Bundling reinforcing vs. minimum spacing 1

[AggieYank]

Is there a penalty in strength for using bundled bars to reinforce concrete beams as opposed to bars spaced out correctly? Other than longer minimum lap length for tension development, I can't seem to find anything.

The basis for my question comes from site visits. Occasionally, in an 8" joist, the two bars in the bottom will be essentially touching. It seems like it would be ok, as this is no different than bundling bars, even though the mininum is 3/4" spacing.

I'd like to hear everyone's thoughts and opinions on this.

 

[JAE]

For flexure in beams, the max. flexure is usually at midspan or at the support and the way the longitudinal steel is detailed, the bundling doesn't affect the
[φ]Mn of the beam. However, ACI does have requirements for developing this longitudinal reinforcing and this is affected by the bundling (as you mention above). A few aspects I can think of:

1. Developing positive moment past supports (ACI 318 - 12.11.3)

2. Cutoff locations for negative reinforcing (12.12)

3. Hook development lengths

4. Class A laps required for structural integrity in 7.13.

5. Distribution of steel per 10.6 would force you into spreading out the bars anyway.

 

[AggieYank]

Thanks JAE. Basically confirming what I knew, just wanted to make sure there wasn't anything I hadn't considered.

 

[HgTX]

Won't it affect your calculated maximum crack width?

Hg

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[UcfSE]

I would either bundle the bars or not. Leaving a small space (less than the minimum but greater than zero) could lead to honeycombing.

 

[AggieYank]

UcfSE, when they're bundled, only one small point is in contact (two circles touching), with a small gap between the rest of it. I'd imagine a 1/4" gap between them would actually be better in terms of honeycombing than no gap at all, as there would be more room for concrete to get into.

 

[henri2]

If the bars are not going to be in contact (bundled in a horizontal plane as illustrated in Fig 3-2 of PCA ACI 318 Notes)then:

1. the clear distance between bars in a layer shall be the larger of db, 1 inch or 1.33 x MSA ..ACI 318 Sec 7.6.1, or

2. if bars are in same vertical plane, then clear distance shall be 1 inch...ACI 318 Sec 7.6.2

The reason for this is to prevent inter-alia, honeycombing as pointed out by UcfSE...ACI 318 R7.6

Is the bar size No.6..what is the MSA of the concrete mix?

 

[bandraoi]

It's better for the bars to be resting against concrete, failing this it's better for the bars to be resting against each other,
what you don't want is air beside the bars.

Unless you've got a very runny mix and a very small aggregate size the bars should not be too close together without touching.

If you end up with air beside the bars you will later end up with water beside the bars and that's BAD.

 

[AggieYank]

Thanks everyone for the advice. I'm failing to understand how two bars with a 1/4" gap between the bars is worse than two bars with no gap. Touching each other doesn't "help", and there is a smaller gap (between the inside faces) if they're touching, meaning it'd be harder for concrete to get between them, meaning there would be more honeycombing? I'm probably off on the likeliness of honeycombing. Can someone explain this better?

 

[JAE]

I think that all this talk about gaps/air etc. is valid, but its primarily valid where you are trying to develop the bars, and has no effect on the flexural calculations, such as at midspan of a beam....agree?