Cast-in-place (Nonprestressed) concrete cover

[engr567]

For concrete elevated slab above parking garage, what is the bottom concrete cover? Can I assume the concrete is not exposed to weather or in contact with ground? Thus, 3/4" cover.

 

[rowingengineer]

is the carpark open? is so what would be the % of openings? how close to the sea are you and is this breaking surf? why would you assume on ground?

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[Teguci]

Yes. Exposure to weather is for alternate wetting and drying surfaces. The soffit above a garage should not be considered exposed (except perhaps at the outside edges).

 

[ron9876]

I agree with Teguci--unless there is some sdditional corrosion concerns.

 

[engr567]

The project is located in New York, and not too close to sea.

 

[msquared48]

You might check the local fire code requirements. For a parking garage with gasoline powered vehicles, the requirement could be greater.

Mike McCann
MMC Engineering

 

[Teguci]

You will be OK with 3/4" cover in NY. If you are placing a business or residence over the garage you will have to sprinkler. Otherwise, your fire separation is 2 hours (as I recall) which can be provided for with a 4" slab.

 

[rowingengineer]

I disagree with the idea that that unless surface is subject to wetting and drying directly that it should have no corrosion considerations.

The amount of times I seen the underside of a slab have cancer issues as opposed to the top side is too numerous to count on two hands. I believe that is has to do with wetting and drying, the top side gets cleaned during the rain event while the bottom side gathers all the dust, chemicals and other not so nice corrosive agents. Then you have all the active ingredients for a nice injection of cancer. If the top of the slab isn’t seal 100% the water will get to the bottom side and create nice damp places for all these ingredients to activate. This of course it relevant for open garages more than enclosed garages and does depend on you top labs water proofing.

The next point I will make is don’t spec ¾” agg, this is to large for ¾” cover if you are going to adopt 3/4" cover. I would recommend a minimum of 1” such that the agg can get under the reo/ducts.



ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[Teguci]

If salty water is getting through the slab from the top side, I don't care what cover you have on the bottom of the rebar.

 

[hokie66]

It doesn't have to be salty water. If the top side is not waterproofed, water will penetrate, and eventually lead to the spalling that RE mentions. Of course, if this is in a climate where salt is used to deice, even worse.

 

[Robvh1984]

Although construction tolerances are supposed to be included in reduction factors, I would consider adding the respective ACI 117 tolerance to your MINIMUM concrete cover.

Unless there is some super stringent architectural or serviceability issue, I do not see how it will make much of a difference

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[ron9876]

All I know for sure is that the 3/4" cover on the bottom is ok for the condo lawyers in South Florida. Must be ok.

I would say that if the concern is for moisture from the top of a slab getting thru to the bottom reinforcement then the problem isn't with the cover on the bottom of the slab. In that case the cover for the bottom reinforcement from the top of the slab is relatively large. Increasing the bottom cover won't help with that problem.

 

[rowingengineer]

If this was a bridge what bottom cover would you spec? and if different why?

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[Ron]

In areas where snow occurs and roadways are salted or otherwise treated, much of the damage of parking garages occurs from the slush water carried into the garage from cars entering from outside...not from simple weather exposure.

Increase the cover since the "exposure" condition is greater than the static minimum condition would imply.

 

[Teguci]

Ron is correct as it regards to top cover. I remember doing an appraisal of a garage structural where the top cover was inadequate. Over 50% of the top surface was spalled and rusted rebar was visible everywhere (hydro demolition becomes feasible). It is also not unusual for considerable concentrations of chloride to saturate the concrete even at 2" deep. However, for the underside of a slab, it is not likely to be a problem (the splash zone for columns is around 30" above the deck) unless you have cracks through the slab.

 

[hokie66]

Don't you always have cracks?

 

[Teguci]

Not in my concrete!

Actually, for conventional concrete, the cracks/control joints should be properly located and sealed. If you have an exposed deck, proper maintenance requires the sealing of unwwanted cracks to prevent deterioration.

For a PT deck, you better not have any unwanted cracks. Otherwise you didn't do your job.

 

[hokie66]

In my opinion, if you depend on a concrete deck to be watertight, then you didn't do your job, regardless of whether it is PT or not.

 

[TXStructural]

ACI 318-08 section 7.7 commentary states in part "The condition 'concrete surfaces exposed to earth or weather' refers to direct exposure to moisture changes and not just to temperature changes. Slab or thin shell soffits are not usually considered directly exposed unless subject to alternate wetting and drying, including that due to condensation conditions or direct leakage from exposed top surface, run off, or similar effects."

This tells me that most parking garages which do not have covered or sealed top decks would require increased cover. Water can be carried into garages on vehicles and by wind in sufficient quantities to induce corrosion.

Also a parking garage in an area where salty ice/water is carried in on vehicles might need to be designed for ACI 318 Table 4.2.1 exposure category C2 (severe for corrosion protection of reinforcement.) This requirement is frequently overlooked.

 

[Teguci]

The OP is asking for the underside concrete cover above a parking deck for nonprestressed concrete. If you have a crack through the slab which allows water and contaminants to leak through, the bottom cover could be 3/4" or 3" and there would still be cracked and spalled concrete due to steel expansion at the crack. Bottom cover is immaterial to this corrosion.

@Hokie - since concrete is usually a water saturated environment for reinforcement, it is not the water exposure that causes corrosion of steel. It is the water in combination with a contaminant (typically chlorides). The better strategies are geared at limiting rebar exposure to chlorides (and other contaminants).
For top rebar this includes cover, tighter concrete (usually with higher strength, fly ash or GGBFS), corrosion inhibitors (DCI), and the sealing/limiting of cracks.
For bottom reinforcement, bottom cover doesn't matter for chloride attacks (water carries the chlorides down) unless you are in a surf zone (New York is not). The worst I can think of would be carbonation from the underside due to high CO2.

@TXStructural - How can you make the statement "This requirement is frequently overlooked"? This table is new (2008) and just now being adopted by building codes.