curing effects is highlighted by the fact that air

curing (AC) is ranked as the best option, which is absurd (Tables 3.14 and 3.15). When considering the

integral initial rate of absorption, the ratio of HC/AC to MC is 1.045 with a confidence on the difference

of 82.9 %. Thus, the accelerated moist curing followed by air curing underperforms the 7-day moist

curing, but only by a small range.



Ranking HC/MC < MC < HC/AC < AC

Sin,integral (mm2) 1.59 < 1.76 < 1.75 < 2.00



Table 3.14: Integral Bulk Electrical Resistivity - Concrete 3

Ranking AC > HC/AC > HC/MC > MC

ρintegral (kOhm.m.mm) 8.92 < 8.91 < 8.49 < 8.39



Table 3.15: Integral Total Charge Passed - Concrete 3

Ranking AC < HC/MC < MC < HC/AC

Qintegral (kC.mm) 52.6 < 55.8 < 55.9 < 58.6



Concluding this round of experiments, the initial rate of absorption is once again deemed to be sensitive

to differential curing in the cover zone and its effect on rate of transport within the pores. Unlike Round

2, the accelerated moist curing followed by air curing (HC/AC) did not provide performance similar to

the prescribed 7-day moist curing (MC). This highlights the differences in interactions between various

concrete mixtures and curing regimes. Thus, it is important to assess every concrete mixture subjected

to the desired curing regime(s) for pre-qualification purposes.

3.6 Conclusions

The method suggested and evaluated in this Chapter is profiling of the initial rate of absorption of a

sodium chloride solution throughout the initial 53 mm from a member’s formed surface. The develop-

ment, backed by a review of fundamentals and the relevant literature, led to the following conclusions:

1. The initial rate of absorption of a 3.0 % by mass sodium chloride solution measured at surfaces at

different distances from the formed surface of concrete slabs is highly sensitive to curing. Profiling

this metric in the cover zone provides an estimate of the depth of the curing-affected zone and its

impact on chloride penetration resistance.

2. The average coefficient of variation of the absorption values is 5.38 %, over 528 samples tested

(each sample comprised of 3 test specimens). The coefficient of variation of the calculated initial

rate of absorption is 5.1 % on average over 48 samples, each comprised of 3 test specimens. This

satisfies the need to estimate the number of samples necessary to test based on a desired confidence

level for comparing curing regimes. Based on the experiments performed, one can conclude with


very high confidence the difference in ability to resist chloride penetration based on the initial rate

of absorption.

3. The rapid chloride permeability (ASTM C1202) and uniaxial bulk electrical resistivity (ASTM

C1876) tests are not sensitive to depth-differential curing. This conclusion is made based on the

inability of the electrical test methods evaluated at differentiating between air curing and moist

cured concrete.

4. The experimental program demonstrated that curing performance equivalent to the prescribed

7-day moist curing can be empirically proven by profiling the initial rate of absorption in the

near-surface zone. The legitimacy of the equivalence between accelerated moist curing followed by

air curing and the prescribed 7-day moist curing in CSA A23.1-19, needs to be assessed for every

given concrete and accelerated curing regime. The test can be done as a pre-qualification measure

and the optimal additional moist curing period post-heat curing can be determined, based on the

desired performance level.

5. Combining the well-established RCPT (ASTM C1202), or uniaxial bulk electrical resistivity (ASTM

C1876), with the suggested absorption profiling test is a plausible solution to assess durability per-

formance of concrete production.

6. As demonstrated by D. P. Bentz et al. (2001), sorptivity can be included in service life predictions.

A mechanistic approach could eventually be adopted rather than a comparative approach involving

the prescribed 7-day moist curing. However, this would require standardization of the test method,

particularly the test specimen conditioning procedure given that the rate of capillary absorption

is highly dependant on the capillary pressure in the pore system (as described previously based on

the Kelvin-Laplace equation).

7. Specifications could eventually incorporate the maximum acceptable value of the initial rate of

absorption, similar to what is already established for the total charge passed as per ASTM C1202.

8. The question of having an individual test depth or a set of depths defining a profile for capillary

absorption arises. The highest sensitivity to curing is detected at the formed surface itself (test

depth 0 mm). Using 100 mm cores, one could obtain test specimens at 50 mm, which would be

beyond the curing-affected zone. Finally, an intermediate depth could be used to get a better

estimate of the extent of the curing effect; 8 mm is a reasonable value based on these experiments.

Obtaining more data is encouraged, especially since the process would only need to be performed

once for pre-qualification of a curing regime for a concrete mixture.

9. When it comes to quality assurance and compliance testing, the number of test depths could be

reduced for simplicity. Performing the test at the formed surface (test depth of 0 mm) would be a

quick way to assess compliance. However, it is the most conservative approach and could lead to

false positives since the formed surface is the most affect by curing.

10. The chloride penetration front determined colorimetrically correlates with the initial rate of absorp-

tion with a coefficient of determination R2 = 0.80. The chloride penetration depth, as formulated

by Kelham (1988), could provide more information, however, the high variability in the data, as

well as the extra work involved, make it impractical for pre-qualification and quality assurance

purposes.

Chapter 4

Formation Factor Characterization

4.1 Introduction

The following presents a methodology for quantifying the effect of curing on the chloride penetration

resistance of the near-surface concrete cover zone by mapping its electrical resistivity. Figure 4.1 sum-

marizes the relation between electrical resistivity, durability performance, and the parameters affecting

interpretation of the measurements, which were introduced in Chapter 2.

Mixture Design/

Proportioning, Curing

History, & Finishing

Performance

Bulk Electrical

Resistivity

Formation

Factor

Frequency &

Shape Factor

Temperature, Degree

of Saturation, & Pore

Solution Resistivity

Figure 4.1: Graphic Representation of the Mechanistic Evaluation Framework

The goal of the development and experimental investigation is to accurately estimate the impact of cur-

ing on the chloride penetration resistance of the near-surface concrete volume by means of the formation

factor. The formation factor can be related to the ionic diffusion coefficient (Snyder, 2001); the Darcian

permeability coefficient (Katz & Thompson, 1986) (Nokken & Hooton, 2008); and the rate of capillary

absorption (Moradllo et al., 2018). The intrinsic characterization would enable a multi-mechanistic per-

formance assessment of curing. The challenges, as presented in subsequent sections of this Chapter, fall

into two categories: first, the design of an experimental setup that would provide accurate and precise

measurements, and second, the extraction of useful information in the form of performance metrics,

47

Chapter 4. Formation Factor Characterization 48

while minimizing effects from extrinsic interfering parameters.

Given a concrete volume, one could discretize it into finite volumes of representative size for which

the bulk electrical resistivity can be measured in each of its three main orthogonal axes. The continuity

of the volume would enable a spatial mapping of the bulk electrical resistivity and its variation through-

out the full volume of interest. Accounting for parameters affecting the resistivity, but independent of

the concrete transport properties, one could translate the experimental result into a map of the chloride

penetration resistance and transport properties of the concrete, by the mean of the formation factor,

and show its variability throughout the volume of interest. This calibration process, based on first prin-

ciples and empirical derivations, would be advantageous in two distinct ways. First, by evaluating the

quality of the concrete, its finishing, and its curing; and secondly, by providing data for the service-life

performance prediction. Equations 4.1 and 4.2 formulate the concept, first qualitatively by mapping the

electrical resistivity (eq. 4.1); and relating it to differential resistance to mass and ionic transport (eq.

4.2).

δCuring

δx∝ δρ

δx(4.1)

δCuring

δx∝ δF

δx= −δD

2

δx= −2

δdcδx

δk−2

δx(4.2)

where:

x the dimension of the curing differential; and

dc the critical pore diameter.

4.2 Review of Electrode Embedment

The electrical properties of concrete have been measured using embedded electrodes in different layouts

and for different purposes. This section presents a review of electrode embedment in concrete both for

mixture characterization at early ages and for long-term monitoring.

