Low Temperature Cracking Performance at MnROAD

The Minnesota Road Research Project (MnROAD) was constructed in 1990–1993 as a full-scale pavement testing facility. Several different cells were built with various materials, mix designs, and structural designs. Two different asphalt binders were used during the original construction: PG 58-28 and PG 64-22. The sections have all shown various degrees of low temperature cracking. In general the cells with stiffer binder (PG 64-22) experienced a higher number and greater severity of thermal cracks than those with the softer binder. The ride quality of the pavements has been adversely affected by the deterioration of the low temperature cracks. In 1999 three cells were reconstructed on the Low Volume Road as a study specifically examining low temperature cracking. These sections were designed using the exact same Superpave mix design except for the asphalt binder type, which differed at the low temperature performance grade. The performance grades for Cells 33, 34, and 35 were PG 58-28, 58-34, and 58-40 respectively. After several years in service these sections have begun to show marked differences in performance. Cell 35 has shown the most cracking, even though it has the softest grade at –40. The cracks on Cell 35 do not look like typical thermal cracks, while Cell 33 exhibits the expected typical thermal cracks. Cell 34 had virtually no distress after six years.