Optimum Proportion of Masonry Chip Aggregate for Internally Cured Concrete
International Journal of Concrete Structures and Materials
Optimum Proportion of Masonry Chip Aggregate for Internally Cured Concrete
Munaz Ahmed Noor
Proper curing of concrete is essential for achieving desirable mechanical properties. However, in a developing country like Bangladesh, curing is often neglected due to lack of proper knowledge and skill of local contractors. Consequently, general concreting work of the country has been found to have both strength and durability issues. Under such scenario, internal curing could be adopted using masonry chip aggregate (MCA) which is quite common in this region. It is observed that saturated MCA desorbs water under favorable relative humidity and temperature. This paper presents the effectiveness of MCA as internal curing medium and recommends a tentative optimum mix proportion to produce such concrete. The experimental study was conducted in two phases. It was found that 20% replacement of stone chips with MCA produced better performing internally cured concrete both in terms of strength and durability. Performance of internally cured concrete with recommended proportion of MCA is comparable to that of normally cured control concrete samples with conventional stone chips. In addition, internally cured concrete performed significantly better than control samples when kept under similar adverse curing conditions. In the absence of supply of external water for curing, compressive strength of internally cured concrete for 20% replacement can be as high as 1.5 times the strength of the control concrete samples. Significant better performance in permeability than that of control samples was also observed for this percent replacement under such adverse curing conditions.
internal curing; lightweight aggregate; temperature; humidity; desorption; strength; permeability
IC Internal curing
LWA Light weight aggregate
IC1 Inside Laboratory with Polythene Cover
IC2 Inside Laboratory without Polythene Cover
IC3 Outside Laboratory with Polythene Cover
IC4 Outside Laboratory without Polythene Cover
IC3a 3 days under normal curing and then Outside
Laboratory with Polythene Cover
IC3b 7 days under normal curing and then Outside
Laboratory with Polythene Cover
IC4a 3 days under normal curing and then Outside
Laboratory without Polythene Cover
IC4b 7 days under normal curing and then Outside
Laboratory without Polythene Cover
RH Relative humidity
SSD Saturated surface dry
Masonry chip aggregate
Light weight aggregates (LWA) absorb considerable
amount of water before mixing which can transfer to the
paste during hydration (Expanded Shale, Clay and Slate
Institute (ESCSI) 2012). On the other hand, internal curing
(IC) is defined as a process where proper hydration of
cement occurs due to availability of additional internal water
within concrete matrix
(Bentz et al. 2005; Bentz 2000)
Therefore, a partial replacement of stone aggregate with
saturated LWA might be an effective means for ensuring
internal curing. Internal curing of concrete is usually done in
two ways. One is by using lightweight aggregates (LWA)
and other way is using super absorbent polymers (SAP).
Both LWA and SAP absorb water before mixing and later,
desorb absorbed water during curing
(Mather 2004; Iffat
et al. 2015; Manzur et al. 2015)
. When LWA is used as
internal curing medium within concrete, usually small
amount is required to supply the additional water (Bentz and
Weiss 2010). A number of research are available that
investigated the effectiveness of LWA as internal curing
medium to produce concrete with desired properties. For
example, structural lightweight aggregate was used on the
Hibernia Offshore Platform in Newfoundland, Canada by
replacing 50% of the coarse aggregate with a pre-wetted
Expanded Shale, Clay and Slate-ESCS (Expanded Shale,
Clay and Slate Institute (ESCSI) 2012). In another research
at Purdue University, suitability of LWA to produce
internally cured concrete was examined
Expanded shale, crushed returned concrete aggregate and
natural river sand were used in that experiment. A study by
(Lura 2003; Lura et al. 2003)
at Delft University
produced internally cured concrete using two different mixes
with 11% and 25% of total volume of aggregate replaced by
shale. Those samples (shale) at Delft released almost all
(96%) of their absorbed moisture and therefore, it was
concluded that water can leave the pores of the LWA if a
large enough suction pressure (or a low enough internal
relative humidity) exists. However, 1.6% reduction in
compressive strength and 3.3% reduction in modulus of
elasticity were observed. De Sensale and Goncalves (2014)
showed the effectiveness of fine LWA and SAP as internal
curing agent to reduce autogenous deformation of concrete.
