Pouring Concrete in Rain

Pouring concrete in the rain is not only miserable, but it can also lead to problems with the final surface strength. Photo by: ESTELLE R/Shutterstock.

Water is a key component of all concrete, mixing with cement and causing it to hydrate and gain strength. But after freshly mixed concrete has been placed, water in the form of a driving rain can do much more harm than good. Pouring concrete in the rain can compromise its strength, increasing the tendency for dusting and scaling to develop.

Once the damage is done, it can be hard to rectify and will often ruin the appearance of the finished surface. Don’t let it rain on your parade. Here are some tips for preventing and troubleshooting rain-damaged concrete.

Advice for Pouring Concrete in Rain

Despite modern weather forecasting technology and radar maps, an unexpected rain shower can develop without warning, especially during the wettest months of the year (see these guidelines for placing concrete by region and season).

If rain is in the forecast, you should postpone a large concrete pour until conditions improve. Even if rain isn’t predicted, you should always be prepared to cover the concrete work with a tarp or plastic sheeting just in case. Make sure to seal the edges so rainwater can’t seep underneath.

Also scope out the jobsite before the project begins, looking for the presence and location of gutters and downspouts. Rainwater gushing from downspouts or cascading over the edge of a roof without gutters will carve a trench into wet concrete if the slab is directly beneath the overflow.

Heavy rain after a concrete pour can be concerning if the concrete hasn’t been finished and had enough time for the intial curing process. Photo by: Mr.Note19/Shutterstock

Tips for Handling Rain After Pouring Concrete

Here’s how to react if it does pour down on your freshly placed concrete:

• Don’t work the rainwater into the concrete surface, this is the biggest mistake you can make.
• Don’t broadcast dry cement onto the concrete to soak up surface water. This will impair the finish and further weaken the top layer.
• Instead, once the rain passes, use a float to push the water off the edge of the slab before you start finishing.

How Long Does Concrete Take to Dry Before Rain?

Even if it starts raining after a concrete pour, the potential for damage may not be that serious. If you had time to complete the finishing process and the concrete has stiffened (typically 4 to 8 hours after mixing), rainwater may cause little if any damage.

In fact, once concrete sets, water on the surface is actually beneficial because it aids in hydration and curing. A simple scratch test, using a screwdriver or Mohs concrete surface hardness scratch test kit, can be performed to assess the integrity of the surface and determine if the rain had any impact.

Solutions for Rain-Damaged Concrete

Surface scaling, or spalling, as seen on this stamped concrete, can be caused by rain weakening the surface strength of the concrete during or shortly after placement.

If the worst happens and the sky opens up on your fresh-from-the-truck concrete, it’s a good bet that the rain will wash some of the cement out of the concrete at the surface.

This is likely to lead to problems such as:

• Dusting
• Surface scaling
• Craze cracking (easily aggravated by freeze-thaw cycles)

If the concrete is integrally colored, rain on the day of the pour can also wash out some of the color and cause streaking.

Short of ripping out the concrete and starting over, the best remedy is to remove the weak surface layer by grinding and then resurface the concrete with an overlay, after making sure that the underlying concrete is structurally sound.

Other Problems Caused by Rain

Even before you place a concrete slab, rain can have a detrimental impact if it oversaturates the subgrade and causes ponding. That’s because the additional water can be absorbed by the fresh concrete and change the water-cement ratio.

If heavy rain is in the forecast a day or two before the pour, cover the ground with plastic sheeting to prevent it from becoming saturated. Learn more about subgrades and subbases for concrete slabs.

Rain can also wreak havoc on freshly sealed concrete, causing the sealer to bubble and blister. Be sure to apply the sealer when it will remain rain-free for 24 hours after sealing. (See Rain Blisters on Freshly Placed Sealer.)