Though a small part of a building’s exterior, sealants perform a very large function.
Joints sealed with an elastomeric sealant usually fail from a combination of factors that can be summed up in six words – a lack of attention to detail. Too often, since the sealants are a small percentage of the work, they are perfunctorily specified, easily substituted, and haphazardly applied. Yet successful joints require meticulous design, precise sealant selection, and painstaking application.
The most common reason for sealant failure is too few or incorrectly sized joints. In order to preserve the esthetics of a building facade, joints may be undersized or, to limit the number of joints, made huge, following the ill-conceived rationale that “if you use bigger joints, you need less of them.”
Many architects, if they size joints at all, only consider movement due to thermal expansion and contraction. However, a number of other factors influence correct sizing and placement. Any change of plane or materials requires a joint. Wind loading affects joint placement not only for structural glazing applications but also for parapet walls. Moisture-related movement of materials also plays a part-concrete shrinks as it dries, brick grows as it absorbs water, and wood alternately shrinks and swells. Differential thermal movement between different but adjacent materials must also be accommodated with joints.
If the joints are detailed too far apart, or are made too small, the building simply creates its own. Most often the new joints appear as cracking in the exterior walls, but incorrectly sized or located joints also manifest themselves by causing bending or bowing out of the walls, crushing at the joints, or shearing of the curtain-wall fasteners or masonry ties.
Joint sizing does not allow much of a margin of error. If the joints are too narrow (less than 1/4 of an inch), the expansion of the substrate can cause the joints to close too much and extrude the sealant. When the building later contracts, the extruded sealant is no longer in the joint and leaks result. If a joint is too wide (more than one or two inches, depending on the sealant), the sealant may sag out of it. In addition, a wide joint requires a deep sealant bead to avoid cohesive failure, and, the deeper the bead, the less able the sealant is to stretch. (This phenomenon can be illustrated by rubber bands. If you have two bands of equal length, the thicker one will not stretch as easily or as far as the thinner.) The forces attempting to stretch the thick bead may then cause undue stress at the bond line and rip the sealant from the substrate (Figure 1). To prevent this action, the joint should be designed so that the depth of the sealant is 1/2 the joint width and never more than 1:1 at mean temperature nor deeper than half an inch (Figure 2).
Figure one shows stresses in sealant and adhesion stresses for deep and shallow beads
after 50 percent movement. In figure two, optimum joint depth is half the sealant’s width.
Selecting the right sealant
When the joint is properly sized and located, the most common cause of failure is that the sealant chosen lacks sufficient movement capacity for its intended use. Part of the problem lies in the manufacturers’ imprecise descriptions of their products. The terms “elongation,” “modulus,” and “performance” are used interchangeably, to the confusion of anyone attempting to evaluate the differences and similarities of the multitude of sealant products and formulations available.
Setting clear definitions of terms is the first step to selection of the proper sealant. Elongation defines the length to which the sealant can stretch, expressed as a percentage of its original size. Modulus defines the tensile strength of a sealant at a given amount of elongation. At 150 percent elongation, high modulus is defined as 100 to 150 psi, medium modulus as 80 to 100 psi, and low modulus as 20 to 40 psi. The modulus has a direct effect on the elongation capacity, since the lower the tensile strength, the easier the sealant may stretch. High-modulus sealants are recommended for uses where high strength is required and little movement is expected. A perfect use for a high-modulus sealant is in structural glazing, where the strength of the adhesive is the highest priority. Low-modulus sealants are used where movement capability is the overriding concern. Low-modulus sealants tend to be easily torn or punctured. An example where low-modulus sealants are suggested is for weather seals on metal curtain-wall buildings, where both high thermal and high wind movement capacity are required.
Performance relates to the amount of expected joint movement and the capacity of the sealant to elongate and recover. Low performance sealants will not work in joints that experience movement greater than ±5 percent of the joint width. Medium performance means that the sealant will work in joints that move up to ±12.5 percent of the joint width. High performance will take movement in joints greater than ±12.5 percent.
Recovery characteristics are also important to proper sealant selection. Some sealants stretch, but once subjected to tensile forces, do not easily return to their original shape, a phenomenon known as stress relaxation (Figure 3). Other sealants experience compression set, meaning they will remain bulged out after being compressed and do not easily stretch out again (Figure 4). In both cases, the sealant has developed a “memory,” which eventually will cause failure. A stress-relaxed sealant will assume a distorted shape when the joint closes. When the joint reopens, the sealant will not stretch as it should from its distorted shape and will therefore fail. A sealant with a compression set will rip from the substrate under the tensile stress developed as the joint opens.
Stress relaxation causes the sealant under tensile stresses to lose its original shape, shown in figure three. In figure four, compression set means the sealant bulges out of shape after being subjected to compression.
