SIKA CORPORATION • ROOFING 100 Dan Road ∙ Canton, MA 02021 ∙ USA Phone: 781-828-5400 ∙ Fax: 781-828-5365 ∙ usa.sarnafil.sika.com MEMO TO Sika Sarnafil Outside and Inside Sales Staff; Manufacture Reps CC Sika Sarnafil Technical Staff FROM Roofing Technical Department PAGES 4 DATE January 28, 2014 Subject: Risks of Concrete Decks 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. Over time this 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 CONCRETE AND WATER 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
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SIKA CORPORATION • ROOFING 100 Dan Road ∙ Canton, MA 02021 ∙ USA Phone: 781-828-5400 ∙ Fax: 781-828-5365 ∙ usa.sarnafil.sika.com
MEMO TO Sika Sarnafil Outside and Inside Sales Staff; Manufacture Reps CC Sika Sarnafil Technical Staff FROM Roofing Technical Department PAGES 4 DATE January 28, 2014 Subject: Risks of Concrete Decks
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. Over time this 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
CONCRETE AND WATER
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
Sika Corporation • Roofing 2/4
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. LWSC has a density between 90 and 115 lb/ft3 (1440 to 1840 Kg/m3) and NWSC has a density range of 140 to150 lb/ft3 (2240 to 2400 Kg/m3). The LWSC can achieve the low “in place” densities by using a lightweight porous aggregate filled with air voids. This aggregate will absorb water so it must be saturated before mixing so as to not affect the cement to water ratio causing issues with the final concrete product. The LWSC aggregate can absorb 5 to 25% of its mass with water. To put this in perspective, the Portland Cement Association Engineering Bulletin 119 states the dry down time for LWSC is many months more than NWSC. To achieve a 75% relative humidity for NWSC it will take approximately three month. To achieve the same 75% relative humidity for LWSC it will take twice as long, almost six months according to testing noted in the PCA Engineering bulletin 119. The test was conducted with an 8 inch (20 cm) slab that had both the top and bottom sides exposed to air to dry. Consider, if a roof membrane is installed over the top surface and the bottom surface is a steel form deck (as is very common), the ability of the concrete to dry will be severely affected. The laboratory ideal conditions for the LWSC drying at six months will be much greater under field conditions.
CONSTRUCTIN GENERATED MOISTURE
Various construction activities such as newly poured concrete, water based construction materials including paint, plaster, and drywall application among others will generate and contribute to the accumulation of moisture within an enclosed building space. Additional moisture will be generated when propane or oil burning heaters are used to condition the interior of the building. This heating of the interior may be to help dry the new construction materials or make the interior space more comfortable. To put this moisture accumulation into perspective a 4 inch (10 cm) thick concrete floor slab will generate approximately 1 ton of water for every 1000 square feet of concrete. For every gallon of oil burned there will be 1 gallon of water produced and a 200 pound tank of propane will produce 30 gallons of water. All of this moisture produced and trapped in an enclosed space will affect the roofing system. Should these conditions exist, the project designer and/or the construction manager/general contractor must take steps to properly vent the moisture out of the enclosed space, or provide for a vapor retarder.
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
Sika Corporation • Roofing 3/4
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.”
Based on the conditioning statement, this test is not viable for concrete slabs exposed to the weather.
Furthermore, even if the amount of moisture could be measured easily and accurately in-situ, the industry has not determined or defined what acceptable moisture content in concrete decks is for the installation of a roofing system.
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
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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.
ATTACHMENTS
• SPRI Industry Info. Bulletin No. 2-13 • Moisture in concrete roof decks, M. Graham NRCA • What You Can’t See Can Hurt You, S. Condren SGH • Reducing The Risk Of Moisture Problems From Concrete Roof Decks, G. Doelp SGH
NO:
Industry Alert SPRI, RCI, and PIMA would like you to be aware that:
The roofing industry is increasingly experiencing roof system performance issues
when roof systems are installed over lightweight structural concrete roof decks.
The potential for high moisture content in this type of deck, coupled with the need for
extended drying times, can pose significant risk to long‐term performance and
possible premature roof failure.
This risk can be significantly increased by the standard practice of installing these
decks over non‐removable, form deck or other non‐permeable substrates.
