2" concrete fill over existing SOG 2

[WWTEng]

Existing construction with 5" concrete slab on grade with 2" mud-set tile. The mud set tiles were removed for a portion of the new slab due to some new work and the question now is how to infill back the 2" with new concrete. To me it appears that the 2" is similar to non-structural topping over precast planks. I don't see any issues with providing such a thin topping granted we specify welded wire mesh and control joints at 15' max.

Is 2" too thin of a topping over existing slab?

 

[SlideRuleEra]

I suggest using a bonded overlay:

1. Scabble or otherwise roughen the entire surface of the existing slab.
2. Carefully clean this roughened surface.
3. Wet-soak the roughen surface for 24 hours.
4. Place the 2" overlay (plain concrete, no wire mesh).
5. Wet cure for 7 days.

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[msquared48]

I've personally laid slab overlays varying in depth from 1" to 3" with no problems other than a little shrinkage which I controlled with pour sequence. Just used a ready mix concrete too, nothing special.

Mike McCann
MMC Engineering

 

[WWTEng]

Slide, thanks for the input. How do you control shrinkage cracks?

 

[SlideRuleEra]

Match the joints and cracks in the existing slab. Since the overlay is only 2" thick AND is bonded to the underlying slab it should actually become a part of the underlying slab. The bond between roughen, cleaned and thoroughly moistened concrete (underlying slab) and the new concrete (the 2" overlay, properly wet cured) is considered by some authorities to be as good as, or better, than an epoxy bond.

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[hokie66]

I agree with SRE, but you still may get some restraint shrinkage cracking. Why is that a problem?

 

[Ron]

Agree with SRE.....bonded overlay, matched joints. As hokie66 says, you'll probably still get some shrinkage cracks. Try to keep the coarse aggregate size as large as possible for the overlay and make the strength of the overlay consistent with the strength of the substrate...meaning the overlay should be weaker at time of placement than the substrate's present strength, but gain strength with time to more closely match existing. This helps to keep differential shrinkage down.

Wet curing is a necessity for a bonded overlay, in my opinion.

 

[WWTEng]

Thank you all.