How to Control Cracking in Racquetball Court

Hotline Problem

Were replacing a concrete slab for a racquetball court. The slab is 20 feet wide and 45 feet long. There are to be no joints in it because they would affect the bounce of the ball. We will use an isolation joint to separate the court from the existing walls and floor.

Were concerned about cracking and someone recommended using glass fibers in the concrete to prevent cracks from forming. Is this a solution to the problem?

Troubleshooting Response

Glass fibers are commonly used in a spray process to produce lightweight cladding panels for building but theyre not commonly used in slabs on grade. Synthetic fibers such as polypropylene are used in slabs on grade to control early cracking of the concrete. However, they wont prevent shrinkage cracks in the hardened concrete or keep the cracks from widening once theyve formed.

Post-tensioning is one way to control cracking. But since this is a replacement slab rather that a new one, post-tensioning may not be feasible.

Your best bet is probably a combination of shrinkage control and crack control methods. To minimize shrinkage, use a low-slump concrete with a low water content. Use only enough water to make the concrete workable. Adding fibers will help to control early cracking caused by plastic shrinkage. So will early curing.

Use either welded wire fabric or rebars to hold together any cracks that might form and to prevent faulting at the cracks. An engineer can calculate the cross-sectional area of steel needed to keep cracks from opening when drying shrinkage or contraction caused by cooling does occur.

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