Plastic Shrinkage Cracking
Careful observation of the development of cracks on a construction project provides a means of differentiating plastic shrinkage cracks from the cracks sometimes occurring from other causes, such as swelling of the subgrade, settlement of the formwork, or contraction caused by a rapid drop in temperature. Plastic shrinkage cracks develop about the time the water sheen disappears from the surface of the concrete. When this type of cracking starts, it will progress rapidly. These are usually random straight line cracks, although occasionally observed in a crow-foot pattern, forming in the interior surface of the slab, but rarely, if ever, extend to the edge.
Plastic shrinkage cracks have considerable depth. On suspended floor slabs these sometimes extend through the floor, and on pavements may penetrate to a depth of four inches.
CAUSE OF PLASTIC SHRINKAGE CRACKS
The major cause of plastic shrinkage and plastic shrinkage cracking is an excessively rapid evaporation of water from the concrete surface. The occurrence of plastic shrinkage cracks is sporadic. Even when the same materials, proportions and methods of mixing, placing, finishing, and curing are used, the cracks may occur on one day, but not the next. This is due to changes in weather conditions that cause variations in the rate of evaporation.
Because the occurrence of plastic shrinkage cracks is sporadic, many different explanations have been advanced as factors responsible for the cracking. The use of modern fine ground cement, hot cement, freshly ground cement, or the use of low slump concrete has been blamed. None of these explanations are tenable. This type of cracking is not new. Plastic shrinkage cracks occurred many years ago when coarse ground cements were used. On one project three different cements were used in an effort to eliminate plastic shrinkage cracking in suspended floor slabs. The first was a Type I cement, freshly ground and delivered to the job at a temperature of 180°F. The second was a Type I cement that had been in storage for six weeks, delivered to the job at 90°F. The third was a Type II cement being used nearby for construction of a floor slab on grade, in an enclosed building, without cracking. Plastic shrinkage cracks developed with all three cements on the suspended floor slabs when exposed to the sun and wind. Later in the day the sky became overcast with a light misty rain. No plastic shrinkage cracks occurred in the concrete placed at that time.
On a paving project the slump of concrete was increased from one inch to three inches in an effort to eliminate cracking. On the first day of use, no cracking was experienced with the high slump concrete. On that day, the relative humidity was high with practically no wind. On the following day, with a lower relative humidity and a wind velocity of 10 to 15miles per hour, plastic shrinkage cracks occurred with the high slump concrete.
After concrete is placed, bleed water usually rises to the surface. As the bleeding water evaporates, the water sheen disappears from the concrete surfaces. The time required to attain this condition will depend on temperature, relative humidity, and wind velocity of the air; as well as the temperature and bleeding characteristics of the concrete. By the time the surface of the concrete has attained an initial rigidity it cannot accommodate the rapid volume change of plastic shrinkage by plastic flow, as it has not developed sufficient strength to withstand contraction tensile stresses. Then plastic shrinkage cracks will develop with continued drying. These can be prevented by applying proper curing procedures prior to that time. The proper curing procedures are those that will stop evaporation from the surface, such as the application of a membrane curing compound, covering with light colored paper, plastic, wet burlap or sand.
RATE OF EVAPORATION
Since the rate of evaporation of water and drying on the surface of the fresh concrete is the major cause of plastic shrinkage cracks, it becomes of interest to examine weather conditions that affect the rate of evaporation and surface drying. Wind velocity, relative humidity and temperature are the major factors that affect the rate of evaporation as shown by the data in Tables I & II.
The data in Table II, Group I, show that as wind velocity increases from 0 to 25 mph, the rate of evaporation increases from 0.015 to 0.135 pounds per square foot per hour, or about nine times. When increases from 0.020 to 0.175 pounds more than eight times. The largest change in the rate of evaporation occurs when two or more factors affecting the rate, occur simultaneously and supplement each other as shown in Group 8. When the temperature of the concrete and the air are high, the relative humidity decreases from 90 percent to 10 percent, Group 2, the rate of evaporation 90°F, the relative humidity is low, 10 percent, and the wind velocity increases from 0 to 25 mph the rate of evaporation increases from 0.070 to 0.740 pounds per square foot per hour. Plastic shrinkage cracks can occur even in cold damp weather, if the concrete is heated and placed at a temperature considerably higher than the air temperature.
