In Situ Measurement of Compression Set in Building Sealants During Outdoor Aging

ABSTRACT: The durability of sealants is dictated by many factors such as joint design, surface preparation, application, formulation, joint movement, and weather. Among these factors the link between formulation (material behavior) and weathering durability is difficult to assess in short term tests. We attempt to address this challenge by monitoring changes in apparent modulus during exposure to outdoor weathering and cyclic strain. This is accomplished via custom built systems that apply cyclic strain to 16 samples simultaneously via- programs that simulate wood (cold compression) and concrete/metal (hot compression) construction materials. A key finding of the research presented here is that changes in apparent modulus are primarily driven by underlying changes in the compression set, a potentially critical contributor to stress in structures during rapid temperature changes. Detection of the compression set is made possible by the in situ material property assessments used in this research. Aging tests that rely on offline evaluation of property changes may miss or underestimate this effect on the sealant’s stiffness due to time delay and/or losing track of the original zero stress-zero strain state.

KEYWORDS: Building sealant, compression set, durability, outdoor aging, strain cycling, apparent modulus

Introduction

Sealants are a critical part of a structure’s moisture and weather barrier enve­lope. Understanding and predicting the service life expectancy of sealants is necessary to prevent serious damage to the cosmetic and structural integrity of buildings and structures. Given this well recognized need, several challenges prohibit the direct assessment of such knowledge. One of the primary chal­lenges is the difficulty in assessing the critical combinations of factors (environ­mental, displacement, fatigue, formulation, material property, etc.) that determine a sealant’s durability in a well characterized service condition. Sec­ondly, pass-fail tests address lifetime prediction by exposing materials to condi­tions much more severe than the conditions of use. Severe conditions are necessary based on a factor of safety approach and the short testing times desired. The approach taken in this research program is to conduct simultane­ous outdoor weathering and cyclic strain aging of sealants while recording envi­ronmental conditions and changes in modulus via in situ measurements. This data set is intended to provide the means for obtaining a dosage versus damage model that may facilitate better predictions of service life for a given application and environment.

The outdoor exposure takes place in Madison, Wisconsin via a custom built computer controlled instrument named the Badger Ilia [1]. The advantage of this approach is that it provides active feedback on incremental material prop­erty changes as a function of weathering and cyclic strain dosage. The disad­vantage of cyclic testing is having sufficient sample throughput to provide statistically significant data. The instrument we designed addresses this issue by applying controlled displacement to 16 sealant specimens simultaneously as a function of temperature. The applied displacement is computer controlled such that custom functions can be developed to fit a service environment. The func­tions are principally temperature based. Consequently, the specimens experi­ence instantaneous and daily diurnal displacement cycles of a controlled magnitude and rate induced by temperature changes within set strain limits. Cycling is stopped once per week to run a stress relaxation profile allowing the calculation of the sealant’s apparent modulus [2].

Compression set is a well-known phenomenon defined as the fraction of applied compressive strain remaining in the sealant after full compressive strain is removed. This, of course, is a time-dependent phenomenon, since upon re­moval of the compressive strain the sealant moves toward the original dimen­sions more slowly over time and may never recover its original shape. In most sealant testing, specimens are allowed to relax between the compression appli­cation and the measurement of properties. This has a very practical benefit, removing a time-dependent factor, making property measurements more repeatable. The sacrifice, however, is that information about the contribution of transient compression set to sealant properties is lost.

Herein we describe the results from two sealants exposed to five months of hot compression cycling and outdoor weathering on an instrument that was designed to enable the measurement of stress relaxation behavior and the calcu­lation of apparent modulus immediately after stopping cyclic strain exposure.

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This test protocol more closely simulates exposure conditions on buildings with exteriors that respond quickly to changes in temperature.