Thin Concrete Overlay 1

[GalileoG]

All,

I am currently evaluating a buried concrete box structure where the roof slab is at grade level. There is concrete deterioration on the roof slab (delaminated concrete, corroded rebar) that will be rehabilitated. This deterioration was caused by water infiltration as the roof slab is exposed to the elements. I want to extend the service life of this structure by adding a thin concrete overlay no more than 3” thick - as I will have issues with grading if I go thicker than that. The structure has the reserve capacity to take the extra weight, so that is not a problem.

I am looking for tips to properly detail this thin concrete overlay so that cracking will be mitigated. I am thinking about reinforcing it with stainless steel welded wire mesh at mid-depth of concrete overlay, and doweling the overlay to the parent concrete at 5’ intervals in both orthogonal directions. The dowels would help prevent debonding between the overlay and the parent concrete.

Now about the joints…
The overlay will have expansion joints (polyurethane caulking) to match location of roof slab expansion joints, which are 80’ apart along the length of the slab. The slab is 20’ wide.
What are the requirements for contraction joints? Do I need them for a thin concrete overlay? How far apart should these joints be? Do I need to terminate the wire mesh at contraction joints (this would be complicated)? How should the joint be detailed? I am familiar with contraction joints for regular slabs, but I have never dealt with a thin concrete overlay before - any help here would be very much appreciated!

 

[damorim]

I'm assuming you will be exposing the roof slab? What sort of materials testing has been done to try to gauge the cause and severity of the corrosion problem? Was there no water proofing detail overtop of the slab?

I would recommend that some chloride test cores are taken to gauge the concrete chloride content and see how far over the 0.025% by mass of concrete threshold your results are. If there are significant levels of chlorides in your roof slab, I would consider the addition of passive cathodic protection through the use of galvanic pucks with whatever repair you implement. How much of the existing concrete will you be removing? If there are significant areas of delam, that all needs to be cleaned out to sound concrete. If you can get an 1" behind the top layer of bars, I wouldn't even bother doweling in. I'd provide a waterproofing layer overtop that slab to protect from future water intrusion...

I wouldn't recommend continuing the overlay or s/s mesh over the contraction joints unless there is evidence that the joints are no longing moving (seized shut for example). Otherwise, the overlay will definitely crack there. Maybe you can simply provide an emseal joint there?

 

[oldestguy]

Regardess of what you do for chlorides and reinforcing, you need to bond the overlay to the old concrete. One way to do this is to vacuum off all the dust from the removal of old weak material. Leave the surface as dry as possible (do not wet the surface). Just before placing a low slump overlay brush on a "cream" of Portland cement and water to try to get as much penetration of cement into the old concrete as possible and then place the overlay. Of course moist cure. My experience with this method has been very successful. You can check it later by taking cores and use a chisel to see if you can break it at the bond.

 

[SlideRuleEra]

See Guide to Concrete Overlays by the American Concrete Pavement Association

and

Resurfacing Concrete Floors by the Portland Cement Association.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[GalileoG]

Thank you all very much for your help.
SlideRuleEra, those documents are exactly what I was looking for - thank you!