The surface of the concrete structure of a new construction is covered with straw/gunny bags by wetting. This is done to:

Covering concrete with wet straw or gunny bags is a standard curing method used to maintain moisture and temperature during the early stages of construction. The chemical process of hydration, which gives concrete its strength and durability, is not instantaneous but continues over a long period. Freshly mixed concrete contains sufficient water for this reaction; however, exposure to the atmosphere leads to rapid evaporation, especially in dry or windy conditions. If water is lost too quickly, hydration ceases, leading to weak, poorly cured concrete. Wet coverings like gunny bags or jute matting act as a barrier to stop the surface from drying out, ensuring that enough water remains available for the cement to hydrate and set completely. This process is essential for achieving the desired structural strength and preventing surface cracks.