Concrete spec. for Slab-on-grade

[wds1]

How do most people specify concrete floor slab-on-grade. Give ACI a little time and they will reverse their decisions. Concrete has too many variables. If you spec a highier strength mix, you get more cracking because of curling. Some want vapor barriers between concrete and granular material. Some want it beneath the granular. With wire mesh, some want it in the top(cracking), some the middle(trade-off), and some the bottom(strength). In reality, it will be on the bottom because no one uses chairs. If we all put as much reinforcing in the slab as the 5 calc. methods indicate, then the floor will cost a fortune. The slab is only as good as the subgrade. It's really a wearing surface. Some are using fibers instead of wire mesh. Some are using fly ash and cement. We avoid them. Each additive will affect the overall mix in different ways. With all the combinations, it's hard to tell what your going to get unless you work with it every day. Some just use the "6" rule, 6" conc., 6"x6"-#6/#6 mesh. Everything seems to be a trade-off. How do you find a happy medium. We do alot of industrial and office floors. We prefer just plain vanilla cement(No/little additives).

 

[miecz]

Here's where I'm at (may change tomorrow):
f`c=3000 psi at 28 days - least shrinkage, lowest cost
Vapor barrier between two 4 inch layers of granular, 3/8" above and 3/4 " below.
Rebars, depends on slab length and thickness, but usually #4@18". No fuss with WWF. No fuss with joints.
No fibers. The fiber people don't guarantee anything.
No air entrainment in slabs. Avoids surface problems.
Mineral Aggregate hardener for industrial floors.

 

[jike]

Believe me, I understand your frustration! I have similarly been frustrated at ACI.

I do not believe it is very practical to put the vapor barrier below granular material. I put it right under the slab. Granted, this may cause plastic shrinkage cracking, but it is the prefered method for a number of reasons. Make sure that your spec. requires the GC to fix all cracks during the warranty period.

I also believe that we should be using 3000 psi concrete for slabs, since it causes less shrinkage.

Other things that should help:

Use the largest aggregate possible to reduce curling. Doweled joints also help.

Make sure that it is properly cured and sealed!

Early sawcutting using "Soff-cut". Some GC's say they can reduce curling by not sawcutting, which might be true, but I believe their real motivation is increasing profit.

Make sure that control joints are cut to proper depth. ACI/PCA recommends 1/4 depth; I prefer 1/3 depth.

Do not allow reinforcement to be lapped at control joints (otherwise the joints will not be fully functional). It is suggested that every other bar or wire should be cut at joints.

I hope this helps!

Jim K.

 

[structuresguy]

We always spec 3000 psi w/ WWF and 1/3 depth saw cuts and 3/4" aggregate. Thickness varies by intended use. My question really is what water/cement ratio to specify. I find we tend to specify too low a w/c ratio, so we generally end up with over 4000 psi concrete. Our senior engineer's motive for the low w/c ratio is reduced drying cracking.

 

[phuduhudu]

Just going back to wds1 initial post. Those who say steel in the bottom for strength are plain wrong because any load on the slab causes flexural tension in the slab in both the top and bottom so if you wanted it for strength you would need a layer top and bottom as with a multi span suspended slab. Has anyone heard of this being done in on grade slabs? The positioning of reinforcement is a problem. Does anyone know of any research showing how often the steel ends up in the top as specified? Should we be using unreinforced slabs with sawcut joints at 4-5m centres more often? It seems that as structural engineers we are reluctant to leave out reinforcement.

Carl Bauer

http://www.bauerconsultbotswana.com

 

[WillisV]

In my experience the WWF reinforcing almost always ends up in the bottom third of the slab. In speaking with a local testing agency experienced in radar investigations etc. they confirmed that in almost all s.o.g. they have ever tested the reinforcing was at the bottom. We have moved to polypropylene fibers and have so far been very pleased with the results in typical office/condo s.o.g. for crack control etc.

