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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


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


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


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


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


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


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


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


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)


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


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


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


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


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


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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