Chloe Zhang
Plastic shrinkage is a well-known problem in the construction industry, and it occurs when there is a loss of water by evaporation from the surface of newly laid concrete or by suction from dry concrete underneath. This results in the concrete contracting and cracking, which can have a significant impact on the final structure. The risk of plastic shrinkage cracking depends on several factors, including the rate of evaporation, temperature, humidity, and wind speed. Understanding the susceptibility of the concrete mixture and taking preventative measures such as covering the surface or using curing compounds can help minimise the risk of cracking. However, if left unchecked, plastic shrinkage can lead to undesirable cracks and potentially dangerous structural issues.
| 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. |
| Conditions | High evaporation rates, caused by a combination of high temperature, low humidity, high wind velocity, and high concrete temperature. |
| Appearance | Plastic shrinkage cracks appear in the first few hours after concrete placement, usually before the finishing operations are complete. They are typically parallel to each other, 2-4 inches deep, and approximately 1/8 inch wide. |
| Prevention | Dampen subgrades and forms, dampen dry aggregates, precool aggregate with water, cool the cement, use chipped ice, shade aggregates, avoid overmixing, place concrete early in the morning or late afternoon, construct temporary walls to reduce wind velocity, use evaporation retardant, use fog sprays, etc. |
| Impact | Plastic shrinkage cracks are considered cosmetic blemishes and are rarely a threat to structural integrity. However, they can create pathways for the penetration of aggressive agents (chlorides, sulfates) and impact the long-term durability of the structure. |
| Mitigation | The American Concrete Institute (ACI) provides guidance on hot weather concreting issues, including strategies to manage plastic shrinkage problems. |
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What You'll Learn
Plastic shrinkage cracking
The risk of plastic shrinkage cracking depends on several factors, including the rate of moisture loss, the rate of evaporation, and the concrete mixture's susceptibility to cracking. Concrete with a higher cement content or lower volumetric aggregate content is more prone to plastic shrinkage cracking. Additionally, the temperature of the fresh concrete, the type and content of Portland cement, and the use of supplementary cementitious materials can also influence the risk 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. Spraying resin-based curing compounds can also help, but only after the free water has evaporated. Understanding the job site conditions and taking proactive measures to minimize rapid moisture loss from the concrete surface is essential to mitigating the risk of plastic shrinkage cracking.
Plastic shrinkage cracks can have significant consequences. They can allow water and deicing chemicals to penetrate the concrete, leading to potential reinforcement corrosion and concrete damage from freezing and thawing. Additionally, any crack in concrete is undesirable as it reduces the effective flexural rigidity and the moment-carrying capacity of the structure.
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Causes of plastic shrinkage
Plastic shrinkage in concrete refers to the moisture loss and contraction of concrete before it sets. It is caused by the loss of water through evaporation from the surface of newly laid concrete or by the suction of dry concrete underneath.
Plastic shrinkage cracking occurs when bleeding ceases and the concrete is neither a liquid nor a solid. At this stage of hardening, the concrete has essentially zero tensile capacity. If concrete hardening is delayed, then the window of vulnerability and the risk of plastic shrinkage cracking increases.
The risk of plastic shrinkage cracking depends on many factors, not just the rate of moisture loss or evaporation from the surface. However, the rate of evaporation is a key factor, and the default maximum allowable rate of evaporation is 0.20 lb/ft2/hr. The rate of evaporation can be estimated using the Menzel Formula, which takes into account air and concrete temperature, relative humidity, and wind velocity.
Other factors that increase the risk of plastic shrinkage cracking include:
- A high cement content mix or low volumetric aggregate content
- A reduction in the amount of bleed water rising to the surface
- Delayed hardening of the concrete
- The type and content of Portland cement and supplementary cementitious materials
- The water-cementitious materials ratio
- The type and dosage rate of chemical admixtures
- The temperature of the fresh concrete
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Prevention of plastic shrinkage
Plastic shrinkage in concrete occurs when the concrete mixture loses moisture too quickly after it has been placed. As the concrete begins to set and harden, if the rate of evaporation is higher than the rate at which water rises to the surface, small cracks called shrinkage cracks may develop. These cracks are typically hairline cracks and do not affect the structural integrity of the concrete, but they can impact its appearance and, if left untreated, may allow water infiltration. Here are several strategies to prevent plastic shrinkage:
The first step in preventing plastic shrinkage is to properly cure the concrete. Curing helps to maintain moisture in the concrete during the early stages of hardening, reducing the rate of evaporation. Immediately after finishing the concrete surface, apply a curing compound or a liquid membrane-forming compound. These products form a protective barrier on the concrete surface, slowing down the evaporation rate and providing adequate time for hydration and strengthening. Make sure to follow the manufacturer's instructions for proper application and coverage rates.
Another effective method is to use windbreaks or sunshades to protect the concrete from rapid moisture loss due to wind or sun exposure. Erect physical barriers such as plywood, straw bales, or plastic sheeting to block the wind and provide shade for the concrete, especially during hot and windy weather conditions. Ensure that the barriers are secure and do not come into direct contact with the fresh concrete surface.
