Gulf Construction Online - Silicone sealants strengthen facades

Security & Safety

Generic design sketch of a stepped IGU in structural glazing.

Silicone sealants strengthen facades

Axel H Giesecke and Kenneth Yarosh of Dow Corning review the developments made with silicone sealants in safety and security systems focusing primarily on modern-day glass facades and how these sealants can open up new possibilities for protective functionality in facade and window systems.

01 January 2006

The loss of life and property following terrorist attacks on buildings has been an eye-opener for developers, building owners, government agencies, insurance agencies and occupants alike – on the vulnerability and false sense of security that such buildings offer – forcing them to seek new solutions to protect people from such attacks as well as from extreme climatic natural disasters.

Being challenged with ever-tighter standards in modern building construction, designers are relying on the unique properties and uncompromising strength of silicone structural glazing products to play an added active role in new protective and safety glazing systems.
Today’s state-of-the-art façade is much more than just a building envelope which protects the building’s occupants from the elements includes multiple functionality such as lighting, heating, cooling, and protection.
With security being a fundamental human requirement, building specifications are becoming more stringent and finding means to protect human life is gaining greater importance.

Protective glazing
In the Middle East and Europe, many new commercial buildings are being specified to have bomb blast-resistant windows and facades. Existing buildings are also being retrofitted with protective film and sealant systems, with the purpose of protecting the occupants from blast-borne debris. Elsewhere in the Americas and Asia, facades have to serve a different purpose – protect occupants from the relentless storms and hurricanes while at the same time facilitate insurance coverage in the event of a disaster.
While protection remains paramount, occupants still need and have the right to enjoy lives in aesthetically-pleasing buildings with a permanent view to the outdoors. Structural silicone glazing is an ideal technology that meets the dual demands.
The project DS2 in London or the Scottish Parliament in Edinburgh are examples of structural silicone glazing facades, that meet these demands. These modern buildings display an aesthetically pleasing design combined with various technical functions, such as wide daylight openings, energy savings and protection against bomb blast.
Silicone sealants offer excellent weather resistance, durability, flexibility, adhesion and a visco-elastic behaviour, which is critical for bomb blast resistant window designs1,2,3. In addition, European Standards endorsing structural glazing systems and structural glazing silicone sealants have been developed4,5. With these new standards, structural sealants that fit the requirements can receive a CE-mark, which recognises that the product as appropriate for the critical requirements of this modern construction design.

Structural sealant adhesives
Design details for structural glazing show that the IGU (insulated glass unit) secondary seal as well as the structural glazing adhesive are fully exposed to weather, which can cause several physical and chemical stress factors on building components. These factors need to be taken into consideration while designing a long-lasting and reliable façade capable of protecting the investment of the building owner. Several examples of these stress factors are shown in Table 1.
The construction design details shown previously clearly necessitate a dual seal system for the IGU, which is capable of resisting the climatic stress factors. Silicone sealants are based on polydimethylsiloxane (PDMS) polymer, which has a completely different polymer structure as opposed to organic polymers, such as those used in polysulfide or polyurethane sealants. The structure of silicone polymer, polydimethylsiloxane (PDMS), is shown in Figure 3.
The key-differentiating feature of PDMS polymer is its inorganic siloxane (Si-O-Si) backbone, which provides the outstanding durability and excellent elastomeric properties.
Its unique polymer structure (Figure 4) provides silicones with an inherent durability, resistance against UV-radiation and high performance properties, which are stable over a wide temperature range, offering the system outstanding weather resistance. These properties make silicones the material of choice when high quality and high durability sealants are required.
The knowledge and experience on the performances of the structural glazing silicones has led to an expansion in the use of silicone structural glazing techniques in new high-performance façade systems – especially where such systems are required to offer additional protection and resistance against bomb blasts and natural calamities.

Bomb blast loads
Properly-designed structural glazing systems can perform exceptionally well under bomb blast loads. Silicone sealants have a long history of performance as an adhesive and sealant for glass. Properly-formulated silicone sealants have high strength, which is necessary to anchor or secure laminated glass to a frame during blast impact. These sealants are also durable and flexible, enhancing the performance of window systems when exposed to the elements for many years.
In July 2000, a 20 kg bomb blast in the centre of Madrid, Spain, destroyed a four-sided structurally glazed building. This building was recently “face-lifted” with a new façade over the existing structure. The float glass was completely shattered but what was observed was that the glass shards were fully attached to the structural silicone sealant around the perimeter of the frame. The sealant maintained full adhesion to the glass and framing through the bomb blast.
Figure 5 shows the result of a bomb blast test on a structural glazing system used on an actual project. The structurally glazed curtainwall system was subjected to a charge of 12 kg of TNT at a distance of 6.5 m from the test specimen at a height of 0.8 m. The peak reflected pressure was 383.7 kPa. Total blast energy was 547.7 kPa-millisec and the duration of the blast load was 6.17 millisec.
After exposure to the bomb blast, the system was evaluated and it was found to have performed successfully since the laminated glass was fully retained in the window openings by the structural silicone sealant.

