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Can superabsorent polymers mitigate autogenous shrinkage of internally cured concrete without compromising the strength?
Construction and Building Materials 31 (2012) 226–230
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Can superabsorent polymers mitigate autogenous shrinkage of internally cured concrete without compromising the strength? q Marianne Tange Hasholt a,⇑, Ole Mejlhede Jensen a, Konstantin Kovler b, Semion Zhutovsky b a b
Department of Civil Engineering, Technical University of Denmark, Building 118, Brovej, DK-2800 Lyngby, Denmark National Building Research Institute, Faculty of Civil and Environmental Engineering, Technion – Israel Institute of Technology, 32000 Haifa, Israel
a r t i c l e
i n f o
Article history: Received 6 July 2011 Received in revised form 15 December 2011 Accepted 19 December 2011 Available online 25 January 2012 Keywords: Concrete Superabsorbent polymers Internal curing Self-desiccation Autogenous shrinkage Compressive strength
a b s t r a c t The paper ‘‘Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks’’ deals with different aspects of using superabsorbent polymers (SAP) in concrete to mitigate self-desiccation. The paper concludes that ‘‘Addition of SAP leads to a significant reduction of mechanical strength’’. The experimental results are in contradiction with several publications and question the appropriateness of using SAP as internal curing agent. However, the observed strength loss – and possibly also other observations – seems to be caused by overestimation of SAP water absorption. This results in an increase in water/cement ratio (w/c) for concrete with SAP. It is misleading to conclude on how SAP influences concrete properties, based on comparison of concrete mixes with SAP and reference concrete without SAP, if SAP mixes have higher w/c than the reference mix. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cementitious materials with low water/cement (w/c) or low water/binder (w/b) ratios are vulnerable to early age cracking due to autogenous deformation caused by self-desiccation. A decade ago, it was suggested and demonstrated that this problem can be solved by mixing superabsorbent polymers (SAP) into the fresh concrete, thereby establishing small reservoirs of internal curing water [1,2]. In the wake of this study, a number of studies have confirmed these findings, as well as examined how SAP influences different concrete properties. A RILEM technical committee is formed to work on the application of SAP in concrete, aiming at among other things recommendations for the use of SAP [3]. To the knowledge of the authors, this RILEM technical committee will also in near future publish a state-of-the-art report, which reviews research studies concerning SAP in concrete from the very first study and up till today. Like this, the topic ‘‘SAP in concrete’’ is a research field with a lot of ongoing activities. It is of course important to ensure that the solution to one problem, here self-desiccation, does not create new problems. When a
q Discussion of the paper ‘‘Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks’’ by Bart Craeye, Matthew Geirnaert, and Geert De Schutter. ⇑ Corresponding author. Tel.: +45 4525 1702. E-mail address: [email protected] (M.T. Hasholt).
0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.12.062
new component like SAP is introduced into the concrete matrix, it is important to critically investigate how this component influences concrete properties. In view of this, the paper by Craeye et al. [4] recently published in Construction and Building Materials is very relevant. Besides autogenous deformation, the authors also examine how SAP addition influences workability of the fresh concrete, mechanical properties and heat development during hydration. Moreover, they simulate stress build-up at early age when casting a bridge deck, where deformation is restrained, thereby testing if the reduction of autogenous deformation is sufficient to avoid crack formation. One of the conclusions of Craeye et al. [4] is that ‘‘Addition of SAP leads to a significant reduction of mechanical strength’’. In comparison with a reference mix made at w/b 0.30, the 28-days compressive strength of 150 mm cubes cured at 20 °C and 90% relative humidity was reduced by 15–28%, depending on the SAP content (0.2% and 0.4% dry SAP relative to cement mass). The higher the SAP dosage, the lower was the 28-day strength. These values of the reported strength loss are indeed high and can compromise the application of superabsorbent polymers as internal curing agents in concrete construction. However, recent results from research made at DTU [5] show that the dependence of strength on the SAP content is more complicated than just a simple relation, where SAP addition leads to strength loss. Actually, in some cases addition of SAP can increase compressive strength, and therefore the results and conclusions by Craeye et al. need to be discussed in more detail.
M.T. Hasholt et al. / Construction and Building Materials 31 (2012) 226–230
2. Previous observations regarding the influence of SAP on compressive strength In the literature, information about the influence of SAP on concrete strength is not extensive and the reported results may seem contradictory. With regard to the 28-days compressive strength, some publications report reduction of compressive strength of mixes containing SAP, in comparison with reference mixes without SAP, whereas other publications demonstrate almost unchanged or even higher strength: Lam and Hooton [6]: The addition of 0.3% SAP (relative to cement mass) to the concrete mix dramatically reduced the compressive strength by more than 50% compared to the control mix made at w/c = 0.35. However, by doubling the SAP dosage, most of the strength loss was prevented. The strength of the mix with 0.6% SAP made at w/c = 0.35 was approximately equal to the strength of another reference mix made at w/c = 0.45. In both SAP mixes, extra water was added. It is common practice to add extra water together with SAP to account for the amount of water absorbed by SAP in the fresh concrete, see Section 3, and therefore the amount of extra water is directly proportional to the amount of SAP. Contrary to this practice, in this study, the content of extra water was the same in both mixes with SAP (equivalent to 10% of cement mass). The lower strength of the system with 0.3% SAP may be due to a too high content of extra water, twice as much as that introduced per unit mass of superabsorbent polymer in the case of 0.6% SAP. Piérard et al. [7]: The strength of concrete made at w/c = 0.35 without SAP and with SAP contents of 0.3% and 0.6% (and extra water equivalent to 2% and 4% of cement mass, respectively), was measured on cubes cured at 20 ± 2 °C and minimum air relative humidity of 95% at ages of 2–28 days. Results showed that the early strength development (2–7 days) was slowed down with SAP, but the reduction in strength decreased at later ages. After 28 days, the reductions in strength were 7% and 13% for concrete mixes with SAP contents of 0.3% and 0.6%, respectively. Lura et al. [8]: For w/c = 0.30, internal curing by means of 0.4% SAP (extra water equivalent to 5% of cement mass) had almost no influence on the compressive strength of mortars, while the strength of cement pastes was reduced by 20% at early ages (up to 7 days) and by 10% at later ages (28 and 56 days). Esteves et al. [9]: In this work mortars with w/c at 0.25, 0.30, and 0.35 were tested. For each w/c, mixes were made without SAP and 0.2% SAP (extra water equivalent to 5% of cement mass), and specimens were cured at 30%, 50%, and 95% relative humidity. Results after 28 days of curing at 95% relative humidity showed a 15–20% reduction of compressive strength for mortar with SAP. But where the strength dropped at lower relative humidity for mortar without SAP, mortar with SAP had almost constant compressive strength no matter the curing conditions. At 30% relative humidity, the strength reduction for mortar with SAP was only 5%. Mechtcherine et al. [10]: Compressive strength was reported for ultra high performance SAP-containing mortars (w/c = 0.22, SAP content 0% (reference mix), 0.3%, and 0.6% relative to cement mass). Extra water was added to SAP-containing mixes to compensate the loss of workability. Noticeable decrease in compressive strength was observed at early ages, e.g. at 7 days, where the strength reductions were 12% and 30% for 0.3% and 0.6% mixes, respectively. At 28 days measurements exhibited only a minor decrease in strength for the 0.3% mixes: 4% (not significant). At 28 days, the strength reduction for the 0.6% mix was 20%.
