TECHNICAL ARTICLE The Effect of Temperature on the Tangent Modulus of Granular Composite Sport Surfaces J.W. Bridge 1,2,3 , M.L. Peterson 2,3 , and C.W. McIlwraith 4 1 Department of Manufacturing and Mechanical Engineering, Oregon Institute of Technology, Klamath Falls, OR 2 Department of Mechanical Engineering, University of Maine, Orono, ME 3 Racing Surfaces Testing Laboratory, Orono, ME 4 Gail Holmes Equine Orthopaedic Research Center, Department of Clinical Sciences, Colorado State University, Fort Collins,CO Keywords Tangent Modulus, Vertical Stiffness, Granular Composites, Paraffin and Microcrystalline Wax, Synthetic Horse Tracks Correspondence J.W. Bridge, Department of Manufacturing and Mechanical Engineering, Oregon Institute of Technology, Klamath Falls, OR 97601 Email: john.bridge@oit.edu Received: March 26, 2012; accepted: July 4, 2012 doi:10.1111/j.1747-1567.2012.00851.x Abstract 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. Introduction To date, nine Thoroughbred horse racetracks in North America have switched from dirt surfaces to synthetic granular composite surfaces in an effort to reduce equine injuries (although one has recently switched back to dirt). Subsequent synthetic track injury reports have been promising; for example, following the 2005–2006 winter–spring meet at Turfway Park in Kentucky, fatal equine racing injuries dropped from 24 to 3. 1 Racing data collected in California between 2004 and 2009 has shown an average reduction in Thoroughbred fatalities from 3.09 per 1000 racing starts on dirt surfaces to 1.95 per 1000 racing starts on synthetic surfaces. 2 These statistics may be attributed, in part, to an improved track base installed during the replacement of the existing dirt tracks but also due to a more consistent racing surface. 3,4 The consistency of the synthetic tracks is influenced by the hydrophobic wax (or polymer)-coated materials that reduce the effect of moisture, a primary variable in traditional dirt and turf surfaces. 5 The construction of the synthetic track systems generally consists of an approximately 250-mm thick layer of synthetic granular track material that rests on porous macadam (asphalt) or other porous geotextile. Under this layer is stone/aggregate that covers a network of drainage pipes. 6,7 The synthetic material differs from dirt used in conventional tracks as it generally contains silica sand, polymer fibers, and rubber particles all of which is coated with a hydrocarbon paraffin-based, high-oil content wax. One representative track manufacturer cites Experimental Techniques (2012) © 2012, Society for Experimental Mechanics 1