Date of Award

Spring 12-24-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Yunping Xi

Second Advisor

Franck Vernerey

Third Advisor

Abbie Liel

Fourth Advisor

Mija Helena. Hubler

Fifth Advisor

Wil V. Srubar III

Abstract

This dissertation is an extensive experimental, theoretical, and numerical study on mechanical properties of Recycled Aggregate Concrete (RAC). Fracture properties of RAC at different loading rates vary comparing with those obtained at quasi-static rate. The present experimental results showed that the brittleness of RAC decreases and R-curve increases corresponding to higher strain rates. Based on the present experimental results and the results in literature, a model was proposed for the fracture energy release rate of RAC at various strain rates. The modulus of elasticity of RAC at different loading rates was also studied. A similar trend with normal concrete was found, that is, the modulus of elasticity increases with increasing strain rates. Viscoelastic theory was used to develop a model for predicting the elastic modulus of RAC at various loading rates. Differing from previous empirical models, the viscoelastic model can cover any selected range of strain rates and more importantly it can distinguish the contribution of the material stiffness in each decade (each order of magnitude of the rate) within the selected loading range. The model predictions were in good agreement with test data. Although the compressive strength and modulus of elasticity of RAC increase quite significantly with increasing loading rate, the increase of compressive strains at peak load was found to be less significant than the strength and stiffness. Recycled aggregate is often treated by a surface coating to reduce its moisture absorbing capacity and to increase the bond between the residual and the new cement paste. To consider this special composition feature of recycled aggregates, a multiphase model based on composite mechanics was proposed to determine the modulus of elasticity of RAC with different strengths and volume fractions of residual cement paste and coating materials. This multiphase model was further integrated with the viscoelastic model such that the generalized comprehensive model, called Strain Rate Multiphase Model (SRMM), can be used to predict stiffness of any multiphase composite material including RAC at different strain rates. The SRMM was validated using the commercial finite element software ABAQUS on crack propagation of RAC notched-beams. The numerical simulations were compared with the present test data, and they agreed reasonably well. Therefore, SRMM is a versatile and reliable model to evaluate the modulus of elasticity of composite materials such as RAC subjected to various loading rates. The experimental and theoretical approaches developed in this dissertation paved the way for further investigation of other properties of composite materials such as RAC under different loading rates.

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