If the rain begins after 2-4 hours from concrete placement, the surface needs to be covered; otherwise, the concrete surface will suffer damages. If rains pound on concrete 12 hours after placement, the likeliness of any damage is negligible.
How Long Should Concrete Dry Before it Rains. If rain is in the forecast for the days or hours after you expect to finish the job, you shouldn't have to worry. As long as the concrete is given 4 to 8 hours after mixing and finishing to set up, rainwater won't affect it nearly as badly as it could.
Plastic sheets and tarpaulin should be kept available on-site to cover concrete and protect it from rainwater. The cover should not be placed directly on the surface of the concrete. Install timber logs or place bricks around the cover's perimeter to create a space between the concrete and sheet surfaces.
Rain on freshly finished concrete can damage the surface, and in extreme cases, wash the surface off the concrete exposing the aggregate. To prevent this, covering the concrete with hessian will protect the surface from running water, reducing the risk of erosion.
The most efficient way to cure concrete is to use a plastic cover. The covers are placed on freshly poured concrete until the concrete has a chance to dry. Plastic concrete curing covers seal off concrete and prevents water from evaporating.
At specified curing temperatures, well-portioned concrete mixtures should attain this strength within 24 to 48 hours. Therefore, it is critical that newly placed concrete be protected from freezing for the first 24 to 48 hours or until the concrete attains a strength of approximately 500 psi.
The first two to four hours lets the concrete “finish”, which refers to the setting process. If it is predicted to rain before the first two to four hours have passed, the slab should be covered to protect it from too much moisture.
Concrete has to withstand all manner of conditions that, over time, do take their toll on the material. Rain will slowly erode the concrete, causing discolouration and weak points.
Concrete that is not moist-cured at all dries too rapidly, and reaches less than half its potential design strength. It will also have a greater number of shrinkage cracks.
Concrete typically takes 24 to 48 hours to dry enough for you to walk or drive on it. However, concrete drying is a continuous and fluid event, and usually reaches its full effective strength after about 28 days.
It depends on the stage
If the concrete is freshly laid and the rain sets in, using a plastic sheet like described is the best way to protect it against the downpour. If the concrete is a little further along the curing process, say around 4 hours and above after pouring, the rain will have less effect.
To protect your new slab and ensure an exceptional finished product you should wait 24 hours for foot traffic (including pets), 10 days to drive light vehicles or add furniture, and 28 days for heavy pick-up trucks and RVs. After 28 days the concrete is cured and you will have a strong and stable slab.
Water repellent sealers can be used to impart water repellency to a variety of porous substrates, including poured-in-place or pre-cast concrete. Silane and siloxane sealers reveal little to no change in the appearance of the substrates to which they are applied.
Usually this white or off-white discoloration to concrete and masonry forms when soluble salts and other water dispersible materials come to the surface through capillary voids as a result of temperature changes, moist migration, condensation, rain, dew, and/or too much water in the mix.
After pouring and finishing the slab, you should wait 24-48 hours before removing concrete forms. You can remove the formwork once the concrete has enough strength to keep it from cracking. Concrete naturally contracts as it cures, so it should pull away from the forms and make them easier to remove.
Physically, concrete that experiences a drying shrinkage of about 0.05 percent (500 millionths or 500 x 10-6) will shrink approximately 0.6 in. per 100 ft (50 mm for every 100 m). In more graphic terms, that is about two inches for the length of a football field. There are several factors that affect drying shrinkage.
Moist curing is a common method of concrete curing. It involves wetting the concrete slab often with water (5-7 times per day) for the first 7 days. This method ensures your concrete slab will be extremely strong and durable, because it allows the moisture to evaporate slowly, preventing cracks and shrinks.
DO spray new concrete with water. One of the most common methods for curing concrete is to hose it down frequently with water—five to 10 times per day, or as often as you can—for the first seven days. Known as “moist curing,” this allows the moisture in the concrete to evaporate slowly.
If your concrete mix is too wet it will also be more porous once cured, making it difficult to achieve the desired finish and potentially making it not fit for purpose. In short, a concrete mix that is too wet could seriously impact the durability, longevity and strength of your project.
Large amounts of rainwater also present the risk of contaminants entering the mix, also jeopardizing the cement. Concrete exposed to an abundance of additional moisture is at a greater risk of cracking due to the uneven surface created. As additional water enters the cracks, the foundation will weaken over time.
Any excess water not absorbed by the sand and cement rise to the surface. This leaves a pool of water on the surface of the concrete, which we call bleed water.
This can happen if the soil is not compact under the poured slab or when water from downspouts or rain seeps through the control joints and washes away the soil underneath the slab. This then causes the heavy concrete to start to sink, and the pressure causes cracking.
Wet the concrete, then place the plastic sheeting over it. Use bricks, boulders, or stakes to hold the plastic in place and keep small animals out. Once a day, remove the plastic sheet and wet the concrete, then replace the cover. Do this for a week for the best results.
Concrete that is newly laid is very vulnerable to many things. When it's curing, we recommend the concrete be covered over with a layer of plastic sheeting which will not only keep the surface clean but will prevent the moisture from escaping from the surface too quickly.
After Lay and Finish is Completed, and Bleed Water has Evaporated, Cover the Concrete with Insulated Curing Blankets to Stabilize at the Constant Optimal Temperature. After about 72 Hours, Remove the Thermal Concrete Blankets and Allow the Slats to Air Dry.