Plastic shrinkage can occur at the surface of fresh concrete within the first few hours after placement. When the rate of evaporation of water from the surface of concrete exceeds its bleeding rate the surface begins to dry resulting in high capillary stress development near the surface. This can be attributed typically to high temperatures, low ambient humidity, high winds, and mixture ingredients and proportions. Plastic shrinkage cracking is a problem for large flat structures, such as bridge decks and pavements, in which the exposed surface area is high relative to the volume of the placed concrete.
Cracks caused by plastic shrinkage can be quite wide on the upper surface 2 to 3 mm (0.08 to 0.12 in.), but their width often decreases rapidly below the surface. Plastic cracks typically do not exceed 10 mm but may pass through the full depth of the member; however the mechanisms leading to the formation of plastic shrinkage cracking does not explain full depth cracks. It is probable that the subsequent events including drying shrinkage and loading can cause the plastic shrinkage cracks to propagate.
The three different curing environments include wet (100% RH), dry (40% RH) and wind [40% RH with wind at 2.5 m/s (8.2 ft/s)]. Holt (2001) suggested that there is a higher risk of early age cracking when the early age shrinkage exceeds 1000 μm/m (0.001 in./in.). This example shows that the construction environment is a major concern when assessing the risk of this early age cracking.
The amount of shrinkage that occurs is directly related to the loss of water from the concrete, greater evaporation leads to greater shrinkage.
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