LABORATORY EVALUATION OF CRUMB RUBBER MODIFIED MIXTURES DESIGNED USING TXDOT MIXTURE DESIGN METHOD

The Texas Department of Transportation has developed a mixture design procedure for crumb rubber modified (CRM) asphalt concrete mixtures. Eight CRM mixtures were designed using this procedure. Four wet process mixtures and four dry process mixtures, in addition to one control mix, were considered for material characterization and performance evaluation using the asphalt aggregate mixture analysis system (AAMAS). The control mixture was designed using the conventional design method. It was determined that CRM has the potential to significantly improve the fatigue cracking performance of asphalt concrete pavements, but only when the wet method is used and the binder is properly designed. The dry process should produce mixtures with a reduced propensity for rutting, but may have an adverse effect on cracking. Fine and coarse rubber can be added dry to the dense graded mix (0.5% by weight of aggregate) without having any adverse effects on the performance. Although state transportation departments must comply with the existing legislative requirements, tire rubber, like any additive, should be used only to address a given mixture deficiency or expected deficiency in a given situation.