ORIGINAL ARTICLE
Effect of pre-soaked superabsorbent polymer on shrinkageof high-strength concrete
Xiang-ming Kong • Zhen-lin Zhang •
Zi-chen Lu
Received: 21 January 2014 / Accepted: 29 May 2014
� RILEM 2014
Abstract Pre-soaked super-absorbent polymer
(SAP) was incorporated into high-strength concrete
(HSC) as an internal curing agent to study its effects on
early-age shrinkage and mechanical properties. On the
basis of the capillary stress based model for shrinkage
prediction of concrete, together with the experimental
results of cement hydration kinetics, evolution of
internal temperature and humidity, development of
pore structure and mechanical properties, the working
mechanism of SAP was discussed. Results indicate that
the addition of pre-soaked SAP significantly reduces
the autogenous shrinkage as well as the early-age
shrinkage of HSC under drying condition. In sealed
HSC specimens, the drop of internal humidity caused
by the self-desiccation effect is notably postponed by
addition of pre-soaked SAP. The addition of pre-
soaked SAP slightly reduces the compressive strength
of HSCs and this effect is more pronounced in early-
age concrete. Furthermore, an insightful comparison of
the behaviours of the internal curing water introduced
by the pre-soaked SAP and the additional free mixing
water in concrete was made. Results indicate that the
internal curing water behaves differently from the
additional mixing water in influencing the cement
hydration kinetics, pore structure of hardened cement
pastes and the mechanical strength of concrete, due to
the different spatial distribution of the two types of
water in the concrete bodies. The shrinkage-reducing
effect on HSC due to the addition of extra internal
curing water incorporated by pre-soaked SAP is much
stronger than that of the additional mixing water.
Besides, the internal curing water shows much less
strength-reducing effect than the additional mixing
water. In virtue of the shrinkage prediction model, the
working mechanism of pre-soaked SAP in reducing
autogenous shrinkage of HSC is proposed on the basis
of the following two aspects. The participation of
internal curing water in cement hydration process leads
to a total volume gain of the hardening cement pastes.
Meanwhile, the release of internal curing water from
the pre-soaked SAP postpones the drop of internal
humidity. The synergistic effect of these two factors
effectively reduces the autogenous shrinkage of HSC.
Keywords High-strength concrete � Super-
absorbent polymer � Autogenous shrinkage � Cement
hydration � Pore structure
1 Introduction
High-strength concrete (HSC), which is defined as the
concrete mixtures with specified strength of 55 MPa
X. Kong � Z. Zhang � Z. Lu
Department of Civil Engineering, Key Laboratory of
Safety and Durability of China Education Ministry,
Tsinghua University, Beijing 100084, China
X. Kong (&)
Collaborative Innovation Center for Advanced Civil
Engineering Materials, Southeast University,
Nanjing 211189, China
e-mail: [email protected]
Materials and Structures
DOI 10.1617/s11527-014-0351-2
or greater by the ACI Committee on High Strength
Concrete in 2002 [1], has been widely applied in the
construction of high-rise buildings, long-span struc-
tures, and many other key applications that demand
long durability. In principle, high strength, low
porosity and low permeability are achieved under the
condition of low water-to-cement ratio (W/C \0.40)
by using chemical additives and mineral admixtures,
such as polycarboxylate superplasticizer and silica
fume. Long durability is usually expected for HSC,
thanks to its very low permeability. However, crack-
ing is often one of the main detrimental factors
affecting the durability of HSC that results from its
low W/C and high shrinkage, especially high autog-
enous shrinkage (AS) [2].
At low water-to-cement ratio, marked self-desic-
cation may occur and leads to severe autogenous
shrinkage. When the tensile stress caused by restrained
autogenous shrinkage is beyond the local tensile
strength, cracking is often observed [3–5]. As well
understood, cracking in concrete structures may
reduce its load carrying capacity and durability, as
the attacking species tend to migrate readily into the
concrete body through the cracks. Despite the highly
dense pore structure, the durability of HSC is greatly
deteriorated due to the cracking issue. Therefore,
mitigation of cracking is an important measure to
ensure the durability of HSC.
Autogenous shrinkage and drying shrinkage (DS)
are among the main causes of cracking in early-age
hardened concrete [6–8]. Drying shrinkage is often the
consequence of non-uniform moisture distribution and
moisture diffusion in concrete [9], while the main
cause of autogenous shrinkage is the capillary tension
in the pore fluid caused by self-desiccation as a result
of chemical shrinkage. Both phenomena are related to
moisture loss inside concrete, either via self-desicca-
tion or via drying. In either case, capillary tension
produced by capillary meniscus plays an important
role in the shrinkage [7, 8, 10]. Water-to-cement ratio
is the most important factor determining their magni-
tudes. In case of normal concrete (W/C [ 0.5) with
strength lower than 40 MPa, the effect of DS is more
considerable than AS in inducing cracks and hence AS
is often ignored [11]. Meanwhile, the risk of early
cracking induced by DS can be effectively minimized
by full water curing after casting [12]. On the other
hand, in case of HSC with a W/C below 0.3, the AS
can account for more than 50 % of the total
contraction deformation. And full water curing has
limited effect in mitigating cracking problems caused
by AS because of its compact pore structure and very
low permeability.
On the other hand, internal curing has been proved a
very promising technique to mitigate the occurrence of
self-desiccation by introducing additional moisture
into concrete [13]. Commonly used materials for
internal curing are porous lightweight aggregates [3,
14–19] and super-absorbent polymers (SAP). SAP is a
class of polymeric material with super-high water
absorption capacity, sometimes even up to 1,000 times
of their own weight. Thus far, several researchers [20–
25] have studied the effects of SAP on the shrinkage
and the mechanical properties of concrete by simple
addition of SAP into concrete mixtures with or without
extra addition of water. During cement hydration, the
water absorbed by SAP is released due to the drop of
the internal humidity in concrete, thereby effectively
reducing the autogenous shrinkage. Jensen and Han-
sen [20] firstly examined the influences of SAP on the
reduction of autogenous shrinkage as well as on the
mechanical strength of cement pastes. In the absence
of SAP, a significant drop of the internal relative
humidity (RH) of the samples was noticed, causing a
considerable amount of autogenous deformation up to
3,700 lm/m after 3 weeks of hardening. With the
addition of 0.3–0.6 wt% of SAP, a notable reduction of
autogenous shrinkage was achieved as facilitated by
the higher internal humidity. The shrinkage reducing
effect of SAP has been confirmed by many other
researchers [22–25]. According to some studies, the
addition of SAP was often found to have a negative
effect on mechanical strength of the hardened cemen-
titious materials [21, 23–25], while some other studies
reported enhancement in compressive strength by the
addition of SAP [22]. So far, it has been well accepted
that internal curing using SAP can be a very promising
method to mitigate cracking, especially in case of
HSC. Recently, some efforts are made to evaluate the
practical feasibility of using superabsorbent polymers
in concrete as a potential strategy to prevent AS [26].
