Date of Award

Fall 1994

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering - (M.S.)

Department

Civil and Environmental Engineering

First Advisor

Mohamed E. Labib

Second Advisor

C.T. Thomas Hsu

Third Advisor

Edward G. Dauenheimer

Abstract

This thesis is designed to investigate the effect of porosity on the mechanical properties (strength and modulus) of concrete, and to evaluate the effect of silica fume on the mechanical properties of the polystyrene foam bead lightweight concrete. In this research we made the lightweight concrete by using polystyrene foam beads as light weight aggregate and studied its mechanical properties in detail. Due to the near zero density and modulus of foam beads, we were able to consider it as "porosity". We could then control porosity in concrete by varying foam bead volume fraction.

In our experiments we used the sand-cement paste as the control matrix, then systematically varied the foam bead volume fraction. Our results showed that the effect of porosity ( foam bead content) and silica fume on the strength of the concrete followed the predictable pattern: increase in porosity decreases the strength of the concrete, increase in silica fume increases the strength of the light weight concrete. The mechanical properties of foam bead concrete was best described by equations governing porosity effects. The strength ( or modulus ) relationships fit very well with the porosity model proposed by Wischers ( 16 ).

In our research we treat the light weight concrete as a composite material considering sand-cement paste as matrix and the foam bead as the second phase. Test results of the modulus showed that only one of the three models of the composite theory approximately fit the relationship between the modulus and the porosity (foam bead volume fraction).The ideal model was the Power's model ( 8 ).This model can also be used to determine the actual modulus of cement paste by extrapolating to zero porosity. We applied this method to cement matrices made with and without silica fume.

The results of this investigation show that the macroporosity of concrete has similar effects to that of microporosity, and those equations describing these phenomena are applicable up to a large volume fraction of porosity. Foam bead particles in concrete can thus be used to study the effect of porosity on concrete properties.

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