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of /home/abouaayt/public_html/aboutcivil/answers/qa-plugin/user-management/layer.php Why do plastic shrinkage cracking occurs in concrete? - Civil Engineering Technical Questions Answers - Ask a Civil Engineer
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Why do plastic shrinkage cracking occurs in concrete?
 

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Plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it has set.  The critical condition exists when the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it. Water receding below the concrete surface forms menisci between the fine  particles of cement and aggregate causing a tensile force  to develop in the surface layers. If the concrete surface  has started to set and has developed sufficient tensile strength to resist the tensile forces, cracks do not form.
  If the surface dries very rapidly, the concrete may still be plastic, and cracks do not develop at that time; but
  plastic cracks will surely form as soon as the concrete stiffens a little more.Synthetic fiber reinforcement incorporated in the concrete mixture can help resist the tension when concrete is very weak.
Plastic Shrinkage Cracks
Conditions that cause high evaporation rates from the concrete surface, and thereby increase the possibility of plastic shrinkage cracking, include:

  1. Wind velocity in excess of 5 mph
  2. Low relative humidity
  3. High ambient and/or concrete temperatures

Small changes in any one of these factors can significantly change the rate of evaporation. ACI 305 provides a chart to estimate the rate of evaporation and indicates when special precautions might be required. However, the chart isn’t infallible because many factors other than rate of evaporation are involved. Concrete mixtures with an inherent reduced rate of bleeding or quantity of bleed water are susceptible to plastic shrinkage cracking even when evaporation rates are low.

Factors that reduce the rate or quantity of bleeding include high cementitious materials content, high fines content, reduced water content, entrained air, high concrete temperature, and thinner sections. Concrete containing silica fume requires particular attention to avoid surface drying during placement. Any factor that delays setting increases the possibility of plastic shrinkage cracking. Delayed setting can result from a combination of one or more of the following: cool weather, cool sub grades, high water contents, lower cement contents, retarders, some water reducers, and supplementary cementing materials.

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