They also found that higher amount of LWA or SAP results
in reduction of compressive strength of concrete. In another
study, effectiveness of internal curing mechanism for making
concrete pavement less susceptible to environmental
conditions was showed by Chun and Kim (2015). Lower
shrinkage and less damage of internally cured mortar were
also observed by
Zou and Weiss (2014)
Zou et al. (2015)
in two different studies. Therefore, benefits of internal curing
can be summarized as enhanced hydration of cement, higher
strength development, reduction of autogenous shrinkage
and cracking, decrease in permeability, and improvement of
(Bentz et al. 2005; Iffat et al. 2015; Manzur
et al. 2015; Bentz and Weiss 2010; de Sensale and
Goncalves 2014; Chun and Kim 2015; Zou and Weiss 2014; Zou
et al. 2015; Geiker et al. 2004)
. The impact of internal curing
begins immediately during the initial hydration of the
cement, with benefits that are observed at ages as early as
two days. However, till now, research on LWA as internal
curing medium have mainly been conducted utilizing two
types of aggregates. One type of LWA is produced by
thermal process like shale and the other type of LWA is
obtained through mechanical treatment of industrial by
products like pulverized fly ash, blast furnace slag, industrial
waste, and sludge
(Obla et al. 2007)
. Also, in some studies,
naturally occurring LWA have been utilized. Utilization of
naturally occurring pumice aggregates for producing
structural lightweight and internally cured concretes has been
observed in some countries like New Zealand and Kenya
(Green et al. 2011; Geoffrey et al. 2012)
Masonry chip aggregates (MCA) are very common in
Bangladesh and in nearby regions due to their relative low
cost and wide availability. MCA are produced from burnt
clay and have been used as coarse aggregate for many years.
Many construction works use MCA as primary coarse
aggregate. However, MCA-concrete exhibits poor
performance, both in terms of strength
(Afroz et al. 2015; Hossain
(Afroz et al. 2015; Bosunia and
. On the other hand, external curing
method is usually practiced in Bangladesh which requires
additional water as well as proper awareness among the
workers and construction supervisors for ensuring required
quality control. Unfortunately, in many instances,
appropriate quality control protocol to ensure proper curing is not
maintained and often not considered as an essential part of
concreting work due to lack of adequate knowledge of local
contractors. There is also scarcity of water in many regions
of the country, particularly in dry season. Therefore, concrete
with MCA as primary coarse aggregate and/or without
proper curing may experience considerable durability issues
(Manzur et al. 2015; Iffat et al. 2016)
. However, MCA are
produced in large quantity in the country and are very
popular among the local contractors. Therefore, the use of
MCA as coarse aggregate would be difficult to control.
Under this circumstance, identification of potential avenues
for using MCA as coarse aggregate is very important from
perspective of Bangladesh. MCA are light weight and have
porous structure. It was also observed that pore spaces of
these aggregates absorb water during saturation and later can
desorb water under favorable humidity and temperature
. It is, therefore, evident that MCA has the
potential to be used as internal curing medium within
concrete and can be considered as an alternative but effective
curing solution for general construction work in Bangladesh.
In a very recent study, (Iffat et al. 2016) showed that
utilization of MCA as internal curing medium can improve the
permeability of concrete under adverse curing conditions.
However, except this recent one, no other significant study is
available on MCA as internal curing agent within concrete.
In this article, the outcome of a comprehensive research
program that studied the effectiveness of MCA as internal
curing medium in terms of compressive strength, split tensile
strength, modulus of elasticity and chloride permeability is
discussed in order to recommend an appropriate mix
proportion to produce internally cured concrete. It has been
found that concrete with MCA as internal curing medium
performed significantly well as compared to conventional
stone aggregate concrete in the absence of proper external
curing mechanism. Moreover, internally cured concrete
having recommended mix proportions can achieve
comparable strength and permeability values of proper externally
cured conventional stone aggregate concrete.
2. Experimental Investigation
Portland composite cement CEM II (2000) produced by a
local manufacturer was used in this study. XRF analysis of
cement was done using LAB CENTER XRF-1800 to evaluate
the composition of used cement and is given in Table 1. The
normal consistency of the cement was measured as per ASTM
C187 (2011). The initial setting time was determined
according to ASTM C191 (2013) and compressive strength of
cement mortar was evaluated following ASTM C109 (2013).
Normal consistency, initial setting time and 28 day
compressive strength of cement mortar was obtained as 27.5%,
3 h and 29.3 MPa, respectively. Crushed stone chip was used
as primary coarse aggregate and MCA (Fig. 1) was used as
partial replacement of stone chips as internal curing medium.
