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Moisture movement within concrete exposed to simulated hotarid/semi-arid conditions
Citation for published version:Alaswad, GA, McCarter, WJ & Suryanto, B 2020, 'Moisture movement within concrete exposed to simulatedhot arid/semi-arid conditions', Construction Materials, vol. 173, no. 6, pp. 298-312.https://doi.org/10.1680/jcoma.18.00012
Digital Object Identifier (DOI):10.1680/jcoma.18.00012
Link:Link to publication record in Heriot-Watt Research Portal
Document Version:Peer reviewed version
Published In:Construction Materials
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Download date: 28. Mar. 2021
Moisture movement within concrete exposed to simulated hot arid/semi-
arid conditions
Author 1: Gasim Alaswad, BSc, MSc
Author 2: William John McCarter, BSc, PhD, DSc, CEng, MICE
Author 3: Benny Suryanto, BEng, MEng, PhD
Author 1: Doctoral Student, School of Energy, Geoscience, Infrastructure and Society,
Institute for Infrastructure and Environment, Heriot Watt University, Edinburgh, EH14 4AS,
UK
Author 2 (corresponding author): Professor, School of Energy, Geoscience, Infrastructure
and Society, Institute for Infrastructure and Environment, Heriot Watt University, Edinburgh,
EH14 4AS, UK.
ORCID: 0000-0002-1949-2856
E-mail: w.j.mccarter@hw.ac.uk
Tel: +44 (0)131 451 3318
Author 3: Associate Professor, School of Energy, Geoscience, Infrastructure and Society,
Institute for Infrastructure and Environment, Heriot Watt University, Edinburgh, EH14 4AS,
UK
ORCID: 0000-0002-3979-9994
Number of Words: 5100
Number of Tables: 6
Number of Figures: 11
Abstract
The ambient environment has considerable influence on the permeation properties of the
near-surface zone of exposed concrete (i.e. the cover zone). Monitoring the mass transport
and flow processes and properties within this region is crucial in evaluating the long-term
performance of concrete for a particular exposure condition. This paper presents an
experimental study on both the spatial and temporal moisture movement within the surface
region of concrete with and without supplementary cementitious materials. Prior to exposure,
the samples were conditioned under two regimes representing poor and good curing; the
samples were then exposed to a simulated hot environment with a diurnal temperature
fluctuation of 20-40C and 60% ambient relative humidity. Moisture movement within the
surface region was monitored using discretized electrical conductivity measurements which,
together with gravimetric measurements, allowed evaluation of the volumetric uptake and
sorptivity of the concrete and the rate and depth of water penetration into the concrete cover-
zone; it is shown that when these are combined, the degree of saturation, effective porosity
and total porosity of the surface region could be estimated. By evaluating the conductivity
prior to and after water absorption, the zone of influence of wetting/drying action (i.e. the
convective zone) could be evaluated.
Keywords: Concrete technology & manufacture / Strength and testing of materials /
Environment
Notation
C = capacitance (pF)
d = depth of penetration of the water front (in mm)
do = depth-based sorptivity coefficient (in mm/h1/2)
f28 = compressive strength at 28 days (MPa)
f180 = compressive strength at 180 days (MPa)
i = cumulative volumetric gain per unit area of inflow surface (mm3/mm2)
io = initial sorption due to the surface effect (mm3/mm2)
k = calibration constant (/cm)
n = saturation coefficient
Rt = thermistor resistance (ohms)
Rc = concrete resistance (ohms)
Sd = depth-based sorptivity coefficient (in mm/h1/2)
Sv = sorptivity based on the volumetric uptake by the concrete
Sr = degree of saturation (%0
t = elapsed time (h)
tm = time of arrival of water-front at electrodes (h)
T = temperature (ºC)
w/b = water-binder ratio
, and = thermistor coefficients (K-1)
= conductivity (S/cm)
o = conductivity just prior to water absorption (S/cm)
ss = conductivity at steady-state (S/cm)
t = conductivity at time, t, after the start of the absorption test (S/cm)
ϕeff = effective porosity (%)
ϕ = capillary porosity (%)
1. Introduction
The movement of water (and water containing dissolved ions) into the cover region of
reinforced concrete plays a significant role in virtually all deterioration processes; for
example, chloride ingress, alkali-silica reaction, carbonation and sulphate attack all depend
on the availability of water. Reinforcement corrosion, either through chloride attack or
carbonation, is considered one of the main causes of the premature deterioration of concrete
structures (Jones et al, 1997). The in-service performance and long-term durability of
concrete is influenced by a number of factors, for example, the quality of construction
materials; quality control in the manufacturing and placement of concrete; poor construction
practices, such as unskilled labour or unqualified supervision; severe climate with large
diurnal temperature fluctuations and the lack of construction standards and specifications
relevant to local environments. In connection with corrosion, as it is the concrete cover which
protects the steel from the ambient environment it is not surprising that the protective
qualities of this zone of concrete have a considerable bearing on concrete performance and
durability. In addition, the use of supplementary cementitious materials (SCM) to form
blended cements are now being increasingly utilized to improve concrete durability.
Regarding durability, in addition to a specified strength requirement, it is the permeation
properties of the cover-zone concrete which determine concrete performance and terms such
as diffusivity, permeability and sorptivity are used in this respect. The service life of
reinforced concrete structures is largely determined by the surface 50-100mm, and an
understanding of water and moisture movement within this zone is crucial in developing
guidelines and performance-based specifications for concreting operations in different
environmental conditions. Measures implemented for concrete performance need to consider
the particular ambient environment prevailing in a country or region as hot-humid climates
will have totally different performance specifications from, for example, cool temperate
climates.
