THE THERMAL-MECHANICAL PROPERTY INTERDEPENDENCY OF WAX-COATED GRANULAR COMPOSITES USED IN THOROUGHBRED HORSE RACETRACKS

The Thermal-Mechanical Property Interdependency of Wax-Coated Granular CompositesUSED IN THOROUGHBRED HORSE RACETRACKS

By John W. Bridge

Thesis Advisor: Dr. Michael “Mick” Peterson

A Lay Abstract of the thesis Presented

 in Partial Fulfillment of the Requirements for the

Interdisciplinary Degree of

Doctor of Philosophy

(in Materials Science and Engineering)

October, 2010

     Wax-coated granular composite materials or “synthetics” are used on the surface of nine Thoroughbred horse racetracks in North America.  The composition of these surfaces differs from that of a conventional dirt track as they contain silica sand, polymer fibers, and rubber particles that are bound together with a paraffin-based, high-oil content wax coating.  It is hypothesized that the melting response of the wax coating has a major effect on the mechanical properties of the track.  

     The purpose of this dissertation is to understand the thermal-mechanical response of a racetrack synthetic surface under operational conditions.  This research investigates; (1) the chemical composition and thermal characteristics of the track wax coating that binds the track constituents, (2) the temperature effects on the shear strength and particle cohesion of the track material, and (3) the temperature effects on the vertical stiffness (hardness) of the track material.  The testing temperatures used in mechanical testing are based on the wax melting transition regions measured in the wax in the first phase of testing. 

          The results of the shear strength tests showed significant sensitivity to the wax melting transitions temperatures.  Strength increased by as much as 29% at these temperatures before sharply dropping off .  This increase is consistent with reduced wax viscosity that allows increased mobility of sand grains to interlock and resist shear.  A test sample which was modified with a high-oil, high-melting temperature microcrystalline wax showed significantly less dependence on temperature.   These materials are also load rate-dependent exhibiting up to 38% higher shear strength as load rates increased when tested below the first melting peak.  In contrast, above the first melting peak temperature, no significant load rate effect on strength was exhibited.  Finally, vertical stiffness (hardness) values were highest at the highest loads and coolest temperatures tested, and decreased (by as much as 54%) as temperature increased through the first melting transition region.

      Understanding the property changes that occur in a synthetic track surface as temperatures change has the potential to determine how maintenance can be used to increase surface consistency which is expected to improve equine safety and performance.