Maya Ellison
Plastic shrinkage cracks are discontinuous cracks that appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks are caused by the rapid evaporation of water from the surface of the concrete, which creates a volume change, known as drying shrinkage. To reduce plastic shrinkage in concrete, it is important to control the rate of evaporation by using fog sprays, dampening the subgrade and formwork, and curing the concrete with a moisture evaporation inhibitor. Other methods include modifying the concrete mixture to increase its water content and incorporating synthetic fibre reinforcement to resist tension.
| Characteristics | Values |
|---|---|
| Cause of plastic shrinkage | Loss of water through evaporation or suction during the plastic state of the concrete |
| When plastic shrinkage occurs | Within the first few hours after placement, while the concrete is still plastic and has little strength |
| How to reduce plastic shrinkage | Maintain appropriate mix design water-cement ratios, use synthetic fibre reinforcement, fog sprays, dampen the subgrade, formwork and reinforcement, use moisture-retaining coverings, spray evaporation retardant on the surface, use moisture evaporation inhibitor or curing compound |
| How to identify plastic shrinkage | Cracks are usually parallel to each other, discontinuous, relatively short (few inches to several feet in length) and fairly shallow |
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What You'll Learn
Limit evaporation from the concrete's surface
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks are caused by the rapid evaporation of water from the surface of the concrete. This rapid evaporation is associated with placing concrete in hot weather conditions. However, plastic shrinkage cracks can also form in cold weather conditions when the temperature of the concrete is high compared to the surrounding air temperature.
To limit evaporation from the concrete's surface, it is important to recognise ahead of time when weather conditions conducive to plastic shrinkage cracking will exist. The rate of evaporation depends on factors such as the temperature of the concrete, temperature of the air, relative humidity, and wind velocity surrounding the concrete. The highest evaporation rates are obtained when the concrete and air temperatures are high, when relative humidity is low, when the concrete temperature is high compared to the air temperature, and when a strong wind is blowing over the concrete surface.
Fog sprays can be used to reduce the rate of evaporation from the concrete surface. Fogging increases the relative humidity in the air above the concrete, thereby reducing the rate of evaporation. It is important to ensure that the fog nozzles create a blanket of fog and not a water spray that can wash away the surface of the concrete. Pressure washers with fine spray nozzles can be used for fogging small and medium-size jobs.
If concrete is to be placed on a dry absorptive subgrade in hot and dry weather, the subgrade should be dampened but not to the point that there is freestanding water prior to placement. The formwork and reinforcement should also be dampened. If delays occur, the concrete should be covered with moisture-retaining coverings such as wet burlap, polyethylene sheeting, or building paper.
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Use synthetic fibre reinforcement
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks are caused by the rapid evaporation of water from the surface of the concrete. The rate of evaporation depends on factors such as the temperature of the concrete, the temperature of the air, relative humidity, and wind velocity. Synthetic fibre reinforcement can be used to resist the tension when concrete is very weak and to reduce the rate of evaporation from the concrete surface.
Synthetic fibres used for concrete reinforcement include materials such as Polyester and Nylon. However, the most used material for concrete reinforcement is Polypropylene, which comes in two types: macrofibres and microfibers. Macrofibres are also called structural fibres because they can replace traditional reinforcement in the form of steel bars or wire mesh. Their length is usually between 30 and 50 mm. Microfibers, on the other hand, do not add any structural capacity to a concrete section. They are typically used for plastic shrinkage crack control and have a diameter of less than 0.3 mm.
For industrial projects, macro-synthetic fibres are used to improve concrete’s durability. They are long and thick and may be used as a replacement for steel bar reinforcement or fabric reinforcement. Adding fibres to the concrete will improve its freeze-thaw resistance and help keep the concrete strong and attractive for extended periods. They also improve mix cohesion and increase resistance to plastic shrinkage during curing.
Euclid Chemical’s TUF-STRAND SF (TSSF) is a patented polypropylene/polyethylene macro synthetic fibre that has been successfully used to replace steel fibres, welded wire mesh, and conventional reinforcing bars. Concrete reinforced with TSSF will have three-dimensional reinforcing with enhanced flexural toughness, impact and abrasion resistance, and will also help mitigate the formation of plastic shrinkage cracking in concrete. TSSF is non-magnetic and non-corrosive, making it a very attractive option for exterior paving projects.
MAPEI also offers Mapefibre synthetic fibres for concrete, which can reduce or eliminate the need for steel reinforcement, saving time, labour, and money on site while achieving superior durability. Micro-synthetic fibres can be added to mitigate plastic shrinkage cracking, while macro-synthetic fibres add tensile strength and flexural toughness in post-cracking conditions.
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Dampen the subgrade before placement
Dampening the subgrade before placement is a crucial step in preventing plastic shrinkage cracks in concrete. These cracks, which can be unsightly and reduce the durability of the concrete, are caused by the rapid evaporation of water from the concrete surface, leading to surface shrinkage and tearing.
