Cutting Rebar in existing slabs 1

[JJF]

I am very familiar with ACI-318 (and several other ACI publications) but have never come across an analysis of the impact of cutting a single piece of reinforcement in an older (+ 50 years) existing slab.

Does anyone have an article on this?

Thanks.

 

[JAE]

JJF -
I don't think there'd be any specific article on this - just engineering "common" sense in that if the bar is a primary reinforcement, and you cut it, then adjacent primary bars would have to take the lost tensile load of the cut bar. This would add stress to these bars which may or may not develop some flexural cracking.

You can simply calculate the required area of steel necessary over the wider width to see if you have a problem. ACI requires a max. spacing of primary flexural steel of 18" so for bars at 12" o.c., with one cut, you'd exceed the 18" (thus the possible cracks).

 

[JJF]

JAE,

Thanks for the response, I was more concerned with judging the impact of this cut with the 50 year old materials.

I agree that a simple one way slab analysis would be a good check, but again, 50 year old concrete/rebar......

If you think of anything else, please let me know.

JJF

 

[JAE]

Well if its old and there is evidence of deterioration throughout the slab, then "depending" on the adjacent rebar may be problematic as the adjacent bars could be partially gone and you end up cutting the only good bar in the vicinity.

Engineering judgement required here - inspect the slab - where the hole is to be cut and decide based on the condition. Old old concrete is actually stronger (f'c keeps increasing slightly over time) and if the rebar is not rusted, it is still performing and able to perform like new (assuming no fatigue issues).

 

[DRC1]

One other note 50 years ago design was done by allowable stress, not ultimate strength, so to be consistent you may want to evluate the impact of the work by allowable stress