MECHANICAL PROPERTIES OF CONCRETE WITH SAP PART I: DEVELOPMENT OF COMPRESSIVE STRENGTH

Transcription

1 MECHANICAL PROPERTIES OF CONCRETE WITH SAP PART I: DEVELOPMENT OF COMPRESSIVE STRENGTH Marianne Tange Hasholt (1), Morten H. Seneka Jespersen (1) and Ole Mejlhede Jensen (1) (1) Department of Civil Engineering, Technical University of Denmark, Lyngby, Denmark Abstract The development of mechanical properties has been studied in a test program comprising 15 different concrete mixes with 3 different w/c ratios and different additions of superabsorbent polymers (SAP). The degree of hydration is followed for 15 corresponding paste mixes. This paper concerns compressive strength. It shows that results agree well with a model based on the following: 1. Concrete compressive strength is proportional to compressive strength of the paste phase 2. Paste strength depends on gel space ratio, as suggested by Powers 3. The influence of air voids created by SAP on compressive strength can be accounted for in the same way as when taking the air content into account in Bolomeys formula. The implication of the model is that at low w/c ratios (w/c < 0.40) and moderate SAP additions, SAP increases the compressive strength at later ages (from 3 days after casting and onwards), because SAP increases the degree of hydration and therefore also the gel space ratio of the paste. However, at high w/c ratios (w/c > 0.45) and addition of large amounts of SAP, this effect cannot counterbalance the strength reducing effect of increased void volume. In these cases, SAP addition reduces the compressive strength. 1. Introduction Superabsorbent polymer (SAP) has been introduced in concrete mix design as a means of internal water curing of cementitious materials with low w/c ratios [1]. For this reason it is only natural that most research concerning SAP in concrete deals with the influence of SAP on autogenous RH change and autogenous deformation, see e.g. [2,3]. However, before SAP can be used in concrete production on a larger scale, it is important to clarify how SAP influences other concrete properties. As regards development of mechanical properties, so far most observations are made in studies, where knowledge about mechanical

2 properties is only a secondary goal. Often measurements only comprise compressive strength, and the number of measurements is insufficient to make proper tests of models. Moreover, observations of compressive strength are contradictory, some showing that SAP addition increases strength, others that SAP decreases strength. The main factors include [1,4,5,6], see table 1: Table 1: Reasons why SAP has an influence on concrete compressive strength. SAP increases compressive strength SAP reduces compressive strength SAP increases degree of hydration, thereby resulting in at denser paste phase Voids created by SAP lower the strength in the same way as traditional air voids SAP prevents microcracks caused by selfdessication, which would otherwise weaken the concrete Moisture condition influences the strength, i.e. higher strength has been observed at lower RH. As concrete with SAP in general has higher RH, it will show lower strength than concrete without SAP The objective of the present project is to study the development of strength and modulus of elasticity for concrete with SAP and see if results are in accordance with models that are already established for conventional concrete. Results concerning modulus of elasticity are described in another paper, see [7]. 2. Theory 2.1 Powers model In 1948 Powers and Brownyard published results showing that when 1 g of cement hydrates, 0.23 g water is bound chemically in the gel solid, and 0.19 g water is bound physically by the gel solid, i.e. it is adsorbed on gel solid surfaces [8]. The adsorbed water phase is called gel water. In this way, the distribution of phases in the hardening cement paste depends linearly on the degree of hydration (the above mentioned constants 0.23 and 0.19 can vary, depending on cement clinker composition). This model is known as Powers model. A graphical version is shown in figure 1 (left). This model predicts that for cement paste with w/c ratio lower than approx. 0.42, which hydrates in sealed conditions, complete hydration is not possible, i.e. the maximum degree of hydration α max < 1. Figure 1 (right) illustrates how Powers model can be extended to account for entrained water [4], e.g. water which in the fresh cement paste is present in swollen superabsorbent polymers. For cement paste with w/c ratio lower than 0.42, access to an entrained water source can increase α max.

