Document Type

Dissertation

Date of Award

5-31-2020

Degree Name

Doctor of Philosophy in Civil Engineering - (Ph.D.)

Department

Civil and Environmental Engineering

First Advisor

Matthew P. Adams

Second Advisor

Matthew J. Bandelt

Third Advisor

Taha F. Marhaba

Fourth Advisor

Selina Cai

Fifth Advisor

Bruno M. Goncalves da Silva

Abstract

Using recycled concrete aggregates (RCA) as a substitution in concrete materials is an excellent way to improve the sustainability of concrete production. However, due to a lack of globally accepted technical guidance, and high material variability, the usage of recycled aggregate concrete (RAC) systems is not readily practiced within most structural engineering applications.

To date, the majority of the experimental findings in the literature indicate that RAC is a weaker composite material when compared to a natural aggregate concrete system in terms of mechanical properties such as compressive strength, elastic modulus, tensile strength, flexural strength, fracture energy, and toughness. However, recent research has attempted to improve the material performance through understanding variabilities of the aggregate properties, and thus, enables one to achieve predictable concrete strength properties that yield satisfactory structural performances in RAC systems.

In this dissertation, the influence of recycled concrete aggregate (RCA) properties associated with mechanical properties, replacement levels, and morphological parameters on RAC strength performance is examined through numerical simulations. The variability of mixture design proportions on RAC hardened strength properties are also statistically investigated. Numerical simulations indicate that the RAC compressive strength and the elastic modulus decrease by 9%, and 22%, respectively, when adhered mortar increases up to 50% within an identical RAC geometry. Additionally, the RAC simulations show material stiffness compatibility with higher adhered mortar contents, and thus, RAC material behavior show adequate`strength performance in uniaxial compression. The statistical analyses show that RAC compressive strength is significantly influenced by the relative stiffness of natural aggregate and mortar matrix, while the elastic modulus is dominated by the stiffness of the cementitious phase alone.

A large statistical database is created by collecting data from a substantial amount of peer reviewed journal articles that include hardened properties of RAC systems. A robust statistical analysis is then performed to examine the variability effects in the concrete systems. The statistical results show that the RAC compressive strength is strongly affected by the total aggregate-to-cement ratio and RCA replacement level. Monte Carlo simulations show that average compressive strength and splitting tensile strength in the database are in agreement with the numerical approach using a novel aggregate generation methodology. This computational RCA random generator accounts for shape factors and size effects in the RAC systems and simulates the material behavior under uniaxial loading environments. The elastic modulus simulation results exhibit high strength predictions because the material geometry and the boundary conditions in the computer simulations are stiffer than the actual configuration of the experiments. The statistical outputs indicate distinct effects on RAC strength due to variability of adhered mortar phases, and require appropriate material characterization methods for predictable strength requirements.

In summary, the research work indicates that the variability of strength properties in RAC systems has high dependency on the mixture design proportions and the mechanical properties of the adhered mortar content. The research further shows that the RAC strength trends vary as a function of RCA size, aggregate shape effects, interfacial transition zone distribution, adhered mortar content, and the location of the adhered mortar in the RCA aggregate system.

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