REMEDIATION OF A LOCKSTRIP GASKET WINDOW ASSEMBLY MATTHEW C. FARMER WISS, JANNEY, ELSTNER ASSOCIATES, INC. FAIRFAX, VIRGINIA S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 FA R M E R • 8 1 ABSTRACT Many façades built since the 1950s employ a twopiece rubber structural gasket system to support glazing. This system is designed to replace a more rigid metal frame. These gaskets are often integrated into a supporting steel or aluminum structure, used as mullions between multiple glazing panels or to tie the glazing into a precast concrete panel wall system. While the system offers many benefits, if poorly installed, severely aged, or damaged during glazing replacement, the system offers no redundancy and minimal protection against uncontrolled water leakage. Repair or remediation of the system is often difficult, due to the inability to reliably adhere sealant to the rubber compounds. This problem, as well as the problems associated with working on occupied buildings, can pose significant repair challenges to buildings. Wiss, Janney, Elstner Associates, Inc.’s recent involvement with one such project illustrates a unique and innovative way to remediate a lockstrip gasket system installed in precast concrete wall panels that suffered from chronic uncontrolled water penetration. Our solution did not require removal of glazing or modifications to the lockstrip gaskets themselves, though it did maintain the integrity of the barrier wall assembly. This paper presents the history of this assembly and its common advantages and disadvantages. It also describes this particular project and the process by which our repair strategy was developed and implemented. SPEAKER MATTHEW C. FARMER — WISS, JANNEY, ELSTNER ASSOCIATES, INC. FAIRFAX, VIRGINIA MATTHEW C. FARMER joined Wiss Janney Elstner’s New Jersey office in 1985. Since then, he has been involved with numerous evaluations of concrete, steel, and timber structures, as well as failures of clay and concrete masonry, stone, and cast stone. Mr. Farmer has concentrated on the area of design, investigation, and repair of new and existing building envelope systems, including engagements as an expert witness for constructionrelated litigation. He was appointed manager of the Washington, D.C. office in 1994. He is a graduate of the University of Colorado and Cornell University and is a licensed professional engineer in the District of Columbia, Virginia, and Maryland. 8 2 • FA R M E R S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 REMEDIATION OF A LOCKSTRIP GASKET WINDOW ASSEMBLY INTRODUCTION Many façades built since the 1950s employ an innovative twopiece rubber structural gasket system to support window and curtain wall glazing. Referred to as a lockingstrip gasket system, the gaskets are designed to replace a more rigid metal frame. The gaskets are often integrated with a supporting steel or aluminum structure and used as mullions between glazing panels or to tie glazing into precast concrete wall panels. Figure 1 – Schematic of Hshaped lockingstrip While the system offers gasket. many benefits, it is highly reliant on installation practices. If improperly installed, severely weathered, or damaged during glazing replacement, the system offers no redundancy and minimal protection against uncontrolled water leakage. Repair or remediation of the system is often difficult, due to the inability to repair the gaskets or reliably bond elastomeric sealants to the rubber compounds comprising the gasket system. These difficulties, along with the problems normally associated with working on occupied buildings, can pose a significant repair challenge to lockingstrip gasket systems that experience uncontrolled water leakage. THE LOCKINGSTRIP GASKET GLAZING SYSTEM HISTORY1 Lockingstrip glazing gasket systems were first introduced for building façades in the 1950s by the General Motors Corporation. Manufactured by its Inland Manufacturing division, the gaskets were an adaptation of GMC’s automotive windshield system. In 1961, Standard Products purchased GMC’s glazing gasket technology and manufactured the gaskets under the brand name StanLock. Griffith Rubber Mills purchased the StanLock Division of Standard Products in 1989 and continues to manufacture and market the twopiece gaskets under the StanLock brand. Although there have been a few other competing systems introduced over the years, StanLock continues to be an industry leader and the most common system in use today. DESCRIPTION The lockingstrip gasket system is a twopiece, preformed, elastomeric mechanical seal used to surround and attach a building panel or glass unit to a supporting structure made, typically, of metal or concrete. The gasket system consists of the gasket itself, which captures the edges of the panel or insulating glass (IG) unit, and a separate locking strip with durometer (Shore A) rating that, when forced into a groove provided in the gasket, puts sufficient compression on the panel to secure it to the supporting structure and create an intended “watertight” seal. The lockingstrip gasket system is also often referred to as a “zipper gasket,” due to its resemblance to a zipper as the locking strip is installed or removed. Although a number of configurations exist, the most common locking strip gasket profile is an “H” shape (see Figure 1). This profile is designed to bridge between a building panel and a higher the flange of a supporting structure, and it is often used for individual window openings. The gaskets are vulcanized into a single square shape by a combination of extrusion and injection molding. An example of this gasket in service is shown in Figure 2. A second common configuration