A Systematic Approach to the Study of Accelerated Weathering of Building Joint Sealants
ABSTRACT: An accurate service life prediction model is needed for building joint sealants in order to greatly reduce the time to market of a new product and reduce the risk of introducing a poorly performing product into the marketplace. A stepping stone to the success of this effort is the precise control of environmental variables in a laboratory accelerated test apparatus in order to produce reliable weathering data that can be used to generate a predictive model. This contribution reports a systematic study, using a novel laboratory test apparatus, investigating the individual and synergistic impacts of four environmental factors (cyclic movement, temperature, relative humidity, and ultraviolet radiation) on the durability of a sealant system. The apparatus used is unique because it not only allows the precise control of environmental factors but also permits in situ characterization tests so that the specimens
need not be removed from the apparatus chamber. Graphical and quantitative statistical approaches have been used to analyze the data. The study shows that the critical role of each individual factor, as well as synergism among the different factors, can be readily quantified, and modes of degradation possibly can be identified.
KEYWORDS: Sealant, Service Life Prediction, model, construction, modulus, SPHERE, Statistics
The accurate prediction of in-service performance in less time than is required for field tests and tests on structures has remained a modern unresolved scientific issue. Reliable performance data still require long-term field exposure. Such tests are needed in order to decrease the risk of introducing a poorly performing product into the marketplace. However, the cost of developing new products is directly related to the product development time and the time to market. The more time a product spends in the pipeline, the greater investment required and the smaller the eventual profit. Furthermore, long test times clearly hamper product innovations. Therefore, extensive efforts have been made to design short-term tests that provide an accurate indication of how well a sealant will perform in actual use. Although modern commercial sealants typically are designed to last for 20 years or more, studies in the construction industry have found a 50 % failure rate in less than 10 years and a 95 % failure rate within 20 years after installation [1-3]. These findings clearly show the inadequacy of current accelerated test methods and the need for the development of a reliable service life prediction methodology based on improved accelerated test methods.
The difficulties that hinder efforts to relate field and laboratory results include (a) unresolved differences between, and a poor understanding of, the failure modes in the two environments [4-6] and (b) a lack of methods with which to accurately quantify the effects of the environmental degradation factors in laboratory and field tests [4,5]. In particular, visual evaluation of physical performance, including crack and chip size, chalking behavior, and color change, is one of the main tests for the effects of weathering [7-9]. Although such a methodology might relate to a customer-perceived failure mode, it is qualitative and time consuming and provides little insight into the mechanisms leading to these macroscopic changes. In this paper and in previous reports [10-19], many of these issues have been resolved. Previous studies examined the impact of temperature [10-17], humidity [13,16-18], applied static and dynamic strain , and outdoor field exposure [10,14,15,17,19] on the durability of sealants, and they also reported the design of novel laboratory and field testing devices [14,15,19,20]. The success of this endeavor depends upon the use of a reliability-based methodology to make rapid, precise, and accurate environmental performance predictions. In this paper, we consider only the problem of identifying and ranking important degradation factors.
The research reported here represents a continuing effort in predicting the service life of building joint sealants. Although temperature, relative humidity, ultraviolet (UV) radiation, and cyclic movement have been identified as
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prevalent aging factors for sealants , there is no study thus far that systematically and quantitatively shows the impact of these environmental factors when they are acting either independently or synergistically on sealant properties. The objective of this study, therefore, is to design a systematic accelerated protocol that can provide a quantitative platform for investigating the individual and synergistic impacts of environmental factors. The study employs a custom-made laboratory apparatus with the capability to control these four environmental factors to high levels of accuracy, precision, and reproducibility . Moreover, because the deformation can be controlled, in situ mechanical characterization tests can be performed without removing specimens from the chamber. Thus, this method permits comparison of the dose of laboratory degrading factors in a quantitative manner. It is demonstrated that the individual and synergistic effects of factors on the durability of sealants can be revealed using this reliability-based approach. The wealth of data generated from the study is expected to facilitate the prediction of potential failure modes and the generation of a service life prediction model for sealant materials.