1.1 Background and Context
The cover-zone of concrete exposed to cyclic wetting and drying action will fluctuate
between fully saturated and partially saturated states which will establish moisture gradients
through this region. Water and moisture movement will thus be a combination of both water
absorption by capillarity and water-vapour diffusive processes. It is not surprising, therefore,
that the water absorption properties of a concrete surface have been used to index concrete
performance and numerous surface-applied tests have been developed in this respect which
include, surface absorption tests (BSI 1996; Wilson et al. 1998; ASTM 2013); Figg
hypodermic methods (Figg 1973); water permeability tests (Basheer 1993); the covercrete
absorption test (Dhir et al. 1987; Meletion et al. 1992; Blight and Lampacher 1995);
cumulative absorption and sorptivity methods (Hall and Yau 1987; McCarter 1993; Classie et
al. 1999; BSI 2011) and the Clam/Autoclam permeation tests (Basheer et al. 1995; Basheer
and Nolan 2001; Yang et al. 2015). These tests quantify the cumulative volumetric uptake of
the concrete surface, however, no information is given on the depth or the rate of penetration
of the water-front into the surface region. Furthermore, evaporative processes are equally as
important as wetting, yet have not received the same attention. A total assessment of the
quality of the surface region should not only consider the drying and wetting response but
also allow evaluation of the spatial distribution of moisture within the concrete during drying
and wetting processes.
Regarding the evaluation of the depth of penetration of the water-front, as yet, there are no
standardized procedures. The most straightforward approach is to subject replicate samples to
a water absorption test, which are then split at specified times and the depth of water
penetration measured. It has been reported, however, that as the strength of the concrete
increases, visual detection of the waterfront can present considerably difficulty (McCarter et
al. 1992). It has been shown that the depth of penetration approximates to a square-root-time
relationship (Ho and Lewis 1984; McCarter et al. 1992) viz,
d = Sd t + do (1)
where d is the depth of penetration of the water front (in mm), t is the elapsed time (in h), do
is a constant (in mm) and Sd is a depth-based sorptivity coefficient (in mm/h1/2). There is a
paucity of data on the depth of water penetration into concrete, for example: Sd values for
concretes subjected to different conditioning procedures were found to be in the range 1.5-
19mm/hr1/2 (Ho et al. 1989) with absorption tests of 24-hours duration and concrete
compressive strengths in the range 30-33MPa; Sd values for plain Portland cement concrete
mixes with strengths in the range 36-59MPa, based on a 24-hour water absorption test, were
reported as being in the range 2.2-4.7mm/h1/2 (McCarter et al. 1992).
It is set against this background that the current study investigates both the spatial and
temporal variations in moisture within the surface region of concrete in response to external
changes in hygrothermal conditions viz. cyclic temperature changes to simulate diurnal
temperature variations experienced in hot arid/semi-arid climates. It must be emphasised,
however, that the temperature regime adopted within this study does not purport to represent
all the climatic conditions experienced in such regions. This study also investigates the
influence of curing conditions and SCM's on the response of the cover-zone to such
environmental action. This is achieved through the use of both gravimetric measurements and
discretized electrical conductivity measurements, the latter being evaluated through the use of
an array of embedded electrodes. Through this combined test-method approach, it is shown
that a number of important parameters related to the long-term performance of concrete can
be estimated.
2. Experimental Programme
In order to monitor both the temporal and spatial changes in moisture movement both in the
short- and long- term, a test-cell was designed such that electrical measurements (resistance
in this instance) and temperature could be made at discrete depths from the exposed surface
of the concrete samples thereby allowing an integrated assessment of the surface-zone. In
parallel, the cumulative volumetric water uptake by the exposed concrete surface and internal
relative humidity (RH) were also monitored.
2.1 Samples
In the current study, the binders comprised ordinary Portland cement clinker, CEM I 52.5N to
EN197-1 (BSI 2011); CEM I cement blended with ground granulated blast-furnace slag to
EN15167-1 (BSI 2006a) and CEM I cement blended with a low-lime fly-ash to EN450-1
(BSI 2012). The oxide analysis of the cementitious materials is given in Table 1 and the
concrete mixes are presented in Table 2, together with their respective 28-day (f28) and 180-
day (f180) compressive strengths after continuous submerged curing at 21C±1°C.
A crushed rock (granite) coarse aggregate and matching crushed rock fines were used
throughout. For the fine aggregate, 100% passed the 5mm sieve, 8% passed the 150m sieve;
additional coarse and fine aggregate properties are presented in Table 3 conducted in
accordance with BS-EN 1097-3 (BSI 1998), BS-EN 1097-6 (BSI 2013) and BS 812-112 (BSI
1990). The aggregate was conditioned to a saturated surface dry state. The aggregate content
was adjusted to ensure that the mass of binder remained constant for each water-binder (w/b)
ratio. Batches were initially dry mixed in a pan mixer for one minute to ensure thorough
mixing of replacement materials prior to addition of water. The following were cast for each
mix: six samples placed in cylindrical test-cells (detailed below) for moisture, relative
humidity and gravimetric monitoring and three, 100mm cubes for compressive strength tests.
The range of mixes used within the experimental programme, in terms of binder-content,
binder composition and water-binder ratio, would satisfy minimum requirements specified in
BS 8500-1 (BSI 2006b) for environmental exposure classes XC (corrosion induced by
carbonation), XS (corrosion induced by chlorides from sea-water) and XD (corrosion induced
by chlorides other than sea-water e.g. deicing salt) for an intended working life of 100 years,
with 50mm cover-to-steel.
2.2 Test Cells
Concrete samples were cast in 182mm (outside diameter) and 200mm (high) PVC moulds.
