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 upper part was cracked in tension, the bond stress between the concrete and the top bar was 2 N/mm2 after applying load, which decreased to almost zero after four months (Figures 17 and 18). On the other hand, the bond stress of the bottom bar was 6 N/mm2 even after four months (Figure 17). As a result, the curvature and the crack widths of the specimen under negative moment were 1.5 times of those of the specimen under positive moment.
(5) The third and fourth conclusions stated above indicate that the effect of the curing condition at the upper or lower surface is larger than that of the age of demolding (3 or 28 days).
(6) The curvatures of all the specimens after 30 years were estimated extrapolating the observed curvatures. The estimated curvatures were between six and twelve times the computed elastic ones as shown in Figure 18. These ratios agreed with the previous researches.
(7) The rainfall of 30 mm caused the decrease of the shrinkage strains both at upper and lower surfaces of the specimens (Figure 19). The rainfall did not affect the tensile strains of the cracked concrete. As a result, the curvatures of the cracked specimens decreased after the rainfall.
