MEASURING AUTOGENOUS STRAIN OF CONCRETE WITH COR- RUGATED MOULDS

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1 13-15 October 28, Nanjing, China MEASURING AUTOGENOUS STRAIN OF CONCRETE WITH COR- RUGATED MOULDS Qian Tian (1, 2) and Ole Mejlhede Jensen (1) (1) Department of Civil Engineering, Technical University of Denmark, Denmark (2) Jiangsu Academy of Building Science Co., Ltd, Nanjing, China Abstract A reliable technique to quantify autogenous strain is a prerequisite for example to reliably carry out numerical modelling of stress build-up in high performance concrete. The introduction of a special kind of corrugated mould enables transformation of volume strain into linear strain for fluid concrete, which not only realizes continuous monitoring of the autogenous strain since casting, but also effectively minimizes the disturbance resulting from gravity, temperature variation and mould restraint on measuring results. Based on this measuring technique, this paper investigates the influence of material properties, entrapped air and size of the corrugated tube on the measuring results. The effect of bleeding water and measuring direction horizontal or vertical on the measurement results is also studied and the mechanism are discussed. 1. INTRODUCTION It is widely recognized that autogenous shrinkage is one of the major causes for significant early age cracking of HPC. A reliable technique to measure autogenous strain is a prerequisite to allow proper establishment and control of shrinkage stress calculations of cement-based materials. Difficulties with the measurement of autogenous strain seem to be the fundamental reason for a large part of the disagreement previously found in the literature. Even for the same mixture the autogenous strain results measured with different kinds of equipment may display a considerable scatter [1]. The strain in the vertical direction is expected to be different from the strain in the horizontal direction due to settlement of the fresh cementitious material before setting. However, only very limited comparison between these two kinds of linear measurement can be found, among which obvious discrepancy is displayed [2-3]. Deformation may be affected by bleeding of the fresh mixture. Not accounting for the possible effect of bleeding on the measured strain may give rise to wrong conclusions as regards the mechanism causing the volume changes. However, it has not been fully examined how bleeding water may affect the measurements 151

2 13-15 October 28, Nanjing, China and the interpretation of them. The absorption of bleeding water by the specimen causes different effects for different measurement techniques. A special corrugated mould system has been designed to enable commencement of linear autogenous strain measurements shortly after casting for cement paste [4]. In further research a concrete dilatometer to measure autogenous strain based on the same principle has been constructed [5]. In order to establish a standard procedure where measurement artifacts are attempted to be minimized a systematic investigation is required for the concrete dilatometer. Based on this measuring technique, this paper investigated the effects of the following factors on autogenous strain measurement of concrete: (i) Stiffness of the mould (restraint); (ii) Size of the specimen; (iii) Internal air content; (iv) Bleeding, (v) Measuring direction (horizontal or vertical). In particular, simple physical models are proposed to elaborate the effect of bleeding both for horizontal and for vertical measurement. 2. EXPERIMENTAL 2.1 Materials Cement: The used cement is a low-alkali Danish white Portland cement produced by Aalborg Portland. Blaine fineness is 42 m 2 /kg. The calculated Bogue phase composition by wt% is C 3 S: 66.1, C 2 S: 21.1, C 3 A: 4.3, C 4 AF: 1.1, free CaO: 1.96, Na 2 O eq:.17. Silica fume: The specific surface of the silica fume is 17.5 m 2 /g (BET method). The chemical composition by wt% is SiO 2 : 94.1, Fe 2 O 3 : 1., Al 2 O 3 :.13, MgO:.71, SO 3 :.43, Na 2 O eq: 1.9. Aggregate: Four sizes of pure quartz sand were used at a certain mix proportion to achieve continuously graded aggregate for the non-bleeding mixture, and discontinuously graded aggregate in order to induce significant bleeding for the bleeding mixture (as seen in Table 1). Table 1: Composition of aggregate for the concrete (in wt%) Size of sand maximum (mm) average Percentage of the Non-bleeding mixture sand (in wt%) Bleeding mixture Superplasticizer: A poly-naphthalene sulfonate superplasticizer (PNS) was used. Water: In all experiments dematerialized water was used 2.2 Mixture proportions and experimental procedure Three kinds of mixtures were studied including one non-bleeding concrete and two bleeding concretes, see Table 2. Mixtures No.2 and No.3 have strong bleeding and sedimentation, which will be demonstrated further in the paper. Table 2: Mixture proportions and basic properties No W/C Silica fume (%, by cement weight) PNS(%, by binder weight) Cement Mix proportions (kg/m 3 ) Silica fume water PNS sand Setting Initial set Final Set Bleed -ing ratio (%) ~ ~87.5 Note: Bleeding ratio is measured according to ASTM C 243, detailed information can be found elsewhere [7] 152