Many studies evaluated the use of early-age electrical resistivity measurements for quality assurance

and as an acceptance criterion for ready-mixed concrete site delivery. Obla et al. (2018) performed

resistance measurements on fresh concrete using a pair of parallel electrodes embedded in cylinders

during the first 90 min after casting, in addition to measuring the electrical resistivity of the corre-

sponding pore solution. A wide range of water-to-cementitious material ratio was used. A linear corre-

lation between the pore solution resistivity and the water-to-cementitious materials ratio was observed.

The measured resistivity correlates well with the computed resistivity based on the National Institute

of Standards and Technology (NIST) pore solution conductivity estimation tool developed by Bentz

(2007) (nist.gov/el/materials-and-structural-systems-division-73100/inorganic-materials

-group-73103/estimation-pore). However, the relationship’s linearity does not hold for concrete

electrical resistance. The average prediction error was 7.4 kg of water per cubic meter of fresh concrete

in the original mixture. Sallehi et al. (2018) adopted a similar approach and computed the formation

factor as a function of time during initial setting and hardening for a variety of pastes containing differ-

nist.gov/el/materials-and-structural-systems-division-73100/inorganic-materials-group-73103/estimation-pore

nist.gov/el/materials-and-structural-systems-division-73100/inorganic-materials-group-73103/estimation-pore


ent cements and pozzolans. The authors observed an increase of the formation factor during hardening

corresponding to the development of the microstructure. The effect of SCMs on the microstructure was

observed and quantified, corresponding to a higher formation factor due to the decrease in porosity and

increase in tortuosity in accordance to Archie’s law. It was suggested that fresh paste samples be used

instead of concrete in order to eliminate the variability for different aggregate sources and gradings,

which do not play a role in hydration kinetics.

Rajabipour et al. (2007) suggested a comprehensive material health monitoring framework for con-

crete structures using a set of sensors that measured the electrical resistivity of concrete, along with 3

sensors, measuring the pore solution resistivity, the temperature, and the relative humidity respectively

enabling calibration of the measurement. The goals of such a monitoring system are material failure

prevention and prediction; service life improvement; and quality control and assurance (Rajabipour et

al., 2007). A similar approach was adopted by McCarter et al. (2017). The approach of the latter is

part of a larger research program initially developed to obtain information on the water front during

absorption testing. McCarter et al. (1995) developed a probe consisting of 1.5 mm diameter stainless

steel rods spaced 5 mm center-to-center and pre-placed before concrete casting at different depths from

the surface, with the first pair of electrodes placed at 5 mm from the surface. The resistances were

monitored over time inside a concrete prism while one surface of the prism was exposed to a constant

head of water, similar to that used for the “Initial Surface Absorption Test”. The general observation

made was a decrease in resistance over time, especially in the near-surface depths due to the ingress

of water. The rate of decrease of the resistance, normalized to the first measurement, is larger as the

measurement is done closer to the surface. However, certain pairs of electrodes detected an increase in

the normalized resistance, which could not be explained by the authors. The same observation was later

noted by McCarter & Chrisp (2000). The initial increase was then followed by a decrease in resistance,

and finally constant resistance readings corresponding to steady-state moisture diffusion (capillary pres-

sure equilibrium). The time at which the water front reached a certain depth was defined by the authors

as the time for which the time derivative of the ratio of the measured resistance to the resistance before

exposure to the absorbed solution is maximized. In addition, the authors suggested that the water-

fillable porosity can be estimated if the depth of penetration was combined with cumulative volumetric

absorption measurements - which was originally suggested by Kelham (1988), as mentioned in Chapter

3. It was also suggested that the degree of saturation evolves as described in Equation 10 where m lies

between 2 and 3. This logarithmic relationship was also considered by Archie (1941) when studying

soils. McCarter & Chrisp (2000) and McCarter et al. (2012) automated the data acquisition process on

concrete in an exposure site in Scotland and suggested performance specifications for the permeability

of the cover zone (McCarter et al., 2017). The authors suggested that the early age permeability should

also be characterized, however no correlation to permeability was suggested by the authors, nor did

they consider the effect of the initial degree of saturation, its differential, nor the differential of the pore

solution conductivity throughout the concrete cover.

St = (RtR0

)1m (4.3)


where:

St the degree of saturation at time t;

Rt the resistance at time t;

R0 the resistance right before the ingress of the absorbed fluid; and

m an empirical coefficient

4.3 Electrode Layout

Given the composite nature of a concrete volume and the complexity that arises from the differential

effects of intrinsic and extrinsic factors in measuring electrical resistivity especially differential hydra-

tion, which in this case is of interest it is important to define a representative volume element (RVE).

The goal of defining an RVE is to ensure, with a certain degree of confidence, that the properties of the

volume of concrete tested depict the conditions and performance of this same volume. A definition of

a representative volume element can be found in Drugan & Willis (1996) as follows: “it is the smallest

material volume element of the composite for which the usual spatially constant macroscopic constitutive

representation is a sufficiently accurate model to represent mean constitutive response”. In this case, the

spatial parameter is the bulk electrical resistivity, assumed to be constant over the volume defined by

two consecutive electrodes. For a concrete volume with non-uniform degree of hydration, and thus a non-

uniform permeability field throughout the volume, in order to characterize permeability as a function of

the formation factor, one would have to optimize the electrode placement. The optimization problem

faced is a dual-objective function where the first is the accuracy of the reading, and the second is the

precision obtained from the discretization. The first is maximized by increasing the size of the discrete

element, i.e., maximize the spacing of the electrodes. The second, on the other hand, is maximized by

minimizing that spacing, providing more data points per unit of spatial dimension (length, area, or vol-

ume). However, as the distance between two consecutive electrodes decreases below a certain threshold

value, the variability of the output increases. This decrease in the discrete element size introduces a bias,

to a certain level that cannot be tolerated defined by the user. The goal is to place the electrodes such

that the volume defined between a set of adjacent electrodes is as small as possible while being equal to

or larger than the minimum RVE. On a meso-scale, looking at discretizing the curing-affected volume

of concrete, the parameters effectively affecting the value of the RVE can be thought of as the volume

of aggregates, their maximum size, and their gradation. An experimental investigation conducted by

du Plooy et al. (2012) reports the error arising from using an electrode spacing smaller than the nom-

inal maximum aggregate size. In their study, external ring electrodes around a concrete cylinder were

used such that the inter-electrode spacing was kept constant at 12 mm and the maximum aggregate

size ranged from 4 mm to 20 mm. The resulting relative error ranged from 0.04 % to 0.24 %, which

is considered to be negligible. However, the concrete volume defined by two consecutive electrodes is

considerably larger than that of interest in the present study given the external layout of the electrodes.

Morris et al. (1996) have investigated the effect of the electrode spacing in a Wenner configuration on

the obtained surface resistivity readings with a variable aggregate type and size. Experimental results

showed a variation of 40 % in the surface resistivity reading induced by varying the maximum aggregate

size from 9.5 mm to 19 mm.

