MOISTURE EXCHANGE AS A BASIC PHENOMENON TO UNDERSTAND VOLUME CHANGES OF LIGHTWEIGHT AGGREGATE CONCRETE AT EARLY AGE

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1 MOISTURE EXCHANGE AS A BASIC PHENOMENON TO UNDERSTAND VOLUME CHANGES OF LIGHTWEIGHT AGGREGATE CONCRETE AT EARLY AGE Pietro Lura and Klaas van Breugel Delft University of Technology, The Netherlands Abstract The early-age volume changes of Lightweight Aggregate Concrete (LWAC) were investigated. Isothermal tests were performed on mixtures with w/c ratio.37, differing only in the particle size of the lightweight aggregates. The lightweight aggregate was Liapor. Mixtures with Liapor F mm, Liapor F8 4-8 mm and Liapor sand, with the same water content in the aggregates and in the mixture as a whole, were investigated. The mixtures with the finer aggregates, and thus a better distribution of the watercontaining particles, showed the highest swelling. The drying shrinkage of specimens made with lightweight aggregates (Liapor F3), glued together with only a very thin cement paste layer, was also investigated. The specimens were first put under water for 24 hours, then exposed to a relative humidity of 5%. While the water was evaporating from the specimen, the lightweight aggregates at first swelled. This first period was followed by a second period of continuous shrinkage. The initial swelling is of the order of magnitude of the swelling of LWAC and occurred in the first twenty hours after casting. Oven-dry specimens made with Liapor F3 aggregate were also exposed to a relative humidity of 5%. The specimens showed a weight increase (due to water absorption) and swelling. Weight increase and swelling were linearly correlated. 1. Introduction Low water/binder ratio concretes are known to be susceptible to autogenous shrinkage [1,2]. By using saturated lightweight aggregate particles the autogenous shrinkage can be reduced [3,4]. The transport of water from the porous saturated aggregate to the matrix has been considered as one of the major reasons for reduction of autogenous shrinkage. This moisture transport is triggered by self-desiccation of the hardening cement paste 533

2 with progress of the hydration process. Because of this moisture flow the relative humidity in the paste will remain higher compared to the situation where no supply of water from the aggregate particles occurs. This situation may result not only in reduced autogenous shrinkage but even in swelling of the concrete [5,6]. Whether the measured swelling is caused by volume changes of the cement paste, of the lightweight aggregate, or more probably of both the cement paste and the aggregate, has still to be established. In order to get a clearer picture of the role of saturated lightweight aggregate on the internal curing and volume changes of the concrete it was considered worthwhile to investigate the deformational behaviour of concrete mixtures made with aggregate particles with different size, i.e. coarse and fine aggregate. A more homogenous distribution of the water containers in the mixture was supposed to result in a more effective internal curing and probably more swelling, i.e. higher reduction of the autogenous shrinkage, of the concrete. Additionally, a study was initiated concerning volume stability of lightweight aggregate when exposed to different ambient moisture conditions. A better understanding of the response of lightweight aggregate particles to moisture changes was considered very helpful, not to say a prerequisite, for correct interpretation of measurement data. Moreover, such a better understanding was also considered of importance in view of studies on mixture optimisation [6]. 2. Experiments on Lightweight Aggregate Concrete 2.1 Test equipment for measuring volume changes in early age concrete The volume changes in hardening concrete were determined on concrete prisms, 1x15x15mm 3. These prisms were cast in a mould that was provided with an external insulating material (see Fig. 1). The inner surface of the mould consisted of thin steel plates, cooled or heated by a system of tubes located between the plates and the insulating material. Temperature differences within the cross-section of the specimen were kept as low as 1.ºC to 1.5ºC. The length changes of the hardening concrete were measured with LVDT's (Linear Voltage Displacement Transducers) over a length of 75 mm. These were positioned outside the mould and registered the relative movement of two steel bars cast in the concrete. Deformation measurements could start when the concrete had gained sufficient strength to keep the steel bars in place. This implied that the very early volume changes that occurred when the concrete was still in a plastic stage were not measured. After casting, the surface of the concrete was covered with a tight insulating cover in order to avoid moisture loss to the environment (curing under sealed conditions). 2.2 Composition of concrete mixtures Concrete mixtures were made with w/c ratio.37. Blended cement was used, consisting of 237 Portland cement (CEM I/52.5 R) and 238 blast furnace slag cement (CEM III/B 42.5). In order to improve the strength, silica fume slurry (5% water and 5% silica fume ) was added to the mixture. The lightweight aggregate was Liapor F8 534

