A Test Method for Monitoring Modulus Changes during Durability Tests on Building Joint Sealants

ABSTRACT: The durability of building joint sealants is generally assessed using a descriptive methodology involving visual inspection of exposed speci­mens for defects. It is widely known that this methodology has inherent limita­tions, including that the results are qualitative. A new test method is proposed that provides more fundamental and quantitative information about changes occurring in a sealant during durability testing. This test method uti­lizes a stress relaxation experiment to evaluate the non-linear viscoelastic behavior of sealants. In particular, changes in the time dependence of the apparent modulus can be observed and related to molecular changes in the sealant. Such changes often precede the formation of cracks and theultimate failure of the sealant. This paper compares results obtained from the new test method and the currently used descriptive methodology.

KEYWORDS: sealant, durability, ASTM C719, ASTM C1519, stress relaxa­tion, modulus

Introduction

Sealants are filled elastomers that are used commonly in structures in order to prevent moisture penetration through gaps, joints, and other openings. These structures span a wide range of diverse applications, including transportation vehicles and medical equipment. The greatest use of sealants, however, is in con­struction. Studies in the construction industry have indicated a 50% failure rate in less than 10 years and a 95% failure rate within 20 years after installation [1-3]. What makes these failures particularly detrimental is that sealants are of­ten used in areas where moisture induced degradation is difficult to monitor and expensive to repair. Consequently, sealant failure is frequently detected only af­ter considerable damage has occurred. In the housing market, premature failure of sealants and subsequent moisture intrusion damage significantly contribute to the $65 x 109 to $80 x 109 spent annually on home repair [4]. The environmen­tal durability, therefore, is the most demanding requirement of a sealant, as it is the property that ultimately determines the long term service life.

Over the past few decades, extensive efforts have been devoted to investigating environmental effects on the long term durability of sealants and to investigating degradation mechanisms [5-8]. However, the accurate prediction of in-service performance in less time than required for field tests and tests on structures has remained an unsolved scientific issue. One of the main stumbling blocks to its so­lution is a lack of reliable methods for accurately quantifying the environmental degradation factors in the laboratory and field. Degradation measurements in the descriptive methodology usually involve visual evaluations of physical perform­ance, including crack and chip size, chalking behavior, and color change. Although such a methodology can 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. This makes it difficult to develop models for accurately predicting sealant service life. An approach embedded in materials science could provide theoretical insight into the degradation mechanisms, help develop predictive models, and facilitate the establishment of a quantitative link between field and laboratory exposure results.

The limitations of descriptive methodology have prompted the sealant com­munity to seek improvements in the testing of sealant materials. A recent paper [9] presented a new method that was developed in cooperation with the sealant industry and which offers a solution to some of the issues inherent to the current approach. In this new method, a stress relaxation measurement was employed that monitors temporal changes in stress for a sealant subjected to a fixed strain. From this information, an apparent modulus versus time curve is generated. The magnitude and time dependence of this apparent modulus are related to the mo­lecular structure of the sealant. By monitoring how this modulus changes with exposure time in a degradation experiment, one can estimate changes in the mo­lecular structure of the sealant. Changes in the modulus over time also provide crucial information about how a sealant responds to the stresses imposed by the expansion and contraction of a structure over the diurnal cycle.

In a recent ASTM round robin for ASTM C1519-10, “Standard Test Method for Evaluating Durability of Building Construction Sealants by Laboratory

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Accelerated Weathering Procedures," four sealant samples were subjected to a series of laboratory accelerated weathering procedures and then evaluated using the usual visual inspection methods. The purpose of this paper is to evalu­ate specimens from this round robin using the new test method and to compare these results against results obtained from conventional evaluations.

Experiment