A simple experimental program was designed in order to estimate the effect of electrode spacing on

the measured electrical resistivity. The goal was to infer on the extent of the effects of the electrode


configuration by comparing the spread (variance) of the electrical resistivity values measured for dif-

ferent electrode layouts. Circular steel electrodes (85 mm long, 3.2 mm diameter, and sealed with

heat-shrink thermoset tubing in the central 65 mm, except for 10 mm exposed at the measurement end)

were mounted along the cylindrical axis of a 100 mm x 200 mm plastic cylinder mold. Two diametrically

opposed arrays of 18 electrodes each were placed such that the embedment length of each electrode

alternated between 10 mm and 36 mm, with an opposite configuration on the other array of electrodes

(shown in Figure 4.2). This resulted in having three different spacing values between electrode tips as

follows: 18 replicates at 54 mm; 34 replicates at 28 mm; and 17 replicates at 30 mm. Note that the

spacing is defined as the distance between the tips of two adjacent electrodes each having a 10 mm

exposed length. The 54 mm spacing is defined by diametrically opposed electrodes at the same vertical

level. The 28 mm spacing is defined by consecutive electrodes of each of the two arrays. Finally, the

30 mm spacing is defined by diametrically opposed and alternating set of electrodes. The cylinder with

this layout is thereafter defined as the “multi-electrode” cylinder. To make sure electrodes stayed in

position, they were supported by an additional 150 mm diameter cylinder which was drilled with holes

in alignment with the holes in the 100 mm diameter plastic cylinder. Silicone gel was used to seal the

holes after the electrodes were placed. The shape factor, as defined in Equation 2.4, was determined

experimentally by filling the cylindrical mold with the fitted electrodes with tap water. The electrical

resistance for each set of probes was measured. Then the electrical resistivity of that tap water was

determined using a commercial resistivity probe. The shape factor for each set of electrodes was then

computed as the ratio of the tap water resistivity to the corresponding measured electrical resistance.

Figure 4.2: Multi-Electrode Cylinder Electrodes Setup


Table 4.1: Mixture Proportions (expressed in kg/m3)

Cement (GU) 573

Coarse Aggregate (25 mm max.) 933

Fine Aggregate (fineness modulus = 2.6) 707

Water 230

Concrete, proportioned as shown in Table 4.1, was used to cast the multi-electrode cylinder. Casting was

done in 3 equal volumes and compacted by rodding while being cautious to not disturb the electrodes.

The surface was then finished and sealed with plastic sheeting. The reason the sample was sealed was

to provide uniform hydration conditions, and ultimately a uniform permeability field, throughout the

depth of the cylinder. Uniform microstructural development throughout the cylinder’s depth is neces-

sary in order to assess the effect of the electrodes layout. The electrical resistivity of the concrete was

computed for each measurement setup at different times between the ages of 1 day and 21 days. The

resistivity was computed based on the electrical impedance determined using a commercial impedance

spectroscopy device with an AC signal frequency of 10 kHz. To assess the effect of AC signal frequency

on the measured electrical impedance, an experiment was carried with embedded electrodes in fresh

concrete. The impedance was measured using different signal frequencies over a few days. The details

of the experiment as well as the data can be found in Appendix F.

0.8 1 1.2 1.4 1.60

0.2

0.4

0.6

0.8

1

Normalized Resistivity

Cu

mu

lati

veP

rob

abil

ity

54 mm30 mm28 mm

Figure 4.3: Cumulative Distribution of Resistivity Measurements for Different Electrode Layouts

At each test date, 18; 34; and 17 measurements were made for each of the 54 mm; 28 mm; and 30 mm

layouts respectively. For each of the three layouts, the measured resistivity values were ordered and

divided by the mean value for each set. Figure 4.3 represents the cumulative frequency distribution of

the observations for each of the 3 measurement layouts at a testing age of 17 days. The trend observed

at this test date is consistent with all the other test dates, thus this data set is used as an example of


the observations. The test data can be found in Appendix D. The following was observed:

1. The electrical resistivity was most consistent throughout the depth of the cylinder when computed

based on the 30-mm spacing layout. The average coefficient of variation of the electrical resistivity

was 6.7 % over all the measurements at different ages. The variability in the data for the 28

mm and 54 mm spacing layouts increased as a function of time, thus making the 30 mm spacing

the most appropriate layout. The results obtained were not initially expected. In fact, it was

initially expected that the coefficient of variation would be the smallest for the 54 mm spacing,

since it defines a larger concrete volume, thus having a higher chance of being a representative

volume. However, the precision of the measurement is also affected by the electrode spatial layout.

The electrode layout with a 30 mm spacing defines a concrete volume over which the electrical

impedance was measured as a function of 2 principal orthogonal axis, mainly horizontal and vertical

- whereas the 54 mm spacing was mainly horizontal. The added dimensionality of the electrode

layout could have contributed to defining the 30-mm layout as the best representative volume

element.

2. The average computed resistivity is strongly affected by the electrode layout. The confidence

level for the equality of means of two sets of resistivity measurements (using the student t-test)

fluctuates and is quite low across the different test ages. The measurements that were the most

consistent were that of the 54 mm and 28 mm layouts, with an average confidence level of 73

%, fluctuating between 34 % and 98 %. The results of this analysis can be found in Appendix

D. However, the variability of the output based on these two layouts makes them less desirable

candidates for adoption. This shows the high impact of electrode layout on the interpretation of

the electrical impedance, and thus the importance of defining a representative volume element for

impedance spectroscopy.

4.4 Accounting for the Differential Hydration Kinetics

Once again, the goal in the development of the electrode sensor is to be able to detect the effect of the

differential hydration kinetics on the fluid penetration resistance of the near-surface concrete volume.

This is done by discretizing the concrete volume and characterizing each discrete volume with a forma-

tion factor, which can be related to transport parameters. The formation factor, as formulated earlier

(Equation 2.3 in Chapter 2), is a function of the electrical resistivity of both the bulk concrete volume

and that of the pore solution. The uncertainties concerning the electrical resistivity of the bulk discrete

concrete volume were discussed in Section 4.3. The following attempts to formulate the challenges aris-

ing from the effect of the differential hydration kinetics on the computation of the formation factor.

At first, it might seem contradictory to consider the effect of the differential hydration kinetics on

the computed formation factor since the goal of computing the formation factor is to detect differences

in degree of hydration and the effect on transport properties in the curing-affected zone. However,

some factors arising from differential degrees of hydration could hinder the adoption of a comparative

approach when assessing the permeability gradient in the near-surface concrete volume by the mean of

the formation factor. As expressed in Equation 2.2 (Chapter 2), the electrical resistivity of a concrete

volume is a function of factors such as temperature, degree of saturation, and pore solution resistivity.


In a discretized concrete volume, and for comparison purposes, the variability of these factors should

either be accounted for or nulled by homogenization. Homogenization can be done by conditioning test

samples in a way that will ensure a uniform degree of saturation of the pores with the same solution is at-

tained. This point is discussed later. The focus here will be on the degree of saturation and the electrical

resistivity of the pore solution since the other factors can be treated as constant throughout the concrete

volume tested. The uncertainties in degree of saturation and pore solution electrical resistivity lead to

uncertainties in the computed formation factor. The resulting uncertainty needs to be estimated, and if

possible minimized, in order to estimate the confidence on the reliability of the performance metric.

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

capillary water

unhydrated cement

hydrated solids

gel water

chemical shrinkage

Degree of Hydration

Volu

me

Fra

ctio

n

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Figure 4.4: Illustration of Powers-Brownyard Model (w/c=0.45)

At a given time of measurement, the degree of hydration (DOH) of the cementitious system is not nec-

essarily constant throughout the distance from the surface (due to loss of moisture to the exterior). This

implies heterogeneous volumetric proportions of the different constituent phases, as well as heterogenous

pore solution chemistry throughout the domain of interest. Proportions of the constituent phases in a

hydrated portland cement system can be estimated using the Powers-Brownyard model, illustrated in

Figure 4.4 for a system with a water-to-cement ratio of 0.45. It shows the evolution of the volume of

liquid water, hydrated solids, unhydrated cement, and shrinkage, as a function of the degree of hydration.

The model applies for portland cement systems and does not account for supplementary cementitious

materials and pozzolanic reactions. Given that the degree of hydration cannot be measured in-situ, the

effect of differential degrees of hydration needs to be accounted for and/or nulled by homogenization.

The following provides a qualitative description of the effects of these differentials on the measured

electrical resistivity and its interpretation:

• Effect of the differential constituent proportions: the total volume of pores, and their degree of

fluid saturation is not uniform due to the curing-affected depth-dependent differential degree of

hydration. This affects the bulk electrical resistivity since it is highly sensitive to the degree of

saturation. A formulation of this effect can be found in Spragg et al. (2013).


