Unpredictable summer thunderstorms can wreak havoc on a concrete pour. Rainwater can cause a new concrete surface to become soft, which in turn decreases the abrasion resistance and strength of the concrete, while increasing the tendency for dusting and cracking to develop. The key to preventing damage to the concrete surface by a rainstorm is proper preparation and timing. Before a storm occurs, a protective enclosure can be built around the work site with wood and plastic sheeting. If you get caught without protection, once it starts raining, it’s best to wait, let the rain pass, and pull or push the surface water off the edge of the slab before completing finishing. Contractors should never work the rainwater into the freshly placed surface or broadcast dry cement on the wet surface in an attempt to soak up the water.
Just because it starts raining during or soon after a concrete pour, does not necessarily mean that your project is doomed. It all has to do with timing, and at what stage in the curing process the concrete is in. If the rain occurs when the concrete is fresh (about 2-4 hours after mixing), the surface should be protected from the rain. If the finishing process was recently completed, rainwater may not cause damage as long as it is not worked into the surface and the slab is left untouched. If the concrete has stiffened to the point where it is ready for grooving and grinding (typically 4-8 hours after mixing), damage due to rain is usually no longer a concern.
After a rainstorm, damage to the concrete should be assessed. A visual survey can be performed to note any obvious defects. A simple scratch test, using a screwdriver, can be performed to compare the relative surface scratch hardness of any areas in question to those slab sections known to be of good quality. A quantitative approach includes removing several core samples and checking them in a lab with an electronic microscope. (See our previous blog entry from February 17, 2012 about petrography)
If the surface quality of the concrete is found to be compromised, remedies are available. Isolated repairs can be made immediately after a storm by using the some of the same concrete that was used for the concrete placement itself. If small areas of a thin slab are damaged, it may be more economical to remove and replace full depth sections. If there are large areas in a thick slab with damage, a thin application of a repair mortar could be applied after the damaged concrete is removed.
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