Nothing upsets concrete contractors quite like the appearance of unexpected storm clouds on the horizon. If there's even a slight chance of rain, most contractors will reschedule their pours for another day. If the weatherman didn't predict rain and you're in the middle of a pour when the drops start falling -- the focus suddenly shifts to damage control.
Slabs are at the greatest risk because rain can permanently damage the concrete. Foundation walls are at minimal risk because it's a simple process to tarp them. The threat to a slab depends on the consistency of the concrete, the intensity and duration of the rain, and what actions the workers take to protect it. Potential types of damage include:
- dusting: a powdery surface.
- spalling: surface flaking.
- pitting: dimples from heavy rain.
- and cracking: loss of slab integrity.
Stop the Pour
If you're in the middle of a slab pour when the rain starts, the best option is to stop, or "break," the pour at a spot where you can pick up later. For a driveway or sidewalk, this means quickly framing a stopping point with lumber, which is best located where you plan to put a control joint.
Tent the Pour
If possible, tent the pour with tarps to keep rain from mixing with the concrete. This is vital if you're in the early stages of a pour when the skies open. The integrity of the slab depends on keeping the water ratio in the concrete mix low. The more rain that mixes with the semi-solid concrete, the more likely it is that the slab will fail.
Cover Fresh Slabs
If you're lucky enough to finish floating and troweling the slab before it rains, but the concrete's still fresh, cover it with plastic sheeting. This can result in the plastic marking the surface of the slab, but it won't compromise the integrity of the concrete. After the slab cures to the point of being able to stand on it, you can grind away any surface ripples left by the plastic.
Fixing Rain Damage
Depending on the extent of the damage, the contractor could choose to wet-grind the surface of the slab, apply a "thin coat" topping or pour a new slab on top of the damaged one. In a worst-case scenario where a large amount of rain compromised the integrity of the slab and widespread cracking occurs, removing the damaged slab and pouring a new one might be the best option.