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Sustainability of glass in construction
M. Achintha
University of Southampton, UK
Abstract:
This chapter discusses the potential applications of glass to deliver dynamic design solutions that enable
buildings to be more energy efficient by making use of the most of daylight and solar gain whilst
protecting the environment and conserving energy. The article also provides an overview of recent
advances in modern glass products, and their potential applications in building envelops that can
engineer reductions in the operational carbon. Glass is a brittle material, and its structural behaviour
poses greater challenges when designing load-bearing structural members in buildings envelops. An
overview of existing design guidelines of structural glass together with the need for detailed finite
element analyses in accurate designs are briefly presented. The sustainability of glass as a construction
material, including methods of recycle and reuse, is discussed. An outline of possible future
developments in the use of glass in buildings is also presented.
Keywords: Carbon reduction, construction materials, embodied carbon, float glass, glass, low energy,
operational carbon, passive house, structural design, sustainability
5.1 Introduction
Glass is one of the most favoured materials with widespread applications, such as a building
construction material, use in containers and vessels, as windows in the automobile industry, in
nanotechnology applications such as optical fibres, etc. Many qualities make glass attractive, as it is
transparent, chemically inert, environmentally friendly, sustainable, strong, easily available and
relatively cheap. There is no other widely available material which possesses these qualities. In the
construction industry, glass has traditionally been used as window panes in buildings, but the use of
glass as a main building material has become increasingly popular during the past 25 years (IStructE,
2014). The recent advances in glass technology and the architectural, sustainability and environmental
considerations promote the use of glass in buildings. It is hard to envisage any modern building without
glass windows/ facades. The use of glass to build well-lit and spacious buildings has become more
prominent. A modern glass building structure is shown in Fig. 5.1.
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Fig. 5.1 Apple Store on Fifth Avenue, New York
5.2 Silica glass
The main compound of glass is silica (SiO2), which is the primary constituent of sand. Natural glass,
which is existing for millions of years, formed when certain types of rocks melted as a result of high-
temperature phenomenon such as volcanic eruptions and lightning, and then cooled and solidified
rapidly (Le Bourhis, 2008). Stone Age people may have used natural glass as tools owing to its high
strength and sharpness. The oldest use of manufactured glass may be in around 3500 BC in Egypt where
coloured glass was used as jewellery and as vessels to store liquids (IStructE, 2014). Manufactured glass
contains considerable amounts of various metal oxides, mainly soda (Na2O) and lime (CaO), in addition
to the main constituent silica. Therefore, it is known as ‘soda-lime-silica glass’ or ‘soda-lime glass’. Soda-
lime glass is the most widely used silica-glass type in the world. Typically, soda-lime glass contains 69–
74% silica, 5–14% lime, 10–16% soda and other minor ingredients such as magnesia (MgO) and alumina
(Al2O3) (Haldimann et al., 2008). The function of soda is to lower the melting point of soda-lime glass to
a value between 400 and 600 oC from that of 1723 oC of silica (Le Bourhis, 2008). This low melting point
enables the bulk production of soda-lime glass. Glass containing only silica and soda will have poor
durability, and it is often water soluble; the addition of lime makes glass more durable. Pure silica-glass
is still used in special applications, for instance, in windows of spacecraft where glass is exposed to
temperatures up to 1200 oC.
Other varieties of silica-glass, which possess specific properties include borosilicate glass, lead-oxide
glass (crystal glass), and aluminosilicate glass. In borosilicate glass, boric-oxide (B2O3) is used instead of
soda in soda-lime glass. Borosilicate glass has a high resistance to temperature changes, as a result, it is
commonly used in household cookware. Lead-oxide glass contains 18–40 % of lead oxide (PbO) instead
of soda in soda-lime glass (IStructE, 2014). Owing to its high density, lead-oxide glass has a high
refractive index, and as a result, lead-oxide glass possesses attractive optical properties. Lead-oxide
glass is used to produce perfectly clear and flawless objects such as glassware. Aluminosilicate glass
contains about 20% alumina and small amounts of lime, magnesia and boric oxide, but only a very small
amount of soda. Aluminosilicate glass has ability to withstand high temperatures and is typically used to
manufacture fibre glass and glass fibre-reinforced polymers.
Soda-lime glass in its flat sheet form is the glass type most used in the construction industry; the
discussion in this chapter is limited to soda-lime, flat glass sheets. The readers interested in other types
of glass should refer to subject specific text (eg, Le Bourhis, 2008).
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5.3 Production of soda-lime-silica flat glass sheets
The float glass process, which was originally developed by Pilkington Brothers in 1959 (Haldimann et al.,
2008), is the most common manufacturing process of flat glass sheets. More than 80–85% of the global
production of float glass is used in the construction industry (Glass for Europe, 2015a). In the float glass
process, the ingredients (silica, lime, soda, etc.) are first blended with cullet (recycled broken glass) and
then heated in a furnace to around 1600 oC to form molten glass. The molten glass is then fed onto the
top of a molten tin bath. A flat glass ribbon of uniform thickness is produced by flowing molten glass on
the tin bath under controlled heating. At the end of the tin bath, the glass is slowly cooled down, and is
then fed into the annealing lehr for further controlled gradual cooling down. The thickness of the glass
ribbon is controlled by changing the speed at which the glass ribbon moves into to the annealing lehr.
Typically, glass is cut to large sheet of 3 m x 6 m. Flat glass sheets of thickness 2–22 mm are
commercially produced from this process. Usually, glass of thickness up to 12 mm is available in the
market, and much thicker glass may be available on request. A schematic diagram of the production
process of float glass is shown in Fig. 5.2.
Fig. 5. 2 A schematic diagram of the production process of float glass
Once manufactured, float glass, which is also known as annealed glass, is sometimes processed further
to produce tempered glass and/or laminated glass. Tempered glass is also known as toughened glass
and is stronger than float glass. The laminated glass has enhanced post-breakage performances, safety
on impact, improved fire resistance and special properties such as noise control. Details of the
manufacturing methods and the mechanical characteristics of tempered and laminated glass are
discussed in Section 5.9.
