Installation and Laboratory Evaluation of Alternatives to Conventional Polymer Modifications for Asphalt

About the report:

The Virginia Department of Transportation (VDOT) specifies polymer-modified asphalt binders for certain asphalt mixtures used on high-volume, high-priority routes. These binders must meet performance grade (PG) requirements for a PG 76-22 binder in addition to elastic recovery requirements. This typically results in the use of binders containing styrenebutadiene- styrene (SBS) modifiers. However, other polymer modifiers may also be used to achieve the PG 76-22 classification. One of these modifiers is a copolymer of SBS and polyethylene (PE) (SBS-PE); another modifier is ground tire rubber (GTR). This study was undertaken to investigate the suitability of SBS-PE–modified PG 76-22 binder and GTR-modified PG 76-22 binder for use in Virginia.

Each modified binder was used in a 12.5 mm nominal maximum aggregate size mixture to pave approximately 2.3 lane-miles. All mixtures were produced as warm mix asphalt using a foaming system. The binders evaluated included a typical SBS polymer-modified binder as a control and binders modified with SBS-PE and GTR. During construction, all processes were documented, and material was sampled for evaluation. Binder and mixture tests were performed. Binder testing included performance grading and multiple stress creep and relaxation testing. Mixture testing included volumetric analysis, dynamic modulus, and flow number tests and cracking, rutting, and fatigue analysis.

Binder testing indicated that the control binder and SBS-PE–modified binders met VDOT specifications for classification as a PG 76-22 binder; the GTR-modified binder graded to a PG 70-22 binder, as it did not meet the PG 76-22 high temperature specification and did not pass the elastic recovery requirement. Laboratory mixture testing indicated that the performance of the SBS-PE–modified mixture should be similar to that of the control mixture. Laboratory test results for the GTR-modified mixture were mixed, with some indicating that the performance was similar to that of the control mixture and some indicating that the performance may be less than that of the control.

Based on the study, SBS-PE–modified binders should continue to be allowed as an alternative to SBS-modified binder provided specifications for PG 76-22 binders are met. However, further investigation of GTR-modified binders is suggested before recommendations can be made. In addition, long-term evaluation of the field site is recommended for validation of the laboratory findings.

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