Micromechanics-based durability study of lightweight thin-sheet fiber-reinforced cement composites.

Abstract

This dissertation is a micromechanics based durability study of the mechanical performance of lightweight thin sheet fiber reinforced cement composites (TSFRCC). Thin sheet fiber reinforced cement composites are used extensively by the building construction industry as roofing tiles and architectural wall panels out-of-doors, so they must be able to resist environmental deterioration processes, specifically water saturation and carbonation aging here. This work provides a framework for the complete experimental and theoretical mechanical durability evaluation of TSFRCC using a fracture mechanics based approach. Three major tasks have been accomplished to achieve this goal. The first task was to experimentally quantify the effects of environmental deterioration processes on the composite's microstructure: the fiber, the matrix and the fiber-matrix interaction region. Fiber strength tests reveal the dependence of fiber strength and stiffness on environmental deterioration processes. Composite fracture toughness testing reveals the influence of changing environment on matrix crack formation. The single fiber pullout test technique reveals the influence of environment on the fiber-matrix interaction micromechanical parameters. The second task was to theoretically model the composite's tensile and flexural behavior as a function of changing environment using the micromechanics based durability model. With the experimentally obtained micromechanical parameters as input, the effect of water saturation and carbonation aging on the composite's constitutive relation, the fiber bridging stress vs. crack opening (sigma_b-delta) relation, was quantified. In addition, the micromechanics based durability model was used to predict the environmental trends in (1) composite tensile and flexural mechanical properties, (2) composite failure (cracking and fiber), (3) crack stability and (4) crack opening for polyvinylalcohol (PVA) TSFRCC, polypropylene (PP) TSFRCC and refined cellulose (RC) TSFRCC as well as for hybrid fiber reinforced (PVA + RC and PP + RC) Hatschek produced composites. In all cases, the model predictions were compared to experimental results to ensure their accuracy. The third task was to apply the micromechanics based durability model to asbestos, glass and carbon TSFRCC for prediction of their environmentally dependent mechanical properties. This demonstrates the model's universality. It is a powerful tool able to identify which of the composite's micromechanical modeling parameters changes as a result of environmental exposure and to quantify this change for any fiber system. This has important implications to the design of durable lightweight TSFRCC because the model can be used to tailor TSFRCC for maximum long-term mechanical durability performance.