Measurement of stress-time avalanches inside polypropylene fiber reinforced concrete beams during flexure

Abstract

In this work, we studied stress variations of loading histories on polypropylene fiber reinforced concrete beams during flexure. A new experimental system was created to collect the stress-time curves at 100 kHz sampling rate to reveal temporal profiles of stress drops in high resolution. Concrete beams prepared at the early age of seven days with different fiber volume fractions were loaded at various rates during four-point bending tests. Stress drops induced by interactions between polypropylene fibers and cementitious matrices or matrix cracking were modeled as avalanches along the time history. We used the mean field model to predict statistics and dynamics of avalanches occurred inside polypropylene fiber reinforced concrete beams during flexure. Good agreement was obtained between measurements and predicted power law exponents and scaling functions from the mean field model. Two types of avalanches were observed for polypropylene fiber reinforced beams during flexure, i.e., small avalanches collapsed onto the scaling regime and large avalanches beyond the regime. Maximum stress drop rates of small avalanches were on the order of kPa/s, whereas maximum stress drop rates of large avalanches could reach the order of MPa/s. Observations of different avalanches revealed insights into the essential failure evolved during flexure concrete beams, i.e., small avalanches with limited plastic deformation and large avalanches with extended incubation periods. Findings from this study could help measure the evolved damage of concrete structural members under service loads, which provide valuable hints to improve the durability of reinforced concrete structures.