of the locking strip gasket system utilizes the “H” shape to receive two building panels and also incorporates a spline to mechanically engage the supporting structure (see Figure 3). These can be completely vulcanized systems or can consist of extruded lengths Figure 2 – Example of Hshaped lockingstrip gasket securing an IG window unit to a precast concrete panel in service. Figure 3 – Schematic of splinetype lockingstrip gasket. S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 FA R M E R • 8 3 replace building panels or glazing; improper installation/removal of the locking strip or the gasket can lead to damage of the system and water leakage. MATERIAL The most common material used for the lockingstrip gasket system is neoprene. It is highly durable as well as flame, oil, and chemicalresistant. It functions well under temperature extremes and remains elastic with low compression set over its service life. Neoprene is typically only available in black. Though the material is stable under exposure to ultraviolet light, the material surface exposed to the environment undergoes oxidation and forms a friable “dust” on that surface as it degrades. This characteristic makes adhesion of elastomeric sealants unreliable, since the bonding surface is difficult to clean and continues to break down over time. The flexibility inherent with the lockingstrip gasket system accommodates variations in alignment between the opening substrate and the building panel, thereby reducing stresses induced by racking and glasstometal contact. It also can be effective in dampening noise and vibration. The gasket joined in the field to molded corners and tees. Figures 4 and 5 illustrate an example of an installation using a splinetype gasket. All of the gasket profile systems rely on the installation of the locking strip into the gasket to secure the glazing or building panels. Special tools and techniques are required to properly install and Figure 4 – Example of splinetype lockingstrip gasket in a curtain wall configuration in service. Figure 5 – Preformed “Tee” gasket as part of a splinelockingstrip gasket system. conductance between the interior and exterior of the wall assembly. It also can easily be designed to allow glass removal from either the interior or exterior. WATER PENETRATION RESISTANCE The locking strip gasket systems are considered barrier systems from a waterresistance standpoint, since they do not offer redundancy against water penetration and do not typically incorporate drainage provisions for water reaching the interior of the assembly. Water is resisted at the interface of the gasket and the building panel by compression of the gasket created by the locking strip. If the gasket compression is compromised by debris, damage, or improper installation, water can collect in the gasket and eventually leak to the interior. Field sealing of splices between extruded sections and molded intersections is also extremely sensitive to installation techniques, as these extend the full depth of the extrusion and represent a direct path by which water can reach the interior of the structure. REMEDIATION OF A LOCKINGSTRIP GASKET ASSEMBLY As previously mentioned, lockingstrip gasket systems rely heavily on knowledgeable technicians for proper installation and maintenance. Over time, these systems can be irreparably damaged during routine glass replacement by glaziers not familiar with the lockingstrip technology or not equipped with the tools necessary to correctly replace failed glazing. Improper maintenance can result in uncontrolled water penetration, damage to interior finishes, and discomfort to building occupants. The author’s recent involvement with one such project illustrates the potential damage from a lack of proper maintenance and incorrect glazingreplacement procedures. It also offers a unique and innovative way to remediate the damaged lockingstrip assembly in a costeffective manner and with minimal disruption to building operations. PROJECT DESCRIPTION The subject project is a 13story, midrise office building located in the midFigure 7 – Lockingstrip gaskets were used to secure glazing into precast concrete wall panels. is also a natural thermal Figure 6 – Office building utilizing a lockingbreak, minimizing thermal strip gasket system to secure glazing. 8 4 • FA R M E R S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 Atlantic region and originally constructed in the 1970s. The structure consists of conventionally reinforced concrete flatplate floor slabs supported by conventionally reinforced columns. The exterior façade consists of precast concrete panels that incorporate rectangular openings recessed between horizontal spandrels and vertical “fins” to accept individual, insulatingglass (IG) units to form the windows (see Figures 6 and 7). The upper floors vary slightly from the typical precast/window configuration in that glazing units, and spandrel panels are stacked to form an uninterrupted “column” of glazing between the precast concrete fins (see Figure 8). An Hshaped lockingstrip glazing gasket system is used to engage the IG units and secure them to flanges on the precast concrete panels at the opening perimeters. Over the past several years, the building suffered from ongoing uncontrolled water infiltration and IGunit failure, along with other deficiencies resulting from deferred maintenance of the exterior wall system. The project objective was to determine the cause of the water infiltration and develop appropriate repairs to address the deficient conditions observed. The following specific conditions were observed with respect to Figure 9 – Example of lockingstrip gasket