3 13-15 October 28, Nanjing, China 2.3 Experiment setup for autogenous strain measurement The horizontal dilatometer was originally developed in 1988 and has been improved at a number of occasions in the following years [6](Fig. 1(a)). The vertical dilatometer is a modified version of the horizontal dilatometer (Fig. 1(b)). It should be noted that the vertical measurements are much more delicate to carry out compared with the horizontal measurements. It is difficult to ensure both proper support of the sample by the dilatometer frame and at the same time avoiding restraint. The timezero of measurement is defined as final set [7]. Fig. 1: Dilatometers measuring in horizontal (left) and vertical direction (right). During measurement both the horizontal and the vertical dilatometer is submerged into a thermostatically controlled bath. 3. RESULTS AND DISCUSSION 3.1 Effect of tube size on autogenous strain measurement Big tube small tube Fig. 2: Corrugated plastic tubes Fig. 3: Setup to compare volume change vs. length change Two sizes of corrugated tubes were used (see Fig. 2). For the big tube the diameters of the inner and outer part of the corrugation are approximately 6 mm and 8 mm respectively, and for the small tube 2 mm and 3 mm respectively. The corrugated tubes are made of low density polyethylene (LDPE) plastic. Due to the corrugation geometry the radial deformation of the mould is very small compared length strain (mm/mm) y small tube =.85x R 2 =.9995 y big tube =.87x R 2 = volume strain (ml/ml) Fig. 4: Ratio between volume change and length change to the longitudinal deformation. For such kind of mould, the volume change of a fresh concrete mixture should ideally transform completely into a length change. However, in practice this is not fully the case - among other things due to some minor geometric compromises in the machining of the mould. In order to check the actual ratio between volume change and length change of the moulds a special experiment setup was used, as shown in Fig.3. Fig. 4 gives the ratio between length change and volume change of the two sizes of tubes. It can be observed that the actual ratio is less than 1.. However, there is a highly liner relationship. The difference in transformation ratio between the big tube and the small tube is insignificant compared with other uncertainties 153

4 13-15 October 28, Nanjing, China related to measurements on a fluid material. For the curves in Fig. 4 the shrinkage behavior is very close to the expansion behavior. 15 small tube 1 big tube small tube big tube first superposition point(t 16.8h) time(h) -1 time(h) (a) effect of mould size (b) first superposition point Fig. 5: Effect of mould size on autogenous strain measurement-no bleeding concrete The measured autogenous strain of the non-bleeding concrete with the two sizes of tubes is given in Fig. 5. In principle, autogenous strain is an intensive parameter and should be independent of specimen size. The measured strain with the big mould is about 2% larger than that with the small mould at the age of 2 weeks. However, this difference is mostly built up before setting. A few hours after final set the two curves develop almost identically. This is shown in Fig. 5(b). The strain curve based on the big moulds is shifted as to make the final values of the two strain curves identical. It can be observed that the two curves begin to overlap after a certain point. The first overlapping point of these two curves is around 16.8 h, which is 5.7 h after final set. From this time the two curves are almost identical till the end of measurement. One of the possible reasons for the slight initial difference may be a delayed equilibrium between the temperature in central part of concrete and that in the surrounding thermostated bath for the big specimen compared with the small specimen. This may accelerate the hydration as well as the autogenous strain development. Another possible reason may be attributed to the wall effect of the corrugated mould. Aggregate particles will pack differently near the mould wall than in the specimen center, and even more so for a corrugated mould. For a given aggregated size such kind of influence will be more pronounced when the mould size is reduced. 3.2 Effect of mould elasticity on autogenous strain measurement Fig. 6: Three moulds with different elasticity Fig. 7: Setup for measuring elasticity of moulds 154

