Principles of durability design for concrete structurestheconcreteportal.com/alexander.pdf · Durable Concrete Construction Through Performance Testing IIT Madras & IIT Bombay: Jan-Feb

Durable Concrete Construction Through Performance Testing IIT Madras & IIT Bombay: Jan-Feb 2014

Principles of durability design for concrete structures

Mark G. AlexanderVisiting Professor, IITM, Chennai

Concrete Materials and Structural Integrity Research UnitUniversity of Cape Town

Presented at Workshop on: “Achieving Durable Concrete Construction Through Performance Testing”

IIT Madras & IIT BombayJan – Feb 2014


Pics of UCT

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New Engineering Building – Upper campus


Outline1. New paradigms in Design for RC structures 2. Design vs. specification3. Durability design Durability design frameworks

4. Performance-based approach5. Performance approach – SA Experience

6. Practical aspects of drafting and implementing performance-based specifications


1. IntroductionNew paradigms in design for RC structures: #1 Durability design This paradigm demands that durability design be undertaken

first, structural design as ‘checks’ Fair progress along this line

#2 Sustainability design Modern ‘buzz’ word – but few understand its implications Engineers need a ‘sustainability design framework’ that can

guide designers through the necessary steps of sustainability design


1. Introduction

New paradigms in design for RC structures: Sustainability design This is intended to be an all-encompassing design

approach, incorporating: Sustainability ‘per se’ – resource conservation, minimisation of

embodied energy, energy efficiency; recycling, etc. Durability Structural strength & stability Any other needed functionalities

Ultimately, all design will be ‘sustainability design’, addressing the above issues

Durability design is crucial to sustainability!


2. Design vs. Specification for RC structures


DesignDesign involves translation

of client’s requirements into practice, e.g. an actual structure such as a bridge

Physical dimensions of project (structure) are set, using calculations, drawings

Explicit requirements of functionality are given e.g. service life


DesignDesign carried out using

design codes which:-a. Reflect good practice

b. Represent accumulated knowledge and wisdom

c. Provide minimum standards of safety and reliability.


Specifications A set of instructions from

engineer/client to enable contractor to execute work in accordance with design requirements

Specifications should:- Provide clear information on

desired nature of construction Ensure a finished product of

adequate quality and value


Construction specifications: two typesPrescriptive specifications: provide a set of

detailed instructions on how something is to be constructed.

Features:-i. Provide a contractor with few options

ii. Inherently non-innovativeiii. May be non-conservative


Construction specifications

Performance specifications: place emphasis on whatis required of final product

Features:-i. Knowledge of what the desired outcome of

construction isii. Must clearly set out measurable performance

criteria that can be proved/verified during construction


3. Durability design


Durability design Aspects that need to be considered:-

a) Environment/exposure conditionsb) Deterioration mechanismsc) Penetrability of concrete cover layerd) Cover thicknesse) Service life: Period in

which structure has adequate resistance to withstand environmental actions that cause deterioration.


Durability design frameworks Current situation: ‘Minimal Design’ Most current methods – no rational or targeted design

approach. ‘Design’ mostly covered by prescriptive specifications

Developing situations: Limit state design approaches At the other end of the scale! Years away still

Service life design approaches More promising in the immediate future Required a) Service life models, and b) suitable tests


Current situation: ‘Minimal Durability Design’ Use of ‘prescriptive’ methods –limiting values on

mix constituents e.g. max. w/c, min. cement content Limiting values based on lab and field tests, empirical

relations and past experience.

Limitations:-a) No reliable prediction of structure’s service lifeb) Inadequate description of environmental conditionsc) Fails to make allowance for changing materials e.g.

new cementsd) No actual way of checking quality of as-built

structures


Current situation: ‘Minimal Design’

No risk of corro-sion or attack

Carbonation-induced corrosion

Chloride-induced corrosion –sea water

X0 XC 1 XC 2 XC 3 XC 4 XS 1 XS 2 XS 3

Maximum w/c --- 0,65 0,60 0,55 0,50 0,50 0,45 0,45

Minimum strength class

C12/15 C20/25 C25/30 C30/37 C30/37 C30/37 C35/45 C35/45

Minimum cement content (kg/m3)

--- 260 280 280 300 300 320 340

Minimum air content (%)

--- --- --- --- --- --- --- ---

Example: extract from EN 206


Current situation: accelerated testing

Use of elevated ‘actions’ (e.g. temperature, concentration of aggressive agent) to accelerate deterioration in short-term tests.

Correlation of short term test results (RAT) with long-term in service tests (RLT) to determine acceleration factor K.

