Pouring Concrete in Rain

Construction never stops, and concreting in the rainy season is inevitable. What happens when it rains on freshly poured concrete, and what could you do to minimize the potential damage?

Let’s find out the effects of rain on concrete.

You can pour concrete in light rain because the curing process is a chemical reaction and not drying. However, if excessive rainwater enters into the concrete mix, it can change the water to cement ratio and compromise the strength of concrete.

Will Concrete Set in the Rain?
A hydration reaction occurs when cement mixes with water, creating a crystal structure. This structure is rigid, and this is what causes concrete to set. If the rain falling on fresh concrete isn’t excessive, washing away the concrete itself, it will set.
In fact, if light rain falls on sufficiently hard concrete, it might even aid the hydration reaction in providing favorable curing conditions.

Can You Pour Concrete in Heavy Rain?
If the job site is not covered, it’s not advisable to pour concrete in heavy rain since the rainwater will mix with the fresh concrete or wash away some of it. Both of these would damage the final product.

It is best to wait until the rain subsides before commencing with the pour. Once the rain subsides, remove the water from the job site before pouring the concrete. Meanwhile, do what you can to protect the site and make it easy to remove the water.
It is essential to keep water away from the pour by protecting the job site. To this end, ensure that any cavities or hollows on the pour are free of water and cover them to keep the water out.

If you plan on pouring concrete in an area prone to rain, plan ahead and be prepared for any eventuality. To this end, check the weather forecast. When rain is forecast, equip the site with plastic sheeting and other waterproof materials to cover the concrete when needed. Also, ensure that the substrate is dry.

Then there is also visibility issues during a heavy downpour. It’s difficult to see where the rebar or wire mesh is placed when it’s raining heavily. If heavy rain is imminent, it would be best to wait until the weather improves. Alternatively, you could make alterations to the site so that you can pour the concrete in an enclosed, roofed area. This will protect it from the weather and improve the quality of your finish.

Don’t attempt to pour concrete on wet surfaces or in trenches filled with water. Here, the water and impurities could mix with the concrete, altering the water to cement ratio and incorporating potentially harmful impurities into the mix design. This would weaken the concrete and cause significant damage.

If you must pour concrete into a submerged area, following underwater concreting best practices is best. This is a complicated and expensive undertaking. Usually, it would probably be more economical to drain the area before concreting and allow water to reenter after the concrete has gained sufficient compressive strength.

How Soon Can You Pour Concrete After Rain?
This depends on the job site. You should only pour concrete on a dry substrate, so the sooner you can dry the substrate, the sooner you can pour the concrete. When concreting in the rainy season, it is best to protect the substrate from the rain using plastic sheeting. This makes it easier to get the surface dry before pouring concrete.

Rain After Concrete Pour

What Happens If It Rains After Pouring Concrete?
Rain can cause untold damage to fresh concrete. When rain falls on freshly laid concrete, the surface could suffer damage in various ways. If it’s heavy rain, the drops themselves could leave indentations in the concrete. Furthermore, the excess water accumulating on the freshly poured concrete would alter the cement:water ratio of the surface layer, similar to having excess bleed water on the surface. This would weaken the concrete and cause cracking or flaking.

Then, there’s the physical damage of water run-off. Excessive rainwater will likely run off the concrete, washing cement paste, sand, and, in extreme cases, even stone with it. The concrete may have to be broken up and the pour redone in this case.
Suppose rain falls on freshly leveled and floated concrete. In that case, all this finishing work could be undone, along with the potential damage outlined above.

Cover the surface to protect it from possible damage if the rain starts within the first two hours after pouring the concrete. Here, plastic sheeting would typically suffice. If you can’t cover the surface before the rain starts, wait until the rain subsides, then remove the excess water from the surface. Here, you could use something simple like a garden hose. Slide it across the surface, pulling the water with it. Note that this will probably only work on concrete that has hardened slightly, not on freshly poured concrete that hasn’t been leveled. Heavy rain will also probably cause damage to uncovered fresh pours.

If the rain starts when the concrete has had sufficient time to set, the effect will be minimal. Here, the concrete should be hard enough to walk on, a point usually reached within four to eight hours after the pour. That is unless you’re concreting in extreme temperatures. The setting time will be delayed in cold weather, while hot weather concreting will accelerate the setting time.

How Long Does Concrete Need to Dry Before Rain?
If the rain starts four to eight hours after the pour, the concrete probably had enough time to gain compressive strength and withstand light rain. Here, the concrete would be hard enough to walk on.

How Do I Know If Rain Damaged the Concrete?
Once the rain stops, inspect the concrete’s surface for any signs of damage. You will clearly see obvious defects here, while some damage may be hidden from the eye. To test abrasion resistance, scratch a mark on the surface using a screwdriver or similar tool. Repeat this on a portion of concrete known to be of good quality. If the results are similar, the concrete is probably fine.
Minor damage can be repaired by pouring extra concrete onto the already laid structure if thick enough. If it’s a thin slab, it will probably be best to remove the concrete and redo the pour.

Whatever you do, don’t work the rainwater into the fresh concrete since this will alter the water: cement ratio, weakening the surface layer. Also, don’t cast cement onto a wet surface since this will lead to cracking and flaking.

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