Potential of Dynamic-Mechanical Analysis Toward a Complementary Material and System Testing Approach for Structural Glazing

ABSTRACT: Dynamic-mechanical material analysis as a basis for a general performance exploration complemented by system testing under superim­posed climatic and mechanical loading seems to be a promising interdepend­ent test approach addressing the performance behavior of construction sealants under more realistic conditions. With this contribution an attempt is made to adapt dynamic-mechanical material analysis, which has been al­ready successfully validated for different construction types of expansion joint systems in road and bridge engineering, to the field of construction sealants for building facades. Test results from dynamic-mechanical material analysis characterizing the temperature-dependent, deformation-dependent, and fre­quency-dependent behavior of structural sealant materials are presented and exemplarily discussed for three different sealant products. An attempt is made to address unknown material characteristics in the multi-dimensional loading matrix representing practical use conditions. Furthermore, the applic­ability of this test approach and its various complex test modes for the explo­ration of technological performance and especially estimation of fatigue behavior is verified in several examples. Based on this fundamental material exploration, it is planned to complement the dynamic-mechanical assess­ment methodology by means of system tests on a section of a structural glaz­ing system subjected to a simplified but superimposed loading function. The technical fundamentals and the procedure proposed to develop an adequate system test mode are introduced. The motivation for these investigations is

Manuscript received June 10, 2011; accepted for publication December 19, 2011; published online April 2012.

‘Working Group “Bituminous Materials and Sealing Technology," Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany, e-mail: christoph. recknagel@bam. de

Cite as: Recknagel, C., “Potential of Dynamic-Mechanical Analysis Toward a Complemen­tary Material and System Testing Approach for Structural Glazing," J. ASTM Inti., Vol. 9, No. 4. doi:10.1520/JAI104124.

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to identify the actual mechanical system behavior under load combinations and for specimens that both closer resemble reality. The objective is to achieve a consistent and interdependent test program complementary to the existing methodology. Finally, the study is meant to initiate further progress toward a performance-related methodology which considers the design, specification, material, and system selection.

KEYWORDS: sealants, structural glazing, performance assessment, mate­rial characterization, dynamic-mechanical testing, system test method, superimposed loading, mechanical characteristics, capability evaluation, du­rability evaluation

Introduction

Cladding elements made of large-scale glass panes that are installed on metal frames are innovative design elements of modern architecture that nowadays are increasingly dominating the inner urban silhouettes of modern cities. Other than the special aesthetical appearance offered by such facade designs and the potential for complex geometrical solutions, such facades may also provide eco­nomical advantages as well as technical benefits in respect to the performance of the building envelope. Enhanced building envelope performance refers to improvements afforded by an in-depth understanding of building physics as applied to, e. g., sealing performance (to control air leakage and water entry), maintaining sustainable energy balances, and noise protection. Such improve­ments are the special features of the structural silicone glazing (SSG) facade technology. Examples of such building facades are given in Fig. 1.

A substantial advancement for this kind of architecture was achieved over the last few decades by improvements of structural silicone-sealant products. New one – or two-part sealants that were developed provided an increase in the options available to architects and engineers for the design of more sustainable technical solutions of structural facades.

According to their safety-relevant function, local building authorities are mandated to ensure the safe use of new and innovative building systems. As such, they are required to verify the capability of products to sustain loads and assess the durability of structural sealant systems; this is achieved on the basis of the application of generalized technical rules for use of the product or sys­tem. In respect to the use of structural sealants in European SSG facades, the harmonized European approval guidelines were introduced in 1998 as ETAG 002 [1]. These guidelines include technical rules for different options of the SSG design with and without additional fixtures, as well as approval requirements and quality-assurance regulations. The technical basis for this guideline was the development of the state-of-the-art in technology of SSG material and systems that occurred in the USA in the late 1980s, as well as the subsequent develop­ment of assessment tools. The transfer of these general European regulations (ETAG 002) and related design options to the respective national building codes is within the responsibility of each national approval body for building and construction.

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Motivation and Methodology for a Complementary Evaluation Approach

With the introduction of the national approval guideline, the German approval body (DIBT) felt impelled to limit possible design options to those systems hav­ing additional mechanical fixtures (retaining and supporting devices). Accord­ing to the requirements of the approval body, the response from the authorities, as well as engineering consultants and the SSG manufacturing industries, a complementary assessment methodology was sought that would overcome defi­ciencies in the material characterization of structural sealants, as well as char­acterization of system performance, especially with regard to durability. It was felt that the lack of proper material characterization, hindered, if not precluded, the use of performance-based design, as well as limited the ability to assess the long-term performance of such products and systems and the risks of prema­ture failure.

The current test methodology (ETAG 002) mainly focuses on the specific material behavior in the non-linear visco-elastic region (NLVE-region), i. e., the tensile rupture of sealants (strength at maximum strain). The discontinuous material response to test loads, as well as the actual visco-elastic material behavior is not adequately considered. In addition, there is no physically defined characterization of the linear load-deformation region (linear-visco­elastic (LVE) region) of sealant materials, as illustrated in Fig. 2. Instead, empir­ical relations are specified with respect to material design stresses using a fixed factor of safety versus the tensile stresses. Primary load dependencies of the materials, as well as dynamic effects or load superposition, are only partially considered (see Fig. 2); this leads to a situation where only single-point loading conditions are verified where, in fact, multidimensional loading conditions exist. This predominantly empirical assessment methodology also affects the general standard of safety represented by the actual test catalogue for initial component testing, factory production control, and third party monitoring. A limited review of the mechanical behavior of the material, generally, also limits the significance of design even if modern technical tools, such as finite-element- method (FEM) analysis, are available. Moreover, an oversimplification of the

FIG. 2—Classification of material characterization with regard to real loading.

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theoretical design model is apparent. Real stress distributions in circumferen­tial adhesion joints (four-sided adhesion bond) and under superimposed load­ing, as well as effects of geometrical defined points of discontinuities are neglected. Furthermore, load transfer (bearing capacity), considering the planar load bearing capability by the pane-framework-interaction, is not sufficiently taken into account.

Another issue relates to the assessment of the long-term performance of the sealant products and of the complete SSG system. Current assessment methods are predominantly based on decoupled loading tests that involve rupture tests after conditioning (accelerated weathering exposure) of the components. There is no simultaneous superposition of weathering loads (e. g., resulting in quasi­static and/or dynamic mechanical loading) to the primary structural loads. What is also missing is a method suitable for estimating the remaining life (du­rability) of deteriorated sealant material when conducting a safety assessment of existing buildings. Simultaneously, the design of the common test specimen, which is representative of the structural sealant joint (SSG), is oversimplified. All these issues suggest that a more detailed knowledge of the expected perform­ance of SSG systems is required, especially with regard to its long-term per­formance (durability) under in-service conditions. Within this context, the fatigue behavior of products and systems should also be explored to ensure their long-term performance and to verify the sustainability of SSG solutions. It must, of course, be considered that SSG solutions, as has been reported, have had good performance over several decades. To explore, in a scientific way, the reasons why is motivation to complement the existing predominantly phenomenological-empirical assessment method by a performance-based method consisting of:

• dynamic-mechanical material analysis (DMA), and

• dynamic-mechanical system analysis.

A schematic of the approach to such a complementary methodology is given in Fig. 3. It is supposed that this methodology would ultimately provide an improved design method,, as well as useful durability criteria for SSG materials and systems that, in turn, would also perhaps further improve acceptance of this innovative technology.

This contribution reports about exemplary first material investigations and describes their integration into a complex system test method according to the methodology proposed.