Construction schedules often fail to allow for proper drying of a concrete slab. Here’s why this occurs and what you can do about it.
Many have seen what happens when a concrete slab has not had the proper moisture measurement or drying time—flooring failure. And more often than not, this is due to lack of understanding, communication, or unrealistic (hasty) scheduling.
An informed, responsible contractor will want to protect the interests of the client. This means helping the client understand the necessity of a realistic construction schedule to allow proper drying time for the concrete slab. Otherwise, the project may be facing:
- Ruined flooring
- Costly reinstallation in an occupied space
- Interior affected from added moisture that escapes from the concrete slab, which can lead to serious health issues
- Expensive legal battles (who’s to blame for construction failure?)
Don’t let this happen to your project and your clients. Many a cautionary tale come from contractors who push for premature flooring installation and lack understanding about the importance of a properly dried concrete slab. For instance:
- In aspiring to please the client, a contractor may aim to complete a project by a certain day—whether the entire building envelope is properly prepared or not.
- This leads to a very unhappy client later on.
- In efforts to cut costs, a contractor may rush completion of a project.
- The cost of flooring failure or excess interior moisture can be much more costly. In fact, it can cost up to $35/sq ft to correct these types of problems.
First Cure the Concrete
Before the concrete drying process can begin, the slab must be cured. Water curing maintains a satisfactory moisture content, allowing the cementitious material to best react to the hydration process. If the concrete isn’t strengthened this way, the surface can be ruined and the odds of future problems with the slab are greatly increased. This helps establish compressive strength and overall durability.
During the first few days of curing, the goal is to keep the concrete saturated as much as possible in order to achieve 70 percent hydration. Fresh concrete needs to be kept moist for a minimum of 5 days.
While the slab is curing, different methods are used to slow down the evaporation process. The wet concrete is covered with:
- Waterproof paper
- Polyethylene sheeting
Many consider wet curing to be superior to other concrete curing methods as the results yield a less permeable slab than when a wax-based or resin-based curing agent is used.
Effective Drying of the Slab
The ability to achieve the optimum length of drying time begins before the slab is even poured. These steps must be considered:
- Incorporating adequate drainage around and under the building
- Use of an adequate capillary break (moisture barrier)
- Installation of an effective vapor retarder underneath the slab
- Obtaining a well-engineered concrete mix
The amount of time required for a concrete slab to properly dry varies, but the ideal drying time is generally one month for every one inch of concrete. Additional factors, however, can indeed affect drying time:
- Ratio of water used in the concrete mix
- Type of aggregate used in the concrete
- Presence of a vapor retarder under the slab
- Temperature and humidity at the site
- Wind and air blowers
- Dehumidification equipment specifically designed for construction sites and for speeding up the drying time of concrete slabs
Ensuring proper moisture levels
To make sure your concrete slab has equilibrated and dried to the appropriate moisture level for flooring installation, adequate testing of the slab is necessary. There are several ways to measure concrete slab moisture, but the relative humidity method has proven to be more reliable.
Find out more about relative humidity meters, and the fastest, easiest relative humidity sensor available.