We have come to regard asphalt as a solid, similar to concrete. Some technical publications even use the term "asphalt concrete" to emphasis the similarity to concrete, as both materials use small aggregate to reinforce it. But, when it comes to attaching anything to asphalt, treating asphalt as concrete will lead to serious failures.
Asphalt is actually a viscoelastic material - a material that is both viscous and elastic. 'Viscous' means that the material will flow when forces are applied for extended period of time. 'Elastic' means that asphalt will partially return to its original shape after the external force was removed.
Source: Appl. Sci. 2017, 7, 142
How Does it Affect You?
If you try to attach a speed bump to asphalt and you use the common method of hammering in a steel rod into the asphalt, the pressure between the rod and the asphalt will loosen in a matter of days. That will cause the speed bump to move more and more as vehicles drive over the speed bump. Eventually the speed bump will slide away.
Our BoltHold asphalt anchors overcome this weakness by making stress-free connections with the asphalt.
The viscosity of asphalt is apparent in hours. In the graph below, the vertical axis shows the percentage compression of the asphalt while the horizontal axis shows the passing of time. Thus the asphalt compressed by 2% within 2 hours when subjected to 17 PSI steady pressure. That is less than the pressure of a car tire (typically 30 PSI).