For large concrete placements or slabs, consider using internal curing methods. One common approach is to incorporate saturated lightweight aggregates or absorbent materials, such as pre-soaked cellulose fibers or sponge-like materials, into the concrete mix. These materials act as internal water reservoirs, slowly releasing moisture as the concrete hardens, thereby reducing the risk of plastic shrinkage cracking.
Properly preparing the subgrade and ensuring it is moist before placing the concrete can also help prevent plastic shrinkage cracks. A damp subgrade will provide additional moisture to the concrete, aiding in the hydration process and reducing the rate of evaporation. Ensure the subgrade is well-compacted and free of debris to provide a stable base for the concrete.
Finally, pay attention to the weather conditions and plan concrete placements accordingly. Avoid pouring concrete when temperatures are extremely high or when it is particularly windy, as these conditions promote rapid moisture loss. Schedule concrete work during cooler times of the day, and consider using cooling techniques such as ice-cooled water for mixing or using chilled aggregates to lower the concrete temperature and slow evaporation.
By implementing these preventive measures, you can effectively minimize the occurrence of plastic shrinkage cracks in concrete and ensure the long-term durability and aesthetic quality of your concrete structures. Proper curing, protection from weather elements, internal curing methods, a damp subgrade, and thoughtful scheduling are key aspects of successful concrete placement and finishing.
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Impact of plastic shrinkage
Plastic shrinkage in concrete refers to the moisture loss and contraction of concrete before it sets. It occurs when water is allowed to escape from the surface of concrete or by suction of dry concrete below. This results in the concrete losing water more rapidly than it is produced by the bleeding process, leading to a net loss of water from the surface layer.
The impact of plastic shrinkage is that it induces tensile stress in the surface layers of concrete. This is because the surface layers are restrained by the non-shrinking inner concrete. As concrete is very weak in its plastic state, this tensile stress can lead to cracking at the surface. These cracks are known as plastic shrinkage cracks and are most common in slabs, occurring randomly, diagonally, and over reinforcement. They are deeper and wider than other early cracks and may extend several feet in length, adopting a crow's-foot pattern.
The risk of plastic shrinkage cracking depends on various factors, including the rate of moisture loss, evaporation rate, and environmental conditions such as temperature, humidity, and wind speed. Plastic shrinkage cracks can also have implications for the durability of concrete structures. For example, if water and deicing chemicals enter the concrete through these cracks, it may increase the potential for reinforcement corrosion and concrete damage from freezing and thawing.
To prevent plastic shrinkage and the resulting cracks, it is crucial to control the rate of evaporation immediately after casting. This can be achieved by covering the surface of the concrete, protecting it from drying winds, and using evaporation retardants or fog sprays to maintain high humidity and low temperatures. Additionally, understanding the susceptibility of the concrete mixture to cracking and monitoring job site conditions are essential to minimize rapid moisture loss.
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Solutions to plastic shrinkage
Plastic shrinkage cracks are caused by the rapid evaporation of water from the surface of newly laid concrete, or by the suction of dry concrete underneath. This occurs when the rate of evaporation exceeds the rate of "bleeding", a process where water is forced upwards as heavier solid particles settle downwards.
To prevent plastic shrinkage, it is important to minimise rapid moisture loss from the concrete surface. Here are some methods to achieve this:
- Cover the concrete surface early: Use physical barriers to cover the concrete surface as soon as possible to protect it from drying winds and direct sunlight.
- Moisten subgrades and forms: Prevent water absorption by wetting the subgrades and forms before placing the concrete.
- Dampen dry aggregates: If using dry aggregates, ensure they are sufficiently dampened to prevent them from absorbing water from the concrete mix.
- Precooling aggregate with water: Cool the aggregates by wetting them before mixing.
- Cool the cement: Use chilled water or ice to lower the temperature of the cement, reducing the rate of evaporation.
- Shading: Provide shade for the aggregates, water tanks, and lines to prevent direct sunlight from increasing the temperature and promoting evaporation.
- Avoid overmixing: Overmixing can introduce excess air and increase the temperature of the mix, leading to higher evaporation rates.
- Timing: Place concrete during cooler periods of the day, such as early morning or late afternoon, to minimise exposure to heat and sunlight.
- Construct temporary walls: Build temporary structures to reduce wind velocity around the concrete, slowing down evaporation.
- Use evaporation retardants: Apply polymers or fog sprays to the concrete surface to reduce evaporation rates by increasing humidity and lowering air temperature.
- Monitor jobsite conditions: Understand the susceptibility of your specific concrete mix to cracking by considering factors such as cement content, aggregate content, and environmental conditions (temperature, humidity, wind speed).
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Frequently asked questions
Plastic shrinkage in concrete refers to the moisture loss and contraction of concrete before it sets. It occurs when water escapes from the surface of concrete or is sucked up by dry concrete below.
Plastic shrinkage cracks are caused by the rapid drying and subsequent shrinkage of the surface of the concrete. This drying can be caused by a variety of factors, including high temperatures, low humidity, and wind.
Plastic shrinkage cracking can be prevented by reducing the rate of evaporation from the surface of the concrete. This can be done by covering the surface early on and protecting it from drying winds. Additionally, it is important to monitor jobsite conditions and take actions to minimize rapid moisture loss.