These moisture issues are not unique to the roofing industry. The flooring industry
has experienced parallel moisture issues with lightweight structural concrete, and
those slabs are not subject to periodic rewetting from being exposed to weather, as
roof decks are.
Roofing stakeholders, including designers, property owners, roofing contractors, and
roofing manufacturers can be at significant risk when installing roofing systems over
lightweight structural concrete roof decks with elevated moisture levels.
Determining when a deck is ready for roofing
Test methods include (but are not limited to):
The spot application of hot bitumen;
Electrical impedance;
ASTM D4263 (Plastic Sheet);
ASTM F1869 (calcium chloride); and
ASTM F2170 (relative humidity probes).
Latent moisture However, latent moisture in the deck material may still be present:
Latent moisture may not be measured by the tests noted above and can affect the
long term performance of roofing systems placed over lightweight structural concrete
decks.
There is no industry agreement concerning methods to detect this latent moisture or
level of moisture that may be tolerable.
Loss of adhesion Experience has shown that high moisture content can lead to compromised adhesion:
Adhesive applied or self‐adhering products may show acceptable adhesion, but can be
comprimised due to high/elevated moisture content and upward vapor drive.
Exposed to high/elevated levels of moisture, insulation facers can deliminate from the
substrate or the insulation core and membranes that appear to be initially adhered
can lose adhesion due to moisture migration.
Date: 07/31/2013
No: 2-13
INDUSTRY INFORMATION BULLETIN
To: Roofing stakeholders, including designers, property owners,
roofing contractors, and roofing manufacturers
Topic: Moisture Concerns in Roofing Systems Applied Over Lightweight
Structural Concrete Roof Decks
Industry Info. Bulletin No. 2-13 Date: 7/31/13
Loss of R Value Upward vapor drive that results in entrapped moisture in insulation can:
Result in significant loss of insulation value; and Possibly increase a buildings energy use.
Mold growth potential
Mold growth can occur:
High/elevated moisture levels can create conditions consistent with mold growth
within the roof system.
Water-based adhesive curing issues
Elevated moisture in these roof decks:
Could compromise the cure time of adhesive; or
Cause rewetting of water–based (low‐VOC) adhesives.
Corrosion of roof fasteners and other ferrous-containing roof components
Mechanical fasteners used to attach roof insulation and membranes to lightweight structural concrete roof decks.
There is the potential for the occurrence of fastener and steel plate corrosion due to the presence of elevated moisture levels.
FM Global FM Global has not specifically addressed the moisture in lightweight structural concrete issues:
It is important to note the lightweight structural concrete does not meet FM’s definition of “structural concrete”.
In the June 2012 version of the FM 4470 standard, FM’s defines structural concrete as having a “density of approximately 150 lbs/ft3”.
Lightweight structural concrete has a density of 90 – 120 lbs/ft3.
Conclusion Because of these performance issues and the potential risk for roof system failure, SPRI, RCI, and PIMA urge building designers to select roofing components and system with great care. Our organizations are continuing to study possible roofing solutions which mitigate the risks associated with the use of lightweight structural concrete. We hope to provide further guidance for proper roof design in the future.
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Moisture in concrete roof decks
Concrete's curing and drying rates can affect roof systems
by Mark S. Graham
lATELY, NRCA has experienced an increase
in reports of moisture-related problems
with low-slope membrane roof systems
applied to newly poured, normal-weight
or lightweight structural concrete roof
decks.
In the reported instances, significant
amounts of water have been found within
roof systems within several months to up
to three years after construction. In most
of the situations reported, it was deter
mined the roof membrane was watertight
and not the source of moisture infiltration.
Nevertheless, NRCA has some recommen
dations for avoiding such problems.
Concrete decks
When mixed, poured and formed, normal
weight and lightweight structural concrete
contain significant amounts of water. As
concrete cures and hardens, it consumes
large amounts of this water through hy
dration and evaporation. For example,
a 4-inch-thick concrete slab will release
about 1 quart of water for each square
foot of surface area.
Historically, the roofing industry has
used a minimum 28-day period as a guide
line for applying roofing materials over
newly poured concrete roof decks. The
28-day period coincides with the curing
time of concrete before it is tested for de
sign compression strength. There is little
correlation between this 28-day period
and concrete's true "dryness."
Professional Roofing February 20 l 0
In some instances, a plastic sheet test has
been used to determine concrete's dryness.
