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.

Figure 1: Likely Top-Down Cracking on I-37 South of San Antonio, TX

Although not fully understood at this time (2002), there are three basic views on the of top-down cracking mechanism:

1. High surface horizontal tensile stresses due to truck tires (wide-based tires and high inflation pressures are cited as causing the highest tensile stresses). 
2. Age hardening of the asphalt binder resulting in high thermal stresses in the HMA.
3. A low stiffness upper layer caused by high surface temperatures.

Likely, the mechanism is some combination of the above.  The bottom line is that HMA top-down cracking is not thoroughly understood and, at this time, is generally not considered as a causative factor for pavement cracking although it probably should be.  Two simple suggestions may help in the identification of top-down cracking. 

1. In thick HMA pavements, consider top-down cracking as a possible cracking mechanism.  Generally, previous research has found that in pavements thicker than about 6 inches top-down cracks can be and often are the dominant form of cracking.  Do not assume pavement cracks are bottom-up. 
2. Before deciding on a maintenance and/or rehabilitation strategy, take a pavement core on a suspect crack (see Figure 2).  Usually, a pavement core will show whether a crack is top-down or bottom-up.  It will also show the extend to which the crack has propagated, thus defining the extend of needed milling prior to overlay.

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