What is plastic shrinkage cracking?

Plastic shrinkage cracking occurs on the surface of freshly placed concrete. Generally, they do not pose an immediate concern regarding the strength of the concrete and are considered more of a cosmetic issue. However, surface cracks may allow for easier penetration from chemicals that could impact the durability and longevity of the concrete.

Why do plastic shrinkage cracks occur?

A rapid loss of water from the surface of the concrete before allowing it to fully set properly can result in plastic shrinkage cracking. If the concrete below the surface remains plastic, yet the surface fully dries, as the concrete below dries, cracking will most likely occur. The conditions under which such cracking most frequently occurs include

• Wind velocity 5+ mph
• Low relative humidity
• High ambient temperature

Additionally, certain mixes with a reduced bleed rate or lower quantity of bleed water are more susceptible to plastic shrinkage cracking, even in the absence of the above listed conditions. Similarly, any factor that delays setting increases the likelihood of plastic shrinkage cracks forming.

How to minimize plastic shrinkage cracking:

• Construct temporary windbreak and/or sunshade to combat the elements
• Fog spray in exceedingly hot/dry weather
• Employ a vapor retarder (may be a necessity in some cases, depending on mix)
• Begin curing as soon as possible
• Consider synthetic fiber use
• Avoid significant temperature differences between concrete and ambient temperature

Adapted from IRMCA – CIP 100

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