Concrete cover violation from spacer reo bar

[geopat69]

Hi I have a query regarding concrete cover, bar chairs and spacer bars made from reo bar.

My slab design requires 60mm cover to the bottom main reo. The steel fixer has placed 40mm bar chairs and placed 20mm spacer bar on top of the chair (spacer bar was not part of my design and is non structural). He has his spacer bars at about 2m centres.

My main bottom reo is on top of the spacer bar and it has correct cover to the bottom (being 60mm)

Is cover violated to my design..by virtue of his "non structural bar" projecting into the 60mm cover zone?

 

[BAretired]

No!

BA

 

[dik]

BART: you don't get much more emphatic than that... Over the last several decades, I've encountered engineers that get so tied up with details that they 'miss the trees'.

Dik

 

[TehMightyEngineer]

I'll counter that it can matter depending on the structure's application and owner requirements (limiting corrosion for appearance, aggressive corrosion environments, etc.). However, in all likelihood this is not a problem. It's also worth noting that in most building codes I'm familiar with stirrups, hoops, and similar bars generally can encroach the clear cover requirements.

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

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

 

[BAretired]

Dik,



BA

 

[geopat69]

Hi TehMightyEngineer.

I have been a bit crook and did not managed to see your response yet. Can you give an indication of which codes permit "stirrups, hoops" to encroach on cover zones? It seems to be common practice for steel fixers to use steel spacers...but don't know why they think it is allowed (ie the Australian code seems to be quite specific that it is from the outer most steel to the surface).

The concern is obviously the "spacer bar" causing spalling...due to it (and only it) being located at the reduced cover.

Appreciate everyone's contributions also!

 

[hokie66]

Yes! Cover is to all reinforcement, structural or not.

 

[Ron]

If I understand the placement, instead of using a single 60mm chair, they have used double 40mm chairs with a 20mm spacer bar spanning the two 40mm chairs. Primary rebar then sits on top of the 20mm spacer.....just a modified chair....no real issue but could be code interpretation issue in some jurisdictions; however, keep in mind that the bottom of a chair is steel and never meets a cover requirement, so strict reading of some code language might conflict even with a steel chair!

 

[TehMightyEngineer]

geo: Apparently I've been doing too much precast, cast-in-place doesn't have the difference for stirrups/ties. Per ACI 318-14 Table 20.6.1.3.3 for precast columns not exposed to earth or weather; primary reinforcement requires 1.5 inches clear cover and stirrups, ties, spirals, and hoops requires 3/8 inches clear cover.

I agree with Ron, CRSI's manual on bar placement indicates that unless the engineer or owner requires differently, it's up to the contractor to determine if using steel bolsters and other rebar supports made of uncoated steel will be acceptable. Unless the contract requires plastic rebar supports or similar, I see no reason they can reasonably accept steel bar supports but reject a steel rebar spacer.

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

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

 

[hokie66]

TME,
Look at it another way. If 1" cover is required, would pieces of #8 bar laid on the form be acceptable as bar supports?

 

[geopat69]

Great advice so far guys.

Just to confirm.. Ron, yes your understanding is correct.

Hokie. Your example is a good one but slightly extreme. As your #8 bar has no cover to start off with. You see in my case... I have 40mm cover to the spacer already.


It is interesting since AS3600 concrete code does seem to indicate "cover to reinforcement" which from a purest viewpoint would be main reo and ligs.

I suspect the real answer is cover to embedded items (where the spacer bar is the 'embedded item'). I am not sure if as3600 covers this.

 

[TehMightyEngineer]

Hokie: Great example, I agree that there's obviously limits to how far you can take this. I think this is one of those cases where the code can't and shouldn't cover every situation, and engineering judgement should prevail.

Obviously a #8 bar at the surface is not acceptable; the amount of steel that can corrode is too great and will likely cause spalling and extensive staining. But little wire "feet" of a unprotected steel rebar chair are acceptable for slabs given the small amount of exposed steel that can corrode and the low risk of spalling.

OP's spacer rebar would be embedded enough that it should have passive protection from the concrete. I'd only consider if spalling is a concern and this could be solved by limiting the length of the bar and ensuring that it's sufficiently deep in the concrete that carbonation or other reduction in the passivization of the concrete still ensures sufficient protection of this spacer bar (40 mm seems enough for this IMO).

For a lot of our precast products we often are required by the owner to ensure that anything embeded in the concrete be non-corrosive within 1 inch of all surfaces. If this isn't required we will generally use our best judgement (plastic supports where the surface is exposed and staining would be unacceptable, otherwise steel chair supports with plastic tipped "feet" if not an issue).

In the end I think it's just what OP's AHJ and owner will accept, other than that it's up to OP's judgement unless he can find code language in AS3600 (which I'm not familiar with) that addresses this.

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

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

 

[hokie66]

geopat69,
Now that you have referred to the Australian code, by memory there are two cover requirements. The first is for corrosion protection, and is to the first steel encountered in the element. The other is related to fire rating, and is to longitudinal reinforcement. Not sure this is true for the current code, as I don't now use it day to day.

 

[geopat69]

Hi Hokie and thanks again. You again mention a good point.."first steel encountered in the element".. But is that specifically referenced anywhere. I know its a bit of a long bow to draw but one could interpret this as corrosion protection to steel that forms part of the reinforcing to that element.

 

[IDS]

I suspect the real answer is cover to embedded items (where the spacer bar is the 'embedded item'). I am not sure if as3600 covers this.

From AS 3600 cl 4.10:
"Embedded items, as defined in Clause 14.2, shall be protected from corrosion or
deterioration. The cover to embedded items that are not corrosion resistant shall be as given
in Table 4.10.3.2 and Table 4.10.3.3, as applicable."

i.e. embedded items that are not corrosion resistant have the same cover requirements as reinforcing steel (both in the 2009 code, and the recently published revision).

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[TehMightyEngineer]

Nice find Doug. That answers that very definitively and also means that any rebar chairs and support of unprotected steel can't be used.

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

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