Development of a new mix design method and specification requirements for asphalt treated bases

Abstract

Asphalt stabilized bases in Texas are usually designed 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 compactor that is not readily available to all districts. Some districts have started using 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 at 96% density and produce a 6 in. by 4.5 in. specimen. 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. A new mix design procedure is needed for this type of material that can use standard equipment such as the Superpave Gyratory Compactor (SGC) to mold the specimens for mix design.

In order to achieve the objective of this project, current TxDOT procedures such as Tex-126-E and Tex-204-F were evaluated and modified in order to propose new Tex-126-H and Tex-204-H specifications. A comprehensive parametric study comparing the results of the two proposed specifications 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.

The most practical set-up for laboratory tests was achieved using Tex-204-H specifications, which proposes preparation of 6 in. diameter and 4.5 in. high specimens using 75 gyrations of the SGC. Furthermore, it is recommended to cure specimens for 24 hrs at room temperature (77°F) before conducting the indirect tensile strength because the results from this procedure were more sensitive to asphalt content while reducing the mix design period. The appropriate asphalt content should satisfy a target indirect tensile strength of at least 85 psi, and a relative density of 97%.