deformation. Figure 8 – Precast concrete wall panels also incorporate vertical groupings of glazing and spandrel panels. the locking strip gasket system: • Many of the gaskets were distorted and misaligned due to a lack of compliance with construction tolerances. This condition resulted in distorted gasket position and poor glass/gasket seals that led to water infiltration (see Figures 9 and 10). • Some gaskets did not completely engage the glazing. Sealant was typically applied to fill the gap between the glazing and the gasket (see Figure 11). • Many of the locking strips were missing, leaving the gasket without adequate compression against the glass and allowing water to enter the gasketglazing pocket (see Figure 12). The gaskets were also often cut at the corners to allow for glazing replacement without removal of the locking strips. This condition also led to reduced glass compression and allowed water in the glazing pocket to leak to the interior. • Water collected in the glazing pockets. Because the IG units were not Figure 10 – Example of lockingstrip gasket deformation. Figure 11 – Sealant was used to fill the gaps between the glazing and the gasket where fit was poor. S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 FA R M E R • 8 5 Figure 12 – Location where the locking strip of the gasket system is missing. Note cut at corner. shimmed, their edge seals remained wet for prolonged periods, leading to eventual failure in the form of condensation between the glass bites and loss of insulating properties (see Figure 13). • Elastomeric sealant was present between the lockingstrip gasket and the Figure 13 – Evidence of water accumulation in precast concrete panels, the glazing pocket including IG spacer as well as between the corrosion. gasket and the glass. This sealant was deteriorated and lacked ditional barrier system repair adhesion to the gasket system (see approaches impractical without Figure 14). substantial associated maintenance. • Although there were relatively few CHALLENGES opening sizes, there were multiple This project presented many challenges configurations of the glazing units with respect to the remediation design. (see prior Figure 8) and variability in These are discussed below: the relationship between the gaskets • The client required that the building and the edges of the precast conremain fully occupied throughout crete openings. It was preferable the course of the work, with minimal from a design and maintenance perdisruption to the tenants. This spective to utilize a similar and connecessitated that the majority of the sistent approach at as many window repair work be conducted from the exterior and eliminated the potential for repairs requiring removal of glass on a large scale. • It was critical to provide an adequate overlap between the IG units and the gasket, or glass bite, to avoid any reduction in resistance to wind loading. • The inability to reliably bond elastomeric compounds to the neoprene gaskets made more traFigure 15 – Overlay extrusion application at typical windowsill. Figure 14 – Sealant between the gasket and the precast concrete panel, as well as between the gasket and the glass, had failed. configurations as possible, regardless of the deviations. • The client appreciated and wanted to maximize the clear glass area at each opening to preserve natural light and desirable views. PROPOSED SOLUTION After reviewing the existing field conditions, it was decided to design an overlay barrier system for the existing lockingstrip gasket system. The final solution consisted of a customfabricated, siliconecompatible rubber extrusion applied directly over the existing glazing gaskets that bridged from the glass to the adjacent precast concrete panels without relying on adhesion to the gasket itself. The final design was an “L”shaped extrusion formed to fit the external profile of the lockingstrip gasket at the glass line. The edge in contact with the glass was Figure 16 – Overlay extrusion application at typical window head. 8 6 • FA R M E R S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 Figure 17 – Overlay extrusion application where glazing meets at inside corners. thickened to allow it to be “bedded” in sealant against the glass surface, but without a visible external seal. The opposite leg length was trimmed to accommodate variable dimensions between the lockingstrip gasket and the surrounding precast concrete. Once in place, a fillet bead of sealant was applied between the new overlay extrusion and the precast concrete panels, creating a watertight seal. Figures 15 through 18 illustrate the final profile concept and its relationship to the existing wall system at typical locations. This repair approach offered several key advantages with respect to the overall project: • The bedding of the extrusion in sealant eliminated exposure of the sealant to the environment, prolonging its service life. It also offered a clean, narrow sight line in comparison to an exposed wet seal using elastomeric sealant. • The installation of the overlay gasket did not require removal of the existing sealant between the existing locking strip gasket and the precast concrete, saving cost and eliminating a laborintensive step in the repair process. • The gaskets are made of siliconecompatible rubber (SCR), a highly durable, environmentally stable material. It has many of the qualities and the appearance of the original neoprene gaskets, but reliably accommodates the application of elastomeric sealants. • The entire installation could be performed from the exterior without glass removal. • Repairs to the original locking strip Figure 18 – Overlay extrusion application