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Gao et al. [11]: An increase in compressive strength was observed, when SAP was added to the aluminate cement paste made at w/c = 0.40, but without adding extra water: from 36.1 MPa (0% SAP) up to 40.5 MPa (0.2% SAP) and 44.4 MPa (0.6% SAP). Bentz et al. [12]: Measurements of compressive strength development were carried out for mortar mixes with w/b = 0.35, with and without SAP (0.4% relative to binder mass). After 7 days, the compressive strength of mortar with SAP was lower than the strength of a reference mortar without SAP, 53 MPa vs. 57 MPa. After 28 days of curing, the picture had changed; mortar with SAP showed higher compressive strength than the reference mortar, 73 MPa vs. 61 MPa (values for compressive strength are approximates, as mixes have been tested at slightly different age, and test results have been interpolated afterwards). 3. Mix design for internal curing The basic principles of internal curing mix design using Powers’ model as design tool were described in [1]. Powers’ model itself is based on the comprehensive experimental work on cement paste systems reported in [13]. Starting with only cement and water, the model predicts the volumetric proportions of different phases in the cement paste depending on the degree of hydration, a. The outer volume of the system is as a good approximation assumed to be constant. Under sealed conditions, the capillary pores are emptied during hydration due to chemical shrinkage, but they can remain water-filled, if water can be imbibed from the surroundings (open system). Powers’ model shows several important implications. Firstly, in a sealed system at a low w/c-ratio (<0.42), complete hydration of the cement is not possible (amax < 1), as the volume of capillary water reaches 0 before a reaches 1. Secondly, if a system with low w/c ratio (<0.42) is in contact with a water reservoir, this can supply water for further hydration and thereby increase amax compared to the sealed situation with no reservoir. However, as hydration products do not grow into the relatively large SAP voids, hydration only proceeds as long as there is space for the cement gel formed (gel solid and gel water), i.e. in the space originally occupied by capillary water. For w/c < 0.36, it is theoretically possible for the cement gel to fill up all available space. Therefore hydration will stop at amax < 1, even though there is at the same time unhydrated cement and free water left in the reservoir. All this knowledge can be used, when SAP is added to a concrete mix: SAP particles absorb water in the fresh concrete and create a lot of small water reservoirs in the concrete. As long as there is water in the SAP reservoirs, the concrete does not desiccate, and it is thereby possible to mitigate autogenous shrinkage. Powers’ model also helps to understand what will happen to e.g. compressive strength, when SAP is added. It seems reasonable to assume a relation between the denseness of the cement paste and the strength. Powers’ suggested an empirical function, which relates compressive strength to the gel space ratio, which he defined as the ratio between volume of gel solid and the volume of available space for it [14]:
fc ¼ A X 3
ð1Þ
where fc is the compressive strength of the paste, A is a constant and X is the gel space ratio. So, when SAP is added to concrete with w/c < 0.42, it increases amax and the amount of gel solid formed and thereby the gel space ratio, so it increases the strength of the cement paste. But at the same time, the reservoirs create voids, and no matter if they are empty or filled with water, this will reduce the strength. These two opposite effects may balance each other or one of them may
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Fig. 1. Powers’ model in 3 different situations. (A) Closed (sealed) system, w/c = 0.32: Hydration stops due to lack of free water. The maximum degree of hydration amax is 0.71. This situation corresponds to mix composition REF in experiments by Craeye et al. [4]. (B) Closed system with a reservoir of internal curing water, w/c = 0.32 and we/c = 0.19: Hydration stops, even though there is access to free water, because there is a lack of space for gel growth. The maximum degree of hydration amax is 0.83, i.e. higher than in situation A, but the pore volume has more than tripled. This situation corresponds to mix composition SAP90 in experiments by Craeye et al. [4], assuming a SAP absorption capacity of 45 g/g. (C) Closed system with a reservoir of internal curing water, w/c = 0.45 and we/c = 0.06: As w/c > 0.42, full hydration (amax = 1) is possible without entrained water, so entrained water only contributes to creating pore volume. Compared to situation B, the total volume is unchanged, but the pore volume is smaller and the paste volume is higher. Moreover, from the starting point, the paste phases have different w/c ratios, i.e. 0.32 in situation B and 0.45 in situation C. This situation corresponds to mix composition SAP90 in experiments by Craeye et al. [4], assuming a SAP absorption capacity of 12.5 g/g. (Both the illustration of situation B and C assume that capillary pores are fully saturated, until internal curing water in the reservoir has been used.)
