Using the Michigan Joint configuration promised improvement in the joint performance. When tested, the joint region did not exhibit increased density as was presumed; however, the joint did perform better and lasted longer.
The case for permeability
With a lack of evidence to support the increased density claim, a different rationale for better performance has been formulated. The main reason for the increased performance in the joint is due to decreased permeability. This is the ability of the joint to withstand water impregnation.
In the past, permeability has been synonymous with density. It is theorized that in fact these may be mutually independent things.
It?s believed that the decreased permeability is in fact due to the physical geometry. That is the notch and wedge form a scarf joint, which increases the distance water must travel to penetrate the joint and get to the subsurface.
Also, the fact that the ramp is not rolled when applied leaves the aggregate exposed. Therefore, when the adjacent lane is placed, the stones in the mix interlock, making the path for water infiltration more difficult. So, it?s permeability, not necessarily density, which gives the improved performance.
Consequently, in order to build a robust longitudinal joint, both permeability and density are important.
The recommended method for building a well performing, long lasting joint is to use the Michigan Joint configuration. This will increase the length of the joint cross section and reduce the permeability.
Also, using a device to apply the configuration that has an approach angle, that can increase the amount of asphalt being forced under the screed in the joint region, will help increase the density of the joint region and give further integrity to the joint.