Fracture characterization of fiber reinforced concrete in direct uniaxial tension

It has been widely accepted that the tensile properties of a concrete can be enhanced substantially by incorporating high strength and small diameter fibers, leading to a fiber reinforced concrete (FRC). In the present study, a new direct tensile test-ing configuration for an unnotched specimen was firstly proposed and implemented to evaluate the tensile behavior of concrete and FRC. In the configuration, a ball-joint loading fixture was developed such that an ideal hinge boundary condition is guaranteed. A complete load-displacement curve can always be obtained by using this technique. Extensive uniaxial tensile tests were then conducted for FRCs with varying fiber types, fiber volume fractions, and matrix composition. Tensile strain hardening responses were for the first time observed for FRCs containing a higher volume fraction of short steel fibers when the matrix was carefully designed. Acoustic emission (AE) technique was then employed to monitor the fracture behavior of concrete and FRC during a direct tensile testing. An adaptive trigger signal identifier was developed and implemented in the AE measurement. An adaptive FIR lowpass filter was proposed to process the AE signals which can remove the measurement noises efficiently and preserve the true signal. A new P-wave arrival time determination method was then developed. AE analyses were then performed to study the damage initiation and propagation by interpreting the AE activities and locations of AE event sources. Microcracking behavior inside a material was found to be in good agreement with the macroscopic response. Based on the experimental observations, an analytical model was also developed to predict the tensile stress-strain behavior of FRC showing a hardening response, according to the principles of the Continuum Damage Mechanics. Fiber-matrix in-terfacial behavior was incorporated in the model. Results predicted by the model show good agreement with those obtained experimentally