Alexei Volkov
Plastic shrinkage cracks are a common issue in concrete construction, occurring within the first few hours of concrete placement. These cracks are caused by rapid moisture loss and the subsequent drying and shrinkage of the concrete surface. When the surface moisture evaporates faster than it can be replaced by rising bleed water, the concrete undergoes a volumetric contraction, leading to tensile stresses that result in cracking. Various factors influence the formation of these cracks, including weather conditions, concrete temperature, relative humidity, and wind velocity. While plastic shrinkage cracks can be challenging to prevent, proper curing procedures and construction techniques can minimize their occurrence.
| Characteristics | Values |
|---|---|
| Time of appearance | Within the first few hours after concrete placement |
| Concrete state | Plastic |
| Crack shape | V-shaped, wide and shallow, normally isolated, occasionally in a crow-foot pattern |
| Crack length | 300-500 mm (can be a few inches to a few feet) |
| Crack depth | 20-50 mm (can be deeper, even extending through the full depth of a member) |
| Crack width | 1-2 mm |
| Cause | Loss of water through evaporation or suction during the plastic state of the concrete, causing volume reduction and the appearance of cracks at the surface |
| Contributing factors | Weather conditions (high temperatures, low humidity, windy weather), rapid moisture loss, bleeding, construction operations, type of cement, temperature of fresh concrete |
| Prevention | Adequate curing procedures, early application of appropriate curing methods (e.g. membrane curing compound, covering with light-colored paper, plastic, wet burlap, or sand) |
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What You'll Learn
Evaporation of surface water
Plastic shrinkage cracks in concrete are caused by the rapid evaporation of surface water, which leads to the drying and shrinkage of the exposed surface of the plastic concrete. These cracks can form within the first few hours of concrete placement, typically before finishing operations are complete. They occur primarily in concrete flatwork but can also be seen on other exposed horizontal surfaces such as beams, foundations, and tops of walls.
The evaporation of surface water from concrete is influenced by various factors, including temperature, relative humidity, and wind velocity. Higher temperatures, low humidity, and windy weather conditions contribute to an increased rate of evaporation. Additionally, the concrete's water content and bleeding characteristics play a role in the evaporation process. Bleeding refers to the appearance of moisture on the concrete surface due to the settling of heavier components in the mixture. When evaporation occurs at a faster rate than the supply of water to the surface through bleeding, it results in a net loss of water from the surface layer.
The rapid evaporation of surface water leads to a reduction in the volume of the concrete. As the surface layer of concrete shrinks, it is restrained by the underlying layers that are not subject to the same volume reduction. This restraint results in the development of tensile stresses in the surface layer, exceeding its tensile capacity, and ultimately leading to cracking. These cracks typically start on the surface and grow downward, forming V-shaped fissures that can reach a depth of a few inches.
The risk of plastic shrinkage cracking can be mitigated by controlling the rate of evaporation from the concrete surface. Proper curing procedures, such as the application of a membrane curing compound, covering with light-colored paper, plastic, wet burlap, or sand, can effectively prevent evaporation and reduce the occurrence of cracks. Additionally, adequate curing and proper construction techniques can significantly reduce the likelihood and severity of plastic shrinkage cracks in concrete.
The rate of evaporation can be estimated using the Menzel Formula, which takes into account measured air and concrete temperatures, relative humidity, and wind velocity at the surface. By understanding the factors influencing evaporation and implementing appropriate preventive measures, the occurrence of plastic shrinkage cracks in concrete can be effectively minimized.
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Rapid moisture loss
Plastic shrinkage cracks in concrete are caused by rapid moisture loss, which occurs when the surface water evaporates faster than it can be replaced by rising bleed water. This rapid evaporation leads to a net loss of water from the surface layer of the concrete, resulting in a reduction in volume. As the surface layer shrinks, it is restrained by the underlying layers, leading to the formation of tensile stresses and, subsequently, cracks.
These cracks typically appear within the first few hours after concrete placement, while the concrete is still in a plastic state and has little strength. The risk of plastic shrinkage cracking is higher during hot weather concreting due to the increased rate of evaporation at high temperatures. However, it is important to note that plastic shrinkage cracks can also occur during cold weather or any other conditions that promote rapid moisture loss.
The rate of evaporation is influenced by various factors, including temperature, relative humidity, and wind velocity. Higher temperatures, low humidity, and windy weather conditions contribute to an increased rate of evaporation, creating favourable conditions for plastic shrinkage cracking. Additionally, the use of modern fine-ground cement, hot cement, or low slump concrete has been associated with an increased risk of cracking.
To prevent plastic shrinkage cracks caused by rapid moisture loss, proper curing procedures must be applied. These procedures aim to stop evaporation from the surface, such as by applying a membrane curing compound, covering with light-coloured paper, plastic, wet burlap, or sand. Adequate curing can significantly reduce the occurrence of plastic shrinkage cracks by minimising the rapid moisture loss from the concrete surface.
Furthermore, the risk of plastic shrinkage cracking can be mitigated by addressing factors that influence the rate of moisture loss. This includes avoiding conditions that promote rapid evaporation, such as high temperatures, low humidity, and windy weather. Additionally, ensuring proper concrete mixing and placement techniques can help reduce the vulnerability of the concrete to rapid moisture loss and subsequent cracking.
