QUANTIFY GRADATION AND STRENGTH FOR LARGE-STONE HMA MIXES

Accurate rating of aggregate gradation and strength — including the effect of stress concentration at contact points of coarse aggregates — is paramount for large stone premium hot-mix asphalt mixes, but methods are available to quantify these variables, say Cesar Alvarado, Fugro Consultants Inc.; Imad N. Abdallah and Soheil Nazarian, Ph.D., P.E., Center for Transportation Infrastructure Systems, University of Texas-El Paso; and Eyad Masad, Ph.D., P.E., Texas Transportation Institute, Texas A&M University, College Station, in their paper, “Feasibility of Characterizing the Role of Coarse Aggregate Strength on Mechanical Properties of Hot Mix Asphalt.”

Today's new large stone mixes include both polymer-modified stone matrix asphalt (SMA) and open-graded friction courses (OGFCs). “The performance of the new generation of hot-mix asphalt mixtures that rely more on stone-on-stone contact is greatly influenced by the properties of the aggregates such as gradation and strength,” the authors write. “As a result, it is important to quantify the quality of aggregates to ensure proper performance of roadways. Aggregate specifications should ensure that particles possess the necessary strengths to avoid degradation during handling, construction and trafficking.”

Several well-known methods are available to determine the aggregate characteristics and their relationship to field performance; aggregate structure in HMA and traffic loading needs to be further investigated and defined, they say; so they undertook an experimental investigation to evaluate the effect of stress concentration at contact points of coarse aggregates that could cause aggregate fracture as presented.

A multifaceted study, which focused on the geological, geotechnical and mix design aspects, was carried out. Their main objective was to identify tests that can characterize the aggregate and the performance of the new generation mix design, and their research effort involved testing rock masses from quarries, individual aggregates, combined aggregates for a specific mix, plus HMA mixes, prepared with different gradations to correlate the performance of particles and blends to their respective characterization test methods.

Three aggregate types were selected from three Texas DOT districts: a granite, a hard limestone and a soft limestone. These aggregates are commonly used in Texas and their performance histories are well-known. Soft limestone aggregates are known to perform poorly in coarser mixtures, while the granite and hard limestone aggregates are classified as high quality in Texas but their performances in coarser mixes have been uneven. “The field performance of coarse mixes with granite has been mixed whereas the hard limestone performs quite well in coarse mixes.”

Correlation analysis among a variety of familiar tests was performed. “Traditional aggregate tests, even though quite valuable, do not seem to provide a full picture of the performance of aggregates,” the authors write. “A series of simple tests is proposed that can be performed on aggregates and rock specimens. Further studies are currently being conducted under the same research project to include more aggregates and mixes so that a larger database can be developed for more conclusive results. The aggregate properties such as stiffness, compressive strength and tensile strength will be used as input to the model to predict performance for mixes under different loading conditions.”

The authors observed:

* The ACV test and its surrogate parameters were found to be suitable for assessing the performance of aggregates.

* The compressive strength obtained using the Schmidt hammer seems to be the most appropriate test for characterizing this parameter in the quarry. This test is not only easier and faster than the compressive strength test, but also eliminates the need for coring the rock and requires minimal training.

* The V-meter seems to be a great tool for estimating the modulus as well as the quality of the rock in tension.

* For the performance characterization of the mixtures, the indirect tensile test and the modulus with the V-meter seem to be the two methods that can be readily implemented and are faster than other performance test used in this study.