Research in Progress: Predicting Bumps in HMA Overlays

The use of hot mix asphalt (HMA) overlays is common in Minnesota. However, while overlaying pavements with sealed cracks, bumps tend to form in the overlay resulting in decreased overlay smoothness and poorer ride quality. Hypotheses as to why (or how) these bumps are caused include:
 Thermal expansion of sealant during overlay
 Roller slip or slide as it rolls over sealed crack
 Differential melting points of the sealant and
asphalt binder
 Vertical compression of sealant under roller.

Prior research (LRRB Investigation 802) studied the thermal expansion hypothesis. Results showed that sealant materials may soften during overlay placement but not expand. It appears likely that the bumps may be formed during rolling operations; either by compression of the sealant or by slipping or sliding of the HMA material. This project conducted field testing to examine these issues, evaluate the probability of the occurrence of bumps, and look at ways of mitigating any bumps that do occur.