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Plastic shrinkage cracking is a condition that arises in concrete that requires special precautions to minimize the effects. Plastic shrinkage is not a result of any change in the concrete production process. Rather, it is a result of certain environmental conditions which frequently occur in the Central Missouri area.  The informations below will provide answers to your questions. 

Some Thoughts on Plastic Shrinkage Cracking

1. Plastic shrinkage cracks are cracks that appear on the surface of concrete before it has hardened. These cracks are typically randomly located in the concrete, are usually parallel to one another, and are from a few inches to a few feet in length. The cracks may be 1 to 3 inches deep. They usually do not extend to the perimeter of the concrete placement.

2. Plastic shrinkage cracking can occur when the rate of a water loss from the surface of fresh concrete because evaporation exceeds the rate at which the water is replaced by bleeding. The drying of the surface concrete develops tensile stresses which can ultimately lead to cracking.

3. The rate of evaporation of moisture from the concrete is governed by air and concrete temperature, wind speed, and relative humidity. The inter-relationships among these factors are shown in the attached chart. Although not shown on the chart, the warming and drying action of direct sunlight also plays an important role in evaporating moisture. For conventional concrete, ACI 305 (attached) recommends taking precautions for plastic shrinkage wherever the predicted rate of moisture loss exceeds 0.2 lb / ft2 / hr.

4. Evaporation of water from freshly placed concrete is driven by concrete temperatures warmer than air temperature, low relative humidity, and surface winds. These conditions can occur at any time of the year. Plastic shrinkage cracks often take contractors and engineers by surprise during the spring and fall.

Example:

Consider a beautiful typical spring day in Jefferson City. The sky is clear-blue, temperature around 60o F, relative humidity of about 30% and a moderate 15 mph breeze. Sound like good weather for placing concrete? Consider that even if the fresh concrete temperature is 70o F, by the middle of the day, the surface of freshly placed concrete could be warmed to 80o F by bright sunlight.

Entering the chart (attached) with these conditions, the evaporation rate is found to be over 0.3 lb / ft2 / hr. Plastic shrinkage cracks will almost certainly occur under these conditions, unless steps are taken to reduce the effective rate of evaporation.

Moral of the Example:

Beware of sunny, breezy weather at any time of the year!

5. There is a general misconception that a high quality concrete will not exhibit plastic shrinkage cracking. In fact, because high-quality concrete typically has low water to cement ratios there is little water available for bleeding to replace that lost by evaporation. Lower quality concrete will typically have a high water to cement ratio and therefore more bleed water will be available.

 

A GENERAL RULE OF THUMB IN REGARDS TO PLASTIC SHRINKAGE

If you feel hot, and if you’re sweating profusely, this probably means that relative humidity is high, the air temperature is higher that the fresh concrete temperature and winds are low. Thus, plastic shrinkage cracks are not likely to occur.

If you think it’s a beautiful day…if it’s cool, sunny and breezy…plastic shrinkage is likely to occur if high quality concrete is being placed and appropriate steps are not taken to reduce evaporation rates from the fresh concrete.

 

 

ACI 305: (American Concrete Institute)

Plastic shrinkage cracking is frequently associated with hot weather concreting in arid climates and may develop whenever the rate of evaporation is greater than the rate at which water rises to the surface of the recently placed concrete (bleeding). High concrete temperature, high air temperature, high wind, and low humidity, or combinations thereof, cause rapid evaporation which significantly increases the likelihood that plastic shrinkage cracking will occur. In humid climates a high concrete temperature is a much less serious factor in causing plastic shrinkage.

Precautions should be taken into consideration when the rate of evaporation is expected to approach 0.2 lb / ft2 / hr. These precautions consist of dampening the subgrade and forms, placing concrete at the lowest practicable temperature, erecting wind-breaks and sunshades, reducing time between placement of concrete and start of curing, and minimizing evaporation, particularly during the first few hours subsequent to placing concrete, by a suitable means such as applying moisture by fog spraying. Plastic shrinkage cracks are difficult to close once they have occurred. Such cracks may be focal points for other forms of deterioration since they allow moisture and dissolved salts to penetrate the concrete, and may impair its performance.

Plastic Shrinkage Calcluations Using Current Weather

Click to download a "Plastic Shrinkage Chart"