A new post-peak behavior assessment approach for effect of steel fibers on bond stress-slip relationship of concrete and steel bar after exposure to high temperatures

摘要

The effectiveness of the widely used admixture of steel fibers in the concrete-rebar bond behavior still has some less-known complexities despite numerous studies in this area. The contradictory results of previous studies serve as evidence for this statement. In this research, through reviewing the CEB-FIP MC2010 and ACI 408-12 codes, comparing the reported observations of previous researchers, and conducting 54 pullout tests, it was shown that the bond stress-slip graph between the rebar and fiber-reinforced concrete has a specific pattern different from that of plain concrete, by which the effectiveness of steel fibers in the bond strength can be evaluated with very good accuracy. In this novel approach, the post-peak behavior of the bond-slip graph is regarded as a measure of the effectiveness of fibers in the bond strength between concrete and rebar. Based on the results of this approach, adding steel fibers to the mix design by a volume ratio up to 1% increased the ratio of the two peaks in the bond-slip graph by 15 to 30%. It was also shown that the effects of steel fibers on the stresses at the first peak and the maximum peak would be different, and thus, the dual behavior of the fibers could be justified. Moreover, the influence of the two parameters of exposure temperature and cover to rebar diameter ratio on the bond behavior was examined. Based on the results of the post-peak behavior approach and using the nonlinear regression method, relationships were presented to model the bond strength as a function of the volume ratio of steel fibers, exposure temperature, and concrete cover to rebar diameter ratio and then assessed against the previous experimental findings and models. (C) 2021 Elsevier Ltd. All rights reserved.