Top-Down Cracking in Asphalt Pavements: Causes, Effects, and Cures

A section of I-25 north of Denver was rehabilitated by cold milling the existing surface to a depth of three in. and replacing with new hot mix asphalt. The contractor received bonuses for quality and smoothness and the mixture passed both Hamburg and French LCPC rut tests. Within 1 year longitudinal cracks appeared in the pavement surface. The cracking appeared in the driving lanes of both the north and southbound directions. The severity of the cracking ranged from low to high. An investigation concluded that cracking on the project was surface initiated and caused by a number of factors. One factor was segregation observed at the bottom of the upper pavement lift that was not visible on the surface. After this so-called “top-down cracking” was discovered on this first project, other pavements began manifesting similar traits. Therefore, a statewide evaluation was conducted to determine the extent of this distress in other pavements. As a result of this study 28 sites were evaluated and of these 18 contained top-down cracking. Based on this finding a change in the mixture design process was implemented to allow for increased asphalt content in hope that the richer mixtures would not be as prone to segregate. In addition, a segregation task force was created with industry to develop a specification for segregation. This task force is in the process of developing a specification to identify subsurface segregation with the elimination of top-down cracking as the goal. Paving equipment manufacturers have also identified areas within the laydown equipment that can promote segregation. As a result manufacturers have taken the initiative to develop an antisegregation retrofit for some laydown machines. As a result of these efforts top-down cracking has been generally eliminated or, at least, greatly reduced.