There are a number of problems which can occur in concrete as a result of improper mixing, placing, or curing. The following are common problems that are easily avoided if proper procedures are followed.
Segregation is the tendency of the various constituents of a concrete mix to separate, especially the separation of the large aggregate particles from the cement mortar. Segregation can result in rock pockets or honeycombs in the hardened concrete, sand streaks, porous layers, scaling, laitance, and bond failure at construction joints. Harsh mixes have a tendency to segregate, usually those that are too wet but sometimes those that are too dry. A well-proportioned mix with a slump of 3 to 4 in. resists segregation, but any mix can segregate if it is not properly handled, transported, and placed. Once segregation has occurred, the aggregate cannot be reintegrated and the mix must be discarded. Segregation can be caused by over-mixing or by improper handling during placement operations.
Bleeding occurs when the cement and aggregate in newly placed concrete begin to settle and surplus water rises to the top surface of the concrete. Bleeding continues until the cement starts to set, until bridging develops between aggregate particles, or until maximum settlement or consolidation occurs. Mix proportions, sand grading, sand particle shape, the amount of aggregate fines, the fineness of the cement, water
content of the mix, admixtures, air content, temperature, and depth or thickness of the concrete all influence the rate and total amount of bleeding. A slab placed on a plastic vapor retarder will bleed more than one placed directly on soil because the soil absorbs some of the surplus water. Some bleeding is a normal part of concrete curing, but excessive bleeding can decrease the durability of the surface, interfere with the
bond of cement paste to reinforcing bars, and increase porosity of the hardened concrete. Air entrainment reduces bleeding, as does a well-graded sand, an increase in cement content, or a reduction in water content. If changes are made to some ingredient quantities, the mix must be adjusted to maintain the proper proportions required for strength and durability. Bleed water must be allowed to dry naturally, as there is little way to remove it from the soft surface of the fresh concrete. Excessive bleeding will delay the start of finishing operations.
Plastic shrinkage cracking is usually associated with hot-weather concreting. It is caused by rapid evaporation of surface moisture from a slab or other flatwork. The procedures recommended for hot-weather concreting will alleviate the possibility of plastic shrinkage cracking.
Dusting is the wearing away of hardened concrete surfaces under traffic. Dusting is caused by mixes with too much water, segregation during the placement and consolidation of the concrete, dirty aggregate, applying water to the concrete surface during finishing operations, or premature or prolonged finishing operations which cause the formation of a weak surface layer called laitance. Laitance is a white or light gray substance which appears on the surface of concrete after it is consolidated and finished and which consists of water, cement, and fine sand or silt particles. Laitance prevents good bond of subsequent
layers of concrete and adhesion of other materials to the concrete such as finish flooring. In an exposed slab, laitance will scale and dust off after the floor is in use, and it can contribute to hairline cracking and
checking. Excessive amounts of rock dust, silt, clay and other similar materials can also contribute to laitance. The same measures that are used to reduce bleeding will also reduce the occurrence of laitance.
Scaling is the flaking or peeling away of a thin layer of cement mortar on the surface of concrete. The aggregate below is usually clearly exposed in patchy areas and often stands out from the remaining surface. Scaling can be paper thin or as deep as 14 in.
One type of scaling is caused by the same things that cause dusting and laitance: mixes with too much water, segregation during the placement and consolidation of the concrete, applying water to the concrete surface during finishing operations, or premature or prolonged finishing operations. Another type of scaling is caused by the use of deicing salts on non-air-entrained concrete, and can be prevented by the use of air-entrained cement or air-entraining admixtures.
In false set, concrete appears to set or harden after only a few minutes. This is a temporary condition caused by hydration of unstable gypsum (calcium sulfate) in the cement. It usually disappears with prolonged mixing or remixing and is generally not a problem with ready-mixed concrete. Do not add more water. After a few more minutes, with or without additional mixing, false set will usually disappear on its own.
In flash set, lumps of dry cement are surrounded by a layer of damp or partially hydrated cement, or solid lumps of partially hydrated cement are formed. Flash set is caused by the use of hot-mixing water in cold weather. To avoid this problem, change the batching sequence so that the hot water and aggregates are put in the mixer first and the cement is added after the water has cooled slightly.