Frequently Asked Questions

How much does Fosroc Conplast RP extend the initial set time of concrete and how is the dosage adjusted for very hot Indian summer conditions (40 degrees or above)?

The retardation effect of Fosroc Conplast RP on concrete initial set time depends on three variables: the dosage rate (higher dosage produces longer retardation); the ambient and concrete temperature (higher temperature accelerates hydration and reduces the retardation effect of the admixture — the admixture must work harder to overcome the accelerated natural hydration rate); and the cement type and C3A content (cements with higher tricalcium aluminate content are more retarded per unit of admixture than low-C3A cements). At the standard recommended dosage of approximately 0.2 to 0.3 per cent by weight of cement and at 25 degrees ambient temperature, Conplast RP typically extends the initial set time of OPC concrete by 1 to 2 hours — extending a normal 2 to 2.5 hour initial set to 3.5 to 4.5 hours. At 35 degrees ambient (typical Indian May-June maximum day temperature), the hydration rate is approximately 30 to 40 per cent faster than at 25 degrees — the accelerated natural hydration partially counteracts the retardation from Conplast RP, and the same dosage provides proportionally less additional set extension relative to the unretarded control concrete at 35 degrees; to achieve the target set extension of 1 to 2 hours at 35 degrees, the dosage typically needs to be increased by 20 to 30 per cent compared to the 25-degree rate. At 40 degrees or above (severe hot weather conditions in Indian summer), the relationship between Conplast RP dosage and retardation becomes increasingly non-linear — the admixture efficiency is significantly reduced by the very high hydration rate, and the required dosage increase to achieve a given retardation target is greater; there is also a risk of over-retardation in parts of a large pour where the concrete is in shadow or cools at night, if the admixture dosage was set high for the peak day temperature. The safest approach for summer hot weather concreting with Conplast RP is to carry out trial mixes at the batching plant at the expected concrete temperature and at the planned dosage, measuring the actual initial and final set times at the trial mix temperatures before committing to the production dosage rate.

What is the risk of over-retarding concrete with Conplast RP and what are the signs of over-retardation on site?

Over-retardation of concrete with Fosroc Conplast RP is a real and potentially serious problem — unlike the more obvious failure modes (slump loss, early stiffening), over-retarded concrete appears normal and workable but fails to develop adequate early strength and may suffer significantly reduced long-term strength and durability if the over-retardation is severe. The risks and consequences of over-retardation are: formwork striking — if the concrete in a column or wall does not develop the required stripping strength within the planned stripping time (typically 10 to 16 hours for column sides, 24 to 72 hours for beam soffits depending on span), the premature stripping of over-retarded concrete leads to plastic collapse or surface defects in the soft concrete; extended setting can mean a slab surface that is still workable the morning after an evening pour — a safety hazard for site workers; strength reduction — severely over-retarded concrete (dosage errors or cold weather after over-dosing) can experience reduced 28-day compressive strength relative to the target value, because the extended period of low early strength leads to early-age damage (from premature loading, thermal gradient formation, or frost exposure) before the concrete has developed adequate strength; delayed construction schedule — a large pour of over-retarded concrete requires extended curing time before the next pour can begin, with knock-on schedule effects on multistorey construction. Signs of over-retardation on site: concrete remains workable and takes no surface footprint after 6 to 8 hours when it should be stiffening; cube testing of 24-hour cubes shows very low early strength (less than 5 MPa at 24 hours for OPC concrete is abnormal); the pour surface cannot be finished or trowelled at the expected time; adjacent concrete from the same pour has stiffened normally while areas with higher admixture concentration remain soft. Over-retardation is best prevented by accurate admixture dispensing at the batching plant (calibrated dispensers, not manual addition) and by reducing the Conplast RP dosage when ambient temperatures are expected to drop significantly overnight after an evening pour.

Is Fosroc Conplast RP compatible with fly ash concrete (PPC) and GGBS blended concrete commonly used in Delhi NCR ready-mix concrete for green building projects?

Yes — Fosroc Conplast RP is compatible with fly ash concrete (using Portland Pozzolana Cement — PPC — or OPC with separately batched fly ash) and with GGBS (Ground Granulated Blast Furnace Slag) blended concrete that are widely used in Delhi NCR ready-mix concrete for green building certification (IGBC, GRIHA, LEED) projects that specify supplementary cementitious material replacement to reduce the embodied carbon of the concrete. The compatibility is well established and Conplast RP is routinely used with blended cement concretes in India, but there are important differences in dosage and performance that must be understood when using Conplast RP with blended cements: fly ash concrete (OPC replaced 20 to 40 per cent with fly ash): fly ash is pozzolanically less reactive in the early hours of hydration than OPC clinker, so fly ash concrete has a naturally longer initial set time than pure OPC concrete of the same w/c ratio; Conplast RP further extends the already-longer set time of fly ash concrete — the same dosage of Conplast RP that provides 1 to 1.5 hours additional retardation in OPC concrete may provide 2 to 3 hours additional retardation in high fly ash concrete (30 to 40 per cent replacement), because both mechanisms are retarding the concrete independently; in hot weather, the longer base set time of fly ash concrete means Conplast RP may be less critical for workability retention than for OPC concrete, but still valuable for large pours; GGBS concrete (OPC replaced 30 to 50 per cent with GGBS): GGBS concrete also has naturally longer setting times than OPC concrete; similar precautions as for fly ash concrete apply regarding over-retardation risk when Conplast RP is added; the dosage for GGBS concrete should be reduced relative to OPC concrete dosage to avoid excessive retardation. The recommended practice for any blended cement concrete project using Conplast RP in India is to carry out trial mixes with the actual cement blend, fly ash or GGBS source, and aggregate used in the project — the retardation response can vary between cement suppliers and fly ash sources, and a trial is the only reliable way to determine the correct dosage for the specific project materials.