February 06, 2020
A series of scattered, connected cracks that resemble a spider web is fatigue cracking. We’ve all seen it before in driveways, roads, and the parking lots of our favorite stores. It looks terrible and is an obvious sign that something is wrong. One small crack will become a larger crack and create another crack, and eventually, the entire surface is breaking apart, clearing the way for potholes and stress cracks. Thankfully, the damage is preventable and repairable with hot crack filler and other means—you just have to know what to look for before the problem gets out of hand. What causes fatigue cracking in asphalt? There are few culprits that we will address, here.
The problem could be as simple as poor construction. The company that laid the asphalt either didn’t know what they were doing or didn’t care. There is a clearly defined process for laying a solid asphalt road, parking lot, or driveway. If installers follow those steps, then it will last for many years with no maintenance. Not leveling the subgrade, using the right stone, and not having a quality asphalt can all lead to early fatigue cracking.
This ties in with the poor construction of the pavement. Making sure that all levels of the asphalt are level and clean will make it strong and durable. Using the right grade stone and the right mix of asphalt for the conditions will, too. If professionals cut corners during the planning stages, then the pavement will not last and it will need redoing soon.
There is a maximum weight that asphalt can support. When vehicles exceed that weight, continuously, the road or parking lot will break down. Asphalt cannot support as much weight as concrete can—that is the biggest drawback for it as a building material. Setting weight limits on the road will help it last longer.
The subgrade under the asphalt is important for the durability of the whole. If the subgrade erodes due to poor drainage, it weakens the whole slab. Think of a sinkhole; on the surface, everything looks solid and as it should. But underneath, there is nothing stable, and it’s a matter of time until your house or car falls in. Structural support of the pavement is vital to keep it from breaking apart.
A thin, top layer of asphalt will not have the strength to support the load demands on it for very long. This goes back to poor construction and design. In thin pavements, cracking initiates at the bottom of the hot asphalt mix (HMA) layer where the tensile stress is the highest, then propagates to the surface as one or more longitudinal cracks. Many know this as “bottom-up” or “classical” fatigue cracking.
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