5 13-15 October 28, Nanjing, China Three moulds with different elasticity were investigated. They are made of Low density polyethylene (LDPE), high density polyethylene (HDPE), and moderate density polyethylene (the used MDPE is a mixture of 4% LDPE and 6% HDPE) (Fig.6). Note that in all other experiments reported in this paper the LDPE mould was used. In order to measure the elasticity of the three different moulds, the setup in Fig. 7 was used. The length change of the mould was measured with a ruler behind the mould under different compression forces provided by weights on top of the mould. Experimental results are given in Fig. 8. The elasticity of the corrugated mould decreases with polyethylene density. At about 5 kpa of equivalent compression stress, the strain of the mould is approximately 11 mm/m for LDPE, 29 mm/m for MDPE, and 2 mm/m for HDPE, respectively. Note that the mentioned compression stress, 5 kpa (.5MPa) is about 1/1 of the ultimate tensile stress for relevant concretes. 5 kpa stress generates a strain in the LDPE mould of 11 m/m. At the total strain observed in Fig.5, approximately 2 m/m, the equivalent stress exerted by the LDPE mould can be calculated as approximately.1 kpa or.1 MPa. strain(mm/m) LDPE y = 21.59x R 2 =.998 y = 5.33x MDPE R 2 =.981 y = 3.52x HDPE R 2 = equivalent stress(kpa) Fig. 8: Elastic behavior of the three kinds of moulds. Equivalent stress is calculated as the compression force divided by the area given by the big diameter of the corrugated mould (8 mm) LDPE MDPE HDPE first LDPE MDPE HDPE superposition point(t 21h) (a) effect of mould stiffness (b) first superposition point Fig. 9: Autogenous strain of non-bleeding concrete with three kinds of moulds 155

6 13-15 October 28, Nanjing, China The autogenous strain of the non-bleeding recipe with the three kinds of moulds are shown in Fig. 9. The measuring values with HDPE and MDPE moulds are slightly lower than with LDPE at 2 weeks. However, this difference is mostly built up at early age. Around 1 hours after final set the three curves develop almost identically, as further indicated by Fig. 9(b). In Fig.9 (b) the strain curves of MDPE and HDPE are shifted as to make their final values overlap with that of LDPE. The first superposition point of the three strain curves is around 21 h, which is 1 h after final set. From this point the three curves are almost identical till the end of measurement. As seen, the measured strain of very early age concrete is affected by the tube to some extent. The higher the tube stiffness the lower the measured concrete strain will be. However, the concrete elastic modulus increases strongly with age, and a few hours after setting the developed concrete stiffness completely overrides the stiffness of the mould. 3.3 Effect of air in mould on measured autogenous strain 2/3 volume full In order to investigate the influence of air in the corrugated mould on measured autogenous strain, two kinds of specimens are prepared with the non bleeding recipe. One is completely filled with concrete (marked as full ), and for the other one is only 2/3 filled with concrete (marked as 2/3 volume ), as shown in Fig. 1. The autogenous strain of the two specimens is shown Fig. 1: concrete specimen in Fig full 15 2/3volume full 1 2/3volume 5 first superposition point(t 21h) (a) effect of air in the mould (b) first superposition point Fig.11: Influence of mould air on autogenous strain of non-bleeding concrete Fig.11 indicates that the autogenous strain of the specimen 2/3 filled is about 13% higher than the one with complete filling. However, about 7 hours after final set the two curves develop almost identically. In principle, the measured autogenous strain should be independent of the filling of the mould. However, at very early age the concrete is not stiff enough to be completely uninfluenced by the mould. For the 2/3 filled sample the shrinkage force resulting from concrete autogenous shrinkage is redistributed along the whole section of the mould and leads to a reduced shrinkage. This shows that the measured concrete strain is influenced by the mould stiffness in this very early age period. However, the stiffness of concrete increases very quickly with hydration and the influence of the mould becomes negligible. A further experimental ana- 156