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Weather conditions
The risk of plastic shrinkage cracking is traditionally associated with hot weather concreting. However, it is important to note that these cracks can also occur during cold weather or any conditions that lead to rapid moisture loss and drying of the concrete surface. The temperature of the fresh concrete itself can influence the risk of cracking, with even construction operations like screeding and finishing potentially impacting the likelihood of cracks forming.
The use of tools such as the Menzel Formula and its associated nomograph can help estimate the rate of evaporation and the potential for plastic shrinkage cracking. This formula takes into account factors such as air and concrete temperatures, relative humidity, and wind velocity. By understanding the specific weather conditions and using these tools, construction professionals can better manage the risk of plastic shrinkage cracks in concrete.
Additionally, proper curing procedures are crucial in preventing plastic shrinkage cracks. These procedures aim to stop evaporation from the concrete surface, such as by applying a membrane curing compound, covering with light-coloured paper, plastic, wet burlap, or sand. Adequate curing can significantly reduce the occurrence of these cracks, minimising the impact of varying weather conditions on the concrete.
Overall, while weather conditions are a contributing factor to plastic shrinkage cracks, proper understanding, planning, and implementation of preventative measures can effectively mitigate the risk.
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Concrete temperature
The temperature affects the rate of evaporation of water from the concrete surface. Higher temperatures and low humidity, particularly in windy weather, increase the rate of evaporation, leading to rapid moisture loss. This, in turn, causes the surface layer of concrete to shrink while being restrained by the underlying layers, resulting in tensile stresses that exceed the concrete's tensile capacity and lead to cracking.
To prevent plastic shrinkage cracks, it is essential to control the temperature during the curing process. Curing concrete in hot weather can accelerate evaporation, resulting in weaker concrete and potential shape abnormalities. On the other hand, curing in cold weather can prolong the curing process, allowing the concrete to shift over time and potentially causing structural damage.
The ideal curing temperature for concrete is between 50 and 60°F (10 and 15.5°C). Maintaining this temperature range helps prevent issues associated with both extreme heat and cold. Insulated blankets, heated enclosures, and curing membranes can be used to regulate temperatures and minimise the risk of cracking.
Additionally, the temperature of fresh concrete during construction operations, such as screeding and finishing, can also influence the risk of cracking. It is important to monitor the concrete temperature throughout the curing process to ensure compliance with the recommended temperature range.
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Concrete placement
To prevent plastic shrinkage cracks during concrete placement, it is essential to consider the following factors and implement preventive measures:
Weather Conditions:
Monitor and plan concrete placement around weather conditions. Hot and dry weather increases the risk of rapid moisture evaporation from the concrete surface. If possible, schedule concrete placement during the later afternoon or early evening when temperatures are cooler. In very hot and dry periods, use fog sprays or misting systems to reduce the air temperature and humidity above the concrete surface, thereby slowing down evaporation.
Subgrade Preparation:
Before placing concrete, ensure that the subgrade is damp but not saturated. This is especially important in hot and dry weather. Also, dampen the formwork and reinforcement to prevent them from absorbing moisture from the concrete.
Concrete Temperature:
Avoid excessively high concrete temperatures, especially in hot weather. The temperature difference between the concrete and the air can impact evaporation rates. Cooler concrete may be used in hot weather to mitigate this issue.
Wind Velocity:
Construct temporary windbreaks to reduce wind velocity over the concrete surface. Wind accelerates the rate of evaporation, increasing the risk of plastic shrinkage cracks.
Moisture Retention:
Use moisture-retaining coverings such as wet burlap, polyethylene sheeting, or building paper between finishing operations to prevent excessive moisture loss. Some contractors also use evaporation retardants sprayed onto the concrete surface to slow down evaporation.
Curing:
Start curing the concrete as soon as possible after finishing. Proper curing ensures the concrete hardens adequately and reduces the risk of cracking. Spray the surface with a liquid membrane curing compound or keep it continuously moist for at least three days.
Concrete Composition:
The composition of the concrete mix can also impact the likelihood of plastic shrinkage cracks. High cementitious materials content, high fines content, reduced water content, entrained air, and high concrete temperature can all contribute to reduced bleeding and increased susceptibility to cracking.
By following these concrete placement guidelines and staying vigilant about potential risks, the occurrence of plastic shrinkage cracks can be significantly reduced, resulting in improved slab integrity and performance.
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Frequently asked questions
Plastic shrinkage cracks are random cracks that appear in the first few hours after concrete placement, while the concrete is still plastic and has little strength. They are typically 1-2 mm wide, 300-500 mm long, and 20-50 mm deep.
Plastic shrinkage cracks are caused by the rapid evaporation of surface water, leading to the drying and shrinking of the exposed surface of the plastic concrete. This evaporation occurs faster than the supply of water to the surface through bleeding, resulting in high-tensile stresses that cause cracking.
Weather conditions, such as high temperatures, low humidity, and windy weather, contribute to the rapid evaporation of water and the formation of plastic shrinkage cracks. The use of modern fine ground cement, hot cement, or low slump concrete may also be factors.
Plastic shrinkage cracks can be prevented by applying proper curing procedures to minimize evaporation from the concrete surface. This includes techniques such as the application of a membrane curing compound, covering with light-coloured paper, plastic, wet burlap, or sand. Adequate curing can significantly reduce the occurrence of these cracks.