Locally available sand, known as Sylhet Sand, was used as
fine aggregate. Gradations of both coarse and fine aggregates
were determined through sieve analysis, according to ASTM
C-136 (2006) and are plotted in Fig. 2. Fineness modulus
(FM) of MCA, stone chips and sand were found as 6.37, 8.42
and 2.43, respectively. Bulk specific gravity of MCA was
found as 1.693 on oven dry basis as per ASTM C128 (2012).
The unit weight of MCA was determined following ASTM
C29 (2009) and was found as 1110 kg/m3.
2.2 Desorption of MCA
The absorption and desorption properties of LWA are
important to determine its effectiveness as an internal curing
Grada on Curve of
Grada on Curve of MCA
Grada on Curve of
Sieve opening (mm) 100
(Kim and Bentz 2008)
. Desorption test is considered
as one of the most effective techniques for evaluating this
property. Therefore, desorption test of MCA was performed
(2012). A dehumidifier with
relative humidity (RH) range from 22 to 90% and temperature
range from 5 to 40 C was used for this test.
A total of 30 experiments were conducted using five
different RHs of 60, 73, 77, 85 and 90% and six different
temperatures of 14.5, 18, 24, 28, 31 and 34 C. The MCA
samples were weighed and made saturated surface dry with
water. Then, they were re-weighed after saturation. It was
found that MCA absorbed nearly 28.6% water in terms of its
oven dry weight. These samples were then placed in the
dehumidifier under different temperatures and relative
humidities as mentioned above. Water loss was measured at
every 30 min interval. It was observed that the rate of
desorption increased with temperature and decreased with
relative humidity (Figs. 3 and 4). It is evident from Fig. 3 that
maximum desorption (minimum absorbed water remained)
occurred when temperature was around 34 C and RH was
around 60%. The maximum and minimum desorption rates
were found as 89.7 and 23.7%, respectively, with respect to
the weight of absorbed water by MCA. According to
(2012), LWA as internal curing medium should
desorb more than 85% of absorbed water under the stated
storage condition. From this experiment, it was observed that
MCA desorbed around 90% of absorbed water at 60% RH
and thereby satisfies the ASTM requirement. Even at higher
RHs of 85 and 90%, MCA can desorb around 50 and 40% of
absorbed water, respectively. So, additional water will be
available from MCA during early curing periods when
internal RH is higher within concrete. Figure 4 shows that,
at early stage, the desorption rate remains low. But this rate
increases rapidly between 10 and 20 h and reaches the
equilibrium condition after around 65–70 h. Similar
desorption behavior of MCA was observed for other
temperatures and RH values, although the rates were found different.
2.3 Concrete Mixtures
Control samples were made with stone chips as primary
coarse aggregate and a mix proportion of 1: 1.5: 2.3 for
design compressive strengths of 20.5 MPa. The mix
proportion was based on weight of the ingredients which means
one proportion of cement was mixed with 1.5 proportion of
sand and 2.3 proportion of conventional stone chips in terms
of weight of the components. The experiments were
conducted in two phases. For the 1st phase of the experiment,
three different water to cement ratios (w/c) of 0.4, 0.45 and
0.5 were used. Compressive strength test, modulus of
elasticity test and RCPT (Rapid Chloride Permeability Test)
were performed in this phase. Downgraded crushed stone of
20 mm size was used as primary coarse aggregate. Nine mixes
with three different partial replacements (10, 20 and 30%) of
conventional stone chips with MCA for each w/c ratio were
prepared using the mix proportion of 1: 1.5: 2.3. The MCAwas
made saturated surface dry (SSD) before mixing. At first,
MCA were kept fully submerged in water for about 24 h. Then
the surface of MCA were wiped properly with cloth and all
surface water was removed to ensure SSD condition. Control
samples with conventional stone chips as coarse aggregate
were also made using the same mix proportions and three
different w/c ratios. For each test age, three identical replicate
samples were prepared. No admixtures were used in the mixes.
In the 2nd phase, only one w/c ratio (0.4) was selected
which produced the best performing internally cured
concrete in terms of compressive strength, elastic modulus and
chloride permeability in the 1st phase. Three different
percent replacements (15, 20 and 25%) of stone chips with
MCA were utilized in order to find more accurate results.
