Why is the ‘Concrete’ a sustainable building material?
Figure 1. House is Lumino, Ticino, Switzerland
Amit Bura University of Portsmouth School of Architecture 2014 Unit 320 Word count: 5436 1
Contents
List of illustrations
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Introduction
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Chapter 1:
The Concrete and its origin
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Chapter 2:
Environmental properties of concrete
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Chapter 3:
Utilization of the thermal mass
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Chapter 4:
Arising difficulties with the concrete consumption & its solutions
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Conclusion
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Bibliography
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List of Illustrations Figure 1, House is Lumino, Ticino, Switzerland, David Macullo Architects. Accessed
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3 Feb 2014 http://www.macullo.com/work.php?pageno=2&c=6&pagenoWork=3&w=30 nd
Figure 2, Early Lime Burning Kiln, British Geological Survey. Accessed 2 Feb 2014. http://www.bgs.ac.uk/mendips/aggregates/history/limeburning.html th
Figure 3, Pantheon, Rome, Travels Rome, Accessed 28 Jan 2014. http://www.travelsrome.com/monuments/the-pantheon/ th
Figure 4, Basilica Nova, Williams,R.W. (14 Sept 2010) Basilica Nova http://architecturalhistoryscad.wordpress.com/2010/09/14/other-mens-buildings -the-emperor-constantine-and-the-city-of-rome/. th
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Figure 5, Fernbridge, California, Structure, (13 Feb 2006), Fern Bridge http://structurae.net/structures/data/index.cfm?id=s0019507.
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Figure 6, Annual CO2 savings, Jacobs, J.P. (February 2009). Annual CO2 savings, Sustainable benefits of Concrete structure.
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Figure 7, Energy of production for common building materials, Concrete CO2 factsheet, (February 2012), Energy of production for common building materials, NRMCA
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Figure 8, Typical consumption of hydraulic cement concrete, Concrete CO2 factsheet, (February 2012). Energy of production for common building materials, NRMCA
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Figure 9, Crush concrete being used in road construction, Jacobs, J.P. (February 2009). Sustainable benefits of Concrete structure, Sustainable benefits of Concrete structure.
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Figure 10, Ultra-thin Reinforced Concrete Project, South Africa,Aurecon. th Accessed (27 Jan 2014). http://www.aurecongroup.com/en/projects/ government/ultra-thin-reinforced-concrete-project.aspx.
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Figure 11, Loading mesh for delivery, Bennett, D.B. (2010).Loading mesh for delivery. Sustainable Concrete Architecture.
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Figure 12, UK Concrete environmental impact, J.P. (February 2009). Annual CO2 savings, Sustainable benefits of Concrete structure.
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Figure 13, Climate change projections (2009). UK Climate Projections Science Report: Climate change projections. Met Office Hadley Centre, Exeter.
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Figure 14, Principals of thermal mass technology, RIBA Journal. (2007). Principals of thermal mass technology. FES low energy cooling of buildings.
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Figure 15, Stabilising effect of thermal mass on internal temperature, The Concrete Centre. (2005). Thermal mass. www.concretecentre.com .
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Figure 16, Bedzed housing development, South London showing sunspaces, The Concrete Centre. (2005) Thermal mass. www.concretecentre.com.
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Figure 17, The Jubilee Library Brighton, The Bennetts Associates Architects, rd Accessed 23 Jan 2014.http://www.bennettsassociates.com/portfolio/9904/.
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Figure 18, The TermoDeck system, MES Energy services. Accessed 23 Jan 2014 http://www.mesenergyservices.co.uk/other-services/mes-termodeck/.
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Figure 19, Reinforced Concrete Columns, Bennett, D.B. (2010).The Jubilee Library, Brighton. Sustainable Concrete Architecture.
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Figure 20, Central concrete structure and the light well, Pile, J.P. Accessed 19 Jan 2014 http://www.mimoa.eu/projects/United%20Kingdom/Brighton/Jubilee%20Library.
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Figure 21, Section sketch, clearly shows the wind towers, Bennett, D.B. (2010). The Jubilee Library, Brighton. Sustainable Concrete Architecture.
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Figure 22, Concrete column structure during construction, The Bennetts Associates Architects, rd Accessed 23 Jan 2014. http://www.bennettsassociates.com/portfolio/9904/.
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Figure 23, Concrete construction in Dubai, Abu Dhabi. TheFuturebuild.com (1 May 2011) Adding Green Concrete to the Mix.
