Q. Half the year we live in Florida, and our development put in a sport court made of cement for pickle ball (a smaller version of tennis using a paddle). The concrete was just the size of the court, and there was no room to run back or to the side without falling off into the grass — a potential hazard. So a crew added more concrete to make the surface bigger, but the edges didn’t meet exactly and had to be grinded. I have several questions. How long should they wait to paint the cement? How do you fix the edges where the concrete meets? Why is the new concrete, just set, already cracking? What do we do about the old bolts that held the net up? They’re just cut down to the concrete, and are beginning to rust.
A. If at first you don’t succeed, try, try again. Or maybe dry, dry again. Since it takes about 28 days for cement to cure, the paint shouldn’t be applied until then, but humidity must be low, and the paint is actually supposed to be an opaque stain manufactured for coating concrete. Concrete must still be able to “breathe,” to take in and allow moisture to freely evaporate. If you use the wrong coating, the surface will bubble, and after a season of temperature fluctuations, will begin to “spall.” Spalling is when the surface pops and pits, leaving impressions that may be the size of your fist. This is the material’s way of letting you know you smothered it.
Cracking in new concrete is generally a combination of improper mix (usually too much water), internal movement due to what’s called the “heat of hydration,” where the internal chemical reaction builds up and releases heat, and weak material below the concrete, not compacted well enough to resist movement. The adjoining surface should have been smoothed, or “screeded,” using a 6-foot-wide blade, called a screed, that should have been half on the old surface and half on the newly poured cement so that the surface heights and edges matched.
To keep the two slabs together and not shifting up and down, steel rods should have been drilled into the vertical edge of the old court so when the two surfaces were joined, the rods would keep them together. I hope the cement personnel did this, and also used correctly engineered reinforcement and thickness. I often see 4-inch thickness where 6 inches was required. There are two reasons for 6-inch thickness: one is for compressive strength, and the other is to cover the reinforcing with enough material to limit continuous wetting and drying. Even though all reinforcing should be rust-protected, it rarely is, so this helps slow the rusting process. Cut bolts also rust, and should be core-drilled out. If not, rusting metal expands and will crack the surrounding concrete. Expansion joints, including where the edges meet, should have a backer rod and polyurethane-based sealant.
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