 

[SlideRuleEra]

For typical slabs on grade, I switched my previous employer (electric utility) from WWF to polypropylene fibers in 1985. Results have been, and continue to be satisfactory.

http://www.SlideRuleEra.net

 

[structural01]

I don't agree that WWF and vapor barrier has to be used in slab on grade.

As many of you already pointed out, most of the time WWF ended in the bottom no matter what you specified. WWF is not for strength, it is for controlling of cracks. I believe WWF in the bottom of the slab is more detrimental to the slab than without any WWF in the slab. This due to the differential drying of the concrete. Of course, the top surface of the slab dry faster than the bottom. if there are more steel at the bottom, then it will resist more shrinkage than the top surface. Therefore, you get might get more concrete curl. To me it is better off to not specify any WWF.

Vapor barrier will only widen the differential drying and shrinkage of the concrete slab.

You can improve the over all slab by improving your mix, using larger aggregate and decreasing cementicious material by adding fly ash. Adjusting joint spacings is also important.

 

[MMOT]

We do mainly schools/commercial (not many heavy industrial floors) and I still have not really come to a good resolution on this item. I started with 3000psi and 6x6-2.9X2.9 then started decreasing the w/c ratio and wound up with 4000psi and the mesh- which as discussed above- never gets in the right place. So, the next move was to increase the steel to bar mats and require frequent supports, and in turn I increased the joint spacings. The result was a lot of changes to slab joints/reinf in addemdum since construction managers would get involved and convince the owner that the bar mats were excessive (ie: not what 'everybody else' was doing). It was a tedious and frequent fight to try to explain why our opinion was that having less joints is better in the long run. I just got tired of walking into that battle. So if the owner wants joints- they can have them all day long- for schools I have dropped back to the aci/pca recommendations for slabs w/out structural reinforcing and let the contractor select from either mesh, bar mats, or blended fibers... To be honest- I have never been called for any issues with curling or with excessive cracking on my jobs. In most areas, once the finishes are down, the owner could care less what the slab looks like beneath and this makes up 90% of our spaces. But the flip side to this is that since 90% of the slab does get a finish I am very particular about having a stone capillary break under a quality vapor barrier (not 6 or 10mil poly...)just below the slab and we require wet curing of all interior slabs.
my 200 cents- sorry for the ramble- ACI's lack of guidance on "run-of-the-mill" SOG's, and meddling CM's are pet peeves. The reinforcing calcs in ACI's new 360 are a drastic increase over the prior edition.

 

[AggieYank]

3000 psi conc.
<1-1/2" agg. size
15 mil vapor barrier directly under slab
#4@12" o.c. E.W. chaired up in center of slab
saw cut joints 1/4 depth of slab, 1/8" wide within 8-12 hrs after pour.

structural01. we use the vapor barrier (and 15 mil at that too) because we'd prefer not to be sued in 10 years when our buildings have mold problems. several companies around us have been through this.

 

[VTPE]

We have had a lot of success with the following:
- 3000 PSI concrete
- 8/18 blended aggregate mix = min of 8% and ma of 18% of total aggregate weight retained on each sieve below 1.50" and above #100 sieve.
- Blended fiber reinforcement = 1.5# of poly fibers + 21.5# of steel fibers. Poly helps plastic cracking, steel helps hold elastic cracks.
- Vapor barrier below 3" of fine granular fill = pea gravel.
- 5" slab thickness
- CJ's at 15'-0" max.
- CURE CURE CURE.

 

[DaveFlax]

The solution is simple and it has been around for decades. Type K, shrinkage-compensating concrete. It has little, if any, curl. It doesn't shrink so saw-cutting isn't required, cracks are almost non-existant, and construction joint spacings well over 100' are common.

 

[structuresguy]

Many of you have discussed rebar/fibers/WWF and saw cutting and vapor barriers, but very few of you have mentioned water cement ratio. This is really key when specifying concrete. It is the w/c ratio that is key to many of concrete's properties. What w/c ratio are you specifying for sog?

