Plastic Shrinkage Cracks

Plastic shrinkage cracks are short, random, often parallel surface cracks that form in fresh concrete within the first few hours — while it is still plastic — when water evaporates from the surface faster than bleed water can rise to replace it. The drying surface contracts against the still-soft mass below and tears. They are typically shallow but can extend deeper in slabs and are most common in hot, dry, windy, low-humidity conditions and with high-cement low-bleed mixes.

They are a curing and protection failure, not a strength problem of the mix: the cure is prevention — start curing/protection immediately, use evaporation retarders/aliphatic-alcohol films, fog spray, windbreaks, sun shading, and pour in cooler parts of the day. IS 456 Cl. 13.5 and good practice require curing to begin as soon as the concrete has set sufficiently; for large flatwork, controlling the evaporation rate (commonly keeping it below ~1.0 kg/m²/hr) is the practical site target. Cracks that have formed in the plastic state can sometimes be closed by re-vibration/re-trowelling if caught early.