To this end, considerable amount of work has been
done from the viewpoints of various aspects. Never-
theless, theoretical perspective of the mechanism
underlying the functioning of SAP in reducing
shrinkage as well as in development of mechanical
strength is still needed to support its practical appli-
cation in concrete [27]. Esteves [28] proposed three
Materials and Structures
interacting mechanisms in water-entrained pastes:
capillary suction, diffusion mode within internal
curing and self-restraint of the bulk paste. So far, a
direct simulation to describe the shrinkage reducing
mechanism of internal curing SAP in real concrete
specimens has not yet been well accomplished,
although some pioneering simulation works in cement
mortars have provided certain valuable results [29].
Various models have been used to simulate or to
predict AS or DS of concrete [30, 31], wherein the
variation of the interior moisture content, as repre-
sented by interior RH, is considered as the major factor
controlling the magnitude of shrinkage. Several stud-
ies have indicated a close correlation between shrink-
age and interior humidity in early-age concrete [31–
33]. For instance, Zhang et al. [8] proposed an
analytical micro-mechanical model (Zhang model)
on the basis of internal humidity for predicting the
shrinkage. This model can be successfully used to
predict the early-age shrinkage of concrete, including
both AS and DS.
In this paper, the effects of pre-soaked SAP as an
internal curing agent on the early-age shrinkage and the
mechanical properties of high strength concrete are
studied. The shrinkage of concrete specimens under
curing conditions of both fully plastic film sealed and
five faces drying is simultaneously measured together
with the interior RH and temperature immediately after
a few hours of casting until 14 days. Compressive
strength of HSC at the ages of 3, 7 and 28 days is also
measured. In addition, cement hydration degree and
pore size distribution of hardened cement paste (HCP)
have been tested to support the application of Zhang
model. Furthermore, model simulation is conducted on
the basis of the experimental data and then the working
mechanism of SAP in shrinkage reduction is discussed
in detail. Specific emphasis has been made to unravel
the behaviours of the internal curing water introduced
by the pre-soaked SAP and the additional free mixing
water, in order to gain deeper scientific insights on the
role of internal curing water in concrete.
2 Experimental
2.1 Materials
In this study, concrete specimens were casted using
P.O. 42.5 (GB175-2007, China) common Portland
cement produced by Jidong Cement Plant, the chem-
ical and mineral composition of which is shown in
Table 1. The coarse aggregate is composed of crushed
granite of size ranging from 5–20 mm. The fine
aggregate consists of washed-out sand with a fineness
modulus of 2.7. Polycarboxylate superplasticizer (SP)
synthesized in our lab was used to guarantee the
workability of the fresh concretes with a controlled
slump of 200–220 mm. SAP was synthesized by
copolymerizing acrylamide and acrylic acid in the
mass ratio of 70:30 via radical polymerization. The
prepared SAP was fully dried and ground into powder
with particle size of 180–420 lm. Several methods
have been proposed to measure the absorption capac-
ity of SAP [34]. Among them, ‘‘teabag’’ method is the
most popularly used method for measuring the
adsorption capacity towards different solutions [2].
In this study, the absorption capacities of the prepared
SAP towards deionized water, tap water and saturated
limewater, as determined by using the ‘‘teabag’’
method, were respectively 200, 80 and 25 times its
own mass. The observed trend in absorption capacities
for the three different solutions could be attributed to
the increase of ion concentration in the solutions, as
well known that the absorption capacity of SAP
decreases for solutions with higher ionic strength [2].
Figure 1 shows the weight loss of pre-saturated SAP
by tap water over time under ambient conditions
(293 ± 2 K, 50 ± 5 % RH), as measured in an
evaporating dish of diameter 145 mm and depth
25 mm. Results indicate that SAP has a high water
absorption capacity and can slowly release water
under low RH condition.
2.2 Mixing proportion and preparation
of specimens
The mixing proportion of concrete used in this study is
shown in Table 2. In the reference concrete mixture
(HSC-0), the W/C ratio was fixed as 0.29. Two
concrete mixtures with addition of pre-soaked SAP
were designed as HSC-S1 and HSC-S2, in which the
ratio of internal curing (IC) water entrained by pre-
soaked SAP to cement (Wic/C) varied respectively as
0.05 and 0.1. The water absorption capacity of the pre-
soaked SAP was fixed as 25, so that the workability of
fresh concrete mixture is minimally affected by the
addition of pre-soaked SAP. If the absorption rate of
the pre-soaked SAP was higher or lower than 25, it was
Materials and Structures
often observed that the slump of the fresh concrete
mixture is enlarged or reduced by the addition of the
pre-soaked SAP, compared to the blank concrete
mixture. Therefore, it is assumed that the pre-soaked
SAP gel with absorption rate of 25, neither absorbed
nor released water when mixed into concrete mixtures,
thereby maintaining the workability of concrete
regardless of the addition of pre-soaked SAP.
Furthermore, in order to compare the effects of IC
water entrained by pre-soaked SAP and free mixing
water, other two concrete formulations, HSC-1 and
HSC-2, were designed with additional mixing water
that was equivalent in quantity to the entrained IC
water by pre-soaked SAP in HSC-S1 and HSC-S2
respectively. That is to say, the total water-to-cement
ratios (Wt/C)s of HSC-1 and HSC-2 were 0.34 and
0.39 respectively. The slumps of the abovementioned
concrete mixtures at 30 min after mixing were con-
trolled in the range of 200–220 mm. For HSC-1 and
HSC-2, the workability of concrete mixtures was
tuned by adjusting the dosage of superplasticizer (SP).
In this study, the concrete specimen was prepared
as follows. The cement and aggregate were first added
into the mixer and mixed for 1 min. Subsequently,
water together with the pre-soaked SAP was added to
the dry mixture and mixed for another 3 min. Thus,
SAP was homogenously distributed in the mixture.
Following that, the mixture was cast into mould for
shrinkage measurement. To measure the degree of
cement hydration and the pore structure of concrete,
cement paste samples were prepared with exactly the
same W/C and dosage of SP as used in the concrete
tests. We assumed that the degree of cement hydration
and the pore structure in the concrete are equal to those
in the corresponding paste made with the same W/C
and dosage of SP.