5.4 Properties having influence on choice of glass as a construction material
A unique combination of fascinating physical, optical, chemical, and thermal properties makes glass the
most preferred construction material in modern buildings. The appropriate use of glass windows, doors,
roofs, staircases, partitions, etc. makes buildings bright, airy, energy efficient, and it also enhances
comfort of the occupants.
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5.4.1 Physical and optical properties
The most striking feature that contributes to the widespread use of glass in buildings is its transparency
to visible light. Owing to the absence of internal subdivisions such as grain boundaries in the
microstructure, glass does not scatter light, and as a result, it is transparent. Glass also has smooth
surfaces, since during the formation the molecules of the super-cooled liquid are not forced to dispose
in rigid crystal geometries and can follow surface tension (Haldimann et al., 2008). The recent
developments of high-tech glass products expands the range of applications of glass beyond the merely
decorative, to functional and structural roles. Light, comfort, well-being, style, safety, security, and
sustainability are among the benefits that can be achieved from the appropriate use of modern glass
products in buildings.
5.4.2 Chemical and thermal properties
One of the key properties of glass is its chemical inertness and general resistance to exposed
environment. Glass is one of the most durable materials used in the construction industry. The chemical
inertness is attributed to its microstructure: an irregular network of silicon and oxygen atoms with
alkaline parts in between. Glass is also an electric insulator, since there are no charged particles such as
free electrons in metals or ions in an electrolyte fluid that can move creating an electric current. The
thermal expansion coefficient of soda-lime glass is 8–9x10-6 K-1, and this is of similar magnitude as that
of the two most widely used construction materials, concrete (12x10-6 K-1) and steel (11–13x10-6 K-1).
The specific heat of glass (the amount of heat required to raise the temperature of unit mass of glass by
1 K) is ~0.8 Jg-1K-1. The thermal conductivity of glass (the amount of heat transmitted through a unit
thickness – in a direction normal to a surface of unit area – due to unit temperature gradient under
steady state conditions) is ~1 Wm-1K-1. The relatively low specific heat, high thermal conductivity, and
the use of less volumes of the material mean glass members have lower thermal mass than that of
equivalent concrete/steel/masonry structures.
5.4.3 Stress corrosion cracking
Despite its well-known chemical inertness characteristics, glass is susceptible to stress corrosion
cracking: small flaws grow slowly when exposed to crack-opening stresses in the presence of water or
water vapour. This phenomenon is also known as ‘static fatigue’ or ‘slow crack growth’. Typically, under
moderate tensile stresses, subcritical cracks in silica-glass can propagate at velocities of 10-12 to 10-5 ms-1
(Lechenault et al., 2011). The mechanism of stress corrosion cracking is complex, and despite the
problem has been studied since 1960s (Wiederhorn, 1968; 1969), the process is not yet fully understood.
One of the most acknowledged mechanisms was proposed by Michalske and Bunker (1984): water
molecules break the Si-O bonds located at the crack tip as a result of a hydrolysis reaction. Under
modest applied tensile stresses, the crack velocity is governed by the rate of the chemical reaction,
which depends on both the degree of humidity and the applied tensile stress. Because of stress
corrosion cracking, the strength of glass depends on the rate of the applied loadings. In-depth
discussions of stress corrosion cracking in glass may be found in elsewhere (eg, Ciccotti, 2009).
5.4.4 Surface Coatings
Properties such as visual appearance, optical, and thermal properties of float glass may be modified by
applying surface coatings. For instance, coatings can regulate certain wavelengths of visible and non-
visible light which are reflected and/or transmitted through glass, and thereby able to control solar
energy passing through the glass or to reflect the heat energy back inside the building. The coatings may
be applied either offline or online. Offline coatings are applied after the glass is manufactured and cut,
usually by dipping glass panes into chemical solutions or by the evaporation of metals onto surfaces in a
vacuum. Coatings which give different colours, reflectivity and thermal properties are applied in this
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way. Online coatings are applied whilst the glass is hot and still in the lehr, and as a result, they form a
strong bond to the glass and are usually more durable than offline coatings. Solar control and low
emissivity (low-e) coatings are mostly applied online. Multifunctional coatings are used in modern high-
tech glass products, the coatings do not adversely affect the desired properties of the original glass. The
coatings are durable and usually last as long as the glass member. The applications of some of the
special surface coatings are discussed in Section 5.6.
5.5 Glass as a construction material
The worldwide increase of CO2 emissions because of the high consumption of energy is signalling an
alarm for society to focus more on low carbon and energy-efficient buildings. There is a need for
professionals in the construction industry and also for governments to be well versed to engineer a
sustainable built environment. Glass proves to be a very attractive building material, which provides
opportunities for the development of innovative, energy-efficient building envelops. Glass also has
invaluable use in renewable solar energy technologies, such as photovoltaic systems and in solar
thermal collectors.
5.5.1 Low carbon and sustainable construction
In simple terms, sustainability is the more prudent use of natural resources for the protection of the
environment. It is anticipated that the global population will be increased to 9 billion people in 40 years
from the current population of about 7.2 billion. This poses great challenges on the development and
maintenance of key infrastructures, such as housing, energy, transport, communications, waste, and
water, whilst offsetting negative economic, social, and environmental impacts. Sustainable
constructions are required to improve the long-term social and ecological health of key infrastructures.
5.5.1.1 UK construction strategy
The global construction market and the sustainable construction industry are forecast to grow by over
70% and 23% respectively, by 2025 (compared to 2012) as a result of the low carbon regulatory
requirements and the greater social demands for greener products (Great Britain, 2013a). A significant
business opportunity exists for low carbon construction; it is expected that the requirement for a green
society will drive the future construction markets over the next few decades. In recent years, the UK
government put forward clear aspirations which provide confidence in businesses and people to invest
in sustainable and low carbon construction. For instance, UK’s Construction Strategy 2025 explicitly
identifies low carbon and sustainable construction together with ‘smart construction and digital design’
and ‘improved trade performance’ as the strategic priorities which underpin sustained growth across
the economy and improved quality of life for people (Great Britain, 2013a).
5.5.2 Low carbon, energy-efficient buildings
The UK goal is to achieve 80% reduction in carbon emissions by 2050 (taking 1990 as the baseline year).