One end of the cylinder was attached to 12mm plywood base-plate and the concrete surface
cast against the plywood was used as the working surface; the plywood had been given a coat
of proprietary release-agent prior to casting. Pairs of stainless steel rod-electrodes, were
inserted through the side of the mould shown schematically in Figure 1. The rod-electrodes
were 2.6mm in diameter and sleeved to expose a 20mm tip. Within each electrode pair,
electrodes were positioned at 12mm centre to centre (c/c) and protruded 50mm into the
sample. With reference to Figure 1, five electrode-pairs were positioned at 10mm intervals
from the working surface with a 30 offset; a pair of electrodes was also positioned at 100mm
and at 175mm from the working surface. This is a variation on other electrode configurations
(Schiessl and Raupach 1996; Rajabipour et al. 2005; McCarter et al. 2015) causing less
interference with the natural distribution of aggregate within the sample. The electrode-pairs
within the test-cell were calibrated by filling the cell with a solution of known conductivity;
hence, if cal is the conductivity of the calibrating solution (calcium hydroxide) and Rcal the
measured resistance across the electrode-pair, then
𝜎𝑐𝑎𝑙 =𝑘
𝑅𝑐𝑎𝑙 (2)
where k is the (geometrical) calibration constant for the electrode-pair. However, as the test-
cell had five (geometrically similar) electrode pairs, an average value of k was evaluated to
give an overall test-cell constant for the electrodes; the value of k was obtained as 0.306 cm-1
(±5%). This allowed the concrete resistance, Rc (ohms), measured across the electrodes to be
converted to conductivity, (S/cm), by,
)cm/S(cR
k (3)
In addition to the electrodes, four, 12mm diameter (50mm long) cavities were formed to
allow relative humidity measurements. The centres of the cavities were positioned at 25, 50,
100, and 175mm from the working surface, as with the electrodes, the cavities had a 30
offset. Pre-formed cavities were chosen in preference to drilled holes due potential damage
caused by hammer-action drilling, particularly those cavities close to the test surface. The
inside of each cavity was lined with a thin plastic sleeve (Kim and Lee 1999; Holmes and
West 2013; Granja et al. 2014) with the end of the cavity exposed to ensure that the RH was
representative of the concrete at that depth. A RH sensor was inserted within each cavity
which was then sealed with a tightly fitting rubber bung.
Two thermistors (for temperature measurements) were cast into the sample and were attached
to electrodes positioned at 10mm and 100mm from the exposed surface.
2.3 Ambient Temperature and Curing Regimes
After casting and compacting, the top (cast) surface of each cell was tightly covered with a
plastic sheet so as to prevent evaporation. The cylinders were immediately placed in an
environmental cabinet which was programmed to give a 24-hour temperature cycle of: 10-
hours at 40±1C, 3-hours cooling to 21C followed by 11-hours at 21±1C (see Figure 2)..
The RH was maintained at 60% over a saturated sodium bromide solution with the air speed
over the samples maintained at approximately 4m/s. As noted earlier, although it is difficult
to generalise diurnal temperature fluctuations and ambient humidity which can be
experienced in hot-arid/semi-arid countries (see, for example, Baghabra Al-Amoudi et al.
2004; Ait-Aider et al. 2007; Haque et al 2007; Pattanaik et al. 2015), the regime adopted thus
represents a specific set of environmental conditions. The samples exposed to this regime
from casting and throughout the testing programme. After 24-hours, the cast surface was
sealed with two coats of a high-build epoxy paint; also, while the final coat of epoxy was still
tacky, it was covered with a layer of cling-film to add an additional vapour barrier to ensure
no moisture movement through this surface.
Each set of six replicates was further sub-divided into two groups of three samples. Each
group was subjected to a specific curing regime:
(i) C1 curing, representing good curing: the plywood was maintained in place for 14 days.
Note: the concrete cast against the plywood formed the working surface.
(ii) C2 curing, representing poor curing: after 24-hours the plywood was removed.
The PVC moulds were maintained in place throughout the testing programme which ensured
uniaxial moisture movement; the samples also remained in the environmental cabinet
throughout and subjected to a wetting cycle of 24-hours duration. In the current work,
samples were subject to a wetting cycle every 60-days (approximately) which allowed
sufficient time for natural drying of the concrete under the simulated environmental
conditions.
2.4 Measurements and Data Acquisition
The electrical resistance (Rc in ohms) of the concrete between each pair of electrodes was
obtained employing a two-point technique using signal amplitude of 350mV at a frequency of
1kHz. This frequency was optimised from a-priori experiments using multi-frequency
measurements (impedance spectroscopy) to ensure electrode polarization effects were
reduced to a minimum (McCarter and Brousseau 1990). The cabling from the cells was
ducted out through a small porthole in the side of the chamber and connected to a resistance
meter and multiplexing unit to record the electrical resistance across the electrode-pairs and
thermistors. A reading cycle, comprising seven electrode-pair measurements and three
thermistor measurements (two embedded within sample and one placed in the environmental
cabinet), was initiated every 2 minutes over the 24-hour absorption cycle.
Thermistor measurements were converted to temperature using the Steinhart-Hart equation,
T = [ + lnRt + (lnRt)3]-1 - 273.15 (4)
where Rt is the measured resistance of the thermistor (ohms); , and are coefficients
which depend on the type of thermistor and in the current work were (Betatherm 2014),
respectively, 1.29×103K-1, 2.36×10-4K-1 and 9.51×10-8K-1and T is the computed temperature
(in ºC ± 0.2°C).