2.3 Testing methods
In this study, shrinkage deformation was monitored
using the method adopted by Zhang et al. [8].
Dimensional changes of the specimens could be
monitored by the readings of linear variable differen-
tial transformer (LVDT) on both ends. To measure the
AS in concrete, specimens of size 80 9 100 9
300 mm3 were completely sealed with plastic film
after casting and stored in a climate room (298 ± 2 K,
50 ± 5 % RH) for 14 days. Perspex plates were
subsequently pulled out, so as to separate the specimen
and the sidewalls of the mould after 2–4 h of mixing,
during which the concrete develops stiffness sufficient
enough to support its own weight. Temperature and
humidity sensors were then plugged into the concrete.
A piece of plastic membrane is placed between the
bottom of the specimen and the mould, which serves to
reduce the frictional force and to eliminate the
restraining effect for free deformation of the concrete
specimen. For each batch of concrete mixture, two
specimens were casted and cured in the same way in
the first 3 days. Sealing membrane of one of the
specimens was peeled off on the third day to allow
drying from the top and side faces in order to obtain
the shrinkage under drying condition, which in turn
provides information on DS of concrete after 3 days.
Shrinkage deformation tests were repeated three times
for each formulation and a representative curve was
chosen for analysis.
Table 1 Composition of Portland cement (wt%)
Chemical composition Mineral composition
SiO2 Fe2O3 Al2O3 SO3 MgO CaO Na2O K2O L.O.I C3S C2S C4AF C3A
21.63 3.22 5.82 2.38 3.25 56.67 0.20 0.88 3.50 50.1 15.8 9.80 9.97
0 7 14 21 280
40
80
120
160
200
Res
idua
l mas
s (g
/g)
Age (d)
Fig. 1 Evolution of the absorbed water content of unit mass of
SAP in a climate room
Materials and Structures
Furthermore, the influence of SAP on cement
dehydration was investigated by performing isother-
mal calorimetry tests on the cement pastes at 298 K
using TAM Air calorimeter (Thermometric AB,
Sweden). Prior to the tests, the calorimeter was
regulated at 298 K and then equilibrated for 24 h.
Thereafter, freshly well mixed cement pastes with
different W/Cs or various contents of IC water were
placed in 20 mL ampoule bottles and then intro-
duced into the channels of the micro-calorimeter.
The resulting heat evolution was recorded for
7 days.
The pore structure of HCP was determined by using
mercury intrusion porosimetry (MIP). After being
cured for 14 days, the HCPs were cut into small pieces
of diameter about 10 mm and thickness 5 mm, and
placed into an alcohol bath (analytical grade) to stop
cement hydration [35]. The resulting samples were
stored for 3 days in an oven at a controlled temper-
ature of 333 ± 2 K, prior to MIP test to determine the
pore structure characteristics using an Hg-porosimetry
(Autopore, IV 9510, USA).
Cubic concrete specimens of dimension
100 9 100 9 100 mm3 were used for measuring the
compressive strength. The concrete specimens were
cured at 293 ± 2 K and relative humidity of
90 ± 5 %. Compressive strength test was performed
in specimens aged for 3, 7 and 28 days, according to
the Chinese standard GB/T 50081-2002. Three spec-
imens were analysed in each test.
3 Results and discussions
3.1 Effects of pre-soaked SAP on the mechanical
properties of concrete
It has been often reported that addition of SAP in
concrete leads to a reduction of compressive strength
in comparison with reference concrete, especially at
early ages [36–39]. The results obtained in this study,
as shown in Fig. 2, are in good agreement with the
observations reported in the literature. As evidenced
from Fig. 2, the compressive strength of HSC-S1 is
lower than that of HSC-0 at all ages before 28 days,
while the 28 days compressive strength exhibits only a
minor decrease with addition of SAP. The strength
reduction becomes more pronounced with higher
dosage of SAP and more entrainment of IC water (as
HSC-S2).
It is meaningful to compare the effect of IC water
and free mixing water on the mechanical strength of
concrete. Comparing HSC-S1 and HSC-1, on the basis
of the concrete mixture formulation of HSC-0, extra
IC water is entrained by the pre-soaked SAP in HSC-
S1 (Wic/C = 0.05), whereas the same amount of water
is added as mixing water in HSC-1. This means that
both HSC-S1 and HSC-1 have the same total water
content. According to Mehta and Monteiro [1], W/C is
the critical factor determining the strength of concrete.
In principle, higher water-to-cement ratio leads to
Table 2 Mix proportion of concrete (kg/m3)
Sample We/C Cement Water Sand Coarse aggregate SP Internal curing water Wic/C
HSC-0 0.29 520 151 750 1,050 4.90 – –
HSC-S1 0.29 520 151 750 1,050 4.90 26.03 0.05
HSC-S2 0.29 520 151 750 1,050 4.90 52.06 0.10
HSC-1 0.34 520 177 750 1,050 3.50 – –
HSC-2 0.39 520 203 750 1,050 2.63 – –
0
20
40
60
80
100
28 d7 dAge (d)
Com
pres
sive
Str
engt
h (M
Pa)
3 d
HSC-0 HSC-S1 HSC-1 HSC-S2 HSC-2
Fig. 2 Influence of SAP dosage on the compressive strength of
HSC
Materials and Structures
lower strength. It is interesting to note that the
compressive strength of HSC-S1 is notably higher
than that of HSC-1 at all ages despite the same W/C
ratio. This suggests that the strength reduction effect
of the SAP entrained water is much lower than that of
the free mixing water at low addition of pre-soaked
SAP. Excessive addition of SAP may lead to many
negative effects, such as appearance of large voids and
very strong retardation effect on cement hydration. For
this reason, HSC-2 and HSC-S2 showed opposite
trend in the comparison of compressive strength.
According to previous studies, the effect of SAP
addition on the compressive strength of concrete could
be a counterbalanced result of several factors [38, 40,
41]. On the one hand, after the pre-soaked SAP gel is
dried out, a reduction in the strength of the concrete
matrix can be generally expected due to the formation of
large voids ([100 lm). On the other hand, at certain
ages, cement hydration may be enhanced by the addition
of pre-soaked SAP that provides extra curing water.
3.2 Effects of pre-soaked SAP on the kinetics
of cement hydration
Several studies have reported the influences of SAP on
cement hydration process [21, 25, 42–44], in which
measurement of non-evaporable water content, ana-
lysis of SEM images and use of TGA-DTA technique
are involved. Enhancement of cement hydration is
usually observed due to the release of water from the
swollen SAP and the increase of RH in cement paste
matrix [25, 44]. In this paper, the hydration kinetics of
cement pastes with and without pre-soaked SAP was
investigated by using isothermal calorimetry (Fig. 3).
