Adding 4.5" of concrete on existing slab? 2

[NewOrleansPE]

a client needs to elevate existing monolithic house slab with 4.5" of concrete to meet flood elevation to get permit from city in order to start building his house on top. The slab is on piles which used to support a 2 story house that was burned down. Just a slab on grade now.

I figured additional weight of concrete would add about 56 lb/ft^2 . slab is 46' x 30' , with 4.5" of concrete. which I don't think would be an issue as for as loading.

but how would you ensure a good bong between the two slabs? should rebar be doweled between the two? or the surface of the existing slab be roughen up? more importantly, should geotechnical tests be done to determine if additional concrete is ok? Not really sure how to approach this one...

thanks

 

[msquared48]

Adding 56 psf to the foundation is no small thing here. If there are pile, then there are, or should be, pile caps and grade beams, as well as the existing slab that spans between them, all with reinforcing. All these elements will have to be analyzed for the new load, with or without any possible composite action from the new slab.

The composite action would only possibly help the existing slab, and maybe the grade beams, depending on the existing slab to grade beam connection.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[csd72]

It is very hard to ensure bond between new and old concrete, I would not rely on this for structural strength.

 

[DWHA]

Well, first I would conside using light weight concrete. As mike stated you need to look at the pile caps and grade beams.

Another option would be to remove the slab and place 4.5" of compaced fill then pour a new slab.

 

[SlideRuleEra]

The key to getting a good bond between old and new concrete is proper preparation. I agree with csd72 that this is hard to ensure, but that is because most Contractors refuse to accept the attention to detail needed to make it happen:

1. Existing concrete must be in good condition (poor material removed).

2. Existing concrete must be properly cleaned... and kept that way.

3. Existing concrete must have the proper moisture content (quite damp) when the overlay is placed.

If these basic steps plus an appropriate concrete mix and good workmanship are used the bond can be more that adequate.

Here is a "Guide to Concrete Overlays" (free .pdf download) by the National Concrete Pavement Technology center. The overlays described here are for highway use, but the principles and measures outlined are appropriate for most situations.

http://www.cptechcenter.org/publications/overlays/index.htm

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[msquared48]

It might be easier to add a 2' high concrete stemwall at the periphery of the slab at the exterior wall lines, and wood frame over that, venting the newly created crawlspace. At the most, you could be adding 10 psf to the foundation - the dead load of the first floor framed wall. A lot better than 56 psf and possibly a lot of unknowns.

If you have the plans of the structure (highly unlikely), or know where the post and bearing lines were (a maybe), it would be best to stick to those support points as closely as possible to avoid having to fully analyze the foundation.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ACtrafficengr]

Was the existing concrete calcined in the fire? It might be worth the effort and cost to remove and replace it.

Rather than trying to bond the two slabs, could you put down a bond breaker between them, similar to white-topping a highway?

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[hokie66]

As others have said, the first step is to determine whether the piles and any substructure are capable of carrying the new load as dead load. If so, then I wouldn't worry about bonding the new to old. As ACTrafficengr implies, bonding the two is probably detrimental, as it will make the new concrete crack more.

If the piles can't carry the load, I like Mike's idea.

 

[csd72]

Why not take the chance to provide additional insulation to the floor. You could provide 3 inches of rigid insulation with a 1.5 inch screed over the top. Just enough room for a fine mesh.

 

[InDepth]

Would avoid light weight concrete if you plan on anchoring your walls and tiedowns to it.