Crumb Rubber Modifier in Asphalt Pavement

Chapter 5

Quality Control / Quality Assurance (QC/QA) Considerations

The purchasing agency (DOT or city or county) can use its standard HMA acceptance procedures with a few modifications. The source acceptance for the asphalt cement and the mineral aggregates will remain the same. Good volumetric quality control procedures should be followed. Some modifications will need to be made for the rubber, the asphalt-rubber binder, and the CRMHMA mix.

In all three States surveyed, the purchasing agencies (cities, counties, or DOT's) require that the CRM supplier furnish the engineer-certified test results covering each shipment of material to each project. They also require that the certification include the manufacturer's batch or lot number. Some agencies do verification testing on the rubber gradation. They also require that the supplier furnish the agency with binder formulation and samples of all the materials to be used, at least 15 working days before construction. This binder formulation should include the following:

The source and paving grade of the asphalt cement.
The source and grade of any additives to the asphalt cement; for example, antistripping agents, extender oils, etc.
The percentages of the asphalt cement and additives being used by total weight of the asphalt-rubber blend.
The source and grade of CRM.
The percentage of CRM by total weight of the asphalt-rubber blend.
Brookfield (or Haake) viscosity of the blended material.

Asphalt Rubber

The production of asphalt rubber should be closely monitored. In each of the three States surveyed, the public agencies (and many of the contractors in those States) monitor the production of the asphalt-rubber blend using a portable viscometer. (See figure 11 for a photograph of a portable viscometer in operation in the field.) For batch-type processes, this test is run on each batch. In Arizona, and with some contractors in California, the trend is to run the ring and ball softening point (AASHTO Test Method T53-89), and cone penetration and resilience tests (ASTM Test Method D3407) on the blended asphalt rubber. The key to running the viscosity tests in the field is good temperature control during the tests. A small variation in the temperature could cause inaccurate test results that may result in changes in the process that are not required.

Insufficient data exist at this time to be able to quantify the variation in the test results from projects.

Asphalt-Rubber Content

Arizona and California utilize a nuclear asphalt content gauge that has been properly calibrated to monitor and control the asphalt-rubber binder content. They all reported problems with calibration. The Florida DOT conducted a study to evaluate the use of the extraction test to determine the binder content in an asphalt-rubber mix. They concluded that since a percentage of the rubber passes through the filter (the amount cannot be determined), the calculated binder content from the extraction test is not accurate⁽¹³⁾. Therefore, the Florida DOT determines asphalt rubber binder content based on certified meter readings at the HMA facility.

Aggregate Gradation

Standard extraction procedures (using either chlorinated or biodegradable solvents) can be used to determine the gradation of the aggregate. The Florida DOT found that even though some rubber particles are left in the extracted aggregate, their weight contribution to any particular sieve is small.⁽¹³⁾