Effect of pre-soaked superabsorbent polymer on shrinkage of high-strength concrete

Transcription

1 DOI /s ORIGINAL ARTICLE Effect of pre-soaked superabsorbent polymer on shrinkage of high-strength concrete Xiang-ming Kong Zhen-lin Zhang Zi-chen Lu Received: 21 January 214 / Accepted: 29 May 214 Ó RILEM 214 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 presoaked SAP slightly reduces the compressive strength of HSCs and this effect is more pronounced in earlyage concrete. Furthermore, an insightful comparison of the behaviours of the internal curing water introduced by the pre-soaked SAP and the additional free mixing X. Kong Z. Zhang Z. Lu Department of Civil Engineering, Key Laboratory of Safety and Durability of China Education Ministry, Tsinghua University, Beijing 184, China X. Kong (&) Collaborative Innovation Center for Advanced Civil Engineering Materials, Southeast University, Nanjing , China kxm@mail.tsinghua.edu.cn 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 Superabsorbent 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

2 or greater by the ACI Committee on High Strength Concrete in 22 [1], has been widely applied in the construction of high-rise buildings, long-span structures, 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 \.4) 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, cracking 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 autogenous shrinkage (AS) [2]. At low water-to-cement ratio, marked self-desiccation 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-desiccation or via drying. In either case, capillary tension produced by capillary meniscus plays an important role in the shrinkage [7, 8, 1]. Water-to-cement ratio is the most important factor determining their magnitudes. In case of normal concrete (W/C [.5) with strength lower than 4 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.3, the AS can account for more than 5 % 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, times of their own weight. Thus far, several researchers [2 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 Hansen [2] 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 3,7 lm/m after 3 weeks of hardening. With the addition of.3.6 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 cementitious 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. Nevertheless, 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 application in concrete [27]. Esteves [28] proposed three

3 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 [3, 31], wherein the variation of the interior moisture content, as represented by interior RH, is considered as the major factor controlling the magnitude of shrinkage. Several studies have indicated a close correlation between shrinkage 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 (GB175-27, China) common Portland cement produced by Jidong Cement Plant, the chemical and mineral composition of which is shown in Table 1. The coarse aggregate is composed of crushed granite of size ranging from 5 2 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 2 22 mm. SAP was synthesized by copolymerizing acrylamide and acrylic acid in the mass ratio of 7:3 via radical polymerization. The prepared SAP was fully dried and ground into powder with particle size of lm. Several methods have been proposed to measure the absorption capacity 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 2, 8 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, 5 ± 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-), the W/C ratio was fixed as.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 presoaked SAP to cement (W ic /C) varied respectively as.5 and.1. The water absorption capacity of the presoaked 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

4 Table 1 Composition of Portland cement (wt%) Chemical composition Mineral composition SiO 2 Fe 2 O 3 Al 2 O 3 SO 3 MgO CaO Na 2 O K 2 O L.O.I C 3 S C 2 S C 4 AF C 3 A Residual mass (g /g) Age (d) Fig. 1 Evolution of the absorbed water content of unit mass of SAP in a climate room 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 (W t /C)s of HSC-1 and HSC-2 were.34 and.39 respectively. The slumps of the abovementioned concrete mixtures at 3 min after mixing were controlled in the range of 2 22 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 differential transformer (LVDT) on both ends. To measure the AS in concrete, specimens of size mm 3 were completely sealed with plastic film after casting and stored in a climate room (298 ± 2K, 5 ± 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.

5 Table 2 Mix proportion of concrete (kg/m 3 ) Sample W e /C Cement Water Sand Coarse aggregate SP Internal curing water W ic /C HSC ,5 4.9 HSC-S , HSC-S , HSC ,5 3.5 HSC , Compressive Strength (MPa) HSC- HSC-S1 HSC-1 HSC-S2 HSC-2 3 d 7 d Age (d) 28 d Fig. 2 Influence of SAP dosage on the compressive strength of HSC Furthermore, the influence of SAP on cement dehydration was investigated by performing isothermal 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 2 ml ampoule bottles and then introduced 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 1 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 temperature of 333 ± 2 K, prior to MIP test to determine the pore structure characteristics using an Hg-porosimetry (Autopore, IV 951, USA). Cubic concrete specimens of dimension mm 3 were used for measuring the compressive strength. The concrete specimens were cured at 293 ± 2 K and relative humidity of 9 ± 5 %. Compressive strength test was performed in specimens aged for 3, 7 and 28 days, according to the Chinese standard GB/T Three specimens 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- 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-, extra IC water is entrained by the pre-soaked SAP in HSC- S1 (W ic /C =.5), 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

