Plastic Shrinkage Cracks in Concrete: Why They Form and How to Stop Them

Plastic shrinkage cracks appear on the surface of fresh concrete in the first few hours after placing, while the concrete is still plastic and has not set. They are common on slabs, pavements and floors, especially in hot, dry or windy weather in India.

These cracks are usually shallow, but they are still a durability risk. Water and chlorides can travel into them, and a crack can grow later under load or temperature change.

Why plastic shrinkage cracks form

Fresh concrete bleeds. Water rises to the surface, and this bleed water keeps the surface wet. If water evaporates from the surface faster than the bleed water can replace it, the top layer dries and shrinks while the concrete below stays the same size. The surface cannot shrink freely, so it cracks.

In simple words: the surface dries faster than the concrete can supply water. See our guide on concrete bleeding for more on how bleed water behaves.

Conditions that increase the risk

• High air temperature and high concrete temperature.
• Low humidity.
• Wind blowing across the slab. Even a moderate wind can double the drying rate.
• Direct sun on a fresh slab.
• Low water-cement ratio, rich mixes and mixes with silica fume or fly ash, which bleed less.
• Dry subgrade or dry formwork that pulls water out of the concrete.

A common rule from ACI 305 says to take extra precautions when the evaporation rate reaches about 1 kg per square metre per hour. You can estimate it from air temperature, concrete temperature, relative humidity and wind speed using the standard nomograph.

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How they look

Plastic shrinkage cracks are short, straight or slightly curved, and often run parallel to each other, a few centimetres to half a metre apart. They may be 0.1 to 3 mm wide and usually only 25 to 50 mm deep. Compare this with structural cracks, which are usually wider, deeper and follow the load path.

How to prevent plastic shrinkage cracking

• Wet the subgrade and the formwork before the pour. Do not leave standing water.
• Place concrete at a lower temperature. Use cold water or ice in the mix, and pour in the early morning or evening.
• Use windbreaks and sunshades over the slab.
• Apply an evaporation-reducing film or spray as soon as the surface is struck off.
• Start curing as early as possible. Cover with wet hessian or plastic sheets as soon as the surface can take them.
• Do not add extra water to the mix on site. It increases shrinkage and weakens the concrete.
• Consider polypropylene fibres in slabs and pavements that are at risk.

What to do if cracks have already formed

If the concrete is still plastic, you can often close the cracks by re-floating or trowelling the surface again, then protect it from drying. If the concrete has hardened, seal the cracks with a low-viscosity epoxy or polymer sealer. Wide or deep cracks should be checked by an engineer, because they may not be plastic shrinkage cracks at all.

Good curing, a controlled mix and attention to the weather on the pour day prevent most of these cracks.