Evaluation of Silicone Sealants at High Movement Rates Relevant to Bomb Mitigating Window and Curtainwall Design

ABSTRACT: Silicone sealants have a long history of successful use in high performance windows and curtainwalls, such as structural glazing systems. With the recent threat of terrorist attacks, there has been an increased use of windows designed to mitigate the impact of bomb blasts. Due to the high strength and durability characteristics of silicone sealants, structural silicone sealants have been utilized in new bomb blast mitigating window designs. Effective bomb blast mitigating window designs allow the window system to withstand a moderate bomb blast without causing significant injury to build­ing occupants from the blast itself or flying glass shards. The occupants are protected because laminated or filmed glass, which can withstand the blast, is attached in the framing with a silicone sealant. Silicone sealants provide unique benefits to these window designs due to their strength properties and their ability to anchor the laminated glass in the framing during a blast situ­ation. In this paper, three commercially available high strength structural sili­cone sealants are evaluated at applied load velocities (movement rates) up to 5.0 m/s. These elevated load velocities are intended to simulate loads encountered during a bomb blast. Sealant joints are fabricated to evaluate the sealant in tension, shear, and combined tension and shear loads. Seal­ants joints are also exposed to accelerated weathering (heat, water, and artificial light through glass). Results show that the sealant strength values

increase substantially at elevated rates of applied load. The paper discusses the effect of joint configuration, load velocities, and accelerating weathering on the performance and durability of the silicone sealants tested.

KEYWORDS: bomb blast, silicone, sealant, movement rate, loading rate, high speed, window, curtainwall

Introduction

The increased threat of terrorist attacks over recent years has highlighted the need for enhanced physical security features to be incorporated into building facades [1]. Historically, the majority of serious injuries or fatalities in blast events sustained by building occupants and the general public in and around the buildings were caused by high-velocity glass fragments originating from the building’s exterior window systems that were fragmented by the forces of the bomb blast (see, for instance, Refs. [2-6]). Figure 1 shows, as an example, the glazed curtain wall of a building adjacent to the Australian Embassy damaged by the Sept. 16, 2004 bomb blast in Jakarta, Indonesia. Nine persons were killed and more than 170 injured in this bomb blast. It is estimated that in a bomb attack up to 80 % of injuries and fatalities are attributable to flying glass shards and falling debris. For example, in the April 19, 1995 bombing of the Alfred P. Murrah Building in Oklahoma City, 40 % of the survivors within the building cited glass as contributing to their injuries. Within nearby buildings, laceration estimates ranged from 25 to 30 % [2].

image204

FIG. 1—Glazed curtainwall damaged by bomb blast (note the remaining shards of glass) (Source: Associated Press).

In addition to posing a hazard to humans, glass fragments can contaminate the work place and cause business disruptions after a bomb blast event. For example, following the 1993 Bishopsgate bombing in the City of London, United Kingdom, the whole air conditioning and ducting system in the building had to be replaced as no other efficient means of removing the contamination was deemed viable [7].

Due to the hazardous behavior of regular glazing systems in bomb blast events, there is a clear conflict between the need to construct secure facilities on the one hand and the importance of designing warm, open, and welcoming buildings such as embassies, hotels, or office blocks on the other. This has led to a renewed effort in the design of transparent facades that mitigate injuries and structural damage from blast loads and has prompted an increase in the use of bomb blast mitigating glazing designs globally during the past decade, espe­cially in new construction for government, institutional, and some high profile commercial buildings. Furthermore, existing window designs, which were not originally designed to withstand bomb blast, also have been retrofitted with specially developed protective films bonded by silicone sealants to the frame [8,9]. In summary, the primary objective of the various blast-mitigating protec­tive glazing systems is to protect people in and around buildings by minimizing the quantity and hazard of broken glass and blast-induced debris.

Despite the amount of research that has taken place into the fracture be­havior of architectural glazing under blast loads, this remains a field in which science is still evolving and the levels of expertise vary greatly. One of the great­est difficulties lies in anticipating the interactions between the high-speed dy­namic behaviors of the various materials involved in these glazing systems. Various factors, such as momentum transfer, natural period of vibration, dy­namic increase in material strength, aspect ratio, energy adsorption, and load path, need to be carefully considered for a balanced design. One of the key materials is the structural sealant, whose proper use and selection is critical to the performance of protective glazing systems.

In this paper, three commercially available high strength structural silicone sealants which are currently marketed for bomb blast mitigating window de­signs are evaluated in high speed testing to simulate loads imposed on the sealant joint during a bomb blast. Sealant joint designs similar to those used in current bomb blast mitigating windows are tested in tension as well as shear and results are analyzed to determine mechanical performance parameters at high loading rates that are useful for optimum sealant joint design.

The performance of a window system is the result of the interaction of all of the components of the system based on their attributes, including design, materials, and construction. The information provided in this paper is intended to help the designer of these systems to better understand the characteristics and behaviors of silicone sealants for bomb-blast mitigating window designs.