be predominant. We have carried out experiments with measurement of compressive strength [5], for concrete with w/c = 0.35, 0.40, and 0.50, and different dosages of SAP. The results showed that as long as SAP addition increases the degree of hydration, it results in a strength increase. But beyond the point, where SAP addition increases the degree of hydration, it leads to a reduction of strength. This is the case when w/c is high (>0.42) where water from the reservoir is not needed at all, and at low w/c if the reservoirs created by SAP contain more water than can be used for hydration. Below is a graphical representation of Powers’ model for cement paste with w/c = 0.32, see Fig. 1A. This corresponds to the reference concrete in the study of Craeye et al., neglecting the silica fume. Silica fume only accounts for 5% of the powder weight, and it has been left out here for simplicity. In Fig. 1B, the model is extended with a reservoir of entrained water. The size of the reservoir corresponds to the mix of Craeye et al. labeled SAP90 with 90 l entrained water per m3 concrete. As can be seen from the model, contact to a water reservoir will increase amax from 0.71 to 0.83, but when hydration stops, the amount of water used from the reservoir only equals 27 l entrained water per m3 concrete. This is also mentioned in the paper by Craeye et al. But still Craeye et al. choose to entrain 50, 70, and 90 l per m3 concrete by the use of SAP. Craeye et al. substantiate that Powers’ model underestimates the amount of water for internal curing, because the ‘‘travel distance’’ for internal curing water should not exceed 200 lm, a distance suggested in [15] based on computer simulations. However, both the suggested ‘‘travel distance’’ of 200 lm and the assumption of underestimation of internal curing water by Powers’ model are in contradiction with other experimental results. It was experimentally demonstrated by several researchers that the amount of internal curing water calculated according to Powers’ model allows significant reduction, and in some cases, complete elimination of autogenous shrinkage [2,16]. Furthermore, the ‘‘travel distance’’ does not need to be as low as 200 lm, it may be up to several millimeters as shown by
both circumstantial evidence [17] and by direct measurements [18]. Because Craeye et al. aims at keeping the travel distance low, they increase the amount of SAP to establish more internal reservoirs. Therefore, with the additional internal curing water required by the SAP, the mixes SAP50, SAP70, and SAP90 get void volumes equivalent to 5%, 7%, and 9% of the concrete volume, respectively. As a rule of thumb, each % of air reduces the compressive strength approximately 5% relative to air-free concrete [19]. So when only approximately 30 l 3% is needed for internal curing, there will be an extra, unnecessary void volume of 2%, 4%, and 6% in the 3 mixes with SAP. According to the rule of thumb, this corresponds to a strength loss in the order of 10%, 20%, and 30%, which is the same magnitude of strength reduction as registered by Craeye et al. (15–28% [4]). 4. The absorption capacity of SAP Super absorbent polymers are called ‘‘super’’, because they can absorb a substantial amount of water, e.g. 1000 times their own weight or even more. However, SAP is just a generic term for a group of materials, where not all group members are equally efficient. Moreover, the absorption capacity may be very dependent on the fluid to be absorbed. The extreme absorption capacities only hold for pure water (distilled or demineralized water). The presence of ions lowers the absorption capacity, especially the presence of divalent ions like Ca++ [20]. The paper of Craeye et al. states a value for SAP absorption capacity equal to 45 g/g SAP. There is no reference for this value or explanation on how it is measured. 45 g/g may hold for absorption of tap water, but it is a very high value for absorption in fresh concrete due to ions in the pore solution. The absorption capacity for SAP in concrete reported in [2] is only 12.5 g/g for SAP type A, which is a suspension polymerized, covalently cross linked acrylamide/acrylic acid copolymer, i.e. same type of SAP as used by Craeye et al. This
M.T. Hasholt et al. / Construction and Building Materials 31 (2012) 226–230
value was based on direct observations of void sizes at cut surfaces of paste samples and the value was supported by absorption experiments with synthetic pore solution. Later, this value was confirmed, e.g. by CT scanning of hardened concrete samples with SAP [21]. If the assumed absorption capacity of SAP is higher than the actual absorption capacity, the extra added water contributes to the w/c ratio. For example, if the actual absorption capacity is only 12.5 g/g and using the quantities stated for mix composition in the paper by Craye et al., the true w/c of SAP90 mix is not 0.32, but rather as follows:
w=c ¼
mwater þ 4512:5 minternal curing water 150 þ 4512:5 90 45 45 ¼ 0:45 ¼ 475 mcement ð2Þ
Powers’ model shows what happens, if the assumed absorption capacity is wrong. If Powers’ model for concrete mix SAP90 in Fig. 1B is redrawn to account for a correction of absorption capacity from 45 to 12.5 g/g, it looks as shown in Fig. 1C. It can be seen that now the SAP reservoir volume is much smaller, as only part of the extra added water is absorbed by SAP (entrained water). The water that is not absorbed by SAP is part of mixing water and contributes to capillary porosity. If it is correct that the absorption capacity is only 12.5 g/g, then the differences between the reference concrete and the mixes containing SAP, for example regarding the mechanical properties, are more likely to be due to differences in w/c ratio than due to the SAP addition. In the results reported by Craeye et al., there are indications that the w/c ratio is higher due to a SAP absorption capacity lower than 45 g/g: In the study by Craeye et al. it was necessary to adjust the amount of superplasticizer to obtain mixes with comparable consistency; mixes with SAP contained less superplasticizer than the reference mix. If the assumed absorption capacity is correct, the volume of dry SAP-particles and internal curing water replace the same volume of sand, so in principle the workability should be virtually unchanged, as the (semi-)hard swollen SAP-particles replace hard sand particles in the same size category. But if the absorption capacity is lower than assumed, then the paste volume has increased and w/c has increased, so the cement paste is much more fluid. Both increased paste volume and increased w/c will result in concrete with higher slump, if the dosage of superplasticizer is not reduced. This situation is opposite to the situation in [22], where SAP is added to mortar without adding extra water; then both the paste content and the w/c of the paste is decreased, and this results in a less fluid mortar. In the study by Craeye et al. concrete with SAP seems to have a much quicker heat development than the reference concrete. As all mixes hold the same amount of cement, the cumulated heat [kJ/m3] reflects the degree of hydration. It is surprising that the rate of the hydration reactions is higher in concrete with SAP in the first hours after casting. If all mixes from the beginning have the same w/c in the paste phase, then the course of hydration should be identical until a later stage, where the availability of capillary water becomes the limiting factor. Only from this point a difference in degree of hydration (and heat development) is expected. In the study on SAP and mechanical properties carried out at DTU [5], for specimens cured at 25 °C, there was no significant difference in degree of hydration before 3 days after mixing. The hydration is probably slower at 25 °C than in the study by Craeye et al., as in the adiabatic test, the temperature quickly exceeds 25 °C, but even so, the difference observed by Craeye et al. a few hours after mixing is remark-
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able. But if the mixes have different w/c, hydration is normally quicker, the higher the w/c ratio, see e.g. [23,24]. Like this, the heat development curves of Craeye et al. support that they are measured for mixes with different w/c ratios. However, the reason why the reference mix lacks behind may also be that it has the highest dosage of superplasticizer of all mixes, as some types of superplasticizer have a retarding effect. Due to the above mentioned circumstances, it seems that the differences in properties observed for the mixes in the study of Craeye et al., at least the mechanical properties, are caused by differences in w/c ratios. 5. Efficiency of SAP as internal curing agent Internal curing as a mitigation strategy for autogenous shrinkage is relatively new in concrete technology, and SAP is an interesting candidate, which may become a popular internal curing agent in tomorrow’s concrete construction. The main question asked by both concrete scientists and practitioners is whether SAP can mitigate autogenous shrinkage in concrete. This is a question of efficiency. A number of experimental works proves that SAP can effectively reduce autogenous shrinkage, e.g. [2]. A thorough review is expected in the RILEM state-of-the-art report mentioned earlier [3]. In the view of the above, it may be questioned why Craeye et al. register substantial autogenous shrinkage for the SAP90 mix: At w/c = 0.45, there should be little self-desiccation and related selfdesiccation shrinkage in a sealed system. However, this observation may be due to the specific testing procedure for measurement of autogenous shrinkage. For example, the results rely on a definition of zero deformation 6 h after casting, which to some extent is arbitrary (by definition, the deformation is zero at set time; set time varies for the different mixes, but it is difficult to measure [25]). The settlement of the fresh mix may give some problems with the vertical position of the measuring surface, which could have been avoided if the experiments had been carried out horizontally [26]. In addition, part of the observed early-age shrinkage may be associated with temperature reduction, which cannot be estimated by the reader, since neither temperature history in the shrinkage specimen, or procedure for accounting for changing temperature in the given autogenous shrinkage curves are provided. 6. Conclusions Craeye et al. reach a misleading conclusion, when they state that: ‘‘Addition of SAP leads to a significant reduction of mechanical strength’’. As shown in the present discussion internal curing with SAP can mitigate autogenous deformation with a minimal loss of compressive strength, and there may even be a small strength gain as shown by other experimental studies. As shown in this paper, the conclusion of Craeye et al. is based on a study, where the use of SAP is not optimal, i.e. the amount of water added for internal curing is too large. Furthermore, the conclusion seems to be incorrect due to an erroneous assumption regarding the absorption capacity of the SAP used, leading to comparison of mixes with different w/c ratios. References [1] Jensen OM, Hansen PF. Water-entrained cement-based materials – I: principle and theoretical background. Cem Concr Res 2001;31:647–54. [2] Jensen OM, Hansen PF. Water-entrained cement-based materials – II: implementation and experimental results. Cem Concr Res 2002;32:973–8. [3] RILEM TC 225-SAP. Application of super absorbent polymers in concrete construction. Chairman: V Mechtcherine.
