Concrete additives are added to the mixture of water cement and aggregate in small quantities to increase the durability of the concrete, to fix concrete behavior and to control setting or hardening. They can either be liquid or powdered additives.
These additives are supplied in ready-to-use liquid form and are added to the concrete at the plant or at the jobsite. Successful use of additives depends on the use of appropriate methods of batching and concreting.
Concrete additives have various functions depending on what the contractor wants to achieve. There are two main types of concrete additives which are chemical and mineral.
Chemical additives reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing/transporting/placing/curing, and overcome certain emergencies during concrete operations.
Mineral additives make mixtures more economical, reduce permeability, increase strength, and influence other concrete properties. Mineral admixtures affect the nature of the hardened concrete through hydraulic or pozzolanic activity. Pozzolans are cementitious materials and include natural pozzolans (such as the volcanic ash used in Roman concrete), fly ash and silica fume. They can be used with Portland cement, or blended cement either individually or in combinations.
Additives are generally classified according to the function that each performs. These are:
These are used to reduce the quantity of mixing water required to produce concrete of a certain slump, reduce water-cement ratio, reduce cement content, or increase slump. They are used extensively on larger projects where reinforcing steel requires high workability. Also used in precast and on site where the large water reduction provides very high early strength and improved durability. Water reducing additives usually reduce the required water content for a concrete mixture by about 5 to 10 percent.
These are used to speed the rate of early hydration of the cement. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather. Calcium chloride (CaCl2) is the chemical most commonly used in accelerating admixtures, especially for non-reinforced concrete.
Air-entraining admixtures are used to purposely introduce and stabilize microscopic air bubbles in concrete. Based on special surfactants, these admixtures cause tiny air bubbles < 0.3mm in diameter to stabilize within the cement paste. This air helps to prevent the concrete from cracking and scaling as a result of frost action. Air also increases cohesion in the mix, reducing bleed water and segregation of the aggregate before the concrete can set.
Shrinkage-reducing admixtures have potential uses in bridge decks, critical floor slabs, and buildings where cracks and curling must be minimized for durability or aesthetic reasons. Concrete shrinks, mainly due to loss of excess water. This causes internal stresses that lead to cracking or curling, especially in slabs. These admixtures reduce the shrinkage stress.