Moisture in Concrete - Part 1
June 26, 2019
This post focuses on the moisture phenomenon in concrete and the difference between lightweight and normal-weight structural concrete. Curing versus drying and the standards used to determine relative humidity levels are also addressed. Part II will address design recommendations and roof assembly selection.

Moisture in Newly Poured Structural Concrete Roof Decks
When investigating roofs for leaks, invariably, moisture is found beneath the roof membrane. However, the source of moisture is not always a roof leak. Newly poured structural concrete could be a contributor to the presence of moisture beneath a new or a replacement roof.

Concrete is a mixture of several components that reaches its optimum strength through a chemical reaction induced by water. Concrete needs water to allow for flowability and workability, however, water also has adverse effects. Once the concrete has cured, the remaining water is considered “free water”, or moisture which is no longer consumed by the curing process. Rain and snow add moisture to exposed concrete roof decks and further prolong the drying. As an example, a 4” slab of structural concrete contains as much as 200 gallons of free water per 1,000 square feet.

Structural Concrete Mix Ratio
The ratio for both normal-weight and lightweight structural concrete (LWSC) is generally the same:
• 10-15% cement
• 60-75% aggregate (fine and coarse)
• 15-20% water

The difference is in the aggregate; the lightweight aggregate is pre-saturated prior to mixing. The lightweight aggregate, which is made up of shale, slate slag, or clay, can absorb 5-25% of its mass. Normal-weight structural concrete, however, utilizes aggregates such as sand and stone, which are not as porous and do not need to be wetted before adding to the mix.

The popularity of LWSC is increasing due to:
• Lower building structural cost;
• Lesser density for reduced dead loads; and
• Environmental and sustainability claims.



Drying Time
To reach a 75% relative humidity for normal-weight structural concrete, it will take approximately three months. However, achieving the same 75% relative humidity for LWSC will take twice as long. According to the Portland Cement Association, the dry-down time for LWSC is more than normal-weight structural concrete.


Standards for Moisture Testing
For many years, the roofing industry has used a curing time of 28 days after the concrete is poured. However, there are test methods published by ASTM for determining the moisture content in concrete.

Qualitative tests, such as the plastic sheet test and electrical resistance and/or impedance are good indicators of the presence of moisture in a given area but are not as accurate as quantitative tests.

Quantitative tests, such as the moisture vapor emission rate test, surface humidity, or in-situ relative humidity tests demonstrate levels of moisture present in the concrete.

The recommended quantitative test is the in-situ relative humidity test (ASTM F2170), in which a sleeved probe is placed in a drilled hole in the concrete and left in place for 24 hours. After the 24 hours, an electronic reader is attached, and the information is read directly from the sensor. The relative humidity reading should be less than 80% at a depth of approximately 40% of the thickness of the slab.

The moisture values and test duration stated above have been slightly modified to better suit outdoor roof conditions. 

Site Considerations
The concrete pour schedule can affect moisture testing and provide inaccurate moisture values. Therefore, in phased construction, the field testing and the roofing installation should be aligned with the concrete pour schedule and ICRI-Certified Concrete Inspectors should be commissioned.

For in-depth information, the International Concrete Repair Institute (ICRI) offers various resources that can aid with the proper steps required for testing and evaluation. Stay tuned for part II for recommendations on design and the selection of an appropriate roofing assembly.


