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Plastic shrinkage cracking

Surface cracks that appear during or shortly after the finishing of freshly placed concrete slabs.

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Plastic shrinkage cracks are cracks that appear on the surface of a freshly placed concrete slab during the finishing operation or soon after. These cracks are usually parallel to each other, 1–3 feet apart and 1–2 inches deep, and rarely intersect the perimeter of the slab. They rarely impair the structural strength of concrete floors and pavements, but can be unsightly. Their development can be minimised if appropriate measures are taken prior to and during construction.

Note: Plastic shrinkage cracks should be distinguished from other early or pre-hardening cracks caused by settlement of concrete on either side of a reinforcing bar, or due to formwork movement.

The most common explanation is that the rate of evaporation of surface moisture exceeds the rate at which it is replaced by bleed water, causing the surface to shrink while the underlying plastic concrete remains the same volume.

Conditions that increase the risk:

  • Decrease in relative humidity: changing from 90% to 50% increases evaporation fivefold.
  • Increase in wind velocity: an increase from 0 to 10 mph quadruples the evaporation rate.
  • Higher temperatures: when both concrete and air temperature increase from 50°F to 70°F, surface evaporation can double.
  • Cold weather with heated concrete: rapid evaporation can occur indoors when heated concrete is significantly warmer than the ambient air.

Key precautions:

  1. Have proper manpower, equipment and supplies ready so concrete can be placed and finished promptly. Cover with wet burlap or polythene between finishing operations.
  2. Start curing with a liquid membrane curing compound as soon as bleed water has gone and the surface is firm enough not to be marked.
  3. Dampen dry subgrades, previously placed concrete, formwork and reinforcement before placement.
  4. If a vapour barrier is required under a slab on grade, cover it with a 2-inch layer of damp sand.
  5. In very hot and dry periods, use fog sprays, erect temporary windbreaks, and provide sunshades to control the surface temperature. Schedule placement in the late afternoon or early evening if conditions are critical.