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I recently received a copy through AASHTO TCRS of the Progress Report for Third Quarter of the Midwest States Regional Pooled Fund Program dated September 15/08. I note your answer and detail for a pipe sleeve under Problem #10 to address the asphalt overlay issue we had discussed previously. Is a 3" asphalt overlay the absolute maximum thickness for this detail or could the thickness of the overlay be increased at least to 4", or even 6"? 

The bolt through tie-down in asphalt retrofit we came up with should work for deeper asphalt depths if needed. The issue is that the pipe size needed and thus the hole in the concrete becomes larger. I have come up with tube specs for larger asphalt depths. There is a spec for the smallest tube size I could make work as well as the smallest standard schedule pipe I could find. These pipes were sized based on expected maximum shear and bending loads expected.

For 4" asphalt

1. 4" Schedule 100 pipe (yield strength = 35 ksi) 2. 3.25" OD x 0.625" thick x 2.00" ID 1026 CD round tube (yield strength = 72 ksi)

For 6" asphalt

1. 5" Schedule 100 pipe (yield strength = 35 ksi) 2. 3.75" OD x 0.625" thick x 2.5" ID DOM (Drawn Over Mandrel) round tube (yield strength = 75 ksi)

Another thing to note is that the larger tube sizes shown here will require a corresponding increase in the plate washers used in the system.