Sometimes you have to dig deep to come out on top. That is what researchers are doing, as they dig into how the lower layers of pavement impact the strength of the road surface you drive on.
In 2014, the Illinois Department of Transportation (IDOT) raised concerns about the requirements for fines content in the coarse unbound aggregates that are used in pavement base layers. Improper silt and clay fines (size less than 75 micrometers) content characteristics may result in premature failure of base and subbase course layers and consequently manifest itself on surface course with rutting (wheel-path depression) and stress cracks.
In this study, the quality control criteria of fines content in unbound aggregate used for base and subbase application were investigated.
“The goal was to either validate the current specification or determine if any updates could be made to the plasticity and dust ratio criteria, which would improve the performance of the aggregates IDOT uses for constructing subbases, bases, surface courses, and shoulders,” Technical Review Panel Chair Heather Shoup said.
Principal Investigator Dr. Abdolreza Osouli and the research team collected samples, ran tests, and analyzed results. The findings of this analysis were published in Project R27-157, “Plasticity Requirements of Aggregates Used in Pavement Base and Subbase Courses.” A series of static and dynamic laboratory tests on various combinations of crushed limestone and gravel materials were conducted to determine the factors impacting strength, stiffness, and deformational behavior of aggregates used in unbound base and subbase applications.
“This project was unique in that it extensively investigated the interplay of several important factors such as plasticity index, fines content, dust ratio, aggregate type, and aggregate gradation on soaked and unsoaked strength of the aggregates for base and subbase layers through laboratory testing,” Osouli said.
The outcome of this study will be used to modify the quality control criteria for fines content characteristics in aggregate materials used for base and subbase layers of highway embankments. The study recommends revising the current specifications. However, verification of the study findings is recommended utilizing vehicular loading of full-scale pavement sections simulated by the ICT’s Accelerated Transportation Loading System (ATLAS) prior to modifying the specifications.
This study is expected to improve Illinois’ pavement performance by implementing a knowledge- and Illinois-based criteria for proper use of fines in unbound layers.