Ethan Clarke
Plastic shrinkage cracking in concrete is a phenomenon that occurs when the rate of evaporation exceeds the rate of bleeding, resulting in a net loss of water from the surface layer. This leads to volume reduction and the development of tensile stresses in the surface layer, causing cracks. These cracks can occur as early as a few hours after the concrete is placed and are typically 1-2 mm wide, 300-500 mm long, and 20-50 mm deep. The risk of plastic shrinkage cracking depends on various factors such as the rate of moisture loss, evaporation rate, concrete mixture, and environmental conditions like temperature and humidity. Understanding the susceptibility of the concrete mixture and monitoring job site conditions are crucial to prevent aesthetic and durability issues caused by plastic shrinkage cracks.
| Characteristics | Values |
|---|---|
| Occurrence | Plastic shrinkage cracking occurs when concrete is neither a liquid nor a solid and has zero tensile capacity. |
| Timing | Plastic shrinkage cracks can form in young concrete within the first few hours after placement. |
| Causes | Plastic shrinkage cracking is caused by rapid moisture loss and drying of the surface. |
| Risk factors | Risk factors for plastic shrinkage cracking include high temperatures, low humidity, and windy weather. |
| Prevention | Plastic shrinkage cracking can be prevented by limiting the rate of evaporation from the surface and covering the concrete to protect it from drying winds. |
| Consequences | Plastic shrinkage cracks can lead to reinforcement corrosion and concrete damage from freezing and thawing if water and deicing chemicals enter the concrete. |
| Dimensions | Plastic shrinkage cracks tend to be 1-2 mm wide, 300-500 mm long, and 20-50 mm deep, but may vary depending on circumstances. |
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What You'll Learn
Plastic shrinkage cracking occurs when concrete is neither liquid nor solid
Plastic shrinkage cracking is a phenomenon that occurs when concrete is in a state of transition between liquid and solid. This typically happens within the first few hours of placing the concrete, when it is still in its plastic state and has little strength. At this stage, the concrete has essentially zero tensile capacity.
The cracking is caused by the loss of water through evaporation or suction from the surface of the concrete, resulting in a net loss of water from the surface layer. This causes the surface layer to shrink while being restrained by the underlying layers, leading to the development of tensile stresses and, eventually, cracks. These cracks can be 1-2 mm wide, 300-500 mm long, and 20-50 mm deep, and they occur randomly, diagonally, and over reinforcement.
The risk of plastic shrinkage cracking depends on several factors, including the rate of evaporation, the rate of bleeding, the temperature and humidity of the air and concrete, and the wind speed. The higher the rate of evaporation and the lower the rate of bleeding, the greater the risk of cracking. Additionally, any factor that increases moisture loss, reduces the amount of bleed water, or delays the hardening of the concrete can increase the risk of cracking.
To prevent plastic shrinkage cracking, it is crucial to cover the surface of the concrete early and protect it from drying winds. Understanding the susceptibility of the concrete mixture and monitoring job site conditions are also essential to minimise rapid moisture loss and the occurrence of cracks. Adequate curing and the use of evaporation retarders can also help reduce the potential for plastic shrinkage cracking.
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Factors that increase the risk of cracking
Plastic shrinkage cracking in concrete is a common issue that can occur within the first few hours of placing the concrete. It is caused by rapid moisture loss and drying of the surface, which creates tensile stresses that exceed the capacity of the plastic concrete. While the rate of moisture loss is a significant factor, other factors also influence the risk of cracking.
One key factor is the water-to-cement ratio in the concrete mix. A higher ratio of water to cement increases shrinkage due to volume loss during evaporation. Therefore, reducing the water content can help mitigate the risk of cracking. The type and content of cement and supplementary materials, such as Portland cement, fly ash, and silica fume, also play a role in the susceptibility of concrete to cracking.
Environmental factors, such as temperature, humidity, and wind, also contribute to the risk of plastic shrinkage cracking. High temperatures, low humidity, and windy weather conditions accelerate the rate of evaporation, increasing the potential for cracking. Additionally, drastic changes in temperature and humidity can generate internal stresses in the concrete, leading to cracking.
Proper construction practices and structural design are crucial in preventing plastic shrinkage cracks. This includes considerations such as adequate curing, proper joint placement, and reinforcement steel spacing. Delayed hardening of concrete, inefficient joints, and inadequate reinforcement can all increase the likelihood of cracking.
Furthermore, concrete placement and ground preparation are essential. Pouring concrete on frozen ground or inadequate compaction of the sub-grade can lead to cracking. Additionally, the concrete mix itself should be monitored to ensure it is not too wet, as this is a common issue in residential work, and can lead to increased shrinkage and cracking.
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How to prevent plastic shrinkage cracking
Plastic shrinkage cracking in concrete is a common issue that occurs when the concrete surface dries out and loses moisture faster than it can be replaced from within the body of the concrete. This rapid loss of moisture, or shrinkage, creates tensile stresses that can exceed the early concrete strength, resulting in fine cracks on the surface. Here are some measures to prevent plastic shrinkage cracking:
One effective way to prevent plastic shrinkage cracking is to slow down the rate of evaporation from the concrete surface. This can be achieved by applying water to the surface soon after paving or finishing. Moist curing helps maintain moisture levels within the concrete, reducing the shrinkage and resulting cracking. This method is particularly useful in hot and windy conditions, which tend to accelerate moisture loss.
