Iowa Department of Transportation
Chris Williamsrwilliam@iastate.edu email >
About the research
Iowa is among the northern US states that experience fluctuating low temperatures that cause low-temperature thermal cracking. Thermal stress buildup in pavements due to low temperatures—and often large, sudden drops in temperatures—result in excessive thermal cracking that requires frequent maintenance work. This increases maintenance costs for pavements and reduces pavement service life, adding an extra cost to departments of transportation (DOTs) budgets. To prevent this distress from occurring too soon in new pavements, there are specifications to guide engineers in designing asphalt pavement mixes. Generally, it is expected that field-produced mixes will have higher variance within their results when compared to laboratory-produced mixes. However, when the variance is too high, mix performance becomes compromised, thus leading to more excessive thermal fracture at low temperatures.
This study evaluated the current performance of field-produced mixtures using various low-temperature cracking methods and recommends necessary adjustments to the limiting criteria for laboratory-produced mixes to enable asphalt pavements to perform better and last longer under low-temperature cracking. Ten different field-produced asphalt mixtures were obtained from paving projects paved within the past seven years. These mixtures were reheated and laboratory-compacted using a gyratory compactor to produce 6 in. (150 mm) diameter specimens with a height of approximately 2 in. (50 mm). To determine the fracture energies of the compacted sample, disk-shaped compact tension (DCT) and semi-circular bend (SCB) tests were carried out as specified by ASTM D7313-13 and AASHTO TP 105-13, respectively. Air voids were determined prior to testing to ensure that specimens used met the air void requirement of 7% for testing. Illinois Flexibility Index Test (I-FIT) Procedure 405 was used for testing at intermediate warmer temperatures to get the flexibility index (FI) as well.
The 10 mixtures evaluated had an average fracture energy ranging from 265–470 J/m2 and 485–905 J/m2 for DCT and SCB, respectively. The FI obtained for the mixtures ranged from 8.36 to 23.32. The DCT fracture energies did not meet the DCT specifications contained in IM510 for the average minimum fracture energies, and the DCT and SCB fracture energies are lower than those produced for approval to pave. A performance criteria adjustment is recommended to ensure that field-produced mixtures meet design specifications from the laboratory to the field.