Archive

All the principal admixture companies supply safety data sheets for their products. These should be carefully checked and advice on handling passed to those working with the products.

Admixtures are usually stored in bulk, in plastic tanks, metal tanks or barrels. Modern bulk plastic storage tanks are double-skinned to minimise the risk of leakage from a skin splitting. If doubled-skinned tanks are not used, the admixture storage area should be bunded to prevent spillage entering the water system.

In general, for optimum performance, admixtures should be added at the end of the batching process along with some of the mixing water, never directly onto dry materials. The timing of the addition affects the performance and so it is important that the admixture is always added at the same point in the batching process to ensure consistent performance.

Admixtures are added to concrete in&nbsp

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 &#180

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.