The mix design, RH and temperature during casting and
curing were kept identical to Phase 1. Compressive strength
and splitting tensile strength tests were performed in this
phase. In addition, the internal relative humidity (RH) of
both control and internally cured samples were investigated.
2.4 Experimental Setup
Adequate normal moist curing (NC) was ensured for
control samples by keeping the samples fully submerged
under water. Beside NC, four different curing conditions
were simulated for both control and internally cured samples
in the 1st phase to represent different field conditions, as
provided in Table 2. Two sets of each type of samples were
kept inside the laboratory to simulate curing under shading.
Among the two sets, one set was covered with polythene
sheets (IC1) and the other set was kept uncovered (IC2). The
other two sets were kept outside the laboratory to simulate
curing under exposed field condition. Exposed field
condition denotes the typical exterior weather condition of the
country during most part of the year. During the experiment,
the average external (Exposed condition-Outside laboratory)
temperature and RH were around 32 C and 71%,
respectively. The average internal (Inside laboratory) temperature
was around 30 C and RH was around 72%. Similar to the
previous case, one set was covered with polythene sheet
(IC3) and the other was placed without covering (IC4).
Control samples (with no MCA) were also kept under
similar (IC1, IC2, IC3, and IC4) curing conditions for
comparison. In the 2nd phase of the study, only IC3 and IC4
conditions were considered since these conditions are mostly
common in construction sites of the country. Moreover,
besides IC3 and IC4 conditions, four additional curing
conditions (termed as IC3a, IC3b, IC4a and IC4b) were
simulated in the 2nd phase and are also listed in Table 2. In
cases of IC3a and IC3b conditions, samples were placed
under water for 3 and 7 days, respectively, and then kept
outside the laboratory with polythene cover. For IC4a and
IC4b conditions, samples were submerged under water for 3
and 7 days and then placed outside without cover.
3 days under normal curing and then outside Exposed field condition under covering after
with cover 3 days of NC
3 days under normal curing and then outside Exposed field condition without covering after
without cover 3 days of NC
7 days under normal curing and then outside Exposed field condition under covering after
with cover 7 days of NC
7 days under normal curing and then outside Exposed field condition without covering after
without cover 7 days of NC
As mentioned above, performance of internally cured
samples were evaluated and compared in terms of
compressive strength, modulus of elasticity, tensile strength and
chloride permeability. Compressive strength test was
performed according to ASTM C39 (2005). Modulus of
elasticity test was performed as per ASTM C469 (2002). For
compressive strength tests, cylindrical concrete samples of
100 mm by 200 mm in size were made and kept under
different curing conditions for up to 28 days. Compressive
strength tests were done at 3, 7 and 28 days. Universal
testing machine was used to apply compressive load on
specimens at a loading rate of 0.15–0.35 MPa per second.
Modulus of elasticity test was also performed using
Universal testing machine with a lower loading rate as per
Code requirement (2002). Axial stress strain method was
used to determine modulus of elasticity
(Iffat et al. 2015)
Three specimens were tested for each variation. Chloride
permeability test known as rapid chloride permeability test
(RCPT) was carried out following ASTM C1202 (2012). For
RCPT, 50 mm diameter cores were cut from top surfaces of
100 mm 9 200 mm cylinders after 28 days of curing.
Epoxy coated and vacuum saturated core specimens
et al. 2014; Grace 2006; Ptefier et al. 1994)
were placed in
the test device as per Code (2012) requirements. The
vacuum process was carried out to remove the air voids and
eventually, to fill those voids with water to make the
concrete sample conductive to electrons. Readings were taken at
every 30 min interval. At the end of 6 h, the sample was
removed from the cell and the amount of coulombs passed
through the specimen was calculated. Splitting tensile
strength test was performed according to ASTM C496/C
496M (2011). Cylindrical samples of dimension 100 mm by
200 mm were used for the tensile strength test. In order to
ensure that specimens remained on the same axial plane,
diametrical lines were drawn on the two ends of the
specimens. Plywood strips were kept on the lower plate of the
testing device and specimens were placed on the plywood
strip. The samples were placed so that the lines marked on
the ends are vertical and centered over the bottom plate.
Another plywood strips were placed above the specimen and
loads were applied continuously till the specimens broke.
