Seamless flooring suppliers generally recommend that any new concrete be at least 28 days old before any application. The reason for this is that the 28-day period is an arbitrary concrete specimen age for testing the compressive strength of concrete. Specification writing authorities chose 28 days as the standard specimen age to establish consistency for testing procedures throughout the industry. That is why it is a “safe” number for flooring suppliers. However, such an arbitrary time frame is not relevant to all mix designs.
In general, after initial hardening, concrete begins to shrink as water not consumed by cement hydration leaves the system. This is known as drying shrinkage. Water above what is necessary to hydrate cement is added for proper workability and finishability. This water is called “water of convenience.” In general, the higher the additional water content, the higher the slab shrinkage potential.
Apart from minimizing the “water of convenience” in the mix, from the POV of a new epoxy or polyurethane topping or coating application, what do we see as being really important?
In our view, the minimum age of the concrete is determined by:
- Time to gain sufficient strength for adequate surface preparation by professional diamond grinding
- Time to have burned off the water required for hydration within the slab itself
- Time for any plastic settlement cracking to be completed
The issue of early application comes up a lot with compressed construction schedules.
In practice we recommend a fast strength development concrete mix that can exhibit compressive results similar to these:
- 30 MPa at 2 days
- 40 MPa at 4 days
- 45 MPa at 7 days
Concrete exhibiting this rate of strength development would be a good candidate for consideration for early topping application (5-7 days) with a moisture tolerant system like EpiMax 222. This product offers greater tolerance to remaining moisture content in the slab.