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.
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.
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.
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.
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.
Accelerating admixtures can be used to increase either the rate of stiffening or setting of the concrete or the rate of hardening and early strength gain to allow earlier formwork striking and demoulding. Most accelerators achieve one rather than both of these functions.
Air-entraining admixtures cause small stable bubbles of air to form uniformly through a concrete mix. The bubbles are mostly below 1 mm diameter with a high proportion below 0.3 mm. The benefits of entraining air in the concrete include increased resistance to freeze-thaw degradation, increased cohesion (resulting in less bleed and segregation) and improved compaction in low-workability mixes.
Water-resisting admixtures are sometimes referred to as ‘waterproofing’ admixtures or ‘permeability-reducing’ admixtures. The guidance in BS 8102 (updated March 2022) terms and definitions for waterproofing defines it as a ´
Sprayed concrete is pumped to the point of application and then pneumatically projected into place at high velocity, frequently vertical or overhead. This calls for a rapid-stiffening mix that will not fall away from the surface. Admixtures can be used in the fresh concrete to give stability and hydration control (a form of retardation) before spraying. An accelerating admixture injected at the spray nozzle then controls the rheology and setting of the concrete to ensure a satisfactory build-up with minimum rebound.
Corrosion-inhibiting admixtures increase the passivation of reinforcement and other embedded steel. This can inhibit corrosion when passivation would otherwise have been lost as a result of chloride ingress or carbonation. They are added to concrete during production and are referred to as ‘integral’ corrosion-inhibitors. So-called ‘migratory corrosion-inhibitors’ applied to hardened concrete are not admixtures.