Foamed concrete admixtures are surfactants that are diluted with water before passing through a foam generator to produce a stable foam, similar to shaving cream. This is then blended into a cementitious mortar in a quantity that produces the required density for the foamed mortar (more usually called foamed concrete).
Polymer dispersion admixtures are aqueous dispersions of synthetic polymers that form a continuous film when sufficient water is lost from the system. Concrete modified with polymer dispersion is known as polymer cement concrete (PCC). Polymer dispersions reduce the permeability of concrete (improving durability), give a more reliable bond to base materials (concrete and steel), increase toughness and resistance to abrasion/dusting and freeze-thaw action.
Modern pumps are able to cope with most concrete mixes but difficulties can be experienced with concretes made from aggregates that are poorly graded, elongated or flaky. Pumping for large distances horizontally or vertically can lead to high pump pressures and the need for improved lubrication within the lines. Adding a polymer-based pumping aid can modify the cement paste rheology to help provide this enhanced lubrication. Pumping aids can also provide better workability retention for long pumping distances and can be particularly effective if interruptions to pumping are likely and when segregation or stiffening in the line could prevent a restart.
With careful mix design, self-compacting concrete (SCC) admixtures produce a concrete that is fully flowing and self-compacting and which can be placed with no vibration.
Shrinkage-reducing admixtures added during batching can significantly decrease both the early and long-term drying shrinkage. This is achieved by treating the cause of drying shrinkage within the capillaries and pores of the cement paste. Such admixtures should not be confused with shrinkage-compensating materials that are normally added at more than 5% by mass of cement to create an expansive reaction within the cement paste to counteract the effects of drying shrinkage.
Underwater concrete anti-washout admixtures increase the cohesion of the concrete in a way that significantly reduces the washing out of cementitious material and sand from fresh concrete placed underwater. Anti?washout admixtures are often used in conjunction with superplasticisers to produce flowing self-levelling concrete to aid placing and compaction underwater.
There is a growing problem with disposal of washwater from cleaning the inside of truck mixer drums at many ready-mixed concrete plants. The Landfill Tax and the prohibition on disposal through the drainage system have increased the cost of disposal. One solution is to recycle the water by the use of a truck washwater admixture.
Retarding admixtures have little or no effect other than to delay the setting of the cement. They do not plasticise significantly and have no effect on the water demand or other properties of the concrete. Retarding plasticising admixtures not only delay the setting of the cement but are also efficient plasticising water-reducers. Most commercially available retarders are of this type. Retarding plasticisers are used to give workability retention to the concrete, delay the setting time and increase initial workability.
Superplasticising/high-range water-reducing admixtures significantly reduce the amount of water needed to achieve a given workability in plastic concrete. This effect can be used to reduce water content (for increased strength and reduced permeability/improved durability), i.e. as a high-range water-reducer, or to increase consistence and workability, i.e. as a superplasticiser.
Normal plasticising admixtures reduce the amount of water needed to achieve a given workability in plastic concrete without significantly affecting the air content or setting characteristics. When water-reducing admixtures are used to increase the workability or consistence of the concrete, they are usually termed ‘plasticising admixtures’. Where no change is made to the water/cement ratio, plasticising/water-reducing admixtures increase workability. Typically the slump will be 60–100 mm higher than that of a plain concrete of equal water content.