3 insulation canals steel plate 15 5 mm 15 mm Fig. 1 Top view and cross section of the Autogenous Deformation Testing Machine used for measurement of the load-independent deformations during hardening. in two of the mixes and Liapor sand in the third mix. In all the mixes normal weight siliceous sand (1-4 mm) was used. Two types of plasticizers were added to improve workability, i.e. lignosulphonate and naphthalene sulphonate. Details of the composition of the various mixtures are summarised in Table 1. In all the mixtures the same volume of lightweight aggregate was used. The water absorbed by the aggregates was fixed as the quantity required to saturate the type of aggregate with the lowest absorption capacity (Liapor F8, 8-16mm, after 24 hours under water). This quantity amounts to Table 1 Mixture composition of LWAC with Liapor F8 or Liapor sand and w/c.37 Type of lightweight aggregate Mixture composition Liapor F8 4-8mm Liapor F8 8-16mm Liapor sand -4mm CEM III/B 42.5 HL HS CEM I 52.5 R Water (incl. water in admixtures) Lightweight aggregate *) Sand -4 mm Lignosulphonate Naphtalene sulphonate Silica fume slurry *) The lightweight aggregates differ in specific weight of the particles (Liapor F8 and Liapor sand). In all mixtures the same volume of lightweight aggregate was used. 535

4 .152 kg of water per kg of dry lightweight aggregates. The total amount of water absorbed by this lightweight aggregate was also added to the lightweight aggregates used in the two other mixtures. In this way the same amount of water was used in all the mixtures, beit that due to the different fineness of the aggregate the distribution of the absorbed water in the different mixtures is not the same. It is noticed that the weight of the lightweight aggregate of different size is different for the three mixtures. The difference was quite substantial in the case of Liapor sand, while the other two mixes were almost equivalent in this respect. 2.3 Non-thermal length changes of concrete The concrete mixtures with lightweight aggregate of different particle size were cured isothermally at 2 C. In Fig. 2 the non-thermal volume changes in the first two weeks are presented. The initial moisture content of the lightweight aggregates was almost the same for the three mixtures. The three mixes showed expansion from the beginning of the measurements, i.e. 16 hours after casting. The expansion of the mix with Liapor F8, 4-8mm, was continuous, reaching a value of.1x1-3 about 138 hours after casting, when the experiment stopped. The expansion of the mixture with Liapor F8, 8-16mm, reached a peak of.85x1-3 about 3 hours after casting. Then a small shrinkage followed, reducing the swelling to about.75x1-3 at 144 hours. At this stage expansion started again, until the Total non-thermal deformation [x1-3 ].12 Liapor sand -4mm.1 Liapor F8 4-8mm.8.6 Liapor F8 8-16mm Temperature 2 o C w/c = Time [hours] Fig. 2 Non-thermal deformation of mixtures with LWA of different particle size. Isothermal curing (mixture composition, see Table 1) 536

5 end of the experiment, 312 hours after casting. The final expansion at the end of the test period was about.8x1-3. The mix with Liapor sand, -4mm, expanded rapidly until 2 hours after casting, then a bump in the curve was observed, which was due to a perturbation on the measuring machine. The expansion restarted at the same rate after 26 hours until 6 hours after casting, reaching the value.95x1-3. Afterwards expansion continued steadily but at a lower rate, reaching a value of.12x1-3 after 312 hours. The measured deformations at early-ages show that in mixtures with homogeneously distributed saturated small lightweight particles the swelling is greater than in mixtures with coarse lightweight aggregates. One of the reasons is that the use of smaller saturated aggregate particles results in a more homogenous distribution of the watercontaining particles and shorter transport distances of the water from the aggregate grains to the drying paste. The extra water is thus present more exactly where it is needed, and the self-desiccation process of the cement paste is thus avoided, eliminating the shrinkage. Moreover, from Fig. 2 it can easily be deduced that the effect of selfcuring via water transport from the saturated lightweight aggregates to the cement paste is active up to at least two weeks after casting, as even then the free deformations are still increasing. 3. Experiments on specimens composed by Liapor aggregates Even in normal weight concrete swelling of the mixture in the very early stage has been observed several times. In the case of LWAC this early swelling far exceeds that of normal weight concrete. Up to now the most plausible reason this phenomenon is the aforementioned moisture flow from the saturated aggregate towards the drying cement paste. To which extent volume changes of the LWA particles could attribute to the explanation of the swelling of the mixture at early ages will be discussed in the next sections. 3.1 Description of the specimens The deformational behaviour of lightweight aggregate particles subjected to moisture changes is not easy to measure directly on particle level with currently used measurement equipment. Nevertheless such measurements on single grains have been performed indeed using an optical microscope [7]. In the case of drying Liapor grains, both swelling and shrinkage were measured. In fact, a lot of grains should be measured with this approach in order to get reliable mean values. For a preliminary indication of the response of LWA particles to moisture changes measurement were, therefore, performed on larger specimens made of lightweight aggregate particles. These specimens consisted of Liapor particles, glued together with only a thin layer of cement paste. The specimens were sawn from commercially available larger blocks of Liapor- Super-K, made with Liapor F3 [8]. The size of the specimen was 3x1x3mm 3. In Fig. 3 a picture of one of the sawn surfaces is shown. It can be seen that the aggregates occupy most of the space, and the cement paste forms only a thin film around them. Since the lightweight aggregates form the principal constituent, it is expected that the behavior of the specimen will depend mostly on the aggregate. For the time being it is 537