K = RAT/RLT (1)


Accelerated testing

Limitations:-a) Selection of suitable exposure factors responsible

for degradation.b) Lack of sufficient long-term data to validate

correlationsc) Assumption of linear relation (equation 1) which is

generally not true.


Developing situations:a) Limit state approach

Durability design criteria and associated specifications given as limit states e.g. ISO 13823.

Limit state needs to be defined – when structure fails to satisfy performance requirements. E.g. for RC structures:

Initiation limit state (ILS): point when corrosion initiates in a certain proportion of concrete structure e.g. 10%

Can be framed in a full probabilistic or reliability-based approach.


Limit state approach

Checks for durability can be done in two formats:-a) Limit state format: Resistance capacity R(t) should

be greater than ‘action’ effect S(t) (environmental load).

R(t) ≥ S(t)

b) Service life format: Predicted service life (tS) should be more than specified design life (tD).

tS ≥ tD


Developing situations:b) Service life design approach Service life design required Service life prediction

models together with key design parameters.

Service life design in fib bulletin 34 (2006) based on four levels:-

a. Full probabilistic design: requires validated mathematical models, full information on variability

b. Partial factor design: Partial factors used to consider variability in action values, materials

c. Deemed-to-satisfy approach: set of rules for dimensioning, execution and material selection.

d. Avoidance of deterioration e.g. through use of non-reacting materials.

Durable Concrete Construction Through Performance Testing IIT Madras & IIT Bombay: Jan-Feb 2014fib Model Code Framework

Assumptions, terms & definitions, administrativeprovisions, principle of service life

Design criteria

Full probabilistic

Probabilistic models-Resistance to attack-Exposure-Geometry

Partial factor design Deemed to satisfy approach Avoidance

Design values-Characteristics values-Partial safety factors-Combination factors

Exposure classes Exposureclasses

Limit states Design equations Design provisions Design provisions

Design / Verification

Project specification for materials selection,execution, maintenance plan

Service life design approach - fib bulletin 34


Service Life Prediction Models (Mathematical modelling)

Based on understanding of deterioration and influencing factors

E.g.: DuraCrete modelsa) Carbonation-induced corrosion model

b) Chloride-induced corrosion model

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Service life prediction models

Key model parameters: cover depth, diffusion coefficients (which can also be used for specification).

Verification of conformance with these parameters using suitable test methods.

Key to success of this approach:

Appropriate test methods!

Application of full probabilistic approach - considers model parameter variability (statistical distribution); Increases reliability of performance approach.


Summary: Current prescriptive approach

q

Further limitations with ‘prescriptive’ provisions:- Lack of clear definition of dual responsibility

between concrete producer and constructor. Increased stringency in prescriptive requirements

Additional improvement in durability performance

To address limitations: • a shift to a performance-based approach required• Service life design lends itself to a performance-

based approach

≠


4. PERFORMANCE-BASED APPROACH TO DURABILITY DESIGN


Performance-based approach to durability design and specifications Prescriptive approach: prescribed mix parameters

Performance approach: prescribed performance criteria

Rational (‘engineering’) approach to durability design and specification

Provides integrated approach – the governing parameters should be used in design formulation, deterioration models and specification values

Deterioration mechanisms are considered

Verification of performance properties that influence durability e.g. penetrability


Performance-based approach

Aspects considered in the approach:-i. Quantification of environmental loads and

predominant deterioration mechanism(s)

ii. Performance criteria for structure e.g. service life

iii. Prediction models for rate of deterioration

iv. Means of considering variability e.g. partial factors.

v. Appropriate specifications and QA systems to verify compliance with required performance.


Performance specifications

Used to assess actual performance of as-built structure for compliance with performance criteria In reality, this is seldom done!

Development of specifications entails:-i) Performance requirements for materials and

actual as-built quality - clearly defined.ii) Appropriate means of verifying compliance –

suitable tests - reliable, repeatable, easily appliediii)Means to enforce compliance with specifications

e.g. suitable penalties in event of failure.iv) p.t.o.


Development of specifications (cont’d):-iv) Definition of functional requirements to ensure all parties

involved in implementation have clear roles

Maintenance

Structural design

Supply of concrete mix

Material properties

Execution

Material specifications

Outlines performancerequirements

Owner

Contractor Design consultants

Concrete producer

Durable Concrete structure

Schematic representation of parties involved in production of RC structure


5. PERFORMANCE APPROACH -SOUTH AFRICAN EXPERIENCE


Performance approach in South Africa

Outline Frameworks: Durability studies;

Performance-based durability design & specification

Premises of “Durability Index Approach” (DI) approach

Review of test methods and service life models

Performance-based durability design and specification – SA developments

Examples of implementation


Provide a framework: For the designer to establish the required level of

performance…AND Within which

the material producer and constructor can produce a structure of desired quality

the owner can be assured that the quality desired is actually achieved

The above is applied in the SA context using the so-called Durability Index approach

