I need to repair my sidewalk and I've seen cement having these little cracks in it after it's dried; anyone knows of a way to prevent this?
No matter what, a sidewalk repair will always crack along the same crack line. One cannot prevent this as the forces of expansion and contraction are along that line and the repair will always crack again. Cement does not dry. Cement polymerizes with water and the water becomes a permanent part of the cured cement itself. On your repair project, take and mix about 5 tablespoons of Latex paint to a batch of repair cement. This will prevent rapid curing of the cement and improve the bond onto the old concrete sidewalk. When you have finished the repair. Cover the repair with corrugated cardboard, two to three layers of it and soak it down. Always do a repair in the late evening as possible to keep the sun off of it.
if it is cracking it is drying or curing to fast spray light water on it then put a sheet of plastic on it to hold the moisture in
The cement cracking you are describing is probably coming from it setting too quickly. You can think of the idea with the analogy of stretching an object. If I take my time, I can stretch an object without breaking it to achieve the desired length. On the other hand if I suddenly pulled that object to get the same length I would tear it. Concrete must be well hydrated to set, and it must have sufficient time to cure! If after mixing the cement, you lay it on a hot, dry object you are taking the water out of the mix, causing it to lose hydrating ability and time to cure! Wet the surface on which you want to place the concrete or cement, then lay burlap or some type of covering to slow the process down. Wet the burlap, but do not spray the cement directly!
There are two things that you have to do: first, before you put pour the cement in, make sure the area it will be touching is wet - underneath, too. Then, after you put the cement down, make sure you tamp it well: this is not only to get out the air bubbles, but also to have the sand, well, fit more tightly together. Finally, and this is the most important part, make sure the cement stays wet for several days. Cement cures very slowly. It is not 100% completely cured even after a month! But in the first 48 - 72 hours, it is still going through quite an active process, and if it becomes too dry in this time, it will develop tiny fissures and be substantially weaker because of that. Plasters do the same thing, but there the culprit is often the surrounding plaster absorbing water from the wet plaster. That is not much of a problem with cement.
