Development of a New Mix Design Method and Specification Requirements for Asphalt Treat Base

摘要

Asphalt treated bases (ATBs) in Texas are usually designed as per Tex-126-E, Molding, Testing, and Evaluating Bituminous Black Base Materials, and constructed as per Item 292, Asphalt Treatment (Plant-Mixed), of the 2004 Standard Specification book. This specification is a hybrid of base and hot mix asphalt concrete procedures and requirements, which are sometimes incompatible. In addition, this Item uses a specific Texas Gyratory Base Compactor (TGBC) that is not readily available to all districts. Some districts use test method Tex-204-F, Part III, Mix Design for Large Stone Mixtures Using the Superpave Gyratory Compactor. However, this procedure was originally developed to design Type A and Type B hot mix on 6 in. by 4.5 in. specimens. Under Item 292, the unconfined compressive strength of the mix (as per Tex-126-E) is used to assess the quality of the mix. Specimens prepared under Tex-204-F are not the appropriate size for this type of testing. As such, the quality of the mix is assessed with the indirect tensile strength. The objective of this project was to propose a new mix design procedure for asphalt treated bases that can use standard equipment such as the Superpave Gyratory Compactor (SGC) to mold the specimens for mix design. To achieve the objective of this project, current TxDOT procedures such as Tex-126-E and Tex-204-F were evaluated and modified to propose new generically-named Tex-126-H and Tex-204-H specifications. A comprehensive parametric study comparing the results of the two proposed specifications with the existing specification was performed. The impact of the number of gyrations, curing temperature, binder grade, and asphalt content variation were evaluated using prepared laboratory specimens. Parameters including density, unconfined compressive strength, indirect tensile strength, and modulus using the existing and proposed specifications were compared. Based on these studies, a new method for determining the optimum asphalt content (OAC) for ATBs was developed. The recommendations were then evaluated at six actual construction projects for reasonableness.