With this test, a plastic sheet (4-mil-thick
polyethylene) is taped to the concrete surface
and the plastic sheet's underside is moni
tored for the presence of condensation.
Up to the publication of The NRCA
Roofing and Waterproofing Manual, Fourth
Edition in 1996, NRCA recommended
the plastic sheet test as a method for deter
mining a concrete surface's dryness.
However, with the publication of The
NRCA Roofing and Waterproofing Manual,
Fifth Edition in 2001 and continuing with
the publication ofThe NRCA Roofing
Manual this year, NRCA no longer con
siders the plastic sheet test a viable assess
ment of concrete's dryness.
Similar to the roofing industry, the con
crete industry has seen significant advances
in technology regarding concrete mix de
sign, placement and curing.
For example, the use of concrete additives
in concrete mix designs and curing com
pounds during concrete placement greatly
can accelerate or retard concrete's curing and
release of free moisture. Similarly, weather
conditions, covering newly poured concrete,
timing of concrete form removal, and tem
porary heating or ventilating of a build
ing's interior after concrete placement can
affect the rate of concrete's upward or
downward release of free moisture.
For these reasons, NRCA no longer sup
ports the 28-day drying period or plastic
sheet test.
Tech Today
NRCA's recommendations
NRCA considers the decision of when it
is appropriate to cover a newly poured
concrete substrate to be beyond roofing
contractors' control. Because of the nu
merous variables associated with concrete
mix design, placement, curing and drying, roofing contractors are not privy
Apparent mold growth on insulation facers in a roof located in the Northeast. The roofing material was applied to a concrete deck without a vapor retarder and became loose. This roof did not leak.
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Deterioration in a gypsum·based cover boord in a roof system in the M idwest. This roof did not leak.
34 www.prafessianalraofing.net AUGUST 2012
requires systematic evaluation of all componenrs in the
roof and involves hygrorhermal modeling calculations.
T he numbers presenred are based on the id~al condi
tions of placing, finishing and curing the concrete without
adding water during construction or rewerring by rain .
In reality, cement will nor be fully hydrated and a deck
likely will be wet-cured or subjecred to some precipita
tion before application of roofing materials. These condi
tions will increase the amount of water entrapped in the
deck by the rime roofing materials are applied .
F ecomrnendol ions
More research is needed ro determine the maximum
amounr of moisture roofing materials can safely toler
are. In the meantime, roof systems should be designed
and installed with vapor retarders to mitigate moisture
entrapped in concrete roof decks. "•"~~-
STEPHEN J. CONDREN, P.E., is a senior project manager
at national engineering fi rm Simpson Gumpertz & Heger Inc.,
Boston; JOSEPH P. PINON, P.E., is a senior project man·
ager at Simpson Gumpertz & Heger, San Francisco; PAUL C. SCHEINER, Ph.D., is staff consultant at Simpson Gumpertz & Heger, Boston.
REDUCING THE RISK OF MOISTURE PROBLEMS FROM CONCRETE
ROOF DECKS
by
Gregory R. Doelp, P.E. and Philip S. Moser, P.E.
ABSTRACT
In recent years, the roofing industry has become increasingly aware of the problems caused by
moisture in concrete roof decks that migrates into the roofing system. Installing a vapor retarder
over the concrete deck is the primary method of addressing this problem. This paper summarizes
some of the challenges associated with incorporating a vapor retarder into the roofing system.
For example, selecting a vapor retarder of the appropriate vapor resistance is challenging due to
the shortage of published data on the acceptable moisture limits of roofing materials. We explore
the question of acceptable moisture limits through an extensive review of published literature,
product-specific recommendations from manufacturers, and some preliminary laboratory testing
of some common roof cover boards. This paper is based on the authors’ experience as designers
and investigators of roofing systems, literature review, and laboratory testing.
1. BACKGROUND
1.1 Consequences of moisture
While the primary function of a roofing system is to prevent water from passing through it into
the building or structure below, water or moisture vapor that collects within the roofing system
can also be detrimental, both to the roofing system’s immediate performance and its long-term
durability. Besides leakage to the interior, moisture in roofing systems can have numerous
negative consequences including the following:
Reduced thermal resistance of insulation.
Loss of strength of the insulation, cover board (Photos 1 and 2), adhesive, or fasteners
(Photo 3); leaving the roofing system vulnerable to uplift damage from wind or