Measurement of the volumetric uptake of the concrete samples during water-absorption was
obtained by filling the Plexiglas cap via a burette until water weeped out of the small hole in
the cap. The supply of water from the burette was then shut and water was absorbed from the
reservoir of water in the cap; at specified times, the burette was opened and the volume of
water required to refill the reservoir recorded from the burette (±0.1ml). Absorption
measurements were taken when the sample was at the constant laboratory temperature regime
of the heating/cooling cycle (i.e. 21±1C). The complete testing arrangement is shown in
Figure 3.
The RH in the cavity was recorded using a capacitive humidity sensor, with the sensor
permanently mounted in each cavity. A LCR meter, operating at a frequency of 10kHz and
350mV signal amplitude, was used to measure the capacitance (C, in pF), which was,
subsequently, converted to %RH (±2%) using the equation (Humirel 2002),
%RH = [-8.8932×10-4×C3] + [0.4651×C2] – [77.970×C] +4202 (5)
3. Results and Discussion
The combination of internal conductivity and gravimetric water-uptake measurements are
presented to highlight the range of parameters which can be evaluated from the dual testing
protocol and highlights the influence of external environment, curing and supplementary
cementitious material (SCM) on water and moisture movement within the surface-zone
(100mm).
3.1 Water Absorption
For illustrative purposes, Figure 4(a)-(c) presents the change in the electrical conductivity of
the concrete during water absorption for the concrete mixes (water/binder ratio, w/b = 0.6)
under curing regime C2. As conductivity values at the different electrode positions can
change by more than an order of magnitude during water absorption, this Figure presents the
relative change in conductivity, o/t , where t is the conductivity at time, t, after the start of
the absorption test and o is the conductivity just prior to the application of water at the
surface. As the water-front moves through the surface zone t will increase and the response
at a particular electrode-pair is characterised by a well-defined, decreasing portion although it
is evident that the gradient of this region becomes flatter (i.e. becomes more negative) with
increasing depth from the surface. The decreasing portion of the curve indicates the arrival of
the water-front into the electrical field between that particular electrode-pair; once the water-
front has moved beyond the electrical field, a steady-state ratio is achieved and denoted
o/ss, where ss is denoted the steady-state conductivity. It could be assumed that at steady-
state, the concrete is in a fully saturated state in the vicinity of the electrode-pair.
A sigmoidal decay curve can be used to describe the o/t versus time (t) response viz.,
s]tg[t
101
c
t
o
m
(6)
The above equation also allows a means of standardising the way in which features of the
response can be quantified. In this equation, c is a constant which ensures that when t = 0,
o/t = 1.0; as t , s is the steady-state ratio,o/ss; g controls the gradient of the
descending portion of the response – as the value of g increases the slope gets more
precipitous; tm represents the time at which d(o/t)/dt maximises on the decreasing portion
of the o/t response. Again, as way of illustration, Figure 4(d) presents four best-fit curves
the form of equation (6) for the response presented in Figure 4(a) over the surface 40mm,
with the values for the modelling parameters c, g, tm and s presented in Table 4. From
equation (5) above, it is proposed that the time of arrival at the depth of the electrode pair
occurs at time, tm, when d(o/t)/dt maximises. The sharpness of the advancing water-front
would be related to the parameter, g; as g decreases, it may imply that the advancing water-
front moves from a sharp wet-front (see, for example, Figure 4(a) at 10mm depth) to one
which becomes increasingly more diffuse with depth as absorption proceeds (see Figure 4(a)
at 40mm depth).
Regarding the steady-state parameter, s, this will be related to the degree of capillary pore
saturation just prior to the start of the absorption test i.e. the residual moisture profile through
the surface region resulting from the preceding drying cycle. With reference to Figure 5, if
concrete (of capillary porosity, ) can be considered as a three-phase system comprising air,
pore-water and solids (i.e. aggregate, products of hydration and unhydrated cement) then, as
the initial degree of pore saturation (Sr) increases, the volume of accessible capillary pore
space (i.e. the effective porosity, eff) which can be filled by the advancing water-front is
reduced. From an electrical point of view, this will imply that the difference between the
conductivity measured at the start of the absorption test, o in Figure 5(a), and its steady-state
value, ss in Figure 5(b), will reduce with increasing degree of initial pore saturation; in terms
of the steady-state parameter, s (= o/ss), this will increase with increasing degree of initial
pore saturation, Sr. With reference to the responses presented in Figure 4(a)-(c), where a
steady-state o/ss value has been achieved, it is evident that this is, indeed, the case as its
value increases with increasing depth from the exposed surface or, in qualitative terms, the
degree of saturation increases with depth from the exposed surface prior to the start of the
absorption test.
It is interesting to note that a saturation function has been presented which relates the
electrical properties of cement-based materials to their degree of saturation (Weiss et al.
2012; Wang et al. 2016). Using the notation defined above, and adapting the work by Weiss
et al (2012), the following relationship has been presented,
𝑆𝑟 = (𝜎𝑜
𝜎𝑠𝑠)
1
𝑛 × 100% (7)
where n is defined as the saturation coefficient. Values of n have been reported in the range
3.5-5.5 (Nokken and Hooton, 2008; Weiss et al. 2012) with a value of 4.0 (Weiss et al. 2012)
suggested for plain Portland cement-based systems. Consider, for example, the steady-state
o/ss values for the PC concrete in Figure 4(a): at 10mm = 0.083; 20mm = 0.26; 30mm =
0.39 and 40mm = 0.43. If a value of n = 4.0 is assumed, this would evaluate the degree of
saturation, Sr, of the concrete just prior to the start of the absorption test as: at 10mm = 54%;
20mm = 71%; 30mm = 79% and 40mm = 81%. Clearly more work is required in establishing
values for n for different concretes with and without SCMs, as this would allow a simple
means of estimating Sr gradients through the surface zone.
