Effects of nonuniform and three-dimensional contact stresses on near-surface cracking

Near-surface cracking, sometimes referred to as top-down cracking, is one of the predominant distress types in flexible pavements. The incidence of near-surface cracking has increased in recent years with the increased construction of relatively thick (hot-mix asphalt layer > 200 mm) flexible pavements. However, understanding the mechanisms of near-surface cracking and its integration into pavement design protocols remains a challenge. Analysis of this problem can be complex because of multi-axial stress states in the vicinity of tires. The near-surface response to nonuniform tire contact stresses is investigated, and the potential for crack occurrence near the surface is analyzed in a typical relatively thick flexible pavement. The generalized finite element method (GFEM) is used to analyze pavement structure. This method provides a computational framework for the arbitrary orientation of cracks in a finite element mesh that is particularly useful for mixed-mode fracture problems. A three-dimensional (3-D) model for a typical pavement structure with a thick bituminous layer is created, and 3-D and nonuniform tire-pavement contact stresses are applied to the pavement surface. Aggregate-scale cracks are inserted at various locations and orientations in the pavement. Results of this numerical study indicate that complex stress states in the presence of strong mode mixity may cause shear or tensile fracture in flexible pavements. The importance of novel computational methods such as the GFEM to the discovery and understanding of mechanisms governing the premature failure of pavements is highlighted.