Frequently Asked Questions

A contractor is pouring a 3,000 square metre rooftop concrete slab on a commercial building in Ghaziabad in the month of May — ambient temperature is 42 degrees Celsius, relative humidity 22 percent, and afternoon wind speed is 25 km/h — what is the complete hot-weather concrete management plan including MC-Crete 620 evaporation retarder use, and how does the contractor calculate whether plastic shrinkage cracking risk is at the critical level requiring intervention?

A rooftop slab pour in Ghaziabad in May at 42 degrees Celsius, 22 percent RH, and 25 km/h wind is among the most severe hot-weather concreting conditions encountered in north Indian construction, and represents a situation where plastic shrinkage cracking is virtually certain without proactive management measures. The complete hot-weather management plan is as follows. Step 1 — Calculate the evaporation rate using the ACI 305 nomograph or formula: for air temperature 42 degrees Celsius, relative humidity 22 percent, wind speed 25 km/h (approximately 7 m/s), and concrete surface temperature approximately 40 degrees Celsius (in direct sun on a rooftop), the ACI 305 calculation gives an evaporation rate of approximately 1.2 to 1.5 kg per square metre per hour. ACI 305 recommends that precautions against plastic shrinkage cracking are taken when the evaporation rate exceeds 1.0 kg per square metre per hour — and requires mandatory measures when the rate exceeds 1.0 kg per square metre per hour. At 1.2 to 1.5 kg per square metre per hour, plastic shrinkage cracking risk is critical and unavoidable without active prevention. The bleeding rate of a typical M30 or M35 rooftop slab concrete (w/c 0.45 to 0.50) is approximately 0.1 to 0.2 kg per square metre per hour — seven to fifteen times less than the evaporation rate. Without intervention, the surface will lose net moisture at 1.0 to 1.3 kg per square metre per hour throughout the finishing period, and plastic cracks will form within 30 to 60 minutes of placement. Step 2 — Pre-pour temperature management: use chilled water (10 to 15 degrees Celsius) for concrete mixing at the batching plant — each 5 degrees Celsius reduction in water temperature reduces concrete temperature by 1 to 1.5 degrees Celsius. Target concrete delivery temperature below 32 degrees Celsius. Use MC-Crete 120 retarder at 0.35 percent by mass of cement to extend workable period and prevent premature stiffening during transit and placement. Plan the pour to start at 6 AM to complete placement before the hottest part of the day (11 AM to 3 PM) — reduce the area of fresh concrete exposed to peak afternoon sun at any one time. Arrange wind barriers (polythene sheeting on temporary scaffolding frames, 1.5 metres high) around the perimeter of the pour and along the direction of the prevailing afternoon wind where feasible. Step 3 — MC-Crete 620 application protocol: have spray equipment (pump sprayers loaded with MC-Crete 620) positioned on the slab perimeter before the pour starts. Assign one dedicated spray operative whose only task is evaporation retarder application — this person does not assist with finishing. The spray sequence is: as each 3 to 5-metre-wide lane of concrete is struck off (screeded level), the spray operative immediately applies MC-Crete 620 at 0.07 litres per square metre as a fine, even mist over the struck-off surface. The mist should cover the entire surface without puddling — if the surface shows wet patches, reduce application rate slightly. After application, the surface will show a faint sheen for 1 to 2 minutes as the film forms, then will appear matt. The finishing crew must not walk on the film surface until it is dry (1 to 2 minutes) to avoid disturbing the film. After the film dries, concrete floating and trowelling proceed normally — the film is thin enough that the float and trowel pass through it into the concrete surface without degrading it. If the finishing is delayed by more than 30 minutes at any area (due to delivery delays, crew bottleneck, or plant breakdown), reapply MC-Crete 620 to that area before the next pass. Step 4 — Wind protection during finishing: position a temporary wind break on the windward side of the fresh concrete in each pour lane — a polythene sheet on a lightweight scaffolding frame 2 metres high is effective in reducing local wind speed at slab level by 60 to 70 percent, which directly reduces the evaporation rate. At 25 km/h ambient wind, a wind break reducing wind at slab level to 8 to 10 km/h approximately halves the evaporation rate contribution from wind, reducing total evaporation from 1.2 to 0.7 to 0.8 kg per square metre per hour — still above the bleeding rate but now manageable with MC-Crete 620. Step 5 — Post-finishing curing: apply MC-Cure acrylic curing compound by sprayer immediately after the final trowelling pass — before any whitening of the surface from drying is visible. On a rooftop in peak summer, the transition from plastic to initial set can happen within 2 to 3 hours from placement, and final trowelling must be completed before this — plan the finishing sequence to stay within 2.5 hours of each concrete pour lane. After curing compound application, cover the slab with two layers of white or silver polythene sheeting (reflective surface facing up) to reduce solar heat absorption — white polythene can reduce the surface temperature of a rooftop slab by 5 to 8 degrees Celsius versus bare concrete or dark plastic. Maintain the curing cover for minimum 7 days. Quality verification: after 24 hours, inspect the slab surface for plastic cracking. Record the crack length and pattern if any cracking has occurred. If the above protocol was followed correctly, plastic cracking should be absent or limited to isolated cracks less than 0.1 mm width. Any cracks wider than 0.3 mm should be sealed with a low-viscosity polyurethane sealant after 28 days. Space Arc Engineering supplies MC-Crete 620 evaporation retarder, MC-Crete 120 retarder, and MC-Cure curing compound as a complete hot-weather concrete management system for rooftop slab and large floor slab construction in Ghaziabad, Delhi NCR, Noida, and Uttar Pradesh.