7 13-15 October 28, Nanjing, China lysis is needed to quantify the exact extent of the influence of the mould in the very first hours after setting. 3.3 Effect of measuring direction and bleeding Non-bleeding concrete The autogenous strain of the non-bleeding concrete measured in horizontal direction and vertical direction is shown in Fig. 12. The measuring results for the two directions are almost identical from final set till the end of the measurement. For the non-bleeding concrete, there is no difference between these two directions. This result is in agreement with the results of Charron et al [3], who measured the deformation of concrete on the three directions of cubes after setting. Based on these results it may be concluded that for a homogenous concrete autogenous strain after setting is isotropic horizontal vertical cavity time(h) Fig. 12: Effect of measuring direction on autogenous strain measurement of non-bleeding concrete Fig. 13: Cavity on the top of concrete specimen It should be noted that, when the end closure of the vertical specimen was opened after the measurement, the anchorage screw of the end closure was found to be only in marginal contact with the concrete. A cavity with a depth of approximately 1 mm was observed between the end closure and the concrete surface, as shown for the left specimen in Fig. 13 (an impression in the concrete left by the anchorage screw can be seen). A perfect bond where the anchorage screw is fully embedded in the concrete of the horizontal specimen is also illustrated in this figure for comparison (as shown for the right specimen in Fig. 13). Possibly the cavity is a consequence of plastic settlement of the fresh concrete when encapsulated air is being released at the top surface due to gravity. The magnitude of plastic settlement may be much higher than autogenous shrinkage at this stage. However, plastic settlement is generally considered to cause little damage to concrete structures in terms of cracking, because it occurs only in the plastic stage of concrete. Moreover, the experimental results indicate that the cavity above the concrete surface did not influence the measurement results of autogenous strain. Due to the corrugated shape the plastic mould is well bonded with the hardening concrete and alleviates transformation of the deformation of the concrete to the end closure despite the weak contact through the anchorage screw. Consequently, the strain of the hardening concrete is fully represented by the external strain of the mould Bleeding concrete 157

8 13-15 October 28, Nanjing, China 15 vertical 1 horizontal vertical 5 horizontal (a) -recipe 2- (b) -recipe 3- Fig. 14: Influence of bleeding and measurement direction on autogenous strain Experimental results of the bleeding concretes (bleeding ratio is given in Table 2) are shown in Fig. 14. Autogenous expansion is observed by horizontal measurement, while autogenous shrinkage is observed by vertical measurement for both these two bleeding concretes. This kind of inverse influence of bleeding on autogenous strain of concrete in the two directions is in agreement with results in the literature [2-3]. (a) Bleeding line (b) Surface defect ( c ) Perfect embedment of the anchorage screw Fig. 15: Details of the concrete in the tube for horizontal bleeding specimen Fig. 16: Investigation of the bleeding concrete by vertical measurement Fig. 15 shows that there is a distinct, horizontal bleeding line along the upper part of the corrugated mould for the bleeding concrete after horizontal measurement (shown by the green arrows). When the plastic tube is removed it is clear to see that there is no concrete above the bleeding line. On the top surface of the concrete specimen there is a thin layer of mud left by the bleeding water. At both ends of the horizontal specimen the anchorage screws are perfectly embedded into the concrete and the side surface is quite similar to non-bleeding concrete. Fig.16 shows that, for the vertical measurement, there is a similar cavity at the top of the bleeding sample like for the non-bleeding sample. However, the depth of the cavity of the bleeding sample is larger, approximately 25 mm and the corresponding volume is about 95 ml. The anchorage screw is fully detached from the main body of the concrete. 158

9 13-15 October 28, Nanjing, China Theoretical interpretation Based on the above observed experimental phenomena two simple theoretical models were conceived to illustrate the influence of bleeding on autogenous strain measurement in the two directions. (i) Parallel connection model for the bleeding specimen in the horizontal direction Fixed end A: Bleeding water Measurement direction of strain B: concrete A:Bleeding water Fig. 17: Parallel connection model for the horizontal measurement of the bleeding specimen As indicated by Fig.17, the whole bleeding specimen can be considered a system composed by A: bleeding water and B: concrete. A and B are parallel and not independent. The direction of measured strain is perpendicular to the direction of gravity. The total strain is dominated by the part with the lowest deformability for such a system. During the fluid stage bleeding water will gradually separate from the concrete body and come to the top of the corrugated mould, while concrete will sediment to the bottom due to gravity. Bleeding water is uniformly distributed on the top of the specimen along its whole length. Once a skeleton develops in the concrete the total length change depends entirely on the strain of the concrete which has a much lower deformability compared with the top bleeding water plus some air which has been released from the concrete due to plastic settlement. After setting, hydration leads to pores in the concrete due to chemical shrinkage by the cement hydration. Water may penetrate back into these pores in the concrete under the driving force of gravity and capillary suction, the latter probably being predominant. The swelling pressure of readsorbed bleeding water will cause concrete expansion [8]. (ii) Series model for the bleeding specimen in the vertical direction Fixed end Direction of strain measurement deformation Deformation direction A: Bleeding water B: B: concrete concrete Fig. 18: Series model for vertical bleeding specimen For the vertical specimen a series model can be considered, as shown in Figure 18. After casting water gradually separates from the concrete body and comes to the top of the specimen, while concrete sediments to the bottom due to gravity. The whole specimen can be considered a system composed of A: bleeding water and B concrete. A and B are connected in series and are not independent. The bottom end of the system is fixed and the top end is free to deform. The direction of strain measurement is parallel to the direction of gravity. For this kind of sys- 159