As can be seen from the comparison of HSC-S1, HSC-
S2 and HSC-0 in Fig. 3a, the addition of pre-soaked
SAP leads to a prolonged induction period and a slight
delay of exothermic peak in the acceleration period.
This suggests a retardation effect of pre-soaked SAP
on cement hydration within 1 day, which is consistent
with the postponed initial setting of concrete observed
during the experiments. The cumulative heat curves
shown in Fig. 3b suggest that the addition of pre-
soaked SAP (as HSC-S1 and HSC-S2) results in a
higher hydration degree at the age of 7 days, when
compared to the reference cement paste in HSC-0. The
observed enhancement in cement hydration by the
addition of pre-soaked SAP is in good agreement with
the results reported in previous studies [43, 44].
Another point worthwhile to discuss is the com-
parison between HSC-S1 and HSC-1 (or comparison
between HSC-S2 and HSC-2). Although the total
water-cement ratios are the same, a certain amount of
IC water is introduced in HSC-S1 (or HSC-S2) with
the addition of pre-soaked SAP, while the extra water
is introduced as free mixing water in HSC-1 (or HSC-
2) based on the reference concrete formulation. It is
clearly seen that the increase in free mixing water
certainly promotes cement hydration, as evidenced
from the advancement in the hydration peaks in
Fig. 3a and the higher hydration degree at the age of
7 days in Fig. 3b (HSC-0, HSC-1 and HSC-2).
Comparing HSC-1 and HSC-S1 (or HSC-2 and
HSC-S2), it can be realised that the enhancement of
cement hydration by the IC water introduced by pre-
soaked SAP is relatively lower than that by the free
0 12 24 36 48-0.001
0.000
0.001
0.002
0.003
0.004
0.005
dQ/d
t (W
/g)
Age (h)
HSC-0HSC-S1HSC-S2HSC-1HSC-2
0 24 48 72 96 120 144 1680
50
100
150
200
250
300
HSC-0HSC-S1HSC-S2HSC-1HSC-2C
umul
ativ
e H
eat
(J/g
)
Age (h)
(a)
(b)
Fig. 3 Effects of the pre-soaked SAP on cement hydration
kinetics. a Differential heat evolution curves; b cumulative heat
evolution curves
Materials and Structures
mixing water. This could possibly be attributed to the
difference in spatial distribution of the two types of
water, as schematically described in Figs. 4 and 5. The
free mixing water is homogeneously distributed in the
whole cement paste and easily reaches the surface of
hydrating cement grains. On the other hand, the IC
water is initially located inside the SAP gel particles,
which subsequently migrates to the cement surface
through slow diffusion with the decrease of RH in the
cement paste.
3.3 Effects of pre-soaked SAP on the pore
structure of cement pastes
Although MIP has been often criticized majorly due to
the misinterpretation of the received data as applying
to pore size, rather than to the volume accessible
through pores of a given pore entry size [45], it is still
the most popular method for characterizing the pore
structure of hardened cementitious materials [46–49].
In this study, the influence of pre-soaked SAP on pore
structure of cement pastes is investigated using MIP
technique, as shown in Fig. 6 and Table 3. According
to Kumar [50], the pores existing in cement paste can
be classified as gel pores, capillary pores and voids.
The gel porosity of the cement paste is directly
proportional to the hydration degree of cement, while
the capillary porosity is closely related to both W/C
and hydration degree. At 100 % hydration degree,
higher W/C leads to higher capillary porosity. From
Fig. 6 and Table 3, it is clearly seen that the addition
of pre-soaked SAP certainly increases the total
(a) HSC-0 (b) HSC-S1 (c) HSC-1
( :Unhydrated cement particle, : Pre-soaked SAP particle, : Free water)
Fig. 4 Schematic diagrams of phase distribution of fresh cement pastes
(a) HSC-0 (b) HSC-S1 (c) HSC-1
( : Hardened cement matrix; : Voids formed after SAP gel particles dry out; : Capillary pores)
Fig. 5 Schematic diagrams of phase distribution of hardened cement pastes
Materials and Structures
porosity of the cement paste when compared to the
reference concrete HSC-0 (HSC-0, HSC-S1 and HSC-
S2), because of the extra IC water entrained by SAP.
HSC-S1 and HSC-S2 are almost equal in capillary
porosity, as the amount of the effective water is
equivalent. As the (Wt/C)s of HSC-S1 and HSC-1
(HSC-S2 and HSC-2) are the same, the total porosities
of HSC-S1 and HSC-1 are approximately identical.
More comprehensively, the threshold pore size and the
porosity of capillary pores in HSC-S1 are much
smaller than HSC-1, while the volume of large voids
in HSC-S1 is higher than that in HSC-1. The smaller
average size and porosity of capillary pores in HSC-S1
could be ascribed to the lower amount of free mixing
water. The formation of large voids in HSC-S1 (or
HSC-S2) is due to the drying of pre-soaked SAP gel
particles. Comparing HSC-S1 and HSC-1 (HSC-S2
and HSC-2), it can be realised that the difference in
spatial distribution of the two types of water (SAP
entrained water in HSC-S1 and the free mixing water
in HSC-1) produces different pore structure in hard-
ened cement pastes. The comparison of pore structure
for HSC-0, HSC-S1 and HSC-1 are schematically
illustrated in Figs. 4 and 5.
Compared with the reference cement paste HSC-0,
the C–S–H gel porosities of HSC-S1 and HSC-1
(HSC-S2 and HSC-2) are noticeably higher. This is
consistent with the higher hydration degree observed
in specimens aged for 7 days, as measured by
calorimetry (Fig. 3b).
3.4 Effects of pre-soaked SAP on autogenous
shrinkage of concrete
In HSC, the amount of water is insufficient to achieve
complete hydration of cement due to the low W/C
(usually W/C \0.4). Self-desiccation during cement
hydration process builds contractive stress in the concrete
body and leads to autogenous shrinkage. In practice,
HSC is prone to cracking caused by autogenous
shrinkage under restraint. In contrast to drying shrinkage,
which occurs due to loss of water from the concrete
surface, autogenous shrinkage occurs over the entire
volume of the concrete body. Consequently, conven-
tional concrete curing methods involving surface treat-
ment like wet curing, cannot substantially contribute to
the mitigation of autogenous shrinkage of HSC. This is
because the very dense microstructure of HSC impedes
effective transport of curing water into the interior of the
concrete body. The use of SAP, together with a certain
amount of IC water, has proven to be an effective strategy
to mitigate autogenous shrinkage of HSC [22, 25, 51].
