Summary of TxDOT Superpave Projects Constructed in 1997
Ten Superpave projects were constructed in 1997 as part of TxDOT's strategic plan to implement Superpave. Approximately 10,000 mg of Superpave mixture was placed on each of the ten construction projects. In addition, a minimum of 5,000 mg of conventional HMAC was placed on each project as control mixtures for comparison purposes. Eight of the ten projects were built with 19.0mm Superpave mixtures and the other two were built with Superpave 12.5mm mixtures. Volumetric mixture designs were completed for each project. For the most part, the mixture design process was fairly simple. In some cases, several trial blends were tested in order to be able to meet the minimum VMA requirements without violating the VFA requirements.
The Texas Hot Mix Asphalt Pavement Association (THMAPA) conducted a survey of the contractors that built the ten Superpave projects. The results of the survey indicate that constructing Superpave mixtures appears to be very similar to conventional hot mix and CMHB mixtures. Tenderness of the mixtures and difficulty achieving in place density, were the two major concerns expressed by the contractors. The condition reported by many contractors and TxDOT employees is the Superpave mix appears to be stable under the breakdown roller; however, after a few passes of the breakdown roller, the mix becomes unstable and begins to move and act much like a tender mix. Further compaction can only be achieved when the mix has sufficiently cooled to be able to sustain additional compaction. Mixtures are reportedly tender in a temperature range of 180F to 270F. Almost all of the 19.0mm mixtures experienced some degree of tenderness whereas neither of the two 12.5mm mixtures experienced tenderness. The tenderness experienced with the 19.0mm mixtures varied from mild to extreme. In a few cases the 19.0mm mixtures developed numerous micro-cracks and some longitudinal cracks in the finished mat. In one instance the movement of the mix under the roller caused the finished mat to be one foot wider than the placement width.
Production Test Results
Extensive testing was conducted during the mixture design and production of each project. Four sets of tests were conducted for each production day. The tests included gradation, asphalt content, maximum theoretical specific gravity (Rice gravity), and bulk specific gravity using both the Superpave and Texas gyratory compactors. Results from the tests indicate that generally, a good degree of production control was exercised for all the projects. In addition to the production tests, in-place air voids were measured on all of the projects. The air voids on seven of the projects exceeded 10.0%. The in-place air voids on the two 12.5mm mixtures and the 19.0mm mixture in El Paso were less than 10.0%. These are the three projects that did not exhibit tenderness during construction.
- Experience with Superpave 19.0mm mixtures in Texas indicate that the mixtures can be very tender. As a result, it is difficult to achieve in-place density on these mixtures.
- Limited experience with Superpave 12.5mm mixtures indicate that the mixtures are stable under the rollers and in-place density can be achieved.
- An acceptable level of production control is achievable on Superpave mixtures.
- There is a good correlation between the densities of specimens compacted with Superpave compared to Texas gyratory compactors when plant produced mixtures are compacted.
When compacting laboratory produced mixtures, the SGC consistently produces mixtures with lower densities than the TGC.
- Due to the tenderness of the 19.0mm mixtures, we recommend that construction of 19.0mm mixtures be avoided until there is a method to predict the "tenderness" phenomena.
- Superpave 12.5mm mixtures should be used when a Superpave surface is desired until the problems with the 19.0mm mixtures can be corrected.
- TxDOT should allow either the TGC or the SGC to be used for field control of HMAC.