Fiber reinforced concrete: Characterization of flexural toughness and some studies on fiber-matrix bond-slip interaction

One major problem associated with the testing of fiber reinforced concrete specimens under flexural loading is that the measured post-cracking response is severely affected by the stiffness of the testing machine. As a consequence, misleading results are obtained when such a flexural response is used for the characterization of composite toughness. An assessment of a new toughness characterization technique termed the Residual Strength Test Method (RSTM) has been made. In this technique, a stable narrow crack is first created in the specimen by applying a flexural load in parallel with a steel plate under controlled conditions. The plate is then removed, and the specimen is tested in a routine manner in flexure to obtain the post-crack load versus displacement response. Flexural response for a variety of fiber reinforced cementitious composites obtained using the Residual Strength Test Method has been found to correlate very well with those obtained with relatively stiffer test configurations such as closed-loop test machines. The Residual Strength Test Method is found to be effective in differentiating between different fiber types, fiber lengths, fiber configurations, fiber volume fractions, fiber geometries and fiber moduli. In particular, the technique has been found to be extremely useful for testing cement-based composites containing fibers at very low dosages (<0.5% by volume).; An analytical model based on shear lag theory is introduced to study the problem of fiber pullout in fiber reinforced composites. The proposed model eliminates limitations of many earlier models and captures essential features of pullout process, including progressive interfacial debonding, Poisson's effect, and variation in interfacial properties during the fiber pullout process. Interfacial debonding is modeled using an interfacial shear strength criterion. Influence of normal contact stress at the fiber-matrix interface is considered using shrink-fit theory, and the interfacial frictional shear stress over the debonded interface is modeled using Coulomb's Law. Stresses required to cause initial, partial and complete debonding of the fiber-matrix interface are analyzed, and closed form solutions are derived to predict the complete fiber pullout response. Analysis shows that the initial debonding stress strongly depends upon fiber length and fiber elastic modulus. The process of interfacial debonding turns catastrophic at the instant the fiber pullout stress begins to drop with an increase in debond length. The magnitude of interfacial frictional shear stress along the embedded fiber length is found to vary as a result of the Poisson's contraction of the fiber. Moreover, Poisson's effect manifests itself in the form of a non-linear relationship between the peak pullout stress versus embedded fiber length plot. Based on energy considerations, an analytical solution is derived to compute the interfacial coefficient of friction. For both steel and polypropylene fibers, interfacial coefficient of friction is found to decrease exponentially with increase in pullout distance. Matrix wear resulting from fiber pullout appears to be responsible for the aforementioned physical phenomena. Parametric studies are carried out to investigate the influence of fiber-matrix interfacial properties (adhesional bond shear strength, normal contact stress and coefficient of friction) and elastic modulus of the fiber. (Abstract shortened by UMI.)...