Omar Khalid
Plastic shrinkage cracks are a common issue in concrete placement, occurring when the concrete is still in its plastic state. These cracks are caused by rapid moisture loss and drying of the concrete surface, resulting in shrinkage and cracking before the concrete has properly set. While these cracks rarely affect the strength or durability of concrete, they are unsightly and can be costly to repair. To prevent plastic shrinkage cracks, it is important to control the rate of evaporation and moisture loss from the concrete surface, especially in hot, dry, and windy weather conditions. This can be achieved through various techniques such as using evaporation retardants, providing shade, and adding polypropylene fibres to the concrete mix.
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What You'll Learn
Dampen the subgrade and forms in hot, dry weather
Plastic shrinkage cracks are caused by the rapid drying of the concrete surface, which can occur when there are high evaporation rates. This is influenced by factors such as high temperatures, low humidity, and strong winds. To prevent this, it is crucial to dampen the subgrade and forms in hot, dry weather.
Firstly, it is important to understand the conditions that lead to plastic shrinkage cracks. These cracks typically appear within the first few hours after concrete placement, while it is still fresh and plastic, and they are usually parallel to each other. The cracks are caused by the rapid evaporation of water from the concrete surface, resulting in the concrete drying and shrinking before it has attained significant strength.
To prevent plastic shrinkage cracks in hot, dry weather, it is essential to dampen the subgrade and forms. If the concrete is placed on a dry, absorptive subgrade, the subgrade should be dampened before placement. However, it is crucial not to over-saturate it to the point of standing water. The formwork and reinforcement should also be dampened to ensure they do not absorb moisture from the concrete.
Additionally, it is recommended to use cooler concrete in hot weather and avoid high concrete temperatures. This can be achieved by scheduling concrete placement during the early morning or early evening, when temperatures are lower. Using fog sprays can also help reduce the rate of evaporation by cooling the concrete surface and the surrounding air.
Proper curing techniques are also crucial. Once the concrete is finished, it should be cured promptly to prevent delays, as this can increase the risk of cracking. Cover the concrete with moisture-retaining materials such as wet burlap or polyethylene sheets between finishing operations to slow down evaporation and maintain adequate moisture levels.
By following these measures, you can effectively prevent plastic shrinkage cracks in hot, dry weather by dampening the subgrade and forms and managing the evaporation rate and concrete temperature.
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Use evaporation retardants
Plastic shrinkage cracks are caused by the rapid evaporation of water from the surface of the concrete. This process, known as "bleeding", occurs when the particles within the concrete settle, displacing the water and causing it to rise to the top. Under most weather conditions, the water on the surface of the concrete evaporates. The rate of evaporation depends on factors such as temperature, humidity, and wind velocity.
To prevent plastic shrinkage cracks, one can use evaporation retardants, which are products used in concrete construction to reduce rapid moisture loss during the curing process. Evaporation retardants form a thin film on the surface of the concrete, slowing down the evaporation of moisture. This helps to maintain the moisture content in the concrete, allowing it to cure properly and prevent issues such as surface cracking.
Evaporation retardants are particularly useful in hot and dry weather conditions, where the risk of rapid moisture loss is higher. They can also be beneficial in cold weather, as maintaining the proper moisture content for curing can be challenging in cold, dry conditions. It is important to follow the manufacturer's instructions for the specific product being used, as the application method may vary. Most evaporation retardants are designed to be compatible with other concrete treatments, but it is always good to check the product's compatibility.
In addition to using evaporation retardants, there are other measures that can be taken to prevent plastic shrinkage cracks. These include dampening the subgrade and forms when conditions for high evaporation rates exist, providing fog sprays to reduce the rate of evaporation, and erecting windbreaks to reduce wind velocity. Proper manpower, equipment, and supplies should be available to ensure that the concrete can be placed and finished promptly.
By using evaporation retardants and implementing precautionary measures, plastic shrinkage cracks can be prevented or minimized. These cracks are unsightly and can affect the appearance and quality of concrete surfaces.
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Use cooler concrete in hot weather
Using cooler concrete in hot weather is a recommended practice to prevent plastic shrinkage cracks. This is because high temperatures increase the rate of moisture loss from the surface, which, in turn, increases the risk of cracking.
Concrete temperature can be reduced during the batching process by lowering the cement and water temperature, as well as cooling the aggregates. It is also recommended to schedule concrete placement during the cooler periods of the day, such as early morning or early evening, or even at night. This helps control the concrete temperature and prevent rapid hardening, which can complicate scheduling.
