Robert Stevens and Dr. W. Spencer Guthrie, Department of Civil and Environmental Engineering
Introduction
The purpose of this project was to investigate selected material properties of polyester polymer concrete (PPC) using laboratory testing. PPC is a synthetic concrete that combines sand and coarse aggregate with a polyester resin binder that takes the place of conventional portland cement. PPC is commonly used as an overlay material on bridge decks. There is some ambiguity about several important mechanical properties of PPC, including coefficient of thermal expansion (CTE), degree of hardening, and modulus of elasticity. The objective of this research was to investigate the effect of temperature on these properties. The results of this research will help bridge engineers better understand the mechanical properties of PPC and appropriate applications for its use.
Methodology
Three PPC specimens were cast in 4 in. by 8 in. cylindrical molds for testing. Specimens were prepared with a resin content of 12% by dry weight of aggregate, which is the optimum resin content for the aggregates used in PPC. After being fully cured, specimens were conditioned at temperatures of 70°F, -4°F, and 140°F for 24 hours (at each temperature). After each conditioning period, tests were performed on the specimens to determine CTE, degree of hardening, and modulus of elasticity.
CTE is a function of how much a material expands and contracts when subjected to temperature changes, so determining the CTE of a material requires measuring length changes. A micrometer was used to determine the initial length of each specimen at 70°F. The length of each specimen was also measured with the micrometer after each conditioning period so that changes in length could be determined. Length measurements were made at three locations on the top of each cylinder.
Degree of hardening was measured using a Schmidt rebound hammer, which gives a relative measurement of the hardness of a material. Hardness was measured at three locations on the top of each cylinder.
A free-free resonant column apparatus was used to measure the resonant frequencies of the cylinders. The cylindrical specimens were acoustically isolated from the test frame using styrofoam. An accelerometer was placed on one end of each specimen, and the other end was struck with an instrumented hammer. The apparatus recorded the resonant frequency of each specimen in hertz. Frequencies were measured at three locations on the top of each cylinder, and modulus of elasticity values, which give a measure of the stiffness of a material, were then computed.
Figure 1 shows the temperature conditioning, length measurements, hardness measurements, and frequency measurements of the specimens.
Figure 1: PPC specimen testing.
Results and Discussion
CTE values were calculated for three different temperature ranges: 70°F to -4°F, 70°F to 140°F, and -4°F to 140°F. The CTE on each cylinder was calculated from the total change in length and the change in temperature of the specimens in each case. The average CTE values for the three temperature ranges are 1.46 x 10-5/°F, 1.15 x 10-5/°F, and 1.19 x 10-5/°F, respectively. CTE values obtained in this study are reasonable for PPC, although they are higher than values that have been reported in previous studies. Higher values indicate that the material will experience greater expansion or contraction for a given change in temperature.
The average rebound number of the specimens is 34 at 70°F, 40 at -4°F, and 29 at 140°F. The rebound numbers measured in this study indicate that PPC hardness is dependent upon temperature, with lower temperatures yielding higher rebound numbers.
Calculated using a standard formula from the resonant frequencies of the specimens, the average modulus of elasticity of the specimens is 2.9 x 106 psi at 70°F and 3.6 x 106 psi at -4°F. Resonant frequencies were not obtained at 140°F, so modulus values could not be calculated at that temperature. In this study, lower temperatures produced higher modulus values (and therefore stiffer specimens), which indicates that PPC stiffness is also dependent upon temperature.
Conclusion
The objective of this research was to investigate the effect of temperature on selected mechanical properties of PPC. Three cylindrical PPC specimens were conditioned at three different temperatures. After each conditioning period, tests to determine CTE, degree of hardening, and modulus of elasticity of the specimens were performed. Testing indicates that these properties of PPC are dependent upon temperature. This area of research could be expanded in future studies.