The cure of the properly finished concrete slab is another important factor that contributes to a successful floor installation.
There is a strong temptation to rush the cure of a new concrete slab to enable brick floor installation to begin sooner. The time saved by reducing the necessary cure time will directly and negatively affect the longevity of the acid brick floor. As concrete cures, it gains strength as the slab completes it’s hydration process. Excess water evaporates by migrating out of the concrete during this process. If the asphalt membrane is applied while there is still excess water present in the top surface of the concrete slab, the 350°F temperature of the liquid asphalt (at initial application) will boil this water, destroying any potential bond. In addition, all the excess water remaining in the slab has been sealed from escaping upward, which will continue to degrade the bond between the membrane and concrete.
Our recommendations for properly curing a new concrete slab are to cover the initially set concrete with polyethylene sheeting (wet cure) for 2 to 3 days to allow the concrete to achieve its desired strength. Following this initial curing period, fresh air needs to be circulated to keep humidity levels low and promote removal of excess water from the slab leaving you with a properly cured concrete floor.
There are several tests to determine if a new concrete floor is ready to receive a membrane, but the simplest is a version of the “rubber-mat test”. Several squares of 4′ x 4′ polyethylene sheeting are taped down to various locations on the new floor and left for 16 hours, ideally overnight. The next day, the areas under the squares are inspected for a darker shade than the surrounding areas or condensation on the underside of the sheets. The presence of either indicates that the slab is still too damp to receive the membrane system. Our experience has shown that most new concrete, cured using the above recommendations, will cure in 10 to 14 days, much less than the safest recommendation of a full 28-day cure.
If the standard cure time is unavailable, options such as specially-designed concrete mixes and polymer concrete subfloors can be used. The installer and material manufacturer should be consulted for proper recommendations. Usually, cure times can be reduced to days instead of weeks, if the additional cost is less important than the time gained.