• Effect of the differential pore solution chemistry: the electrical resistivity of the pore solution is

highly dependent of the degree of saturation and the extent of the pozzolanic reaction(s), such as

formulated by D. P. Bentz (2007). In addition, moisture variation during curing (due to desiccation,

or supply of additional curing), can cause dilution and ionic leaching.

As mentioned earlier, homogenization of these parameters is useful in order to reduce the uncertainty of

their effects on the interpretation of the results. Homogenization can only be done on lab test specimens,

i.e. when the test is used for quality assurance and pre-qualification and not for in-situ quality control

or real-time curing optimization. The first step in validating the suggested test method is to assess its

quality assurance capability and its reliability.

For quality assurance purposes, the test specimen needs to be conditioned prior to testing. In this

particular case, the conditioning involves submerging the test specimen in a simulated pore solution of

a known resistivity for a sufficient time. The main objective of this treatment is to increase the degree

of saturation of the pores, and make it uniform throughout the test specimen. In order to avoid ionic

leaching from the highly concentrated electrolyte in the pores, the saturation is done using a simulated

pore solution, similar to the method adopted in the ASTM C1876 (2019) standard. Submerging the test

specimen for a sufficient period of time helps guarantee reaching full saturation of the capillary pores

and also in obtaining a homogenous pore solution composition. However, prolonged submersion also

causes additional hydration and potentially pozzolanic activation of the cementitious system, inducing

a bias in interpreting the results. Thus, the submersion period of the test specimen needs to be optimized.

Due to the complications described, the goal of this development is refined to the validation of the

concept for use in quality assurance and pre-qualification, rather than for quality control. The degree

of saturation and pore solution composition are homogenized by controlling the test sample condition-

ing procedure. The procedure adopted in the ASTM C1876 was adopted here, as described in Section 4.5.

On the other hand, if the test is to be used in-situ, the degree of saturation needs to be estimated

and used to calibrate the measured electrical resistance, based on a power-law saturation function vali-

dated by Weiss et al. (2013). In addition, the chemistry of the pore solution would need to be considered.

This would involve either direct measurement of the electrical resistivity/conductivity of the pore solu-

tion (Rajabipour et al., 2007); or by estimating it and then quantifying the corresponding uncertainty

and its impact on the performance metric derived. A suggested data processing framework can be found

in Appendix E.

4.5 Proof of Concept

In order to prove the concept, and to obtain replicate results, a model “array of sensors” was fabricated,

as well as fabricating a concrete mold into which the sensor array can be inserted, as shown in Figure

4.5. The sensor array was mounted on plywood (20 mm x 50 mm x 100 mm). Stainless steel threaded

rods (2.6 mm diameter 35 mmm long) were placed in drilled holes with a center-to-center spacing of 8

mm. The sensors were then placed into opposite facing sides of the 50 by 50 by 100 mm high concrete

mold (see Figure 4.5). Although having identical electrode spacings, one array of sensors had its first

electrode 4 mm away from the cast surface, while the other at 8 mm from that same surface. This


layout was selected based on the results of the experiment presented in Section 4.3. This layout was

determined to have the lowest coefficient of variation, as concluded earlier. Each electrode had a 25 mm

long heat-shrink tube applied over the center of its length such that the exposed electrode tip was 5 mm.

Measurements were performed using the same electrical resistivity meter used previously, at a frequency

of 1 kHz. Electrical impedance measurements were made using facing electrodes. The shape factor of

each measurement setup was determined following the same procedure described in Section 4.3, except

that since the electrode ends were embedded in the concrete in the setup adopted here, the shape factors

were predetermined using a block of Styrofoam as an analogue.

Figure 4.5: Concrete Mold and Sensor Arrays Used for the Proof of Concept Tests. Note: one of thesensor arrays has not yet been fully inserted.

The first step was to determine the systematic error of the designed system. This was done adopting a

similar approach to that presented in Section 4.3. Concrete (400 kg/m3 GU cement; 0.5 w/c; 895 kg/m3

14 mm crushed limestone aggregates; and 927 kg/m3 sand) was used to fill the mold with the installed

sensor arrays. Once the cast surface was leveled, it was sealed using a thick plastic sheet held in place by

duct tape, minimizing potential moisture loss. It is assumed that the setting and curing conditions are

uniform throughout the depth of the concrete specimen, leading to a uniform porosity and pore structure

tortuosity. The output of the electrical resistivity mapping in that case was expected to be uniform,

and all variability quantified by the coefficient of variation is considered as the systematic error of the

system for future curing-effect detection and for statistical testing. The specimen was sealed until 7 days

of age, after which the specimen is placed in a simulated pore solution, as described in the ASTM C1876

standard (ASTM C1876, 2019). Electrical resistivity was computed for each measurement setup, and

specimen mass measurements were taken on a daily basis. Simultaneously, a replicate concrete specimen


was cast but the cast surface was left uncovered for 7 days in air at 23 °C and 30% RH. Once demolded,

all other specimen faces were sealed using plastic sheeting. This allowed for unidirectional loss of mois-

ture during setting and early-age curing. At 7 days of age, the electrical resistivity was computed for

each measurement setup, and the mass of the specimen was measured. Once measured, the specimens

were then placed in the simulated pore solution.

Figure 4.6 shows the computed electrical resistivity values as a function of depth for both the sealed

and air-cured specimens. The plot shown is at 7 days after submersion in the alkaline storage solution,

corresponding to 14 days of age. This was done to bring the pore structure to a comparable level in

terms of its degree of liquid-saturation. The systematic error (coefficient of variation) was determined

to be 5.54 % based on data for the sealed specimen (data provided in Appendix E). It is expected that

the systematic error can be reduce as the hardware used for the embedded resistivity measurement is

improved. In addition, it is a function of the electrode layout and aggregate size distribution and maxi-

mum size, as discussed in Section 4.3.

0 100 200 300 400

0

10

20

30

40

50

60

70

80

90

Electrical Resistivity (Ohm−m)

Dep

thfr

omC

ast

Su

rface

(mm

)

SealedAir-cured

Figure 4.6: Depth-dependent Electrical Resistivity of Concrete Specimens Fitted with Embedded Elec-trodes

The air-cured resistivity profile in Figure 4.6 shows a gradient in the first 12 mm from the cast surface.

This differential is due to the drying during setting and early-age curing. Ignoring the systematic error

of the system, the drop in resistivity over the initial 12 mm from the cast surface is 24.4 %, which is

considerable. However, the intrinsic variability of the measurement system needs to be considered. This

implies assessing the confidence on the severity of the gradient and setting bounds for interpretation.

Confidence testing can be done using the results of the sealed specimen. For instance, the baseline for

comparison can be the average electrical resistivity of the sealed specimen (352.8 Ohm −m), and the

standard deviation of each measurement in the air-cured sample can be equal to the standard deviation

corresponding to a coefficient of variation of 5.54 % (assuming any variability observed is systematic


error). As an example, and applying this to the measurement at 4 mm in the air-cured specimen (263.76

Ohm−m), the confidence level on the difference between this value and the baseline is extremely high

(99.99 %). The confidence on inferences on curing quality is improved as the testing scheme and hardware

used is more robust. This point is discussed in Chapter 5.

4.6 Conclusion

The effect of early-age drying during setting and curing can be detected by mapping the electrical resis-

tivity, after conditioning in simulated pore solution. By measuring electrical impedance and correcting

for the shape factor and homogenizing the degree of liquid saturation, a qualitative appreciation of the

effect of curing through depth can be obtained. By considering the pore solution composition, a map

of the formation factor could eventually be obtained, enabling a multi-mechanistic assessment of the

transport properties and its variation in the near-surface depth of concrete. The work presented in this

Chapter is primitive, however an analytical framework is presented for future work.

