3/8" ACCEPTABLE TOLERANCE ON CONCRETE? 3

[structurebeton]

A residential contractor is telling me that 3/8" differential for a 32" distance on concrete is an acceptable construction tolerance. Where would he come up with something like that? Can someone point me which standard he could be referring to? He claims his concrete floor can have that much diferential and it is still an adequate construction.

 

[HouseBoy]

"How can the HBA standards be less than IRC/ACI?

HBA standards are written by the HBA! No surprise that they are as lenient as they are. Once "everybody" builds to that standard, it becomes the "industry standard".

If you're asking how they can get away with that....human nature.

I'm not advocating them. I am saying that they are prevalent.

 

[structurebeton]

I checked their standards on the 2/10 warranty, and the defect that triggers repairs is a 1/4" deflection in 32" for floors, and 3/8" in 32" for out of plumb walls.

That is the final word. Now, that floor deflection criteria corresponds to a 1/240 deflection.

 

[concretedoc]

Ron,

Sorry for the delay in responding to your response to my comment. We're in agreement that 3/8 in. in 32 in. is deplorable concrete construction if we're talking about the gap under a 32-inch-long straightedge for a floor that is supposed to be level and reasonably flat. A slope to drain of 1/8 to 1/4 in. per ft, however, is suggested in ACI 302.1R-04. The lower number works out to less than 3/8 in. in 32 in. and the higher number is in excess of 3/8 in. in 32 in. So if the floor is flat but sloped to drain, it may not be bad construction. There's a difference between flatness and levelness.

My point in the earlier post was that a flatness of 1/8 in. in 10 ft is rarely needed in residential concrete floors and it takes considerable skill to produce that result. You say "With a little skill and readily available tools, 1/8 in. in 10 ft is achievable." So here's my question:

Given: You're finishing a 46 x 25 ft basement slab with the poured concrete walls already in place. You're using a 5-in. slump and adding no water to the 3000-psi concrete.

What "commonly available tools" will you use to achieve the 1/8 in. gap under a straightedge in 10 ft, and what are the finishing steps that comprise your "with a little skill" statement? Contractors will appreciate learning from you how they can achieve 1/8 in. in 10 ft flatness results on residential concrete.

 

[msquared48]

Sorry, but I have done a lot of flatwork in my time too, and 1/8" in 10 feet is not difficult to achieve with a proper form/screed setup and a reasonable degree of due dilligence.

Been there. Done that.

Mike McCann
MMC Engineering

 

[Ron]

Concretedoc...two points..

You mention that there's a difference between levelness and flatness. There is and you've confused the two.

You can have a 1/4" per foot slope to drain that is constructed to a flatness of 1/8" in 10 feet. Slope to drain has nothing to do with flatness, other than presenting another variable to counter in the setup and finishing process.

You point out that contractors would "appreciate learning from you how they can achieve 1/8 in. in 10 ft flatness results on residential concrete". No they wouldn't. That would demand that they learn something about concrete technology and placement techniques that require doing more than chuting 8" slump concrete into a form set off batter boards. Residential concrete work is some of the worst concrete placement, finishing, and curing that you'll ever run into. Something better than that is rare and commendable; but don't expect to see it often. I could go on...but I won't.

 

[concretedoc]

Ron,

I'm aware that it is "possible" to have a flatness of 1/8 in. in 10 ft with a slope of 1/4 in. per foot. Other people who posted were confusing levelness and flatness, so I discussed that issue.

Mike illustrates my point perfectly. Proper form/screed setup is related primarily to levelness. So Mike, we're left with "due diligence" whatever that means. Let's see your data for "been there, done that." And I'd be interested in more detail than "due diligence."

However, you both avoided my question. How would you achieve 1/8 in. in 10 ft. for the conditions I gave you, which didn't include 8-in.-slump concrete and contractors who don't know anything about concrete technology? Get rid of the red herrings and tell me the tools and methods you would use. Share your knowledge of concrete finishing with all of us.

 

[msquared48]

1/8" in 10 feet has nothing to do with levelness or slope per se, only with the adherence to a design constraint. I have matched both to the standard here whether sloped or flat. It makes no difference. It's all relative and that is the point. It's all in the formwork setup and screeding process. That''s what laser levels and stiff forms are for.

Sorry. but that was said with a smile. It's been a tough day, as you may have picked up from some of my other posts today.

Mike McCann
MMC Engineering

 

[msquared48]

As a corollary here, you, as an engineer, should not have to educate a contractor in how to lay a slab to the contstraints set. He has to figure that out, not you. The specification or consraint is not out of the norm. If he does not have the technical expertise, experience or proficiency to do the job, he should not be bidding the project. I have done that with my limited experience, and to me, that is all that matters here.

And by the way, an 8" slump is ridiculous. May as well be working with water. Sounds like a little extra water was added by the contractor at the site to make it more workable.



Mike McCann
MMC Engineering

 

[racookpe1978]

Since the floor "flatness" problem is in the basement, and assuming that the "slope" is correct for flooding drainage, washing machine overflow drainage, water heater breaks drainage, second floor toilet pipe breaks drainage, (I've had all four!) ... then consider just demanding the contractor add a self-leveling concrete/grout on top of the bad floor slab.

By the way, even without a drain or sump pump, it is easier to mop up/suck up water from a wet basement if there is a definitive "low spot" (near the washer or water heater?) that collects water. But a floor where the bed needs to be shimmed into "level" ? No. Don't accept it.

I bought the house used, and my basement slab started with a bad slope similar to your case. The spare bedroom in the basement could not be used without propping up one end of the bed so it laid flat. Get it corrected, even if you only allow him to pour a floor leveling grout. See a carpet or wood floor installer.

 

[Ron]

concretedoc....here's what I would use, procedures and equipment...

1. Make sure the subgrade is stable, compacted, can support the screed rails and other equipment without deformation, and graded to 1/4"-1/2" in 10 feet.

2. Set a screed rail along the basement wall, preferably a steel angle, leg up, fastened at 24 inches o.c. Set to tolerance of 1/16" in 10 feet.

3. Set interior screed rails at no greater than 10 feet, to tolerance of 1/16" in 10 feet.

4. Maintain the slump of the concrete at 4 to 5 inches.

5. Use portable vibrating screed such as Black Beauty or similar.

6. Power trowel.

7. Moist cure for at least 7 days, then measure.

 

[msquared48]

Nice post Ron.

All I ever used though was wood screeds and forms plus a jitterbug hand vibrator, bull float and hand trowels, with 4" slump concrete. Very primitive, but it got me the results I wanted every time. This does not have to be rocket science.

Getting back to the OP's question though, if the 3/8" in 32" is along the top of a stem wall, I have no problem with that result, although framers might a little. But not so when applied to a slab, flat or sloped.

Mike McCann
MMC Engineering

 

[concretedoc]

Thank you all for your insight and helpful posting.