Although extensive measures towards low carbon footprint have already been undertaken in some
industries (eg, electricity generation), the construction industry has not yet made significant progress.
Buildings presently account for nearly 45% of the total carbon emission (The Royal Academy of
Engineering, 2010). The scale of the challenge in reducing the carbon footprint of buildings is immense;
the construction industry must adapt the low carbon paradigm. The design of low carbon, energy-
efficient buildings is quite distinct from the designs of traditional 20th-century ‘energy hungry’ buildings;
creative designs and rigours engineering analyses are required. A ‘green’ building should encompass six
main features of sustainability: sustainable sites, water efficiency, energy and atmosphere, material and
resources, indoor environmental quality, and innovation in design (US Green Building Council, 2012).
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The carbon footprint of a building is generally the CO2 equivalent of all greenhouse gases associated
with the construction and the operation of the building throughout its life time (Great Britain, 2013b).
The carbon footprint is twofold: (1) Capital carbon (also referred to as embodied carbon) is the total
embodied carbon of the materials and the construction process, and (2) Operational carbon is the
carbon associated with the operation and maintenance, for instance, fossil fuel required for lighting,
heating and all other day-to-day operations of the building. Reducing the carbon footprint is
fundamentally important to long-term global economy, social, and environmental sustainability.
Although the low carbon paradigm is an enormous challenge, its pursuance stimulates innovation in
design and construction where the reductions in capital and operational carbon ensure resource
efficiency and economic benefits.
5.6 Applications of glass to engineer reductions in operational carbon
Many buildings constructed in the 20th-century are mainly dependent on fossil fuel energy to make
them habitable. As the need for sustainable buildings is more pressing than ever, energy efficiency is the
principal driver of modern buildings. Energy-efficient buildings are required to limit the growing gulf
between the carbon regulations and what is being actually delivered at present. Although zero sum
energy buildings may be built by installing renewable energy sources alongside conventional buildings,
such designs are expensive and are not sustainable. Glass offers dynamic design solutions which have
potentials to make buildings to be energy efficient through the use of daylight and solar gain whilst
conserving energy. Glass is the most striking feature in modern building designs.
5.6.1 Features and benefits of glass in buildings
Energy required for heating, cooling, lighting, and ventilating of conventional buildings contributes to a
major portion of the total carbon footprint. By combining the knowledge of ‘Building Engineering
Physics’ (ie, exploitation of natural science that relates to the performance of buildings and their indoor
and outdoor designs) together with the creative use of glass in building envelops, it is possible to reduce
the demand for artificial energy.
5.6.1.1 Daylighting
The most striking property of soda-lime glass is its transparency to visible light (wavelength, 380-
750 nm). Glass has a refractive index of 1.5, and the reflection of visual light is about 4% per surface;
hence, the transmissivity of one glass sheet (ie, two surfaces) is more than 90% (Haldimann et al., 2008).
By applying special surface coatings, the transmissivity of glass can be further improved and such high-
transmissivity glass are available in the market. Because of transparency of glass, whether a window or
a fully glazed facade, can provide invaluable daylighting into buildings. For an example, Fig. 5.3 shows a
modern building which is lit well using daylighting. Daylighting is essential for the function of the
buildings, and it also helps to improve the health and productivity and to regulate the biological clock of
the occupants. Proper daylighting designs can avoid the need for artificial lighting for a majority of the
day/year, and consequently, can lead to 30–50% savings in the total energy bill of certain buildings
(Aboulnaga, 2006).
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Fig. 5.3 An example of a modern building which is lit well using daylighting
Although daylighting can easily be provided through glass windows/facades, it is necessary to monitor
the intensity, distribution, glare, colour rendering, etc., to create stimulating high quality interior
environments. To achieve a good lighting design, many factors must be taken into account:
characteristics of glazing and its orientation, solar control elements such as blinds and louvers, the
geometry and the space organisation of the building, surface properties of internal partitions, and
distance and orientation with respect to windows/glazing. Inefficient designs could lead not only to
poor daylighting, but could also adversely affect the comfort and the productivity of the occupants.
Daylighting systems should be free from too much solar gain, brightness, glare, nonuniform lighting, etc.
Solar control can be achieved by using specially designed solar control glass. It is possible to offset glare
interfering with work tasks such as computer screens by the use of exterior shading, window blinds,
reflective louvers, low-transmission glass, and optimal placement of windows/glazing. The provision of
high windows with sloped celling together with light shelves that redirect light brings daylight deep into
interior spaces without overheating and glare. Readers interested in in-depth discussions of daylighting
designs should refer to subject specific text (eg, Baker and Steemers, 2014).
5.6.1.2 Solar control glass
Visual light passes through glass, and heats up the interior. The emitted long-wave thermal radiation is
unable to escape through glass because it is absorbed by Si-O groups; this origins the greenhouse effect.
In winter, the fact that glass allows solar gain as well as light into the building is beneficial, but in
summer months, without solar control, it can become uncomfortably warm. Glass building envelops
should be able to ensure maximum comfort, aesthetic appearance whilst minimising the energy
consumption throughout the year irrespective of the climatic conditions. The solar control glass
products available in the market can be used to avoid overheating of buildings whilst still maintaining
high levels of daylight when exposed to sun. Solar control glass is required in buildings which have large
areas of glass facades such as the modern commercial building shown in Fig. 5.4.
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Fig. 5.4 Solar control glass is required in buildings which have large areas of glass facades
Solar control glass regulates solar radiation by managing reflection, transmittance and absorption. In
the past, highly reflective glass was used in windows to control solar gain, but the use of reflective glass
also reduces daylight entering the buildings and the mirror-like buildings, as a result of reflection of light,
are not architecturally pleasing. Modern solar control glass uses tinted/translucent/opaque/patterned
coatings or interlayers to regulate the passage of solar radiation. Solar control glass with interlayers that
blocks UV (ultra violet) light can be used to protect materials which are sensitive to UV (Nitz and
Hartwig, 2005). Generally, modern solar control glass products possess multifunctional benefits, such as
low-e, thermal insulation, noise/sound control, etc. In-depth discussions of solar-controlling
technologies used in modern glass products can be found in subject specific text (eg, Nitz and Hartwig,
2005; Smith et al., 2002).
