Increase thickness of slab on grade 1

[bvbuf]

I have an existing building being repurposed to a storage facility. The existing slab on grade is 4" It needs to be 7" for the interior wall loading. The owner does not want to remove and replace or cut up the existing to add thickened areas. The existing building was retail.

Can I place 3" on the existing 4" slab and call it good?
Would I need to bond the slabs? If so, roughen slab and add epoxy?
Would I also need to mechanically attach the topping? I do not expect forklift traffic.
If bonded, I understand about reflective cracking.

If I go with unbounded, does the "topping slab" have to be the full 7" thick?

Is there a publication that specifically addresses this?

Thanks

 

[dik]

Depends on the condition and prior use of the 4" slab... If you can hydrodemo it to provide a clean surface, and if the surface is OK, then you can dry it and put a concrete patch on it... Check loading so that horizontal shear is low and you should be good to go. One of the other posters have outlined a manner of adding a conc patch in one of the forums. I think it was SRE.

Dik

 

[MotorCity]

What sort of new interior walls are we talking about? CMU? Concrete? Loadbearing? It seems like overkill to add 3" over the entire floor when a thicker section is only required around the walls. Maybe the cost will persuade the owner to consider thickening the slab at the walls only.

If the existing slab is in good condition, I would not bond the two slabs together. In fact, I would provide a polyethylene sheet bond breaker between them and maybe some wire mesh for shrinkage reinforcement.

On a sidenote, is there a potential for the increased slab thickness to affect door swings, head clearance, etc?

 

[SlideRuleEra]

bvbuf - Per Dik's comments, the condition of the slab is most important. A 4" thick existing slab may be too thin to survive surface preparation for a bonded overlay.

If the full strength of 7" inch (bonded overlay) slab is not needed, a 3" unbonded overlay should work.

Go through the "Guide to Concrete Overlays, Third Edition" (free .pdf) by the U.S. National Concrete Pavement Technology Center, for info on both bonded and unbonded oevrlays.

This thread may help, too.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[bvbuf]

Thanks Guys - I appreciate your help.

dik - I haven't see the slab itself. But the VCT looks pretty good. It was a retail store (K-Mart?)in continuous operation.

Motor City - The interior walls are prefab steel wall that comprise the storage unit walls. I'm thinking they are like a shipping container. They continuously run the entire length of the building spaced at about 10' o.c. DOOR SWINGS! Good point.

SlideRuleEra - A 7" slab on grade is required for the wall loading of 1650 PLF. I'm not sure if that is what you mean by " the full strength of 7" inch (bonded overlay) slab". Thanks for the publication and the thread.

bvbuf

 

[SlideRuleEra]

I'm not sure if that is what you mean by " the full strength of 7" inch (bonded overlay) slab".

With a bonded overlay, the concrete (should) perform as if it was at 7" thick monolithic slab:

Page 2 of the "Guide to Concrete Overlays"

An unbonded overlay is expected to perform as a 3" thick slab that is supported by really good base material (the existing 4" slab)

Page 3 of the Guide

A 3" (unbonded) slab is pretty thin, even on good base. Is that good enough for this project... maybe, that's your call.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

I dug a little through the reference by SRE but found no method on how to bond the new to the old. My experience with this method has found it to work well. You roughen the old surface with jack hammers or other tool to get rid of any weak concrete. Vacuum off all the dust. Make sure that old surface is BONE DRY! Have your new concrete layer batch on hand ready to apply it to the old. DO NOT WET THE ROUGHENED SURFACE. Now brush on a Portland cement paste of cement and water at the consistency of cream. Work it into that roughened surface. Then add your new concrete mix immediately. Moist Cure with surface covered for 7 days minimum. Later take a few cores to see how well you did. Use a chisel and see if you can break the core at that bond. My bet is it will break elsewhere.

 

[SlideRuleEra]

Portland Cement Association (PCA) offers a free paper, "Resurfacing Concrete Floors", that includes the bonded overlay method OG recommends along with other info.

bvbuf - Using the dry-method for this project - a 4" thick slab will not survive a jack hammer. Scrabbling or abrasive blasting... maybe.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

SRE again shows his know how of using less impact than jack hammers from roughening as well as using low slump concrete.

 

[dik]

maybe it was OG, sorry Guy... someone did... I thought I was the only guy that used that method...

MotorCity: I would definitely bond the two layers and would not use PEVB as a separation. I'd also make sure the sawcuts aligned with the existing jointing.

Dik

 

[MotorCity]

dik - curious, why would you recommend bonding the layers?

 

[dik]

to engage the horizontal shear, from flexure, and, provide added strength and stiffness.

Dik