The answer is yes—concrete can be mixed in the rain. However, concrete must be allowed to cure for at least 4 to 8 hours before it becomes sufficiently strong and hard. Concrete that is still fresh during a rainstorm must be covered with plastic to avoid wetness. Fresh concrete cannot withstand rain if the pouring is finished before 2-4 hours. Although concrete has water as a vital ingredient, water in the form of a driving rain can damage the structure. It can weaken freshly mixed concrete after it has been put, increasing the propensity for dusting and scaling to develop.
