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.
In this study, longitudinal strain distributions of rebars in the uniaxial tension test are evaluated with an image processing method (IPM). The measurements of strain are conducted till rebar is ruptured. As a result of the test, the detailed longitudinal strain distributions of rebar in the tension test are clarified and the results show that the strain at necking region are almost 2-3 times of average strain in the gauge length of specimen. After the stress of rebar is reached at ultimate strength, the most of elongations are concentrated at necking region. In the other region except necking region, therefore, the strains are merely increased till the end of test. From these results, it is estimated that the necking region can make the over estimation of elongation of rebar, therefore, the estimation except the necking region can be suggested to evaluate the true elongation of rebar. Also the measured results with IPM are compared with the measured results by using conventional strain gauge. It is estimated that the IPM shows some noise in data, but the IPM has advantages to measure the strain distribution to the range of rupture. Therefore if the image acquisition of the better resolution and better quality are possible, it can be a good alternative measuring method for the strain measurement.
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