5.6.1.3 Thermally-insulated glazing and low-e glass
Thermal insulation is an issue of great interest in colder countries where energy is required for space
heating. Depending on the exposed environment, up to 25% of the heat from residential and
commercial buildings may escape through the windows (Jelle et al., 2012). To minimise the
environmental impacts and the rising energy bills, it is essential to save energy. The United Kingdom and
other governments regulate minimum requirement for the energy efficiency. For instance, the ‘Green
Deal’ introduced in the United Kingdom encourages home owners to enhance energy efficiency through
the provision of an upfront finance to undertake energy improvement measures, with repayments over
time offset by savings on energy bill (Great Britain, 2010). Thermally-insulated glass can stabilise the
internal temperature, and consequently be able to reduce the energy need for heating and cooling.
Improvements in thermal insulation also allow the incorporation of larger areas of glazing for
daylighting and solar gain.
The recent advances in thermally insulated glazing technology include insulating glass units (IGU) and
low-e glass along with the improvements in frames and spacing designs (Sadineni et al., 2011). In an IGU
unit, two or more glass panes enclose a sealed air space whilst the whole unit is assembled by a
secondary edge seal, usually silicone. IGUs have low heat transfer coefficients (U-values) of about 1
Wm 2K-1 and about 0.7 Wm-2K-1 for double glazed and triple glazed units, respectively; the values are
significantly lower than the U value of 5.8 Wm-2K-1 of conventional single glazing (Haldimann et al.,
2008). Low-e glass has an invisible coating (usually tin oxide or a silver-based coating) (Hammarberg and
Roos, 2003) that regulates wavelengths of energy, thereby reducing the heat transfer and reflecting the
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heat back into the interior. Low-e glass is more suitable for rooms/buildings which have high
proportions of windows/glass doors. Manufacturers of low-e glass products (Pilkington, 2015b; Dupont,
2015) claim savings up to 75% compared to conventional single glazing. In-depth discussions of thermal
insulating technologies used in modern glass products can be found in subject specific text (eg, Sadineni
et al., 2011).
5.6.1.4 Noise control glass
Since the windows is the primary path through which noise enters a building, it is necessary to have
sound insulation glass in nosier environments, such as those close to airports, highways, cities, etc.
Propagation of sound may be retarded by either reflecting the noise back towards the source, or by
absorbing the energy within the glass. Resin-based interlayers, those bonded between the glass sheets
of laminated glass, are used to reduce the propagation of sound through glass windows. Damping
effects of the interlayer retard the vibrations, thereby suppressing the acoustic noise. The mass of the
glass also has a significant effect on the sound attenuation; thick glass laminates usually possess
satisfactory acoustic properties. Noise control, laminated glass are available in combination with other
special properties such solar control and low-e. In-depth discussions of noise-controlling technologies
used in modern glass products can be found in subject specific text (eg, Zhu et al., 2004).
5.6.1.5 Vibration control glass
As in noise control glass, dissipation of energy is required to control vibrations. Viscoelastic materials
are widely used in many industries to damp vibrations; laminated glass with interlayers, which possess
damping properties, are widely used in glass members subject to vibration-induced loadings, for
instance, in floor plates and treads for staircases in buildings (Haldimann et al., 2008). A more in-depth
discussion of contemporary glass products is beyond the scope of this chapter. Interested readers
should refer to detailed text on the subject (eg, Koutsawa and Daya, 2007).
5.6.1.6 Self-cleaning glass
Build-up of dirt (soiling) and the subsequent decay of optical properties is one of the main problems
encountered in high-rise buildings with large glazing. Fig. 5.5 shows a glass window which has lost
optical properties as a result of the built-up of dirt. Due to the cost and the challenge associated with
cleaning of dirty glass, the use of self-cleaning glazing has become popular in recent years. A special
coating (usually, nanostructured TiO2), which has an innovative dual action, is used in modern self-
cleaning glass products. Once exposed to sunlight (UV radiation), the coating chemically reacts with
oxygen and the water molecules present in the atmosphere, and subsequently breaks down the organic
dirt deposits, for example, bird droppings (Haldimann et al., 2008; Chabasa, 2008). The rain water then
easily washes away the loosened particles. Self-cleaning glass also functions well in prolonged dry spells
and in areas those protected from direct rainfall; it is only necessary to washes down with water. Similar
to other special coatings, self-cleaning glass can be combined with other properties such as solar control.
In-depth discussions of modern self-cleaning glass technologies can be found in subject specific text, (eg,
Chabasa, 2008).
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Fig. 5.5 A glass window which has lost optical properties due to the built-up of dirt
5.6.1.7 Fire resistance glass
The resistance of glass to high temperatures is low, and glass transmits heat rapidly. Float glass usually
breaks due to thermal shock when the temperature difference is about 40 oC, whereas toughened glass
can withstand temperature differences up to 200oC (IStructE, 2014). Glass also starts to soften and loses
stiffness at temperatures above the glass transition temperature (~500oC). Special fire-resistance glass is
required to achieve satisfactory performances against fire. Borosilicate glass has relatively high
resistances to thermal shocks as opposed to soda-lime glass, since its thermal expansion coefficient
(9x10-6 K-1) is higher than that of soda-silica glass (6 x10-6 K-1). The modern fire-resistance glass products
use laminated glass with intumescent interlayers (Haldimann et al., 2008). When one side of the
laminate exposes to a fire, the interlayer expands into an insulating foam after absorbing heat from the
fire, and subsequently protects the second glass sheet (Haldimann et al., 2008). The modern glass
products have potential to withstand moderate fires expected in residential/commercial buildings for
up to 3 hours (Pilkington, 2015a). Old technologies of fire-resistance glass include laminated glass with a
wire mesh (see Fig. 5.6), in which the wire mesh keeps glass in place after it cracks. Although the
application of a wire mesh has potential to improve the fire resistance, it weakens the glass due to the
surface flaws induced by the wire mesh.
