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Specifying Joint Sealants to Maximize Performance

THURSDAY, JULY 6, 2017

By Wiss, Janney, Elstner Associates Inc.


More items for Building Envelope

The specification of joint sealants used as weather seals in building construction is as simple as choosing a sealant material from a manufacturer’s catalog, right? Well, maybe.

As much thought should be given to choosing a sealant and specifying the conditions under which the sealant will be installed, as with any other building envelope material. Joint sealants used in building envelopes are not as exciting as the cladding material and other more aesthetic aspects of the design, but are no less important, and are often expected to do more than other building materials. A thin bead of joint sealant is expected to keep air and water from moving through the joints in the exterior envelope, while undergoing thousands of extension and compression cycles, and being exposed to heat, cold, water, ice, ultra violet radiation and pollution.

Images: Courtesy of Wiss, Janney, Elstner Associates Inc.

If the manufacturer adhered the sealant to glass or aluminum, and the substrates on your project are precast concrete and brick masonry, you should have sealant adhesion evaluated for the specific substrates of your project, both with and without primer. Pictured here is an adhesion failure between the sealant and substrate.

Care in selecting the joint sealant material, and specifying parameters that address specific common conditions that can be anticipated during its installation, can more likely provide for a satisfactory sealant installation, and one with a longer service life.

All joint sealant materials intended for use as weather seals are not equal. If all sealant materials were equal, each manufacturer would have a single product in their product line to be used as a weather seal. Therefore, choose a sealant that will provide the performance necessary for the application.

Generally speaking, this means selecting a sealant that will have sufficient movement capability, to accommodate cyclical dimensional changes due to expansion and contraction of the building structure and of the other building materials to which the sealant is adhered, and remain adhered to the substrates. There are other performance criteria to consider in selecting a sealant; however, this article isn’t going to delve into how to choose the proper sealant for a specific application, but will touch on a few items to consider generally when specifying joint sealants, so they have the best opportunity to perform well.

Substrate Primers

One consideration relative to installation of sealant is whether a substrate primer should be used. First, always consult the sealant manufacturer. The sealant manufacturers and their literature might say something such as: “Our product provides adequate primerless adhesion.” However, you need to pay attention to which substrates the manufacturer obtained “adequate primerless adhesion.”

Sealant installed when joint dimension is narrow will result in cohesive and/or adhesive failure when the substrates contract.

If the manufacturer adhered the sealant to glass or aluminum, and the substrates on your project are precast concrete and brick masonry, you should have sealant adhesion evaluated for the specific substrates of your project, both with and without primer. ASTM C1521 Standard Practice for Evaluating Adhesion of Installed Weatherproofing Sealant Joints provides a method for evaluating sealant adhesion. Keep in mind that you should evaluate each sealant you are considering, and that the sealants may require weeks to achieve final cure before adhesion evaluations can be performed.

Substrate Preparation

Consider specifying specific parameters and conditions at the job site under which the installation of the sealant can occur. The current position among several sealant industry experts is that sealant should only be installed within the same day that the substrate was cleaned and primed. ASTM C1193 Standard Guide for the Use of Joint Sealants currently states that the sealant should be installed within the same day that the primer was installed. However, several industry experts suggest more stringent requirements in that primer should be installed within hours of cleaning the substrate, and the sealant be installed within hours of applying the primer.

This is particularly important for dusty project sites where airborne particulate can contaminate a cleaned and primed bond surface prior to installing the sealant, thereby compromising adhesion. Even though a specifier may do everything correctly on a project—specifying an appropriate sealant product, identifying the best substrate cleaning process, and determining whether or not a primer should be used—if the substrate bond surface becomes contaminated prior to the sealant installation, premature adhesive failure may still end up occurring. It is certainly possible that a prepared substrate remains exposed too long.

Therefore, it is prudent to specify specific installation requirements stipulating that if the sealant is not installed within a specific time period, (i.e. the same day) after which the primer was installed, that the primer be reapplied.

Installation Temperatures

Be aware of application temperatures. Some sealant manufacturers allow application of their sealant materials down to minus 20 degrees Fahrenheit. Keep in mind that the cure time of many sealant materials will be extended as the temperature decreases. Generally, it is accepted that 40 degrees Fahrenheit is a safe minimum application temperature. Also keep in mind that substrates need to be moisture and frost free for the sealant to adhere properly. A thin layer of frost or moisture on the substrate at the bond line will result in a separation between the sealant and the substrate.

Sealant installed when joint dimension is wide will result in extrusion of the sealant when the substrates expand

Some sealants also have a published maximum surface temperature. A dark-colored aluminum curtainwall may have a surface temperature of 180 degrees on a clear day when the ambient air temperature is 75 degrees Fahrenheit. Some sealants may contain volatiles that will vaporize upon contact with a hot substrate resulting in spherical void in the sealant at the bond surface. Such voids result in bond surface area being less than 100 percent, thereby compromising the sealant’s adhesive bond to the substrate. Specifying the minimum ambient air temperature and the maximum surface temperature for installation of the sealant can help avoid these types of problems.

Joint Width

Related to installation temperature, and to movement capability, is joint width at the time of sealant installation. The sealant industry recommends that sealants be installed when the joint is at its neutral position; that is, when the joint is not at its narrowest position due to high ambient temperatures that expand the substrates, nor when the joint is at its widest position due to low ambient temperatures that cause contraction of the substrates.

This condition is often out of the specifier’s, and, to a lesser extent, the contractor’s control. The specifier cannot possibly know on what day, at what time and under what environmental conditions the sealant will be installed. Similarly, the contractor cannot wait around, not installing sealant, until conditions are ideal. So if there is a suspicion that the sealant may be installed under either extreme, you may want to specify more sealant movement capability than you might otherwise need.

Auxiliary Materials

Lastly, specifiers should be aware of the auxiliary materials that are commonly used by the installer and identify those which are acceptable to the sealant manufacturer for use with their sealant products and specify only those auxiliary materials be used. Manufacturers will often recommend specific substrate cleaners, primers and backing materials to assure compatibility and proper cure time.

For example, moisture cure sealant materials may require an open cell backing material to allow atmospheric moisture to reach the back surface of the sealant to promote more rapid and full cure. Incompatible materials in contact with, or even in close proximity to, some sealant materials, may cause failure of the sealant material, or adhesion failure.

Careful specifying of joint sealant materials and requiring specific installation conditions be present could mean the difference between a sealant installation that provides long-term, durable performance, or premature failure.

About the Author


Christopher Sass, an associate principal-level architect with Wiss, Janney, Elstner Associates Inc. has worked on a variety of projects related to the investigation and repair design of distressed conditions in buildings. He has directed and participated in numerous investigations and design of repairs involving plazas, roofing, masonry walls, EIFS cladding, curtain walls, windows, sealants and water infiltration. His work has been focused on water infiltration, masonry, curtain walls and roofing. Sass is the Chairman of ASTM C24 Building Seals and Sealants, and the President of the Board of the Masonry Institute of Michigan. You can reach him at csass@wje.com.

ABOUT THE BLOGGER

Wiss, Janney, Elstner Associates Inc.

“Solving for Why” is written by professionals at Wiss, Janney, Elstner Associates Inc. Since 1956, WJE’s primary goal has been to provide the best solutions for its clients’ new and existing construction-related problems. The firm’s highly qualified engineers, architects, and materials scientists possess a collective knowledge gained from solving, as well as helping clients avoid, thousands of problems. Author information is available at the bottom of each blog entry.

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