As far as the lean concrete is concerned, it serves no purpose other than to create a flat surface for the insertion of rebars, hence there is no need for curing. The same goes for plain concrete. It needs to be cured to make it strong enough for its intended use. Concrete that is too soft requires more time to cure than soft sand or clay, which means that it can be used immediately after mixing. Concrete that is too hard requires less time to cure than softer materials, so it can be used later after some time has passed. Usually, concrete cures in three days if the temperature stays below 90 degrees F and there is adequate moisture present, but this can take up to six months at temperatures above 100 degrees F or when it rains often. Curing concrete also helps prevent the growth of bacteria that can cause disease. The type of cement used in concrete affects how it cures. Ordinary portland cement will keep the concrete solid until it reaches 120 degrees F, at which point it begins to melt slightly and then turns into a harder stone-like material. This is called "cured" concrete and is suitable for most applications where strength is not an issue.