The subgrade, which is the natural soil or compacted fill material beneath the concrete slab, must be properly prepared to ensure the concrete's integrity and longevity. One essential aspect of subgrade preparation is moisture management. If the subgrade is too dry, it can lead to concrete shrinkage and cracking. Therefore, it is recommended to dampen the subgrade before placing the concrete, especially in hot and dry weather conditions. However, it is crucial not to over-saturate the subgrade to the point of standing water, as this can also negatively impact the concrete.
Proper drainage is also critical to preventing moisture accumulation, which can adversely affect the subgrade and concrete slab. Installing effective drainage systems, such as perforated pipes or French drains, helps redirect water away from the subgrade area. Additionally, a thicker sub-base can be used to better support higher loads and provide a capillary break, preventing water from the groundwater table from rising into the slab.
The subgrade should also be compacted properly to prevent cracking. There are two methods of compaction: static force, which relies on the weight of the machine, and vibratory force, which uses a mechanism to vibrate the soil and reduce friction between soil particles. The choice between the two depends on the type of soil or subgrade material.
By following these steps and paying close attention to moisture management and proper compaction, you can effectively reduce the potential for plastic shrinkage cracks in concrete and ensure a durable and aesthetically pleasing final product.
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Use a moisture evaporation inhibitor
Plastic shrinkage cracks occur when the rate of evaporation from the surface of the concrete exceeds the rate at which moisture is supplied to it, typically through bleeding. These cracks appear in the first few hours after concrete placement, usually on horizontal surfaces.
To prevent this, it is important to use a moisture evaporation inhibitor or a curing compound. This will help to immediately cure the surface of the concrete and reduce the potential for cracking. Fog sprays, for example, increase the humidity in the air above the concrete, thereby reducing the rate of evaporation. This method can also help replace the bleed water that may have prematurely evaporated from the surface. It is important to note that the fog spray should create a blanket of fog and not a water spray, which could wash away the surface of the concrete.
In addition to fog sprays, other methods to prevent moisture evaporation include using temporary windbreaks to reduce wind velocity, placing concrete at the coolest time of the day, and using sunshades to reduce concrete surface temperatures.
The use of an evaporation retardant or inhibitor is particularly important when placing concrete in hot and dry weather conditions, as the concrete surface may lose moisture faster than it can tolerate, leading to plastic shrinkage cracking.
By employing these strategies and using a moisture evaporation inhibitor, the potential for plastic shrinkage cracks in concrete can be significantly reduced.
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Monitor jobsite conditions
To prevent or at least minimize plastic shrinkage cracks, it is important to monitor jobsite conditions and implement precautionary measures. Plastic shrinkage cracks are caused by the rapid evaporation of water from the surface of the concrete, which can occur in hot weather conditions, or when the temperature of the concrete is high compared to the surrounding air temperature. Concrete placed in hot and dry weather is particularly susceptible to plastic shrinkage cracks.
To monitor jobsite conditions, it is important to keep track of the temperature of the concrete, the air temperature, the relative humidity, and wind velocity. The highest evaporation rates occur when concrete and air temperatures are high, when relative humidity is low, when there is a significant temperature difference between the concrete and the air, and when wind speed is high. Therefore, it is important to monitor these factors and take steps to reduce evaporation rates if they are likely to exceed the rate of water produced by the bleeding process.
If the concrete is to be placed in hot and dry weather, it is important to dampen the subgrade before placement, without creating freestanding water. The formwork and reinforcement should also be dampened. Ensure that you have the proper manpower, equipment, and supplies on hand so that the concrete can be placed and finished promptly. If delays occur, cover the concrete with moisture-retaining coverings, such as wet burlap, polyethylene sheeting, or building paper, between finishing operations.
Additionally, fog spraying can be used to increase the relative humidity in the air above the concrete, reducing the rate of evaporation. It is important to locate the water sprayer upwind of the concrete and discharge the spray into the air, creating a blanket of fog rather than a water spray that can wash away the surface of the concrete.
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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 the rapid evaporation of water from the surface of the concrete.
The rate of evaporation depends on factors such as the temperature of the concrete, temperature of the air, relative humidity, and wind velocity surrounding the concrete.
Fog sprays can be used to reduce the rate of evaporation from the concrete surface. The formwork and reinforcement should also be dampened. Additionally, ensuring adequate curing can significantly reduce the occurrence of plastic shrinkage cracks.
Plastic shrinkage cracks may create aesthetic concerns, especially for architectural concrete. In some cases, they may also lead to durability issues depending on the severity, width, depth, and exposure conditions of the cracks.
Maintaining the appropriate mix design water-cement ratios is critical. It is also important to ensure that the subgrade is moistened and not dry, as dry soil can absorb water quickly. Additionally, using admixtures instead of water can increase workability without negatively impacting the concrete mixture.