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Figure 24, Limestone quarrying, Jefferies, A.J. (2nd May 2009) Is it green? Concrete. http://inhabitat.com/is-it-green-concrete/.
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Figure 25, Fly Ash bricks, Sheth Industries. Accessed 21 Jan 2014. http://shethindustries.in/flyash.php.
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Figure 26, Similarities between Cement and Fly Ash, Lehigh Heidelberg Cement Group, nd Accessed 22 Jan 2014 http://www.lehighnw.com/canada/Frames/index7ac.htm.
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Figure 27, RockTorn cement recycling centre, The Construction Index. rd Rocktron’s profit-building low CO2 concrete.3 Feb 2014 http://www.theconstructionindex.co.uk/news/view/rocktrons-profit-buildinglow-co2-concrete.
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Figure 28, Water curing off the concrete surface, Chitra, S.C. Myreality.in. Accessed 3 Feb 2014 http://www.myreality.in/2012/08/which-is-best-time-for-constructing-is.html.
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Introduction Concrete is one of the most important building materials in the world. Throughout history, concrete has contributed significantly towards the built environment. Examples of various forms of concrete can be found as far as the early Egyptian civilisation. Significant concrete buildings and remnants still exists from the Roman civilisation. The Pantheon in Rome is a great example of a building built with concrete which still exists to the present day nearly after 2000 years! Sustainability regarding the use of concrete is moderately debated. Burntland report defines sustainable development as ‘development that meets the needs of the present without compromising the ability of future generation to meet their own needs’. In resources (concrete) perspective, we need to be careful that the future generation should also be able to benefit from the same amount of resources we are consuming today. In addition to this, for material to be truly sustainable it has to address the three pillars of sustainability: Social, Environmental, and Economic. This dissertation will explore the material concrete and its efficiency in achieving a sustainable development through responsible design and construction. A series of real life examples and a case study on a Jubilee Library, Brighton will be explored. Sustainable properties of concrete e.g. thermal mass will be discussed to provide a compelling indication to show concrete’s competence in achieving a sustainable design and construction. On the other hand, of the study also explores the existing difficulty of concrete in achieving sustainability globally. High energy consumption and CO2 emissions associated with the production of Portland cement remains one of the major obstacles that concrete currently faces in terms of sustainability. Additionally, the large amount of water needed for the curing of the concrete, and reckless extraction of aggregates are issues that have also been identified as problems. Therefore, investigation into these complications will be carried out and possible sustainable solutions towards the much debated material will be discussed and presented.
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1. Concrete and its origin Building construction - one of the oldest activities of humankind - has a complex and often nonlinear history. The materials and process used for millennia, for instance, have sometimes suddenly become objects of unexpected and surprising invention. Such is the case of concrete, a major component of Roman building that today is almost certainly the world’s most widely used structural material. (Cohen, 2006) Although the Romans are generally credited as being the first concrete engineers, a form of concrete dating to 6500 BC was discovered by archaeologist in Syria (Steiger, 1995). This reveals that during the Stone Age Syrians had built permanent fire pits for heating and cooking. This resulted in primitive calcination of the surrounding rocks which led to the accidental discovery of lime as a primary material for making concrete. This newly discovered technology soon gained a wide spread use. Central lime burning Kilns (Figure 1) were constructed to supply mortar for rubble wall house construction, concrete floors and waterproofing cisterns (Steiger, 1995)
Figure 2. Early lime burning kiln
For instance, the extensive use of lime and development of well-built kilns has been attributed to the Romans and the earliest archaeological evidence for lime burning in Britain come from the Roman period (43-410AD) (Smith, 2011).Another form of early concrete was discovered in the banks of Danube River in Yugoslavia in 5,600 BC. Stone Age hunter and fisherman used the material to make floors in huts. Analysis into this discovery indicates red lime was mixed with sand, gravel, and water to form the earliest concrete.
The use of lime based mortar and concrete continued in lands around the eastern Mediterranean Sea. About 600 B.C the Greeks discovered a natural pozzolan on Santorini Island that developed hydraulic properties when mixed with lime. This made it possible to produce concrete that would harden under water as well as in the air. The chemical reaction between the lime and slicia alumina in pozzolan made this possible. It was then Romans in 300 BC who “not only improved the concrete technically, they also gave it a name�. The word concrete comes from the Latin word Concreteus, meaning grown together or compounded. The mixture consisted of a ground mix of lime and volcanic ash containing silica and aluminium found near Pozzuoli, Italy hence the name, pozzolanic cement. Although the Roman concrete was unreinforced, it still stands today and was used in wide scale (Steiger, 1995).
