Estimation of Punching Shear on UHPC Slab

Abstract

Nowadays, UHPC (High Performance Fiber Reinforced Concrete) is used widely with its remarkable performance, such as strength, ductility and durability. Due to the fibers in the UHPC which can control the tensile crack, the punching shear capacity of UHPC is higher than that of the conventional concrete, but the structural behavior of UHPC has not been determined completely. In this study, the data were analyzed for confirming the feasibility of the existing equations. In the preliminary study, the punching shear data of fiber-reinforced concrete slabs were analyzed for understanding the fiber effect. By analyzing experimental data, the equation that calculates the concrete, reinforcement and the fiber separately shows more accurate results. However, the method has problem that the coefficients for each terms are usually determined by experimental results. Therefore the relationship between the deformation based on material properties and the punching shear strength should be examined. In this paper, seven slabs with different thickness and fiber volume ratio were tested. In direct tension test, the crack width does not show the relationship according to the fiber volume ratio, but the tensile strength was increasing as the fiber volume ratio was increasing up to 1%. For the fiber content, 1% and 1.5% UHPC do not have big differences in tension test and punching shear test. However, the ratio of tensile strength and the punching shear strength was not proportional. The design tensile strength seems more proportional to the punching shear strength. It is assumed for the reason of un-proportional relationship between the direct tensile strength and punching shear strength that the un-unified crack width and the fibers directivity difficult to control. The thicker slab thickness causes the increment of the punching shear strength with decrement of deformation capacity. The UHPC flat plate shows the wider punching shear area than the conventional concrete. The gentler strut angle for UHPC slab can be assumed in further study for UHPC failure mode. The punching shear strength equations tend to overestimate the thin slabs. From observing the deformation curve and the failure section, it is insisted that the thin slabs have flexural failure behavior. Due to the flexural behavior of the thin slabs, it could not resist the punching load as much as predicted. The minimum slab thickness should be proposed thicker than 40mm. By analyzing the test data, the JSCE code shows the least standard deviation, but it overestimates the punching shear strength. K-UHPC code results are close to the experimental value. However, the study for finding the relationship between the stress and the material property should be performed with effort to reduce the variance.