Laminated glass comprises of two or more layers of glass which are permanently bonded together using an interlayer material. Without the interlayers glass is fragile and will shatter and fall from its frame if impacted, whilst laminated glass will be held in place. Therefore, laminated glass is often specified in the construction of buildings for many safety and security reasons.
Laminated glass can deliver protection against blasts through careful selection of the type, thickness and numbers of layers of both the glass and the interlayer material and the associated manufacturing process. Therefore, it is recommended that laminated glass is specified when designing glazing in buildings for blast loading. The right combination of glass and interlayers is particularly important and, if held securely within a frame, it can provide the appropriate level of mitigation required for your building against blast threats.
Additional advice should be sought from a blast engineer from the Register of Security Engineers and Specialists (RSES) if a higher level of risk from improvised explosive devices has been identified in relation to the building you are concerned with. In this case a more detailed blast protection strategy will need to be developed to bring down the higher levels of risk.
The main considerations in selecting and using laminated glass for blast resistances are:
- laminated glass must be specified when designing for a blast loading, and if used as part of a double glazed unit it must always be on the inside face
- a wide range of interlayers exist and are used to produce laminate glass, to meet varying acoustic and strength requirements, however not all interlayers are suitable for blast loading applications
- it is very important that the right standards for both the production process and the performance are specified
- laminated glass can be bolted, sealant bonded or held with a gasket in a frame. The type of fixing will impact the level of retention.
There are many types of interlayer and to achieve blast protection the selection and specification of the right interlayer is crucial. The main points to consider are summarised below, with important additional detail provided in the guidance note at the bottom of this page.
Specification of interlayers to achieve blast performance
There are a wide range of interlayers that have been developed for different applications. The following points should be considered when blast performance is required:
- EVA (ethylene-vinyl acetate) is not recommended for use in laminated glass where blast performance is required
- PVB (polyvinyl butyral) interlayers should always be specified
- SentryGlas® may be used for blast resistance, however it is much stronger than PVB and its response differs significantly and specialist advice should be sought if this interlayer is proposed.
- the type and thickness of interlayer must be clearly defined to the supply chain (if the wrong interlayer type has been installed, it is difficult to detect after the process)
- check that the laminated glass supplier has the appropriate expertise and quality assurance in place
- care must also be taken to ensure the durability of the interlayers after installation
- laminated glass bonded to a frame with structural silicone will provide a greater level of retention than a gasket seal
When selecting blast resistant glazing, it is very important to make certain that it meets the appropriate standards in relation to both:
- the production process (which covers factory production control standards)
- the performance standard (which sets out the specific performance criteria, for example the predetermined level of resistance required to a person trying to break the glass)
Compliance with these standards must be checked. Best practice is for a BSI Kitemark or branding to be displayed on the glass.
Over time, delamination may occur around the edges of the glass pane. This is a known phenomenon which, as well as being unsightly, may impact the blast performance of the window.