Proper sealant selection also entails avoiding incompatibility between the sealant and the substrate. Improperly chosen sealants can cause straining or etching of the substrate and loss of sealant adhesion. Other symptoms of incompatibility problems include disintegration, discoloration, or hardening of the sealant. When any chance of incompatibility is present, the architect should request that the manufacturer test the actual components to be used in the assembly to determine possible detrimental effects before construction begins.
The specifier also needs to consider appropriateness of the sealant for the environment in which it is expected to perform. Important characteristics include:
- resistance to puncture and vandalism
- ultraviolet resistance
- chemical resistance
- abrasion resistance
- ability to withstand extreme weathering
- ease of access for application and repairs
- reaction to continued submersion
- ability to resist dirt pickup due to slow cure or dust attraction.
Failure to take these factors into consideration can result in adhesive failure, cohesive failure, craze cracking, hardening, disintegration, color changes, or other types of premature failure.
Sealant selection does not end with preparing specifications. The same consideration must be given to any proposed “or equal” substitution as for the original specified product. Keep in mind that, in general, different brands of sealant are not equal, and never assume that one formulation of sealant can be substituted for another.
Avoiding application failures
The sealant’s main function, accommodating movement, is more easily performed if the sealant does not do double duty as weather protection. But, as in most cases, when the sealant does form the joint’s sole weather protection, careful spacing, location, and sizing of the joint are even more critical to the ultimate success of the seal. Even the finest set of specifications proves useless if it is not followed, or if the application is faulty. Consequently, architects need to design around the potential sealant problems that could develop in the field.
For instance, all sealant manufacturers state in their recommended application procedures that the substrate must be clean and dry for the sealant to properly adhere, and the most common of all adhesion problems begins when this basic rule is violated. Cleaning deficiencies include not cleaning at all, using contaminated or dirty solvent, using the wrong solvent for a particular sealant, allowing contaminated solvent to dry on the substrate, using contaminated rags or brushes, or using a rag containing lint.
The second most prevalent adhesion problem is caused by the improper use of primers-not using the primer at all, using too much primer, using the wrong primer for the specified sealant or substrate, or not allowing the primer to dry completely before applying the sealant.
The weather on the day of application can also cause problems in the field. Beware of applying an organic sealant at too low a temperature-cold causes the sealant to lose viscosity, making it difficult to apply without gaps and voids, and too thick to properly tool. In addition, the cold air is usually less humid, and less available moisture may retard the cure. Also, if the substrate has contracted due to cold temperatures, the joint will be wide open. If the sealant is applied then, there may be trouble later when the substrate warms up and expands, causing the joint to close and the sealant to be squeezed out entirely.
Too high a temperature can cause the sealant to sag or even flow out of the joint. If the sealant sags, the bead formed is of uneven thickness, causing differential stresses in the bead and eventual failure. High temperatures can also cause a premature skinning over the sealant bead. This can result in craze cracking of, or blistering below, the surface. If the joint is smaller due to thermal expansion of the substrate, when the temperature drops and the substrate subsequently contracts, the small bead of sealant may rip apart or be pulled away from the substrate. Even a spring or fall day may not be ideal-a large temperature swing while the sealant is curing may cause adhesive failure and/or craze cracking in the partially cured sealant.
Weather factors other than temperature are also at work in the field. With very few exceptions, sealants must be applied to dry surfaces. Applying the sealant when there is dew, frost, or any sort of precipitation will guarantee adhesive failure. A sealant applied when the substrate is still damp from a previous rain or from insufficient curing time will share the same fate.
Even if the weather is perfect, field applications can go awry -the most common cause being unauthorized substitution of the specified product. Many sealants look alike, but they do not perform in the same way. Also, improper storage of the materials can cause sealants to freeze, prematurely cure, or exceed their shelf life.
As for painting, there is only one word to say: don’t. Sealants are used for the specific reason that they are able to, and supposed to, move. Most paints will not adhere to most sealants, and, in general, are not formulated nor intended to take the kind and amount of movement to which sealants are subjected. Thus, when the joint moves, the paint will crack. If the paint is adhering to the sealant, and the paint cracks, the sealant will also. If a colored sealant is desired, talk to the manufacturer-sealants now are available in a range of colors in addition to the usual black, bronze, white, and transparent.
To allow sealants to perform their jobs properly, attention must be paid to the joint surface. The edges of the joint must be smooth cut and without jagged edges so that sealant doesn’t develop air pockets during installation. Any mortar must be removed from the joint or cohesive failure can result. Backer rods form another component of sound joints. Backer rods create the proper depth to width ratio, act as a bondbreaker, and provide a firm surface against which tooling can be done. Failure to specify an appropriate bondbreaker to prevent adhesion of the sealant on more than two sides of the joint creates a nightmare. A joint with three-sided adhesion will fail – cohesively, adhesively, or both ways (Figure 5).