Fig. 5.6 Wire mesh can improve the fire resistance but the surface flaws caused by the wire mesh
degrade the strength of the glass
5.7 Use of glass in low energy, passive house buildings
At present, a significant portion of the total production of electricity/gas energy is consumed by
buildings, especially in developed countries. For instance, about 40% of the total US primary energy is
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consumed by buildings (United States, 2009). There is an imperative need for energy efficiency in
buildings. Energy efficiency is one of the key attributes of a green building. In recent years, there is a
radical overhaul in the approach for building designs where reductions in the demand for artificial
energy required for lighting, cooling/heating, ventilation, etc., are intended. The recent developments in
energy-saving technologies are mainly twofold: (1) active technologies, such as heat pumps coupled
with air/ground water heat sources, solar thermal collectors, renewable energy sources such as solar
photovoltaic panels and wind power, etc., and (2) passive technologies, which include increased
insulation, efficient use of daylight and solar gain, heat recovery from ventilation air and/or waste water,
etc. (Sartori and Hestnes, 2007).
The use of active technologies such as renewable energy sources is expensive, and the solutions may
also increase the embodied carbon of the buildings. On the other hand, passive technology based
building designs (eg, passive houses), in which the design exploits passive technologies to diminish the
energy demand, can significantly reduce the total energy requirement without increasing the embodied
carbon or the total construction cost. Passive house systems outperform conventional buildings in
terms of living conditions and energy efficiency due to the heat recovery, good thermal insulation and
the overall optimisation of the buildings (Sartori and Hestnes, 2007). Some renewable energy sources
may also be used in these buildings to generate the small energy demand, and consequently, to fulfil
zero sum total energy.
Development of passive house buildings is a win-win situation due to the potential reductions in the
energy demand and the carbon footprint. The recent innovative advances in glass products (Section 5.6)
mean glass has become the most vital building material in low energy, passive house buildings. As
described previously, appropriate use of glass can reduce heat loss but allows solar gain to heat
buildings. Proper use of solar control glass whilst maintaining the transmittance of daylight can
eliminate the need of artificial air-conditioning systems. In a properly designed passive house building,
the internal temperature can be kept between 20°C and 26°C and the indoor relative humidity between
30% and 60%, ensuring the building is comfortable in all seasons. Passive house buildings are now
constructed all around the world; significant energy savings and high levels of occupant satisfaction are
already being noted. In-depth discussions of passive house designs can be found in subject specific texts
(eg, Sartori and Hestnes, 2007; Baetens et al., 2010; Cheung et al., 2005; Pacheco, 2012).
5.8 Glass: A sustainable construction material
The applications of glass in buildings to engineer reductions in the operational energy have already been
discussed in this chapter. The embodied energy/carbon impact of glass and the sustainability of glass as
a construction material, including reuse and recycling, are discussed in the following sections.
5.8.1 Embodied energy and carbon
The embodied energy of a building is the energy consumed by all the materials and the processes
associated with the construction of the building – from the mining and processing of natural resources
to manufacturing, transport, and product delivery (Sattary and Thorpe, 2012). Similarly, the embodied
carbon of a building is the total carbon associated with all the materials and processes used over the
total life cycle of the building. The annual use of over 400 million tonnes of materials in the UK
construction industry (Langdon, 2009) indicates the high embodied energy/carbon impact of the
industry. If the UK is to achieve the ambitious target of 80% reductions in carbon emissions by 2050,
greater reductions in the total embodied carbon of buildings are required. This can be achieved by: (1)
reducing the amount of materials used and minimising the waste, (2) lessening the use of impact
materials and energy-intensive manufacturing methods, and (3) using good environmental management
methods including reuse and recycling of the materials (Waste & Resources Action Programme, 2015).
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5.8.2 Embodied energy and carbon of common construction materials
The embodied energy/carbon of buildings been somewhat neglected in government regulations as the
current regulations mostly focus on the reduction of operational energy/carbon. According to Rawlinson
and Weight (2007), the embodied energy of a typical complex commercial building in the United
Kingdom may be equivalent to 30 times its annual operational energy use. Sturgis and Robert (2010)
estimated that the embodied carbon can account for up to 45% of the total carbon impact of a building
over its life cycle. Although an analysis of embodied energy/carbon is required in order to evaluate the
total impact of a given building, a reliable investigation of the embodied energy/carbon is not trivial. For
instance, transportation can affect the embodied energy – a material manufactured and used in London
has an embodied energy impact different from the same material is transported by road to Edinburgh.
Recycled materials are sometimes used for the manufacturing of new products, and these products
usually have a lesser carbon impact. It is also difficult to take into account the energy required for the
maintenance, repair, and refurbishment of a building over its life cycle.
Despite the difficulty of accurate analyses of the embodied energy/carbon impact of construction
materials over the life cycle of a given building, few methods have been reported in the literature. One
such method is the ‘University of Bath’s inventory of carbon and energy database’ (Hammond and Jones,
2006), and this inventory provides an open-access database of energy/carbon impact of over 400
materials (Hammond and Jones, 2008). This database has been employed by various researchers and
developers of carbon footprint calculators, including the UK’s Environmental Agency’s carbon calculator
for construction (Hammond and Jones, 2008).
5.8.3 Embodied energy/carbon of glass
Table 5.1 shows a comparison between the embodied energy and carbon values of glass and the two
mostly-used construction materials, concrete and steel.