Objectives


South African Framework – “Durability Index Approach”

DIRECT DURABILITY TESTING

----Correlations----Long-term tests (lab or site-based)

Aggressiveness of micro- and macro-environment

CorrelationsENVIRONMENT

MATERIAL INDEXINGCharacterization of concrete (surface layer) using easily measured physical properties, such as permeability and sorptivity

FUNDAMENTAL MECHANISTIC STUDIES

STRUCTURAL PERFORMANCEEvaluation of structural performance; Consequences of deterioration; Management of economic strategies

PREDICTION

Suite of accelerated tests (lab)

Correlations

QUALITY CONTROL


The durability of RC structures depends on the ability of the cover to protect the reinforcing steel,i.e. the quality and thickness of the concrete cover

Improved durability can be assured if relevant durability parameters reflecting the quality of the cover layer can be measured

In South Africa, we have developed such durability parameters and tests

– so-called ‘Durability Indexes’

Premises of SA Durability index approach for R.C. structures

39


1. A Robust Quality Control Test or Tests Routine, easily-carried out, reliable measure of

resistance (e.g. to chloride ingress)2. Service Life Models Relates performance to the quality control test (e.g. in

terms of limiting material parameters)3. A means to account for differences (i.e. ‘Margins’)

between ‘Material Potential’ and ‘As-Built’ values In order to differentiate between areas of responsibility

(e.g. material supplier & constructor)

Criteria for establishing a performance approach


Review of:

DI Tests

Service Life Models

(SA Durability Programme)


Oxygen Permeability Index (OPI) Test

Perforated cover plate

Concrete sampleRubber O-ringPressure gauge

Gas outletSpecimens: 70 mm dia. x 30 mm discs,pre-conditioned

Used to control carbonation resistance

Plastic pipe section and PVC spacerSilicon rubber ring

Gas inlet“OPI” = -log10k

Common values OPI: 8.5 …. 10.8

D’Arcy coefft. of permeability k


Chloride Conductivity Index (CCI) TestUsed to control chloride resistance

Plastic tubes Silicone

rubber ring

Concrete sample

Carbon anode

V

A

DC power source

5M NaCl 5M NaClSteel cathode

AVti⋅⋅

=σCommon values CCI: 0.5 – 2.5 mS/cm


Repeatability & reproducibility of DI tests

k-value OPI CCI

• Repeatability 1s%

Laboratory data 30 - 40 1.00 - 2.00 5.00-10.00Ready mix concrete

data - 1.00 - 2.00 5.00-10.00

Site data 40 - 50 1.50 - 3.00 10.00 - 15.00

• Reproducibility 1s%

Laboratory data 30 - 50 1.00 - 3.00 20.0


Initiation models: SLM for carbonation resistance, using 28-day OPI as a

parameter One SLM for chloride resistance, using 28-day CCI as

input to a Fickian model Account for material type and environment

Integrated approach: DI parameters are used In design, via the SLMs In specification – min. required values For quality control on site – checks on as-built

values

Service life models


Examples of Implementation:

Performance-based durability design


1. ‘Deemed to Satisfy’ approach (based on ‘standard’ sets of design conditions) – some examples given

Design methodology can be applied to two conditions:

2. Rigorous approach – not covered here


U’Shaka pier - Durban


U’Shaka pier

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

A BAv

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Adj A

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AugustCube 1

AugustCube 2

AugustElement 1

AugustElement 2

September Element 1

September Element 2

October Element 1

OctoberElement 2

OctoberElement 3

Chloride Conductivity ResultsMarine Pier

Individual Disc ResultsArithmetic MeanAdjusted Mean

Full Site Acceptance Limit

Laboratory Limit

Site Acceptance w ith Penalties

0.75

1.50


Gauteng Freeway Improvement Project

Major freeway upgrade programme in Gauteng Province

ca. 2.5 b US$ (1st phase)


Example of Implementation: GFIP (SANRAL) DIs used to specify carbonation resistance of bridge structures

Oxygen Permeability Index Concrete cover

OPI Percentage Overall cover Percentage

(log scale) payment (mm) payment Full acceptance > 9.70 100% ≥ 85% 100%

< (100%+15mm)

Conditional acceptance a > 8.75 ≤ 9.70 80% < 85% ≥ 75% 85%

Conditional acceptance b - - < 75% 70%

Rejection < 8.75 Not applicable < 65% Not applicable


Example of Implementation: GFIP (SANRAL)