3.4.1 Curves of total deformation and choosing
of initial points
The one-dimensional deformation of HSC was mea-
sured in situ on the closely sealed concrete specimens,
starting from 2–4 h after mixing. Simultaneously, we
monitored the development of temperature and rela-
tive humidity in the interior of the concrete specimens.
One of the critical factors to quantify the autogenous
shrinkage of concrete is to determine the starting point of
autogenous shrinkage. According to Weiss [52], the
starting point of autogenous shrinkage is defined as the
time at which the cement matrix develops sufficient
0.00
0.04
0.08
0.12
0.16
0.20HSC-0 HSC-1 HSC-S1 HSC-2 HSC-S2
Vol
ume
(mL
/g)
Diameter (nm)
(a)
1 10 100 1000 10000 100000
1 10 100 1000 10000 1000000.0
0.1
0.2
0.3
0.4
Vol
ume
(mL
/g)
Diameter (nm)
HSC-0 HSC-1 HSC-S1 HSC-2 HSC-S2
(b)
Fig. 6 Effects of SAP on pore structure of the cement paste at
the age of 14 days. a Cumulative pore size distribution;
b differential pore size distribution
Materials and Structures
strength to enable tensile stress transfer. As a rough guide,
time zero can be estimated as the beginning of initial
setting. Several techniques can be adopted to estimate the
starting point, such as the needle penetration test,
ultrasonic measurement, hydraulic pressure change and
temperature measurement [53–55]. Another issue asso-
ciated with the quantification of autogenous shrinkage is
how to extract autogenous shrinkage from the directly
measured total deformation. This is often difficult,
because several physical and chemical processes are
involved in the time period of concrete setting, including
thermal deformation due to inner temperature change,
drastic change in thermal expansion coefficient of
hardening concrete, autogenous shrinkage and so on
[1]. Therefore, it is a highly challenging work to
accurately determine the autogenous shrinkage from
the very start of initial setting, on the basis of the total
deformation. Given these constraints, most studies
measure the autogenous shrinkage after demoulding the
hardened specimens at age of one day or even later [56].
As evidenced from Fig. 7, the interior temperature
of the concrete body starts to increase upon initiation
of the water-cement contact. This is mainly due to the
difference in temperature between the climate room
and the raw materials (water, aggregates and cement)
of concrete. The coarse aggregate and fine sand were
stored outside and their temperature can be
290–292 K. The temperature of tap water could be
as low as 288 K. The temperature of the climate room
for AS measurement was regulated at 298 K. These
temperature differences cause the initial rise of the
internal temperature (before point A) of fresh concrete
mixture, as seen from Fig. 7. After an inflection point
A, as marked in the temperature curves, the inner
temperature begins to increase rapidly, which is
believed to be the result of the exothermal reaction
of cement hydration. This suggests that the cement
hydration enters into the acceleration period at time
point A. Accordingly, this time point should corre-
spond to the initial setting of concrete, which is also
confirmed by the calorimetry results as shown in
Fig. 3a. The temperature peak (B) arises at 15 h for
HSC-0, while slightly postponed temperature peaks
(by 1.5–3 h) are observed for HSC-S1 and HSC-S2,
which is consistent with the calorimetry results shown
in Fig. 3a. From the time point of initial setting A to
temperature peak B, the total temperature rise is about
4.0–5.0 K for HSC-0, HSC-S1 and HSC-S2. The
thermal expansion coefficient of cementitious material
severely varies around the time period of initial setting
and then stays relatively constant (8–12 lm/m/K) after
complete hardening [57]. According to the studies
reported by Zhang [58] and Zhang et al. [8], thermal
expansion coefficient of concrete during early age can
be calculated by using the following equation:
bT ¼ C � expð�c � teqÞþb0 ð1Þ
where bT (910-6/K) is the thermal expansion coeffi-
cient of concrete at a given age; C, c and b0 are
constants determined from the experimental results; teq
is the equivalent age at reference temperature, consid-
ering the influence of temperature on cement hydra-
tion. According to Zhang, the values of the constants C,
c and b0 are set as 48, 0.235 and 8, respectively, for
HSC. It is found that the thermal expansion coefficient
of HSC starts to drastically drop from 56 lm/m/K to
about 8 lm/m/K after complete hardening. The
thermal deformation eT can be calculated from the
following Eq. (2), on the basis of the development
curve of internal temperature in concrete:
eT ¼ZT
T0
bTdT ð2Þ
Table 3 Results of MIP analysis for cement pastes at age of 14 days
Samples Total pore volume (mL/g) Porosity (%) Threshold radius (nm) Pore size distribution (mL/g)
3–10 nm 10–
00 nm
100–1,000 nm [1,000 nm
HSC-0 0.124 22.7 90 0.023 0.080 0.018 0.004
HSC-S1 0.170 28.7 86 0.031 0.115 0.005 0.018
HSC-S2 0.195 30.8 79 0.028 0.136 0.007 0.024
HSC-1 0.173 29.3 482 0.031 0.086 0.048 0.007
HSC-2 0.207 32.8 685 0.039 0.077 0.085 0.005
Materials and Structures
The thermal expansive deformation of an early-age
concrete with temperature rise of 5.0 K right after
initial setting is about 50–60 lm/m, while the total
deformation between time point A and B is about 120
lm/m. Upon deduction of the thermal expansion from
the total deformation, a residual expansive deforma-
tion is obtained rather than shrinkage. This suggests
that there exists another expansive deformation after
initial setting, the mechanism of which is still unclear.
Several other researchers have earlier reported such
early-age expansion [28, 59, 60]. For instance,
Baroghel-Bouny and Kheirbek [59] and Barcelo
et al. [60] reported expansive deformation after initial
setting of concrete, which originates from the inten-
sive formation of ettringite and portlandite C–H
during the early cement hydration process. It is also
possible that the IC water stored in pre-soaked SAP
participates in the cement hydration process and thus
produces volume-gain. With the subsequent drying of
pre-soaked SAP, the volume occupied by the SAP gel
becomes internal pores. This may produce apparent
volume expansion. On the other hand, such expansive
deformation of concrete after initial setting is not
harmful in terms of mitigation of cracking at early age.
The main cause of cracking is the shrinkage under
restraint. Therefore, for simplification, we have
discussed the autogenous shrinkage of concrete start-
ing from the time point of temperature peak (B), rather
than from the initial setting.