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[4] Craeye B, Geirnaert M, De Schutter G. Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks. Constr Build Mater 2011;25:1–13. [5] Hasholt MT, Jespersen MHS, Jensen OM. Mechanical properties of concrete with SAP. Part I: Development of compressive strength. In: Jensen OM, Hasholt MT, Laustsen S, editors. RILEM Proc. PRO 74 ‘‘Use of superabsorbent polymers and other new additives in concrete’’, 15–18 August, 2010, Lyngby, Denmark. Bagneux (France): RILEM Publications SARL; 2010. p. 117–26. [6] Lam Hoa, Hooton RD. Effects of internal curing methods on restrained shrinkage and permeability. In: Persson B, Bentz D, Nilsson L-O, editors. Proc. 4th int. sem. on self-desiccation and its importance in concrete technology. Lund, (Sweden): Lund University; 2005. p. 210–28. [7] Piérard J, Pollet V, Cauberg N. Mitigating autogenous shrinkage in HPC by internal curing using superabsorbent polymers. In: Jensen OM, Lura P, Kovler K, editors. RILEM proc. PRO 52, volume changes of hardening concrete: testing and mitigation. Bagneux (France): RILEM Publications SARL; 2006. p. 97–106. [8] Lura P, Durand F, Loukili A, Kovler K, Jensen OM. Compressive strength of cement pastes and mortars with suberabsorbent polymers. In: Jensen OM, Lura P, Kovler K, editors. RILEM proc. PRO 52, volume changes of hardening concrete: testing and mitigation. Bagneux (France): RILEM Publications SARL; 2006. p. 117–26. [9] Esteves LP, Cachim P, Ferreira VM. Mechanical properties of cement mortars with superabsorbent polymers. Adv Constr Mater 2007:451–62. [10] Mechtcherine V, Dudziak L, Hempel S. Mitigating early age shrinkage of ultrahigh performance concrete by using super absorbent polymers (SAP). In: Tanabe T, et al., editors. Creep, shrinkage and durability mechanics of concrete and concrete structures – CONCREEP-8. London, UK: Taylor & Francis Group; 2009. p. 847–53. [11] Gao D, Heimann RB, Alexander SDB. Box-behnken design applied to study the strengthening of aluminate concrete modified by a superabsorbent polymer/ clay composite. Adv Chem Res 1997;9:93–7. [12] Bentz DP, Geiker M, Jensen OM. On the mitigation of early age cracking. In: Persson B, Fagerlund G, editors. Proceedings ‘‘3rd international seminar on self-desiccation and its importance in concrete technology. Sweden; 2002. p. 195–204. [13] Powers TC, Brownyard TL. Studies of the physical properties of hardened Portland cement paste (PCA Bulletin 22). Portland Cement Association; 1948. [14] Powers TC. The physical structure and engineering properties of concrete (PCA Bulletin 90). Portland Cement Association; 1958.
[15] Bentz DP, Snyder KA. Protected paste volume in concrete. Extension to internal curing using saturated lightweight fine aggregate. Cem Concr Res 1999;29:1863–7. [16] Lura P, Durand F, Jensen OM. Autogenous strain of cement pastes with superabsorbent polymers. In: Jensen OM, Lura P, Kovler K, editors. RILEM proc. PRO 52, volume changes of hardening concrete: testing and mitigation. Bagneux (France): RILEM Publications SARL; 2006. p. 57–65. [17] Zhutovsky S, Kovler K, Bentur A. Assessment of water migration distance in internal curing of high strength concrete. American Concrete Institute, Special Publication SP-220; 2004. p. 181–97. [18] Lura P, Bentz DP, Lange DA, Kovler K, Bentur A, van Breugel K. Measurement of water transport from saturated pumice aggregates to hardening cement paste. Mater Struct 2006;39:861–8. [19] Popovics S. Strength and related properties of concrete – a quantitative approach. John Wiley & Sons Inc.; 1998. [20] Buchholz FL, Graham AT. Modern superabsorbent polymer technology. Wiley VCH; 1998. [21] Laustsen S, Bentz DP, Hasholt MT, Jensen OM. CT measurement of SAP voids in concrete. In: Jensen OM, Hasholt MT, Laustsen S, editors. RILEM Proc PRO 74 ‘‘Use of superabsorbent polymers and other new additives in concrete’’, 15–18 August, 2010, Lyngby, Denmark. Bagneux (France): RILEM Publications SARL; 2010. p. 153–62. [22] Paiva H, Esteves LP, Cachim PB, Ferreira VM. Rheology and hardened properties of single-coat render mortars with different types of water retaining agents. Constr Build Mater 2009;23:1141–6. [23] Geiker M, Knudsen T. Chemical shrinkage of Portland cement paste. Cem Concr Res 1982;12:603–10. [24] Bentz DP. A three-dimensional cement hydration and microstructure program. In: Hydration rate, heat of hydration, and chemical shrinkage. Report NISTIR 5756. Building and Fire Research Laboratory, National Institute of Technology; 1995. [25] Weiss J. Experimental determination of the ‘‘time zero’’ t0 (maturity zero M0). In: Bentur A, editor. Chapter 6.1 in report of RILEM technical committee 181EAS: early age cracking in cementitious systems. Bagneux, (France): RILEM Publications SARL; 2003. p. 195–206. [26] Qian T, Jensen OM. Measuring autogenous strain of concrete with corrugated moulds. In: Sun W, Van Breugel K, Miao C, Ye G, Chen H, editors. RILEM proc. PRO 61 ‘‘1st international conference on microstructure related durability of cementitious composites’’, October 13–15, 2008, Nanjing, China. Bagneux (France): RILEM Publications SARL; 2008. p. 1501–11.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Can superabsorent polymers mitigate autogenous shrinkage of internally cured concrete without compromising the strength? q Marianne Tange Hasholt a,⇑, Ole Mejlhede Jensen a, Konstantin Kovler b, Semion Zhutovsky b a b
Department of Civil Engineering, Technical University of Denmark, Building 118, Brovej, DK-2800 Lyngby, Denmark National Building Research Institute, Faculty of Civil and Environmental Engineering, Technion – Israel Institute of Technology, 32000 Haifa, Israel
a r t i c l e
i n f o
Article history: Received 6 July 2011 Received in revised form 15 December 2011 Accepted 19 December 2011 Available online 25 January 2012 Keywords: Concrete Superabsorbent polymers Internal curing Self-desiccation Autogenous shrinkage Compressive strength
