Spalling of concrete slab soffit - residential garage "roof"

[Brandon]

A friend's house has a garage with a concrete slab "roof" that is the floor of the living space above. House built probably in the 1920s. Most of the concrete cover has spalled off the slab soffit in lines following the rebar (orthogonal grid)and the rebar is obviously corroded. I haven't been over to see it yet, so from what I know at this point, the concrete is mostly intact around the perimeter of the slab near the supporting walls.

I'm going over to take a look and can post pictures later. I can get some measurements and probably even get a good idea of the thickness, then do the capacity calculations easy enough. But in terms of should this slab be repaired/strengthened my thinking is that as long as there is at least a development length of the bars surrounded by sound concrete at the supports, then the capacity of the slab should be mostly intact? In this case, the repair objective would be to simply protect the bars and prevent any further concrete spalling, rather than a full-scale strengthening and/or structural repair. Am I off base here?

I read a couple of technical papers that examined loss of bond of the tensile steel of reinforced concrete beams, and the conclusion was that the ultimate flexural capacity of the slab was only reduced about 15% even if the tensile steel had lost bond over 75% of it's length.

Thoughts/advice appreciated. Pictures to come.

 

[JAE]

Wheel load punching shear or possibly loose chunks of concrete falling below is possible as well.
Safety hazard for those below.

I'm not sure I'd be comfortable inhabiting a space below a fully spalled slab with rusted rebar and bond loss along 75% of its length.
Unless of course I was down there watching TV and constantly bombed out of my mind....which I never am...really.

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[dik]

Any idea of why the corrosion, if that's the cause? What is the extent of the corrosion? Can you check for concrete cover to reinforcing. Any insulation, vapour barrier? Long term carbonation because of a garage environment and loss of alkali corrosion resistance?

Dik

 

[KootK]

What you have, and what was discussed in the paper that you referenced, is a tied concrete arch. It can be a viable mechanism but you need to keep in mind that the span to depth ratio on those beams were likely much lower than the span to depth ratio for your friends slab. That worsens things. Ensure that difference is accounted for in any evaluation.

I'd be seeking to repair the spalled concrete.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Spalling results from corrosion. That means loss of section of the reinforcing steel. In order to know the remaining capacity, you need to know how much steel remains. And if the house was built almost 100 years ago, the type of steel bar would be an important consideration. Don't assume you just need to protect the remaining steel, when you may find there is none to protect.

 

[Brandon]

dik/hookie 66- I am going to take a look at it this evening and will check as many of those things as I am able to. The house is almost 100 years old and I think the corrosion is just the result of dampness and low concrete cover, probably some freeze the contributing as well. The space above is living space, and the garage below is unheated.

KootK - can you expand on the span-to-depth ratio concept with respect to the effective tied arch?

Thanks all!

 

[KootK]

Imagine a tied arch where the height of the arch is 10' and the span is 20'

Now imagine another tied arch where the height of the arch is 1' and the span is 20'.

You can see how the tension and compression forces in the second arch are much greater, yes?

The house is almost 100 years old and I think the corrosion is just the result of dampness and low concrete cover, probably some freeze the contributing as well.

Just the result of dampness?!? Take care not do underestimate the consequences of dampness. In various forms, it's responsible for most structural failures.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Amen, KootK. One of the first lessons I learned in a beginning structural class was that "water is the greatest enemy of buildings". Unfortunately, that lesson does not seem to come through in architecture classes.

 

[epoxybot]

You will want to look for signs the concrete was ever painted. A light coat of flat white to lighten things up would not be so bad but a paint formulated for something like a concrete patio or any heavy coating would have trapped moisture. Assuming it is a cold climate, were past occupants likely in the habit of starting their cars & letting them warm up in the garage. Is it a garage with a door or a carport open to the elements? Does the overhead slab include outside walkways, terraces or a patio area that might indicate moisture from above is permitting moisture ingress. What is above the garage, kitchen, bath or laundry room? Is the laundry located in the garage? Ideally, you would want some 1% phenolphthalein indicator solution to test the alkalinity of the concrete & a Schmidt hammer. You would want to access if the concrete beyond the rebar is carbonated. A phenolphthalein solution will show a pink, red or magenta color if the alkalinity is 9.5 or higher. Colorless, other than appearing damp indicates the concrete is carbonated. You can't use the Schmidt hammer on carbonated concrete because it can yield high values. Once you have exposed concrete of good alkalinity, the Schmidt hammer will provide proper results. Depending upon where you live, phenolphthalein can be purchased in small 1 to 2 oz. quantities from chemical supply shops that provide discrete quantities for schools. I'm envisioning just long-term freeze-thaw with snow from wheel wells being the main moisture contributor.