The variation in weather conditions cited above occur on job sites from season to season, as well as from day to day. Thus it is apparent why the development of plastic shrinkage cracks is sporadic. They occur some times, and not at others, even when the same construction practices are used.
Plastic shrinkage cracks can be minimized by the application of appropriate construction practices. Most of the procedures required to prevent plastic shrinkage cracks are just good construction practices that should be used at all times. Under extreme conditions a few special precautions may be required. The procedures to be used should be considered prior to starting the job. Construction practices that should be applied to prevent plastic shrinkage cracks are:
- Saturate the subgrade and forms.
- Dampen the under-slab aggregates if they are dry and absorptive.
- Lower the temperature of the concrete in hot weather when necessary.
- Avoid overheating the concrete in cold weather.
- Reduce the time between placing and final finishing to a minimum.
- Apply a membrane curing compound, wet burlap, damp sand, light colored paper or other curing procedures as soon as possible after final finishing.
- Protect the concrete with temporary coverings, or apply a fog spray during any appreciable delay between placing and finishing.
- Provide sun shades to control the temperature at the surface of the concrete, or
- Erect windbreaks to reduce the wind velocity over the surface of the concrete.
- These procedures have been used to prevent the occurrence of plastic shrinkage cracks.
A few examples are cited to show how these procedures have been used to eliminate plastic shrinkage cracking on construction projects.
On a number of paving projects it has been observed plastic shrinkage cracks occurred before the curing was applied. By the simple procedure of an earlier application of curing, the cracks were eliminated. On one such project, plastic shrinkage cracks were observed intermittently for several weeks. One day when the temperature was 93 to 96°F, the relative humidity 30 to 45 percent, and the wind 5 to 8 mph there were no cracks. On the following day when the temperature was the same, the relative humidity was 15 to 25 percent and the wind 10 to 15 mph, cracking was prevalent. A liquid membrane curing compound was applied about one to two hours after finishing. It was observed that the cracks occurred before the curing compound was applied. Arrangement was then made to apply the curing compound immediately after the final belting, and the cracking problem was solved.
On another project where the curing compound was applied early, plastic shrinkage cracks did occur on some areas. An examination of the surface clearly indicated a very thin layer of curing compound had not been applied over the areas that cracked. This example illustrates the advantage of a visual pigmented curing compound, and the need for rigorous inspection.
On exposed surfaced requiring a hard steel-trowel finish somewhat different corrective measures are required. On one such project it was observed plastic shrinkage cracks appeared before the final troweling could be completed. Obviously final curing could not be applied prior to that time. Experience has shown that the application of a fog spray or a cover with wet burlap, during the initial hardening period, or between successive passes of the trowel, will eliminate such cracking.
In very hot, arid areas sun shades have been used, and sometimes night construction has been required to prevent plastic shrinkage cracks and otherwise improve the quality of the work. Such procedures reduce the maximum temperature in the concrete during the initial hardening and thereby also decrease the thermal contraction and cracking of the hardened concrete on subsequent cooling.
Group 4:Concrete at 70°F Decrease in Air Temperature
Group 5:Group 5 Concrete at High Temperature Air 40°F and 100% Relative Humidity
Group 1:Increase in Wind Velocity
Group 2:Decrease in Relative Humidity
Group 3:Increase Concrete and Air Temperature
Group 4: Concrete and Air at High Temperature, 10% Relative Humidity, Wind Variable
Thanks to Kaiser Cement who supplied information for this document.
Table 1 - Effect of Variations in Concrete and Air Temperatures, Relative Humidity, and Wind Speed on Drying Tendency of Air at Job Site From ACI Journal, February 1957
|Case No.||Concrete Temp (°F)||Air Temp (°F)||Relative Humidity (%)||Dew Point (°F)||Wind Speed (MPH)||Drying Rate (lb/sq ft/hr)|
Table 2 - Effect of Variations in Concrete and Air Temperatures, Relative Humidity and Wind Velocity on Drying Tendency of Air at Job Site From ACI Journal, February 1957
|Case No.||Concrete Temp (°F)||Air Temp (°F)||Relative Humidity (%)||Wind Speed (MPH)||Drying Rate (lb/sq ft/hr)|