It is interesting to note that some of the responses displayed an initial increase in resistance
on application of water at the surface of the samples (i.e. o/t > 1.0) before decreasing. This
feature is particularly evident in Figure 4(c) for the GGBS concrete mix. As the resistance
over this time has increased, a possible explanation of such a feature would be as a result
from air dispersing into the capillary pores in the vicinity of the electrodes: as the water from
moves into the partially saturated concrete, a volume of air will be pushed ahead of the
advancing the water-front which disperses into the pore system which, in this instance, is
sufficient to cause a transitory increase in resistance. Further work is required, however, to
fully explain this feature.
3.2 Water Penetration and Volumetric Gain
Using the definition of tm defined above to estimate the time of arrival of the water-front at an
electrode-pair, Figures 6(a) and (b) present the water penetration depth within the initial 16
hours of the absorption cycle and plotted in the form of equation (1). Error bars on the data
markers represent ±1.0 standard deviation, where the error bars appear to be missing, the data
marker is larger than the error bar. This has only been undertaken for C1 and C2 cured
concrete mixes having w/b = 0.6 as the water-front had not penetrated sufficiently within the
24-hour absorption period for w/b = 0.4 mixes to enable a relationship to be established.
Regarding samples with w/b = 0.4, for C1 cured samples, the depth of water penetration was,
typically, 20mm and for C2 cured samples 30mm. This, in itself, highlights the influence
of w/b ratio on water ingress for the curing and exposure conditions detailed above.
When using SCM's good curing, particularly during the early stages of strength development,
is essential to ensure that water remains within the capillary pore structure due to the much
slower pozzolanic reaction. This is clearly highlighted in Figure 6(b) whereby C2 curing in
the simulated climatic conditions results in a rapid movement of water through the surface
50mm as depth-based sorptivity values (Sd) have increased when compared to their C1
cured counterparts (Figure 6(a)). Particularly striking is the FA concrete mix where the Sd
value has increased by more than 150%; furthermore, the high replacement GGBS mix gives
the highest Sd values under both curing regimes whereas the PC mix performs best at this w/b
ratio under both C1 and C2 curing. Generally, there is good repeatability and only the C1
cured FA mix displays scatter at depths of 30mm and 40mm.
Figure 7(a)-(b) shows the cumulative volumetric gain per unit area of inflow surface (i,
mm3/mm2) for all concretes during the initial 6 hours absorption with the values at 24-hours
presented in Table 4. The data have been plotted on a square-root-time axis and it is evident
that the best-fit line takes the form,
i = Sv t + io (8)
where Sv is the sorptivity based on the volumetric uptake by the concrete and io is the initial
sorption due to the surface effect. Considering the Sv values (presented in Figure legend) and
the 24-hour cumulative absorption (Table 5), the concretes containing GGBS and FA
outperform the plain PC mixes when used at the low w/b ratio (0.4) with C1 curing; however,
this reverses when these concretes are used at w/b = 0.6 with C2 curing.
The depth of penetration of the water-front, d (in mm), and the volumetric gain of the
concrete, i (in mm3/mm2) can be combined to estimate the effective porosity, eff, of the
surface region. As noted above, the effective porosity represents the fraction of empty (or air-
filled) capillary pores within the binder just prior to the start of water absorption (see Figure
5) and is the void space that can be invaded by the advancing water-front, hence,
%100d
ieff
(9)
For example, consider Figure 8 which presents the depth of penetration and volumetric gain
results for the FA concrete with w/b = 0.6 under C2 curing: when the water-front, d, has
penetrated to 30mm, the volumetric gain at this time is 1.17 mm3/mm2, hence eff for the
surface 30mm is 3.9%. In this way, the effective porosity of the surface region could be
estimated and, when combined with the degree of saturation, Sr, discussed above in relation
to the steady-state conductivity ratio, o/ss , the total porosity, , of the surface region could
then be evaluated i.e. = eff/(1-Sr).
This combination of techniques offers a methodology for studying and evaluating the depth-
related features which, ultimately, are of considerable important in relation to concrete
performance.
3.3 Conductivity profiles and the Convective Zone
As water evaporates from (or is absorbed into) the concrete surface, a moisture gradient will
be established through the concrete cover-zone. As the electrical conductivity of the concrete
will be directly related to the volumetric water-content (or the degree of saturation) of the
capillary pore network, the conductivity profile through the sample should mimic the
moisture gradient. As way of illustration, Figures 9-11 present the conductivity profiles
through the concrete samples both before and after water absorption for two cycles of
drying/wetting. As water is absorbed into the concrete, it is evident that the conductivity after
the 24-hour absorption cycle has increased relative to the value before absorption. This
simply reflects the influence of the advancing water-front; however, it is also noticeable that
this effect diminishes with increasing depth from the concrete surface. It could be postulated
that, where the conductivity of the concrete remains unaltered before and after absorption,
this would indicate the maximum potential extent of the zone most influenced by
drying/wetting action i.e. the convective zone. The convective zone, or influential depth of
moisture transport (Li et al. 2009), would lie within a range bounded by the depth at which no
change in conductivity is observed at the electrode-pair, and the electrode-pair positioned
above. As the absorption period is 24-hours, any hydration and pozzolanic reaction occurring
in this time will have negligible influence on the conductivity of the concrete and changes in
conductivity will be solely due to the moisture-state of the capillary pore system. The
influence of curing regime, w/b ratio and SCM, coupled with the environmental exposure
conditions, is clearly evident from these Figures with, in some cases, the convective zone
extending through the surface 100mm (see, for example, Figure 11(d)).