3 Figure 1: Powers model for cement hydration in sealed conditions. Left: w/c ratio = Right: w/c ratio = 0.30, entrained water w e /c = Due to entrained water α max is increased. Illustrations from [4]. 2.2 Strength development In [8], in part 6 titled Relation of physical characteristics of the paste and compressive strength, Powers and Brownyard suggested a relation, which links paste strength and degree of hydration: It thus appears that the increase in strength is directly proportional to the increase in V m /w 0, regardless of age, water-cement ratio, or identity of cement (V m is proportional to the amount of non-evaporable water, i.e. water bound in cement paste, and w 0 is the initial water content in the cement paste). 10 years later, Powers modified this relation [9]: The compressive strength of the hardening cement paste depends on the amount of gel and the space available for it, and he suggests the following empirical power function: f c 3 A X, X volume of gel volume of space (1) where A is a constant, and X is the gel so-called gel space ratio. The constant A can be interpreted as the intrinsic strength of the gel, as A simply equals the compressive strength, when the gel space ratio X is unity. The paste may contain voids, e.g. due to insufficient compaction or deliberate air entrainment. Powers suggested that such voids are taken into account by including the air volume in the space available to the gel space, though in reality gel will not intrude these voids. However, in concrete technology, a linear correction is often used, e.g. in the Bolomey formula for estimating compressive strength f c from the w/c ratio [10]:

4 f 1 K Bolomey 0.5 B a a w c 1 (2) correction related to air content c 0 where K Bolomey and B are constants, a is the actual air content (% relative to concrete volume), and a 0 is a reference air content, typically the natural air content without air entrainment. It is often assumed that B takes the value of 0.04, but it can vary, depending on e.g. paste content. 3. Materials and methods 3.1 Test specimens The test programme is based on 3 reference concrete mixtures, see table 2: Table 2: Mix design for reference concretes and example of mix design for concrete with SAP. All values are stated as [kg/m³ concrete]. Aggregates are saturated and surface dry. Constituent Reference mixes (0 % SAP) Example: w/c = 0.35 w/c = 0.40 w/c = 0.50 w/c = % SAP Cement Water SAP Sand Coarse aggregate, 4-8 mm Coarse aggregate, 8-11 mm w/c w/c entrained Note 1: Aalborg Portland Rapid (CEM I 52.5 N (MS/LA/<2)) Note 2: Suspension-polymerized covalently cross-linked acrylamide/acrylic acid copolymer. Amount stated is amount dry SAP. It is assumed that when SAP is mixed in concrete, 1 g SAP absorbs 12.5 g water. For each w/c ratio, 4 mixes are prepared with different amounts of SAP, up to 0.6% of cement weight. Extra water is added to take into account that some of the mixing water is absorbed by SAP, see example in table 2, which has a starting point in the reference mix with w/c = 0.35 and modifies it with 0.6% SAP addition. Paste samples (approximately Ø20 x 120 mm) are stored at 25 C, sealed in the moulds until testing. Concrete specimens (Ø100 x 200 mm cylinders) are demoulded 1 day after casting and stored in a 25 C water bath until testing (standard procedure for strength testing). See also section 4 for comparison of curing conditions.