Design values
The design of structural glazing systems, which are intended to withstand bomb blast loads, can be a complex process. The performance of the system is dependent on many variables such as the structure, rigidity or flexibility of the framing system, size of the window openings, type of glass and laminate used, explosion pressure loads and finally the type and design of the structural silicone attachment. Systems designed to mitigate a bomb blast must be tested and evaluated as a system.
Although the structural silicone sealant is a critical component of these designs, it alone will not determine the successful performance of a system. ASTM C1564 Standard Guide for the Use of Silicone Sealants for Protective Glazing Systems discusses design considerations for protective glazing systems.
Structural silicone sealants have been proven through actual high-speed testing to have the strength properties needed for these designs. In a recent study3, two structural silicone sealants were evaluated in a high-speed tensile test, which simulates the high-speed blast loads on a window system. These two structural silicone sealants have an allowable design stress of 140,000 Pa in structural glazing applications. At normal tensile testing pull rates of .00083 m/s, these sealant have peak tensile values in excess of 840,000 Pa for a safety factor of 6. At the high-speed testing at pull rates of 1.1 m/sec to 5.0 m/sec, these same sealants had peak tensile values over 1,600,000 Pa on average for the various joint dimensions tested. Figure 6 shows the results of high speed testing of one of these sealants. This testing once again demonstrates the suitability of high performance structural silicone sealants in bomb blast mitigating window designs.

Protective glazing
Of the thousands of structurally glazed buildings worldwide, only a small but significantly growing percentage of those are designed for protective function. It is fair to say that almost all new government buildings including office buildings, courthouses and embassies utilise a bomb blast mitigating window design. Many existing buildings have been retrofitted using protective film and silicone sealant. Also, many new commercial buildings such as banks and hotels in high-profile locations have been designed or retrofitted with protective glazing systems.
Existing buildings can be renovated with the use of a protective film and a structural silicone corner bead, which is applied from the film to the perimeter framing system (Figure 9). These systems are tested and approved by the protective film manufacturer.

Summary
Structural silicone sealants can provide a product with a proven track record of performance for bomb blast mitigating window systems. Silicone chemistry is appropriate for these demanding designs. High-performance structural silicone sealants provide outstanding durability and adhesion, high strength and flexibility for long term performance. Structural silicone sealants have been independently tested and certified to comply with major European standards. When used with appropriate materials such as laminated glass, high-performance window systems using structural silicone sealants can provide not only an aesthetically pleasing building but one that is much safer for the building’s occupants in the event of bomb blast or natural disaster.

References
[1] Wolf A T, “Durability of Silicone Sealants”, Durability of Building Sealants Rilem state-of-the-art report, A T Wolf, Ed, Rilem Publication, Bagnaux, France 2000, pp. 253-273;
[2] Wolf A T, “Silicone Sealed Insulating Glass Units”, Proceedings of ISAAG – International Symposium on the Application of Architectural Glass – Engineering and architectural design of glass – Munich, Germany, November 15 to 16, 2004, Universität der Bundeswehr München, Neubiberg, Germany (2004);
[3] Yarosh K, Braeuer G, Sitte S, “Behaviour of Silicone Sealants in Bomb Blast Mitigating Windows,” Durability of Building and Construction Sealants and Adhesives, ASTM Standard Technical Publication, Andreas Wolf, Ed, ASTM International, West Conshohocken, PA 2005;
[4] ETAG 002 – European Guideline for Technical Approval for Structural Silicone Glazing Systems SSGS, Part 1 (24 September 1998), Part 2 – Coated Aluminium Systems (16 January 2002), Part 3 – Systems incorporating profiles with thermal barrier (25 May 2002), EOTA, European Organisation for Technical Approvals, Avenue des Arts 40, 1040 Brussels, Belgium;
[5] Pr-EN 13022–2, Glass in Construction – Structural Sealant Glazing – Part 2, Product Standard for Ultraviolet Resistant Sealant and Structural Sealant, CEN European Committee for Standardisation, Brussels, Belgium, 2002.