6 Fig. 3 Picture of the sawn surface of the specimen. Lightweight aggregates appear in gray, cement paste in white, pores in black. assumed that the presence of the cement paste does not strongly influence the results. The use of blocks of aggregates instead of the single grains has the advantage of averaging the behavior of the individual grains. Thus, fewer tests are needed and the scatter in the results is expected to be less significant compared to measurements on single particles. A drawback of the specimen concerns the fact that the outer Liapor grains have been cut, and the core of the grain is exposed. Thus it is expected that the moisture exchanges with the ambient medium will be more intense than in the case of whole grains, i.e. in the way they are present in a real concrete mixture. 3.2 Description of the first series of tests A first series of tests was aimed to investigate the deformations of wet Liapor specimens exposed to drying in a 5% RH environment. Three specimens of Liapor F3 were sawn from larger blocks of Liapor-Super-K and dried for 24 hours at 15 C. Subsequently they were weighed (dry weight) and immersed in water at 2 C for 24 hours. At this point the surfaces were dried with a piece of cloth and the specimens were weighed for the second time (wet weight). Then they were exposed in a room with controlled 5% relative humidity (RH) and a 538

7 temperature of 2 C for five days. At the end of the tests, the specimens were weighed again in order to determine the water loss to the environment. To each specimen two LVDT s were fixed, on the two larger sides. The measuring length of each LVDT was 2mm, with an accuracy of 1µm. Under the specimens a teflon foil was set, in order to reduce the friction with the base. One of the specimens was set on a balance and its weight was continuously monitored. 3.3 Test results In Table 2 the measured weights (oven-dry, wet after 24 hours under water, after 12 hours in 5% RH) are given. Table 2 Weight of the specimen in different moisture conditions Specimen number Dry weight (24 hrs oven) [g] Wet weight (24 hrs under water) [g] Weight after 12 hrs in 5% RH [g] Specimen Specimen Specimen The values are quite similar for the three specimens. This means that the mechanisms of water transport (both the suction of liquid water and the water release to the surrounding during the drying process) are quite similar for the three specimens. Specimen 1 was set on a balance and its weight was continuously monitored for the 12 hours of drying. The loss of weight of specimen 1 is shown in Fig. 4. The drying process begins very fast. After 24 hours most of the water (8% of the final value) is evaporated already. After 8 hours the process is terminated and the specimen is in hygral equilibrium with the environment. In Fig. 5 the measured strains of the three specimens, exposed to 5% RH at 2 C after 24 hours under water, are plotted versus time. In the beginning the three specimens show swelling, which continues up to about 24 hours. Specimen 1 and specimen 3 reach a value of swelling of about.15x1-3 and specimen 2 a value of.7x1-3. It must be noticed that the specimens are swelling while losing water to the surrounding. After about 24 hours exposure at 5% RH the specimens start to shrink. At 12 hours from the beginning of the test the shrinkage is almost over. The final shrinkage for the three specimens is about x

8 1 weight [g] Time [hours] Fig. 4 Loss of weight of specimen 1 exposed to 5% RH at 2 C..2.1 Strain [x1-3 ] Specimen 1 Specimen 2 Specimen Fig. 5 Time [hours] Deformation of the three wet specimens subjected to 5% RH at 2 C. 54

