DURABILITY OF GLASS
Float glass is highly durable in most environments encountered in construction applications, and it does not degrade or change over time, other than the accumulation of microscopic surface damage, which reduces the strength of new glass but tends to stabilise after several years. Therefore monolithic single glazing can usually be regarded as maintenance free for the lifetime of a glazing system, aside from routine cleaning and replacement in the event of breakage. Some environments can have a detrimental effect on glass.
Most notably, alkaline conditions created by contact with cementitious materials can lead to etching of the surface. Even a puddle of water on a glass roof that is too flat to drain properly can become highly alkaline around the perimeter as it dries because some of the alkali fluxes dissolve from the glass. This results in permanent etched marks around the puddle.
When glass is assembled into a double-glazed unit, the edge seals are expected to offer a limited service life of up to 25 years or so. Similarly, interlayers in laminated glazing may not be as durable as the glass itself owing to mechanisms such as yellowing when exposed to UV radiation, loss of plasticisers or delamination.
Glass is usually cleaned with a solution of mild detergent in water, using a mop and squeegee. The mop wets the glass and loosens the dirt, aided by the detergent, and the squeegee is used to sweep the dirty water off the glass, leaving it dry.
The process does not use a great quantity of water, and because it is not left on the glass, it does not matter that the water becomes dirty. If droplets of water are left to dry on glass, they leave faint white rings where salts are leached from the surface and deposited as the water dries.
Self-cleaning treatments on glass, either hydrophobic or hydrophilic, rely on regular wetting by rain to carry away dust and dirt. The drying out of droplets is avoided, either by forcing them to run off as beads or by drawing them out over the surface of the glass until dry, according to the technology employed.
Protection on Site
Glass can easily be damaged on a construction site by impact, particularly on the edges and corners, and minor damage before glazing can result in premature failure in service when thermal and other stresses start to act on the glass.
Welding and grinding works pose a less obvious risk to unprotected glass because sparks, spatter and dust can fuse to the surface of glass. Hot metal particles cause pits and can initiate vents (cracks) into the surface, which may substantially weaken glass, cause scratching when dislodged by window cleaning, or create rust stains when exposed to the weather.
Stacked glass also has to be kept dry because it can be permanently marked if wetted when in contact with other glass or packing materials, especially in the presence of cementitious dust.
Failure of Double Glazed Units
Insulating glass units eventually absorb enough moisture that condensation occurs within the cavity during cold weather, which damages or negates the effect of any coating and spoils the view out. The service life is affected by the quality of the original manufacturing and the conditions in which the unit is used.
If the edge seals are exposed to liquid water for long periods, the life can be dramatically shortened.
Delamination of Laminated Glass
Laminated glass is generally resistant to occasional wetting of its edges, if they are allowed to dry out but, like insulating units, can be rapidly damaged by standing in water. Early effects may be seen as a white ‘fogging’ of the interlayer, followed by progressive loss of adhesion between the interlayer and the glass.
WHAT TO DO IF GLASS BREAKS
Broken glass is often viewed as dangerous rubbish to be disposed of as swiftly as possible, but in many cases someone will want to know why the glass broke, and that can only be determined if the evidence is preserved. If glass presents a safety risk because it might fall and cause injury then the first priority should be to exclude people from the area where it might fall. If it has to be removed for safety, it is usually helpful to photograph the breakage pattern as clearly as possible and collect all the glass fragments for later examination.
Cracks in all materials have markings that relate to the manner of breaking and the properties of the material. The study of those markings is known as fractography, and it is especially useful in the understanding of brittle materials like glass. Whenever glass is tested to destruction or glass breaks in service there are lessons to be learnt by the designer from a study of the fracture markings.
There are several texts on the subject of fractography, which should be required reading for any engineer wishing to work with glass or other brittle materials (e.g. Quinn, 2007).
DISPOSAL AND RECYCLING OF GLASS
The raw materials required to produce glass are available in abundant supply. However, the energy cost of actually producing glass from the raw materials is high owing to the temperatures involved. Further high-energy procedures such as toughening and heat-soak testing may be carried out once the float process has been completed.
This should be set against the fact that glass is a durable material, and therefore offers the benefit of prolonged, lowmaintenance service superior to many of the possible alternatives. Such materials, for example historic stained-glass windows, can be seen in many old buildings, where material hundreds of years old has survived since construction and continues to function as intended. The stability of glass makes disposal difficult, as it will not readily break down.
Recycling is a viable option, by crushing, re-melting and reforming waste glass into a new product. This process is not difficult to carry out, however it tends to lead to contamination and it is difficult to produce recycled glass of the highest optical quality. Therefore recycled material is mostly used in non-architectural applications, such as coloured drinks bottles, where visual quality is less critical. Reusing crushed glass directly, for example as a secondary aggregate for concrete or screed, is also possible though the level of demand is limited.