I own a home with a suspended slab in the garage that has cracked so snow melt off the cars has seeped into the basement. The corrigated metal sheeting that held the pour is badly corroded so I can imagine the rebar in the concrete is in similar shape. A contractor says rthe cheapest solution is to pour another slab over the existing one and seal it. The support in the basement are 12" I beams so the added weight should not be an issue. Is cutting out the slab a preferred fix?  

You really should get a structural engineer involved.  Just because there's 12" beams doesn't mean they can support another topping of concrete and the live loads that'll be applied.

Cheapest might be to leave the concrete alone and cover it with continuous plastic flooring (to keep out any future water).

I would be concerned with the structural integrity because of the corrosion observed.

I would agree with two things brought up.  One get an engineer familiar with corrosion to double- check.  But I doubt you'll have an issue with the rebar.  Just because the exposed corrugated sheet looks bad, it may be only surface corrosion.  But the bigger thing is that the rebar is encased in concrete.  Even though it spans the crack, it is in a highly alkaline environment, which greatly reduces the corrosion rate.  You see bridges torn up for pot hole repaire that are 50 years old and they often don't replace the rebar, just jack hammer the concrete around it and repour.

The second, if sound, a plastic membrane as Jon bought up would be the simplest solution, to the seepage.

Hi Larry,
 
That might be true about bridges in warm climates, but in regions that have icing and salt is used, it'll more than likely corrode the bar.  Why else would epoxy coated bar be used?

Posted By LarryT on 18 Jul 2010 10:56 PM
I would agree with two things brought up.  One get an engineer familiar with corrosion to double- check.  But I doubt you'll have an issue with the rebar.  Just because the exposed corrugated sheet looks bad, it may be only surface corrosion.  But the bigger thing is that the rebar is encased in concrete.  Even though it spans the crack, it is in a highly alkaline environment, which greatly reduces the corrosion rate.  You see bridges torn up for pot hole repaire that are 50 years old and they often don't replace the rebar, just jack hammer the concrete around it and repour.

The second, if sound, a plastic membrane as Jon bought up would be the simplest solution, to the seepage.

i agree with you :D

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May I suggest using natural products the are both corrosion proof and Alkaline resistant. Basalt products have been used in Europe and are now gaining acceptance in the North American market.

Basalt Rebar will not corrode as steel does and has a lifecycle of 100+years, Being a ceramic it shares many of the same properties of the concrete and aggregate responding at similar rates of expansion and contraction. Check out ENR oct 2010 for details of the bridge made using "ROCKBAR" this also allows for thinner slabs as the coverage requirments when using steel reinforcement does not apply

On the use of epoxy bar, in some areass is being phased out due to excessive corrosion within the bars when the surface of the epoxy is nicked, it allows formoisture and chemicals to penetrate into the bar around the epoxy and then the epoxy itself holds the moisture in place and can spped up the corrosion process.

Just some food for thought.

I agree, lots of research regarding salt and rebar - galvanized, stainless steel and non-metallic rebars, impressed current protection, admixtures, arc spraying with zinc, porosity, etc, etc.