3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
(OP)
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.






RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
Ask him where it shows it is acceptable.
I haven't looked at the ACI or codes, but they should have the answer. What do your specs say?
Dick
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
I was just wondering if any has seen other references for concrete tolerance for floor slabs.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
Needless to say that this slab is a little out of standard, just a little. Like 12 times +-.
Mike McCann
MMC Engineering
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
If the slab is not cracked, there probably is not settlement there, but just the surface variation. Since the first and second floor were framed on the slab there was either no corrections made during construction or there was additional settlement or shrinkage. If there is just one 3/8" variation in the foundation slab, you could not expects this to telegraph up to the second floor, but would be distributed in a larger general area.
It does not seem logical that a 3/8" slab variation would cause a major settlement in a structural apparently built to standards unless there were other factors that caused an owner to hire an engineer on a hunt for a single probable cause. - Is the owner in a conflict with the builder over other issues?
Dick
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
There is also an Aussie publication on floor/finish tolerances - I'll try to find the url and post it.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
If you could get a peak at the IRC code, there may be additional guidance in there for you on this.
Greg
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
As others have noted, 1/8" in 10 feet is a common tolerance and achievable without much difficulty. The noted 3/8" in 32" is absurdly bad construction for a floor slab. Chairs won't sit flat, tables will rock, people will stumble.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
1/8 in. in 10 ft is a common tolerance only because it was in use for many years when no one measured floors, and is now in use because some specifiers don't understand floor flatness. And it definitely is not "achievable without much difficulty," especially for residential construction. In fact, ACI 117-06 doesn't even include that tolerance. The tightest straightedge tolerance in ACI 117-06 is 3/8 in. in 10 ft for 100% compliance. There is virtually no need for a residential floor that is flat to within 1/8 in. in 10 ft or with an FF number of 60, the superflat category. As stated in ACI 117-06, "Only skilled contractors, using sophisticated equipment, will be able to achieve this level of quality." That level of quality is specified for high-rack warehouses, TV studios, or other floors that must be superflat to perform well.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
Dik
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
The grout or second pour of low strength concrete is also common in India where the "wet rooms" (kitchens, entries, bathroom, showers)in a home where the floors are tile. Additionally, there they also have the dual waste situation where they have "gray" and "black" water separated.
I think in this case, the 3/8" variation on the basement floor slab is being looked at as a the possible cause of settlement on the second floor (unless I missed something).
Dick
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
1/8" in 10 feet isn't anywhere near an Ff of 60...that requires extraordinary techniques, attention to mix design characteristics and skilled craftsmen. With a little skill and readily available tools, 1/8" in 10 feet is achievable, but you can't start with a slump of 6 inches then add water to get it even higher and expect to achieve that tolerance and a durable finish.
My whole point is that 3/8" in 32" is deplorable concrete construction and should be rejected, residential or not.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
I just wanted to know if the argument of an 3/8" concrete differential is an argument that can be thrown out right off the batt.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
With that said, it is likely that any agreement between the builder and the homeowner has some language referring to HBA standards as the measure that applies.
It might be "sloppy" and it might not conform to anything by ACI, but it is likely to be the legal standard that applies in this situation.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
That is the final word. Now, that floor deflection criteria corresponds to a 1/240 deflection.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
Been there. Done that.
Mike McCann
MMC Engineering
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
Sorry. but that was said with a smile.
Mike McCann
MMC Engineering
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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.
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?
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
RE: 3/8" ACCEPTABLE TOLERANCE ON CONCRETE?