DI prediction model for inland exposure conditions, R.H. = 60%,OPI = 9.70, and 100% CEM I binder

OPI=9.7

OPI=8.75

OPI=10.2


GFIP (SANRAL)


GFIP (SANRAL)


N’ganga (2012) Study to evaluate practicality of the DI performance-

based approach, by considering: Extent of variability in test results Applicability of the test in laboratories Perception of resident engineers on the approach

• Data obtained from:− GFIP work packages − Questionnaires sent out to

resident engineers− Review of a laboratory audit report


Statistical summary (OPI):

OPI (log scale)

Proportion of

Project ID n Mean Max Min s

CoV (%)

defectives %

1 172 9.75 10.41 9.07 0.28 2.84 40.1

2 94 9.91 10.42 9.37 0.22 2.24 13.8

4 116 9.87 10.40 9.39 0.23 2.33 18.1

6 91 10.06 11.10 8.83 0.46 4.60 26.4

9 132 10.25 10.70 9.85 0.18 1.75 0• Project 1, 2, 4, 6: in-situ samples; Project 9 - precast element samples.


Statistical summary (Cont’d):

• All mean OPI values comply with limit value of 9.70

• Nevertheless, large ‘proportion of defectives’ for certain as-built structures

• Variability (CoV) and proportion of defectives lowest for precast samples

o Better control exercised in precast unit manufacture than for in-situ construction


8.8 9 9.2 9.4 9.6 9.8 10 10.2 10.4 10.6 10.80

0.4

0.8

1.2

1.6

2

2.4

OPI (log scale)

Den

sit

y

Project 1: n =172

8.8 9 9.2 9.4 9.6 9.8 10 10.2 10.4 10.6 10.80

0.4

0.8

1.2

1.6

2

2.4

OPI (log scale)

Den

sit

y

Project 2: n = 94

8.8 9 9.2 9.4 9.6 9.8 10 10.2 10.4 10.6 10.80

0.4

0.8

1.2

1.6

2

2.4

OPI (log scale)

Den

sit

y

Project 4: n = 116

8.8 9 9.2 9.4 9.6 9.8 10 10.2 10.4 10.6 10.8 11 11.20

0.4

0.8

1.2

1.6

2

2.4

OPI (log scale)

Den

sit

y

Project 6: n = 91

8.8 9 9.2 9.4 9.6 9.8 10 10.2 10.4 10.6 10.80

0.4

0.8

1.2

1.6

2

2.4

OPI (log scale)

Den

sit

y

Project 9: n = 132

Histogram plots illustrating variability of OPI values in projects


6. PRACTICAL ASPECTS OF DRAFTING AND IMPLEMENTING PERFORMANCE-BASED SPECIFICATIONS


Implementation of Performance-based specifications

Some practical effects of introduction of these specifications included:-

i) Suspicion and slow uptake by industry.ii) Ready Mix concrete industry:-

a) Mix designs no longer disclosedb) Application of approach dependent on size of contractc) Want responsibility to end on deliveryd) Expected owners to undertake verification as before

iii) Lack of performance dataiv) Main contractors not fully engagedv) Problems of initial high variability between different labs


Perception of resident engineers

Questionnaires sent out to REs for information on site construction practices, source of samples, problems encountered, and improvements with implementation

Responses from REs: Difficulties in manufacture of test panels - generally poorly

handled on site Difficulties in transport of panels - large size and poor

communication between laboratory and site staff Some REs felt approach had no effect on construction

practices, while others felt that stricter controls were exercised.


More work required on test/sample variability: between batch variability, and in-situ variability

This will give more confidence in relationships between target and characteristic material value

Very little information on magnitude of reduction in values between lab standard cured samples and in-situ achievements

Need information on actualas-built values, to confirm validity of approach

Current limitations in application


Implementation of Performance-based specifications

Factors to consider for further development of the approach:-

a) In situ testing and obtaining ‘representative’ samples

b) Testing is critical; test methods required which need not be ‘perfect’

c) Penalties are needed

d) Greater technical knowledge and competence of all parties essential

e) Important aspect of proper communication to ensure that all parties involved are aware of their responsibility


Closure The next decade or so will see major developments in RC

design – related mainly to Durability design Sustainability design

Current codes and standards will need rapidly to change and reflect these developments

As a first step, performance-based design and specifications are now possible, provided suitable test methods are available for specification and verification of construction quality

A concerted effort is needed among owners, specifiers, designers, materials suppliers, and constructors to move the industry forward


Thank You!

Documents

Principles of durability design for concrete structurestheconcreteportal.com/alexander.pdf · Durable Concrete Construction Through Performance Testing IIT Madras & IIT Bombay: Jan-Feb