Frequently Asked Questions

What is the maximum temperature at which Conplast PA21S remains effective and what happens at very high concrete temperatures?

Conplast PA21S provides effective plasticising and retarding action at concrete temperatures up to approximately 40 degrees Celsius. At concrete temperatures above 40 degrees Celsius (which can occur in very hot summer conditions when the mixing water, aggregates, and ambient air are all at high temperature simultaneously), the rate of cement hydration is so fast that even a retarding admixture may not provide adequate workability retention for all applications. At these extreme temperatures, additional hot weather concrete measures are needed alongside Conplast PA21S: cooling the mixing water with ice (adding crushed ice as part of the water addition to reduce concrete temperature at mixing); shading aggregate stockpiles to prevent solar heating; minimising the time between batching and placement; and using cold water for curing immediately after compaction. The dosage of Conplast PA21S may need to be increased toward the upper end of the recommended range in extreme summer conditions — but never exceeded without prior trial mix confirmation of setting time and strength at the elevated temperature.

Can Conplast PA21S be used in mass concrete pours for a large raft foundation in summer in Delhi?

Yes. Conplast PA21S is well-suited for mass concrete pours such as large raft foundations and pile caps in Delhi and surrounding areas during the summer construction season. Mass concrete pours for thick raft foundations require extended workability for two reasons: the pour volume is large and takes many hours to complete, so the last concrete placed must remain workable for the duration of the pour; and mass concrete generates significant heat of hydration internally, which can raise the concrete temperature and accelerate slump loss in the middle of the pour. Conplast PA21S provides the 1 to 3 hours of additional set retardation needed to keep the concrete workable through a long pour in hot conditions. For very large raft pours exceeding 500 cubic metres, the Conplast PA21S dosage should be calibrated by trial mix at the peak summer ambient temperature to confirm adequate retardation for the expected pour duration. The use of chilled water and pre-cooled aggregates should also be considered to manage the concrete temperature during large summer pours in Delhi.

What is the risk of over-dosing Conplast PA21S and how long can the concrete remain unhardened?

Over-dosing Conplast PA21S beyond the recommended dosage range carries the risk of excessive retardation — the concrete remains in a plastic or semi-plastic state for much longer than intended. In a raft foundation or slab pour, over-retarded concrete that does not set within 24 to 36 hours can cause significant problems: the formwork cannot be removed; the construction programme is delayed waiting for the concrete to achieve stripping strength; and the extended setting time may coincide with a temperature drop (such as an overnight temperature reduction in summer) that further extends the retardation. In extreme cases of gross over-dosing, concrete has remained unhardened for 48 to 72 hours. To avoid this risk, the dosage must be established by trial mix at the target ambient temperature, and on-site verification of the setting time (using penetration resistance testing on a sample of the concrete) should be carried out on the first major pour before production batching at the full volume.