The simplest answer is not to pour concrete when it is raining. Water is added in the right proportion to make the concrete; we don’t want any more water in it. Particularly, we do not want rain on the surface of our newly poured general concrete project. Rainwater will have a tendency to mix with the concrete on the surface of the pour and weaken the surface.
Preferably, we deal with rain before pouring concrete. We might postpone the pour if possible. This postponement will be done by the concrete contractor, if we have one. The concrete pumping company (if involved) must confirm. At least we can plan what we will do if it does rain while we are pouring. We can prepare plastic sheets or tarps to cover the concrete after it has been placed and rodded off. This assumes that we are planning on only a shower of rain, not a continuous, hard rain. The intent is to constantly protect the placed concrete from rain.
Rain Before Placing Concrete
If rain comes before our pouring hours, we can prepare by covering the general concrete area with tarps or plastic. We want the area to be poured to be free from standing water. Damp is OK; saturated is not OK. Also, we must pay attention to lower areas to be poured, like footings and foundation areas. These should not have any water in them. Standing water will tend to mix with the concrete and weaken it.
Rain While Pouring Concrete
If a rainstorm should come up while we are pouring concrete, we may feel we have no choice but to continue. If we have a concrete contractor, we can ask him/her if additional concrete is already on the way. If a concrete pumping company is involved, that can affect the plans. If it does rain while pouring, we can place the concrete, and rod it off. Then cover the placed concrete with plastic to protect it from the rain. We do not want to work any rainwater into the surface of the concrete. If rainwater is left standing on the surface, we can push the water off the slab with a float, and then do our finishing work, assuming it has ceased to rain.
Sometimes it is possible to protect fresh concrete from rain with a temporary roof of plastic. That is often ideal, as it allows us to work while it rains.
Rain After Placing Concrete
If it should rain after we have placed concrete and finished it, what we do depends on how set up the surface is. If the general concrete project has had a chance to set up for several hours after finishing, the rain may not adversely affect the concrete. The question is, has rain become absorbed into the surface? If it has, it will affect the strength of the surface and cause dusting, scaling, and cracking. In this case, there must be remedial action to the surface of the concrete. In general, the top of the concrete must be ground down, and a layer of repair mortar or other filler material laid over the surface.