Omar Khalid
Concrete is often covered with plastic sheeting, known as visquene in the industry, to protect it from the elements during the curing process. The plastic sheeting helps to maintain a certain level of heat, providing insulation and protecting the concrete from colder temperatures. It also prevents water from evaporating, ensuring that there is adequate moisture in the concrete as it dries. However, applying plastic too early can cause issues with late bleed water, and wrinkles in the plastic sheeting can cause discoloration.
| Characteristics | Values |
|---|---|
| Purpose | Protect concrete from cold and/or wet conditions |
| Maintain optimal moisture content during curing | |
| Prevent concrete from drying out too quickly | |
| Protect against rain | |
| Maintain heat | |
| Prevent water from evaporating | |
| Protect from hot weather | |
| Disadvantages | Can cause discoloration |
| Can cause marks on the concrete | |
| Can cause air pockets if not in full contact with the surface | |
| Timing | Should not be applied too early when finishing and late bleed water are still present on the surface |
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What You'll Learn
Concrete covered in plastic can discolour
Concrete is covered with plastic to protect it from adverse weather conditions and to prevent it from drying out. However, covering concrete with plastic can sometimes lead to discolouration. This happens when the plastic sheeting is in direct contact with the concrete, resulting in streaks or a swirling pattern. The discolouration is caused by the moisture coagulating with the contact surfaces of the plastic. If there are areas where the plastic is not in full contact, air pockets will form, leading to uneven discolouration.
Several factors can contribute to concrete discolouration, including inconsistent curing, variations in cement alkali content, delayed hydration of cement paste, and the use of calcium chloride or chloride-bearing chemical admixtures. Inadequate mixing time, improper timing of finishing operations, and the type and condition of formwork can also play a role in discolouration.
To minimise discolouration, it is important to cure the concrete uniformly. Proper curing methods include wet curing with tepid water, using a liquid curing compound, or covering with polyethylene sheeting kept in uniform contact with the slab surface. Discolouration can be reduced or removed through repeated washing with hot water and a scrub brush or by grinding or sanding down the top layer of the concrete.
While plastic sheeting can be used to protect concrete from the elements, it should be applied carefully to avoid discolouration. Alternative methods, such as misting with water or using curing compounds, can also be considered to prevent discolouration and ensure uniform curing.
Overall, while plastic sheeting can be beneficial in protecting concrete, it is important to be cautious of the potential for discolouration and to take the necessary steps to minimise this issue. Proper curing techniques and alternative protective measures can help ensure the concrete maintains its intended colour and appearance.
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Plastic prevents concrete from drying out
Concrete is covered with plastic to prevent it from drying out prematurely. This process is known as "curing" and it is essential in the concrete hardening process. If concrete dries out too much, hydration (curing) will stop, causing the concrete to crack.
Plastic sheeting, such as visquene, is placed over freshly poured concrete to trap water inside, ensuring gradual curing. This prevents water from evaporating and helps maintain optimal moisture content during the curing process. The plastic sheeting also helps to maintain a certain level of heat, protecting the concrete from colder temperatures. In colder weather, the plastic can help keep the heat generated during cement hydration, while in hotter weather, the concrete may need to be misted with water daily to prevent it from drying out.
Applying plastic too early, when finishing and late bleed water are still present on the surface, can cause issues. When plastic is applied to a "still wet" surface, moisture can coagulate with the contact surfaces of the plastic. However, if there are areas where the plastic is not in full contact, air pockets will form. This can lead to discoloration and uneven absorption of stains or sealers, affecting the final appearance of the concrete.
The use of plastic sheeting for curing concrete requires careful consideration of timing and technique to ensure effective results. It is important to allow the concrete to cure gradually and evenly to achieve its required strength and durability.
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Concrete covered in plastic can crack
Concrete is covered with plastic to protect it from the elements during the curing process. Cold, wet, and freezing weather conditions can damage concrete that has not yet set, so covering it with plastic sheeting (known as "visquene" in the industry) helps to keep the heat generated during cement hydration and prevents the concrete surface from drying out.
However, concrete covered in plastic can still crack. These cracks are known as "plastic shrinkage cracks" and are caused by a rapid loss of water from the surface of the concrete before it has set. This occurs when the rate of evaporation of surface moisture is faster than the rate at which rising bleed water can replace it. As water evaporates from the surface, the concrete tries to shrink, but it is restrained by the underlying layers, resulting in tensile stresses that lead to the formation of cracks. These cracks tend to be 1-2 mm wide, 300-500 mm long, and 20-50 mm deep, but they can sometimes extend through the full depth of the concrete.