Chapter 5

Conclusions and Recommendations

The objective of the work presented in this thesis was to develop a test method to accurately detect

the effect of curing on the chloride penetration resistance of the near-surface concrete. Use of a perfor-

mance test would allow the adoption of equivalent or more efficient curing methods, such as provided

by accelerated moist curing often used in precast concrete production. Loss of moisture in the concrete

cover causes depth-dependent differential porosity, and thus depth-dependent fluid penetration resis-

tance. Therefore, the performance test needs to be able to detect the depth-dependent gradient in fluid

penetration resistance. Two methods were investigated in this thesis, both based on discretizing the

concrete cover over its depth.

5.1 Conclusions

1. The initial rate of absorption (sorptivity) of a 3.0 % by mass NaCl solution initiated at a cut

plane in the near-surface of a core is highly sensitive to the type of curing. By testing several

test planes, profiling the sorptivity with depth can provide an estimate of the extent and depth of

curing, making it a good candidate for curing pre-qualification and quality assurance.

2. Sorptivity values at specific depths can be used for acceptance, as well as a weighted average value

over the whole cover depth, as shown in Section 3.5 of Chapter 3. Accelerated moist curing followed

by ambient air curing can be equivalent, or better, than the 7-day moist curing period prescribed

in CSA A23.1-19 for concrete exposed to chlorides (CSA A23.1, 2019).

3. The performance equivalence of different curing regimes depends on the mixture design and pro-

portioning, as well as the accelerated moist curing regime, and the prescriptive moist curing regime

adopted as the baseline.

4. Mapping the electrical resistivity with depth over time using embedded electrodes appears to be

promising for possible quality assurance and pre-qualification testing. It requires minimal sample

preparation, testing is quick and non-destructive, and it allows monitoring of the effect of curing

over time.

5. The effect of the electrode layout on the measurement output is the main experimental conclusion

from the work performed, and is necessary for the interpretation of results and confidence testing.

59

Chapter 5. Conclusions and Recommendations 60

5.2 Recommendations

1. In terms of setting acceptance criteria for alternative curing regimes, two approaches could be

adopted. First, is a comparative approach where the comparison baseline is the prescriptive curing

regime. Although simple, this approach does not guarantee a particular performance level since

the baseline itself is prescriptive. The second is a mechanistic approach considering the service

conditions and fundamental mass and ionic transport formulations. However, it requires better

characterization of the concrete, as well as a good service life modeling framework. Both methods

suggested in this thesis can be used for comparison purposes as well as providing a mechanistic

approach. However, the formation factor mapping would provide a more comprehensive multi-

mechanistic characterization of the concrete cover, whereas sorptivity profiling solely characterizes

capillary absorption while ignoring other transport mechanisms.

2. Standardizing the sorptivity test procedure described in Chapter 3, and incorporating it in the

relevant CSA standard, would enable the adoption of wider range of curing regimes, such as

accelerated moist curing. However, given the variety of specimen conditioning procedures used in

other standards and by different research groups, an optimal conditioning procedure needs to be

developed and adopted for this test. It is believed that the conditioning procedure adopted in this

work (drying at 60 °C until constant mass) is a good candidate given its simplicity and relative

rapidity. That being said, more work should be performed to validate this procedure.

3. Given the early stage of the development of embedded electrode arrays, and the complexity of the

challenges arising from the differential degree of hydration and saturation, more work needs to be

done before validating the potential for quality control and assurance. This includes improving

the sensing hardware to obtain more repeatable and reliable results, adjusting for the degree of

liquid saturation based on relative humidity measurements, and accounting for the pore solution

composition (refer to Appendix E).

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Appendix A

Cementitious and Aggregates

Properties

Cementitious Materials

General use (GU) cement: CRH Canada GU cement.

Ground granulated blast furnace slag (GGBSF): Holcium GranCem® Cement.

Silica fume blended cement (GUbSF): CRH Canada GUbSF cement.

Aggregates

Fine Coarse 1 Coarse 2

Type Quartz river sand Crushed limestone Crushed limestone

Relative Density (SSD) 2.66 2.72 2.71

Fineness Modulus 2.64 - -

Nominal Maximum Size (mm) 5 14 25

Absorption (%) 1.00 2.00 1.29

Dry-rodded Density (kg/m3) - 1573 1684

Admixtures

Air-entraining admixture: BASF Master Builder® Master Air AE200

Water-reducing admixture: BASF Matser Builder® Master Pozzolith 210.

67

Appendix B

Chapter 3 Data

Fresh Properties

Round 1 Round 2 Round 3

Temperature (°C) 28 28 23

Slump (mm) 90 58 120

Air content (%) 8 9 5

For each round of experiment, five tables are shown; one for each of the initial rate of absorption (in

mm/min1/2), total charge passed (in Coulomb), bulk electrical resistivity (in Ohm.m), chloride penetra-

tion depth determined colorimetrically (in mm), and the compressive strength (in MPa). The standard

deviation of each set of measurement, denoted as s, is also shown. In addition, for each of rounds 2 and

3, an absorption plot for each curing regime is produced, showing the time evolution of absorption.

