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.


August 21, 2019
Understanding FM 1-52

There are two recognized field test methods for determining uplift resistance of adhered membrane roof systems, both of which can be problematic: ASTM E907, "Standard Test Method for Field Testing Uplift Resistance of Adhered Membrane Roofing Systems," and  FM Global Loss Prevention Data Sheet 1-52 (FM 1-52), "Field Verification of Roof Wind Uplift Resistance."  Both test methods provide for affixing a 5’ x 5’ dome-like chamber to the roof’s surface and applying a defined negative (uplift) pressure inside the chamber to the roof system's exterior-side surface using a vacuum pump, like in the photo below.  An example of a test chamber used for negative-pressure uplift testing However, ASTM E907 and FM 1-52 differ notably in their test cycles and maximum test pressures for determining roof system deflections and whether a roof system passes or is “suspect”. Using ASTM E907, a roof system is “suspect” if the deflection measured during the test is 25 mm (about 1 inch) or greater.  Using FM 1-52, a roof system is “suspect” if the measured deflection is between ¼ of an inch and 15/16 of an inch, depending on the maximum test pressure; 1 inch where a thin cover board is used; or 2 inches where a thin cover board or flexible, mechanically attached insulation is used.  Test results' reliability  The reliability of the results derived from ASTM E907 and FM 1-52 is a concern, especially when the tests are used for quality assurance purposes. A note in ASTM E907 acknowledges its test viability. "Deflection due to negative pressure will potentially vary at different locations because of varying stiffness of the roof system assembly. Stiffness of a roof system assembly, including the deck, is influenced by the location of mechanical fasteners, thickness of insulation, stiffness of deck, and by the type, proximity, and rigidity of connections between the deck and framing system." For example, when testing an adhered roof system over a steel roof deck, placement of the test chamber relative to the deck supports (bar joists) can have a significant effect on the test results. If positioned between deck supports, the test chamber's deflection gauge will measure roof assembly deflection at the deck's midspan, which is the point of maximum deck deflection. Also, in many instances, field-uplift testing results in steel roof deck overstress and deck deflections far in excess of design values, which can result in roof system failure. These situations can result in false “suspect” determinations of a roof system. Industry position/recommendations Because of the known variability in test results using ASTM E907 and FM 1-52 and the lack of correlation between laboratory uplift-resistance testing and field-uplift testing, the roofing industry considers field-uplift testing to be inappropriate for use as a post-installation quality-assurance measure for membrane roof systems. Conclusion FM 1-52 is an FM Global-promulgated evaluation method and not a recognized industry-consensus test standard. The scope of FM 1-52 indicates that it’s only intended to confirm acceptable wind-uplift resistance on completed roof systems in hurricane-prone regions, where a partial blow-off has occurred, or where inferior roof system construction is suspected or known to be present. FM 1-52 was originally published by FM Global in October 1970. The negative-pressure uplift test was added in August 1980 and has been revised several times. The current edition is dated July 2012 and includes an option for "visual construction observation (VCO)" as an alternative to negative-pressure uplift testing. VCO provides for full-time, third-party monitoring to verify roof system installation is in accordance with contract documents. For more information, contact Craig Tyler.

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August 7, 2019
LEED, Green Globes, and Living Building Challenge

For years, “going green” initiatives have been popping up in all aspects of our lives. In the construction industry, there are three main programs pushing buildings to be greener: LEED®, Green Globes, and Living Building Challenge (LBC). All three of these programs push for more sustainable buildings, but each takes a different approach to accomplish this goal. In this SpecTopics post, we will look at some of the differences between these three programs. Put simply, LEED and Green Globes are working toward making buildings more sustainable by improving existing standards, while LBC takes things a step further and promotes buildings that have little to no negative impact on the environment.  Overall Mission LEED wants to make buildings better for the environment, community, and those who use the building Green Globes wants to make buildings more environmentally efficient based on commonly valued environmental outcomes LBC promotes buildings that have a positive environmental impact   Certification Methods/Requirements  LEED  Points-based rating system (100 points possible)  Four levels of certification: Certified, Silver, Gold, and Platinum Green Globes  Points-based rating system (1,000 points possible)  Four levels of certification: 1 Globe, 2 Globes, 3 Globes, 4 Globes LBC  Seven petals (like those of a plant or flower) broken down into 20 imperatives; number of petals/imperatives completed determines award  Three awards are available (based on which path you take)  A Zero Carbon Certification and a Zero Energy Certification are available Main Points of Focus  LEED Location & Transportation, Sustainable Sites, Water Efficiency, Energy & Atmosphere, Material & Resources, Indoor Environmental Quality, Innovation, Regional Priority, and Integrative Process Green Globes Project Management, Site Energy, Water, Materials & Resources, Emissions, Indoor Environment LBC Place, Water, Energy, Health & Happiness, Materials, Equity, and Beauty

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July 24, 2019
Solar Ready Roofing

As designers and architects try to meet green construction or net-zero energy goals, on-site power generation in the form of photovoltaic (PV) arrays is becoming more and more common. Preparing your roof to accept a PV system in the future is also increasingly commonplace. In general, robust roofs with extended warranties are the best candidates for PV systems. While a normal roof may only experience light foot traffic from the building owner or maintenance personnel a few times a year, a PV installation may need more frequent inspections (i.e. after significant storms or heavy snowfall, monitoring, etc.). So starting out with a thicker membrane, a cover board, and walkway pads are a must. It’s important that the PV array does not interfere with the roof’s drainage. This may mean some of the array will have to be supported by structural steel tubing or piping in lieu of traditional racking (which may rest on the roof utilizing ballast). It’s important to leave room for roofers to inspect or repair seams and flashings, so it may be necessary to raise the height of the array as not to obstruct roof penetrations. Always verify the type of PV array system to be installed on your roof and coordinate with your local Carlisle Field Service Representative (FSR) to assess the new or existing roof’s condition prior to beginning any work. Consult the SOLAReady Specification on the Carlisle SynTec website at or contact Craig Tyler with questions.

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