Fat Spots

Description

Isolated areas in the mat where excess asphalt binder is visible on the surface.

Fat spots in a newly constructed SMA
Figure 1: Fat spots in a newly constructed SMA

Fat spots seen in a core showing a SMA top layer
Figure 2: Fat spots seen in a core showing a SMA top layer

Problem

A few scattered fat spots may not significantly affect mat durability or pavement structure. A large number of fat spots may affect:


  • Pavement performance. Air voids in fat spot locations may be excessively low leading to potential shoving and rutting.
  • Skid resistance. Fat spots have a smooth surface texture and will decrease skid resistance, especially when wet.

Possible Causes

Causes can be related to HMA moisture, petroleum/diesel contamination or construction:

  • Excessive moisture in the HMA. Aggregate with significant moisture can cause fat spots because the moisture vapor will force asphalt binder to the mat surface behind the paver as it escapes from the mix.
  • Petroleum or diesel contamination. This can occur when petroleum products are spilled onto the underlying surface in isolated areas before paving or when these products contaminate the mix.
  • Construction. Mixes that use heavily modified asphalt binders, such as SMA, tend to generate accumulations of asphalt binder on construction equipment like pavers and MTVs. As binder accumulates, it eventually breaks free of the equipment or drips onto the mat below causing a fat spot.

Solutions

Solutions differ depending upon the cause:


  • Excessive moisture in the HMA. Ensure all aggregate is dry when combined with asphalt binder during the HMA process. Moisture content should be as low as possible but generally not more than 0.5 percent.
  • Petroleum or diesel contamination. Be especially careful when using petroleum or diesel products around a paving site. These products are banned by most agencies as a truck bed release agent because of their destructive effect on HMA mats.
  • Construction. When paving with modified binders, be particularly careful that excess asphalt binder does not accumulate on paving equipment and drip onto the newly placed mat.