UC Davis Institute of Transportation Studies

This report is the fifth in a series of first-level analysis reports that describe the results of HVS testing on a full-scale experiment being performed at the Richmond Field Station (RFS) to validate Caltrans overlay strategies for the rehabilitation of cracked asphalt concrete. It describes the results of the fifth HVS reflective cracking testing section, designated 586RF, carried out on a 45-mm half-thickness MB4 gap-graded overlay with 15 percent recycled tire rubber. The test forms part of Partnered Pavement Research Center Strategic Plan Element 4.10: “Development of Improved Rehabilitation Designs for Reflective Cracking.”

HVS trafficking on the section commenced on May 25, 2006, and was completed on November 21, 2006. A total of 2,492,387 load repetitions, equating to 87.9 million ESALs and a Traffic Index of 15.3, was applied during this period. Temperatures were maintained at 20°C±4°C for the first one million repetitions, then at 15°C±4°C for the remainder of the test. Caltrans and the UCPRC jointly agreed to halt HVS trafficking at this point as there was no indication of the failure criteria being reached in the near future.

Findings and observations based on the data collected during this HVS study include:

• No cracking was observed on the section. This implies that the MB15-G overlay successfully prevented reflective cracking.

• The average maximum rut depth across the entire test section was just 4.6 mm, considerably lower than the failure criterion of 12.5 mm. The maximum rut depth measured was 7.7 mm. The MB15-G overlay thus did not appear susceptible to rutting at the temperature range under which the test was conducted.

• Ratios of final-to-initial elastic surface deflections under a 60 kN wheel load increased by between 1.4 and 1.9 times along the length of the section. The ratios for in-depth deflections show that damage increased at all depths in the pavement structure by the end of trafficking. Loss of stiffness was highest in the area of most severe cracking in the underlying DGAC layer.

• Analysis of surface profile and in-depth permanent deformation measurements indicates that most of the permanent deformation (approximately 55 percent) occurred in the asphalt-bound surfacing layers (overlay and cracked DGAC) with the remainder mostly in the aggregate base layer.

No recommendations as to the use of the modified binders in overlay mixes are made at this time. These recommendations will be included in the second-level analysis report, which will be prepared and submitted on completion of all HVS and laboratory testing.