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.
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.