Sofia Martinez
Plastic shrinkage in concrete is a well-known problem in the construction industry, which can cause undesirable cracks in the concrete. This phenomenon occurs due to the loss of water through evaporation or suction during the plastic state of the concrete, leading to volume reduction and the appearance of cracks on the surface. These cracks can be shallow or full-depth, typically forming within the first few hours after concrete placement, when the concrete is still soft and has little strength. The cracks tend to be around 1-2mm wide and 300-500mm long. Various factors influence the occurrence and extent of plastic shrinkage, including the rate of evaporation, the cement content of the mix, and the temperature and humidity conditions. Understanding these factors and implementing precautionary measures can help minimize the formation of plastic shrinkage cracks in concrete.
| Characteristics | Values |
|---|---|
| Definition | Plastic shrinkage refers to the moisture loss and contraction of concrete before it sets. |
| 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. |
| Timing | Plastic shrinkage cracks appear in the first few hours after concrete placement and typically before finishing operations are complete. |
| Prevention | Covering the surface of the concrete as early as possible and protecting it from the effects of drying winds. Spraying of resin-based curing compounds. |
| Size | Plastic shrinkage cracks tend to be 1-2 mm wide, 300-500 mm long and 20-50 mm deep, though they may extend through the full depth of a member. |
| Pattern | Plastic shrinkage cracks occur in both random and parallel patterns. They are discontinuous, relatively short, and typically fairly shallow. |
| Curing | The occurrence of plastic shrinkage cracks can be significantly reduced by adequate curing. |
| Bleeding | Bleeding is caused by mix water being forced upwards when heavier solid particles settle downwards. Greater bleeding capacity reduces evaporation from the concrete surface, reducing plastic shrinkage cracking. |
| Cement Content | Plastic shrinkage is greater the larger the cement content of the mix or lower the larger the volumetric aggregate content. |
| Temperature | Evaporation of water is more rapid at high temperatures and/or low humidity, particularly in windy weather. |
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What You'll Learn
Evaporation and suction
Plastic shrinkage in concrete refers to the moisture loss and contraction of concrete before it sets. It is caused by the loss of water by evaporation from the surface of newly laid concrete or by the suction of dry concrete underneath. Evaporation occurs at the surface and is more rapid at high temperatures and/or low humidity, particularly in windy weather. If evaporation occurs at a faster rate than bleeding, there is a net loss of water from the surface layer of concrete, leading to a net reduction in volume.
The surface layer of concrete tries to shrink but is restrained by the underlying layers, which do not experience the same reduction in volume. As a result, tensile stresses develop in the surface layer, and cracks form. These cracks can be discontinuous, relatively short (a few inches to several feet in length), and typically shallow, but they can occasionally become full-depth cracks. The cracks tend to be 1-2 mm wide, 300-500 mm long, and 20-50 mm deep.
To prevent plastic shrinkage cracking, the surface of the concrete should be covered as early as possible to protect it from the effects of drying winds. Spraying resin-based curing compounds (unless in emulsion form) is another prevention method, but this cannot be done effectively until the free water has evaporated.
The occurrence of plastic shrinkage cracks can be significantly reduced by adequate curing. One method is to trowel the freshly cast concrete surface after the initial set of the concrete (normally 30 minutes) so that the crack developed due to shrinkage can be removed by trowelling the surface. This process is to be continued until the concrete becomes hard enough to resist the crack.
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Cement content
Plastic shrinkage in concrete is caused by the loss of water through evaporation or suction during the plastic state of the concrete, leading to volume reduction and cracking on the surface. This is influenced by factors such as air temperature, concrete temperature, humidity, and wind speed, which affect the rate of evaporation.
The cement content of the concrete mix plays a significant role in determining the extent of plastic shrinkage. As the cement content increases, the plastic shrinkage also increases. This relationship is evident in studies where the plastic shrinkage of cement paste was found to be significantly higher than that of concrete.
The water-to-cement ratio (w/c) is a critical factor in understanding the susceptibility of concrete to plastic shrinkage. For conventional concrete with a w/c ratio beyond 0.4, drying shrinkage becomes predominant, and when the w/c ratio exceeds 0.42, water content is in excess, impacting cement hydration.
The use of recycled aggregates in a saturated state can act as a water supply, reducing plastic shrinkage compared to non-recycled aggregates. Additionally, the type and content of cementitious materials, such as Portland cement or alkali-activated cement, influence the bleeding and evaporation rates, impacting plastic shrinkage cracking.
To minimise plastic shrinkage cracks, adequate curing, proper jobsite monitoring, and precautionary measures are essential. These cracks typically occur within the first few hours of concrete placement and can be influenced by wind direction, temperature, and humidity. By understanding the factors contributing to plastic shrinkage, such as cement content and its interactions with other components, effective measures can be implemented to mitigate this issue.