3.4.2 Effects of SAP on the autogenous shrinkage
and RH of concrete
The autogenous shrinkage of early-age concrete after
the temperature peak is calculated by subtracting the
thermal deformation from the total deformation, as
shown in Fig. 8a. Simultaneously, the relative humid-
ity inside the concrete body is measured, as shown in
Fig. 8b. As can be seen from Fig. 8a, the autogenous
shrinkage within 14 days is almost eliminated by
addition of pre-soaked SAP with IC water of Wic/
C = 0.05 or 0.1. Compared to HSC-0, HSC-1 and
HSC-2 present smaller autogenous shrinkage due to
their higher W/C, which is in agreement with litera-
tures [25, 61].
Compared to HSC-0, the addition of pre-soaked
SAP significantly delays the reduction of RH inside
the concrete body and increases the RH value at a
certain age. This suggests that the pre-soaked SAP
directly interferes in the development of relative
humidity inside the concrete body at early ages by
releasing the absorbed water to the surroundings. It is
well known that the autogenous shrinkage of concrete
is highly related to the development of RH inside the
concrete body [61–64]. In principle, slower drop of
RH leads to smaller autogenous shrinkage. Therefore,
the addition of pre-soaked SAP effectively reduces the
autogenous shrinkage of HSC via effective tuning of
its internal RH, as indicated in Fig. 8.
On the other hand, the absorbed water by the pre-
soaked SAP is much more effective in increasing the
RH inside the concrete than the additional mixing
water, as evidenced by comparing HSC-S1 and HSC-1
(or HSC-S2 and HSC-2) in Fig. 8b. This should be
again related to the spatial distribution of the two types
of water, as discussed earlier. The difference in the
distribution of the two types of water (IC water
introduced by SAP and the extra mixing water) results
in different pore structure of hardened cement pastes
and different kinetics of cement hydration.
0
50
100
150
2000
50
100
150
2000
50
100
150
200
0 12 24 36 48
0 12 24 36 48
B
Def
orm
atio
n (µ
m· m
-1)
TD
TD-TE
HSC-0
Tem
pera
ture
(K
)
Age (h)
A
299
B
A
HSC-S1
B
A
HSC-S2 299
Temperature
301
303
305
307
299
301
303
305
307
301
303
305
307
Fig. 7 The total deformation and inner temperature variation of
concrete at early age (the time zero in X-axis is corresponding to
the time point of water-cementcontact;TD total deformation, TE
thermal expansion due to temperature rise)
Materials and Structures
3.5 Effects of SAP on the drying shrinkage
of concrete
As described in Sect. 2.3, drying shrinkage is
measured by removing the sealing membrane from
the top and side faces of the specimen, after curing
time of 3 days. Thus, the total shrinkage deformation
under drying condition should contain information of
both the AS within age of 3 days and the DS of
concrete after 3 days, as shown in Fig. 9. As seen from
Fig. 9a, the development of total shrinkage under
drying condition is significantly depressed by the
addition of pre-soaked SAP (HSC-S1 and HSC-S2),
analogous to the evolution of the autogenous shrink-
age (Fig. 8a). Similar trend was observed in the
development of internal RH under drying condition
(Fig. 9b) to the sealed condition (Fig. 8b). The
existence of pre-soaked SAP remarkably mitigates
the rapid drop of the internal RH during hardening
under drying condition.
Figure 10 shows the absolute magnitude of drying
shrinkage from the age of 3–14 days, which is
determined by subtracting the autogenous shrinkage
(Fig. 8a) during the same period from the total
shrinkage under drying condition (Fig. 9a). It is seen
that the drying shrinkage of the blank concrete HSC-0
is small due to its very low W/C. With increase in
W/C, larger drying shrinkage is observed (HSC-1,
HSC-2). The addition of pre-soaked SAP (HSC-S1
and HSC-S2) slightly increases the drying shrinkage
compared with HSC-0. However, comparing HSC-S1
and HSC-1 (or HSC-S2 and HSC-2), it is observed that
the drying shrinkage in HSC-S1 is much lower than
HSC-1, despite the same Wt/C. This is again due to the
difference in spatial distribution of the two types of
water, namely, the IC water introduced by pre-soaked
SAP and the extra mixing water.
0 48 96 144 192 240 288 336
0 48 96 144 192 240 288 336
-250
-200
-150
-100
-50
0
50
Age (h)
Def
orm
atio
n (µ
m·m
-1)
HSC-S2, S1, 2, 1, 0
80
85
90
95
100
Age (h)
RH
(%
)
HSC-S2, S1, 2, 1, 0
(a)
(b)
Fig. 8 Influences of SAP dosage and W/C on a autogenous
shrinkage and b internal humidity of HSCs
0 48 96 144 192 240 288 336
-300
-200
-100
0
Def
orm
atio
n (µ
m· m
-1)
Age (h)
HSC-1, 0, 2, S2, S1
(a)
(b)
0 48 96 144 192 240 288 33670
75
80
85
90
95
100
Age (h)
RH
(%
)
HSC-0, 1, 2, S2, S1
Fig. 9 Influences of SAP dosage and W/C on a shrinkage
deformation and b internal humidity of HSCs under drying
condition
Materials and Structures
3.6 Discussion
Jensen has comprehensively discussed the mechanism
underlying the effect of SAP on the pore structure and
self-desiccation in water-entrained cement pastes [61].
On the other hand, to the best of our knowledge, a
quantitative explanation on the shrinkage-reducing
mechanism of SAP as internal curing agent in the
practical concrete with relatively bigger size, other
than cement paste or mortars has not yet been fully
established. So far, several models have been devel-
oped to predict AS and DS of concrete, with an aim to
interpret the cracking mechanism in concrete [65–69].
For instance, Chen simulated the evolution of RH in
self-desiccating cement pastes by using the pore size
distribution measured by MIP and the chemical
shrinkage as input [70]. Furthermore, Zhang et al.
[8] developed a micromechanical model for predicting
concrete shrinkage that is induced by moisture loss in
concrete. The model proposed by Zhang is based on
the calculation of the contractive stress driven by the
capillary force in concrete, which originates from the
drop of internal humidity in concrete body. It is
particularly useful for understanding and predicting
concrete shrinkage in early ages. In order to gain
deeper insights on the shrinkage reducing mechanism
of pre-soaked SAP, we have adopted the model
proposed by Zhang and calculated the AS with
experimental results as inputs, such as the develop-
ment of elastic modulus, cement hydration degree,
pore distribution of cement paste matrix and internal
RH.
