Where did the 28-day rule come from?

The 28-day period for concrete to fully cure originates from the civil engineering aspect of construction is the consensus standard in the construction industry. The four-week waiting period is often referenced by the protective coatings industry allows enough time for Portland cement concrete to achieve its full strength and hydration, thus avoiding latent water that could negatively impact the application and performance of fluid-applied coatings.

What are some options if you can’t wait the full 28 days?

If it’s not possible to wait the full 28-day period, it’s recommended to ensure the concrete’s moisture content is within the tolerances of the protective lining. If there is excess moisture, there are primer systems that can be applied to the concrete that can help withstand either the high alkalinity or the higher moisture vapor transmission levels. Some coatings are designed to tolerate higher moisture content, so as long as the moisture content falls within the coating manufacturer’s published information, then we can coat before 28 days.

It should be noted that, as concrete cures, it shrinks, and some random shrinkage cracks could occur that transfer up through the coating.

It is important to test different methods to determine the best possible outcome for your moisture. There are three different test methods to determine if moisture is present. If it’s a slab on grade, then we can use the plastic sheet test or calcium chloride, which provides a quantifiable result. The in-situ probe that measures relative humidity is also commonly used on horizontal surfaces. For vertical surfaces, we’re limited to just two of those methods: plastic sheets and in-situ probes.

Need more assistance?

If you need help evaluating your Portland cement concrete before applying a coating system, contact your local coatings representative. They can help you determine which coating would be ideal for your project’s given concrete condition.