Crumb Rubber Modifier in Asphalt Pavement

Chapter 6

Performance

Arizona, California, and Florida all have several years' experience with CRM materials. In particular, Arizona and California placed numerous CRM-HMA overlays beginning in the late 1970's. Many of the early, experimental placements included SAM or SAMI layers and/or proprietary products. Florida DOT's work with ORM mixes began in 1989. CRM performance from each State is summarized below. Only performance history considered relevant to the use of CRMHMA and currently available technologies are included.

Arizona

Arizona, arguably, has the longest sustained experience with CRM mixes of any State. Though many of their older projects used SAM's or SAMI's exclusively to mitigate reflection cracking, these products were expressly excluded from study in the current project, which focuses on traditional HMA sections and applications. Much of the CRMHMA performance information available is from projects initiated in the late 1980s.⁽¹⁷⁾

Currently, the city of Phoenix uses significant quantities of gap-graded CRM mixes in overlays of residential streets. Prior to 1992, the CRM mixes used a patented asphalt-rubber binder. During the 1995 construction season about 26 km (16 mi) of CRM mix will be placed. Typical overlay thicknesses are 30 mm. Overall, performance is reported to be better than conventional mixes. Recently, some early reflection cracking has been reported.

Arizona DOT uses both open- and gap-graded mixes on existing rigid and flexible pavements. Open-graded mix overlays are generally less than 25 mm, whereas the gap-graded mixes are placed in thicknesses of about 50 mm. Since 1989, approximately 40 projects have been placed using open-graded mixes. All are reported to be in excellent condition. Of the 16 gap-graded mix projects placed since 1989, all are performing satisfactorily. Some bleeding (presumably due to over-asphalting) and premature reflective cracking has been reported on gap-graded mix projects.

California

California has performance history on CRM materials dating back to 1978. Both wet and dry process mixes have been placed over existing flexible and rigid pavements with and without SAMI's. Evaluation of these projects has led CALTRANS to use only asphalt-rubber (wet process) gap-graded and dense-graded mixes in nonexperimental work. Investigation of other CRM mix types is continuing; however, this work is limited.

Performance of dry mixes has been variable with potholing in some projects. The potholing was attributed to variation in rubber content. Older, wet-process mixes sometimes raveled or cracked prematurely. Some of this distress was associated with low binder contents, or, in one case, with overlay placement on an existing pavement that was too flexible. Rutting and bleeding has been reported on some newer gap-graded projects. The cause of these problems is being investigated.

It should be noted that although distresses have been reported (rutting, bleeding, and raveling), the majority of projects are performing well. CALTRANS reports improved durability, reflection crack control, and resistance to chain wear when asphalt-rubber hot mixes are used. Side-by-side performance comparison of thinner CRM and conventional overlays led CALTRANS to reduce required overlay thickness when CRM is used, as described in chapter 2.

Florida

Although Florida first placed CRM material in the 1970's, the bulk of their performance experience is limited to projects placed since 1989. Both open- and dense-graded friction course mixes are used. To date, performance has been good.

Beginning in January 1994, all OGFC's and DGFC's require an asphalt-rubber binder. Florida DOT expects improved durability and better temperature susceptibility performance from the CRM mixes.

Summary

Performance of CRM mixes in these three States has generally been good overall. California and Arizona have longer performance records, particularly with wet process mixes. Occasional problems have been reported as being attributed to mix design problems (over- or underasphalting), lack of contractor experience, and use of newer processes. The continued use of wet process CRM materials is perhaps the strongest indication of their satisfaction with CRM performance. (See table 10.)

Table 10
History of CRM-HMA Projects in Arizona, California and Florida

Agency First Use of HMA Type of Mix Estimated Total Number of Projects Approximate Tonnage Since First Use Proposed Tonnage in 1995

Arizona 1989
1975 Gap-graded
Open-graded 16
41 212,000
470,000 50,000
150,000

California 1978 Gap-graded
Dense-graded 76
35 820,000
180,000 200,000
15,000

Florida 1989 Dense-graded
Open-graded 255
306 726,000
766,000 400,000
500,000