Table 5.1 – Embodied energy and carbon values of glass, concrete, and steel (Hammond and Jones,
2006)
Material
Embodied Energy
MJ/kg
Embodied Carbon
MJ/kg
Float Glass
Toughened glass
Reinforced concrete
Steel (Bar and Rod)
15
23.5
1.39
24.6
0.232
0.346
0.057
0.466
Although the exact embodied energy/carbon impact of construction materials depends on the actual
application, the values quoted in Table 5.1 may be used to investigate the relative impact of glass
compared to concrete and steel. Float glass has an embodied energy/carbon of 15/0.232 MJ/kg which is
less than that of steel (24.6/0.466 MJ/kg), but greater than reinforced concrete (1.39/0.057 MJ/kg)
(Table 5.1). A major portion of the embodied energy/carbon of glass is attributed to the high
temperature production process. Owing to the secondary heating process used in toughened glass, it
has greater embodied energy/carbon (23.5/0.346 MJ/kg) than float glass. It should be noted that,
although concrete has a relatively low embodied energy per unit mass, its global impact is greater than
that of glass due to the large volumes of concrete used in the construction industry (note: reinforced
concrete is the most widely used construction material in the world with an estimated annual
consumption of over 12 billion m3 (~25 gigatonnes) of concrete (Gursel et al., 2014) and ~200 billion kg
of steel (World Steel, 2010)). In the United Kingdom, the current average embodied carbon impact of
concrete is around 100 kg of CO2 per tonne (The Concrete Centre, 2015). On the other hand, the
13
mass/volume of glass needed to construct members of buildings is less than that required for an
equivalent concrete member. In addition, glass is more durable than steel and concrete, and the use of
glass also has potential to reduce the operational energy/carbon impact of buildings. Thus, glass is a
more sustainable construction material than concrete and steel, in spite of the embodied energy of
glass is greater than that of concrete in unit mass basis. More in-depth discussions on embodied energy
and the sustainability of construction materials can be found elsewhere (Hammond and Jones, 2008,
2009; Khatib, 2009)
5.8.4 Reduction of the impact of embodied energy/carbon of glass
Buildings which are efficient in terms of the amount of materials used are also efficient in terms of
embodied energy and carbon. The amount of materials required for construction can be optimised by
improving the overall efficiency of the designs, for example, better specification of the design guidelines,
optimal structural designs, avoiding over-engineering, designs for future use including adaptability and
flexibility, etc. It is also important to reduce the waste of the materials, for instance, through the use of
skill workmanship and off-site construction. Another important way to reduce the impact of embodied
energy/carbon is to recycle and reuse the materials.
5.8.4.1 Recycling glass
Although glass bottles are usually recycled, glass sheets those used in buildings are not recycled. This is
mainly due to the difficulty of removing the coatings and other materials (eg, adhesives, metals, glass
produced by other manufactures, etc.) mixed with waste glass. The low energy savings that can be
achieved from recycling is another reason for the lack of recycling of glass. Table 5.2 provides the
potential energy savings that can be achieved by recycling glass and a few other materials, aluminium,
plastic and cardboard (Sattary and Thorpe, 2012).
Table 5.2 – Potential energy savings by recycling glass, aluminium, plastic and cardboard (Sattary and
Thorpe, 2012)
Material
Energy required to produce
from virgin material: MJ/kg
Energy savings by using
recycled materials (%)
Float Glass
Aluminium
Plastic
Cardboard
15
250
98
26.5
5
95
88
24
Recycling of glass can only save up to 5% of the energy required in the original production using the raw
materials (Table 5.2). This is a very low energy saving compared to that can be achieved from recycling
aluminium and plastic, where the potential energy savings are 95% and 88%, respectively. During
recycling of glass, the cleaned old glass crushed into small glass pieces and mix with the original raw
materials (silica, lime, and soda, etc.). The mix is then heated and annealed in the same way as the
production using pure raw materials (see Fig. 5.2). Due to the high energy required for this process, the
energy saving that can be achieved by recycling is limited to 5%. Although recycling of waste glass is not
particularly appealing, waste glass can be reused in many ways.
16.8.4.2 Reuse of glass
In Europe, each year over 1.2 million tonnes of waste are generated by the demolition of and
renovation of buildings in which the waste glass is only about 0.6% (Glass for Europe, 2015b). Glass
manufacturers usually do not recycle most of the waste glass, and therefore the reuse of the material is
14
important. Since glass is a hard, relatively inert material, it naturally lends itself to use as aggregate in
concrete. Owing to the different colours and glistening when sunlight falls on glass, concrete with glass
aggregate possess excellent aesthetics (see Fig. 5.7). However, glass aggregate in concrete has durability
issues due to the alkali-silica reaction. Research investigations, however, showed that the alkali-silica
reaction may not happen if the waste glass is finely grounded, usually smaller than 75–100 m size
(Corinaldesi et al., 2005).
Another successful application of waste glass is its use as an alternative aggregate in bituminous
materials in road construction, where the term glassphalt is being used (Khatib, 2009). The amount of
glassphalt replacement of conventional aggregate ranges up to 50%, but more commonly about 10% is
used (Khatib, 2009). Fig. 5.8 shows a close-up of the surface of a road constructed using glassphalt.
Waste glass, after being crushed to a size specified by the end-user, can also be used as glass beads for
reflective paint, as pipe cushion for French and storm drain systems, and as an abrasive (such as
sandblasting grit) (Khatib, 2009). Other useful applications of fine waste glass are ashtrays, filter media
for swimming pools, golf course sand traps, aquarium sand, etc. (City and County of Honolulu, 2005).
The reuse of waste glass is greener than recycling, and the availability of many reuse applications mean
almost all waste glass is reusable.
Fig. 5.7 Use of glass aggregate in concrete
Fig. 5.8 Surface of a road constructed using
glassphalt
5.9 Glass as a structural material
The recent architectural and technological developments in the use of glass in building envelops pose
the challenge for structural engineers to design large areas of glass panels, roofs, floors, staircases and
partitions. All these glass members will have structural roles compared to the small glass panes used in
the traditional four-edge supported windows that engineers familiar with for centuries. Since glass is a
brittle material, its structural behaviour is significantly different from that of more familiar materials
such as steel and reinforced concrete. The structural designs of glass must take into account the
fundamental material behaviour.
5.9.1 Mechanical properties of glass
Glass is a perfectly linear-elastic and isotopic material. The typical values of Young’s modulus (E), Shear
modulus (G), Poisson’s ratio (), and density () of glass are given in Table 5.3.