a b s t r a c t The paper ‘‘Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks’’ deals with different aspects of using superabsorbent polymers (SAP) in concrete to mitigate self-desiccation. The paper concludes that ‘‘Addition of SAP leads to a significant reduction of mechanical strength’’. The experimental results are in contradiction with several publications and question the appropriateness of using SAP as internal curing agent. However, the observed strength loss – and possibly also other observations – seems to be caused by overestimation of SAP water absorption. This results in an increase in water/cement ratio (w/c) for concrete with SAP. It is misleading to conclude on how SAP influences concrete properties, based on comparison of concrete mixes with SAP and reference concrete without SAP, if SAP mixes have higher w/c than the reference mix. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cementitious materials with low water/cement (w/c) or low water/binder (w/b) ratios are vulnerable to early age cracking due to autogenous deformation caused by self-desiccation. A decade ago, it was suggested and demonstrated that this problem can be solved by mixing superabsorbent polymers (SAP) into the fresh concrete, thereby establishing small reservoirs of internal curing water [1,2]. In the wake of this study, a number of studies have confirmed these findings, as well as examined how SAP influences different concrete properties. A RILEM technical committee is formed to work on the application of SAP in concrete, aiming at among other things recommendations for the use of SAP [3]. To the knowledge of the authors, this RILEM technical committee will also in near future publish a state-of-the-art report, which reviews research studies concerning SAP in concrete from the very first study and up till today. Like this, the topic ‘‘SAP in concrete’’ is a research field with a lot of ongoing activities. It is of course important to ensure that the solution to one problem, here self-desiccation, does not create new problems. When a
q Discussion of the paper ‘‘Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks’’ by Bart Craeye, Matthew Geirnaert, and Geert De Schutter. ⇑ Corresponding author. Tel.: +45 4525 1702. E-mail address: [email protected] (M.T. Hasholt).
0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.12.062
new component like SAP is introduced into the concrete matrix, it is important to critically investigate how this component influences concrete properties. In view of this, the paper by Craeye et al. [4] recently published in Construction and Building Materials is very relevant. Besides autogenous deformation, the authors also examine how SAP addition influences workability of the fresh concrete, mechanical properties and heat development during hydration. Moreover, they simulate stress build-up at early age when casting a bridge deck, where deformation is restrained, thereby testing if the reduction of autogenous deformation is sufficient to avoid crack formation. One of the conclusions of Craeye et al. [4] is that ‘‘Addition of SAP leads to a significant reduction of mechanical strength’’. In comparison with a reference mix made at w/b 0.30, the 28-days compressive strength of 150 mm cubes cured at 20 °C and 90% relative humidity was reduced by 15–28%, depending on the SAP content (0.2% and 0.4% dry SAP relative to cement mass). The higher the SAP dosage, the lower was the 28-day strength. These values of the reported strength loss are indeed high and can compromise the application of superabsorbent polymers as internal curing agents in concrete construction. However, recent results from research made at DTU [5] show that the dependence of strength on the SAP content is more complicated than just a simple relation, where SAP addition leads to strength loss. Actually, in some cases addition of SAP can increase compressive strength, and therefore the results and conclusions by Craeye et al. need to be discussed in more detail.
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2. Previous observations regarding the influence of SAP on compressive strength In the literature, information about the influence of SAP on concrete strength is not extensive and the reported results may seem contradictory. With regard to the 28-days compressive strength, some publications report reduction of compressive strength of mixes containing SAP, in comparison with reference mixes without SAP, whereas other publications demonstrate almost unchanged or even higher strength: Lam and Hooton [6]: The addition of 0.3% SAP (relative to cement mass) to the concrete mix dramatically reduced the compressive strength by more than 50% compared to the control mix made at w/c = 0.35. However, by doubling the SAP dosage, most of the strength loss was prevented. The strength of the mix with 0.6% SAP made at w/c = 0.35 was approximately equal to the strength of another reference mix made at w/c = 0.45. In both SAP mixes, extra water was added. It is common practice to add extra water together with SAP to account for the amount of water absorbed by SAP in the fresh concrete, see Section 3, and therefore the amount of extra water is directly proportional to the amount of SAP. Contrary to this practice, in this study, the content of extra water was the same in both mixes with SAP (equivalent to 10% of cement mass). The lower strength of the system with 0.3% SAP may be due to a too high content of extra water, twice as much as that introduced per unit mass of superabsorbent polymer in the case of 0.6% SAP. Piérard et al. [7]: The strength of concrete made at w/c = 0.35 without SAP and with SAP contents of 0.3% and 0.6% (and extra water equivalent to 2% and 4% of cement mass, respectively), was measured on cubes cured at 20 ± 2 °C and minimum air relative humidity of 95% at ages of 2–28 days. Results showed that the early strength development (2–7 days) was slowed down with SAP, but the reduction in strength decreased at later ages. After 28 days, the reductions in strength were 7% and 13% for concrete mixes with SAP contents of 0.3% and 0.6%, respectively. Lura et al. [8]: For w/c = 0.30, internal curing by means of 0.4% SAP (extra water equivalent to 5% of cement mass) had almost no influence on the compressive strength of mortars, while the strength of cement pastes was reduced by 20% at early ages (up to 7 days) and by 10% at later ages (28 and 56 days). Esteves et al. [9]: In this work mortars with w/c at 0.25, 0.30, and 0.35 were tested. For each w/c, mixes were made without SAP and 0.2% SAP (extra water equivalent to 5% of cement mass), and specimens were cured at 30%, 50%, and 95% relative humidity. Results after 28 days of curing at 95% relative humidity showed a 15–20% reduction of compressive strength for mortar with SAP. But where the strength dropped at lower relative humidity for mortar without SAP, mortar with SAP had almost constant compressive strength no matter the curing conditions. At 30% relative humidity, the strength reduction for mortar with SAP was only 5%. Mechtcherine et al. [10]: Compressive strength was reported for ultra high performance SAP-containing mortars (w/c = 0.22, SAP content 0% (reference mix), 0.3%, and 0.6% relative to cement mass). Extra water was added to SAP-containing mixes to compensate the loss of workability. Noticeable decrease in compressive strength was observed at early ages, e.g. at 7 days, where the strength reductions were 12% and 30% for 0.3% and 0.6% mixes, respectively. At 28 days measurements exhibited only a minor decrease in strength for the 0.3% mixes: 4% (not significant). At 28 days, the strength reduction for the 0.6% mix was 20%.
