Water-cured concrete made with Portland cement will harden even when submerged in water. This is because the cement particles are hydrated and react chemically with the water, joining sand and gravel together. This curing process, also known as hardening, takes almost a month and causes the concrete to set. The biggest challenge when pouring concrete under water is movement.
If the water is calm, then it's not a problem at all. However, if the water moves, it can remove the cement paste that holds the sand and gravel together. Concrete is among the most durable and attractive building materials, and its use is much greater than that of steel or wood. It does not dissolve in rain, but cement does, and since cement is the “glue that holds everything together”, concrete becomes useless when flooded with rain.
To prevent this from happening, create temporary berms around the new concrete slab, and then flood the interior area with water up to one foot (30.48 cm). In very hot and humid regions, heat loss and continuous evaporation lead to a lack of moisture from the concrete surface, and therefore lead to cracks. To avoid this, locate polyethylene or burlap sheets and stretch them over the entire area of wet concrete by spreading them over the edges for at least 6 to 12 inches to prevent rain from splashing underneath. Dampen the concrete thoroughly and then cover it with the sheets of your choice, using bricks, stones or other heavy objects to keep it in place.
While curing concrete will greatly contribute to strengthening the finished project, many concrete slabs will still crack despite all the precautions due to concrete shrinkage as water is consumed in the hydration process and temperature fluctuations. This technique can also be used for vertical concrete columns and walls by wetting them and wrapping them with a curing blanket or plastic sheets. An adequate supply of water and keeping the concrete moist after laying is important so that the concrete does not crack and strength is not compromised at all. Then another compound in the concrete reacts at a slower rate and reaches its final hardness 28 days later. To ensure that the drying process goes smoothly and that you get your concrete to its final position while preventing it from falling through the water, use soluble emulsions that form a protective layer on freshly poured concrete slabs or walls. Concrete hardens over time, as long as the correct amount of water is maintained in the concrete mix.