For investigating internal RH of samples, concrete cubes
(150 mm 9 150 mm 9 150 mm) were prepared, each
having three circular hollow sections (18.75 mm in
diameter) inside. These hollow portions were held in reserve in
order to measure the internal RH of concrete. The cubes
were made with similar mix proportions as considered in this
study. The hollow sections were sealed properly with cotton
using conduit so that sensor can be inserted easily. The cube
specimens were placed in the dehumidifier at constant
temperature and RH of 34 C & 65%, respectively.
3. Results and Discussion, Phase 1
3.1 Compressive Strengthand Modulus
Variations in compressive strengths are shown in Fig. 5. It
was found that most of the internally cured samples under
four different simulated conditions of the 1st phase achieved
less compressive strength than that of control samples under
NC. This pattern of behavior was expected since concrete
with stone chips as coarse aggregate usually have higher
compressive strengths due to greater unit weight. However,
20% replacement with MCA resulted in similar compressive
strength of NC control samples under IC3 curing condition
(Fig. 5a, c). Figure 6 shows the effect of MCA replacement
on compressive strength of samples having w/c ratio of 0.40.
This w/c ratio resulted in the maximum compressive
strength. It may be observed that for lower w/c ratio, higher
compressive strength was achieved for all samples. Samples
under IC3 condition achieved the highest compressive
strength for all MCA replacements. This is due to the fact
that samples under IC3 condition were exposed to relatively
higher temperature which was beneficial for proper
hydration. In addition, polythene sheet covering prevented water
W/C ra o
W/C ra o
Fig. 5 Effect of w/c ratio on 28 day compressive strength of samples a control samples with 0% replacement b IC samples with
10% replacement c IC samples with 20% replacement d IC samples with 30% replacement.
loss due to evaporation. It is clearly evident that, with
increase in percent replacement, compressive strength
increased up to a certain value, and then declined. Maximum
strength is observed for 20% MCA replacement.
Another significant observation of the study is that
internally cured samples achieved considerable higher strength as
compared to control samples when kept under similar curing
conditions with no external supply of curing water. Control
samples were kept under the four simulating curing
conditions to study the effect of different curing condition on
compressive strength of conventional concrete. Similar to
internally cured concrete, the IC4 condition resulted in
minimum compressive strength for control samples (Fig. 6).
Unfortunately, curing condition analogous to IC4 prevails in
some practical instances due to the absence of proper quality
control. Usually, a layer of external water is poured on
concrete and left without any covering. This external water
evaporates very quickly, particularly during summer, leaving
the concrete in almost similar to IC4 condition. Internally
cured samples with 20% replacement under IC4 condition
achieved 46% higher compressive strength than that of
control samples. The 10 and 30% MCA replacement under
IC4 condition also produced significantly higher strength
(about 38 and 36%, respectively) as compared to control
samples. Under IC3 condition, 20% partial replacement
resulted in 42% more compressive strength than control
samples. Table 3 shows increase in compressive strength for
internally cured samples in comparison with control samples
under similar curing conditions.
Modulus of elasticity of control and internally cured
samples under different curing conditions are plotted in
Fig. 7. Similar to compressive strength, it was found that
20% replacement under IC3 condition produced comparable
modulus of elasticity of normally cured control samples.
Control samples under IC4 condition produced the least
modulus of elasticity (Fig. 8). The best performing internally
cured samples (20% partial replacement and under IC3
condition) achieved about 19.5% higher elasticity than that
of control samples placed under IC3 curing condition. The
W/C ra o
W/C ra o
W/C ra o
W/C ra o
Fig. 7 Effect of w/c ratio on modulus of elasticity of samples a control samples with 0% replacement b IC samples with 10%
replacement c IC samples with 20% replacement d IC samples with 30% replacement.
least performing internally cured samples (30% partial
replacement and under IC4 condition) had about 11% higher
modulus of elasticity than IC4-control samples. Samples
with 20% replacement under IC4 exhibited 17% higher
elasticity value as compared to IC4-control samples.
Similarly, 10% replacement also achieved relative high modulus
of elasticity. Table 4 shows increases in modulus of elasticity
for internally cured sampled as compared to control samples
under similar curing conditions.
3.2 Chloride Permeability Test
The RCPT test results of control and internally cured
samples for 0.4 w/c ratio are shown in Fig. 9. As per ASTM
C1202 (2012), samples allowing less than 4000 C charge to
pass through are termed as ‘‘moderate chloride permeable’’,
and more than 4000 C to pass through as ‘‘high chloride
permeable’’. In Fig. 9, the bar corresponding to NC
represents RCPT test results of control samples (samples with
100% stone chips) under NC condition. These samples
(NCcontrol samples) experienced the least coulomb charge
passed through them and fall into moderate chloride
permeability region. Only internally cured samples with 20%
replacement under IC1 and IC3 conditions showed
permeability values comparable to NC-control samples. All control
samples under simulated four curing conditions showed
significantly high chloride permeability. In addition, all
samples without polythene cover, both control and internally
cured, experienced considerably high chloride permeability.