Another strategy is to use windbreaks or sunshades to shield the concrete from drying winds and direct sunlight. By creating a barrier, you can reduce the speed of air movement over the concrete surface and lower the rate of evaporation. This technique is especially beneficial when placing concrete in windy conditions or when there is direct sunlight on the pouring site.
The use of shrinkage-reducing admixtures is also recommended to lower the risk of plastic shrinkage cracking. These admixtures work by reducing the surface tension of the mixing water, which in turn reduces the capillary forces that cause shrinkage. They also improve the cohesion and workability of the concrete mix, making it less susceptible to cracking.
Properly preparing the subgrade is crucial in preventing plastic shrinkage cracking. The subgrade should be compacted and shaped to drain water away from the concrete slab. A damp subgrade can help provide moisture to the concrete, reducing the rate of moisture loss from the surface. Additionally, ensuring the subgrade is free of debris and loose material will create a clean and smooth surface for the concrete to be placed on.
Finally, paying attention to the weather conditions is essential when pouring concrete. Avoid placing concrete on hot, dry, and windy days as these conditions promote rapid moisture loss. Schedule concrete placement for cooler and more humid times of the day, such as early morning or late afternoon, and monitor the weather forecast to minimize the risk of unexpected weather changes.
By implementing these measures, you can effectively reduce the likelihood of plastic shrinkage cracking in concrete. Proper curing, protecting the concrete from extreme weather, using appropriate admixtures, and carefully preparing the subgrade all contribute to a more durable and crack-resistant concrete surface.
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Plastic shrinkage cracks can form in young concrete
Evaporation of water occurs at the surface, and several factors influence the rate of evaporation, including temperature, humidity, and wind speed. If evaporation occurs faster than the rate of bleeding, there is a net loss of water from the surface layer of concrete, leading to a reduction in volume. As a result, the surface layer tries to shrink, but it is restrained by the underlying layers that are not subject to the same reduction in volume. This restraint causes tensile stresses to develop in the surface layer, leading to the formation of cracks.
The risk of plastic shrinkage cracking depends on various factors beyond just the rate of moisture loss or evaporation. The type and content of cement, water-cement ratio, admixtures, and temperature of the fresh concrete all play a role in the susceptibility of concrete to cracking. Additionally, the concrete mixture's susceptibility to cracking and job site conditions should be monitored to minimise rapid moisture loss and the potential for reinforcement corrosion and concrete damage.
Plastic shrinkage cracks tend to be 1-2 mm wide, 300-500 mm long, and 20-50 mm deep. They can occur as isolated single cracks or in patterns, and they are most commonly found in slabs, occurring randomly, diagonally, and over reinforcement. Adequate curing and prevention of evaporation immediately after casting are crucial in reducing the occurrence of plastic shrinkage cracks. Covering the surface of the concrete early on and protecting it from drying winds can also help prevent cracking.
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Plastic shrinkage cracks tend to be 1-2mm wide
Plastic shrinkage cracks are caused by the loss of water through evaporation or suction during the plastic state of the concrete. This leads to a volume reduction and the appearance of cracks on the surface. The cracks tend to be 1-2mm wide, 300-500mm long, and 20-50mm deep. In some cases, they may even extend through the full depth of a member.
The risk of plastic shrinkage cracking depends on several factors, including the rate of moisture loss, evaporation, and the concrete mixture's susceptibility to cracking. Rapid moisture loss and drying of the surface have traditionally been considered the primary causes of plastic shrinkage cracking. However, any factor that increases moisture loss, reduces bleed water rising to the surface, or delays concrete hardening can increase the risk of cracking.
To prevent plastic shrinkage cracks, it is essential to understand the concrete mixture's susceptibility to cracking and monitor job site conditions. Covering the concrete surface early and protecting it from drying winds can also help. Additionally, controlling the rate of evaporation from the surface is crucial. The Menzel Formula, for example, can be used to estimate the rate of evaporation and the potential for plastic shrinkage cracking.
The occurrence of plastic shrinkage cracks can be significantly reduced by adequate curing. This involves providing suitable moisture and temperature conditions for hydration and strength development. Concrete with higher water content or the use of recycled aggregate in its saturated surface dry (SSD) state can also reduce the effect of plastic shrinkage.
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Frequently asked questions
Plastic shrinkage cracking occurs when the rate of evaporation exceeds the rate of bleeding, leading to a net loss of water from the surface layer of concrete. As a result, the surface layer tries to shrink but is restrained by the underlying layers, causing tensile stresses and cracks to form.
The risk of plastic shrinkage cracking depends on various factors such as the rate of moisture loss, evaporation rate, temperature, humidity, and wind speed. The type of concrete mixture and jobsite conditions also play a role in the susceptibility of concrete to cracking.
Plastic shrinkage cracking can be prevented by minimizing rapid moisture loss from the concrete surface. This can be achieved by covering the surface early and protecting it from drying winds. Additionally, adequate curing and the use of evaporation retarders can help reduce the occurrence of plastic shrinkage cracks.