In addition to using cooler concrete, there are other measures that can be taken to mitigate the effects of hot weather on concrete placement. For instance, using a concrete consistency that allows for rapid placement and consolidation can help reduce the risk of cracking.
Furthermore, it is important to consider the evaporation rate of the concrete mixture. A high evaporation rate during placement and initial curing increases the likelihood of plastic shrinkage cracks. To control the evaporation rate, it is recommended to dampen the subgrade, formwork, and reinforcement. Additionally, using evaporation retardants can help prevent rapid evaporation and shrinkage cracking by forming a thin film across the treated concrete surface.
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Add polypropylene fibres to the concrete mix
Plastic shrinkage cracks are discontinuous, relatively shallow, and usually appear in fresh concrete soon after it is placed, while the concrete is still plastic. They are caused by rapid moisture loss and the drying of the surface, which results in the concrete shrinking and cracking. This is more common in hot and dry weather, but can occur in cold weather too, and is influenced by factors such as wind velocity.
To prevent plastic shrinkage cracks, polypropylene fibres can be added to the concrete mix. Polypropylene fibres bind the surface of the concrete together and provide strength to the plastic concrete to resist shrinkage forces. They act as secondary reinforcement, reducing the likelihood of cracking and controlling the spread of cracks. The fibres ensure even distribution of stresses throughout the mix, thereby reducing the chances of cracks due to greater moments of inertia.
The addition of polypropylene fibres has been found to reduce the crack area by between 40% and 85% compared to plain concrete. The use of 0.1% of fibres resulted in an average reduction of 60% in the number of cracks and crack area, while the use of 0.5% of fibres was found to be successful in combating plastic shrinkage cracking in all concrete mixes tested. The longer the fibres, the stronger the bond between the fibres and the paste, and the greater the pull-out strength. However, longer fibres can be difficult to distribute uniformly, so the length of the fibres should be considered in relation to the size of the aggregates.
Polypropylene fibres also improve the workability of the concrete mix, making it easier to handle, pour, and finish. They are cost-effective, as they are cheaper than steel as a reinforcing material, and they increase the durability of the concrete.
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Schedule concrete placement in the early morning or early evening
Scheduling concrete placement in the early morning or early evening is a crucial step in preventing plastic shrinkage cracks. These cracks occur when the surface of freshly placed concrete dries and shrinks before it has set, resulting in unsightly cracks that can appear within six hours of laying the concrete.
Concrete placed in the afternoon during very hot weather conditions is more susceptible to plastic shrinkage cracks due to the rapid loss of water from its surface. By scheduling concrete placement in the early morning or early evening, we can mitigate this risk by taking advantage of cooler temperatures, reducing the rate of evaporation, and allowing the concrete to retain moisture.
In addition to temperature control, scheduling concrete placement at optimal times enables better control over other factors that contribute to plastic shrinkage cracks. For example, high wind velocities can accelerate evaporation and increase the likelihood of cracks. By placing concrete during the early morning or early evening, we can benefit from typically calmer wind conditions, reducing the risk of rapid moisture loss.
Furthermore, the early morning and early evening are often chosen for concrete placement as they offer improved site conditions. Adequate lighting is essential for ensuring the quality of concrete placement and providing a safer working environment for construction crews. By avoiding the extreme temperatures and glare of the midday sun, as well as the potential darkness of late evening, these periods offer optimal lighting conditions for effective concrete placement and enhanced job site safety.
Scheduling concrete placement in the early morning or early evening is a proactive strategy that helps address the root causes of plastic shrinkage cracks. By considering the environmental factors that influence evaporation rates and concrete temperatures, this scheduling practice plays a vital role in preventing plastic shrinkage cracks and contributes to the overall success of the construction project.
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Frequently asked questions
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. They are caused by rapid moisture loss and drying of the surface.
Understanding the causes of plastic shrinkage cracks and monitoring jobsite conditions can help prevent or minimize these cracks from occurring. You can also take precautionary measures such as:
- Using polypropylene fibres in the concrete mix
- Dampening the subgrade, formwork, and reinforcement
- Scheduling concrete placement in the early morning or early evening to avoid hot weather
- Using fog sprays to reduce the rate of evaporation
- Erecting windbreaks to reduce wind velocity
- Providing shade to ensure the surface does not get too hot
You can use evaporation rate solvers, such as the Kestrel 5200, to monitor real-time weather data and environmental factors that may contribute to plastic shrinkage cracks. You can also use products like SINAK AntiVap and Finishing Aid to slow evaporation and retain surface moisture.
