analysis of concrete slab 1

[vincentpa]

Hello,
I am working with a contractor to rehabilitate a nuclear facility built in the 1950’s which was contaminated in a small meltdown in 1960. The majority of the building is concrete and heavy construction. The contamination in the concrete is removed by scabbling or shaving the concrete surface. The surface in question is a concrete beam and one-way slab system with the slab poured integrally with the beams. The slab is 8” thick with ¾” cover top and bottom and the beams are 20” x 36”. What is the minimum cover to still have the top and bottom bars effective in bending? Can we shave or remove the top bars completely in the one-way slab? I’m not sure where the crack will occur if we remove the top bars in the slab entirely. It seems the compression strut will still be in the slab and the slab will just act as a simple support if we remove the top layer of rebar. Is this true?
We are also analyzing a 1’-6” retaining wall that spans horizontally with 2” of cover retaining 32’ of soil. The walls are contaminated. Can we remove the cover to the bar?
I have a complete set of ACI manuals. I just don’t know where this information would be contained. I see bridges with spalled abutments, columns, and beams to the rebar and they seem to function well. What are your thoughts regarding the effectiveness of the rebar with minimal cover or no cover?
Thank you,

 

[Bagman2524]

Looking at ACI-318-99 section 7.7.1. 3/4" cover is required in slabs for #11 & smaller bars. 1-1/2" cover is required for beams. However, Nukes usually require to maintain the design per the year of code that the plant was designed to, so whatever edition of ACI-318 was in effect in the 50's probably applies.

 

[vincentpa]

The test reactor is out of service and the building use will change. We are just doing an analysis for structural safety and eventual load rating of the components. For the slab I was thinking about assuming a crack at the beam/slab interface. I would ignore the top steel because it might not be there. A hinge would develop and shear friction could carry the slab. What does anyone think about this? I can't demo the floor slab because it helps brace the walls.

 

[SlideRuleEra]

1950's concrete would have been designed using Working Stress Theory (Strength Design was not formally introduced until 1963). Reinforcing is likely different that what would be done today or in the recent past.

I was involved in the total rebuild of a (new) concrete cast-in-place commercial building that had a number of design & construction problems. We removed all concrete cover on the main supporting members - all the way down to the principal rebar. Cover was then replaced with shotcrete. This whole procedure was reviewed and accepted by three independent engineering consulting firms. What you are talking about can be done.

http://www.SlideRuleEra.net

 

[vincentpa]

There is no minimum for development of bond stress between the rebar and surrounding concrete? How is the bond stress developed between the shotcrete, rebar, and old concrete?

 

[SlideRuleEra]

In this case, the rebar remained bonded to the existing concrete. The rebar cover was removed with controlled sandblasting.

If performed correctly, the bond between old and new concrete (shotcrete in this case) is as good as or better than using epoxy compounds. These repairs were performed in 1991, the building is still in service. The secret is water.

Four years later I had the opportunity to work on major repairs to a severely damaged electric turbine/generator pedestal. We called in a national infrastructure consulting firm. Their top concrete engineer also insisted that we did not need epoxy or other compounds to repair concrete cover over not only rebar, but also the main turbine/generator anchor bolts (4" diameter, about 6 feet embedment). He was on-site, with us, for several months and I learned from him what to do - it is so simple, but it HAS to be done right (that is why he was on-site)

Soak the the existing concrete with water - I mean really soak it, by ponding (if possible), or wet rags (keep them wet), or a steady steam of water. Do this for at least 12 hours (24 hours is better). Remove (or turn off) the water and as quickly as practical place the new concrete (wet cure for 7 days). This unit (Rated 270 MW Electric) is still in service.

http://www.SlideRuleEra.net