where glazing and spandrel panels meet vertically. figurations on the project. Contract documents were developed for the design to provide sufficient detailing and information for the repair contractors to competitively bid the fabrication and installation of the overlay extrusions and associated work. Once the bids were received, each bid was carefully examined to assure that it recognized the intricacies of the repair process necessary to arrive at a final overlay extrusion product. Each contractor was asked to submit his preferred extrusion manufacturer as part of his bid. After awarding the contract gaskets were not required. for the repairs to the successful bidder, the • All of the various glazing configuraspecified overlay extrusion manufacturer tions could be addressed using comwas contacted to perform field measurebinations of the custom overlay ments and develop a prototype extrusion for extrusion and widely available stantrial installation (see Figure 19). dard preformed SCR shapes. A trial installation proved critical to • New sealant installed in joints refine the final extrusion design and develbetween the precast concrete panels could be seamlessly integrated into the sealant joints around the window openings. Despite the many advantages of this remediation design, one major drawback to this solution is the need to access the exterior and remove/reinstall the overlay extrusions to replace failed IG units in the future. The owner was provided with attic stock for future repairs /replacement. IMPLEMENTATION The first steps toward the repair implementation began with design of the concept. This involved field measuring and documenting the various glazing conditions and components to assure that the design concept could address the multiple glazing conFigure 19 – An early schematic of the overlay extrusion profile proposed by the manufacturer. Figure 20 – Final overlay extrusion profile. Note the modifications to vertical leg (area circled). S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 FA R M E R • 8 7 Figure 22 – Example of final overlay extrusion corner detail developed during the trial installation. sult, it was discovered that the extrusion leg bearing on the glass was not being fully bedded, leading to water entrapment behind the overlay extrusion and uncontrolled water leakage (see Figure 23). In response, the installation methods were adjusted to assure a waterFigure 21 – Trial installation of overlay tight bedseal that could be extrusion was used to optimize corner and visually verified from the overlap detailing. building interior. RESULTS Once an effective installation procedure was estabop an efficient and reliable installation procedure. Minor modifications included a longer leg to accommodate greater deviations in distance between the original gasket and the precast concrete panels, as well as a shorter leg bearing on the glass to assure it did not displace outward when compressed (see Figure 20). The trial installation also allowed for the opportunity to experiment with different corner treatments to maximize gures 21 and 22). Once the overlay extrusion profile and the installation procedure were finalized, a mockup installation was performed to establish the standards for the project and for the client’s aesthetic approval prior to implementing repairs buildingwide. Wat e r p e n e t r a tionresistance testing was conducted to verify that the overlay extrusion concept would be successful. As a relished, the overlay extrusions were applied with surprising quickness. They maximize clear window area while substantially reducing water penetration. This method avoided expensive deglazing Figure 23 – The success of the overlay system is heavily dependent upon full bedding of the extrusion to the glass. Figure 25 – Example of completed overlay extrusion installation at upper floors where glazing and Figure 24 – Example of completed overlay extrusion at a typical punched window opening. wa t e r t i g h t n e s s while simplifying the installation (see Fispandrel panels are joined vertically. 8 8 • FA R M E R S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 and gasketreplacement options, while having a minimal impact on the overall wall assembly. Overlaying the damaged lockingstrip glazing gasket system resulted in a crisp, clean line around the glass that is very similar in appearance to the original lockingstrip gasket system installation (see Figures 24 through 27). The overlay extrusions integrated into the existing wall system should continue to serve well into the future as an effective barrier against uncontrolled water penetration with minimal maintenance requirement. ACKNOWLEDGEMENTS The author wishes to recognize the contributions to this project and paper from Griffith Rubber Mills, of Eugene, Oregon; Tremco Sealant/Weatherproofing Division, of Ashland, OH; Suzanne Thorpe, project associate (WJE); and Rita Sparacino, project associate (WJE). 1 StanLock Lockstrip Gasket Systems product literature, available at www.stanlock.com. EDITOR’S NOTE: A shortened version of this paper, titled, “Repairing a Gasket with a Gasket; Remediation of a LockingStrip Window Gasket Assembly Provides Similar Appearance, Effective Water Barrier,” appeared in the Spring 2007 issue of Applicator magazine, published by the Sealant Waterproofing & Restoration Institute. S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 FA R M E R • 8 9 Figure 27 – Completed building exterior renovation that incorporates overlay extrusions to remediate a damaged lockingstrip gasket system. Figure 26 – Typical appearance of the completed overlay extrusion installation at a punched window opening.
