Increasing existing concrete slab thickness

[WWTEng]

Is it practical to install a new layer of rebar and increase thickness of a concrete slab for the underside? The rebar at the underside of the existing has corroded and concrete spalled. Also, the top 2” of the 7” slab are to be removed and replaced due to extensive damage (50 year old slab). For these reasons I am wondering if it would be a good idea to reinforce from the bottom. I want to mechanically connect the two pours with SS U pins (see attached) and roughen the existing slab from the bottom and apply some sort of boding agent between the two pours. Does this seem like a practical approach

 

[hokie66]

Possible, yes. Practical, I don't think so. Can't you just start over?

 

[msquared48]

I agree with Hokie here.

Also, I would be concerned with what caused the corrosion and spalling in the first place and try to mitigate that prior to any repairs to the slab.

Mike McCann
MMC Engineering

 

[MiketheEngineer]

Fix the reason. Replace the slab. Done and over. Probably cheapest method.

 

[Teguci]

No, not practical. Have you ever watched someone try to jack hammer concrete over their heads? How do you place concrete flush with the underside of a soffit? How are the forms supported? Most importantly, why? Addressing the top and bottom of the slab suggests to me a full depth repair is in order.

 

[Once20036]

I agree with all of the above re: practicality, especially the thought of jack hammering over head.

Assuming that the slab is already roughened and jack hammering wouldn't be required, I think placing the concrete would be possible: Traditional formwork, core drill at some spacing (5' oc), at every other core hole build a "hopper" to add a couple feet of pressure head to the fluid concrete and pour concrete until it comes out the next adjacent hole. Use superplasticizers and/or vibrate from below.

I don't think it would be feasible to use a bonding agent because anything out there would cure in the time it takes to build the formwork & pour.

If this slab is below grade - be mindful that if you remove and replace the slab, you're removing a support from the wall that the slab previously provided and significant temporary bracing may be in order.

 

[SkiisAndBikes]

In my experience the cost of a topping plus the cost of the soffit repairs would exceed the cost to drop the entire slab and replace. The design life of the cheaper option, i.e. dropping the entire slab, would far exceed that of the repair option you describe.

At 50 years old, I would be worried about possible chloride contamination or other deleterious issues there may be in the existing concrete you propose to leave in place. A new slab, with quality concrete, proper concrete cover and a waterproofing membrane is the way to go.

 

[WWTEng]

I agree that it would be more practical and economical to remove and replace the entire slab. But the problem is with some existing limestone columns. See attached. The column sandwich the existing slab between itself and the brick pier below. I don't believe that there is any positive connection between the brick pier and the column. I am afraid that if I take out the entire slab, the columns and brick pier might be compromised because of bracing issues.

 

[csd72]

Nothing that a good scheme of needling and underpinning wont fix.

I agree wih the others, replacing top and bottom slabs wont fix the root cause and will normally cost more money.