Spalling caused by insufficient concrete cover

Question

Spalling caused by insufficient concrete cover

pepe1234 (Civil/Environmental)

(OP)

17 Oct 13 14:47

Anyone have any insight of the correct method of concrete restoration for the following scenario:

A reinforced concrete tank, about 40 years old, has several areas experiencing spalling. The owner wants to address this problem becasue the spalling has become so prevalent, that it is now a hazard to walk near the tank as you never know when another piece of concrete will be falling off. After examining the tank, the spalling is occuring in multiple locations where the circumfrential hoop steel was incorrectly placed during original construction. There are many areas where there is on;y 1/2" to 3/4" concrete cover over the hoop bars.

What would be the repair strategy here? I understand that typically in concrete restoration, you want to saw cut perimeters of unsound areas of concrete so that the repair material has a minimum 1" thickenss (avoiding feathered edges). I would assume that to have a long lasting repair, the thickness of cocnrete cover will need to increase in these areas where minimal concrete cover was provided at original construction. But in 'building up' the thickness with repair material...how could you avoid a feathered edge conditon with the repair material???

Maybe my approach isn't even the best one for this type of problem. I'm open to totally new strategies if anybody has suggestions. Thanks!

Ron (Structural) · Answer 1 · 2013-10-17 17:54:43

You need to determine the cause of the cracking before you develop a repair strategy. Has carbonation developed in the concrete to the depth of the rebar? Can you fully expose the rebar to safely effect a repair?

You might consider a shotcrete repair if there are large areas. You could then use the existing tank as the inside form and reinforce as necessary within the shotcrete.

JedClampett (Structural) · Answer 2 · 2013-10-17 17:59:21

After removing any loose concrete, I would treat the entire exterior of the tank of Sika Armatec Epocem. This material says it has a corrosion proof material that migrates through the concrete to the reinforcing. Then I would shotcrete the exterior of the tank with 1 1/2 inch thick (to make up the missing cover) material or whatever the minimum amount the shorcrete guy will stand behind.
A more expensive alternate is to wrap the tank with prestressing tendons and then shotcrete. This will assure that the spalled material is held in place.

pepe1234 (Civil/Environmental) · Answer 3 · 2013-10-18 13:44:33

The casue of the spalling is the insufficient concrete cover provided at orignal construction. With such a thin colver, through either solar expansion or stress concentrations at the hoop steel, the concrete has developed hairline cracks at the locations of most hoop steel members. That hairline crack allows moisture to reach the reinforcing steel.

The contractor the owner first engaged about repairs wants to address only the superfical after effect of the problem - they propose chipping away unsound concrete and patching back in with shotcrete and polymer concretes. I have consulted with the owner that this repair doesn't address the root of problem and would likely be a repair that last a few years.

I guess the 1-1/2" shotcrete external liner would be a solution. The owner may poo a brick when we talk costs on that repair strategy. Any other ideas out there?

The areas where the insufficent concrete cover exists is in small isolated areas around the perimeter. But there are a lot of these small isoated areas and they are pretty eveny distributed everywhere...if this makes any sense.

dik (Structural) · Answer 4 · 2013-10-18 14:17:52

With 1/2" to 3/4" cover, the spalling of the concrete is likely due to corrosion of the reinforcing steel and the corrosion products expanding and popping off the concrete. What does the tank contain? anything corrosive? Is the existing tank strong enough?

It's a matter of checking if the existing rebar is OK and if so, cleaning the surface to accept a patch... a couple of good products have been mentioned. The quality of the existing concrete should be determined also prior to patching...

Dik

bridgebuster (Civil) · Answer 5 · 2013-10-18 15:33:29

Ditto on the shotcrete if you have a large quantity and appearance wise it will blend in better over large areas; remove all unsound material first. Shotcrete doesn't pay - at least in NYC - for small quantities. High mobilization cost for the equipment and the labor for loading the hopper.

If you were going to create "blisters" or "build up" you avoid feathered edges by maintaining a constant overlay thickness. However, it looks ugly. If you're ever in Queens look at the piers for the Long Island Expressway/Van Wyck Expressway and the bridges over the LIE to the east of there, you'll see what I mean.

We avoided the blister problem on one project by painting the bars; don't recall the product but it was made by Fosroc, that was 20 years ago.

pepe1234 (Civil/Environmental) · Answer 6 · 2013-10-19 17:41:03

The tank is an agricultural tank that stores grain. If you've ever worked with ag sector jobs, you'll know these owners most often don't care one iota about appearance of the finished product. They just want a repair at the lowest possible cost. The tank is actually in good condition despite the spalled areas. There isn't much loss of diameter on the rebar that is exposed - it will just need the little rust that is on the rebar removed prior to repairing.

So the 'blisters' you are describing, bridgebusters....I assume that means that at the perimeter of the build up area, you will literally have a 1-1/2" (or whatever thickness of material you are adding) step down to the original material at the patches. Correct?

If we went the route of a full 1-1/2" exterior liner around the entire perimeter...Would you recommend placing reinforcing (WWF) in the new 1-1/2 shotcrete liner?

Thanks!

bridgebuster (Civil) · Answer 7 · 2013-10-19 20:39:33

Pepe1234 -yes, the build up area would simply step down. I don't see the need for WWF with a 1 1/2" thick overlay. It's important to chip out about 1" behind the rebar to get a good mechanical bond.

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Spalling caused by insufficient concrete cover