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Gao et al. [11]: An increase in compressive strength was observed, when SAP was added to the aluminate cement paste made at w/c = 0.40, but without adding extra water: from 36.1 MPa (0% SAP) up to 40.5 MPa (0.2% SAP) and 44.4 MPa (0.6% SAP). Bentz et al. [12]: Measurements of compressive strength development were carried out for mortar mixes with w/b = 0.35, with and without SAP (0.4% relative to binder mass). After 7 days, the compressive strength of mortar with SAP was lower than the strength of a reference mortar without SAP, 53 MPa vs. 57 MPa. After 28 days of curing, the picture had changed; mortar with SAP showed higher compressive strength than the reference mortar, 73 MPa vs. 61 MPa (values for compressive strength are approximates, as mixes have been tested at slightly different age, and test results have been interpolated afterwards). 3. Mix design for internal curing The basic principles of internal curing mix design using Powers’ model as design tool were described in [1]. Powers’ model itself is based on the comprehensive experimental work on cement paste systems reported in [13]. Starting with only cement and water, the model predicts the volumetric proportions of different phases in the cement paste depending on the degree of hydration, a. The outer volume of the system is as a good approximation assumed to be constant. Under sealed conditions, the capillary pores are emptied during hydration due to chemical shrinkage, but they can remain water-filled, if water can be imbibed from the surroundings (open system). Powers’ model shows several important implications. Firstly, in a sealed system at a low w/c-ratio (<0.42), complete hydration of the cement is not possible (amax < 1), as the volume of capillary water reaches 0 before a reaches 1. Secondly, if a system with low w/c ratio (<0.42) is in contact with a water reservoir, this can supply water for further hydration and thereby increase amax compared to the sealed situation with no reservoir. However, as hydration products do not grow into the relatively large SAP voids, hydration only proceeds as long as there is space for the cement gel formed (gel solid and gel water), i.e. in the space originally occupied by capillary water. For w/c < 0.36, it is theoretically possible for the cement gel to fill up all available space. Therefore hydration will stop at amax < 1, even though there is at the same time unhydrated cement and free water left in the reservoir. All this knowledge can be used, when SAP is added to a concrete mix: SAP particles absorb water in the fresh concrete and create a lot of small water reservoirs in the concrete. As long as there is water in the SAP reservoirs, the concrete does not desiccate, and it is thereby possible to mitigate autogenous shrinkage. Powers’ model also helps to understand what will happen to e.g. compressive strength, when SAP is added. It seems reasonable to assume a relation between the denseness of the cement paste and the strength. Powers’ suggested an empirical function, which relates compressive strength to the gel space ratio, which he defined as the ratio between volume of gel solid and the volume of available space for it [14]:
fc ¼ A X 3
ð1Þ
where fc is the compressive strength of the paste, A is a constant and X is the gel space ratio. So, when SAP is added to concrete with w/c < 0.42, it increases amax and the amount of gel solid formed and thereby the gel space ratio, so it increases the strength of the cement paste. But at the same time, the reservoirs create voids, and no matter if they are empty or filled with water, this will reduce the strength. These two opposite effects may balance each other or one of them may
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Fig. 1. Powers’ model in 3 different situations. (A) Closed (sealed) system, w/c = 0.32: Hydration stops due to lack of free water. The maximum degree of hydration amax is 0.71. This situation corresponds to mix composition REF in experiments by Craeye et al. [4]. (B) Closed system with a reservoir of internal curing water, w/c = 0.32 and we/c = 0.19: Hydration stops, even though there is access to free water, because there is a lack of space for gel growth. The maximum degree of hydration amax is 0.83, i.e. higher than in situation A, but the pore volume has more than tripled. This situation corresponds to mix composition SAP90 in experiments by Craeye et al. [4], assuming a SAP absorption capacity of 45 g/g. (C) Closed system with a reservoir of internal curing water, w/c = 0.45 and we/c = 0.06: As w/c > 0.42, full hydration (amax = 1) is possible without entrained water, so entrained water only contributes to creating pore volume. Compared to situation B, the total volume is unchanged, but the pore volume is smaller and the paste volume is higher. Moreover, from the starting point, the paste phases have different w/c ratios, i.e. 0.32 in situation B and 0.45 in situation C. This situation corresponds to mix composition SAP90 in experiments by Craeye et al. [4], assuming a SAP absorption capacity of 12.5 g/g. (Both the illustration of situation B and C assume that capillary pores are fully saturated, until internal curing water in the reservoir has been used.)
