Cracking can be classified into two general types: one form appears before the concrete hardens; the other appears after the concrete hardens. In each case, and regardless of whether or not the structure is weakened, the cracks are unsightly and undesirable. The most common flaw showing up in newly placed concrete is plastic cracking. As the name indicates, this type of cracking shows up before the concrete hardens. The cracks begin to from soon after a slab is placed and while the concrete is still plastic. Plastic cracking appears most frequently on horizontal surfaces, but the cracks take no particular shape. There is no mystery concerning the cause of true plastic cracking. The cracks show up when the surface of newly placed concrete dries too rapidly. Drying, in turn, comes about too swiftly when the evaporation of water from the surface of the fresh concrete gets out of hand. The best way to avoid this is to start curing as soon as possible and use a either a white membrane curing compound or sand to keep in moisture. Another cause of plastic shrinkage cracking is known as syneresis. This is a chemical reaction in which the cement gel shrinks and water escapes rapidly. Usually cracks that occur as a result of syneresis can easily be erased by reworking the plastic concrete and closing the cracks. They seldom reappear. Contraction cracks are among those occurring most frequently after the concrete hardens. Temperature variations are largely responsible for these cracks. Such cracks can be avoided by the proper spacing of contraction joints. Make sure that all exterior slabs are sectioned off in reasonable lengths. Stress concentrations are another major cause of after-hardening cracks. Tensile cracks can almost always be anticipated in reinforced concrete. Efforts should always be made to control the width of these cracks by increasing the bond between the concrete and the steel.