Slab Curling

[Falconite]

I have a question about slab curling. It happened in a retail strip mall in Northern Kentucky. The building is divided into 4 sections with windows completely across the front and 30 ft down each side. A 4 ft strip was poured to set the windows on. One slab was poured in June, the 2nd poured in August, the 3rd poured in Sept, and nothing in the other section. The curling happened in Jan where the slabs have raised up 1/4 to 1/2 inch above the 4 ft strip on all three slabs. There is no evidence of any movement against the back walls of the building. I have only heard of slabs curling during the initial curing. Is this still considered slab curl? I am thinking that the Sunlight coming in the windows heating the slab has caused this. I have worked on several building with the same set up and haven't had this problem. Give me your thoughts.

 

[rowingengineer]

sorry Falconite,
Too little info to help much, but have you had any rain or someone started to water the area. If so water could pass through the joints and wet the clay below causing localized heaving?

Maybe if you give us some more info, we can come up with some more ideas.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[Ron]

Sunlight would cause the surface to expand. Curling is typically caused by the surface shrinking.

Have a slab surface profile done (Face profiler or similar), including across the joints. This will tell you if curling is the mechanism. Do the profile at several different exposure times, at least one very early in the day and one at 3:00 or 4:00 in the afternoon. This typically gives the most significant thermal extremes. Compare the results.

After doing this, if you still see no pattern, you'll have to start evaluating the subgrade for movement.

 

[Ron]

Sorry...hit the button too quickly...

There are several other considerations when trying to determine if curling is occurring. For instance, is the space air conditioned? What was the water-cement ratio of the concrete when placed? Is the subgrade wet and does it remain so? How much faulting occurs at the joints, both longitudinal and transverse to the 4-foot strip? What type of coarse aggregate was used in the concrete? Was there any slag cement or flyash used?

Are you sure the slabs moved up relative to the strip? What about settlement of the strip? Any distress in the storefront glazing or gaskets? (If they are typical storefront sections, pop the "beauty cap" off and look at the bottom rubber spacers holding the glass. If they are differentially "bulged" check why). Do you have active or expansive clay in the subgrade? What has been the temperature differential profile of the building (inside to outside)? What is the crack pattern in the slab, if any? What is the joint spacing? When were the joints cut in the slab?


Lots of things to consider...curling doesn't seem like the likely cause, but could contribute.

 

[MiketheEngineer]

I am betting on frost heave or plastic soils getting wet...

I have seen frost heave especially where the slab is next to a conditioned place. Like the driveway apron slab heaving up above the condtioned (and dry) garage floor slab..

 

[Falconite]

The building is 200 ft. x 60 ft. and is 80 ft. from the road. It sits about 4 ft lower then the road with 95% of the area surrounding the building covered with concrete or asphalt. This site was used as a gas station before with the tank area filled with engineered fill. The area where the tanks were removed is located in the middle of the front wall under the 3rd slab.

The first slab (5000 sq ft) was pumped with a 2" pump line on a 5" slump. The mix was 4000 psi straight cement, 1% HE, and small aggregate. 6 mil poly and wire mesh.
The second slab (2000 sq ft) used Georgia Buggies on a 4" slump. The mix was 4000 psi straight cement and 5/8 aggregates. 6 mil poly and wire mesh.
The third slab is unknown.
Joints were cut same day with green cut saw at 12' spacing.

The drywall above the windows and at the columns show no cracking or any signs of settling of the foundation or the 4 ft strip. No distress to the glazing. All three slabs are covered with carpet so no exposed slab to examine.

Here is some info from an Engineering Consultant:
On Feb 22, 2010, the building was inspected for evidence of foundation settlement. No cracks were noted in the masonry, building exterior, or the interior drywall. It is our feeling that the foundations have not settled, and the building is structurally sound.
Crack monitors installed in the vacant suite have shown no additional movement between the original 4 ft. perimeter slab, and the interior slabs. However, the crack monitors have only been in place for 19 days.


It is scheduled this Sunday to take out a 3 ft strip of concrete next to the 4 ft strip under the windows. I will be there to oversee the repair work.