For those mixes with w/b = 0.4 and C1 or C2 curing regimes, the conductivity values at
electrode positions 50mm, 100mm and 175mm are, in most cases, within ±10% of the mean
of the two respective values. This would imply that the curing regime applied at the surface
does not have any significant influence on the concrete at these depths where the concrete is
virtually self-curing. Considering the conductivity profiles between the 1st and 2nd absorption
cycles, it is also apparent that they become displaced downwards indicating a decrease in
conductivity over the intervening period. This decrease can be attributed to ongoing
hydration and pozzolanic reaction which modifies the microstructure resulting in a more
tortuous/disconnected pore network. Considering the electrodes positioned at 100mm from
the surface, at the end of the 2nd absorption cycle mixes with SCMs exhibit conductivity
values which are, typically, 2.5-3.0 times lower than the respective PC mix, regardless of w/b
ratio or curing regime.
For comparative purposes, Table 6 presents the relative humidity at cavities positioned at 25,
50, 100 and 175mm from the concrete surface for C1 and C2 curing. It was observed that the
RH sensor had a relatively slow response to changes in moisture state of the concrete, whilst
the electrical conductivity responded more rapidly. As can be seen from Table 6 (C1 curing
regime) the RH at 25mm and 50mm displays an increase for all mixes; for C2 curing,
however, mixes with w/b = 0.6 display an increase in humidity extending to 100mm. Whilst
there are similarities between the conductivity profile and RH profile, it must be emphasised
that conduction through the specimens will be directly related to the degree of saturation of
the aqueous phase in the capillary pore network, whereas RH is related to the water vapour
present in the air within the preformed cavity.
4. Conclusions and Concluding Comments
A combined testing procedure, using gravimetric and discretized, in-situ, conductivity
measurements, was used to study both the spatial and temporal distribution of moisture
within the surface 175mm of concrete samples. Concrete mixes, with and without SCMs,
were subjected to a simulated ambient environment which was representative of an arid/semi-
arid region. The work has clearly highlighted and quantified the influence of w/b ratio and
curing on the permeation properties of blended systems exposed to diurnal fluctuations in
temperature during the post-placement phase of concreting operations. For the curing and
simulated temperature and humidity regime used in the current study, it was observed that,
Under the environmental exposure regime, the influence of C1 curing and C2 curing on
both the volumetric-gain sorptivity (Sv) and the depth-related sorptivity (Sd) was clearly
evident. When the concrete is cured at a condition in which sufficient moisture is
retained within the capillary pore network, as in the case with C1 curing, both sorptivities
are significantly improved relative to C2 curing.
The incorporation of SCMs can beneficially reduce volumetric gain and depth of
penetration with the proviso that, under the environmental exposure conditions, these
materials are used in conjunction with a low w/b ratio (=0.4) and C1curing. Due to the
slow pozzolanic reaction, mixes containing SCMs with a high w/b ratio (0.6) and C2
curing resulted in the highest volumetric gain and rate/depth of water penetration
Electrical measurements taken at discrete depths from the exposed concrete surface
showed that during absorption, the penetration of water into concrete could be modelled
by a sigmoidal curve and indicated that the advancing water-front became increasingly
more diffuse with depth.
The depth of water penetration (d) together with the cumulative volumetric water gain of
the concrete, i (mm3/mm2), could be combined to estimate of effective porosity of the
surface zone.
The convective zone can be estimated by evaluating the conductivity profile through the
surface zone prior to and after water absorption. It was shown that the convective zone
extended over the surface 50-100mm when the concrete with the high w/b ratio was
subjected to C2 curing; concrete at greater depths is virtually self-curing. The RH profile,
although showing similar trends to the conductivity profiles, did not have the same level
of precision (or response rate) in estimating the convective zone.
The testing methodology presented offers the potential in evaluating a range of parameters
which are important in relation to the long-term performance of concrete; furthermore, as
measurements are taken at discrete locations within the concrete cover, it allows an integrated
assessment of this region. For example, it was shown that through the use of a saturation
function, the steady-state conductivity-ratio could be used to estimate the degree of saturation
of the concrete and, when this was combined with the effective porosity, the total porosity
could be evaluated.
Acknowledgements
One of the Authors (GA) would like to thank Libyan Embassy-Cultural Attaché for their
financial support (Grant Reference 11049).
Conflict of Interest
The authors declare that they have no conflict of interest.
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Captions for Figures
Figure 1 Schematic diagram showing positioning of electrodes, humidity cavities and
thermistors within the test cell (a) sectional view, and (b) plan view.
Figure 2 Thermal cycling regime within environmental chamber (RH=60%).
Figure 3 Water absorption and monitoring arrangement.
Figure 4 Relative change in conductivity o/t for C2 cured concrete mixes (w/b = 0.6)
during the initial 24-hour absorption cycle for (a) PC mix, (b) FA mix, (c)
GGBS mix (Legend on Figure 4(b)) and (d) simulated response for PC mix at
10, 20, 30 and 40mm using equation (5) and fitting parameters in Table 3.
Figure 5 Schematic diagram of concrete (a) three-phase system comprising air, pore-
fluid and solids just prior the start of water-absorption, and (b) after 24-hour
water absorption test where the pore system is assumed to be fully saturated
and o/t has reached the steady-state condition (o/ss). In this Figure, Sr is the
degree of pore saturation before absorption, is the total porosity and eff the
effective porosity.
Figure 6 Water penetration depth versus square root of elapsed time for w/b = 0.6
concrete mixes (a) C1 cured and (b) C2 cured.
Figure 7 The cumulative water gain per unit area of inflow surface (mm3/mm2) versus
square root of elapsed time for C1 and C2 (a) w/b = 0.4 and (b) w/b = 0.6.
