RELATIONSHIPS BETWEEN MIX CHARACTERISTICS AND LOW-TEMPERATURE PAVEMENT CRACKING (WITH DISCUSSION)

Low-temperature cracking of asphalt pavements occurs over much of the United States and Canada. The effects are costly in terms of accelerated deterioration, depending upon the rainfall, frost, subgrade, traffic and other conditions at the particular site involved. Other implications include the need for earlier-than-planned rehabilitation as well as significant maintenance costs. In addition, there is the future implication of reflective cracking through any overlays placed, which is, in effect, a continuation of the problem unless complete reconstruction is carried out. The basic mechanisms of low-temperature crack initiation and propagation were hypothesized a number of years ago, and the following conditions were identified under which such cracking can occur: (a) excessive thermal stresses in the bituminous layer, (b) freezing shrinkage and cracking of the base, subbase or subgrade, and (c) differential frost heave of the subgrade. Excessive thermal stresses in the bituminous layer represent by far the most prevalent condition. In this case, under decreasing temperatures a micro-crack initiates at the surface when the thermally induced stress exceeds the tensile strength. The crack then propagates through the full depth of the layer with one or more thermal cycles. Another, similar phenomenon occurs when an existing, cracked pavement is resurfaced and these underlying cracks propagate up through the new layer under one or more cycles of thermal shrinkage. It is commonly termed "reflection cracking" and represents a serious problem over much of the U.S. and Canada. A large amount of research has been carried out on low-temperature cracking. It indicates that the nature of the asphalt itself is a most important variable in low-temperature cracking. However, mix characteristics can also play an important role and that is the focus of this paper. More specifically, the paper addresses the following: (1) Mechanisms and factors associated with low-temperature cracking; (2) Methods for characterizing the cracking susceptibility of mixes; (3) Design alternatives to minimize or reduce cracking; and (4) Recommendations for obtaining more crack resistant mixes.