冬季に材齢4日で脱型したRCスラブの持続荷重下における挙動

Removing formworks early is in great demand for reduction of construction period. It has been studied from both material and structural fields and is accepted in the AIJ Guidelines. However, the detailed effects of early removing such as bond stress are not well known. In the previous experiment conducted in summer, compressive strength of the upper layer of the reinforced concrete (RC) slab was found to be a half of that of the lower layer, which resulted in smaller bond stress between the upper reinforcement and the concrete and larger flexural crack width at the upper surface than the lower surface. In this research, eight specimens of RC slab are tested by applying sustained bending moment outside in winter. The specimens are sprinkled with water in the morning after the concrete casting and demolded after four or twenty-six days. The upper surfaces of the specimens are exposed to sunlight and rain for five or eight months 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. Figure 2 shows the reinforcement. Figures 3 and 4 show the loading setup. After the exposure of five months, two of the specimens were cut as shown in Figure 5 to investigate the strength of the concrete near the top and the bottom of the specimen. 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.The following findings are obtained from the results of the experiment.(1) At the age of nine months, the compressive strength of the concrete of the upper layer of the specimen was 0.8 times of that of the lower layer (Figures 7b and 7c).(2) The observed crack widths and the bond stresses of the specimens with early demolding and +1.5 Mc moment were similar to those with -1.5 Mc moment (Figures 19, 20, and 21). The observed crack widths agreed with the predictions by the AIJ Guidelines (Figure 19).(3) Assuming curvature change to continue for 30 years, the curvatures after 30 years were predicted. The predicted curvatures of the specimens with early demolding fell within 6 to 12 times the elastic curvatures as indicated by the AIJ Guidelines (Figure 17).(4) In the specimens with early demolding, the incre