Minimum Concrete Cover for Rebar Development

[TS59401]

I am reviewing a structural analysis report for a client regarding some existing concrete pipe on their property. The report is stating that concrete reinforcement requires a minimum cover of 1/2" to develop the full strength of the reinforcement (not code minimums for construction). I think this is ringing a bell in my memory, but I cannot find that information in ACI 318. Does anyone out there know if this "minimum cover" statement true and where it comes from.

Thanks in advance,
Tom

 

[TehMightyEngineer]

I would imagine it's coming from the 3/4" minimum tolerance for slabs and walls per CRSI minus the 1/4" clear cover placing tolerance.

It might also be referenced by whatever ASTM standard covers concrete pipe (we don't manufacture RCP so I'm not familiar with the specification details).

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[JoelTXCive]

I'm not sure this is the answer you are looking for....but maybe it is a clue?.......

In ACI 318-14 commentary for tension development lengths (Commentary section 25.4.2.2 pg 415), they say you need clear cover of at least a bar diameter (d_b). That could be the 1/2" that your report is mentioning? Also, If your pipes are precast, they might need less cover than Cast in place concrete?

 

[dreber]

I can't vouch for it's accuracy, but this could help:

https://www2.strongtie.com/webapps/rebardevelopmentlengthcalculator

For rebar development length, see ACI318-14 §25.4.2.3

Hopefully, that simpson calculator uses the provisions from ACI318-14.

 

[BridgeSmith]

Development of reinforcement requires confinement by the surrounding concrete, enough so that the concrete does not split and allow the reinforcing to become unbonded from the concrete.

 

[TS59401]

Thank you everyone. The specific rebar that I am looking it is WWF in a pre-cast concrete pipe (off the self stuff). The half in sounds reasonable and is will within the bar diameter minimum referenced above.

 

[TLHS]

Why are you doing a report for off the shelf precast pipe? They're designed and certified to standards. Presumably it also touches on cover requirements.

https://precast.org/pipe-specifications/

There are also design manuals for pipes built to these standards:

https://www.concretepipe.org/pipe-box-resources/design/design-manual/

 

[TS59401]

This is a review of another firms work related to corroded existing concrete pipe. The report investigates pipe stresses with varying levels of concrete cover corrosion all the way down to 0" of cover, thus eliminating the effectiveness of the inner rebar layer. The report scales a reduced reinforcement strength for concrete cover below 1/2". I was just trying to determine where this concept was documented.

 

[TLHS]

Fair enough, I guess...

I'd question a linear distribution of capacity. Stuff's going to start getting wonky as you approach the aggregate size I'd think. There's definitely some point where you'd consider reinforcing basically ineffective prior to complete loss of cover.

By the time we're down in that area, though, you're well into corrosion of the reinforcement, leading to expansion, leading to cracking of the concrete and debonding with the reinforcement.

Of course, localized damage also would need to be looked at differently that constant damage over the whole section.

Interesting research:

https://www.icevirtuallibrary.com/doi/10.1680/jstbu.14.00060

 

[cvg]

ACI is structural code for buildings, not reinforced concrete pipes and is totally irrelevant. RCP is designed and fabricated according to ASTM standards (C76 or C655). ASTM does not allow less than 3/4 inch cover from either the outside or inside pipe wall for single layer reinforcement or 1 inch for double layer. anything less than 3/4 inch I would say is not acceptable under any circumstances. compressive strength of the pipe for trench and traffic loads (ring compression) requires the concrete. The steel wire prevents cracking and leaking. RCP is not designed for any significant amount of tension loading. the pipe will be unable to handle the traffic loads without it. to determine if the existing wall thickness is enough, than the pipe must be analyzed for ring compression.

 

[TehMightyEngineer]

ACI is structural code for buildings

Not entirely; a lot of ASTM precast standards utilize ACI 318 for design of custom engineered items. Manhole flat tops, buried utility vaults, etc.

But, yes, I would expect ASTM standards specifically cover precast concrete pipe.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[cvg]

unless this is not RCP, suggest reading the ASTM standards which define the design requirements. it could also be reinforced concrete pressure pipe (such as AWWA C300, C301, C302, etc) which would fall under AWWA standards. not ACI

 

[TehMightyEngineer]

it could also be reinforced concrete pressure pipe (such as AWWA C300, C301, C302, etc) which would fall under AWWA standards. not ACI

Was not aware of this; thanks for the tip!

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/