January 8, 2020
Air and Vapor Barriers for Roofs

In 2012, the International Energy Conservation Code (IECC) introduced the continuous air barrier requirement for new commercial construction. This meant that air and vapor barriers were now required for walls, and they must be tied to both the roofing assembly and the foundation. For years, many architects and designers only utilized an air and vapor barrier on the roof deck for high-humidity occupancies, such as swimming pools or food processing facilities. But the new requirement meant taking a hard look at the needs of all buildings and what a roof assembly could do for the building envelope. A single-ply membrane, as stated in the IECC and as tested utilizing the ASTM E2178 standard, qualifies as an air barrier and can satisfy the requirement for an air barrier on any given project. So why would you consider adding an additional air and vapor barrier to the roofing assembly? There are a couple of very simple reasons: Reason 1: Air Intrusion. While a properly installed roofing system will not allow air leakage (e.g., conditioned indoor air from exiting the building thermal envelope), it does allow air movement within the roof assembly. As the single-ply roof membrane is on the top of the assembly, indoor conditioned air can infiltrate into the roofing system and travel into the layers of insulation or cover boards. Why is this an issue? See Reason 2… Reason 2: Moisture Migration. Adding a deck-level air and vapor barrier is a great solution to prevent air intrusion and moisture migration. This also allows the wall air and vapor barrier to be tied together at the deck level, which allows the roof to be replaced more easily in the future. The contractor will not be modifying the continuous air barrier when re-roofing, as the roof is no longer that barrier. Carlisle SynTec provides many options for deck level air and vapor barriers: VapAir Seal MD for steel deck construction, direct to deck; VapAir Seal 725TR for Concrete Decks; VapAir Seal Flashing Foam for sealing around penetrations such as pipes; Go to the Air and Vapor Barriers Product Page on the Carlisle SynTec website for more information, specifications, and details. Contact Craig Tyler at [email protected] with further questions.

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December 18, 2019
Cold Weather Installation Tips Part 2 - Membranes and Insulation

As discussed in the previous SpecTopic, "Cold Weather Installation Tips Part 1 - Low-VOC Bonding Adhesives and Primers", specifying and handling of building envelope products is challenging during the colder winter months. Single-ply membranes and rigid insulation boards need some extra consideration, as they can be adversely affected by outside temperatures. For starters, all membranes will need time to "relax" after being unrolled from the original packaging; this applies to EPDM, TPO, PVC and KEE HP. It is also suggested that membrane widths be limited to a maximum of 10 feet for adhered roofing systems. Treat flashing products and accessories as you would adhesives and primers, by utilizing heated storage enclosures or "hot boxes". This practice is strongly recommended when ambient temperatures are expected to fall below 40°F for an extended period of time. In all applications, but especially in cold conditions, insulation and underlayments must be stored so that they are kept dry and protected from the elements. Insulations should be stored on a skid, covered with a breathable tarp, and weighted to prevent wind damage. In winter months, ice and frost can form on the membrane. This can be difficult to see and can remain on the roof well into the day, especially on white membranes. This can be especially hazardous when working near the edge of the roof. Additionally, frost on metal edges and copings can create a very slick surface and cause ladders to slide and shift. Never step onto a metal coping when it is frost- or snow-covered. So for your next cold weather specification for single-ply membranes and rigid board insulation, include some installation precautions as mentioned. Contact Craig Tyler at [email protected] with further questions.

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December 4, 2019
Cold Weather Installation Tips Part 1 – Low-VOC Bonding Adhesives and Primers

As temperatures fall and winter approaches, specifying and handling building envelope products – especially adhesives and primers – becomes a concern. Low-VOC adhesives and primers contain more water than standard adhesives and primers and can be adversely affected by outside temperatures. When specifying a low-VOC bonding adhesive or primer for a winter installation time frame, make sure to include information in the specification regarding cold weather application. This should include heated storage enclosures, or "hot boxes", for jobsite adhesive storage. This practice is strongly recommended when ambient temperatures are expected to fall below 40°F for an extended period of time. Adhesives and primers should be stored in locations where temperatures are between 60°F and 80°F. While working with adhesives, they should be rotated in hot boxes to ensure the temperature of the product stays above 40°F. Adhesives may appear gelled or lumpy when left for extended periods of time at temperatures below 40°F. If this occurs, return the material to room temperature for a minimum of 24 hours prior to use. In all applications, but especially in colder conditions, make sure you achieve the proper coverage rates for the adhesive or primer being used. Following coverage rates for Low-VOC adhesives and primers allows proper flash-off and reduces the trapped solvents which could lead to membrane blistering. For applications in very cold temperatures, Flexible FAST™ Adhesive may be necessary. Flexible FAST is a two-part polyurethane foam adhesive which is spray-applied and used with a fleece-backed single-ply membrane. The advantage of this system is that it can be sprayed using 15- or 50-gallon drums of Part A and Part B, which can be heated using drum or band heaters. This allows the material to stay warmer during application and lowers the minimum application temperature to 25°F. So for your next cold weather specification of Low-VOC adhesives and primers, include some installation precautions as mentioned. Contact Craig Tyler at [email protected] with further questions.

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