Four scaled-down pavement sections were prepared in the laboratory using a conventional asphalt concrete mixture and a commercially produced crumb rubber modified (CRM) mixture, which is used routinely in city streets. The pavement sections were then subjected to simulated traffic loads using a Model Mobile Load Simulator (MMLS) device with two load levels. In addition, specimens were prepared and tested in the laboratory for resilient modulus, tensile strength, bulk specific gravity, fatigue resistance and creep compliance. The laboratory measured material properties were input in a computer simulation program (KENLAYER) to provide theoretical prediction of performance.
Gradual increase in rut depth with load repetitions was observed in the wheel paths. Light loads resulted in smaller rut depth than heavy load, while asphalt concrete pavement experienced larger rutting under heavy loads than the CRM pavement. Rutting was mostly due to densification in the case of light loads, while it was mostly due to lateral flow in the case of heavy loads. No fatigue cracking was observed. The computer simulation showed that it takes 85 x 1015 axle load repetitions for CRM under heavy load to cause 12.5 mm (0.5 in.) rutting, while it takes only 6.9 x 1015 repetitions for asphalt concrete to cause the same amount of rutting. Both field and laboratory observations have similar trends with a shift factor of about 3 to 5.