How can I accelerate concrete set time, and what is the difference between Calcium Chloride and Non-Chloride Accelerators?
Calcium Chloride is the most commonly used accelerating admixture and is used to accelerate strength development of concrete at an early age. The strength development of concrete can also be accelerated by (1) Using Type III Cement, (2) Lowering the water-cement ratio by increasing classification, or (3) Curing at higher temperatures (steam, applied heat, hot water).
The widespread use of Calcium Chloride has provided much data and experience on their effect on the properties of concrete. Besides accelerating strength gain, calcium chloride causes an increase in drying shrinkage, potential reinforcement corrosion, and discoloration (darkens concrete). Calcium chloride is not an antifreeze agent. When used in allowable amounts, it will not reduce the freeze point of concrete by more than a few degrees. The amount of calcium chloride added should be no more than is necessary to produce the desired results and in no case exceed 2% by weight of cement.
Non-Chloride Accelerators should be used for the following applications; (1) Concrete slabs supported by permanent galvanized-steel forms, (2) Colored concrete, (3) Pre-stressed concrete, (4) Concrete containing embedded aluminum or steel since serious corrosion can result, (5) In floors intended to receive dry-shake metallic finishes. The maximum chloride-ion content for corrosion protection of reinforced concrete is recommended by ACI 318. Resistance to the corrosion of embedded steel is further improved with an increase in the depth of concrete cover over reinforcing steel, and a lower water-cement ratio. Several non-chloride, noncorrosive accelerators are available for use in concrete where chlorides are not recommended. However, most non-chloride accelerators are not as effective as calcium chloride and are more expensive.
Whichever acceleration the application warrants almost all accelerating admixtures need the initial “kick” of hot water to get started, and to be as effective as possible in cold weather concreting.