Plastic shrinkage cracking refers to cracks that develop before the surface gets the opportunity to hydrate appropriately. They’re caused by rapid loss of water from the surface of the concrete before setting occurs and before concrete can resist the tensile forces of it’s own weight. Cracks form after placement/consolidation while concrete is still plastic. Shallow in depth, form in parallel, typically 2-30 inches in length. Rarely cause damage to strength or durability to floor due to shallow depth.
Some causes for plastic shrinkage cracking might be: low relative humidity, high ambient air or concrete temperatures, high winds are blowing across the slab surface, limited bleed water is present in the concrete resulting in rapid surface moisture loss. Below are some ways to reduce the likelihood of such problems: use wind screens, sunshades, or fog spraying to reduce surface evaporation rate, pre-dampen subgrade when hot/dry to reduce moisture loss into subgrade, use fiber mesh which reduces tensile forces that are present during plastic shrinkage period, ‘Off-hour’ placement to avoid high temp, high wind, high sun, use evaporation retardant sprays