Volume 39, Issue 5, May 2004
Why Construction Sealant Joints Fail
by James Moody
The sealant applied in the joints between building components, generally known as caulking, is an often misunderstood component of modern construction. The cost of this component is roughly half of 1 percent of a building’s total construction cost, but without proper sealing it can consume a disproportionate amount of the building maintenance budget to repair.
Water is a building’s primary enemy, so ensuring it does not penetrate the building envelope is essential. Water that penetrates walls from sealant joints can lead to serious problems such as corrosion of concealed metal components, rotting wood and saturated insulation. This, in turn, can lead to excessive heat loss in winter, concealed mold growth leading to a “sick building” complaint and costly structural problems. Damaged drywall and stained ceiling tiles, annoy building tenants and could eventually lead to vacant office space as tenants may not renew leases. These problems are compounded by the fact that leaks don’t always make themselves known until considerable damage has already been done.
A construction joint sealant requires gap filling, the ability to move with the building under thermal cycles and building deflections, adherence to substrates and prevention of water and air intrusion. Given these requirements and the potential for mishap if they fail, it makes sense for glazing contractors, engineers and architects to pay close attention to the design, material selection and application of this vital building component.
Sealant failures can occur as a result of inadequate design, defective workmanship or inappropriate materials. Generally, the failure is made up of a combination of these factors. The sections below address how to avoid problems in these areas.
The sealant installed in the joint must be able to expand and contract to accommodate thermal movements and building deflections without cracking or breaking away from the substrate. The demand on the sealant is great because it must expand to its maximum limit when the substrates to which they adhere experience maximum movement. This is the time when flexible materials are at their highest stress. Expansion and contraction occur on a seasonal cycle with a smaller superimposed daily cycle.
The width of the sealant joint needs to be sized to accommodate the full range of expected movements, including thermal movements and building movements. It also needs to account for some fabrication and erection tolerances, which can affect the final installed joint size.
For thermal movement the width of the joint should be sized according to three factors:
• The temperature range of the region;
• The length of the members being sealed; and
• The type of material being sealed.
With these factors we can calculate the amount of expansion and contraction to which the joint will be subjected. As an example, a 10-foot panel of concrete will expand by 3/32 inches with a temperature range of 100 degrees Fahrenheit. The same panel made of aluminum will expand by 3/16 inches. It’s important to size the joint and select the correct sealant to tolerate the movement. Most of the silicone sealants available today have a 50-percent movement capability, are highly resistant to temperature cycling and have a life expectancy of 20 years or more. Polyurethane sealants are only able to accommodate 25 percent movement typically.
Sealants that can’t tolerate the right amount of movement can tear cohesively (the sealant pulls apart from itself) and adhesively (the sealant pulls away from the substrate).
The sealant cross section should look like an hourglass with a 2:1 minimum ratio as shown in figure 4. Bead profiles that are too thick increase bond line stress, which decreases movement capability, resulting in tears and splits. Bead profiles that are too thin have a tendency to tear or split too easily.
When it comes to selecting material it’s best to contact a sealant representative or building enclosure consultant. There will probably be a variety of products that will work once the variables of joint movement, substrates and environment are known.
A good rule to keep in mind with sealant selection is the keep-it-simple principle. If two sealants will work and only one requires priming, choose the primerless sealant. This cuts down on labor and also prevents misapplication of primer. The same concept applies to one- or two-part mixes. Choose the one-part sealant, as there’s no possibility of introducing air during the mixing process.
Some Common Mistakes
• Using a urethane sealant in an area of high ultraviolet (UV) exposure. This can lead to a type of failure known as reversion, a chemical breakdown of the sealant that turns it back into a liquid state over time, as shown in figure 5. The silicone family is generally the product of choice in this environment given that it is not affected by prolonged exposure to UV light;
• Using a sealant beyond its shelf life; and
• Substrate incompatibility. Sealant manufacturers should indicate clearly in their technical data sheets substrates with which the product is compatible.
Because testing is generally carried out under optimum conditions, not all sealants perform as stated in their manufacturer’s specifications. For instance, to state that a sealant has 50-percent movement capacity can be misleading. The sealant may have this capability at room temperature, but the capacity at cold temperatures may be a lot less.
The Sealant, Waterproofing and Restoration Institute (SWRI) seal is a good indication that sealant performance matches manufacturers’ specifications. SWRI certifies that the product performs to the manufacturers claims through independent testing.
Proper surface preparation is key. All surfaces must be clean, dry, dust- and frost-free prior to sealant application. This means removing all dirt, dust, oils and other forms of surface contamination that could interfere with bonding. These contaminants can be removed with solvents, wire brushing or by blasting with abrasives. The appropriate method for removal depends on the substrate.
After the sealant is shot into the joint it should be tooled to force it against the backer rod, which functions as a backstop for controlling the joint depth and forces the sealant into the joint surfaces to fill any voids.
Sealant installed in an unclean environment can lead to adhesive bond loss failure.
The sealant should be installed and cured only during appropriate weather conditions. The optimum time to apply the sealant is when the width of the joint is at its middle range, which is generally in spring or fall. During summer months metal parts have enlarged and the joint width is minimum; during winter the parts have probably shrunk and the joint width is maximum. Unfortunately, this is not always possible due to construction schedules.
As sealant technology is a dynamic field, new products are being introduced frequently. Glazing contractors should pay close attention to application instructions when using new products, as there may be changes.
It is difficult to assess the adequacy of sealant work after it has been completed. It is advisable to specify and institute a quality control program that includes testing finished joints (water testing and field adhesion testing) as construction progresses. This allows a mistake to be caught early on while deficiencies are still corrected easily.
It is important to resist pressure from anyone involved in the building process to cut corners in this important step of ensuring long-term protection, given the potential costs that are incurred in fixing the problem, should one occur.
Conversely, if the construction sealant joint has been shown as defective, all causes of failure should be considered in determining a solution. Failure causes can occur during design, material selection or the installation stage. Any problem solving process must identify the root cause and correct it to truly solve the problem.
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