The contractor or cement mason who wants to be in full command of most job situations should be able to identify and dispose of the potential difficulties. Common ones related to finishing problems and surface defects in flatwork, their prevention and remedies are discussed here. These include: segregation, excessive bleeding, plastic cracking, surface blisters, pickup or peeling, scaling, dusting, and crazing.
Segregation of concrete is a separation of coarse and fine materials in the fresh concrete. Excess water in the mix, producing too high a slump, will usually cause segregation of aggregate during placing. Segregation of sand and cement fines at the surface, on the other hand, is usually caused by use of grate tampers, mesh rollers, darbies or bull floats while there is bleed water on the surface. This remixes the water with the sand and cement fines, causing them to segregate into distinct horizontal planes.
Segregation during placing may be prevented by correcting a slump that is too high. Generally concrete for flatwork should not be placed with more than a 4-inch slump. Segregation of sand and cement fines in subsequent work can be avoided by not doing any finishing operation while there is water on the surface.