NCAT invites your comments and questions, which may be submitted to Christine Hall. Questions and responses are published with editing for consistency and space limitations.
Randy West, NCAT
Are the Gsb values used in mix designs in your state determined by the highway department or by the contractor (or mix design lab)? How often are the Gsb values tested?
Jerry Geib, Minnesota DOT
In 2016, eight of the HMA cells at MnROAD will be reconstructed as part of the NCAT-MnDOT cracking study. The MnROAD experiment will focus on thermal cracking.
MnDOT will be using a 4.75 mm mix for a thin overlay of PCC on the Interstate in Duluth.
The ultrathin bonded wearing course (UTBWC) reacts differently than our dense graded HMA in rural areas during snow and drifting events. We are discussing a future effort to look into this matter. This is not an issue in our metro area.
The following responses have been received to questions shared in the previous issue.
MnDOT is moving toward implementation of the disk-shaped compact tension (DCT) test. This test measures fracture energy. High fracture energy provides greater resistance to thermal cracking. Are other DOT’s looking at new performance tests?
Michael Stanford, Colorado DOT
Colorado DOT is currently performing the DCT test for information only. We test at a frequency of one test per mix for each project.
Eric Biehl, Ohio DOT
In Ohio, we are looking at mix fatigue tests for high RAP and RAS mixes through a research project that just kicked off. The Illinois Flexibility Index Test (I-FIT or also known as SCB-IL) and Texas Overlay Test are probably the front-runners for us. We have not looked at low temperature mix tests yet, but could see that coming at some point in the next few years. For PG binders, we’re hoping to see what comes out of NCHRP 9-59 and 9-60 in a few years.
James Williams, Mississippi DOT
Currently, Mississippi DOT is not looking at any particular performance test for cracking. Of particular interest is research into various cracking tests and their relationship with field performance. These relationships and the associated performance models are key to ME design.