Measurement on stress-time avalanches of fiber reinforced concrete beams during flexure
Stress variations of loading histories on fiber reinforced concrete beams during flexure was investigated in this dissertation. A new experimental system was developed to collect the stress-time curves at 100 kHz sampling rate to measure temporal profiles in high resolution. Concrete beams prepared at the early age of seven days with different fiber types and fiber volume fractions were loaded at various rates during four-point bending tests. The four types of fiber used to make reinforced concrete were: steel fibers, basalt fibers, polypropylene (PP) fibers and recycled polyethylene terephthalate (PET) fibers. Stress drops induced by interactions between fibers and cementitious matrices or matrix cracking were modeled as avalanches along the time history. Mean field model was used to predict statistics and dynamics of avalanches occurred inside 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 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 of concrete beams, i.e., small avalanches with limited plastic deformation and large avalanches with extended incubation periods. For avalanches at the fracture of bottom concrete, the large stress drop rate decreased to zero via a prolonged period. When the peak stress was reached after the fracture of bottom concrete, the post-peak avalanche size could exceed the size of avalanches occurred at the bottom fracture. Findings from these studies could help clarify the evolved damage of concrete structural members under service loads, which provide valuable hints to improve the durability of reinforced concrete structures.