Frequently Asked Questions

During a summer roof slab pour in Ghaziabad at 42 degrees C and 20 percent relative humidity with the loo wind blowing, what is the evaporation rate from fresh concrete and why is MC-Fibre PP essential to prevent plastic shrinkage cracking on this pour?

Calculating the evaporation rate from fresh concrete in Delhi NCR summer conditions is the starting point for understanding why plastic shrinkage cracking is practically inevitable on large concrete pours without proper precautions and why PP fibre addition is not optional in these conditions. The Menzel-Lerch nomograph (standardised in ACI 305R Hot Weather Concreting Guide) relates concrete surface temperature, air temperature, relative humidity, and wind speed to the evaporation rate in kg per square metre per hour. For a typical Ghaziabad summer noon condition — concrete surface temperature 40 degrees C, air temperature 42 degrees C, relative humidity 20 percent, wind speed 20 km per hour (the loo wind from the west) — the evaporation rate from the ACI 305R nomograph is approximately 1.0 to 1.2 kg per square metre per hour. This can be compared to the bleeding rate of a typical Indian structural concrete mix (M30 grade, water-cement ratio 0.45, OPC cement, plasticised with Centrament Flow 100) of approximately 0.1 to 0.3 kg per square metre per hour. The evaporation rate (1.0 to 1.2 kg per square metre per hour) substantially exceeds the bleeding rate (0.1 to 0.3 kg per square metre per hour), meaning that the concrete surface is drying out faster than the bleeding water can replenish it. ACI 305R states that when the evaporation rate exceeds 1.0 kg per square metre per hour, plastic shrinkage cracking is virtually certain in the absence of protective measures — and the Delhi NCR summer noon condition exceeds this threshold comfortably. The mechanism of plastic shrinkage cracking then proceeds as follows. As the surface moisture is evaporated faster than it is replenished by bleeding, the surface layer (the top 10 to 20 mm of the concrete) begins to dry and contract. The underlying concrete is still fluid and cannot provide tensile resistance to this surface contraction. The surface contraction creates tensile stress in the surface layer. When the tensile stress exceeds the tensile strength of the fresh concrete (which in the plastic stage is essentially zero — fresh concrete has no tensile resistance before initial set), cracks form. These cracks are typically 1 to 3 mm wide on the day of pouring, narrowing to 0.1 to 0.5 mm after initial set and early strength gain. MC-Fibre PP at 0.6 to 1.0 kg per cubic metre addresses the cracking mechanism directly: the randomly distributed fibres at extremely high numbers per cubic metre (monofilament fibres at 0.6 kg per cubic metre of 12 mm length and 20 micron diameter give approximately 15 to 20 million individual fibres per cubic metre of concrete) create a dense three-dimensional reinforcement network in the plastic concrete that bridges every incipient crack as it begins to open at the microscopic scale, providing enough tensile resistance to prevent the crack from widening to a macroscopic visible size. However, MC-Fibre PP alone is not sufficient in the Delhi summer noon evaporation rate condition — it must be combined with protective measures to reduce the evaporation rate to below the ACI 305R threshold. The correct protocol for a Ghaziabad roof slab pour in summer is: time the pour for pre-dawn (3 to 7 am) to avoid peak evaporation conditions; add MC-Fibre PP at 0.6 to 0.8 kg per cubic metre; erect windbreaks on the windward side to reduce wind speed at the concrete surface; have shade cloth or tarpaulins ready to cover the placed concrete if evaporation conditions worsen; use an evaporation retarder (aliphatic alcohol monomolecular film spray) applied to the concrete surface after finishing and before initial set to further reduce surface moisture loss; and apply curing compound or wet hessian immediately after initial set. With these combined measures, plastic shrinkage cracking is effectively eliminated even in the most aggressive Delhi summer pouring conditions. Space Arc Engineering provides admixture, fibre, and curing compound supply for summer concrete programmes in Delhi NCR.