ICT and IDOT projects develop accelerated experimental procedure to assess aggregate properties

Aggregate properties have an impact on several aspects of asphalt pavement performance. One of the most important performance parameters of aggregate properties is frictional (skid) resistance, which is a factor in travel safety. Aggregates must also be tough and abrasion resistant to prevent crushing, degradation, and disintegration during production, transportation, and construction.

In 2012, the Illinois Center for Transportation (ICT) initiated a study at the request of the Illinois Department of Transportation (IDOT). The study focused on implementing a second-generation Aggregate Imaging System (AIMS-II) and the Enhanced University of Illinois Aggregate Image Analyzer (E-UIAIA) to develop a procedure to measure aggregate characteristics. The study was titled “Implementation of AIMS in Measuring Aggregate Resistance to Polishing, Abrasion, and Breakage” (R27-129). Enad Mahmoud, assistant professor in the Department of Civil Engineering, University of Texas, Rio Grande Valley (formerly at Bradley University, Peoria, Illinois), was principal investigator. Erol Tutumluer, professor in the Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, served as a technical advisor for the project.

In 2014, ICT and IDOT undertook a follow-up special project to conduct a comprehensive analysis of AIMS-II and Micro-Deval shape properties and the data collected in the first project. Mahmoud also served as the principal investigator on that project, “Investigation of Relationships Between AIMS Shape Properties and VST Friction Values,” (R27-SP27).

Sheila Beshears, Aggregate Technology Coordinator for IDOT’s Bureau of Materials and Physical Research, chaired the Technical Review Panels for both projects.

The major outcome of the research was development of an experimental procedure to evaluate aggregate friction properties. Mahmoud says, “One of the main advantages of the AIMS/Micro-Deval method is its ability to separate the effects of aggregate angularity and surface texture, which the current method cannot do. In addition, the time required to prepare and test the samples is significantly shortened under the new method, allowing several aggregate sources to be tested within a day or two.”

He says the method can also be used to identify polishing characteristics of aggregates such as intimal texture, final texture, and polishing rate.

The research relied on measuring aggregate particle angularity and texture directly with AIMS-II, which captured images of the aggregate shape and surface texture. The images were then analyzed to produce shape indices, and the Micro-Deval was used as a polishing mechanism. Aggregate shape properties could be measured before polishing, after intermediate polishing, and after terminal polishing in any of the following combinations:

• Measure aggregate properties after terminal polishing only: This technique can obtain quick results to assess terminal friction for a particular aggregate.
• Measure aggregate properties before and after terminal polishing: This technique can assess initial and terminal friction properties and provide an indication of the initial service (friction) quality.
• Measure aggregate properties before polishing, at intermediate polishing, and after terminal polishing: Full characterization of aggregate polishing characteristics can be obtained this way—not only to predict terminal polishing but also how quickly it will be reached.

As part of the ICT research, a classification chart was developed to aid IDOT in assessing friction characteristics of aggregates. The research team recommended that an aggregate source have a minimum 140 texture index and 1240 angularity index after 105 minutes of polishing in the Micro-Deval.

Beshears says that the new test will provide a quicker, more accurate test for friction. “Our current test takes much longer to run, so our friction policy had to be based on lithology of the aggregates. IDOT has known for years that some aggregates perform outside of the ranges set in our current policy.” She adds that as result of this research, IDOT will be able to use more local aggregates at a reduced rate, which will save money for the agency and the State of Illinois.

She notes that testing will need to be conducted on materials finer than those tested as part of the research project. Once the new testing is finished, she says, IDOT will implement a new friction policy based on individual source results rather than what type of aggregate the source produces.