be predominant. We have carried out experiments with measurement of compressive strength [5], for concrete with w/c = 0.35, 0.40, and 0.50, and different dosages of SAP. The results showed that as long as SAP addition increases the degree of hydration, it results in a strength increase. But beyond the point, where SAP addition increases the degree of hydration, it leads to a reduction of strength. This is the case when w/c is high (>0.42) where water from the reservoir is not needed at all, and at low w/c if the reservoirs created by SAP contain more water than can be used for hydration. Below is a graphical representation of Powers’ model for cement paste with w/c = 0.32, see Fig. 1A. This corresponds to the reference concrete in the study of Craeye et al., neglecting the silica fume. Silica fume only accounts for 5% of the powder weight, and it has been left out here for simplicity. In Fig. 1B, the model is extended with a reservoir of entrained water. The size of the reservoir corresponds to the mix of Craeye et al. labeled SAP90 with 90 l entrained water per m3 concrete. As can be seen from the model, contact to a water reservoir will increase amax from 0.71 to 0.83, but when hydration stops, the amount of water used from the reservoir only equals 27 l entrained water per m3 concrete. This is also mentioned in the paper by Craeye et al. But still Craeye et al. choose to entrain 50, 70, and 90 l per m3 concrete by the use of SAP. Craeye et al. substantiate that Powers’ model underestimates the amount of water for internal curing, because the ‘‘travel distance’’ for internal curing water should not exceed 200 lm, a distance suggested in [15] based on computer simulations. However, both the suggested ‘‘travel distance’’ of 200 lm and the assumption of underestimation of internal curing water by Powers’ model are in contradiction with other experimental results. It was experimentally demonstrated by several researchers that the amount of internal curing water calculated according to Powers’ model allows significant reduction, and in some cases, complete elimination of autogenous shrinkage [2,16]. Furthermore, the ‘‘travel distance’’ does not need to be as low as 200 lm, it may be up to several millimeters as shown by
both circumstantial evidence [17] and by direct measurements [18]. Because Craeye et al. aims at keeping the travel distance low, they increase the amount of SAP to establish more internal reservoirs. Therefore, with the additional internal curing water required by the SAP, the mixes SAP50, SAP70, and SAP90 get void volumes equivalent to 5%, 7%, and 9% of the concrete volume, respectively. As a rule of thumb, each % of air reduces the compressive strength approximately 5% relative to air-free concrete [19]. So when only approximately 30 l 3% is needed for internal curing, there will be an extra, unnecessary void volume of 2%, 4%, and 6% in the 3 mixes with SAP. According to the rule of thumb, this corresponds to a strength loss in the order of 10%, 20%, and 30%, which is the same magnitude of strength reduction as registered by Craeye et al. (15–28% [4]). 4. The absorption capacity of SAP Super absorbent polymers are called ‘‘super’’, because they can absorb a substantial amount of water, e.g. 1000 times their own weight or even more. However, SAP is just a generic term for a group of materials, where not all group members are equally efficient. Moreover, the absorption capacity may be very dependent on the fluid to be absorbed. The extreme absorption capacities only hold for pure water (distilled or demineralized water). The presence of ions lowers the absorption capacity, especially the presence of divalent ions like Ca++ [20]. The paper of Craeye et al. states a value for SAP absorption capacity equal to 45 g/g SAP. There is no reference for this value or explanation on how it is measured. 45 g/g may hold for absorption of tap water, but it is a very high value for absorption in fresh concrete due to ions in the pore solution. The absorption capacity for SAP in concrete reported in [2] is only 12.5 g/g for SAP type A, which is a suspension polymerized, covalently cross linked acrylamide/acrylic acid copolymer, i.e. same type of SAP as used by Craeye et al. This
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value was based on direct observations of void sizes at cut surfaces of paste samples and the value was supported by absorption experiments with synthetic pore solution. Later, this value was confirmed, e.g. by CT scanning of hardened concrete samples with SAP [21]. If the assumed absorption capacity of SAP is higher than the actual absorption capacity, the extra added water contributes to the w/c ratio. For example, if the actual absorption capacity is only 12.5 g/g and using the quantities stated for mix composition in the paper by Craye et al., the true w/c of SAP90 mix is not 0.32, but rather as follows:
w=c ¼
mwater þ 4512:5 minternal curing water 150 þ 4512:5 90 45 45 ¼ 0:45 ¼ 475 mcement ð2Þ
Powers’ model shows what happens, if the assumed absorption capacity is wrong. If Powers’ model for concrete mix SAP90 in Fig. 1B is redrawn to account for a correction of absorption capacity from 45 to 12.5 g/g, it looks as shown in Fig. 1C. It can be seen that now the SAP reservoir volume is much smaller, as only part of the extra added water is absorbed by SAP (entrained water). The water that is not absorbed by SAP is part of mixing water and contributes to capillary porosity. If it is correct that the absorption capacity is only 12.5 g/g, then the differences between the reference concrete and the mixes containing SAP, for example regarding the mechanical properties, are more likely to be due to differences in w/c ratio than due to the SAP addition. In the results reported by Craeye et al., there are indications that the w/c ratio is higher due to a SAP absorption capacity lower than 45 g/g: In the study by Craeye et al. it was necessary to adjust the amount of superplasticizer to obtain mixes with comparable consistency; mixes with SAP contained less superplasticizer than the reference mix. If the assumed absorption capacity is correct, the volume of dry SAP-particles and internal curing water replace the same volume of sand, so in principle the workability should be virtually unchanged, as the (semi-)hard swollen SAP-particles replace hard sand particles in the same size category. But if the absorption capacity is lower than assumed, then the paste volume has increased and w/c has increased, so the cement paste is much more fluid. Both increased paste volume and increased w/c will result in concrete with higher slump, if the dosage of superplasticizer is not reduced. This situation is opposite to the situation in [22], where SAP is added to mortar without adding extra water; then both the paste content and the w/c of the paste is decreased, and this results in a less fluid mortar. In the study by Craeye et al. concrete with SAP seems to have a much quicker heat development than the reference concrete. As all mixes hold the same amount of cement, the cumulated heat [kJ/m3] reflects the degree of hydration. It is surprising that the rate of the hydration reactions is higher in concrete with SAP in the first hours after casting. If all mixes from the beginning have the same w/c in the paste phase, then the course of hydration should be identical until a later stage, where the availability of capillary water becomes the limiting factor. Only from this point a difference in degree of hydration (and heat development) is expected. In the study on SAP and mechanical properties carried out at DTU [5], for specimens cured at 25 °C, there was no significant difference in degree of hydration before 3 days after mixing. The hydration is probably slower at 25 °C than in the study by Craeye et al., as in the adiabatic test, the temperature quickly exceeds 25 °C, but even so, the difference observed by Craeye et al. a few hours after mixing is remark-
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able. But if the mixes have different w/c, hydration is normally quicker, the higher the w/c ratio, see e.g. [23,24]. Like this, the heat development curves of Craeye et al. support that they are measured for mixes with different w/c ratios. However, the reason why the reference mix lacks behind may also be that it has the highest dosage of superplasticizer of all mixes, as some types of superplasticizer have a retarding effect. Due to the above mentioned circumstances, it seems that the differences in properties observed for the mixes in the study of Craeye et al., at least the mechanical properties, are caused by differences in w/c ratios. 5. Efficiency of SAP as internal curing agent Internal curing as a mitigation strategy for autogenous shrinkage is relatively new in concrete technology, and SAP is an interesting candidate, which may become a popular internal curing agent in tomorrow’s concrete construction. The main question asked by both concrete scientists and practitioners is whether SAP can mitigate autogenous shrinkage in concrete. This is a question of efficiency. A number of experimental works proves that SAP can effectively reduce autogenous shrinkage, e.g. [2]. A thorough review is expected in the RILEM state-of-the-art report mentioned earlier [3]. In the view of the above, it may be questioned why Craeye et al. register substantial autogenous shrinkage for the SAP90 mix: At w/c = 0.45, there should be little self-desiccation and related selfdesiccation shrinkage in a sealed system. However, this observation may be due to the specific testing procedure for measurement of autogenous shrinkage. For example, the results rely on a definition of zero deformation 6 h after casting, which to some extent is arbitrary (by definition, the deformation is zero at set time; set time varies for the different mixes, but it is difficult to measure [25]). The settlement of the fresh mix may give some problems with the vertical position of the measuring surface, which could have been avoided if the experiments had been carried out horizontally [26]. In addition, part of the observed early-age shrinkage may be associated with temperature reduction, which cannot be estimated by the reader, since neither temperature history in the shrinkage specimen, or procedure for accounting for changing temperature in the given autogenous shrinkage curves are provided. 6. Conclusions Craeye et al. reach a misleading conclusion, when they state that: ‘‘Addition of SAP leads to a significant reduction of mechanical strength’’. As shown in the present discussion internal curing with SAP can mitigate autogenous deformation with a minimal loss of compressive strength, and there may even be a small strength gain as shown by other experimental studies. As shown in this paper, the conclusion of Craeye et al. is based on a study, where the use of SAP is not optimal, i.e. the amount of water added for internal curing is too large. Furthermore, the conclusion seems to be incorrect due to an erroneous assumption regarding the absorption capacity of the SAP used, leading to comparison of mixes with different w/c ratios. References [1] Jensen OM, Hansen PF. Water-entrained cement-based materials – I: principle and theoretical background. Cem Concr Res 2001;31:647–54. [2] Jensen OM, Hansen PF. Water-entrained cement-based materials – II: implementation and experimental results. Cem Concr Res 2002;32:973–8. [3] RILEM TC 225-SAP. Application of super absorbent polymers in concrete construction. Chairman: V Mechtcherine.