According to Zhang model [8], the early shrinkage
is divided into two parts based on different driving
forces, as indicated in Eqs. 3 and 4. Under saturated
moisture condition (RH = 100 %), the chemical
shrinkage is partly delivered to the macroscopic
deformation. When the internal RH drops below
100 %, capillary suction becomes the main driving
force of shrinkage. Proceeding from the constitutive
model, a humidity deformation equation is derived
with internal RH as the main parameter. The complete
expressions are as follows:
ew¼g 1�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� ðVcs � V0Þ3
p� �for RH ¼ 100 %
ð3Þ
ec �SmpqRT
3M
1
K� 1
Ks
� �ln (RH) for RH\100 %
ð4Þ
where ew is the moisture-loss induced shrinkage
(including AS and DS) of HSC; g is the stiffness
coefficient, which is defined as the ratio of macroscopic
deformation and total chemical shrinkage; Vcs is the
chemical shrinkage of volume for the moment; V0 is
originally the chemical shrinkage of volume at the time
point of setting, but here it represents the chemical
shrinkage at the time point of temperature peak since it
is chosen as the starting time for determining AS in this
study; ec is the moisture-loss induced shrinkage when
the RH begins to drop from 100 %; S is the correction
factor of saturation, reflecting the effective area ratio of
capillary pressure action; mp is the influential coefficient
of pore structure, which is a parameter related to the
effective pore volume ratio of cement matrix, and could
be obtained from the pore distribution curves; q is the
density of water; R is the molar gas constant; T is the
absolute temperature; M is the molar mass of water; K
and Ks are respectively the bulk and skeletal volume
modulus that can be reckoned from the linear modulus
of concrete.
Applying Zhang model [8] using hydration degree,
pore size distribution, elastic modulus and the devel-
opment of internal RH as inputs, the full autogenous
shrinkage can be successfully calculated. Detailed
explanation of the methodology can be found in the
literature. The parameters used for calculating the
autogenous shrinkage are listed in Table 4. As can be
Fig. 10 Drying shrinkage caused by water evaporation during
the age of 3–14 days
Materials and Structures
seen from Fig. 11, the calculated shrinkage fits well
with the experimental data for all HSCs. Although the
experimental determination of autogenous shrinkage
starts from the time point of temperature peak, the
autogenous shrinkage in the time span from the initial
setting to the temperature peak can be obtained from
the calculation by using the model. Based on the
calculated autogenous shrinkage, the working mech-
anisms of SAP in shrinkage reduction are further
discussed for periods before and after the RH starts to
drop from 100 % respectively.
3.6.1 RH = 100 %
With the progress of cement hydration, the internal
RH starts to drop from 100 %, when the water stored
in the capillary pores of size smaller than 100 nm
starts to be consumed by cement hydration. Figure 12
presents the autogenous shrinkage at the moisture
saturated stage (RH = 100 %), obtained from the
model calculation. It is clearly found that the addition
of pre-soaked SAP extremely reduces the AS during
this stage and meanwhile visibly prolongs the period
for which the internal humidity of the concrete
remains 100 %. During this period, the macroscopic
deformation of the concrete originates from the
chemical shrinkage, which is due to the reduced
volume of hydration products from the hydrating
cement and water. On the other hand, in case that pre-
soaked SAP is incorporated into the concrete, it is
believed that a part of water absorbed in SAP
participates in cement hydration in the adjacent
region. This extra water reacts with the neighbouring
cement and the produced hydrates have larger volume
than the reacted cement. One should note that the
consumption of the water absorbed by SAP does not
bring visible changes in volume that is originally
occupied by the pre-soaked SAP gel particles due to
the well-established skeleton of concrete after
hardening, as observed by SEM in Fig. 13. Upon
drying up of the pre-soaked SAP particles, the
remaining space becomes voids with almost the same
volume as that of the originally added SAP gel
particles. Thus, a total volume increase is obtained, as
the IC water involves in cement hydration. This
volume gain may compensate the ordinary chemical
shrinkage and allows significant reduction in autoge-
nous shrinkage during the period of RH = 100 %.
Jensen and Hansen [21] also reported an expansive
swelling peak in the first few hours of hydration. They
explained such expansion as a result of absorption of
water by the cement gel, under conditions of contin-
uous water supply to the concrete during hydration.
This result is in good agreement with the abovemen-
tioned findings observed in the present study.
3.6.2 RH \ 100 %
As obviously observed in Fig. 12, the addition of pre-
soaked SAP remarkably postpones the drop of internal
RH from 100 %. This is again indicative of the fact
that a part of the SAP absorbed water participates in
cement hydration during the period of RH = 100 %.
On the other hand, with the release of water to the
adjacent concrete body, the addition of pre-soaked
SAP also largely heightens the RH level at a certain
age, after RH is below 100 %. In the second part of
Zhang model (Eq. 4), the critical radius of capillary
pores, rc, is an important intermediate parameter
correlating the internal RH and the contractive stress at
a certain age of concrete. According to Zhang model,
rc is defined as the critical capillary pore radius, pores
of size smaller than which are filled with water, while
those of size larger than which are dried out due to
either self-desiccation or drying process. After reach-
ing thermodynamic equilibrium, rc could be calcu-
lated from the RH value, according to the following
Kelvin equation:
Table 4 Parameters used in the calculation of autogenous shrinkage
Concrete sample HSC-0 HSC-S1 HSC-S2 HSC-1 HSC-2
g 0.0083 0.0004 0.0002 0.0061 0.0035
Elastic modulus E28 (GPa) 44.1 41.5 39.8 40.6 38.8
Es (GPa)a 72.9 72.9 72.9 72.9 72.9
a Es is the ultimate elastic modulus of concrete
Materials and Structures
rc ¼ �2cM
ln ðRHÞqRTð5Þ
where c is the surface tension of water, which is
typically 0.073 N/m. Thus, the critical pore radius of
concrete can be deduced from Fig. 8b, as shown in
Fig. 14. The corresponding calculation results are
listed in Table 5.
As can be seen from Fig. 14, the critical pore radius of
the reference concrete HSC-0 sharply drops after aging
for 36 h. This corresponds to the rapid development of
autogenous shrinkage. The addition of pre-soaked SAP
significantly delays the decline of critical pore radius. As
listed in Table 5, the critical pore radius of HSC-0, 1, 2,
S1 and S2 aged for 14 days is respectively 6, 8, 10, 34 and
59 nm and the corresponding values of capillary nega-
tive pressure (r) are 24.3, 18.3, 14.6, 4.3 and 2.5 MPa
(according to Eq. 3). Thus, the capillary pressure of
concrete with SAP is far smaller than that without SAP
and the driving force of AS is greatly reduced by the
addition of SAP. This is the most fundamental cause of
shrinkage-reducing effect for the SAP internal curing.