Figure 8 Showing how the depth of penetration and cumulative volumetric gain results
(FA concrete; w/b = 0.6 and C2 curing) can be used to evaluate the effective
porosity.
Figure 9 Conductivity profiles through the concrete samples before and after 1st and 2nd
water absorption cycles for the PC concrete mixes: (a) w/b = 0.4: C1 curing,
(b) w/b = 0.4: C2 curing, (c) w/b = 0.6: C1 curing, and (d) w/b = 0.6: C2
curing. (Legend on Figure 9(a))
Figure 10 Conductivity profiles through the concrete samples before and after 1st and 2nd
water absorption cycles for the FA concrete mixes: (a) w/b = 0.4: C1 curing,
(b) w/b = 0.4: C2 curing, (c) w/b = 0.6: C1 curing, and (d) w/b = 0.6: C2
curing. (Legend on Figure 10(a))
Figure 11 Conductivity profiles through the concrete samples before and after 1st and 2nd
water absorption cycles for the GGBS concrete mixes: (a) w/b = 0.4: C1
curing, (b) w/b = 0.4: C2 curing, (c) w/b = 0.6: C1 curing, and (d) w/b = 0.6:
C2 curing. (Legend on Figure 11(a))
Captions for Tables
Table 1 Oxide composition of cementitious materials
Table 2 Summary of concrete mixes (w/b = water-binder ratio, FA = fly ash, GGBS =
ground granulated blast-furnace slag). Figure in brackets is the standard deviation
for the compressive strength.
Table 3 Properties of Coarse and Fine Aggregates
Table 4 Modelling parameters for equation (5) (CoD = coefficient of determination for
fitting curve).
Table 5 Cumulative volumetric gain, i (mm3/mm2) after 24-hours absorption.
Table 6 Relative humidity before/after 1st absorption cycle for C1 and C2 curing.
Table 1 Oxide composition of cementitious materials
Oxide % SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 SO3 LOI
CEM I 20.6 4.8 3.17 63.9 2.53 0.54 0.08 - 2.8 -
Fly Ash 48 29 9 2.5 3 3 1.3 0.9 0.7 3.3
GGBS 34 12.6 0.6 41 8.3 0.47 0.25 - - -
Table 2 Summary of concrete mixes (w/b = water-binder ratio, FA = fly ash, GGBS = ground granulated blast-furnace slag). Figure in brackets
is the standard deviation for the compressive strength.
Mix
Designation
w/b CEM
I
kg/m3
GGBS
kg/m3
FA
kg/m3
20mm
kg/m3
10mm
kg/m3
Fine
(<5mm)
kg/m3
Slump
(mm)
f28
MPa
f180
MPa
Plasticiser
l/m3
PC
0.40 370 - - 687 459 766 60 88.5
(0.95)
99.2
(1.44)
3.0
0.60 290 - - 687 458 765 50 43.4
(0.87)
54.7
(2.51)
0.5
FA35
0.40 240 - 130 672 448 744 65 51.4
(0.38)
80.0
(1.15)
3.5
0.60 188 - 102 674 450 748 50 25.6
(0.35)
40.8
(0.95)
0.6
GGBS65
0.40 130 240 - 681 454 757 65 45.6
(2.26)
66.7
(0.58)
2.5
0.60 102 188 - 682 455 758 50 30.1
(0.79)
47.7
(2.31)
0.6
32
Table 3 Properties of Coarse and Fine Aggregates
Aggregate
Type
Absorption
(%)
Bulk density
(loose)
(kg/m3)
Specific
gravity
Fineness
Modulus
AIV*
(%)
Coarse 1.02 1450 2.63 6.04 8.2
Fine 2 1520 2.63 2.89 +
*AIV: aggregate impact value
Table 4 Modelling parameters for equation (5) (CoD = coefficient of determination for
fitting curve).
Depth (mm) c g (hrs-1) tm (hrs) s CoD (r2)
10 1.22 1.57 0.46 0.08 0.97
20 0.74 0.60 3.17 0.26 0.99
30 0.61 0.24 9.07 0.40 0.99
40 0.56 0.28 10.8 0.45 0.99
Table 5 Cumulative volumetric gain, i (mm3/mm2) after 24-hours absorption.
Mix C1 C2
w/b = 0.4 w/b = 0.6 w/b = 0.4 w/b = 0.6
PC 1.01 1.30 1.46 2.02
GGBS/65 0.63 2.90 1.96 5.15
FA/35 0.74 1.77 0.97 4.01
33
Table 6 Relative humidity before/after 1st absorption cycle for C1 and C2 curing.
C1 Curing
Depth
(mm)
PC GGBS/65 FA/35
w/b = 0.4 w/b = 0.6 w/b = 0.4 w/b = 0.6 w/b = 0.4 w/b = 0.6
Before After Before After Before After Before After Before After Before After
25 88 100 87 100 88 100 87 100 89 100 89 100
50 87 96 87 100 90 100 87 100 90 100 89 100
100 88 88 87 87 90 90 88 92 89 89 90 94
175 87 87 87 89 90 90 90 91 89 89 90 90
C2 Curing
25 80 100 80 100 81 100 86 100 85 100 85 100
50 87 100 83 100 87 100 87 100 90 100 88 100
100 88 88 88 97 90 90 88 100 90 90 90 94
175 88 88 87 87 90 90 90 92 89 89 90 93
34
(a)
(b)
Figure 1
Thermistor
Curved inner surface of cavity lined with plastic sleeve
Thermistor
Sealed surface
Mastic seal
Electrode-pairs
Rubber
bung
RH sensor
35
Figure 2
15
20
25
30
35
40
45
0 10 20 30
Time (hrs)
Te
mp
era
ture
( C
)
36
Figure 3
Water reservoir
Test cell
Graduated
Burette.
