Exposed Rebar 2

[KHoff]

I recently worked on the structural design for a renovation project for a cast-in-place concrete building built in 1965. The renovation project is currently under construction, and as you might expect they are finding numerous issues as the concrete structure is exposed. I received an RFI with the attached photos showing rebar exposed on the underside of some concrete joist/beams. The RFI states "Throughout the building there are multiple scenarios where rebar is exposed either in a concrete beam or in the bottom of the slab". My initial thought was this looks like the result of pour concrete placement during the initial construction, not the result of a structural failure. If this condition occurs at a large number of locations patching this concrete would come at a large cost to the owner. Of course, in a perfect world the concrete would be patched so the beams can function as intended. I would like to hear the opinions of other engineers on whether or not to recommend patching the concrete. Would there be long term concerns if the concrete is left as is? Any input is much appreciated

 

[Lomarandil]

Absolutely, that appears to be a construction defect, not a structural failure. (Others here will know the technical terms and likely causes).

If the defects only occur in positive flexure zones (e.g. the exposed rebar is in tension), there wouldn't be any immediate loss of capacity or structural issue due to the missing concrete -- at least not the apparent missing concrete. Is it possible that there are other void regions in compression zones?

However, there is definitely an accelerated risk of rebar corrosion -- which will eventually have some effect on structural capacity. Is there any sign of corrosion now? Just patching over the concrete might not be enough if that's the case.

 

[SlideRuleEra]

Our company (the owner) had to rehabilitate an office building that had wide-spread similar construction defects. We came up with a 3-step process to proceed:

1. An independent engineering review of the original design performed to ensure the design was adequate.

2. An ACI load test on a typical defective portion of the building to verify the construction was at least minimally competent.

3. A time and material contract to repair the damage (a lot of shotcrete, plus numerous structural modifications).

Very expensive, but our conclusion was the only other options were to either demolish the building or repair it and gamble on its long term safety.

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[JAE]

This is simple honeycombing and poor workmanship.

In the photos you included I didn't see anything that would be a public safety concern with regard to strength/capacity.....as Lomarandil said, unless the honeycombing was in a compression zone (i.e. near the ends of the spans on the beam/joist bottoms.

One concern, though, would be fire protection - the bars don't have the cover typically required for fire...but this also depends on the building occupancy, type, egress, etc. that would require a code analysis by an architect - so it may not be a code issue at all.

If there are holes/honeycombs like this in the sides of the beams/joists then shear may be an issue and the steps that SlideRuleEra suggests might be required.

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[Ingenuity]

Concrete looks like crap!

Is that aluminium electric conduit in the slab/beam?

Mid-1960's was ripe with alum conduit in concrete construction...often with disastrous results.

 

[TehMightyEngineer]

Picking up the Concrete Repair Manual would be a great resource for you.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[canwesteng]

I'm substantially more concerned about the first picture than others seem to be - I think having rebar unbounded for such a length would affect the assumption that plane sections remain plane no? On the upside, the rebar itself looks good, so probably no more work than just patching the concrete.

 

[sandman21]

It's also a little hard to lap bars in air, which appears to occur in at least one picture. This should be fixed, considering you own this now.

 

[racookpe1978]

Hmmmmn.

Rebar exposed to "in air" ...
Is it interior (A/C and humidity controlled) with NO salt or exterior exposure? (An interior building "hole" and not an exterior parking garage?

That rebar shows NO signs of rust or problems. How many others are like it?
Now, it ALSO does not act in the proper plan and elevation level inside the concrete, but that is a separate analysis.

Why not - if the exposed rebar in each "hole" is NOT corroded, just shotcrete the hole shut?

Then re-analize the building for the reduced strength of the out-of-position rebar?

 

[Ron]

This is a miserable construction defect of poorly consolidated concrete. There also appears to be a cold joint above this, with numerous areas of underconsolidation (honeycomb). The conduit is problematic if aluminum, but likely galvanized.

All underconsolidated concrete should be removed back to solid concrete and then a shotcrete repair should be done. I agree with SRE that an ACI load test should be done.

 

[SwinnyGG]

Then re-analize the building

Sounds painful.