Slab curling limits?

[Bernied]

I have a warehouse with a 6" concrete slab on 12" compacted gravel. Control joints were made within 12 hours at approximately 15'-0" x 20'-0" panels. (No vapor barrier was necessary.)
The slab is reinforced with Fiber mesh and it was poured with the building fully enclosed using chutes (no pumping). Construction joints divided the slab into about 10,000 sq. ft. pours. The slab was wet cured for seven days.
Considerable curling has occurred (upward at the edges). Enough to snap an electric feed line driving rack off-loaders and resulting in consideration of making expensive repairs (drilling holes around the panel edges and pressure grouting).
Does anyone have: a) suggestions why this may have occurred, b) any other ideas on "fixes" and c) is there any ACI or other specification with allowable/acceptable tolerances for slab curling?

 

[rowingengineer]

This is an Australian pub,but the best I know of for explaining curling.

http://www.concrete.net.au/publications/pdf/Curling.pdf

I would suggest you could saw cut in a few nominated positions at the tension face of the curl. would need more info to give better ideas, aka a picture or similar.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[Den32]

a.) I would have guessed improper curing, but if you sayed it was 7-day wet-cured, then you should look at Concrete Mix (water content) next.

Second guess would be saturated porous fill when they poured. This would 'trap' water on bottom of slab. I would still question the 7-day wet cure......what exactly did they do? and did it comply with ACI Specs for 7-day wet.

See ACI 360, Section 13.3. There is also information in ACI 302.1, Section 11.11.

Also, what does your Control Joint detail look like? Dowels? Is it curling up on both sides of control joints or just one?

Did they have alot of surface cracking?

b.) Fixes would be removing and replacing, grouting beneath as you are doing. Note if you have hard-wheeled traffic, consider a semi-rigid joint filler to protect concrete edges
c.) Check ACI Tolerances for what is acceptable

Hope this helps.

 

[BAretired]

Here are a few links discussing slab curling:

http://www.google.ca/search?hl=&q=s...lient-ff&rlz=1B3GGLL_en-GBCA404CA404&ie=UTF-8

It seems to be a pretty common problem. The only standards I know are in the job specification...usually about 1/4" tolerance in 10'.

BA

 

[JAE]

The curling limits would depend on the forklift type, storage rack heights, and general acceptance by the owner. By specification, the F numbers might control as well - all those "hot" corners would skew the F(flatness) numbers up.

The concern - as you've suggested with the grouting, is the future diagonal cracking at the corners when the upturned corners are loaded.

Some possible fixes:
1. Mill down the high spots after grouting any voids beneath.
2. Determine where voids are occurring - cut out completely the slab corners at those locations and replace the full depth concrete.

 

[JAE]

As to how it possibly could have occurred - I'd concur with Den32 - look at the mix - high cement content and/or small aggregate can accentuate shrinkage and thus the curling.

 

[demayeng]

As curling is caused by a differential in shrinkage and there was no vapour barrier used, it is likely that there was a high moisture content in the subgrade, as Den32 said.

 

[BAretired]

That brings up a frequently argued topic. When should vapor barriers be used under grade slabs and when should they be omitted?

BA

 

[Ron]

Since you used a compacted gravel subbase, I doubt that a wet subgrade is the problem. As others have noted, check the mix design...in fact, post the mix design so we can take a look as well.

Your joint spacings are stretched to the limit. I would have gone to 12 to 15 feet max, and your panel ratio is 1:1.33...also a bit high.

 

[a2mfk]

BA- I cannot fully answer that question, but I seem to remember reading an article on it not that long ago. I know with our climate and water tables here in Florida, if you have an interior space you better have a very good vapor barrier. I have investigated a lot of flooring problems caused by vapor transmission through SOG floor systems.

Not sure if it would affect curling, but most publications recommend sawcutting your control joints as soon as you can get a Soffcut blade onto the slab without disturbing or removing aggregate (raveling), the consensus being within 4 hours or so o finishing ("early-entry dry-cut")...

Also, most recommendations for CJs (just had to do a ton of research on this):
-Space no greater than 15', or 24-36 x thickness. A 4" slab < 10'..
-Avoid odd shapes and L shapes, square as possible, limit the L:W to 1: 1.5

Though these have nothing to do with the actual shrinkage, perhaps more CJs would decrease the magnitude of the curling, though not the quantity.

Sorry I just realized I offered you no help at all for your problem!

 

[Ron]

a2mfk...actually you did offer something of value...more control joints. You're right. Since curling occurs at the joints, and curling is affected by the length of the section, then reducing the length with more control joints will decrease the magnitude of the curling...possibly to tolerable limits.

 

[hokie66]

I never like to miss an opportunity to push the cause of jointless floors. The more joints, the more chance of problems, maybe not with curling, but with other issues. Jointless floors reinforced with steel fibres and/or relatively large amounts of reinforcing bars are making headway in the warehousing and industrial floor market. Restraint shrinkage cracking occurs, but the cracks do not cause serviceability or maintenance issues. After all, joints are cracks, just straight ones.

With curled edges, grinding and grouting are generally the remedies to restore usable slabs.

 

[dik]

Specifying a flatness may not be of much use. Flatness is generally measured within a short time period of finishing and will likely occur prior to any curling.

Proper placement and curing often minimise curling issues. Remedy must include pressure grouting of the underside at the joints. The joint areas generally take the most abuse and require sound support.

Dik