Control Joints & Curling in Gymnasium Concrete Slab on Grade
Control Joints & Curling in Gymnasium Concrete Slab on Grade
(OP)
A gymnasium addition I designed is currently under construction. The slab on grade in the gym consists of 4" concrete with fiber mesh over a 15 mil vapor barrier and granular fill. In addition, I show sawed control joints at 12 feet on center in both directions. The contractor recommended to omit the control joints under the gym floor to prevent curling. I understand curling can occur at a construction joints, so I specify dowels at construction joints. Should I be that concerned about curling at control joints as well? Should I be more concerned about the random cracking that will occur without joints?






RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
If they will cure the slab properly, preferably with moist curing for at least 7 days, there will likely be minimal curling. It is more often a problem in exposed slabs. Also, pay attention to the mix design that the contractor will be using. If he is pumping the concrete, he will likely use a smaller aggregate. This requires more cement and more water...thus more shrinkage. If you have not already done so, require that they use the largest aggregate practicable for the placement. No. 57 stone would be the minimum size I would use for this application.
Make sure they positively cure the slab!!
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
Without adding reinforcement, if control joints are omitted it seems that random cracking in the slab is a certainty. However, with proper curing the slab curling at the control joints seems like it could be controlled (or at least minimized).
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
I'm not a fan of WWF. It is usually not placed properly and does little to help the concrete. When I specify WWF, I specify it in sheets, not rolls and I specify that it be placed in the second of two lifts of concrete. Almost never gets done, but that's the only way it actually works.
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
Curling is caused by differences in moisture content and/or temperature between the top and bottom of the slab. When the top surface dries and ultimately shrinks, while the bottom dries at a slower rate, the slab can exhibit deformations at the edges and corners. Nighttime temperatures can cool the top surface of the slab while the mass or bottom is not cooled, creating or adding to the curling or deformation. The benefit of Fibermesh fiber reinforcement is the internal support provided in the critical green state. Uniform bleeding limiting the formation, scope, and migration of the bleed water hinders the conditions conducive to curling.
This was a publication from a company selling fiber mesh, so they aren't exactly unbiased. While I don't agree with their reference to the product as fiber "reinforcement", I wonder if the fiber mesh does help prevent curling. Does this seem reasonable?
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
If you omit the joints, the slab will crack more randomly. At these cracks, and more so where the cracks intersect (forming corners) you will still get curling unless you
keep the shrinkage down and cure very well as mentioned above.
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RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
proper curing methods at the proper time is the best thing the Contractor can do to prevent curling. i say proper time meaning they have to finish that PORTION of the slab immediately after bleed water goes and start curing the finished parts. many finishers don't want to start curing until they have all their finishing equipment loaded up in the truck... meanwhile at least half of the slab could have already started curing while they were finishing the other side. There is a diminishing return on curing... more evaporation occurs earlier than later. Contractors will scratch their head about curling because they know they eventually followed the curing procedures when they were done, but don't realize that the pump breaking down 3 or 4 times during the pour made one area sit for too long before they finished it and then again so from curing.....
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
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RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
Mike McCann, PE, SE (WA)
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
Thanks for your comments everyone. It seems that the control joints bring more positive effects to the slab than negative.
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
As a concrete contractor, on a gym floor I am most corn earned with flatness. I would use a laser screed and pans on the largest ride on machine in the fleet.
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade
Agree with you on laser screed.
RE: Control Joints & Curling in Gymnasium Concrete Slab on Grade