Graduation Semester and Year

2012

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Pranesh B Aswath

Abstract

Scrap tires continue to be a nuisance to the environment and this research proposes one way of recycling them as a lightweight aggregate which can substitute for mineral aggregates in concrete. Aggregates derived from scrap tires are often referred to as Tire Derived Aggregate (TDA). First, the focus is how much mineral aggregate can be replaced by these waste tires and how the properties of concrete are affected with the introduction of rubber. This is being mindful of the fact that for a new material to be acceptable as an engineering material, its properties and behavior has to be well understood, the materials must perform properly and be acceptable to the regulating agencies.The role played by the quantity of TDA and Crumb Rubber replacing coarse aggregate and fine aggregate respectively as well as different treatment and additives in concrete on its properties are examined. Conventional concrete (without TDA) and concrete containing TDA are compared by examining their compressive strength based on ASTM C39, workability based on ASTM C143, Splitting Tensile Strength based on ASTM C496, Modulus of Rupture (flexural strength) based on ASTM C78 and Bond strength of concrete developed with reinforcing steel based on ASTM C234.Through stress-strain plots, the rubberized concrete is compared in terms of change in ductility, toughness and Elastic Modulus.Results indicate that while replacement of mineral aggregates with TDA results in reduction in compressive strength, this may be mitigated by addition of silica fume or using a smaller size of TDA to obtain the desired strength. The greatest benefit of using TDA is in the development of a higher ductile product with lower density while utilizing recycled TDA. From the results, it is observed that 7-10% of weight of mineral aggregates can be replaced by an equal volume of TDA to produce concrete with compressive strength of up to 4000 psi (27.5 MPa). Rubberized concrete would have higher ductility and toughness with better damage tolerance but the Elastic Modulus would be reduced. After evaluation of rubberized concrete at elevated temperatures, it has been found that very high temperature would have adverse effects to the concrete like excessive spalling, pop-outs and cracking on the surface and therefore it is proposed to use this kind of concrete where temperature would not exceed 100°C (212°F) for extended periods.Observation of concrete at microscopic level showed that it consists of three phases; interfacial transition zone (ITZ), bulk hydrated cement paste and aggregate. The ITZ was seen to contain micro pores and microcracks and was considered the weakest phase in concrete therefore exercises a far greater influence on the mechanical behavior of concrete than is reflected by its size. Existence of the ITZ explains why concrete strength is lower and behaves inelastically while the aggregate and cement paste if tested separately behave elastically and have higher strength than concrete.A 3-Dimensional nonlinear Finite Element Model (FEM) for a concrete beam is proposed and developed using ABAQUS. Smeared crack model in ABAQUS is used to define material properties. The developed FEM is capable of predicting the ultimate load, deflections, Stress-deflection/strain curves and crack initiation which are all verified against the experimental tests. ABAQUS was found to be a useful tool for modeling of concrete. In conclusion, this research provides a clear understanding on the effects of using scrap tires as an aggregate in concrete. The pros and cons of TDA are explored, ways of overcoming the shortcomings suggested and a way of predicting concrete properties when using TDA provided.

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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