9 In Fig. 6 the measured strains is presented as a function of the loss of water. Up to a loss of water of 6 g (which corresponds to 8% of the total loss to the environment and also to the maximum swelling of the specimens) the relationship between weight loss and swelling is linear..2 Strain [x 1-3 ] weight [g] Fig. 6 Strain in specimen 1 as a function of the water loss 3.4 Comments about the first test series The measured strains in wet specimens of Liapor F3 dried in 5% RH, see Fig. 4, 5 and 6, show swelling of the specimens while they lose moisture to the environment. After 24 hours the swelling changes into shrinkage and the shrinkage proceeds very rapidly also when the drying process (observed through the weight change) seems to be almost over. A possible reason for the observed behaviour could be that the LWA particles were not fully saturated at the moment they were exposed to the ambient condition of 5% RH. Having been under water for only 24 hours, not all the pores of the aggregate are possibly filled with water. Thus, when exposed to a drier environment the larger pores may lose water to the environment while at the same time the smaller pores, still dry, are absorbing moisture from the (still partly) saturated outer shell of the grains. The latter uptake of water in the small pores may result in swelling of the specimen. In this respect it is noticed that that the smaller the pores, the more pronounced the effect of water absorption by the pores [9]. 541

10 3.5 Description of the second series of tests A second series of tests was performed in order to check the deformational behaviour over a wider range of relative humidity. In this series the specimens were dried for 48 hours at 15ºC. The dry weight was almost the same as in the case of drying for 24 hours. Then the dry specimens were exposed to 5% RH at 2ºC. Both the deformations and the weight were monitored in the same way as described for the first series of tests. 3.6 Results of the second series of tests Up to now only the results of one specimen is available. In Fig 7 the weight increase of this specimen is shown. The specimen absorbed moisture from the environment and increased in weight. The process started very fasts and has almost ceased after 5 days. By that time about 15 g of water was absorbed from the air. 15 weight [g] Time [hours] Fig. 7 Weight increase versus time of an oven-dried specimen, exposed to 5% RH at 2ºC. In Fig. 8 the strains measured at two sides of the specimen are shown. The dried specimen swelled as soon as it was exposed to the environment with RH = 5%. After about 5 days the swelling amounted to.23x1-3. In Fig. 9 the relationship between the strains and the weight increase is shown. The relationship appears to be linear almost from the beginning of the process. 542

11 Strain [x1-3 ].2 Specimen 3 right Specimen 3 left Time [hours] Fig. 8 Strain versus time of specimen 3, first oven-dried and then exposed to 5% RH at 2ºC. Strain [x1-3 ].2.1 Specimen 3 average weight [g] Fig 9 Strain versus weight increase of specimen 3, first oven-dried and then exposed to 5% RH at 2ºC. 543

12 3.7 Comments about the second series of tests The second series of tests showed that an oven-dry specimen made of Liapor F3, exposed to an environment with 5% RH and constant temperature, absorbs moisture and swells. If the assumption were correct that the deformations are mainly attributable to the deformations of the lightweight aggregate particles, it is clear that these particles are not volumetrically inert when submitted to moisture changes. Furthermore, the swelling of the specimen appears to be linearly correlated to the water absorption (see Fig. 9) and lasts for more than 12 hours (Fig. 8). Even after 12 hours the specimen in not in hygral equilibrium with the ambient RH of 5%. The smaller pores are considered responsible for this slow water absorption. 4. Conclusions By using saturated lightweight aggregate in low water/cement ratio concretes, i.e. w/c <.4, autogenous shrinkage can significantly be reduced or even swelling may occur. Moisture transport from the aggregates to the drying cement paste is assumed to be the cause of this swelling. If this is the cause indeed, then the effectiveness of this moisture transport phenomenon will be higher in case the aggregate particles are more homogeneously distributed throughout the mixture. This hypothesis was verified experimentally by comparing the volume changes (more precisely the length changes) of three LWAC s. Three LWAC mixtures were tested, one made with Liapor F8, 4-8 mm and one with Liapor F8, 8-16 mm. In the third mixture the coarse aggregate was replaced by lightweight aggregate sand. The same volume of lightweight aggregate was used in all mixtures, whereas also the same amount of water was contained by the aggregates. So, the amount of water contained by the lightweight aggregates was the same, but the distribution within the mixture was different. The test confirmed that the swelling of the mixture made with lightweight sand was highest, followed by the mixture with Liapor F8, 4-8 mm and Liapor F8, 8-16 mm. In a second part of this paper it was attempted to verify whether the aggregate particles could be considered as volumetrically inert when exposed to moisture changes. If they were not, the volume changes of the aggregate have to be considered when interpreting the early volume changes of LWAC mixtures. In a preliminary test series the length changes were measured of a specimen produced with Liapor F3 particles, glued together with a thin cementitious layer. It was assumed that the length changes of the specimen, if present, would only originate from the lightweight aggregate particles (which has still to be verified). A first series of tests was performed with specimens with (partly) saturated aggregates (after 24 hours under water), which were subsequently dried at 5% RH. At first the specimens exhibited swelling, followed by shrinkage, while the specimens were constantly losing water to the environment. The first swelling (which reached its peak 24 hours after the beginning of the experiment) is of the order of magnitude of the swelling of the LWAC mixtures tested in the first part of this study. The first swelling is assumed to be caused by suction of water in the smaller pores of the Liapor grains. At the same time the specimen loses water to the environment via the bigger pores, so that the weight 544

