heated sidewalk transition to unheated concrete asphalt

[bta15]

Hi,

I have been working on the design of several heated slabs for both sidewalks and loading dock ramp areas. One issue that is arising on projects around my local community is heaving of the unheated surface (AC pavement or concrete) adjacent to the heated concrete. This is due to runoff from the heated concrete draining into the seam between the heated and unheated surface and then freezing and heaving.

Generally we place 2-4" of rigid board insulation right below heated concrete and then place another piece of insulation either at a 45deg angle or vertically at the transition and it has had varying levels of success. Have any of you come up with solutions for this? We have tried to specify sealants at the seam but I am not sure this will do the trick. Any ideas of what else could be used?

Thanks,

bta15

 

[CarlB]

Are you certain the water drainage is causing the heaving? Does it only heave for the few feet next to the heated portion? I ask that since it could just as well be that all the unheated area is frost heaving, and the displacement is just obvious at the transition because the heated sidewalks/slab do not heave.

A detail sometimes used is to extend the insulation horizontally some distance (say 4') under the unheated section, just to reduce the heave there. However that may just move the problem to the end of the insulation.

 

[bta15]

The issue does seem to be localized to unheated sections adjacent to heated sections. In areas where unheated sidewalk is adjacent to asphalt the heaving is not noticeable.

 

[oldestguy]

If you want to make the transition longer consider this. The material under the unheated walk should be non-frost susceptible at the junction and tapering to regular subgrade say 10 feet out. The question is how deep is freezing temp in clean granular (less than 5% P-200). Four feet may not be sufficient, since the moisture content of granular soil is low and that means frost can penetrate deep. Even though clean granular soil is below freezing, it won't heave. I've done this commonly for entrance sidewalks to buildings.

For typical northern US conditions, I'd go at lest 4 feet down with granular at the junction, preferably 6 feet to be sure. Taper up to zero thickness 10 feet out. I'd skip insulation there, since exactly what it dues is questionable here.

 

[bta15]

We see frost depths down to 10 feet. Typically we are shooting for 11 feet min bury on water line services and mains. I don't think it is economical to ex out that deep and replace with NFS material.

Generally we are using 6 inches of an NFS 1" min base with 18-24" NFS 3" minus classified fill. So I think without excavating really deep we are going to be running into these issues if we are not using insulation and using it correctly, but I could be wrong.

Am I just approaching this completely wrong?

 

[oldestguy]

That's some cold area. Along the line of tapering the effects of frost, considering your area and the use of heated walks, what about tapering the effect of heat also?

Combine some use of tapered thickness clean granular to a reasonable depth along with tapering a heat usage out there also. That is, could not your heat cables be gradually spaced farther apart going out from the protected slabs? Maybe also taper the insulation effects also. My past experience shows 10 feet works OK in my areas.

As a side issue if you ever have freezing rain, that can be deadly when there are sharp differences like you may have. Going from the heated area to the non protected slabs or pavement can mean falling accidents or skidding vehicles if the break is sharp. This has happened when roadways are constructed on insulation next to ordinary subgrades. Makes for accidents.