It is, therefore, obvious that absence of proper external
curing mechanism severely affect the durability performance
of concrete. However, 20% replacement of stone chips with
MCA showed potential to resolve such problem if proper
covering can be ensured. The chloride permeability of these
samples (under IC3 condition and having 20% replacement)
was almost one-third of the chloride permeability of the
control samples under IC3 condition. Similar to previous
observations in strength and elasticity tests, control samples
under IC4 condition showed the worst permeability
performance as compared to all samples considered in this study.
Thus, internal curing showed significant better performance
in permeability tests in comparison with control samples
when subjected to curing condition without supply of
Although all partial replacement under IC3 condition
showed higher compressive strength at lower w/c ratio, only
20% replacement exhibited comparable chloride
permeability performance with respect to control samples. Such
improvement in permeability can only be explained by better
hydration of cement that produced denser concrete. It is
obvious that better hydration was the result of internal curing
ensured by MCA since addition of MCA was the only
difference between the control and internally cured samples. It
is also evident from the observed results that 20%
replacement can be considered as the tentative optimum proportion
of MCA to produce internally cured concrete. This amount
of replacement appears to be resulted in better hydration of
cement by providing required amount of internal water.
Moreover, porosity of this amount of MCA had insignificant
effect on permeability of concrete.
4. Results and Discussion, Phase 2
In order to reach more conclusive outcome, narrower
bands of partial replacement (15, 20 and 25%) were used in
the 2nd phase of the study. The w/c ratio was kept constant
at 0.4 for all samples. Compressive strength and splitting
tensile strength tests were performed. Moreover, only
exposed conditions (IC3 and IC4) were considered in this
phase, since such conditions mostly prevail in actual
construction sites of the country. However, four additional
curing conditions were investigated with samples having 3
and 7 days of proper external curing before exposed to
outside conditions. The details of these additional curing
conditions are listed in Table 2. The internal Relative
Humidity (RH) of both control and internally cured samples
were also studied in this phase for different partial
4.1 Internal Relative Humidity (RH)
Relative Humidity readings inside the hollow sections of
cubes were taken at each day using hygrometer with digital
sensor. Experimental setup for internal RH test is shown in
Fig. 10. It is observed that RH of internally cured specimens
was greater than RH of control samples for all replacement
levels. The RH data of control samples and internally cured
samples are plotted in Fig. 11 with respect to time. It is
observed that maximum RH was obtained from samples
with 25% replacement levels and the minimum RH was
found from control samples with no replacement. It is,
therefore, obvious that MCA as internal curing agent within
concrete supplies additional water that eventually increases
the internal RH within the concrete matrix.
4.2 Compressive Strength
Variations in compressive strengths with replacements of 15,
20 and 25% are shown in Fig. 12. It is again evident that the
maximum compressive strength was achieved by the samples
having 20% replacement for all curing conditions considered in
phase II. Control samples under NC exhibited the highest
strength which is expected. In the phase I, 20% replacement
produced slightly higher compressive strength under IC3
condition than that of NC control samples. Such increment
could be due to sample variation. However, it is apparent from
outcomes of both phases that 20% replacement under covering
could produce compressive strength comparable to that of
control samples under NC. Further studies with more sample
size would provide statistically significant difference between
NC control samples and internally cured samples with 20%
replacement under IC3. Covered samples exhibited higher
compressive strength than samples without covering, when
kept under similar curing conditions. Samples subjected to
external curing for seven days and then kept outside under
covering (IC3b) produced the maximum compressive strength.
In fact, internally cured samples with 20% replacement under
proper covering achieved almost similar compressive strength
of control samples under normal curing. Control samples
without any external curing and placed outside without cover
(IC4) exhibited lowest compressive strength. However, under
IC4 condition, internally cured samples with 20% replacement
showed about 29% higher compressive strength than control
samples. For both IC4a and IC4b conditions, 20% replacement
resulted in about 12% and 25% higher compressive strength as
compared to control samples, respectively.
