Frequently Asked Questions
Cement & Concrete
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fresh concrete
Q: How does rain affect fresh concrete?
A contractor placed a concrete driveway for me last night. Before
they could start the finishing work, it rained for several minutes.
We covered everything we could but the concrete still got quite
wet. In the end, they were able to finish (floating and edging),
but I am really concerned how this will hold up long term.
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 specific
situation will fall. Descriptions of a best case scenario and a
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 (w/c) of the concrete at the surface
of the slab, energy must be added to the system, typically in the
form of troweling passes with excess water on the concrete surface.
If the water is allowed to evaporate, the w/c remains reasonably
low, and since w/c 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 w/c ratio in the near surface of the concrete reducing
its strength and thus its durability. In the worst situation, 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. (If the surface is dusty after 14 days of curing there a
likely to be a problem.) If the surface strength is only slightly
affected, the long term durability of the concrete may be reduced
as evidenced by a general loss of the surface mortar (scaling) after
the concrete has been through a winter season of freezing and thawing
cycles; however, the concrete strength and durability below the
surface would not be affected.
In most cases concrete is warranted for one year, which will allow
you to assess the potential durability of the concrete surface,
and in instances similar to this one many contractors are willing
to extend the warranty for an additional period of time (an extra
year or two) to settle the doubt.
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