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Temperature and humidity
Plastic shrinkage cracks typically appear within the first few hours after concrete placement, during the initial setting time when the concrete is still in a plastic state and has little strength. At this stage, water can move relatively freely and tends to migrate upwards towards the surface, a process known as bleeding.
The rate of evaporation exceeds the rate of bleeding when the surface moisture of recently placed concrete evaporates faster than it can be replenished by the rising bleed water. This imbalance leads to a net loss of water from the surface layer, resulting in a reduction in volume. As the surface layer attempts to shrink, it is restrained by the underlying layers that are not subject to the same reduction in volume, giving rise to tensile stresses in the surface layer and, ultimately, cracking.
The occurrence of plastic shrinkage cracks is influenced by temperature and humidity conditions. High temperatures and low humidity levels accelerate the evaporation process, exacerbating the risk of cracking. Conversely, higher humidity can mitigate this issue by reducing the rate of evaporation. Additionally, covering the surface of the concrete as early as possible and protecting it from drying winds can help prevent plastic shrinkage cracks.
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Concrete curing
Moist Curing
One of the most common methods for curing concrete is moist curing or "hose and cover". This involves hosing down the concrete with water frequently—five to ten times per day—for the first seven days. This slow evaporation process allows the concrete to gain strength, and moist-cured concrete can be up to 50% stronger than uncured concrete. If the weather is cold, spraying water may not be effective, and it is recommended to cover the concrete with a curing insulating blanket or polyethylene sheeting to trap and slow moisture evaporation.
Pond Curing
Pond curing is another effective curing method, especially for large slabs. It involves forming temporary berms around a new concrete slab and then flooding the area inside with one foot of water. This method requires less daily attention than moist curing, but it may not be feasible for all projects due to the space and soil requirements for forming the berms.
Curing Compounds
Curing compounds are liquid chemical formulations sprayed onto the concrete slab to form a protective film that slows the dehydration process. These compounds can be purchased from DIY stores and ready-mix concrete companies. Some curing compounds are designed to dissipate after a few weeks, while others need to be removed after curing. It is important to carefully read the manufacturer's instructions before choosing and applying a curing compound.
Temperature Control
Maintaining the proper temperature is crucial during the curing process. Ideally, the concrete should be kept between 50 and 85 degrees Fahrenheit to prevent it from getting too cold or too hot, which can weaken the concrete.
Timing
Curing must be started as soon as possible after the concrete is placed. Concrete is sensitive and easily ruined during the initial stages, so proper curing is essential to ensure the quality of the finished product. The concrete should be kept damp and cured for about 28 days to allow the chemical reaction between cement and water to fully complete.
By following these concrete curing instructions, you can help prevent plastic shrinkage cracks and ensure the strength and durability of your concrete project.
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Cracking and tensile stress
Plastic shrinkage cracks are a well-known problem in construction works. They are caused by the loss of water through evaporation or suction during the plastic state of the concrete, leading to volume reduction and the appearance of cracks on the surface. These cracks can occur in both random and parallel patterns and are typically fairly short, ranging from a few inches to several feet in length. They are usually fairly shallow, but they can occasionally become full-depth cracks.
The cracking is caused by tensile stress in the surface layers of the concrete, which occurs when the rate of evaporation exceeds the rate of bleeding. As the surface layer of concrete shrinks due to water evaporation, it is restrained by the underlying layers that are not subject to the same reduction in volume. This restraint results in the development of tensile stresses in the surface layer, leading to cracking. The concrete has very low tensile strength at this stage, making it susceptible to cracking.
The risk of plastic shrinkage cracking increases when concrete hardening is delayed, providing a larger window for evaporation to occur. Rapid moisture loss from the surface of the concrete, high temperatures, low humidity, and windy weather conditions all contribute to increased evaporation and, consequently, a higher risk of cracking. Additionally, factors that reduce the amount of bleed water rising to the surface or delay concrete hardening also elevate the likelihood of cracking.
To prevent plastic shrinkage cracking, it is crucial to cover the surface of the concrete as early as possible to protect it from drying winds. Adequate curing methods, such as trowelling the surface during the initial setting of the concrete, can also help remove cracks that develop due to shrinkage. By understanding the susceptibility of the concrete mixture to cracking and monitoring job site conditions, precautionary measures can be implemented to minimise the occurrence of these unsightly and potentially dangerous cracks.
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Frequently asked questions
Plastic shrinkage in concrete refers to the moisture loss and contraction of concrete before it sets. This can lead to cracks in the concrete, known as plastic shrinkage cracks.
Plastic shrinkage in concrete is caused by the loss of water through evaporation or suction during the plastic state of the concrete. This evaporation occurs at the surface of newly laid concrete and can be influenced by factors such as temperature, humidity, and wind.
Plastic shrinkage cracking can be prevented by understanding the susceptibility of the concrete mixture and monitoring job site conditions. Covering the surface of the concrete early and protecting it from drying winds can also help. Adequate curing and proper labour techniques during the casting process are crucial in preventing and minimising plastic shrinkage cracks.
