The On The Face Of It blog series has been written to give property professionals an overview of the curtain wall façade, differences in system types and defects which are commonly observed during inspections.
Blog #5 in this series will look at some glass types and provide a quick overview on common glazing defects.
Common glass types
Glass is a very specialised area, so this section is just a high-level overview of some common glass types. For more detailed information on the various glass types (there are loads), take a look at some glass manufacturer websites, where you’ll usually find information about performance and the manufacturing processes.
Annealed – This is a budget glass made from soda lime silicates. The problem with annealed glass is it breaks into sharp shards. This is why it is not appropriate when designing for fire or potential breakage situations.
Toughened – Toughened glass is a safety glass, manufactured using annealed glass which is heated and rapidly cooled. This process creates a compressive stress in the surface. During the heating cycle of the manufacturing process, nickel sulphate impurities (found in all annealed glass) change state and can cause spontaneous breakage of the glass. To reduce the likelihood of this type of failure when the glass is in service (installed), heat soaking can be undertaken, and this will encourage the impurities to revert to their low temperature state. Toughened glass is about 4-5 times stronger in compression and bending than annealed glass and when is breaks, it shatters into small fragments.
Heat Strengthened – Manufactured by heating annealed glass and cooling it under controlled conditions. Heat strengthened glass is not as strong as toughened glass but the likelihood of failure from nickel sulphide inclusions is reduced because the tensile stress in the glass is lower.
Laminated Glass – Considered a safety glass, laminated glass comprises of two or more panes of glass (annealed, toughened or heat strengthened) that are bonded together by a plastic interlayer. When one of the layers break, the interlayer holds the shards together. If both layers break, the structural integrity will be compromised, and the unit can fall out if it is not properly secured in position.
Insulated Gazing Units (IGU) – Formed using two or more panes of the glass types above and separated and sealed to create a cavity between the panes.
Breakages are one of the most common defects I see, and they happen for a number of different reasons. All potential causes need to be reviewed as part of any review and can include:
- poor glazing tolerances with inadequate allowance for movement
- framing deflection, corrosion of the frame (steel) or deflection of the frame (concrete)
- thermal movement
- air pressure changes
- nickel sulphide inclusions – this is not common
When breakages occur, the glass is often removed before crack patterns can be reviewed and this can make identification of the cause very difficult. Crack patterns are incredibly informative:
- impact or point loading can be identified by crush marks
- toughened glass will display wedge-shaped fragments which will extend from the initial point of fracture – this can effectively point to a rusting section of frame, perished gasket etc.
- where a nickel sulphide inclusion expands and causes a breakage, the glass shards adjacent to the inclusion have a hexagonal shape and a butterfly wing crack pattern is visible. Whilst this issue is relatively well known, it is not common so make sure other causes are thoroughly reviewed
Delamination of laminated glass
Besides breakages, the most common defect I see during surveys is delamination. Delamination occurs where the bond provided by the interlayer between the glass layers is lost. The reasons why this occurs, varies.
Minor delamination can occur during manufacturing (autoclave process) and can present as small bubbles along the edge or a cluster of bubbles. It should be noted that this type of delamination is not necessarily a defect. Some manufacturers state that they will not replace new glass where edge delamination is 6mm or less. However, if delamination is more pronounced and is more obtrusive, replacement may be appropriate.
If delamination is not characteristic of a manufacturing fault (as above), the extent of the delamination will be a key factor in determining the timing for replacement. It is best to consult with a glazier to help with these types of determinations.
The cause of the delamination will also need to be reviewed. For example, if there is an underlying defect that has allowed moisture to come into contact with the edges of the unit, this will need to be remedied. The image to the right shows extensive delamination to the glazing. The delamination was present to the full perimeter of the unit and the same characteristics were observed in almost all units in that specific bay of the stick system curtain wall (full height of building). For this issue to be so widespread and for there to be no obvious external defects, this issue must be originating from inside the building. Directly behind these units is a service riser and it is likely the airflow is not sufficient and humidity in the riser has caused the delamination.
Movement and gaps
Now this isn’t strictly a glazing defect but this blog feels like the best place to cover it. Now and then I will come across a glazed unit that is not being properly supported in its frame.
It’s pretty unlikely that the glass would have been installed small, so it is usually the case that the gap I’m seeing is the result of movement. The cause of that movement needs to be understood to make sure the correct repair approach is adopted – simply replacing the unit with a larger one is not going to cut it.
By Victoria Richardson