10 13-15 October 28, Nanjing, China tem, the total strain depends on both the strain of A (bleeding water) and the strain of B (concrete). In the fluid stage, the concrete will shrink due to chemical shrinkage by the cement hydration. Settlement of the concrete at this stage leads to a cavity on the top of the mould. Once the skeleton has developed, the system is composed of three phases: air at the top, concrete at the bottom, and water in between them. If there is no bleeding water, the measured strain should be identical in both horizontal and vertical direction. Because of the re-absorption of bleeding water, the upper part of the concrete may expand under swelling pressure. However, the lower part of the concrete, into which the bleeding water does not re-penetrate, may undergo autogenous shrinkage with the hydration process. The total strain of the concrete will be influenced by the strain of these two parts of the concrete. As the height of the vertical specimen is much larger than the influencing range of the bleeding water, the expansion of the upper part concrete may be overrided by the shrinkage of the lower part of the concrete. The measured strain will depend on the embedment of the anchorage screw. If the anchorage screw is detached from the concrete the measured strain will be directly dominated by reabsorption of bleeding water. If the anchorage screw is in contact with the concrete the shrinkage of the concrete outside the re-penetration zone will most likely dominate. In conclusion, the bleeding concrete the measured autogenous strain in the vertical direction is shrinkage contrary to the expansion which is observed in horizontal measurement. 4. CONCLUSIONS Based on the above observations and discussion the following conclusions are drawn for the corrugated mould dilatometer: Stiffness and size of mould does influence the autogenous strain measurement. However, the influence of these factors is negligible a few hours after final set. After that, the measuring results are independent of these factors. A very large amount of air inside the mould may indirectly have an influence on the measuring results in the first hours after setting. If the concrete does not bleed, the measured autogenous strain after final set in the vertical direction is identical to the measured autogenous strain in the horizontal direction. Before setting the measured autogenous strain may be influenced by plastic settlement. Bleeding promotes shrinkage in the vertical measurement and swelling in the horizontal measurement once a solid skeleton is developed. Settlement of concrete in the fluid stage leads to a cavity on top of the mould in vertical measurement. Horizontal measurement in general will be influenced by artifacts to a lesser extent than vertical measurements. However, in some cases settlement phenomena may also need to be measured and in this case vertical measurement may be necessary. REFERENCES [1] Hammer, T. A., Bjøntegaard, Ø. and Sellevold, E. J., Measurement methods for testing of early age autogenous strain RILEM report 25: Early Age Cracking in Cementitious Systems, RILEM TC181-EAS RILEM, 22(Cachan, France), eds. Bentur A., [2] Barcelo, L., Boivin, S., and Rigaud, S. et al., Liner vs. Volumetric autogenous shrinkage measurement: Material Behaviour or experimental artefact? Proc. 2 nd Int. Res. Sem. on self-desiccation and its Importance in Concrete Technology, 1999(Lund, Sweden), [3] Charron, J. P., Marchand, J. and Bissonette, B., Early-age strain of hydrating cement systems: comparison of linear and volumetric shrinkage measurements, pro. RILEM Int. Conf. on Early Age Cracking in Cementitious Systems (EAC 1), 21(Haifa, Israel),

11 13-15 October 28, Nanjing, China [4] Jensen, O. M. and Hansen, P. F., A dilatometer for measuring autogenous strain in hardening Portland cement paste, Materials and Structures 28(181), 1995, [5] Pornain, D., Measurement of autogenous strain of concrete, Final year project report, Technical University of Denmark, Department Civil Engineering, Lyngby, Denmark, 25. [6]Jensen, O. M., Dilatometer - further development, Report, Technical University of Denmark, Department of Structural Engineering and Materials, 1996, Lyngby, Denmark [7] Tian, Q and Jensen, O. M., Measuring Autogenous Strain of Concrete, technical report, Technical University of Denmark, Department of Civil Engineering, 27, Lyngby, Denmark. [8] Powers, T. C., Mechanisms of shrinkage and reversible creep of hardened cement paste, in Proc. Int. Symp. Structure of concrete and its behavior under load, Cem & Concr. assoc., London, 1965,