Development of New Test Methods and Performance-Based Specifications

The weatherability of construction sealants is a highly important per­formance criterion for the prediction of their aesthetic and functional service lives. Currently, the evaluation of a sealant’s surface degradation is carried out mainly by qualitative visual assessment against pictorial references. Enomoto, Ito and Tanaka present information on the weatherability of construction sealants based on a recently developed test specimen design that allows simultaneous exposure of the sealant to forced compression and extension movement in a single specimen with cyclic movement and weath­ering carried out simultaneously. A quantitative method for the assessment of surface cracks is employed and the relationship between outdoor and ac­celerated weathering exposure is evaluated by using metrics that indicate the degree of surface cracking as a new semi-quantitative criterion of sur­face degradation.

Recently, ASTM International published a standardized methodology suitable for the evaluation of joint seal continuity, ASTM C 1736-11 Stand­ard Practice for Non-Destructive Evaluation of Adhesion of Installed Weath­erproofing Sealant Joints Using a Rolling Device. This standard practice was created under the jurisdiction of ASTM committee C 24 on Building Seals and Sealants, and the direct responsibility of Subcommittee 30 on Adhesion. It was approved shortly before the symposium on July 1, 2011. In his paper, Huff discusses some of the technical questions raised during the develop­ment of this standard.

Today there are fifty-nine completed buildings globally that stand over 300 meters tall, a height generally considered super-tall, and dozens more

are under construction or being planned. The trend towards super-tall build­ings is driven by scarcity of available land, economic prosperity with dra­matic population growth within the big cities, and high economic value of the super-tall buildings. Nowhere is the trend towards super-tall buildings more evident than in Asia, especially in China and South Korea, as well as in the Middle East. Structural silicone glazing is a curtain wall technique com­monly used in South Korea and this glazing method is also considered for many of the future super-tall buildings. However, there is no industry-wide guideline or specification for structural silicone sealants in South Korea. In order to prepare for such a specification, Jung, Hahn and Lee report on a comparative evaluation of locally available structural silicone sealants that employs artificial weathering protocols adapted from various global industry standards, such as ASTM C1135 and EOTA ETAG 002. While silicones in general are known to have excellent resistance to weathering, some silicone products included in the study still show noticeable degradation of proper­ties, since the weathering performance of a sealant is affected by its overall composition and not just by its polymer type.

The strength of autoclaved lightweight concrete (ALC) is evidently lower than that of traditional concrete. When movement occurs at a sealed joint between ALC panels, the sealant is required to deform without causing damage to the ALC substrate. However, there is currently not sufficient in­formation permitting the selection of suitable sealants for ALC substrates. Miyauchi, Lacasse, Murata, Enomoto and Tanaka report on a study comprising both static and dynamic tests carried out to obtain an indication of the modulus of a sealant that can be expected to provide long-term per­formance when applied to an ALC substrate. Using two-part polyurethane sealants of different elastic modulus, the authors determine the relationship between shear and tensile stresses and the type of joint fracture. The results reveal that the ALC substrate is increasingly likely to fail when the sealant stress exceeds about 0.6-0.7 MPa.

The design criteria for structural silicone glazing (SSG) applications re­quire adhesive systems that maintain their functionality for longer than twenty years in actual field installations. Silicone sealants have well demon­strated their ability to effectively and reliably perform in long-term exterior structural applications. The first-ever four-sided SSG facade, completed in 1971, is still operational today. Still, estimation of the service life of SSG systems based on accelerated testing is difficult, since, in principle, it is nec­essary to test to failure in order to allow service life prediction, which, for systems designed for long-term durability, imposes practical difficulties. Fur­ther complications arise during the transfer of information gained on small scale test specimens to the actual performance of SSG systems as a whole. In his paper, Recknagel makes an attempt at adapting dynamic-mechanical

material analysis for the performance characterization of structural silicone sealants. The results obtained are reported and discussed for three structur­al silicone sealants, and characterize their temperature-, deformation – and frequency-dependent behavior. The applicability of the dynamic mechanical material analysis approach and of its various complex test modes for the exploration of the technical performance and the estimation of fatigue life is evaluated for the three sealants investigated. The author intends to comple­ment the dynamic-mechanical assessment methodology with suitable sys­tem tests on a section of a structural glazing system that will be subjected to a simplified load function representing the superposition of actual loads act­ing on the system. The technical fundamentals and the procedure proposed for the development of adequate system tests are discussed.

As already described for the Enomoto et al. paper, the durability of build­ing joint sealants is generally assessed using a descriptive methodology in­volving visual inspection of aged specimens for defects. This methodology has inherent limitations and the results are qualitative in nature. White, Hunston and Tan propose a new test method that utilizes stress relaxation to evaluate changes in the viscoelastic behavior occurring in sealants dur­ing durability testing. In particular, changes in the time dependence of the apparent modulus can be observed and related to molecular changes in the sealant. According to the authors, such changes often precede the formation of cracks and the ultimate failure of the sealant. The paper compares results obtained with the new test method and the currently used descriptive meth­odology.

During the symposium, a panel discussion was held regarding the impact of the newly developed ASTM C1736 Standard Practice for Non-Destructive Evaluation of Adhesion of Installed Weatherproofing Sealant Joints Using a Rolling Device. The panel consisted of three members of ASTM C24 commit­tee, who had direct involvement with the creation, oversight, and/or passage of C1736, plus one panelist representing a sealant applicator. Context to the discussion is provided by the editor, who has added a short introduction to the topic.