Development of New Test Methods and Performance Based Specifications
The final section of the collated symposium papers reviews attempts at developing new test methods for assessing the durability of sealants and adhesives, and, at reaching the ultimate goal, the development of performance-based specifications.
Structural silicone sealants are used to attach glass or other panels to curtainwall framing systems. These sealants must possess sufficient structural strength to carry the wind-loads but must also have sufficient movement capability to resist the fatigue caused by cyclic shear movement. Cyclic shear movement is induced in the structural sealant by differences in the thermal expansion between the glazing panel and the curtainwall substructure undergoing variations in temperature. Temperature variations occur in response to changes in atmospheric conditions (clouds, rain, etc.), as well as diurnal or seasonal climate changes. Assuming the occurrence of a cyclic shear exposure event twice a day, a structural sealant is exposed to 36 525 cycles over a period of 50 years. In their paper, L. D. Carbary, E. D. Bull, and S. S. Mishra expose nine silicone sealants with rated movement capabilities of 12.5%, 20%, 25% and 50% to at least 36 000 cycles of cyclic shear movement with 15% strain at a rate of five cycles per minute. It should be noted that realistically, thermal movement displacements for in-service curtainwalls will rarely approach strains of 15%. Products with a movement capability of 12.5% have been performing without failure for many years lending credence to this point. While the tested structural silicone sealants experience degradation as a result of the cyclic shear exposure, the degradation is small enough not to induce rupture, if the sealants are not strained beyond their movement capability. The sealants with higher movement capability show less susceptibility to degradation than sealants with lower movement capability. The authors suggest that this test method can provide a basis and a model from which to further study fatigue and to provide some guidelines and understanding of how the structural silicone sealants react to repetitive loading. They also speculate that further development of this method may result in its inclusion in the ASTM Cl 184 Specification for Structural Silicone Sealants.
The expectations of building owners for waterproofing systems are simple. Waterproofing must prevent the passage of water (impermeability), must be capable of successful installation under typical construction conditions (constructability), and must continue to provide waterproofing for the life of the structure (durability). The realization of these expectations is thwarted, in part, by a lack of consensus on how these features are defined, tested, and compared to performance-based criteria. J. C. Strong, J. R. Kovach, and V. S. Eng in the first part of their paper investigate important properties of waterproofing materials commonly used on plazas and below-grade walls and review the acceptance criteria of ASTM International, Canadian General Standards Board (CGSB), and International Code Council (ICC) Evaluation Services (ES) standards for waterproofing systems. In order to overcome the discrepancies between these standards, the authors recommend an open dialogue between the groups responsible for waterproofing material specifications in ASTM, CGSB, and the ICC ES. In the second part of their paper, the authors contribute two exploratory studies on the initial leakage resistance of bentonite waterproofing and the water absorption in cold liquid-applied waterproofing. Based on this testing, the authors suggest specific improvements in the relevant standards.
In November 2000, the Architectural Institute of Japan (AIJ) established a subcommittee chartered with developing an accelerated weathering test method suitable for assessing the durability of sealants. In their paper, N. Enomoto, A. Ito, I. Shimizu, T. Matsumura, Y. Takane, and K. Tanaka report interim results obtained with the proposed test method. In this method, the weatherability of sealants is studied using newly developed test specimens, which enable exposure of the cured sealants to simultaneous compression and extension in a single test specimen. The study comprises twenty – four sealants of seven chemical types commercially available in Japan. Interim results are reported after twelve months of natural outdoor weathering at three exposure sites in Japan (north: Hokkaido, central: Chiba, and south: Okinawa) and 3500 hours of artificial accelerated weathering with xenon lamp and carbon flame weathering devices. The interim results confirm that the surface degradation of sealants is accelerated by the additional movement cycles, and that the differences in the degradation among the sealants are becoming observable after the current exposure durations.
In their paper, H. Miyauchi and K. Tanaka propose a new design and service life assessment method for sealed joints exposed to seismic events. The paper is the culmination of several years of research by the two authors. In this research, the fatigue resistance of sealed joints to relative story displacement movements caused by earthquakes was studied experimentally and analytically. The authors now propose a new joint design method, which provides adequate sealed joint performance over the joint’s service life. The design method is based on three criteria, i. e., type of sealant, effect of crosssectional size and shape of the sealed joint, and fatigue resistance of the sealant at intersectional zones of sealed joints to the sliding and rocking motions of curtain wall panels. The process of sealed joint design considers the relationship between the number of cyclic movements to which the sealed joint is exposed during its service life and the number of cycles to crack initiation in the sealed joint as observed in the fatigue test method developed by the authors. Finally, the approach suggested by the authors allows the calculation of the accumulated damage level and the expected service life of a sealed joint.
In their paper, A. T. Wolf, S. Sugiyama, and F. Lee report on the use of an optical imaging and image analysis system in the assessment of surface changes induced in sealants by outdoor weathering. The method allows quantification of four distinct surface defects in the samples, namely cracking (crazing), visual color change, spatial uniformity of deterioration (due to dirt pick-up and uneven color change, or both), and overall surface texture. Chalking and dirt pick-up, as rated visually prior to the evaluation, cannot be accurately assessed with the digital imaging technique employed. The analysis shows that surface cracking and crazing generally can be well characterized using the automated image analysis system. While this study represents a step in the right direction, the authors suggest that further investigations are needed to develop an automated surface characterization method for sealants.
With current expectations for building exteriors to prevent all air and water entry into the building interior, the need for a near perfect seal of weatherproofing sealant joints has reached new levels of intensity. The need for better field tests has increased accordingly. To reach these goals, ASTM C – 1521 -02a Standard Practice for Evaluating Adhesion of Installed Weatherproofing Sealant Joints has been developed and adopted. The practice outlines a nondestructive procedure. The advantage of this methodology is that it allows an unlimited amount of testing to be conducted. While the procedure does not specify a specific instrument to induce the strain on the sealant/substrate bond-line, a device able to accomplish this procedure in a uniform, controlled, and calibrated fashion has been developed. The paper by D. Huff outlines a description of the device and its capabilities. The paper also provides a discussion of the use of statistical sampling when the option of complete testing is not feasible or required.