Round 1

Depth (mm) 0 2 8 14 20 50

MC10 Sinitial 0.0965 0.103 0.102 0.112 0.111 0.111

MC10 s 0.00662 0.0148 0.00609 0.00573 0.000998 0.00163

MC23 Sinitial 0.102 0.0996 0.107 0.138 0.124 0.132

MC23 s 0.00579 0.0101 0.00575 0.0140 0.00868 0.00486

AC23 Sinitial 0.108 0.103 0.116 0.123 0.107 0.113

AC23 s 0.00944 0.00768 0.00438 0.00825 0.00938 0.00971

HC10 Sinitial 0.122 0.108 0.112 0.109 0.109 0.110

HC10 s 0.00477 0.00453 0.00771 0.00596 0.00603 0.00468

68

Appendix B. Chapter 3 Data 69

Depth (mm) 0 2 8 14 20 50

MC10 RCPT 3433 3747 4030 3668 3425 3383

MC10 s 562 294 62 456 61 277

MC23 RCPT 2768 3120 2598 3136 2539 2857

MC23 s 336 207 676 73 131 366

AC23 RCPT 3983 7211 2774 3076 2748 6853

AC23 s 661 3780 165 146 199 1159

HC10 RCPT 3891 4336 4334 3853 4698 4090

HC10 s 517 134 1064 1183 705 170

Depth (mm) 0 2 8 14 20 50

MC10 BR 69.13 76 69.5 66.7 73.4 73.6

MC10 s 1.6 0.53 1.2 0.87 1.3 2.1

MC23 BR 66.8 62.3 74.6 74.2 69.2 70.9

MC23 s 2.1 0.58 0.59 0.95 0.22 1.9

AC23 BR 56.8 56.2 86.1 82.7 72.9 49.1

AC23 s 3.5 3.2 0.95 5.6 0.29 4.7

HC10 BR 55.5 54.5 54.3 56.5 63.2 53.3

HC10 s 0.43 7.9 1.1 5.9 3.0 2.26

Round 2

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

AC 0 mmAC 2 mmAC 8 mmAC 14 mmAC 20 mmAC 50 mm

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

MC 0 mmMC 2 mmMC 8 mmMC 14 mmMC 20 mmMC 50 mm

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

HC/AC 0 mm

HC/AC 2 mm

HC/AC 8 mm

HC/AC 14 mm

HC/AC 20 mm

HC/AC 50 mm

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

HC/MC 0 mm

HC/MC 2 mm

HC/MC 8 mm

HC/MC 14 mm

HC/MC 20 mm

HC/MC 50 mm


Depth (mm) 0 2 8 14 20 50

AC Sinitial 0.0680 0.0611 0.0663 0.0564 0.0555 0.0491

AC s 0.00144 0.00727 0.00732 0.00242 0.00207 0.000918

MC Sinitial 0.0562 0.0491 0.0528 0.0481 0.0459 0.0494

MC s 0.00296 0.00401 0.000779 0.00405 0.000323 0.00185

HC/AC Sinitial 0.0443 0.0467 0.0381 0.0417 0.0350 0.0360

HC/AC s 0.000556 0.00231 0.00258 0.00207 0.00270 0.00239

HC/MC Sinitial 0.0414 0.0422 0.0366 0.0332 0.0334 0.0358

HC/MC s 0.000410 0.00247 0.00177 0.00175 0.00223 0.00128

Depth (mm) 0 2 8 14 20 50

AC RCPT 623 530 707 692 592 720

AC s 30.0 36.0 74.3 9.8 17.8 1.8

MC RCPT 479 327 494 438 364 578

MC s 34.3 3.6 111.9 5.2 6.3 11.8

HC/AC RCPT 462 382 418 436 319 548

HC/AC s 1.31 33.3 12.9 40.0 127.6 28.0

HC/MC RCPT 615 493 786 638 504 501

HC/MC s 30.0 10.9 5.8 73.9 8.3 16.6

Depth (mm) 0 2 8 14 20 50

AC BR 329.12 382.73 282.86 307.47 355.53 310.17

AC s 7.79 26.64 14.70 31.50 13.22 53.18

MC BR 501.29 568.59 469.52 471.72 510.21 389.95

MC s 8.62 22.71 12.86 2.46 11.00 16.78

HC/AC BR 499.69 535.34 449.28 433.21 494.62 366.27

HC/AC s 16.46 34.50 29.10 19.60 3.56 35.55

HC/MC BR 361.69 403.04 332.24 318.78 406.79 286.87

HC/MC s 2.02 20.47 1.89 9.15 15.90 11.02

Round 3

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

AC 0 mmAC 2 mmAC 8 mmAC 14 mmAC 20 mmAC 50 mm

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

MC 0 mmMC 2 mmMC 8 mmMC 14 mmMC 20 mmMC 50 mm


0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

HC/AC 0 mm

HC/AC 2 mm

HC/AC 8 mm

HC/AC 14 mm

HC/AC 20 mm

HC/AC 50 mm

0 5 10 15 200

0.2

0.4

0.6

0.8

1

1.2

Time1/2 (min1/2)

Ab

sorp

tion

(mm

)

HC/MC 0 mm

HC/MC 2 mm

HC/MC 8 mm

HC/MC 14 mm

HC/MC 20 mm

HC/MC 50 mm

Depth (mm) 0 2 8 14 20 50

AC Sinitial 0.0669 0.0515 0.0366 0.0381 0.0382 0.0392

AC s 0.002328 0.00239 0.00160 0.000847 0.000709 0.000463

MC Sinitial 0.0504 0.0404 0.0316 0.0329 0.0317 0.0391

MC s 0.00447 0.00190 0.00286 0.00155 0.00250 0.000797

HC/AC Sinitial 0.0558 0.0431 0.0329 0.0370 0.0338 0.0397

HC/AC s 0.00273 0.000679 0.00119 0.00336 0.00181 0.000469

HC/MC Sinitial 0.0421 0.0360 0.0295 0.0322 0.0280 0.0353

HC/MC s 0.002971 0.00136 0.00185 0.00202 0.00222 0.000600

Depth (mm) 0 2 8 14 20 50

AC RCPT 1163 1011 1024 1050 1050 1068

AC s 71.8 56.6 99.5 6.3 29.1 32.7

MC RCPT 1229 959 1004 966 1162 1207

MC s 71.8 237.7 139.0 199.9 31.1 81.4

HC/AC RCPT 1284 1323 1115 1429 1204 1006

HC/AC s 124.4 120.5 31.7 76.1 27.7 50.9

HC/MC RCPT 1171 1060 1130 1105 1087 1161

HC/MC s 278.9 193.6 101.0 57.6 10.1 372.4

Depth (mm) 0 2 8 14 20 50

AC BR 177.56 166.99 178.86 172.96 176.40 185.62

AC s 5.65 7.75 6.78 2.16 3.95 14.15

MC BR 161.23 166.73 181.40 163.28 172.27 159.69

MC s 1.42 7.38 7.74 3.48 6.91 11.25

HC/AC BR 160.52 156.15 186.18 155.96 190.94 175.93

HC/AC s 3.41 17.48 0.85 0.56 7.23 28.10

HC/MC BR 174.97 158.35 192.99 156.25 168.57 176.04

HC/MC s 4.03 3.98 23.55 0.38 2.27 15.54

Appendix C

Absorption Profiling Test Method

The following is drafted based on the ASTM and CSA standard test methods layout.

Test Method for

Detection of Curing Effect by Profiling Rate of Absorption of a Sodium Chloride So-

lution by Hydraulic-Cement Concretes

1. Scope

(a) This test method is used to determine the effect of curing on the chloride penetration resis-

tance of the near-surface concrete using the initial rate of absorption (initial sorptivity) of a

sodium chloride solution by hydraulic cement concrete. The initial sorptivity is determined

by measuring the increase in the mass of a specimen resulting from the absorption of a 3.0 %

m/m sodium chloride solution as a function of time when only one surface of the specimen

is exposed to the solution. Given that the absorption behavior is affected by the sample

conditioning, a standard specimen conditioning procedure is adopted. Initial absorption is

dominated by capillary absorption, which is highly sensitive to the quality of curing.

(b) The values stated in SI units are to be regarded as standard. No other units of measurement

are included in this standard.

2. Referenced Documents

(a) ASTM Standards:

i. C21/C31M Practice for Making and Curing Concrete Test Specimens in the Field

ii. C42/C42M Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of

Concrete

iii. C125 Terminology Relating to Concrete and Concrete Aggregates

iv. C192/C192M Practice for Making and Curing Concrete Test Specimens in the Laboratory

v. C642 Test Method for Density, Absorption, and Voids in Hardened Concrete

vi. C1005 Specification for Reference Masses and Devices for Determining Mass and Volume

for Use in the Physical Testing of Hydraulic Cements

72

Appendix C. Absorption Profiling Test Method 73

vii. C1202 Test Method for Electrical Indication of Concretes Ability to Resist Chloride Ion

Penetration

viii. C1585 Test Method for Measurement of Rate of Absorption of Water by Hydraulic-

Cement Concretes

3. Terminology

(a) Definitions for definitions of terms used in this standard, refer to Terminology, ASTM C125.

4. Significant and Use

(a) The performance of concrete subjected to many aggressive environments is a function, of the

fluid penetration resistance of the pore system. In the near-surface region of concrete elements,

early-age curing (moisture desiccation) affects the hydration of the cementitious materials and

imperfect curing causes depth-dependent changes in fluid penetration resistance of aggressive

agents, such as chlorides. The variable quality of the curing-affected zone reduces the chloride

penetration resistance of the reinforcement cover. This test method measures the initial rate

of absorption of a sodium chloride solution of test specimens taken at different depths from a

formed or finished surface to detect depth and extent of curing on the near-surface concrete. In

most structures, the concrete cover is only partially unsaturated, making capillary absorption

the dominant initial mechanism for penetration of aggressive solutions. This test method is

based on series of modified tests using ASTM C1585 “Standard Test Method for Measurement

of Rate of Absorption of Water by Hydraulic-Cement Concretes”.

(b) The absorption behavior of a concrete surface depends on many factors including: (a) concrete

mixture proportions; (b) the presence of chemical admixtures and supplementary cementitious

materials; (c) the composition and physical characteristics of the cementitious component

and of the aggregates; (d) the entrained air content; (e) the type and duration of curing; (f)

the degree of hydration or age; (g) the presence of microcracks; (h) the presence of surface

treatments such as sealers or form oil; and (i) placement method including consolidation and

finishing. The rate of absorption is also strongly affected by the moisture condition of the

concrete at the time of testing.

5. Apparatus

(a) Pan, a watertight polyethylene or other corrosion-resistant pan large enough to accommodate

the test specimens with the surfaces to be tested exposed to water.

