What are the Effects of Rain on Freshly Placed Concrete?

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The sudden outpouring of rain on freshly placed concrete can give rise to unexpected challenges. The severity of the damage is dependent on the extent of surface protection and the curing procedure when water sets in. If deadlines permit, contractors should consider the postponement of concrete pouring when heavy rains are expected.

However, if unforeseeable rains occur, contractors and engineers should recognize and estimate the extent of concrete damage. While rainfall can cause severe damage to freshly placed concrete, hardened concrete may benefit from little rainwater on its surface.

Rain can influence the surface appearance of the concrete and its internal strength. It can cause the concrete surface to become soft, which reduces the abrasion resistance and strength of concrete. At the same time, it increases the tendency for dusting and cracking to develop. 

What are the Effects of Rain on Freshly Placed Concrete?

1. If rain pounds on freshly placed concrete, the result can range from detrimental damage to a weakening of the concrete surface.
2. If too much rain falls on concrete during its placement, it alters the concrete mixture proportions and, subsequently, influences concrete consistency.
3. Rainfall during concrete pouring leads to the mixing of rainwater with concrete. This leads to concrete bleeding, laitance of concrete, and weakening of the overall surface of the concrete.
4. When the concrete surface is not appropriately hardened, rainfall may wash out cement from its surface and expose aggregates, see Figure-2. This may not affect the structure's safety, but it can lead to uneven surface and color streaking if the concrete is integrally colored.
5. If the concrete surface is washed out, it wears off rapidly and attracts moisture. As a result, it may suffer from freezing and thawing cycles, and subsequently experience cracking. So, the loss of surface strength leads to more problems in the future.
6. Another effect of rainfall on freshly placed concrete is surface scaling which is peeling off of near-surface concrete. This occurs due to weakening of the concrete surface, see Figure-3.
7. If the rain continues for several days, water accumulation on a concrete surface can slow down the curing process. As a result, the internal bond between mixture particles will be influenced, and concrete strength will be weaker than the designated strength.
8. If the rain begins after 2-4 hours from concrete placement, the surface needs to be covered; otherwise, the concrete surface will suffer damages.
9. If rains pound on concrete 12 hours after placement, the likeliness of any damage is negligible.
10. Rainwater is composed of various chemicals from dissolved particulate materials in the atmosphere. So, the composition of rain is different from one place to another based on human activities, factories, and local climate conditions. If such water comes in contact with the concrete, it may activate chemical processes like carbonation, acid, and sulfate reactions that all contribute to concrete deterioration. 
11. Inspectors can check the edges of freshly placed concrete for possible damages.
12. Uneven surfaces, lumps, and bumps in concrete are signs of damages due to rain.
13. A scratch test is an excellent way to determine whether a concrete surface is adequately hard or not.
14. If the concrete surface feels rough and powdery, and a little brittle, contractors can remove the weak concrete surface and resurface it again.  

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