In addition to correct joint design, selection of a sealant product with the ability to withstand anticipated building movement is critical to avoid premature failure. Inadequate provision for movement, either by underestimating the amount of movement or by using a sealant with insufficient movement capability, can cause even correctly proportioned joints to fail.
Fountains, parking garages, plazas, schools, and other sites subject to vandalism, water, and weather conditions demand sealant with superior abrasion resistance. High-heeled shoes—the bane of the precast concrete parking garage owner—are notorious for puncturing sealant at joints. Special additives and hardeners are manufactured specifically to resist damage from high heels. Pick-proof sealants could withstand vandalism, but these harder materials tend not to accommodate much movement.
Substrate compatibility is another major consideration when selecting a sealant. Some products can leach chemicals, discoloring or degrading porous substrates, such as brick masonry and stone. Under stress, a sealant stronger than its substrate, can cause cracks and spalls, as force is dissipated within the weaker material. This phenomenon is common with exterior insulation and finish systems (EIFS).
Existing coatings can also prove problematic for sealant performance. Fully removing such coatings—or selecting a sealant compatible with the existing product—is necessary to achieve adequate adhesion. As many surface sealers are clear and, therefore, difficult to detect, it is important to conduct a field adhesion test before full-scale sealant replacement.
The number one concern in the sealant application process is surface preparation. Given how often sealant failures relate to poor surface preparation, and how costly such failures can be to rehabilitate, it would be reasonable to assume a top consideration for sealant installers would be the diligent and thorough cleaning and, if required, priming of joint surfaces. However, this is not necessarily so. Especially for workers who have “always done it this way,” manufacturers’ recommendations for preparing the substrate may have little bearing on what is actually done in the field. At a minimum, surfaces must be clean and dry. Too often, though, even this simple stipulation is ignored. Dirty rags, incorrect or contaminated solvent, lint, and residue from existing sealant are just some of the many ways in which a sealant joint can be compromised.
A number of sealant types require surface primers prior to application, depending on the substrate. Primers can enhance adhesion, prevent the sealant from diffusing into the substrate, and emulsify dirt particles remaining on the surface. The advantage to using a sealant that does not require a primer is there is less room for error on the part of the installer, and skipping the priming step cuts down on application time and cost. However, some of the sealants requiring a primer perform better overall than their direct-application counterparts, so the additional time and oversight required for primer use may be worthwhile in the long run.
Weather conditions on the day of application can affect sealant performance. A significant percentage of sealant failures may be attributed to noncompliance with manufacturer instructions regarding installation. Ideally, sealant should be installed at the median of the design range, meaning the sealant has room to elongate or compress to accommodate fluctuations in temperature. If the sealant is installed in very cold weather, for instance, the substrate has shrunk and the joint is at its widest. As the weather warms and the substrate expands, compressive forces may exceed the sealant’s tolerance, leading to failure. The converse is also true; sealant installed in hot weather may stretch beyond capacity as the weather cools and the substrate contracts. Sealant installed at moderate temperatures retains the flexibility to accommodate the upper and lower ends of the design range.
Sealant viscosity also varies with temperature. If the temperature is very hot, sealant may sag; cold sealant, on the other hand, may be thick and difficult to tool. High humidity, frost, dew, or dampness can also lead to failure, as sealant will not adhere properly to a surface that is not dry.
Correct joint preparation and tooling are essential. Using a backer rod prevents three-sided adhesion for moving joints, and it helps to achieve correct sealant depth and profile. Without a bond-breaker at the back of the joint, sealant adheres to all three sides, leading to adhesive or cohesive failure—or both. To understand how this works, picture stretching a rubber band. This is how a sealant joint is meant to operate: one’s hands are the substrate, stretching and relaxing the rubber band, which represents the sealant. Now imagine grasping the rubber bands with the hands very close together, leaving only a tiny bit of the band to stretch and contract. Similarly, three-sided adhesion restricts elongation, as the bond area imposes additional stress on the sealant.
Sloppy tooling may result in voids, gaps, and irregular sealant thickness, causing stresses to act unevenly along the joint. Ideally, sealant should follow the curve of the cylindrical backer rod, with a concave tooled surface, such that it resembles an hourglass in a cross-section.
Some organic sealants, especially polyurethanes, have the potential for reversion failure, in which they return to an uncured or gummy state in response to UV light exposure and moisture. Although manufacturers became aware of this problem more than a decade ago and have modified their products accordingly, owners and managers of buildings with older sealants should be on the lookout for signs of reversion. Keeping tabs on the consistency and performance of sealants should be part of a routine maintenance program.