13 of the specimen decreases while expanding. At later stages the ordinary process of drying shrinkage dominates the deformational behaviour. A second series of experiments with Liapor F3 insulating blocks showed that oven-dry specimen (dried during 48 hours at 15 C) absorb water when placed in a room with 5% RH and swell. The swelling and the increase of weight of the specimen are almost linearly correlated in the test period of 12 hours. This points to a very slow process and confirms earlier observations that the uptake or loss of water by lightweight aggregate particles may continue for a long time [5]. From the executed tests the tentative conclusion is drawn that the early volume changes of LWAC are not only caused by moisture flow from the lightweight aggregate particles to the drying cement paste, but that additionally volume changes of the aggregates may have to be considered. More solid conclusions on this point have still to be made. For that purpose it has to be checked whether the length measurements on the Liapor F3 insulating blocks might have been influenced by the hygral properties of the cementitious glue between the aggregate particles. In this respect it is remarked that, as can be seen from Fig. 3, the pore structure of the aggregates is coarser than that of the cementitious glue. It might be that the cementitious matrix, due to it fine pore structure, significantly influences the deformational behaviour. Another way to check the deformational properties of the lightweight aggregate particles is to carry out measurements directly at individual particles in a way recently reported by Schmidt-Döhl et al. [7]. The authors are now preparing similar tests. The final aim of these studies is to rationalize procedures for mixture optimization in view of the strength and deformational behaviour of concrete mixtures. 5. References 1 Tazawa, E., Miyazawa, S., Effect of constituents and curing conditions on autogenous shrinkage of concrete, in Autogenous Shrinkage of Concrete, Proceedings of an Int. Workshop on Autogenous Shrinkage, Hiroshima, 1998 (E&FN Spon, 1999) Bjøntegaard, Ø., Sellevold, E., Hammer, T.A., High performance concrete (HPC) at early ages: Self generated stresses due to autogenous shrinkage and temperature, in Self-desiccation and its importance in concrete technology, Proceedings Int. Workshop, Lund, 1997 (Ed. Persson et al.) Weber, S., Nachbehandlungemphindlicher Hochleistungsbeton. Institut für Werkstoffe im Bauwesen, PhD, Stuttgart, 1996, 221 p. 4 Vaysburd, A.M., Durability of lightweight concrete bridges in severe environments, Concrete International, 18 (7) (1996) Sickert, G., Schwesinger, P., v. Haza-Radlitz, G., Creep, Shrinkage and creep recovery of HPLWA-Concrete, in Proc. 5th Int. Symp. on Utilization of High Strength/High Performance Concrete. Sandefjord, Vol. 2, June 1999,

14 6 Breugel, K. van, de Vries, J., Potential of mixtures with blended aggragates for reducing autogenous deformation in low water/cement ratio concrete, in Proc. 2nd Int. Symp. on Structural Lightweight Aggregate Concrete, Kristiansand, June Schmidt-Döhl, F., Thienel K.C., Measurement of Swelling and Shrinkage of Lightweight Aggregate, in Proc. 2nd Int. Symp. on Structural Lightweight Aggregate Concrete, Kristiansand, June 2, Liapor: Liapor Vollwärme-Block. Brochure 3/96. Liapor-Werk Pautzfeld. 9 Bentz, D.P., Jensen, O.M., Hansen K.K., Olesen J.F., Stang. H., Haecker C.J., Influence of Cement Particle Size Distribution on early Age Autogenous Strains and Stresses in Cement-Based Materials, 1999, NIST report 546