4.3 Splitting Tensile Strength Test Results
The splitting tensile strengths of control and internally cured
samples are shown in Fig. 13. Similar trend of compressive
strength test results is observed for splitting tensile strengths .
For all curing conditions except NC, internally cured samples
with 20% replacement exhibited higher tensile strength than
that of control samples (Fig. 13). Covered samples with 20%
replacement produced comparable tensile strength of control
samples under NC. When samples were kept without covering
at exposed condition, significantly higher tensile strength was
observed for 20% replacement as compared to control samples.
It is obvious from the above discussion that internally
cured concrete performed better as compared to control
samples when subjected to curing conditions without the
supply of external water. The only difference between
control samples and internally cured samples was the presence
of saturated MCA. Therefore, the reason behind such
improved performance was the additional water supplied by
saturated MCA. The RH data of control and internally cured
samples (Fig. 11) also reinforces this inference. Moreover,
the internal temperature and RH of concrete also facilitate
desorption of MCA since desorption rate of MCA increases
with increase in temperature and decrease in RH. The
additional water provided by desorption of MCA ensured
proper hydration and eventually, resulted in concrete with
better mechanical properties.
5. Economics of Internal Curing
Internal curing using MCA and polythene sheet covering
is also a less costly method to implement. Both MCA and
polythene sheets are cheap and readily available in
Bangladesh. Typical price of MCA is $3.5 USD per cubic meter
and price of polythene is $1.0 USD per square meter. Also,
polythene covering can be re-used. So, production of this
kind of internal cured concrete can be a promising solution
to improve the quality of general concreting works of
developing countries like Bangladesh.
The following conclusions may be made based on the
findings from the current study:
• MCA has required desorption capacity to be considered
as an effective internal curing medium. It is observed that
desorption capacity of MCA depends on temperature and
relative humidity. Higher temperature (in the range of
30–34 C) and lower relative humidity (in the range of
60–73%) conditions are favorable to desorption by
MCA. However, it is also found that considerable
amount of water can be desorbed by MCA even at
higher relative humidity of 85% or more.
• It is apparent from sample internal RH that internal
curing ensures additional water within concrete. At all
ages, internally cured samples experienced relatively
high internal RH than control samples.
• Strength and durability of internally cured samples having
MCA as internal curing medium and polythene sheet
covering are significantly higher than those of control
samples with conventional stone chips as coarse aggregate
in the absence of proper external curing. The best
performance was observed when 20% stone chips was
replaced by MCA, which produced comparable
compressive strength, elastic modulus and tensile strength as
obtained by normally cured control specimens.
• It is evident from past experience that proper external curing
is not practiced in number of general construction sites in
Bangladesh which may result in poor performance. Field
conditions in several construction sites, particularly in
outskirts of major cities, are similar to IC4 condition
considered in the study. In such cases, internal curing can
produce about 30% or more increase in compressive strength
at 28 days. Moreover, about 30% and 10% increase in
modulus of elasticity and tensile strength, respectively, can be
achieved through internal curing, as found from this study.
• Durability performance of MCA concrete is a concern
because MCA as coarse aggregate increases the
permeability due to its porous structure. Consequently, most of
the internally cured samples showed higher permeability.
However, moderate chloride permeability was achieved
from 20% replacement of stone chips with MCA. This
means that such proportion of MCA within concrete
desorbs sufficient water to ensure proper hydration which
eventually produces better performing concrete without
requiring any external water. Proper covering should be
employed for ensuring better performance through
internal curing mechanism with MCA.
• Internal curing can be recommended in adverse curing
conditions, because internally cured samples showed
significantly better performance than that of control samples under
similar curing conditions without supply of external water.
• Twenty percent partial replacement of stone chips with
MCA, 0.4 w/c ratio and utilization of proper covering
(preferably polythene) are recommended as optimum
combination for producing internally cured concrete.
• Internal curing technique, considered in this study, can
be very effective where curing water and skilled labor are
not easily available, which is very common in many
parts of the world.
• Internal curing using MCA is also a simple and
inexpensive method to execute. Therefore, internally
cured concrete can be a considered as a promising
solution for improving the overall quality of general
concreting work of Bangladesh.
The authors express their profound thanks to the Concrete
Laboratory and the Strength of Materials Laboratory,
Department of Civil Engineering, Bangladesh University
of Engineering & Technology, (BUET), Dhaka, for the
assistance in this research work.
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