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Pitfalls of Mounting to Asphalt


Asphalt is not Concrete!

Asphalt and concrete have one property in common -- you can drive on either of them. But when it comes to attaching anything to asphalt, anchoring has nothing in common with concrete.

The difficulty with anchoring to asphalt is that it will yield, over time, to continuous pressure. If you were to leave your car in the same spot for a year, the tires will leave a depression in the asphalt. And that is even though the pressure on the surface by the tires is very low -- around 35 PSI (pounds per square inch).

Concrete anchors rely on the expansion of the anchor as it is tightened. The expanded anchor pushes against the walls of the hole around the anchor. The friction prevents the anchor from rotating or pulling out. But if the concrete was to yield by as little as 1/16", the holding forces will drop precipitously. Thus the resistance of the walls to pressure determines the pull resistance of the anchor.

A good indicator of the rigidity of materials is their tensile or shear strength. The tensile strength of concrete is about 5,000 PSI. The tensile strength of asphalt is about 200 PSI at 70 degrees F when measured for short-term forces. Under continuous forces, the asphalt "creeps" and the resistance drops with time. The creep starts as soon as 10 seconds.

It follows that one can not use expansion anchors in asphalt. Even if it held at first, the creep would loosen the anchors within hours.


So what is a viable solution? Using "chemical" anchors.

Chemical anchors use grout (epoxy or special cement) to bind the anchor to the asphalt. The grout flows into the crevices in the asphalt and the gravel below it, and bonds to the asphalt on the one side, and the the specially-designed anchor on the other side. This bond is stress-free, and thus not subject to creep.

More on this subject in future blogs.

References: The 2 research documents below provide detailed insight into the properties of asphalt.

http://pedago.cegepoutaouais.qc.ca/media/0260309/0378334/SCGC-BON/Documents/TR149-Abd-El-Naby.pdf

https://engineering.purdue.edu/NCSC/documents/presentations/MO_Creep_Compliance_and_Tensile_Strength_of_HMA.pdf


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