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slab on slab

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DanatDynamic

DanatDynamic

Structural
Joined
Apr 10, 2014
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I am looking for standards/specifications/recommendations/design guide for pouring a 4" concrete slab over an existing interior 4" concrete slab (foot traffic)...if one exists. Typically, we do not recommend or specify this, due to lack of knowledge of the existing slab and sub-base, however, a current client has specifically requested it, for financial reasons. Therefore, before I discount this option, I am wondering if there is any protocol that can be followed for successfully pouring a slab over a slab...with the confidence that the end product will perform. I have not been able to locate any information with ACI. Does a guide/protocol exist? If not, what are your thoughts/recommendations? I can provide more specific project information, if necessary. Thank you.
 
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Is the existing slab structural or on-grade?

 
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It is an interior slab on grade. Again, we know nothing about the material under the existing slab.
 
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What's the reasoning for the replacement/covering? If the old slab is heaving or overly cracked from anything other than shrinkage cracks then I wouldn't touch it with a 100 foot pole.

If it is just to have a nice new concrete floor and the old slab seems to be in generally good working order than I don't see why you can't just through the new slab on top of the old one.
 
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The existing concrete slab is in great condition. The reason for slab on slab is simply to raise the elevation of the existing slab area to connect spaces that are at varying elevations (4" difference) for accessibility purposes. The floor area to be raised 4" is approximately 380 s.f. (19ft x 20ft).
I am not able to locate any standards for "slab on slab" construction, if any exist. Should the new slab be independent from the existing slab?...slip sheet between....or
Should the new slab be bonded to the existing slab?...bonding agent and/or reinforcement ties.

 
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The real problem is that most slabs on grade develop soft spots, or areas where the materials underneath have settled away from the slab. These can cause serious problems, as well as any equipment driven onto the slab can cause temporary elastic deflection which can rebound and wreak havoc with your new slab.

I am speaking from personal experience. One of my jobs in New Zealand was nearly ruined by construction equipment causing a later rebound. This was likely due to an insufficiently compacted base under the existing slab. There is no way to ensure the quality of the new slab when you blindly place over old.

I have been told that the same preparation normally used for adding loads to an exising slab (ie: low pressure grout injection) works, but with no way to do any quality control worth a damn.
 
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19' x 20', I would just do it. A slip sheet would help with shrinkage cracking.
 
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Not all that different than a topping slab. You can try to match control joint locations and count on the adhesion between the old and new slab and with some luck get decent shrinkage cracking control. Or you can do something like a slip sheet to keep the two slabs from bonding together, but unless things are perfectly smooth you will be some interlock that could cause cracking.
 
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Either tie them both together COMPLETELY or separate them COMPLETELY. Any half-assed attempt at either will be a problem.
 
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