Effect of Freeze/Thaw Cycles on Reinforced Raft Slab with No Drainage Slopes

[ajk1]

For a reinforced concrete raft slab supporting an open-air above grade parking structure, is there any concern about water getting getting into cracks in the concrete top surface of the raft and causing concrete spalling due to freezing? The top of the raft slab has no drainage slopes so water can lie there. Over top of the raft slab is a granular layer of several inches thickness and on top of that is asphalt pavement. I would take it that water can get thru the asphalt and the underlying granular fill and reach the top of the raft slab. I am sure that is a common scenario. How has it been dealt with? Or is it not a concern, and if not, why not?

 

[oldestguy]

Maybe a common system and maybe not. Regardless there are several factors involved. Presumably the cars will bring in salty water or even salt may be used as a deicer there. I'd not depend on any asphalt pavement to be waterproof, so the surface (salty) water is very likely to be in contact with the concrete below. Then comes the concrete itself, the cover over re-bars, air entrainment effecvinesss, etc. Spalling of concrete probably not big deal, but re-bar corrosion is anther story.

 

[ajk1]

No one wants to comment??

 

[oldestguy]

One more. Freeze - thaw cycles with salty water occur more frequently than the air above, due to low freeze temp of salty water.

 

[ajk1]

Thank you oldestguy. We have the rebar corrosion issue in hand ... we specified 125 mm cover which gives us about 100 year life (if I remember right) when modelled on Life 365.

Yes you are right about the number of freeze-thaw cycles, I realize that there are more freeze-thaw cycles in the presence of chlorides.

It just occurred to me that even for pavements with drainage slopes,the water still can get into any cracks and freeze. I have not seen freeze-thaw breakdown of the cracks in exterior concrete pavements, so does it matter that much if there are no drainage slopes? The only thing that seems to me to be different is that we have large cover which may result in wider cracks. But I don't think that should matter because the strain in the ice when it freezes is the same regardless of the thickness of the ice. Therefore the stress and force is the same regardless of the ice thickness. Right?

 

[allgoodnamestaken]

I wasn't directly involved but I sat near a coworker who was dealing with a parking garage experiencing extreme corrosion issues within a few years of its construction. The culprit ended up being faulty installation of an elastomeric sealant that they used on gaps between slabs and other similar places.

The architect was screaming, the contractor was screaming, the engineer was yelling. It was a bad day for everyone. I think if you aren't familiar with high corrosion risk concrete construction you should consult with an engineer who is familiar with it and learn some tricks. You don't want it coming to haunt you years later.

 

[oldestguy]

What's goofy about this is that suppose you do have deterioration of concrete and maybe re-bar corrosion. How does anyone know this is going on before a failure occurs? Is this sort of surface over concrete really need to be present? Can a practical method be set up for periodic inspection?

 

[ajk1]

To allgoodnamestaken - LOL. I have specialized in parking garage design, construction, inspection and repair for almost 40 years, and provided expert testimony in the largest litigation case in Canadian history at that time, which involved a parking garage and corrosion. Your advice is excellent, but in my humble opinion is directed at the wrong person.

To oldestguy - your comments are well taken (that is why we have adopted a 100 year design life criteria). But your concern would also pertain to other building elements not accessible for inspection, such as multistorey basement walls where one cannot readily access the earth face of the wall to inspect the negative reinforcing, building spread footings, piles, etc.


I think this is wandering from the specific question posed, namely: is there any concern about water getting into the cracks in the top surface of a raft slab with no drainage slopes (and 5" cover to the top bars), and freezing? I think that this is not a concern, for the reasons I stated earlier, but I am interested in what others with experience with this kind of situation think.