Prestressing force is induced in reinforced concrete (RC) structures to improve their load-carrying capacity. Generally, the prestressing strand of an RC structure is tensioned using a hydraulic jack, which decreases its workability. In this study, we evaluate the application of prestressing force by using a shape memory alloy (SMA), as has been actively studied in civil engineering. Experiments were conducted to measure the multi-stepwise prestressing force introduced in a hybrid SMA wire composed of two different types of SMA wires. The experimental parameters were determined based on the combinations of the SMA wires and the heating temperatures. The results of the experiments show that the prestressing force was induced in a sequence. The magnitude of the prestressing force generated by the hybrid SMA wire was equal to the sum of the prestressing forces generated by the NiTi50 and NiTi90 SMA wires. In conclusion, this study verified the applicability of the proposed concept of multi-stepwise prestressing by using hybrid SMA wires. Further research is required to measure the effect of prestressing by locally heating the center of a girder with the aim of expanding the applicability of this concept.
This study conducted uniaxial tension tests on D10, D19, D29, and D35 SD400 steel-grade rebar and evaluated the strain distributions and necking regions to provide basic data for resolving differences between evaluation methods. Owing to the limitations of the existing measurement methods, this study conducted detailed evaluations of the strain of the rebar and necking regions using image processing, which is almost limitless on the measurement range and can easily distinguish measurement regions. The strain was concentrated at the region where necking occurred when the rebar approached its ultimate strength, which was successfully confirmed through image processing. The correlation between the length of the necking region and the diameter of the rebar could be analyzed by evaluating the necking region that occurs during the ultimate behavior of the rebar. According to the results, the length of the necking region is around 1.5~2.5 times the diameter of the rebar.
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