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Plastic cracking occurs when the concrete you have just placed and finished is exposed to warm drying wind, and then develops cracking over the surface of the concrete.

The plastic shrinkage cracks will appear in the first few hours of the slab's life. These cracks are either irregular in shape and appear as tears in the concrete, or they are extremely regular in that they follow the outline of the mesh and other reinforcing.

Factors which can lead to plastic cracking:

  • Warm wind
  • Warm concrete
  • Low humidity
  • Low bleed concrete
  • Exposed sites
  • Broom finishes

How can I recognise the conditions likely to lead to plastic cracking?
As a general rule: If it is good weather to hang out washing to dry it is ideal weather for plastic cracking to occur as well. Use our Evaporation Calculator below to assess your risk of cracking on any given day.  Just enter the weather data, concrete temperatures and assess the risk accordingly.

When the concrete temperature is unknown, add 5 degrees to the air temperature if the sky is cloudy, or 10 degrees if it is a sunny day. These values are conservative and are likely to overstate the risk of plastic cracking.  For greater certainty we recommend that a digital thermometer be bought to give a more accurate assessment of the plastic cracking risk.

 

Evaporation Rate Calculator

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When the evaporation rate exceeds 0.5 Litres per square metre per hour, the risk of cracking is significant and precautions need to be taken to address this. See below:

How to minimise the risk of plastic shrinkage cracking?
Do not place concrete when the weather conditions are too severe for you to control the evaporation rates of water from your concrete. This means that in high winds and warm temperatures, the pour should be delayed unless you can protect the slab accordingly. Moisten the sub-grade and forms prior to starting the pour. Use polypropylene fibres in your mix. Use an anti-evaporative spray to prevent excessive water loss from the slabCover the concrete with polythene or membrane cure immediately after finishing.

Allied Concrete staff are more than happy to help you with any problems or enquiries.

For more information or assistance, please don't hesitate to call. Your call will be automatically connected to our nearest plant. (Calls from mobile phones will be directed for Auckland, Wellington or Christchurch.)

DISCLAIMER: Allied Concrete has endeavoured to present the best possible information. However we accept no responsibility for the application of the principles discussed.