You may not have intentionally chosen to pour on a day forecast to rain but now it is raining and you need to know what to do to save the pour. The last thing that you want to do is to have to spend the time and extra money to repair a concrete finish that was ruined by the rain. There are some simple steps you can follow that will help you pour concrete in the rain with a minimum of damage to the final product.
Review your project plans and know what is allowable for breaking a pour should the rain become a downpour. "Breaking a pour" means to stop a pour before it is complete so you can finish it on another day. There will be specific guidelines in the front of your plans (in the General Instructions section) that will tell you how far away you must be from any corners or openings to stop a pour. Devise a plan of several different points in the pour where you can stop it before the concrete even arrives. While you can pour in rain, something akin to a tropical downpour requires that you cancel the pour.
Begin your pour. As you fill your forms to grade and level the concrete with your screed boards, cover the form with a concrete insulating blanket immediately. Try to cover the pour every ten feet. The insulating blanket will trap the heat that will build as the concrete cures and keep the rain from saturating the top layer.
Float finish your concrete with a magnesium trowel. Hold the trowel at a 35-degree angle to the concrete and move it over the surface in broad sweeping arcs. This will bring excess water to the surface of the concrete, and you then sweep it off with the movement of your trowel. Only uncover small sections of the pour at a time and then recover them as you finish floating the section.
Monitor the curing rate of the concrete before attempting to do the final finish (broom or float for example). The curing rate will be very different and slower because of the extra moisture in the air. Apply your final finish in small sections, uncovering the pour and then covering it again when you are done. A good way to think of how to pour concrete in the rain is to approach it in the same way that you would pour concrete in extreme cold weather. The concrete insulating blankets are what will allow your pour to be completed.
Delay your pour if at all possible. It can save you time and money in the long run to cancel a pour due to rain and combine it with a later scheduled pour. Having to go back and resurface a finished pour if the rain becomes too heavy for you to protect it with blankets and smooth floats can be a very costly venture.
Do not call for a lower slump (less water) when you pour in the rain thinking that the rain will provide enough moisture to adjust the concrete. Too low a slump will create a dry mix on the interior of your concrete, which will lead to it breaking down faster.