Plastic sheeting can contribute to plastic shrinkage cracks if it is applied too early when the concrete is still wet. When the plastic makes localized contact with a wet surface, moisture coagulates with the contact surfaces of the plastic. If there are areas where the plastic is not in full contact, air pockets will form. This uneven coverage can cause discoloration and mottling in the finished concrete.
To prevent plastic shrinkage cracks, it is important to start curing the concrete as soon as possible. In hot and dry climates, spraying an evaporation retardant on the surface can help. Additionally, covering the concrete with wet burlap or polyethylene sheets and keeping it continuously moist for at least three days can help prevent cracks. Proper manpower, equipment, and supplies are also necessary to ensure that the concrete can be placed and finished promptly without delays.
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Concrete should be covered in plastic in cold and/or wet weather
Concrete should be covered with plastic in cold and/or wet weather to protect it from the elements during the curing process. The curing process is when concrete hardens and gains strength over time. It is a gradual process that requires the maintenance of optimal moisture content to prevent cracking, scaling, and other issues.
Concrete covered with plastic sheeting, also known as "visquene" in the industry, can protect the concrete from the cold by helping to keep the heat generated during cement hydration. It also provides curing protection by preventing the concrete surface from drying out too quickly. This is especially important in cold and/or wet weather, as the concrete is more susceptible to rapid moisture loss and temperature fluctuations.
However, it is crucial to be cautious when applying plastic to concrete. Applying plastic too early, when finishing and late bleed water are still present on the surface, can cause issues. When the plastic makes localized contact with a "still wet" surface, moisture can coagulate, creating uneven coloration and texture. This can result in an undesirable aesthetic finish and affect the concrete's functionality.
Additionally, concrete covered with plastic should be monitored to prevent excess moisture accumulation. While maintaining moisture is essential for curing, intermittent wetting and drying can be detrimental. Constant wet curing is ideal, and concrete should be kept wet for 3 to 6 days to ensure proper hydration.
In summary, covering concrete with plastic in cold and/or wet weather is a protective measure to maintain optimal curing conditions. However, it requires careful timing and management to avoid issues related to excess moisture and discoloration. Properly cured concrete will result in a durable and long-lasting surface.
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Applying plastic too early can cause late bleed water issues
Concrete is covered with plastic to protect it from the elements, such as cold temperatures, rain, and snow. Plastic sheeting can also help to maintain a certain level of heat and prevent the concrete from drying out before it cures. However, applying plastic too early can cause late bleed water issues.
Late bleed water occurs when the concrete vibration/consolidation process brings mix liquid to the surface. Finishers prefer a wetter surface as it makes floating the concrete easier. However, excess water or cement paste on the surface can cause scaling. Applying plastic sheeting too early can trap this excess water, leading to potential issues with the concrete's surface.
The timing of applying plastic sheeting to concrete is a delicate balance. If applied too early, when the concrete is still wet, the moisture will coagulate with the plastic, forming air pockets in areas where there is no full contact. This can cause discoloration and uneven staining of the concrete surface.
To avoid late bleed water issues, it is important to allow the concrete to dry slightly before applying plastic sheeting. Additionally, ensuring that the plastic lays flat and is wrinkle-free can help prevent discoloration. It is also recommended to mist the concrete with water before applying the plastic to maintain optimal moisture levels.
In summary, while plastic sheeting is useful for protecting concrete and maintaining moisture levels, applying it too early can lead to late bleed water issues and discoloration. Proper timing and preparation are crucial to ensuring successful concrete curing.
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Frequently asked questions
Covering concrete with plastic sheeting, known as "visquene" in the industry, is done to protect the concrete from cold and/or wet weather conditions. Plastic helps to maintain the heat generated during cement hydration and prevents the concrete surface from drying out too quickly.
Applying plastic too early, before the concrete has finished curing and late bleed water is still present on the surface, can cause issues. This can lead to uneven coloration and discolouration where the plastic made contact with the concrete.
Instead of using plastic, one can add water to the surface to maintain water levels in the concrete and prevent it from drying out. Another method is to use a chemical curing compound applied with a garden sprayer, which eliminates the need for a "wet" cure.
The best temperature for concrete to cure is 40 Fahrenheit. The concrete should be misted with water occasionally for the first day or two after it is poured. Constant wet curing is preferable to intermittent wetting and drying.
