The Effect of Temperature on the Tangent Modulus of Granular Composite Sport Surfaces

A series of tests were conducted to determine the tangent modulus (vertical stiffness) values for a wax-coated granular composite material. This material is commonly used as the surface for Thoroughbred horse racetracks. The tangent modulus is important in the vertical loading of the surface by the hoof for a highly nonlinear material. Test temperatures span a range from 0 to 64°C and include the thermal transition regions of the wax coating obtained from differential scanning calorimetry tests. These temperatures also include the range of temperatures that are encountered during use. Creep tests were conducted to obtain steady-state strain conditions at loads ranging from 0.89 to 4.45 kN, the latter load approximating the weight of a Thoroughbred racehorse. Through-transmission ultrasonic waves were utilized to determine the tangent modulus values. The tangent moduli ranged from 74 to 573 MPa for the conditions tested. For all loads tested, a large decrease in modulus and decrease in material nonlinearity occurred as temperatures increased from 20 to 32°C. This temperature range matches the first thermal transition temperature for the wax coating of the track material. The results of this work provide a basis for racetrack maintenance decisions that can eliminate potentially adverse temperature effects and may reduce equine injuries.