Crumb Rubber Modifier in Asphalt Pavement

Chapter 7

This report summarizes the current CRM practices in HMA for the States of Arizona, California, and Florida. The major items addressed were:

Thickness design considerations. Much of the use of CRM-HMA is as functional overlays. Only California utilizes a layer equivalency to reduce the thickness of the CRM-HMA.
Materials. The different agencies generally use ambiently ground CRM with asphalts that will produce an asphalt-rubber binder with a certain viscosity. Dense-, open-, and gap-graded aggregates have been used.
Mix design considerations. Arizona and Florida have modified the Marshall procedure to obtain their mix designs. California has modified the Hveem procedure. All have produced successful mixes.
Construction process. The addition of CRM in HMA makes use of much of the equipment used for conventional mixes. However, there is an additional requirement for a blending reaction vessel and some increases in mixing temperatures.
Costs. The increased cost of the CRM-HMA (compared with conventional HMA) ranges from as little as 15 percent (in Florida) to 70+ percent in Arizona, depending on the mix type.
Quality control. This issue is an important one. The uniformity of the asphalt rubber is generally checked using viscosity tests; however, there is a definite need for improved procedures to evaluate the binder uniformity as well as the mix properties.
Performance. All agencies feel they are obtaining an added value. However, inadequate data are available to quantify these benefits.