The following is from an internal memo from Sarnafil to inside Sales Staff and Manufacturer Reps. I have removed some of the dryer information but if you are in construction and a “data driven guy” you will want to read the complete report. You can Download Concrete Decks Drying Times
Over the years, the construction industry has been aware of moisture issues due to freshly poured concrete as well as the ability of concrete to hold and absorb great amounts of water. We are referring to light weight concrete roofs and concrete structures that need to be waterproofed. Over time water may migrate into the roof system, saturating the insulation and cover boards or causing adhered systems to become dis-bonded and potentially cause corrosion to metal components.
Many papers and articles have been written discussing the issues of moisture and concrete. These papers identify some of the reasons and issues related to the moisture in concrete, and why they appear to be more prevalent than in the past, such as eliminating vapor retarders, especially ones that are adhered to the concrete deck and the practice of keeping the concrete forms in place, which are typically metal pans.
The most common ways excess water in concrete is generated includes;
- Mixing and pouring new concrete decks/slabs
- Interior finish work, including new concrete pours, water based construction materials including paint, plaster, and drywall application among others and heating the interior with propane or oil burners
- Concrete decks that are exposed to standing water which may come from various sources including exposure to long term leakage into existing roofs, rain or snow and other sources
Risks of Fresh Concrete Decks and Concrete Drying Times
The main issue our industry has regarding water and moisture in concrete is there is not a good, practical, consistent and viable test to determine the moisture content or relative humidity of a concrete roof deck.
Concrete is a combination of cement, aggregate (fine and coarse) and water, which typically has about 10–15% cement, 60–75 % aggregate and 15–20 % water. Studies have shown that there may be between 0.9 to 2.6 quarts (0.85 to 2.5 l)of excess water per square foot of concrete surface present in a one month old, 6 inch thick concrete roof deck. This does not include possible water from rain or snow or water from the curing process. This excess water may migrate into a roof system after the concrete has reached sufficient strength or cure which typically is 28 days. With this large amount of free water available it must be noted that cure time (28 days) does not mean the concrete is dry enough to cover.
In addition to normal weight structural concrete (NWSC), there is more lightweight structural concrete (LWSC) being used. The differences between the two structural concretes are the “in place density” and the type of aggregate used. (see report)
WATER ABSORBED INTO CONCRETE
Water, sitting on the concrete deck, as precipitation on new decks, or through long term leakage into existing systems being re-roofed, will typically be absorbed into the concrete deck. The top surface may appear dry, giving a false sense that a roof system can be installed. After the installation of the roof membrane, which will act as a vapor retarder, the moisture within the concrete will migrate into the roof system. The rate of the water migration will depend on the local climate. Often the migration of the water out of the concrete will be greater than the moisture vapor passing through the roof membrane. The accumulation of water may affect moisture sensitive products such as adhesives, paper faced insulation boards and gypsum boards.
DETERMINING MOISTURE CONTENT
The main issue our industry has regarding water and moisture in concrete is there is not a good, practical, consistent and viable test to determine the moisture content or relative humidity of a concrete roof deck. The plastic film test (ASTM D 4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method) is no longer considered a good valid test, especially with LWSC. This is also true for the calcium Chloride test (ASTM F 1869). Independent testing has shown these test methods often give misleading results.
The flooring industry, which has concerns with moisture in concrete, uses a moisture probe test, ASTM F 2170 Standard Test Method for Determining Humidity in Concrete Floor Slab using In-Situ Probes to determine if the moisture in the concrete slab has reached a level where the flooring material can be adhered. This test uses probes that are set into cores of the concrete slab and sealed for 72 hours. This test works relatively well for flooring due to the more consistent indoor temperatures and humidity. For concrete slabs that are exposed to the weather, such as roof decks, the temperature and humidity will vary, which will affect the readings from the probes. The conditioning section for ASTM 2170 states; “9.1 Concrete floor slabs shall be at service temperature and the occupied air space above the floor slab shall be at service temperature and service relative humidity for at least 48 h before making relative humidity measurements in the concrete slab.”
CONCLUSION
Moisture and concrete decks will continue to be an issue for the roofing industry, based on current practices of not including vapor barriers and leaving the metal pan/forms in place. In some sense we may see more issues as there are energy savings realized when the LWSC is used (reduced transportation costs, handling and weight) which may be used to accumulate some LEED points. As noted above, there is currently no acceptable test method to determine the moisture content or relative humidity of a concrete deck that is exposed to the weather.
The 28 day “cure” time commonly referenced with structural concrete is for testing the design compressive strength of the concrete and has no correlation with the moisture content or drying time.
The Portland Cement Association has testing that shows it takes up to 3 months to reach a 75% relative humidity level with NWSC and twice as long with LWSC. Their test was done in a laboratory setting, constant temperature and humidity levels and all sides of the concrete exposed, and without any additional moisture, which often occurs in the field due to precipitation.
Although surface dryness can generally easily be determined the remaining free moisture that is within the concrete slab cannot readily be assessed. The decision of when a concrete deck may be roofed should include the project designer, general contractor, the concrete contractor and suppliers as they will have more knowledge of the concrete formulation, and moisture release rates.
This design and management group should communicate with the roofing contractor when they can proceed. The designer of projects that include concrete decks, should strongly consider including in the roofing specification an adequate vapor retarder on the top side of the deck to prevent any water that may be retained in the concrete from migrating into the roofing system over time.