Three sided adhesion of the
sealant may result in both
adhesive and cohesive failures.
Backer rods are of two types, open-cell polyurethane and closed-cell polyethylene. Closed-cell backer rods should not be used with sealants that cure by reacting with moisture in the air because the closed cells will not allow air in from the back of the sealant bead, in turn retarding the cure. Closed-cell rods must also be protected from puncturing, to avoid formation of gas bubbles in the sealant, known as outgassing. Open-cell backer rods should not be used where moisture absorption into the backer rods can be a problem, including horizontal joints and submerged joints.
Since backer rods are held in place by compression, it is important that the rod selected is about 20 percent larger than the maximum expected joint opening. If the rod is too small, it will not function as intended and proper tooling will be impossible.
Bondbreaker tape should be used only where there is a firm bottom surface and when the joint is so shallow that a backer rod will not fit. Sizing of the tape is very important-if it is too small, three-sided adhesion will result; if it is too large, the tape will wrap around the bottom and sides of the joint, eliminating some of the bonding area required for good adhesion.
Tooling also helps protect the weathertightness of the joint by eliminating air pockets created behind the sealant during gunning. If the air pockets remain, they may expand and rupture during hot weather. Tooling also forces sealant contact with the sides of the joint, promoting good adhesion. During the process, the sealant is pushed against the backer rod and pressed into an hourglass shape, allowing it to stretch properly. Without tooling, the bond area between the sealant and the sides of the joint may not be sufficient to prevent the sealant from pulling away from the sides of the joint (Figure 6).
The materials to which a sealant may be applied carry individual sets of precautions. Masonry and concrete substrates connote special care in choosing a compatible sealant. Because masonry and concrete are porous, some sealants may bleed into them, causing unsightly discoloration. Acid cure sealants can etch the surface of limestone or marble. In concrete applications, if the stress on the sealant is stronger than the tensile strength of the concrete, the thin, sharp edges of the concrete joint will spall. One way to minimize spalling is to chamfer the edges of the joint. However, the sealant bead must be kept below the bottom of the chamfer or tearing may occur (Figure 7). In all applications, the concrete must be sufficiently cured and dry before sealant is applied.
Aluminum coatings contain mill contaminants, oils, graphite, and carbon residues and oxides that act as release agents for elastomeric sealants. Some baked-on coatings will also make adhesion difficult. Adhesion testing to determine the suitability should be done by the sealant manufacturer for each installation because variations in fabrication make each batch of coating different.
Galvanized steel has a history of poor sealant adhesion. Due to the sacrificial nature of the galvanizing, the surface gradually erodes, making long term performance of an elastomeric sealant difficult. Galvanized steel normally requires an alternate form of joint seal.
Wood substrates demand that joint movement due to swelling and shrinkage be accommodated. Wood is best painted before sealing; paint must be compatible with the sealant and firmly adhered to the wood. Unpainted wood does not make a good substrate for any sealant. Unpainted wood will absorb moisture, and adhesion problems will eventually result. Woods that are naturally resistant to water, such as redwood and teak, have sealant problems-they contain natural oils that may affect the adhesion. Most softwoods such as pine contain natural resins that may bleed out under the sealant and affect adhesion.
Surface sealers and coatings may also prove unfriendly to joint sealants. Masonry sealers, anti-graffiti compounds, and waterproofing compounds and coatings vary widely in composition and formulation. Their main purpose is to provide a surface that will repel dirt, paint, and/or water. Unfortunately, these compounds often end up repelling sealants. Some sealant manufacturers, recognizing the widespread use of such compounds, have pretested them to determine compatibility with sealants. However, surface compounds undergo frequent and drastic formulation changes and can also vary widely from batch to batch. The application thickness can affect the sealant adhesion. Some sealers and coatings even have a base compound with low solvent resistance that dissolves into a clear gummy residue upon contact with sealant or primer, making adhesion impossible. Visually, many of these coatings are clear and difficult to detect. If a masonry or concrete surface is suspected of having been treated with one of these compounds, an adhesion test should be performed prior to sealant application. (Whenever possible, sealers and coatings should be applied after the sealant is cured, as it is extremely difficult to apply such materials onto the surface without contaminating the joints, no matter how careful the applicator.)
Form release agents, if applied according to the manufacturer’s recommendations, usually do not affect sealant adhesion. However, too thick of an application may leave a brittle film that can flake off, thereby creating loss of bond in that area. Petroleum-based release agents may be incompatible with the sealant. Testing should also be done on the actual materials used in construction to be sure that there will be no detrimental effects on the bond.
Testing can be a good idea in a range of other situations as well, following the time-honored advice “better safe than sorry.” Though sealant joints are only a small part of a building’s exterior, they are asked to perform a very large function: keeping the interior draft free and dry. They are not a component where chances should be taken, and their specification and application must never be treated lightly.