5 3.2 Experimental program The following methods have been used, see table 3. Measurements of compressive strength and degree of hydration are carried out 1, 2, 3, 7, and 28 days after mixing. Table 3: Test methods. Property Method Compressive strength EN [11] (3 cylinders for each test). Air content (hardened concrete) Degree of hydration The mass of 3 cylinders cast for strength tests is registered when demoulding 24 h after casting. The air content (air voids other than SAP voids) is calculated from this mass measurement, so density of theoretical mix design and actual density match. Air content was also measured in fresh concrete, but they showed to be unreliable. 1. Paste sample is crushed immediately after demoulding, weighed, and vacuum dried for 1 h to stop further hydration. 2. Sample is subsequently dried 24 h at 105 C, and weighed again (measurement used to determine amount of evaporable water, i.e. capillary water + gel water). 3. Sample is finally heated at 1050 C for 1 h, and weighed (measurement used to determine amount of chemically bound water). 4. Results Measurements showed that air content (not including SAP voids) varied from %. In order to study the pure effect of SAP, all values measured for compressive strength are corrected on line with the correction stated in equation (2). As a rule of thumb, the value of B is equal to 0.04, when the paste phase occupies 25 % of the concrete volume. The mix design shown in table 2 is relatively paste rich due to small maximum aggregate size and angular aggregates, in the reference mixes the paste content (volume of cement and water) is 37 %. For this reason 0.01/0.37 = is used as correction factor. When calculating the degree of hydration, loss on ignition (LOI) of cement has not been taken into account. LOI of cement is primarily due to water chemically bound in gypsum and unintended pre-hydration. The cement producer has declared LOI to be in the interval %. For that reason the degree of hydration is only a rough estimate. Results for compressive strength are shown in figure 2, and results for degree of hydration are shown in figure 3. Control measurements of the weight of concrete cylinders showed a weight gain from 1 day (time of demoulding) to 28 days. The weight gain is most probably due to absorption during water storage. The weight gain is approximately 20 kg/m³ concrete, a little bit less for concrete with w/c = 0.35 and a little bit more for w/c = kg of water is significant compared to the amount of water entrained by SAP (up to 42 kg/m³), so it is a serious source of uncertainty. More research is needed to estimate how much this influences the results.

6 Figure 2: Compressive strength vs. age at different w/c (compressive strength corrected for presence of air voids, but not SAP voids).

7 Figure 3: Degree of hydration for SAP mixes relative to the degree of hydration for the reference mix at the same age. Results for different w/c are shown.

8 5. Discussion For w/c = 0.50, SAP addition reduces the compressive strength. The trend is that the strength reduction is larger, the larger the amount of SAP. The addition of SAP has only a minor effect on the degree of hydration, so the presence of SAP voids reduces the paste strength. For w/c = 0.35 at 1 day, all concrete mixes with SAP have lower strength than the reference mix, but at later tests, the picture is more diverse. There is no significant effect of SAP on hydration the first 1-2 days after casting, so like for w/c ratio 0.50, SAP voids reduces the compressive strength. This is also to be expected from Powers model: there is still free capillary water in the ordinary paste structure, so the internal curing water in the SAP voids does not promote hydration at this time. But after 3 days, where free water in the reference mix becomes the limiting factor for the reaction rate, SAP addition promotes hydration. After 28 days, the degree of hydration is up to 6 % larger for concrete with SAP. Then space for the hydration products starts to become a limiting factor. Hydration products precipitate in the water-filled space created by chemical shrinkage inside the cement paste. Therefore the degree of hydration for 0.6 % SAP addition is on the same level as for 0.4 % SAP addition (theoretically, the upper limit corresponds to 0.43 % SAP for w/c = 0.35). The gel solid will have approximately the same strength for 0.4 % and 0.6 % SAP, but due to larger void volume, the overall concrete compressive strength is lower for 0.6 % SAP. Results for w/c = 0.40 are in between results for w/c = 0.35 and Mixes with SAP show strength reduction at all times like w/c = Addition of SAP increases degree of hydration as for w/c = 0.35, but only up to 4 %, and it is only significant at later ages (7 and 28 days). In figure 4, the compressive strength has been corrected, assuming that SAP voids have the same weakening effect as air voids (for the actual concrete 2.7% strength reduction for each % SAP void volume relative to concrete volume, cf. calculations in section 4). For each w/c ratio, there is a close relation between degree of hydration and corrected compressive strength. A combination of the void volume correction from Bolomey s formula and Powers model based on gel space ratio, seems to be a good way to model the effect of SAP on compressive strength. Both the Bolomey correction and Powers gel space ratio function including SAP void volume in available space will always predict that SAP addition lowers compressive strength. But in the described combination, the model predicts an increase of strength at low w/c ratios and moderate SAP addition, which is also observed experimentally. The proposed model does not take into account that SAP may influence early age cracking and the moisture state of concrete, and through this influence the compressive strength. However, the model works very well without these effects. It seems that at least the effect of changed early age cracking is minor compared to the effects described by the model. The water storage of specimens for strength measurements may have blurred an effect of changed moisture state.

