Insufficient concrete cover 1

[mats12]

I am dealing with an existing RC slab that is 200 mm thick and is reinforced sufficiently.
But concrete cover at the bottom is only 10 mm. Slab is inside residental buidling so it is not exposed to the weather.

What to expect when concrete cover is that thin? What is a smart thing to do here?

 

[BridgeSmith]

If the cover is intact, there's probably nothing that needs to be done. If it's cracking or spalling, and cannot be left in that condition, a common repair method I can think of is shotcrete. I suppose fiberglass wraps (FRP) could work for it, as well, but they are typically fairly expensive. I'm sure there are other repair strategies for overhead concrete repairs.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

How old is the slab? Is it suspended, or a slab on ground? If on the ground, is there a vapor barrier? Some engineers will reduce the minimum clear cover if a vapor barrier is installed, but not by that much. If it's elevated, can you inspect it? Any signs of flexural cracking?

For an interior application with good climate control, my primary concern would be the ability of aggregate to get around the bar and allow the wet concrete to fully enclose and bond with the bar. There's a chance for segregation of the mix and a lower strength composite connection between the steel and the concrete than might normally be expected. Not sure if that is quantifiable, though.

When you say you're "dealing" with it, what do you mean? What's the end goal?

 

[dik]

and more importantly,what is the bar size... with small bars, the lack of cover is less consequential... maybe at worst, some spalling that would have to be repaired due to bond issues.

Dik

 

[phamENG]

Good point, dik. Though if that sketch is to scale I'd say we're looking at welded wire...

 

[r13]

It really depends on your scope of work, if you are going to alter the load, even the existing reinforcing is adequate, the thin cover might not be sufficient to resist the increased stress. I think BridgeSmith's suggestions should be considered.

 

[Hokie93]

Is the existing slab experiencing any visible distress? Are you increasing the total design load on the slab? Are other trades cutting holes or otherwise adding penetrations to the existing slab? Is the 10mmm cover taken from original construction drawings or an in situ measurement? If the latter, how many measurements were taken? With the information given (adequate strength and interior exposure), I would leave the slab as-is if you are not increasing the load, the slab does not appear to have signs of distress, and the governing building code does not require you to bring the slab up to current code. Is the client aware of the condition?

 

[dik]

Is the cover needed for fire resistance rating?

Dik

 

[Agent666]

You can certainly address parts of the durability aspects with things like silane based surface treatments or similar.

You can test the cover concrete to determine the carbonation depth and chloride ion content to see if you think durability is currently an issue, or is likely to be in the future to enable you to determine if something should be done.

Dik raises a good point, seems like that might be a governing requirement.

 

[rapt]

Yes, Fire would be the major consideration.

Bar development length also increases with reducing cover especially if less than the bar diameter, so any development or lapping requirements would be problematic, but there probably are not many places this would be a problem in a residential building.

 

[msquared48]

Attach a layer of 5/8 GWB on RC channels to the underside. Add an extra layer of GWB if needed for fire.

Mike McCann, PE, SE (WA, HI)

 

[mats12]

Thank you for feedback.

Reinforcement is welded wire - bars are 8mm in diameter.
Slab is 30 years old.
There are not cracks/spalling yet, but load on the slab will be increased.

Good point about fire resistance rating.

 

[dik]

M^2: Type X... another couple of pounds a square foot... but, it hides the cracks

Dik