6 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, 4, 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 ([1 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, analysis 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- 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 presoaked 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-. 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]. (a) dq/dt (W/g) (b) HSC- HSC-S1 HSC-S2 HSC-1 HSC Cumulative Heat (J/g) HSC- HSC-S1 HSC-S2 HSC-1 HSC Fig. 3 Effects of the pre-soaked SAP on cement hydration kinetics. a Differential heat evolution curves; b cumulative heat evolution curves Another point worthwhile to discuss is the comparison 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-, 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 presoaked SAP is relatively lower than that by the free

7 (a) HSC- (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- (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 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 [5], 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 1 % 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

8 (a) Volume (ml/g) (b) Volume (ml/g) HSC- HSC-1 HSC-S1 HSC-2 HSC-S Diameter (nm) HSC- HSC-1 HSC-S1 HSC-2 HSC-S Diameter (nm) 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 porosity of the cement paste when compared to the reference concrete HSC- (HSC-, 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 (W t /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 hardened cement pastes. The comparison of pore structure for HSC-, HSC-S1 and HSC-1 are schematically illustrated in Figs. 4 and 5. Compared with the reference cement paste HSC-, 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 \.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, conventional concrete curing methods involving surface treatment 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] Curves of total deformation and choosing of initial points The one-dimensional deformation of HSC was measured in situ on the closely sealed concrete specimens, starting from 2 4 h after mixing. Simultaneously, we monitored the development of temperature and relative 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

9 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 1 nm 1 nm 1 1, nm [1, nm HSC HSC-S HSC-S HSC HSC 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 associated 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 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 correspond 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-, while slightly postponed temperature peaks (by 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 K for HSC-, 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: b T ¼ C expð c t eq Þþb ð1þ where b T (91-6 /K) is the thermal expansion coefficient of concrete at a given age; C, c and b are constants determined from the experimental results; t eq is the equivalent age at reference temperature, considering the influence of temperature on cement hydration. According to Zhang, the values of the constants C, c and b are set as 48,.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 e T can be calculated from the following Eq. (2), on the basis of the development curve of internal temperature in concrete: e T ¼ Z T T b T dt ð2þ

10 Deformation (µm m -1 ) TD Temperature TD-TE A A A B B B HSC HSC-S HSC-S1 Temperature (K) The thermal expansive deformation of an early-age concrete with temperature rise of 5. K right after initial setting is about 5 6 lm/m, while the total deformation between time point A and B is about 12 lm/m. Upon deduction of the thermal expansion from the total deformation, a residual expansive deformation 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, 6]. For instance, Baroghel-Bouny and Kheirbek [59] and Barcelo et al. [6] reported expansive deformation after initial setting of concrete, which originates from the intensive 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 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) 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 starting from the time point of temperature peak (B), rather than from the initial setting 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 humidity 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 W ic / C =.5 or.1. Compared to HSC-, HSC-1 and HSC-2 present smaller autogenous shrinkage due to their higher W/C, which is in agreement with literatures [25, 61]. Compared to HSC-, 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 presoaked 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.

11 (a) 5 HSC-S2, S1, 2, 1, (a) Deformation (µm m -1 ) Deformation (µm m -1 ) HSC-1,, 2, S2, S1 (b) HSC-S2, S1, 2, 1, (b) RH (%) 9 85 RH (%) HSC-, 1, 2, S2, S Fig. 8 Influences of SAP dosage and W/C on a autogenous shrinkage and b internal humidity of HSCs Fig. 9 Influences of SAP dosage and W/C on a shrinkage deformation and b internal humidity of HSCs under drying condition 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 shrinkage (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 1 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- 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-. 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 W t /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.

