Pullout damage analysis of steel fiber with various inclination angles and interface states in UHPC through acoustic emission and microscopic observation

Abstract

The bond properties of fiber-matrix interfaces can significantly affect the tensile performance of ultrahigh-performance concrete (UHPC), and the fiber pullout test is the most direct method to study these properties. In this paper, in combination with acoustic emission (AE) and microscopic observation, an appropriate test method was set up to study the pullout damage process of brass-coated steel fibers, especially in the initial stage (slip of 0–0.2 mm) involving fiber debonding and matrix spalling. In addition to the fiber inclination and the loading rates, the potential corrosion, abrasion, and curvature of fibers in the actual casting process were considered in the tests. Moreover, a force function was introduced to an existing analytical model to describe the force provided by fiber end deformation, and the bond stress of aligned fibers was analyzed with the modified model. The test results indicate that: fiber with rougher surfaces has higher bond strength but a lower bond modulus and more severe damage on the fiber-matrix interface; the fiber curvature can both enhance the bond strength and alleviate the damage caused by snubbing effect; the corrosion product can rough the fiber surface and weaken the force provided by fiber end deformation. It is also found that the AE cumulative signal strength (CSS) is a helpful index to evaluate the matrix damage caused by snubbing effect in inclined fibers.