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Rainwater effects on fresh concrete

Rainfall over fresh concrete can result in a severe reduction of strength and durability. Therefore, we should take all the prevention measures to avoid the rains from falling on newly placed concrete. We should monitor the weather and follow the weather forecasts closely. If the rain expected to fall, we should change the date of placing concrete. But if the concrete placed and the rain fell suddenly on the contrary to forecasts, we should be prepared to protect the placed concrete.

The protection of fresh concrete in rainy weather can be done by providing a protective enclosure to the working area. The enclosure could be a temporary framework with plastic sheeting that prevent the rains from falling on fresh concrete. Field engineers and supervisors should ensure enough overlapping and taping of plastic sheets. Gaps in plastic sheets will result in rain dropping over new concrete, which can cause concrete damage. Also, the plastic sheeting should be extended sufficiently beyond the placement area to avoid the draining of rain into the concrete area.

If the concrete placed on an open field and over a large area. The protecting of the concrete areas may not be plausible. For a similar situation, workers should not mix rainwater with fresh concrete. Also, they should not pour fresh concrete on a wet surface in an attempt to soak up the water. In this situation, we should wait until the rain is a pass. Also, we should herd off the surface water from the slab edge before completing the surface preparation. Monitoring the weather and avoiding the placing of concrete in rainy weather would protect our concrete from getting damaged, and it is eliminating the need for repairing damaged concrete. The repairing of concrete is expensive. In some cases, we need to remove part of concrete if it is damaged to the extent that it is jeopardizing the structural integrity and functionality.

Hydration is a chemical reaction that occurs as soon as the concrete is mixed. After the mixing, concrete enters a dormant period of approximately 2 hours. During this interval, concrete is still plastic, and it can be placed, compacted, and finished. During this period, concrete should be protected from rainwater. The falling of rain following the completion of surface preparation may not cause serious damage. The surface of the concrete shall not be worked again when the rainfall. Any finishing of the surface will mix the rainwater with concrete, this will increase the water-cement ratio. Increasing w/c will reduce concrete strength, and it will increase permeability. Increasing permeability will reduce concrete durability, and concrete will be vulnerable to the ingress of corrosive chemicals such as chloride.

Concrete will harden at a higher rate in hot weather. While the hardening process is decreasing by the decreasing of the surrounding temperature. If the concrete stiffens and it is in, or past stage 3, as shown in figure no:2, the falling of rain is no longer a concern. Actually, the falling of rain at this stage will help in the curing of concrete. 

Figure 2

The evaluating of damaged concrete due to rainfall should start after the curing period is completed. The evaluating can be performed by visual inspection. The visual inspection should be performed by competent persons to determine the damaged area. If they fail to agree or they are uncertain about the location and extent of the damage. Schmidt hammer can be used to determine the location of damaged concrete. A more accurate approach is by exerting representative cores from slab concrete and examine them according to ASTM C856, “Standard Practice for Petrographic Examination of Hardened Concrete.” examination will reveal the surface quality and the depth of any rainfall affected zone.

Once the damaged area is located, the repairing process will start. The damaged area should be removed. The removal of the damaged areas may perform by shot-blasting, grinding, or hydro-blasting if the surface of the concrete is soft (depth approximately 3 mm). Damaged concrete surface shall be removed totally until the sound concrete is exposed. Then the surface will be cleaned and roughened to ensure the maximum bonding between the concrete surface and repairing materials. for thick slabs with the large damaged areas, a competent contractor shall carry out the repairing work and restore the concrete to the intended strength and durability.


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