Rain falling on top of freshly laid concrete can damage the surface and compromise a level and floated finish. Even worse, if too much extra water works its way into the concrete mix, it can alter the carefully proportioned water-cement ratio, disrupt the curing process and result in weak concrete overall – every concrete contractor’s nightmare.
For this reason, it’s important when pouring concrete in the rain to never lay the concrete onto wet surfaces or trenches filled with water if there’s a risk that the water may mix into the concrete.
The concrete mixture can often displace the rain water, pushing it upwards and out of the cavity into rainwater pools. Don’t try to ‘soak up’ the rain water with the concrete or dry cement, or work it into a freshly laid surface as this excess water will compromise the concrete mix and diminish the top layer.
How rainwater will affect concrete depends on the stage in the curing and pouring process that was reached before the rain set in. If the concrete is still fresh (around 2-4 hours after pouring), it’s important to cover the surface to protect it.
How long does concrete take to dry after rain?
Once the concrete has been finished (between 4-8 hours after pouring) and has set hard enough for walking on, the effects of rain should be minimal. The curing process is a chemical reaction, with ready mix concrete taking up to 28 days to fully cure. However, as long as you let your fresh concrete cure under waterproof covers with good drainage for a minimum of 4-8 hours, this should be sufficient to prevent damage.