Wrong size welded wire in foundation slab is it a problem? 1

[adurbin]

I have a subcontractor that installed 6x6 - W1.4xW1.4 welded wire fabric instead of the 6x6 – W6xW6. This does not meet the temperature and shrikage requirements of ACI 318.

The owner is concerned that his concrete will crack because of this error. the slab measures 6 inch deep on 1 foot of gravel, and is 20' x 20'. with all the piping installed. There is no structural loading concern as it will be used to store misc. lightweight equipment.

I recommended an epoxy coating on top, however the owner feels that this is still not what he paid for so he wants to demo (remove) and reinstall with the correct size wire.

Is this excessive or needed?

 

[AggieYank]

I suppose the owner is technically correct, in that he paid for the W6xW6, and got the lighter mesh. The lighter gage steel means the pad will have more shrinkage cracks.

If he wants LESS (Not NONE) shrinkage cracks in his storage building, then he should replace the slab. The obvious best option, in my opinion, is for the owner to accept that there will be MORE shrinkage cracks, but since it is a storage building, cosmetics probably won't be an issue, and the contractor should give him a credit for the difference in steel.

Whether it is excessive or needed really depends on the owner. If he really doesn't want more than usual shrinkage cracks because he's worried about cosmetics, then it isn't excessive. If he isn't sure what the difference between the two meshes is, and just wants to do it on principle, then yes, it's probably excessive.

As an aside question: When you recommended the epoxy coating, did you recommend they fill in any cracks that appear with the epoxy? Or did you recommend laying it on the surface of the existing slab immediately? I haven't seen someone layer it on a whole slab, but then I don't usually see incidents like this. Thanks.

 

[adurbin]

As for the epoxy, I would of recommended that they lay it down at least 28 days after the pour. Basically add a 1/4" to 1/2" to the top of the entire slab. Figured that this would actually be a big benefit to reducing all crack mitigation.... The owner really is doing it out of principle and really is unfamiliar with concrete. In reality even with the correct size wire mesh, concrete will crack...a lot depends on the finishing techniques, curing etc.

 

[SlideRuleEra]

This is more of a contract issue than a technical problem. The Owner is most likely under no obligation to accept any work that does not meet the contract requirements, even if it is technically acceptable.

With that said, I suggest two options to avoid having to remove and replace the slab. Both will have costs in time and money that may not be worth the trouble.

1. Negotiate a financial settlement (reduced cost to the Owner) to accept the slab "as-is".

2. Retain a qualified third party to evaluate, and possibly test the slab to offer an independent opinion on it's acceptablity. Even then, a financial settlement may still be required.

I recommend that you make a brief attempt at Option 1. If unsuccessful, for a slab that size the most cost-effective thing will probably be removal/replacement, then get on with the project.

Note: At various times, I have been on each of the three sides of the situation, Contractor, Engineer, and Owner's Representative.

http://www.SlideRuleEra.net

 

[JAE]

6x6-W6xW6?

That's a lot of shrinkage/temp. steel in a 6" slab - its .0077 x Ag.

Its similar to #4 @ 6" o.c.

 

[adurbin]

Ok, correct me if I'm wrong...but I was using ACI 318 7.12.2.1
0.0018

Per ACI 318 code section 7.12.2 ( = 0.18%)


As = 6 inch (slab depth) * 12 (in-to-ft)*0.0018 = 0.12 (sq. in. per one foot of slab) REQUIRED


The 6x6 - W1.4xW1.4 provides 0.029 sq. in. per foot < 0.12 (sq. in. per one foot of slab)


Therefore we are undersized according to ACI 318


The 6x6 – W6xW6 provides 0.12 sq. in. per foot REQUIRED.

Where is the 0.0077 Ag coming from?

thanks

 

[JAE]

Nominal wire diameter for a W6 wire = .276 inches

so for a 6x6 mesh, you have two wires per foot = 2 x .276
= 0.552 sq. inches per foot.

0.552 / (12 x 6) = 0.0077

 

[JAE]

Also, ACI 318 7.12.2.1 does not apply to slabs-on-grade.

 

[adurbin]

ok, then what are steel reinforcing requirements for slab on grade. and what code applies?

 

[JAE]

Here's a couple of threads on this topic:

thread592-4955

thread507-20302

There are a number of methods of determining steel in SOG - I can't get to my reference materials right now - later today I'll try to look some up - PCA has a good book along with ACI 360R.

 

[Taro]

JAE, you need to use Pi*D^2/4 to get the area of the wire (0.06 in^2).

 

[henri2]

adurbin: I always thought ACI 318 Sec 7.12 applied structural slabs and footings but not SOGs. Ref: 10.5.4, 7.12.1, and R7.12.1. I do not think ACI 318 is the appropriate code for determining shrinkage/temp reinforcement for non-structural SOGs.

JAE: Isn't the area of the WWR 0.12 sq.in/ft? i.e. 2 x the csa of each wire = 2 x 0.06 = 0.12 sq.in. I think you inadvertently multiplied the diameter of the wire rather than the csa. I always remember the wire gage size as being the csa x 100, which conversely means the csa = wire gage size/100. BTW, your contribution to thread 592-4955 was very informative and worthy of all those stars...plus one more for interjecting it in this thread, because of its elucidative value.

 

[adurbin]

I think the best resource I found for design of concrete slab on grade that will not experience a wheel loading or heavy loading.

http://www.wirereinforcementinstitute.org/HTDocs/PDFs/TF 705-R-03.pdf

Kind of along the same info I found from JAE's thread link... dealing with the subgrade drag formula.

 

[DRC1]

Just acouple of notes: a) there are a lot of 6" slabs out there that do not have any reinforcement. b)Even properly reinforced slabs will develop cracks. c)Subgrade preperation proably more important than T&S. d) There is a theory called economic waste that says that forcing a contractor to redo work to comply with the letter of the contract when the non conforming material is servicable for the intended use is a compensible change to the contractor. Granite Construction was the landmark case. So be careful telling the contractor to go rip it out until you are sure it is materially deficent.

 

[adurbin]

Oh, I certainly did not tell the contractor to demo and reinstall. They just requested calcs. relating to how I determined the min. reinforcement required. On the fly I grabbed ACI 318 and used 0.0018 figuring that this would be more than sufficient even if the subgrade is not suitable...basically very conservative approach.

However, the owner sees this as a black and white. Basically its stated on the drawings. That's what they want. (period)

 

[JAE]

Hey all...my apologies for the mix up in my math. Yes indeed I did use the diameter vs. the area (per Taro's comment) - I've been fighting a real bad head cold, with certain drugs, and just wasn't thinking too clearly.

Sorry for the confusion, adurbin. 6x6-W6xW6 is indeed 0.12 sq. in / foot.

 

[dik]

From a contractual vantage, it's a matter of reviewing the drawings and spect to see what is required. Hopefully there's an ambiguity that may influence evaluation of the contract. Seems like a shame to cut out a perfectly good slab...

From a technical point of view, it depends on the loading anticipated. Generally, a plain concrete slab 6" thick if cast using a reasonable low slump will likely perform OK. I've got piles of info on slab on grade construction and a 20'X20' 6" thick is not unreasonable. You might want to research the ACI or PCA data on the effect of slab reinforcing.

 

[dicksewerrat]

Well you shared info. Now share the drugs

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[JAE]

dicksewerrat - I tried to upload them to your website but your system doesn't take them in pill form. I've got a call into my IT guys to see about conversion software to translate into some other format. Stand by adventure fans!

 

[dik]

Sorry JAE..., but unless I'm drinking, I don't advocate using mind altering drugs... Dik