Also, the discussion of the vapor barrier is really the architect's decision. Moisture control, IMHO, is not the structural engineer's responsibility. THe vapor barrier, as far as I am concerned, is just something the architect puts under my slab. What thickness it is does not make any difference to me, or the performance of my slab. If mold problems occur, or delamination of epoxy floors happens, the structural engineer will not be held responsible.

 

[casseopeia]

Structuresguy,

You could not be more mistaken. In my particular field, my primary responsibilities is acting as expert witness for subcontractors and design professionals involved in litigation. Not only are the Architect and Engineer held proportionally responsible for mold or moisture issues related to the slab, I have seen the geotechnical engineer, mass and fine graders, concrete supplier, installer and testing lab, flooring manufacturers and their installers also held responsible. I've even defended framers whose insurance companies had to pony up a proportion of the consequential damages related to moisture in the slab (10% of the Plaintiff's exorbitant estimate for repair).

If I have learned one thing, it is that litigation has nothing to do with technical logic and everything with getting money from whatever source is there and structural engineers are required to carry E&O insurance and they generally do. Many (not all) subcontractors out there do not carry insurance, or let it lapse once a project is started. It's plain and simple extortion. It's cheaper for your insurance carrier to pay a small sum than fight the claim. And remember that your insurance carrier has no problem settling a claim equal to or less than your deductible.

Cracks are generally cited in Plaintiff's list of defects, but serious mediation about money generally focuses on consequential damages caused by moisture migration issues, i.e. staining, carpet wrinkles, mold and discoloration of vinyl flooring, etc. Cracks are defects, but legally, they must cause a consequential damage to something other than the concrete in order to have a viable claim.

A typical 4" SOG for residential construction takes about 1 year to reach moisture equilibrium where inflow equals outflow. Until then, concrete is emitting water from its original placement. Impermeable flooring interfers with the drying process, which can lead to flooring failures and mold growth.

Concrete mix design has a direct impact on moisture transmission as does the presence of granular base and vapor barrier.

Best performing residential slabs that we have seen have water/cement ratio =0.40, 4,000 psi, 15% flyash, 7% silica fume, placed directly on the vapor barrier (10-mil or greater) on min 4" granular fill. Do not place concrete on a 'sand cushion' that is on top of the vapor barrier. It only serves to hold in moisture. Control joints at 15' o/c and at every reentrant corner.

WWF is worthless, but if you use it, place it on supports. Personlly I like fibre, unless its a slab for a burnished industrial floor. Fibres will gum up the finisher and you don't get a good burnish. (Incidentally, the big steel plates used to get that burnished finish are called pizza pans by the field guys. I've always liked that.)

 

[Rjeffery]

caviat: I am an Inspector NOT an Engineer.

My experience is only about 12 years of testing and inspection of concrete placements.

The truth is:

WWF is great IF it is placed properly and useless if not.
If you specify WWF, specify chairs. (not only height but positioning)

Poly or steel fibers are the way to go as it makes the placement a "no brainer".

0.4 or 0.45 W/C with class F flyash replacing 15-25% of the cement will make the placement easier and will tend to discourage "blessing" the slab during finishing. The flyash
tends to make the concrete more fluid (and "Pumpable") without additional water. Avoid Specifying flyash in areas where the concrete plants do not have the ability to heat the water and aggregate or in winter.

A vapor barrier is a waste of time and effort on a slab that will not have a floor covering, but is essential on slabs that will or in a structure (like a wood working shop) where keeping humidity at even levels is essential.

Specify 1/3 depth construction joints saw cut within four to six hours of final finishing.

Specify wet curing in summer weather, or film forming curing compound (when you have an inspector to verify full coverage).

Always (in every case) prohibit the addition of water at the site and the use of water (blessing) to finish the slab.

Always have a ACI/NICET/WACEL certified concrete inspector on hand before, during and after placement even for residential concrete.