Top down cracking appears to be a common mode of HMA pavement distress in at least several states and countries. Traditionally, pavement cracking is thought to initiate at the bottom of the HMA layer where the tensile bending stresses are the greatest and then progress up to the surface (a bottom-up crack). Most traditional transfer functions used in mechanistic-empirical structural design are based on this concept. However, the late 1990s saw a substantial focus on a second mode of crack initiation and propagation: top-down cracking.
Based on measurements of roughness, surface distress, skid resistance and deflection, pavements can be assigned a score that reflects their overall condition. This score, sometimes called a pavement condition rating, quantifies a pavement's overall performance and can be used to help manage pavement networks. By carefully choosing the rating scale (called the condition index), pavement condition scores can be used to (Deighton 1997):
Water bleeding (left two photos) occurs when water seeps out of joints or cracks or through an excessively porous HMA layer. Pumping (right-most photo) occurs when water and fine material is ejected from underlying layers through cracks in the HMA layer or out the sides of the HMA layer under moving loads.