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The Builder's Guide

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How does rain affect freshly poured concrete?

Q We've recently had our driveway concreted by a building contractor, while they were smoothing off the concrete, rain began to fall and it rained for several minutes, we covered what we could but the concrete still got wet. The contractor were able to finish (floating and edging), but I am really concerned as to how does rain affect freshly poured concrete and will it cause problems to the driveway in the future? T Holmes, St. Patricks

 

A Rainfall during placement of concrete flatwork can present challenges to achieving a quality concrete, potential outcomes range from no damage to a weakened nondurable surface, only time will tell at which end of the range your situation will fall.

 

Best case and worst case scenario follow: Best case: The concrete is protected as much as possible from the falling rain. After the rain has stopped, the water that has fallen on the surface is allowed to evaporate just as bleed water from the original concrete mixture must be allowed to evaporate prior to proceeding with finishing operations. To substantially change the water-cement ratio of the concrete at the surface of the slab, energy must be added to the system, typically in the form of towelling passes with excess water on the concrete surface. If the water is allowed to evaporate, the water-cement ratio remains reasonably low, and since the ratio governs the strength of the concrete there is no substantial damage to the finished surface.

 

In extreme cases it is not uncommon to physically remove excess water from the slab surface by dragging a garden hose or a broom across the concrete surface to lower the volume of water that must evaporate. With proper timing and process, the durability of the concrete is not affected.

 

Worst case: the concrete is not protected from the rain; the water is not allowed to evaporate from the slab surface; and multiple passes of the floats and trowels used to finish the surface are made with the surface moisture in place. The energy supplied by the finishing operations mixes the excess water into the slab surface creating a high water-cement ratio in the near surface of the concrete reducing its strength and thus its durability.

 

The damage to the concrete surface is readily apparent since the texture of the surface is easily damaged or removed after the initial curing period. (Driveway is dusty after 14 days of curing there is likely to be a problem with the concrete.) If the surface strength is only slightly affected, the long term durability of the concrete may be reduced, however, the concrete strength and durability below the surface would not be affected.