Pouring concrete in the rain can reduce its strength, increasing the likelihood of dusting and scaling. Once the damage has been done, it can be difficult to repair and can frequently harm the aesthetic of the final surface. Don't let that ruin your day. The best option is to wait for clear weather before you pour that concrete.
However, if you are going to pour concrete anyway, there are some things you can do to minimize the effects of the rain on your project. First, use a plastic tarp to cover any materials you don't want wetted during the pouring process. This will help prevent any stains from being transferred to the new floor. Second, choose a concrete color that will not show moisture-induced discoloration such as white or light blue. Finally, add several drops of black oxide dye to each gallon of concrete to create an even darker colored floor that will hide scuff marks and other defects better.
The most important thing to remember is that you should never pour concrete on any soil that is more than 6 months old because it won't hold enough water to properly cure the cement. If necessary, add a few bags of premium grade sand to ensure that the finished product has the desired density.
Concrete is a very durable material that can last for hundreds of years if properly treated.
If rain falls when the concrete is still new (approximately 2-4 hours after mixing), the surface should be covered. Rainwater may not cause harm if the finishing procedure was recently finished, as long as it is not worked into the surface and the slab is left untouched. However, if you try to wash it off, this could lead to cracking.
If rain falls on concrete that has been allowed to harden (6-24 hours after mixing) or mature (4 weeks or more after mixing), water may penetrate deeper into the material, causing it to expand and possibly crack. The older the concrete, the greater the risk of damage. Concrete that gets wet and isn't dried within 24 hours can become moldy, especially in warm climates. The mold will grow harder than normal concrete, which could cause it to break down under pressure.
Concrete that gets wet and isn't dried within a day should also be painted to protect against growth of algae or other organisms. This prevents them from forming a layer of organic material that can trap moisture underneath, causing more serious problems for wood framing or internal support beams.
Concrete that gets wet and isn't dried within a week needs additional protection against cracks developing due to swelling. For dry climates, add 1/8 inch of gravel to each fresh dump of concrete for optimum air circulation and reduced heat buildup.
If the concrete is still new (about 2-4 hours after pouring), it is critical to preserve the surface. However, the impacts of rain should be negligible until the concrete has been poured (between 4–8 hours after pouring) and has hardened sufficiently to walk on. At that point, water will be absorbed into the concrete causing voids that can lead to corrosion if not done properly.
Concrete continues to harden as it ages. If it is not exposed to air, it will eventually become as hard as stone. However, if it is not also subject to some form of stress, such as a tensile force, it will fail. Concrete will fail when its tensile strength is reached; this occurs when enough of the material has been removed that the remaining mass is under too much strain. Once this happens, more serious problems may arise, including spalling and cracking of the surface. These issues are generally associated with old or degraded concrete.
As mentioned, water is the key ingredient in concrete's recipe. Without it, there is no reaction that forms a solid substance. Concrete needs water in order to set up shop as an expansive material. As the mixture cures, the cement particles attract each other and form a mesh like structure that allows water to penetrate the material. This process helps the concrete develop its strength over time.
If the concrete has solidified to the point where it can be grooved and ground (generally 4–8 hours after mixing), rain damage is usually no longer a worry. However, if it starts to re-solidify, cover it again until it reaches an acceptable working condition.
Concrete that gets wet will eventually dry out. If you must walk on it within 24 hours, add more water to thin it out. But if you have time, let it cure completely before walking on it.
Curing concrete requires time to allow the cement paste to develop its full strength. The amount of time required depends on the temperature of the air and the humidity level. Curing takes place primarily through evaporation of moisture from the concrete surface but also may occur due to heat transfer from the surrounding environment. Concrete that has not cured enough to be walked on is called "green" concrete. This page describes how long green concrete needs to cure before it can be walked on.
The key to avoiding problems with fresh concrete is to allow it adequate time to cure before any traffic passes over or steps in it. If you are waiting on other work to be completed before you use the concrete, wait no longer than two days since the last water was added. Add more water if necessary to maintain a workable consistency.