(b) Support Device, rods, pins, or other devices, which are made of materials resistant to corrosion

by water or alkaline solutions, and which allow free access of water to the exposed surface of

the specimen during testing. Alternatively, the specimens can be supported on several layers

of blotting paper or filter papers with a total thickness of at least 1 mm.

(c) Top-pan Balance, complying with Specification C1005 and with sufficient capacity for the test

specimens and accurate to at least ± 0.01 g. Timing Device, stop watch or other suitable

timing device accurate to ± 1 s.

(d) Paper Towel or Cloth, for wiping excess water from specimen surfaces.


(e) Water-Cooled Saw, with diamond impregnated blade to cut test specimens from larger sam-

ples.

(f) Oven, an oven allowing for air circulation and able to maintain a temperature of 60 ± 2 °C.

(g) Polyethylene Storage Containers, with sealable lids, large enough to contain at least one test

specimen but not larger than 5 times the specimen volume.

(h) Caliper, to measure the specimen dimensions to the nearest 0.1 mm.

6. Reagents and Materials

(a) Sodium Chloride, Reagent Grade, required to prepare the absorbed solution.

(b) Sealing Material, strips of low permeability adhesive sheets, epoxy paint, vinyl electrician’s

tape, duct tape, or aluminum tape. The material shall not require a curing time longer than

10 minutes.

(c) Plastic Bag or Sheeting, any plastic bag or sheeting that could be attached to the specimen

to control evaporation from the surface not exposed to water. An elastic band is required to

keep the bag or sheeting in place during the measurements.

7. Test Specimens

(a) The standard test specimen is a 100 ± 6 mm diameter disc, with a length of 50 ± 3 mm.

Specimens are obtained from drilled cores according to Test Method C42/C42M. The cross-

sectional area of a specimen shall not vary more than 1 % from the top to the bottom of the

specimen.

Note 1 - This procedure was validated by assessing the differential effects of curing at the

formed surfaces of concrete slabs. In order to isolate the effect of drying, the cast surface of

each slab is sealed with plastic sheeting after finishing to avoid moisture loss. In addition,

once demolded, to prevent evaporation from the formed surfaces, the side formed surfaces are

also covered with plastic sheeting.

(b) The average test results on at least 2 specimens with test surfaces at the same distance from

the original exposed surface of the concrete (Note 2) shall constitute the test result.

Note 2 - In order to detect the depth-dependent effect of curing on the initial rate of absorption,

the tests should be carried on core segments with test surfaces at different depths from the

outer surface. Depth increments of 0, 2, 8, 14, 20, and 53 mm from the formed surface were

used during test development (Figure C.1)

Note 3 - Test specimens are obtained from the cored concrete using a water-cooled diamond

saw. If test surfaces at 2 mm depth are unattainable with a saw, end grinding can be adopted.

The ground surface needs to be carefully brushed to remove any slurry blocking the surface

pores.

8. Sample Conditioning and Preparation

(a) Rinse the test specimens with tap water, blot off excess water, and measure the mass of each

specimen to the nearest 0.01 g.

Note 4 if the test specimens are obtained from a structure, saturate them in accordance

with the vacuum-saturation procedure in Test Method C1202, but omit the step for coating


specimen side surfaces. After saturating, measure the mass of each test specimen to the

nearest 0.01 g.

(b) Place test specimens in the oven at 60 ± 2°C.

(c) Measure the mass of each specimen on a daily basis. Remove the specimens from the oven once

the daily mass change does not exceed 0.2 %. Place the test specimen in a sealed polyethylene

container and store at 23 ± 2 °C for at least 12 h before the start of the absorption test to

allow for internal moisture redistribution. Remove the specimen from the storage container

and record the mass of the conditioned specimen to the nearest 0.01 g before sealing of side

surfaces.

(d) Measure at least four diameters of the specimen at the surface to be exposed to water. Measure

the diameters to the nearest 0.1 mm and calculate the average diameter to the nearest 0.1

mm.

(e) Seal the side surfaces of each specimen with two layers of vinyl electrician tape. Seal the end

of the specimen that will not be exposed to water using a loose plastic sheet (see 6.b). Then

secure the plastic sheet using an elastic band.

Figure C.1: Cored Slab for Test Specimen Extraction

9. Test Procedure

(a) The initial rate of absorption is determined as a function of time at 23 ± 2 °C using a 3.0 %

by mass NaCl solution conditioned to the same temperature.

(b) Absorption Procedure:

i. Measure the mass of the sealed specimen to the nearest 0.01 g and record it as the initial

mass for water absorption calculations.

ii. Place the support device at the bottom of the pan and fill the pan with the NaCl solution

so that the liquid level is 1 to 3 mm above the top of the support device, as shown in

Figure C.2. Maintain the liquid level 1 to 3 mm above the top of the support device for

the duration of the tests.

iii. Start the timing device and immediately place the test surface of the specimen on the

support device. Record the time and date of initial contact with water.

iv. Record the mass at the intervals shown in Table C.1 after first contact with the NaCl

solution. Using the procedure in 9.b.v, the first point shall be at 60 ± 2 s and the second

point at 5 min ± 10 s. Subsequent measurements shall be within ± 2 min of 10 min,

20 min, 30 min, and 60 min with the actual time recorded to within ± 10 s. Continue

the measurements every hour, ± 5 min, up to 6 h, from the first contact of the specimen

with the solution and record the times to within ± 1 min. For each mass determination,


remove the test specimen from the pan, stop the timing device if the contact time is less

than 10 min, and blot off any surface solution with a dampened paper towel or cloth.

After blotting to remove excess solution, invert the specimen so that the wet surface does

not come in contact with the balance pan. Within 15 s of removal from the pan, measure

the mass to the nearest 0.01 g. Immediately replace the specimen on the support device

and restart the timing device.

Table C.1: Measurement Schedule and Tolerance

Time 60 s 5 min 10 min 20 min 30 min 60 min Every hour up to 6 h

Tolerance 2 s 10 s 2 min 2 min 2 min 2 min 5 min

Figure C.2: Sorptivity Experimental Setup (adapted from ASTM C1585 (ASTM, 2013))

10. Calculations

(a) The absorption, I, is the change in mass divided by the product of the cross-sectional area of

the test specimen and the density of the absorbed solution. The units of I are mm.

I =mt

ad(C.1)

I the absorption;

mt the change in mass, in grams, at time t;

a the exposed area of the specimen, in mm2; and

d the density of water in g/mm3.

(b) The initial rate of water absorption (mm/min1/2) is defined as the slope of the line that is

the best fit when plotted against the square root of time (min1/2). Obtain this slope by using

least-squares, linear regression analysis of the plot of I versus time1/2. For the regression

analysis, use all the points from 1 min to 360 min. If the data between 1 min and 6 h do not

follow a linear relationship (a correlation coefficient of less than 0.98) and show a systematic

curvature, the initial rate of absorption cannot be determined.

11. Report


(a) Report the following:

i. Date when concrete was sampled or cast.

ii. Source of sample.

iii. Depth of each test surface from the exposed surface.

iv. Relevant background information on sample such as mixture proportions, curing history,

type of finishing, and age, if available.

v. Dimensions of specimen before sealing.

vi. Mass of specimen before the start of conditioning, before sealing, and after sealing.

vii. A plot of initial rate of absorption, S, versus test depth.

12. Precision and Bias

(a) Precision - The repeatability coefficient of variation has been determined to be 4.9 % in

preliminary measurements for the initial rate of absorption as measured by this test method

for a single laboratory and single operator.

(b) Bias - The test method has no bias because the rate of water absorption determined can only

be defined in terms of the test method.

Appendix D

Chapter 4 Data

Representative Volume Element Experiment

The following summarizes the output of the electrode layout experiment (Section 4.4) where each value

corresponds to the average electrical resistivity, expressed in Ohm.m with the corresponding coefficient

of variation in parentheses. The value n is the sample size for each of the three layouts.

