BEHAVIOR OF RC SLAB DEMOLDED AT THREE DAYS IN SUMMER AND SUBJECTED TO SUSTAINED LOADING

Removing formworks early is in great demand for economic reasons, because removed formworks can be reused for upper stories and construction period can be shortened. Many researches on this topic have been done in both material and structural fields. However, the bridges between the two fields are rare. In this research, five specimens of reinforced concrete (RC) slab are tested applying sustained bending moment in summer. The specimens are sprinkled with water in the morning after the concrete casting and demolded after three days. The upper surfaces of the specimens are exposed to sunlight and rain for 138 days while loading. The test parameter is the magnitude of the bending moment: zero, -0.5Mc', -1.0Mc', -1.5Mc and +1.5Mc', where Mc is the moment at flexural crack and “–” or “+” indicate the upper surface is subjected to tension or to compression, respectively. Figure 2 shows the reinforcement. Figures 3 and 4 show the loading setup. After the exposure of five months, one of the specimens is cut as shown in Figure 5 to investigate the stress-strain relationship of the concrete near the top and the bottom of the specimen. The following findings are obtained from the results of the experiment. In the following discussion, the computed values are obtained using AIJ Guidelines and assuming that the concrete of each specimen is homogeneous and that the humidity at the upper and lower surfaces of each specimen is same as that of the open air. (1) At the age of 194 days, the compressive strength of the concrete of the upper part of the specimen was about 50% of that of the lower part (Figure 8). Young's modulus of the upper part was about 60% of that of the lower part (Figure 8). (2) The shrinkage strain of the lower part of the specimen without bending moment agreed with the computation, whereas that of the upper part was negligible (Figure 19a). As a result, the specimen warped to upwardly convex. (3) The compressive strains of the lower part of the specimens with negative bending moment less than cracking moment agreed with the computation, whereas the tensile strains of the upper part were larger (Figures 12b and 12c). As a result, the curvatures of the specimens were approximately twice of the computed values (Figure 13a). The differences between the observed curvatures and the computed ones were much larger than those between the computed values assuming the demolding at the age of three days and those at 28 days. (4) In the case of the specimen where