This page puts forth some basic guidance for use when designing HMA pavements and is meant to be used as a checklist. When designing a pavement, the guidance listed here should at least be considered. The guidance contained on this page is not absolute, however design and construction in conflict with this guidance should (1) only be undertaken for a specific reason and (2) only carried out once the associated risks are understood.
Top Down Cracking
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.
Condition Rating Systems
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):