Traffic noise is a major setback to the life quality of residents living in the vicinity of highways. Over the past 40 years, much legislation has been passed to regulate sound levels to which residents are exposed. Among the many approaches adopted to alleviate the problem, crumb rubber modified asphalt has been proposed as a measure to reduce highway traffic noise.;The use of crumb rubber modified asphalt to reduce traffic noise on pavements had not been evaluated in great detail prior to this study. This research investigates the effects of crumb rubber particles on asphalt pavement noise absorption ability. Both dense and porous Superpave gyratory compacted specimens were prepared and evaluated. To evaluate the effect of crumb rubber, several types of specimens were evaluated with and without the presence of crumb rubber with air void contents of 3.5%+/-0.5% and 16%+/-0.5% for dense and porous mixtures, respectively. The effect of asphalt binder source and the effects of crumb rubber grinding procedure, concentration, and gradation were also evaluated in order to better understand the factors that affect the sound absorption of pavements. In addition, the effect of pavement thickness was evaluated. The non-acoustical properties of the specimen were measured and correlated with sound absorption coefficients obtained from the compacted specimens.;The results indicated that the effect of crumb rubber as an energy absorbent (damping) material is minimal and does not significantly increase the sound absorption. However, the presence of crumb rubber particles affected volumetric mixture properties, which consequently affected the sound absorption. Strong correlations were found between permeability and binder content with sound absorption. However, permeability was affected by binder content in this study, with permeability decreasing as binder content increased. This was attributed to the presence of crumb rubber in the higher binder contents. Results also indicated that sound absorption is correlated to pavement thickness, although the effects were different for dense and porous mixtures as well as different for slow and high frequency ranges.;In summary, crumb rubber was shown not to directly affect sound absorption, but did indirectly affect sound absorption through other changes in mixture properties like permeability and binder content. Pavement thickness did affect the sound absorption differently for porous and dense mixtures, and an optimum thickness yields the highest sound absorption for a given type of mixture.
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