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[4] Craeye B, Geirnaert M, De Schutter G. Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks. Constr Build Mater 2011;25:1–13. [5] Hasholt MT, Jespersen MHS, Jensen OM. Mechanical properties of concrete with SAP. Part I: Development of compressive strength. In: Jensen OM, Hasholt MT, Laustsen S, editors. RILEM Proc. PRO 74 ‘‘Use of superabsorbent polymers and other new additives in concrete’’, 15–18 August, 2010, Lyngby, Denmark. Bagneux (France): RILEM Publications SARL; 2010. p. 117–26. [6] Lam Hoa, Hooton RD. Effects of internal curing methods on restrained shrinkage and permeability. In: Persson B, Bentz D, Nilsson L-O, editors. Proc. 4th int. sem. on self-desiccation and its importance in concrete technology. Lund, (Sweden): Lund University; 2005. p. 210–28. [7] Piérard J, Pollet V, Cauberg N. Mitigating autogenous shrinkage in HPC by internal curing using superabsorbent polymers. In: Jensen OM, Lura P, Kovler K, editors. RILEM proc. PRO 52, volume changes of hardening concrete: testing and mitigation. Bagneux (France): RILEM Publications SARL; 2006. p. 97–106. [8] Lura P, Durand F, Loukili A, Kovler K, Jensen OM. Compressive strength of cement pastes and mortars with suberabsorbent polymers. In: Jensen OM, Lura P, Kovler K, editors. RILEM proc. PRO 52, volume changes of hardening concrete: testing and mitigation. Bagneux (France): RILEM Publications SARL; 2006. p. 117–26. [9] Esteves LP, Cachim P, Ferreira VM. Mechanical properties of cement mortars with superabsorbent polymers. Adv Constr Mater 2007:451–62. [10] Mechtcherine V, Dudziak L, Hempel S. Mitigating early age shrinkage of ultrahigh performance concrete by using super absorbent polymers (SAP). In: Tanabe T, et al., editors. Creep, shrinkage and durability mechanics of concrete and concrete structures – CONCREEP-8. London, UK: Taylor & Francis Group; 2009. p. 847–53. [11] Gao D, Heimann RB, Alexander SDB. Box-behnken design applied to study the strengthening of aluminate concrete modified by a superabsorbent polymer/ clay composite. Adv Chem Res 1997;9:93–7. [12] Bentz DP, Geiker M, Jensen OM. On the mitigation of early age cracking. In: Persson B, Fagerlund G, editors. Proceedings ‘‘3rd international seminar on self-desiccation and its importance in concrete technology. Sweden; 2002. p. 195–204. [13] Powers TC, Brownyard TL. Studies of the physical properties of hardened Portland cement paste (PCA Bulletin 22). Portland Cement Association; 1948. [14] Powers TC. The physical structure and engineering properties of concrete (PCA Bulletin 90). Portland Cement Association; 1958.
[15] Bentz DP, Snyder KA. Protected paste volume in concrete. Extension to internal curing using saturated lightweight fine aggregate. Cem Concr Res 1999;29:1863–7. [16] Lura P, Durand F, Jensen OM. Autogenous strain of cement pastes with superabsorbent polymers. In: Jensen OM, Lura P, Kovler K, editors. RILEM proc. PRO 52, volume changes of hardening concrete: testing and mitigation. Bagneux (France): RILEM Publications SARL; 2006. p. 57–65. [17] Zhutovsky S, Kovler K, Bentur A. Assessment of water migration distance in internal curing of high strength concrete. American Concrete Institute, Special Publication SP-220; 2004. p. 181–97. [18] Lura P, Bentz DP, Lange DA, Kovler K, Bentur A, van Breugel K. Measurement of water transport from saturated pumice aggregates to hardening cement paste. Mater Struct 2006;39:861–8. [19] Popovics S. Strength and related properties of concrete – a quantitative approach. John Wiley & Sons Inc.; 1998. [20] Buchholz FL, Graham AT. Modern superabsorbent polymer technology. Wiley VCH; 1998. [21] Laustsen S, Bentz DP, Hasholt MT, Jensen OM. CT measurement of SAP voids in concrete. In: Jensen OM, Hasholt MT, Laustsen S, editors. RILEM Proc PRO 74 ‘‘Use of superabsorbent polymers and other new additives in concrete’’, 15–18 August, 2010, Lyngby, Denmark. Bagneux (France): RILEM Publications SARL; 2010. p. 153–62. [22] Paiva H, Esteves LP, Cachim PB, Ferreira VM. Rheology and hardened properties of single-coat render mortars with different types of water retaining agents. Constr Build Mater 2009;23:1141–6. [23] Geiker M, Knudsen T. Chemical shrinkage of Portland cement paste. Cem Concr Res 1982;12:603–10. [24] Bentz DP. A three-dimensional cement hydration and microstructure program. In: Hydration rate, heat of hydration, and chemical shrinkage. Report NISTIR 5756. Building and Fire Research Laboratory, National Institute of Technology; 1995. [25] Weiss J. Experimental determination of the ‘‘time zero’’ t0 (maturity zero M0). In: Bentur A, editor. Chapter 6.1 in report of RILEM technical committee 181EAS: early age cracking in cementitious systems. Bagneux, (France): RILEM Publications SARL; 2003. p. 195–206. [26] Qian T, Jensen OM. Measuring autogenous strain of concrete with corrugated moulds. In: Sun W, Van Breugel K, Miao C, Ye G, Chen H, editors. RILEM proc. PRO 61 ‘‘1st international conference on microstructure related durability of cementitious composites’’, October 13–15, 2008, Nanjing, China. Bagneux (France): RILEM Publications SARL; 2008. p. 1501–11.