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, this can result in weak concrete overall.
For this reason, it’s important 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. It is often possible for the concrete to displace the water, pushing it upwards and out of the cavity. Don’t try to ‘soak up’ the water with the concrete, or work it into a freshly laid surface as this excess water will compromise the concrete mix.
How rainwater will affect concrete depends on the stage in the curing 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.
However, 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.