Fracture resistance of engineered fiber cementitious composites and implications to structural behavior.

Abstract

This dissertation focuses on the optimization of Engineered Cementitious Composites (ECCs) by the use of developed micromechanical models which relate the macroscopic behavior of the composite to the micro-properties (fiber, matrix, and fiber/matrix interface properties). Specifically, the composite properties are optimized in terms of tensile strength, flexural strength, and fracture energy. An experimental program is designed to validate the developed analytical models and introduce ECCs to a structural application, specifically, reinforced concrete (R/C) flexural members. In the analytical program, a model for the composite bridging stress-crack opening relation ($\sigma\sb{\rm c}$-$\delta$) was developed. The model accounts for both fiber pull-out and tensile rupture. The deduced $\sigma\sb{\rm c}$-$\delta$ relation was used to compute the composite tensile strength and bridging fracture energy. Then, the fictitious crack model was used to relate the composite flexural strength (MOR) to the $\sigma\sb{\rm c}$-$\delta$ relation as predicted by the derived $\sigma\sb{\rm c}$-$\delta$ model. The derived relationship can also be used to predict the flexural strength of any fiber reinforced composite provided that its tension softening relation is known. Furthermore, an analytical model which predicts the flexural strength in strain hardening ECC was also developed. The experimental program includes uniaxial tension test, to validate the derived $\sigma\sb{\rm c}$-$\delta$ relation; flexure test, to validate the flexural strength models, and fracture test to demonstrate and validate an extended J-based technique, and measure the fracture energy absorbed by ECC during the multiple cracking process. The second part of the experimental program includes flexural testing of R/C beams with a bottom layer of ECC material. The beams were designed for targeted durability properties, adopting a Performance Driven Design Approach. There was a reasonable agreement between the predictions of the derived analytical models and the experimental measurements. It was concluded that by the use of the developed analytical models, ECCs can be efficiently designed to satisfy any specific structural performance requirement where the modeled properties are governing. It was concluded that the idea of using an ECC layer around the main reinforcement in R/C flexural members could be proven effective in extending the life cycle of R/C structures while maintaining the associated increase in material cost at a minimum.