12 Fig. 1 Drying shrinkage caused by water evaporation during the age of 3 14 days 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 developed 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 [7]. 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 development 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 = 1 %), the chemical shrinkage is partly delivered to the macroscopic deformation. When the internal RH drops below 1 %, 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: p e w ¼g 1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 1 ðv cs V Þ for RH ¼ 1 % ð3þ e c Sm pqrt 1 3M K 1 ln (RH) for RH\1 % K s ð4þ where e w 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; V cs is the chemical shrinkage of volume for the moment; V 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; e c is the moisture-loss induced shrinkage when the RH begins to drop from 1 %; S is the correction factor of saturation, reflecting the effective area ratio of capillary pressure action; m p 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 K s 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 development 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

13 Table 4 Parameters used in the calculation of autogenous shrinkage Concrete sample HSC- HSC-S1 HSC-S2 HSC-1 HSC-2 g Elastic modulus E 28 (GPa) E s (GPa) a a E s is the ultimate elastic modulus of concrete 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 mechanisms of SAP in shrinkage reduction are further discussed for periods before and after the RH starts to drop from 1 % respectively RH = 1 % With the progress of cement hydration, the internal RH starts to drop from 1 %, when the water stored in the capillary pores of size smaller than 1 nm starts to be consumed by cement hydration. Figure 12 presents the autogenous shrinkage at the moisture saturated stage (RH = 1 %), 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 1 %. 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 presoaked 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 autogenous shrinkage during the period of RH = 1 %. 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 continuous water supply to the concrete during hydration. This result is in good agreement with the abovementioned findings observed in the present study RH \ 1 % As obviously observed in Fig. 12, the addition of presoaked SAP remarkably postpones the drop of internal RH from 1 %. This is again indicative of the fact that a part of the SAP absorbed water participates in cement hydration during the period of RH = 1 %. 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 1 %. 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, r c 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 reaching thermodynamic equilibrium, rc could be calculated from the RH value, according to the following Kelvin equation:

14 Table 5 Results of model calculation Sample RH at 14 days/ (%) Critical radius (r c /nm) Capillary stress (r/mpa) m p Contractive stress m p (r/mpa) HSC HSC-S HSC-S HSC HSC Shrinkage (μm m -1 ) Experimental curve of HSC- Experimental curve of HSC-1 Experimental curve of HSC-2 Experimental curve of HSC-S1 Experimental curve of HSC-S2 Model curve of HSC- Model curve of HSC-1-2 Model curve of HSC-2 Model curve of HSC-S1 Model curve of HSC-S Fig. 11 Full autogenous shrinkage obtained by model calculation Large void introduced by SAP 5.μm Shrinkage (µm m -1 ) HSC- HSC-1 HSC-2 HSC-S1 HSC-S Fig. 12 Autogenous shrinkage from initial setting to the moment RH dropping below 1 % obtained from the model calculation 2cM r c ¼ ð5þ ln ðrhþqrt where c is the surface tension of water, which is typically.73 N/m. Thus, the critical pore radius of Fig. 13 SEM image of cement paste with addition of presoaked SAP at 28 days 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- 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 listedintable5, the critical pore radius of HSC-, 1, 2, S1 and S2 aged for 14 days is respectively 6, 8, 1, 34 and 59 nm and the corresponding values of capillary negative 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 autogenous shrinkage is also related to the number and size

15 Critical radius (nm) distribution of pores smaller than the critical radius. These two factors can be indicated by the parameter m p, representing the ratio of the volume of stressed pores to the total pore volume. Accordingly, the total contractive stress is represented as m p r, which is the driving force for the moisture-loss induced shrinkage. As can be realized from the comparison of HSC-, 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 shrinkage, 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 HSC- HSC-S1 HSC-S2 HSC-1 HSC-2 Fig. 14 Evolution of the critical radius over age under sealed condition Based on the aforementioned experimental results as well as the model calculation, the following conclusions can be drawn. 1. Addition of pre-soaked SAP in HSC can firmly alleviate the early-age shrinkage related to moisture loss (mainly consisting of AS and DS), which is expected to be beneficial for mitigating cracking 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 autogenous 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 relatively less strength-reducing effect than that of the additional mixing water. 5. The evolution of contractive stress in concrete body during hardening was quantitatively simulated 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