15
Table 5.3 – Basic mechanical properties of glass (BS EN 16612, 2013)
Mechanical property
Value
Young’s modulus, E
Shear modulus, G
Poisson's ratio,
Density,
70 GPa
28.7 GPa
0.22
2500 kg/m3
5.9.2 Strength of glass
Since glass is a brittle material, its tensile strength depends on the inevitably present surface flaws. The
geometry and the distribution of the surface flaws are unknowable, and therefore the prediction of the
strength of glass is challenging. The analysis of molecular forces indicates the tensile strength of glass is
as high as 32 GPa (Haldimann et al., 2008), but since the surface flaws cause fracture the actual tensile
strength of float glass is in the range of 20 to 45 MPa (IStructE, 2014). The strength of glass also depends
on the duration and the spatial distribution of the applied loads. The compressive strength of glass is
much greater than the tensile strength. However, the compressive strength is not relevant in practical
structural designs as the compression members will prematurely fail due to buckling or due to the
tensile stresses developed owing to the Poisson’s ratio effects. A comprehensive analytical or numerical
method to predict the strength of glass structural members is lacking. Consequently, the use of glass as
a structural material is not being exploited as effectively as it might be.
5.9.2.1 Practical strength of glass
Experimental methods those recommended in design codes to determine the strength of glass include
4-point bending tests and coaxial double ring tests (BS EN 1288-2, 2000). However, the strength values
obtained for a number of test specimens made from one single glass sheet usually show a significant
scatter. This may be attributed to the different distributions of the surface flaws present in each test
specimen. The strength values obtained from an experiment may not align with any accepted
probability distribution; Weibull probability density functions are usually proposed in the design codes,
e.g. BS EN 12600 (2002). The current design codes provide typical strength values of different types of
commercially available glass products. For instance, Table 5.4 shows the characteristics bending
strength values recommended in the recently published IStructE structural glass design guidelines
(Haldimann et al., 2008). However, due to the differences in the manufacturing processes used by
different manufacturers, the quoted strength values must be considered cautiously. In-depth
discussions on investigation of strength of glass under different loading conditions can be found in
subject-specific text (eg, Haldimann et al., 2008)
Table 5.4 – Characteristic bending strength of different glass types (Haldimann et al., 2008)
Glass type
Characteristic bending
strength, MPa
Float (annealed)
Heat-strengthened
Fully-toughened
45
70
120
16
5.9.3 Glass in load-bearing structural members
Although float glass is the most readily available glass type, it is rarely used in load-bearing structural
members owing to its low tensile strength and the brittle failure behaviour. Toughened and laminated
glass are mostly used in load-bearing glass structural members.
5.9.3.1 Toughened glass
During the tempering (toughening) process, float glass is heated to about 620–675 °C in a furnace and
then quenched by jets of air. When cooled the glass surface solidifies first whilst the interior remains
hot. When the interior cools the thermal shrinkage is resisted by rigid solid surface, and as a result a
residual stress field with tensile stresses in the mid-thickness region is developed. The subsurface tensile
stresses are then balanced by compressive residual stresses developed in the surface regions. Typically,
surface compressive stress of 80–150 N/mm2 is present in fully-tempered glass. Owing to the surface
compression, toughened glass is about five times stronger than float glass. The depth of the surface
compression zone is ~0.2 times the overall thickness of the sheet (Castellini et al., 2015), and the
compression layer will often retard the potential propagation of the surface flaws. Fig. 5.9 shows the
typical parabolic shape of the residual stress depth profile that exists in fully-toughened glass. Generally,
modern toughened glass is strong enough to withstand sledgehammer attacks. Toughened glass is
about 3–5 times more expensive than float glass (on a unit area basis). The use of tempered glass also
adds additional design/construction challenges; penetrations beyond the surface compression layer will
lead to fragmentation. In addition, no alterations (eg, cutting, drilling, grinding, etc.) can be done in
tempered glass.
Fig. 5.9 Parabolic residual stress (prestress) depth profiles present in fully-toughened glass pieces of
thickness 10 mm and 4 mm respectively
5.9.3.2 Heat-strengthened glass
Heat-strengthened glass is produced in the same as way as the production of fully-toughened glass, but
the heated float glass is quenched at a slower cooling rate than that used for full-tempering. As a result,
heat-strengthened glass has a low surface precompression of 40-80 MPa compared to that of 80-150
N/mm2 in fully-tempered glass.
5.9.3.3 Laminated glass
Laminated glass is produced by combining two or more sheets of float/tempered glass with one or more
interlayers, and is processed by autoclaving at 1400 oC and pressure up to 14 bar (Haldimann et al.,
17
2008). PolyVinylButyral (PVB) is the most common interlayer used in laminated glass. As described in
Section 5.6, alternative transparent interlayers may be used in laminated glass to obtain special
properties, such as solar control, thermal insulation, fire protection, etc. Laminated glass also provides
satisfactory structural behaviours under extreme loading conditions such as explosions. One of the main
reasons for the use of laminated glass in building envelops is its safe failure mode compared to that of
float glass and tempered glass. Recent developments include high-tech ionoplastic interlayers.
According to the manufacturers, laminated glass with ionplastic are lighter and stronger than
conventional laminated glass, and can withstand against storms, impacts and powerful blasts (Dupont,
2015). It is also worth noting that toughened glass with polycarbonate interlayers is used in bullet-proof
glass.
5.9.4 Failure mode and post-fracture behaviour of glass
The tensile stresses which cause failure of glass specimens may be generated in one of many possible
ways, for example, as direct tensile stresses, bending stresses, temperature gradients, due to the initial
lack of fit in the structures, etc. Float glass shatters into large pieces of sharp shards as shown in
Fig. 5.10(A). Shards falling from the top of a building or those flow at high speeds are a threat to the
occupants of the building. In tempered glass, the residual stress affects the fragmentation. When
tempered glass cracks, it releases the residual stress and the cracks progress rapidly, repeatedly
bifurcating and causing complete fragmentation of small dice of about 100 mm2 (Haldimann et al.,
2008), as depicted Fig. 5.10(B). Because of the fracture pattern and the high strength of the material,
tempered glass is often known as ‘safety glass’. Nevertheless, even small glass dice can cause serious
injuries if they flow at high velocities. On the other hand, laminated glass with PVB interlayer is often
able to provide adequate level of postbreakage performance, since the PVB interlayer locks together
the broken glass pieces and interact with the remaining unbroken glass sheet/s (See Fig. 5.10(C)). The
PVB interlayer has a certain degree of tensile strength, and this strength may be utilised during a severe
breakage where broken glass pieces can be locked in compression as a result of the arching action.
