Structures: KNOW YOUR SEALANTS

It was a clear cold day on January 28, 1986 when the space shuttle Challenger waited on the launch pad at Cape Canaveral for take-off. For all its majestic power and symbolism, it was nonetheless a scene that had been repeated many times. On this day however a series of devastating events unfolded, including the rupturing of a seal in the spacecraft’s booster rocket. Seconds into the flight the spacecraft was destroyed and all seven astronauts on board lost their lives. While many important factors contributed to the fiery demise of the Challenger and its crew, the importance of the failure of a seal between sections of the booster rocket cannot be overstated.

In the 1990s and early 2000s the leaky condo syndrome in southern British Columbia made national headlines. The extent and amazing speed with which building envelopes of newly constructed residential condominiums failed during this period of construction was unprecedented. Water leaks causing rot and mould growth in the walls of these buildings resulted in hundreds of millions of dollars of damage and created potentially unhealthy environmental conditions for their occupants.

A common thread in these two very different stories is the importance of seals in structures; what materials are used, how they are used, and what environmental conditions they are exposed to, are questions that must be answered to achieve a successful design.

A Brief History

Prior to the 1950s, oil and resin based caulks were commonly used in building construction and wall systems were primarily load bearing masonry. Post 1950s saw drastic changes. Building designs began to incorporate curtain wall systems, natural stone veneer claddings and various other building envelope options. These new wall systems behaved differently from heavy masonry walls and they required sealants with much greater movement capability. To cope with these new requirements, elastomeric joint sealants were introduced and manufacturers started producing silicone, urethane and polysulfide based sealants to accommodate cyclic joint movements.

Without question, silicone and urethane based sealant products now dominate the market. Even though the majority of sealants are based on these two materials, current formulation advancements and various additives have generated a wide selection.

Today’s sealants can be broken down into three main categories: low performance, medium performance and high performance. The governing factor among the categories is the sealant’s ability to accommodate joint movement. Typically, low performance sealants can accommodate movement up to 10% of the joint width dimension, medium performance sealants around 10% to 25% of the joint width dimension, and high performance sealants greater than 25%. Not only is cyclic movement accounted for, but many of today’s sealants offer non-staining, ozone and UV resistances, along with the ability to adhere to various substrates.

In addition to the three levels of performance, sealants are differentiated by their application and curing method. A “single component” product depends on moisture and oxygen in the ambient air to cure. “Multi-component” sealants are mixed on site immediately prior to application. Two-component sealants consist of the base polymer and the curing agent that is the catalyst for the curing process. The curing of the sealant is a critical process since it is at that moment the polymeric chains extend and link and dictate the overall characteristics, such as strength and elasticity, of the final product. Furthermore, with a shift towards sustainable design in all aspects of construction, more and more sealants are being engineered with minimal volatile organic compounds (VOCs).

As well there are specialty sealants, which are manufactured with either a butyl, acrylic latex or various synthetic rubber chemical bases. Each sealant has its own specific properties that dictate its appropriate application. Whether dealing with an expansion joint or miscellaneous glazing, it is the responsibility of the designer to be aware of what conditions the sealant will be exposed to and what characteristics it should have to ensure proper performance.

Common Failures

Since sealants have been extensively used over decades, common failures have been identified. There are four basic types of sealant joint failures: adhesive failure, cohesive failure, substrate failure and degradation of sealant properties. Adhesive failure is when the sealant separates from the surface of the substrate. This failure is generally caused by the surface being improperly prepared, the substrate becoming contaminated, or the sealant being improperly installed. During the application, the substrate surface may be covered with dirt, oils, dust, coatings and various contaminants that can interfere with the sealant’s adhesion. Each type of substrate has its own characteristics and requires proper surface preparation. The designer should be aware of compatibility issues between the sealant and its substrate.

Cohesive failure occurs when the sealant tears within itself. It is usually a result of improper joint design, a joint width that is inadequate, and/or the improper selection of a sealant. The selection of a sealant should be based on the anticipated movement (amount and direction) at the joint, along with other critical factors. Given that numerous factors influence joint movement, sealants that can tolerate all potential movements should be applied to avoid any cohesive failures. Substrate failure occurs, for example, when the substrate edges crush or spall.

Exposure to the weather causes sealants to lose their properties. They may wither, harden, dry out and lose some or all of their elasticity. This can lead to the formation of cracks and crazing (fine spiderweb cracking) and can initiate further failure of the sealant. At the same time, sealants may fade or chalk, resulting in poor aesthetic appeal.

A failure that is receiving much more attention is staining. Many buildings have natural stone veneer claddings with sealant used to fill the joints. However, the plasticizers in certain sealants can migrate into the surrounding porous stone causing stains (halo effect) that are impossible to remove.

Proper Joint Geometry

The joint geometry is important to how the seal will perform. The type of joint and configuration dictate what type of sealant may be required. Building joints are either static (not subject to movement) or dynamic (joints that change width or shape). Static joints, even though they remain constant in width and shape, must have the proper configuration for the sealant. Whether it is a rectangular or angled bead application, there have to be proper width-to-depth ratios for the sealant to fully adhere to the substrates and to ensure the sealant will maintain its own integrity under the design exposure conditions.

Moving joints are the most critical as they can introduce tension/compression, shear and flexural stresses, and combinations of these, into the sealant. Not only must the designer know the range of temperatures that the joint will be exposed to, but also he or she must estimate what the temperatures will be when the sealant is installed in order to properly specify the joint width and geometry. All building materials have inherent coefficients of thermal expansion that can be used to predict their movement caused by temperature changes. Designers must consider the materials that are being used and also assess the extreme temperatures that the sealant and joint will be exposed to. With accurate modelling of the conditions, they can make reasonable estimates of the actual joint movements and select the most appropriate sealant.

In sum, the proper design of joints and selection of sealants for the building envelope is critical in order for the building and its components to achieve their intended serviceability and durability.

Matthew Lipiec, M. Eng. is an engi
neering graduate with Halcrow Yolles in Toronto, and Peter Di Lullo, P. Eng. is a senior principal in the firm.

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DESIGN CONSIDERATIONS

Premature failure of joints can lead to serious damages and repair costs many orders of magnitude greater than the initial cost of the sealant supply and installation. Designers must therefore consider carefully a range of factors when specifying a joint sealant, including:

Anticipated joint movements

Joint geometry

Adhesion to substrates and substrate tension and compression strength

Exposure conditions/Temperature ranges

Compatibility with substrates and other materials with which they may come into contact

Aesthetic requirements/Colour requirements

Life cycle cost