9 Figure 4: Compressive strength vs. degree of hydration. Compressive strength has been corrected, so the weakening effect of the total void volume (air + SAP) is taken into account. In figure 5, the same corrected values for compressive strength are plotted as function of the gel space ratio calculated from the degree of hydration, not including SAP and air voids in the available space. There is a close fit to a power function. The exponent value of 2.88 found here is close to the value 3 as proposed by Powers (see equation (1)). Figure 5: Compressive strength vs. gel space ratio. Compressive strength has been corrected, so the weakening effect of the total void volume (air + SAP) is taken into account. 6. Conclusion The effect of SAP on concrete strength has been modelled by combining Bolomeys formula and Powers model. The former predicts that SAP voids reduce strength. The later predicts that SAP promotes a higher degree of hydration for concrete with low w/c ratio. This leads to

10 increased strength. Whether the net effect of SAP is positive or negative depends on the actual mix design and age. If the w/c ratio is high (> 0.45), SAP addition will have very little effect on hydration, and therefore SAP addition will generally reduce strength. The same is true for low w/c ratios at early ages, and if the amount of added SAP is large. Then the strengthincreasing effect due to increased degree of hydration cannot counterbalance the strengthreducing effect of the volume of SAP voids. However, for low w/c ratios and SAP addition below a certain limit, SAP increases compressive strength at later ages. References [1] Jensen, O. M., and Hansen, P. F.: 'Water-entrained cement based materials II. Experimental observations', Cement Concrete Res 32 (6) (2002). [2] Lura, P., Durand, F, and Jensen, O. M.: 'Autogenous strain of cement pastes with superabsorbent polymers', Proceedings, International RILEM Conference on Volume Changes of Hardening Concrete: Testing and Mitigation, RILEM PRO 52, RILEM Publications S. A. R. L., (2006). [3] Igarashi, S., and Watanabe, A.: 'Experimental study on prevention of autogenous deformation by internal curing using super-absorbent polymer particles', Proceedings, International RILEM Conference on Volume Changes of Hardening Concrete: Testing and Mitigation, RILEM PRO 52, RILEM Publications S. A. R. L., (2006). [4] Jensen, O. M., and Hansen, P. F.: 'Water-entrained cement based materials I. Principles and theoretical background', Cement Concrete Res 31 (4) (2001). [5] Lura, P., Durand, F, Loukili, A., Kovler, K., and Jensen, O. M.: 'Compressive strength of cement pastes and mortars with superabsorbent polymers', Proceedings, International RILEM Conference on Volume Changes of Hardening Concrete: Testing and Mitigation, RILEM PRO 52, RILEM Publications S. A. R. L., (2006). [6] Bartlett, F. M., and MacGregor, J. G.: 'Cores from high-performance beams', ACI Mater. J. 91 (6) (1994). [7] Hasholt, M. T., Jespersen, M. H. S., and Jensen, O. M.: 'Mechanical properties of concrete with SAP Part II: Modulus of elasticity', International RILEM Conference on Use of Superabsorbent Polymers and Other New Additives in Concrete, RILEM PRO 74, RILEM Publications S. A. R. L (2010). [8] Powers, T. C., and Brownyard, T. L.: 'Studies of the physical properties of hardened Portland cement paste', Bulletin 22, Research Laboratories of the Portland Cement Association, Chicago (1948). [9] Powers, T. C.: 'The physical structure and engineering properties of concrete', Bulletin 90, Research and Development Laboratories of the Portland Cement Association, Chicago (1958). [10] Herholdt, A. D., Justesen, C. F. P., Nepper-Christensen, P., and Nielsen, A.: 'Beton- Bogen', Cementfabrikkernes tekniske Oplysningskontor, 2nd edition, (1985). [11] EN :2002: 'Testing hardened concrete - Part 3: Compressive strength of test specimens' (2002).