(A)
(B)
18
(C)
Fig. 5.10 Post-fracture behaviour of (A) float, (B) fully-tempered, and (C) laminated glass
5.10 Glass structural design criteria
The increasing use of glass as a load-bearing material has led to the development of design standards,
technical guidelines, and recommendations in recent years, for example, IStructE (2014). A
comprehensive overview of all current design standards and design methodologies is beyond the scope
of this chapter. A brief overview of the limit state design methodology recommended in the draft
Eurocode, BS EN 1288-2 (2000), is described here. Readers interested in in-depth reviews of design
codes and design methodologies should refer to subject specific text (eg, IStructE, 2014).
The ultimate limit state design adopted in BS EN 1288-2 (2000) compares the applied tensile stress with
the design glass strength. The applied stresses may be determined using standard methods of structural
analysis that are based on loads multiplied by partial factors as defined in the current Eurocodes: (1)
‘Basis of Structural Design’ (BS EN 1990:2002, 2010), and (2) ‘Actions on Structures’ (BS EN 1991:1-
1:2002, 2010). The design strength of glass is determined by applying a series of partial factors to the
characteristics strength of glass (the characteristics strength values of different glass types are
presented in Table 5.4). These partial factors usually account for: (1) load duration, (2) surface profile of
glass, (3) material partial safety factor, (4) partial safety factor for surface prestress (if any), (5) partial
safety factor for method of prestressing, etc. The typical values of the material partial factors can be
found in BS EN 1288-2 (2000). This design code also recommends the serviceability limit state design
guidelines.
5.11 Connections in glass
One of the main challenges that limits the effectiveness of load-bearing large glass structural members
is the lack of an effective connection system. The most popular mechanical connection is ‘point fixing’
(an example is shown in Fig. 5.11) in which a stainless steel bolt is used in a countersunk hole with an
intermediate softer liner material (eg, aluminium), which reduces the bearing stresses (Overend et al.,
2011). The current design guidelines are based on rules of thumb where simply a bearing capacity of 1
kN/mm is assumed (IStructE, 2014) without explicitly taking into account the surface flaws induced by
the drilling process or the high stress concentration presents in the vicinity of the hole. Since glass
cannot yield, the development of high-stress concentrations cause failure. The current design guidelines
also do not explicitly take into account the residual stress relaxes around the edge of a hole in tempered
glass (Achintha and Balan, 2015; Balan and Achintha, 2015).
On the other hand, adhesive-bonded joints are structurally superior to bolted joints for applications
ranging from glass–glass to glass hybrid joints as they do not cause stress concentrations or the
19
development of new surface flaws. However, the use of stiff adhesives is a relatively unproven
technology for joints in glass. Adhesive joints subject to complex 3D stress and strain states, and these
stresses may lead to premature failures. A widely accepted experimentally validated design
methodology for the analysis of the load response and the failure mechanism of glass–adhesive joint
configurations has not been reported in the literature.
Fig. 5.11 Although bolted-joints are widely used they limit the effectiveness of load-
bearing glass structural members
5.12 Detailed finite element analysis
The propagation of surface flaws under applied tensile stresses causes failure of glass; therefore, the
theory of linear elastic fracture mechanics (LEFM) provides a reliable method to design glass structures.
According to LEFM, a brittle material fails when the stress intensity factor at the critical crack reaches
the fracture toughness of the material, which is a known value for a given material. A range of values
has been reported in the literature for the fracture toughness of soda-lime glass; the value of 0.75 MPa
m0.5 is recommended in the design codes (Haldimann et al., 2008). The growth of the critical flaw and
the complete lifetime of a glass member may be modelled using LEFM. However, LEFM analyses require
the knowledge of the geometric properties of the critical crack, such as its location, crack size and crack
shape; in practice, these are unknowable and should be assumed by the analyst/designer. In addition,
finite element (FE)-based analyses are required to take into account the effects of the residual stress in
a complex geometry. For instance, Fig. 5.12 shows the results obtained from an FE analysis for the
residual stress distribution in the vicinity of a hole in a tempered glass plate (note: due to symmetry,
only a quarter of the plate is shown in the figure).
Fig. 5.12 Residual stress distribution in the vicinity of a hole in a fully-tempered glass plate
20
5.13 Future trends
As the need for sustainable buildings is more pressing than ever, energy efficiency is the principal driver
of the new buildings and the existing buildings as well. Glass has potentials to offer dynamic design
solutions that enable buildings to be more energy efficient by making use of the most of daylight and
solar gain whilst protecting the environment and conserving energy. Future trends in the use of glass in
buildings are twofold:
(1) The use of smart and truly responsive glass facades in which the properties change to actively
control solar gain, daylight, glare, and thermal emissions in passive house buildings
Passive technology-based building designs can significantly lower the total energy demand without
increasing the embodied carbon or the total construction costs. The use of passive or self-adjustable
glass products which control light transmittance by responding to the amount of UV radiation in the
exposed daylight will be able to regulate the solar gain. The coatings/interlayers which contain
chemicals sensitive to the temperature can be used to control thermal properties of glass facades, and
thereby to maintain satisfactory indoor environments. Photovoltavic glass, which consists of laminated
glass with integrated solar cells, may be used to supply the moderate energy demand of passive house
buildings.
(2) The use of glass and glass hybrid systems as load-bearing structural members
Structural engineers will be required to design new large areas of glass panels, roofs, floors, staircases
and partitions which all have load-bearing structural roles. Finite element analysis-based designs may be
used to incorporate the effect of residual stress and also to predict the ultimate strength and the
lifetime. The use of adhesive-bonded joints, which are structurally superior to bolted-joints, will enable
the construction of unconventional and large glass structures of complex geometries. The growing use
of glass as a load-bearing material means comprehensive design standards, technical guidelines and
recommendations will be required.
It is also anticipated that the waste glass from construction/building sites may be reused to a great
extent. As described in this chapter, waste glass can be reused in a number of ways, such as aggregate
replacement in concrete and bituminous mixtures, ground glass as cement replacement, etc.
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