Question

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The primary cause of "plastic shrinkage" cracks is the rapid evaporation of water from the surface of the concrete. Immediately after the concrete has been placed, the particles within the concrete begin to settle. When the particles settle, the water within the concrete displaces and rises to the top. This process is better known as "bleeding." Not all of the water within the concrete displaces. Under most weather conditions, some of the water on the surface of the concrete evaporates. The rate of evaporation depends on factors such as the temperature of the concrete, temperature of the air, relative humidity, and wind velocity surrounding the concrete. Table 1 shows how different weather conditions and properties effect the rate of evaporation. The highest evaporation rates are obtained when the concrete and air temperatures are high, when the relative humidity of the air is low, when the concrete temperature is high compared to the air temperature, and when a strong wind is blowing over the surface of the concrete. The rapid evaporation of water at the surface is most associated with placing concrete in hot weather conditions. However, plastic shrinkage cracks can also form in cold weather conditions when the temperature of the concrete is high compared with the surrounding air temperature. When the temperature at the surface creates an evaporation rate that exceeds the rate of water produced by the bleeding process, the water film disappears and the top surface of the concrete begins to shrink. When the evaporation rate exceeds1.0 kilogram per square meter per hour, it is almost certain that plastic shrinkage cracks will develop (Figure 1). When the evaporation rate is greater than 0.5 kilogram per square meter per hour cracking is possible.

Plastic shrinkage cracks typically occur on horizontal surfaces exposed to the atmosphere. These cracks are different from other early cracks because they are deeper and wider. Plastic shrinkage cracks are typically two to four inches deep and approximately one-eighth inch wid.

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Answer 1

Solution

  Plastic Shrinkage Cracks

“Plastic shrinkage” cracks are common in fresh concrete and occurs mainly due to rapid evaporation of water from the surface of concrete causing the concrete to shrink and develop cracks. These cracks differ significantly from other cracks in terms of their higher depth and width. They can extend up to 2 to 4 inches deep and 1/8th of an inch in width.

As soon as a concrete is placed, the concrete particles begin to settle down under gravity causing the displaced water to rise up to the surface. This process is termed as “bleeding”. During this process the surface water also keeps on evaporating and when the evaporation rate is higher than bleeding, the water film from the surface disappears causing the concrete begins to shrink.

Generally shrinkage cracks are observed on flat surfaces exposed to atmosphere where evaporation rate is higher than 0.5 kilogram per square meter per hour and at rates greater than 1 kilogram per square meter per hour, the formation of cracks become certain.

All the factors that promote rate of evaporation also favours occurrence of shrinkage cracks. Weather conditions and other properties affecting evaporation are shown in detail in Table1. Evaporation is observed to be maximum at conditions where air temperature is high, relative humidity of air is low, wind speed near the surface is high and concrete’s temperature is observed to be higher than air. So such conditions also favour formations of plastic shrinkage cracks. These conditions are common in hot weather and therefore shrinkage cracks are common in concrete placed in hot weather conditions.

Though hot climatic zones are favourable for such cracks but sometimes these cracks can also be observed in colder areas where the surface temperature of concrete is high compared to surrounding temperature.