Resistance
measurement
and multiplexing
system.
37
0
0.2
0.4
0.6
0.8
1.0
1.2
0 5 10 15 20 25
(a)
Time (hrs)
o /
t
0
0.2
0.4
0.6
0.8
1.0
1.2
0 5 10 15 20 25
10mm20mm30mm40mm50mm100mm175mm
(b)
Time (hrs)
o /
t
38
Figure 4
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 5 10 15 20 25
(c)
Time (hrs)
o/
t
0
0.2
0.4
0.6
0.8
1.0
1.2
0 5 10 15 20 25
(d)
40mm
30mm
20mm
10mm
Time (hrs)
o /
t
39
Figure 5
(a)
Solids
s
Pore-water
Air
eff
Sr
Solids
s
Just prior to absorption:
Conductivity = o
After absorption:
Conductivity = ss
Pore-water
(b)
40
Figure 6
0
10
20
30
40
50
0 1 2 3 4 5
PC: d = 8.53t1/2
- 1.87 (r2 = 0.92)
FA: d = 10.8t1/2
- 0.143 (r2 = 0.98)
GGBS: d = 18.8t1/2
- 3.72 (r2 = 0.94)
(a) C1 curing
(Time, t)1/2
(hrs1/2
)
De
pth
, d
(m
m)
0
10
20
30
40
50
0 1 2 3 4 5
PC: d =9.61t1/2
+ 3.04 (r2 = 0.98)
FA: d = 26.5t1/2
- 3.59 (r2 = 0.98)
GGBS: d = 25.6t1/2
+ 0.876 (r2 = 0.96)
(b) C2 curing
(Time, t)1/2
(hrs)1/2
De
pth
, d
(m
m)
41
Figure 7
0
0.5
1.0
1.5
2.0
0 1 2 3
C1/PC: i = 0.166t1/2
+ 0.044 (r2 = 0.99)
C2/PC: i = 0.235t1/2
+ 0.093 (r2 = 0.98)
C1/FA: i = 0.106t1/2
+ 0.021 (r2 = 0.99)
C2/FA: i = 0.222t1/2
+ 0.023 (r2 = 0.99)
C1/BS: i = 0.082t1/2
+ 0.052 (r2 = 0.99)
C2/BS: i = 0.739t1/2
+ 0.163 (r2 = 0.94)
(a)
(Time, t)1/2
(hrs)1/2
i (
mm
3/m
m2)
0
0.5
1.0
1.5
2.0
2.5
3.0
0 1 2 3
C1/PC: i = 0.182t1/2
+ 0.022 (r2 = 0.99)
C2/PC: i = 0.533t1/2
- 0.016 (r2 = 0.99)
C1/FA: i = 0.661t1/2
- 0.103 (r2 = 1.0)
C2/FA: i = 1.09t1/2
+ 0.006 (r2 = 99)
C1/BS: i = 0.468t1/2
+ 0.11 (r2 = 0.99)
C2/BS: i = 1.03t1/2
+ 0.261 (r2 = 0.99)
(b)
(Time, t)1/2
(hrs)1/2
i (
mm
3/m
m2)
42
Figure 8
0
10
20
30
40
50
0 1 2 30
1
2
3
(Time, t)1/2
(hrs)1/2
Dep
th,
d
(mm
)
i (
mm
3/m
m2)
43
0
0.5
1.0
1.5
0 50 100 150 200
Before: 1st Cycle
After: 1st Cycle
Before: 2nd
Cycle
After: 2nd
Cycle
(a) w/b = 0.4: C1
Depth (mm)
( 1
0-4
S/c
m)
0
0.5
1.0
1.5
0 50 100 150 200
(b) w/b = 0.4: C2
Depth (mm)
( 1
0-4
S/c
m)
44
Figure 9
0
0.5
1.0
1.5
0 50 100 150 200
(c) w/b = 0.4: C2
Depth (mm)
( 1
0-4
S/c
m)
0
1
2
3
0 50 100 150 200
(d) w/b = 0.6: C2
Depth (mm)
( 1
0-4
S/c
m)
45
0
0.5
1.0
1.5
2.0
0 50 100 150 200
Before: 1st
Cycle
After: 1st Cycle
Before: 2nd
Cycle
After: 2nd
Cycle
(a) w/b = 0.4: C1
Depth (mm)
( 1
0-4
S
/cm
)
0
0.5
1.0
1.5
2.0
0 50 100 150 200
(b) w/b = 0.4: C2
Depth (mm)
( 1
0-4
S/c
m)
46
Figure 10
0
0.5
1.0
1.5
2.0
0 50 100 150 200
(c) w/b = 0.6: C1
Depth (mm)
( 1
0-4
S
/cm
)
0
0.5
1.0
1.5
2.0
0 50 100 150 200
(d) w/b = 0.6: C2
Depth (mm)
( 1
0-4
S/c
m)
47
0
0.5
1.0
1.5
0 50 100 150 200
Before: 1st Cycle
After: 1st Cycle
Before: 2nd
Cycle
After: 2nd
Cycle
(a) w/b = 0.4: C1
Depth (mm)
( 1
0-4
S
/cm
)
0
0.5
1.0
1.5
0 50 100 150 200
(b) w/b = 0.4: C2
Depth (mm)
( 1
0-4
S/c
m)
48
Figure 11
0
1
2
3
0 50 100 150 200
(c) w/b = 0.6: C1
Depth (mm)
( 1
0-4
S
/cm
)
0
0.5
1.0
1.5
0 50 100 150 200
(d) w/b = 0.6: C2
Depth (mm)
( 1
0-4
S/c
m)
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