RETARDING WATER-REDUCING CONCRETE ADMIXTURE
Retarding water-reducing admixtures are chemicals that slow down the initial reaction between cement and water by reducing the rate of water penetration to the cement and slowing down the growth of the hydration products. The concrete therefore stays workable longer than it would otherwise.
The length of time during which a concrete remains workable depends on its temperature, consistence class, and water-cement ratio, and on the amount of retarder used. Although the occasions justifying the use of retarders in the UK and US are limited, these admixtures may be helpful when one or more of the following conditions apply:
- In warm weather, when the ambient temperature is higher than about 20oC, to prevent early stiffening (going-off) and loss of workability, which would otherwise make placing and finishing difficult.
- When a large pour of concrete will take several hours and must be considered without already placed concrete hardening before subsequent concrete is merged with it (i.e. without a cold joint).
- When the complexity of slip-forming demands a slow rate of rise.
- When there is a delay of half an hour or more between mixing and placing –for example, when ready mixed concrete is being used and when there may be traffic delays and/or long hauls. This can be seriously aggravated during hot weather, especially if the concrete has a high cement content.
The amount of retardation can be varied – usually upto about four to six hours – by altering the dosage, but longer delays can be obtained for special purposes.
While the early strength of concrete is reduced by using a retarder, which may affect formwork striking times, the 7 and 28 day strength are not likely to be significantly affected.
Retarded concrete needs careful proportioning to minimize bleeding due to the longer period during which the concrete remains fresh.