Time from cast (day) 54 mm (n=18) 28 mm (n=34) 30 mm (n=17)

1 11.10 (7.5%) 10.99 (9.1%) 11.26 (7.5%)

2 16.08 (7.9%) 15.97 (10.4%) 16.43 (6.4%)

3 18.13 (8.9%) 17.93 (11.7%) 18.48 (6.7%)

4 19.68 (4.5%) 20.02 (13.6%) 20.58 (6.5%)

8 24.35 (12.3%) 24.06 (15.3%) 24.76 (6.5%)

9 25.41 (12.7%) 25.38 (20.1%) 25.81 (6.5%)

10 26.22 (13.7%) 26.04 (17.7%) 26.54 (6.4%)

14 28.81 (15.0%) 28.48 (17.7%) 29.58 (7.6%)

15 29.40 (15.2%) 29.06 (18.4%) 29.62 (6.4%)

17 30.91 (16.5%) 30.51 (18.9%) 30.99 (6.6%)

21 31.98 (14.9%) 31.77 (19.3%) 32.19 (6.4%)

Probability of equivalence (equality) of the electrical resistivity measured using the different layouts

tested.Test date (d) 1 2 3 4 8 9 10 14 15 17 21

54 to 28 0.611 0.789 0.698 0.508 0.764 0.983 0.875 0.806 0.807 0.800 0.894

54 to 30 0.517 0.387 0.474 0.026 0.613 0.643 0.734 0.512 0.847 0.948 0.862

28 to 30 0.225 0.240 0.243 0.329 0.350 0.658 0.573 0.287 0.586 0.665 0.718

Proof of Concept

Electrical resistivity (Ohm-m) at 14 days (7 days submerged).

78

Appendix D. Chapter 4 Data 79

Depth (mm) Sealed Air-cured

4 n/a 263.76

8 342.09 312.12

12 344.19 349.05

16 354.37 377.65

20 334.67 354.29

24 325.16 362.59

28 360.51 352.61

32 364.84 329.91

36 383.31 353.95

40 332.44 349.10

44 355.24 362.08

48 319.97 336.40

52 332.08 344.92

56 339.45 342.96

60 373.59 343.84

64 359.99 n/a

68 336.04 n/a

72 387.48 331.06

76 346.53 337.28

80 348.49 346.83

84 371.72 331.06

88 383.54 343.17

92 365.91 342.68

Appendix E

Data Processing for In-situ QC

For quality control purposes, or in-situ measurements, and as described in Section 4.4 of Chapter 4, the

degree of fluid saturation and pore solution electrical resistivity need to be known. For this purpose, in

addition to electrical impedance measurements, the temperature and relative humidity need to be de-

termined and mapped with depth. The electrical resistivity of the pore solution could either be mapped

with depth or assumed to be constant. Regarding the former option, a pore solution sensor would need

to be used and embedded in the concrete at different depths from the surface. It could consist of a porous

material with embedded electrodes such that the shape factor is known. The electrical resistance would

then be measured and the resistivity derived, similarly to that presented in Rajabipour et al. (2007).

In the case where the pore solution resistivity would be considered constant, the electrical resistivity

profiling suffices to obtain a qualitative appreciation of curing and obtain a gradient of the formation

factor, without quantifying it. In both cases, the electrical resistivity needs to be corrected for the degree

of liquid saturation (S). Weiss et al. (2013) suggest a power law with an exponent parameter n that

would need to be determined for each concrete mixture. The value of the parameter n is a function

of the concrete mixture and is found to exhibit little variation with respect to varying water-to-cement

ratio and degree of hydration (Weiss et al., 2013). This implies that a unique value of n can be used for

specific cementitious materials used. It can be determined experimentally by fitting to the power law

the relationship between the electrical resistivity and degree of saturation of uniaxial test samples. The

coefficient δ is a correction factor for the ionic concentration of the pore fluid. It can also be determined

experimentally by measuring the electrical resistivity of a pore solution at different dilution levels and

fitting the trend to a power function (E.1) (Weiss et al., 2013). The value of the degree of saturation can

be estimated based on relative humidity measurements and the Brunauer, Emmett, and Teller (BET)

theory. Jiang & Yuang (2013) obtained empirical coefficients (λ1; λ2; and λ3) to fit the BET desorption

isotherm curve (E.2). Although limited to portland cement systems, their work could be of use.

F =ρbulk,measured

ρsolSn−δ−1 (E.1)

S =λ1RH

(1− λ2RH)(1 + λ3RH)(E.2)

80

Appendix F

AC Signal Frequency for Impedance

Measurements

In order to have an idea of the effect of signal frequency on the measured electrical impedance of con-

crete using embedded electrodes, this simple experiment was carried. A 50 mm diameter, 100 mm high

cylindrical mold was filled with concrete (400 kg/m3 GU cement; 0.5 w/c; 895 kg/m3 14 mm crushed

limestone aggregates; and 927 kg/m3 sand). The mold was then covered with a plastic lid fitted with

2 parallel 3 mm diameter steel electrodes than were embedded in the fresh concrete. The electrodes

were 20 mm long, and had 10 mm embedded in the fresh concrete. The electrical impedance of the

concrete was measured through the electrodes using a commercial concrete resistivity measuring device.

At each measuring date, the AC signal frequency took 5 values (100, 500, 1000, 10000, and 30000 Hz).

The following table lists the measured electrical impedance, expressed in Ohm, and the phase angle in

parenthesis, expressed in degrees. Measurements were performed starting 5 min after cast and until 22

days of age.

The effect of the signal frequency is very pronounced at early ages. In fact, the electrical impedance

doubled when the frequency varied from 30 kHz to 100 Hz. In addition, the polarization at the concrete-

electrode interfaces is pronounced at early ages for frequencies lower than 10 kHz, as the phase angle

was as high as 44 °. The difference between the impedance measured at 10 kHz and 30 kHz is small

at all test dates, even at early age. The effect of signal frequency progressively diminishes with time as

hydration advances and the liquid volume diminishes. The ASTM C1876 standard prescribes a signal

frequency of 1 kHz (ASTM, 2019) which works well for mature concrete. However, given the context of

the experiment performed in Section 4.3, and that the measurements were performed starting early age,

a signal frequency of 10 kHz was picked for the multi-electrode cylinder test.

81

Appendix F. AC Signal Frequency for Impedance Measurements 82

Time (min) 100 Hz 500 Hz 1000 Hz 10000 Hz 30000 Hz

5 73(44) 53.1 (31) 43.8 (25) 30.9 (8) 29.4 (6)

46 74.4 (44) 44.6 (23) 39.7 (17) 31.6 (8) 29.7 (8)

82 62.4 (40) 41.2 (18) 37.9 (13) 31.2 (7) 29.5 (5)

435 118 (18) 105 (6) 103 (4) 100 (0) 99 (2)

1781 283 (5) 275 (2) 274 (1) 269 (0) 269 (1)

4290 472 (3) 464 (1) 463 (1) 457 (0) 454 (1)

5520 526 (3) 518 (1) 515 (1) 510 (0) 507 (0)

5760 534 (3) 527 (1) 525 (1) 518 (0) 515 (0)

7020 578 (3) 570 (1) 568 (1) 561 (0) 557 (0)

8590 617 (2) 608 (1) 607 (1) 599 (0) 595 (0)

9960 659 (2) 650 (1) 648 (1) 641 (0) 636 (0)

14240 755 (2) 746 (1) 743 (1) 734 (0) 728 (0)

15850 775 (2) 765 (1) 763 (1) 752 (0) 745 (0)

17180 811 (2) 800 (1) 797 (1) 788 (0) 779 (0)

18960 847 (2) 836 (1) 832 (1) 822 (0) 815 (0)

31740 944 (2) 932 (1) 928 (1) 916 (1) 908 (1)