According to Zhang model, the magnitude of autoge-
nous shrinkage is also related to the number and size
Table 5 Results of model calculation
Sample RH at
14 days/ (%)
Critical
radius (rc/nm)
Capillary
stress (r/MPa)
mp Contractive stress mp
(r/MPa)
HSC-0 84 6 24.3 0.85 20.6
HSC-S1 97 34 4.3 0.96 4.2
HSC-S2 98 59 2.5 0.98 2.45
HSC-1 87 8 18.3 0.88 16.1
HSC-2 92 10 14.6 0.92 13.4
0 48 96 144 192 240 288 336-250
-200
-150
-100
-50
0
Experimental curve of HSC-0 Experimental curve of HSC-1 Experimental curve of HSC-2 Experimental curve of HSC-S1 Experimental curve of HSC-S2
Age (h)
Shri
nkag
e (μ
m⋅ m
-1)
Model curve of HSC-0 Model curve of HSC-1 Model curve of HSC-2 Model curve of HSC-S1 Model curve of HSC-S2
Fig. 11 Full autogenous shrinkage obtained by model
calculation
0 6 12 18 24 30 36 42 48-140
-120
-100
-80
-60
-40
-20
0
Shri
nkag
e (µ
m· m
-1)
Age (h)
HSC-0 HSC-1 HSC-2 HSC-S1 HSC-S2
Fig. 12 Autogenous shrinkage from initial setting to the
moment RH dropping below 100 % obtained from the model
calculation
Large void introduced by SAP
500.00μm
Fig. 13 SEM image of cement paste with addition of pre-
soaked SAP at 28 days
Materials and Structures
distribution of pores smaller than the critical radius.
These two factors can be indicated by the parameter mp,
representing the ratio of the volume of stressed pores to
the total pore volume. Accordingly, the total contractive
stress is represented as mp�r, which is the driving force for
the moisture-loss induced shrinkage. As can be realized
from the comparison of HSC-0, HSC-S1 and HSC-S2
shown in Table 5, the contractive stress is substantially
reduced by the addition of the pre-soaked SAP.
Mechtcherine et al. [26, 27] has provided an overview
of the effects of SAP on various types of shrinkages in
concrete, including plastic shrinkage, chemical shrink-
age, autogenous shrinkage and drying shrinkage. It is
stated that there is still limited knowledge on the
mechanism of internal curing of concrete using SAP,
especially in the first few hours after the setting. The
outcomes of the present study are consistent with the
results summarized in the abovementioned literature. In
particular, the theoretical modelling performed in the
present study attempts to explain the issues put forth in the
abovementioned literature. These inferences substantiate
the fact that the addition of SAP significantly reduces the
autogenous shrinkage because of the participation of the
internal curing water in cement hydration process and the
postponed drop in the critical pore radius, especially in
the first few hours after the setting of concrete.
4 Conclusions
Based on the aforementioned experimental results as
well as the model calculation, the following conclu-
sions can be drawn.
1. Addition of pre-soaked SAP in HSC can firmly
alleviate the early-age shrinkage related to mois-
ture loss (mainly consisting of AS and DS), which
is expected to be beneficial for mitigating crack-
ing in HSC during the early ages.
2. Addition of pre-soaked SAP affects the cement
hydration process, the pore structure of hardened
cement pastes, as well as the evolution of internal
humidity in the concrete bodies. The reduction of
relative humidity in concrete caused by cement
hydration is significantly postponed with the
addition of pre-soaked SAP and therefore the
relative humidity inside concrete body at a certain
age is greatly heightened.
3. Addition of pre-soaked SAP slightly reduces the
compressive strength of HSCs, and this effect is
more pronounced in early-age concrete. It is
believed that the negative effect of pre-soaked
SAP on the strength development of HSC can be
minimized with the choice of appropriate dosage.
4. Furthermore, we scientifically compared the
behaviour of the internal curing water introduced
by the pre-soaked SAP with that of the additional
free mixing water. It was found that the autoge-
nous shrinkage-reducing effect of the internal
curing water incorporated by the pre-soaked SAP
is much stronger than that of the additional mixing
water. Pre-soaked SAP changes the kinetics of
cement hydration and pore structure of cement
pastes with respect to systems with higher W/C
ratio. Thus, the internal curing water and the
additional free mixing water act differently in
influencing the development of internal humidity
in concrete and the development of compressive
strength. The internal curing water shows rela-
tively less strength-reducing effect than that of the
additional mixing water.
5. The evolution of contractive stress in concrete
body during hardening was quantitatively simu-
lated when internal curing agent is incorporated.
In virtue of the shrinkage model, the mechanism
underlying the function of pre-soaked SAP in
reducing autogenous shrinkage is proposed. Two
facts are responsible for the shrinkage reducing
effect of the pre-soaked SAP. One is the volume
gain due to the participation of the internal curing
water introduced by the pre-soaked SAP in
cement hydration process. The second on is the
postponed drop of the internal humidity due to the
0 48 96 144 192 240 288 3360
200
400
600
800
1000
1200C
riti
cal r
adiu
s (n
m)
Age (h)
HSC-0 HSC-S1 HSC-S2 HSC-1 HSC-2
Fig. 14 Evolution of the critical radius over age under sealed
condition
Materials and Structures
release of internal curing water from the pre-
soaked SAP, which reduces the driving force of
autogenuous shrinkage.
In this paper, we investigate the effects of pre-
soaked SAP as internal curing agent, on cement
hydration, pore structure, strength development and
shrinkage of high strength concrete. Pre-soaked SAP,
rather than SAP powder, is used due to its less impact
on workability of fresh concrete. More systematic
investigation is needed to quantitatively compare the
rheological properties of fresh concrete with addition
of pre-soaked SAP versus dry SAP powder. For future
study, optimization of the chemical composition and
particle size of SAP could be a potential technical
approach to minimize the negative effects on strength
development and workability of concrete. Durability
related issues of the concrete containing SAP internal
curing agent, including freeze–thaw stability and
transportation process of attacking species such like
chloride ions and sulphate ions, should be given more
emphasis in the future research, which should be
valued for practice of SAP internal curing technology.
Acknowledgments The support from the National Natural
Science Foundation of China (Grant Nos. 51173094 and
U1262107) is appreciated.
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