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The Pantheon in Rome (Figure 2) and the Colosseum which were both completed in A.D 82 contain large amount of concrete. The Basilica of Maxentius and Constantine (Figure 3) and foundations of the Roman Forum buildings were also built from concrete.
Figure 3. Pantheon, Rome
Figure 4. Basilica of Maxentius and Constantine
In this way, lime and pozzolana concrete continued to be used intermittently for nearly two millennia. The next major development occurred in 1825 when Joseph Aspdin of Leeds, UK patented the manufacture of Portland cement. Aspdin’s cement - a mixture of clay and limestone, which has been crushed and fired in a kiln - was an immediate success. (Lambert, 2002) More than 190 years on, although, many developments in cement production have since been made, the basic ingredients and the processes of manufacture remain the same. The current Portland cement consists of mixture of calcium carbonate, silica, iron oxide and alumina. The primary raw material used in the production process is limestone which is the source of calcium. The raw materials are mixed and place in a high temperature kiln which is fuelled by coal, natural gas or other fuels and heated to around 1450C. This transforms them into chemically and physically into a grey pebble like material called ‘clinker’. In the final process, clinker is ground into a very fine powder and mixed with a small amount of ground gypsum to produce Portland cement.
Portland cement manufacture
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2. Environmental properties of concrete Earlier chapter shows, concrete has managed to revolutionise from its early age to now a material that is widely available everywhere. Therefore, it might be worth looking into why the material has gained popularity among many generations. Furthermore, there are clear environmental benefits that add to the popularity of the concrete and shall be discussed in this chapter. Durability Concrete has always been known for its strength and durable aspect. “Many temples, roads and aqueducts constructed during Roman times have held up remarkably well, despite the wear-andtear in form of military invasions, tourist’s mobs and natural disasters such as earthquakes- they’ve had to endure” (Pruitt, 2013). Durable concrete means the structure lasts longer avoiding the need for frequent repairs and rebuilding of the structure. Concrete also do not rust, rot or burn and requires less energy and fewer materials to repair or replace it. For this reason majority of buildings, bridges, tunnels and dams are constructed using concrete. Pantheon in Rome is a fine example that shows the durability aspect of the concrete. The building structure has survived for centuries with very little or no maintenance. The dome of the Pantheon is an unreinforced concrete structure, maintained in compression and with no reinforcing steel to corrode. Use of concrete in the bridge construction, for example the Fernbridge (Figure 4) that crosses Eel River in Fernbridge, California is another example that exhibits the durability of concrete. The concrete arch bridge was designed by American engineer John B.Leonard which opened in 1911. It was named the ‘Queen of Bridges’ and is “still the longest, functional poured concrete bridge in operation in the world” (aaroads, 2007)
Figure 5. Fernbridge, California
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Energy efficient use & production
Figure 6. Annual CO2 savings chart
In terms of thermal mass, concrete structure manages to be more energy efficient compared to other building materials. The concrete walls and floors slow the passage of heat moving through, reducing temperature swings. This reduces energy needs for heating and cooling of the building, offering year round energy savings over the life time of the building. This area of concrete will be discussed in more detail in the next chapter. Additionally, in terms of production efficiency, concrete has embodied less energy compared to other material e.g. steel. For instance, a test was carried out by Leslie Struble and Jonathan Godfrey of University of Illinois to determine the efficiency in production of concrete. (Godfrey, Centre for Transportion Research and Education)
Figure 7. Energy of production for common building materials
Figure 8. Typical consumption of hydraulic cement concrete
The test was carried out by designing a “simple reinforced beam and a steel I-beam with the same amount of capacity�. The environmental impact of each of these two beams was estimated using a computer program designed for this purpose. The estimation showed that energy required to produce a concrete I beam was 109 MJ compared to 237 MJ for the steel. The energy to produce reinforced concrete beam was about half the energy to produce similar steel I beam. This suggests concrete has environmentally beneficial production. 9
It is also important to note that cement is only one of the constituents of concrete. However, as a whole, cement makes up only approximately 10% of concrete (Figure 7), while water and aggregates and make up the other 90% of mass of concrete. This is why concrete has relatively low embodied energy, even though it contains cement, which has a relatively significant embodied energy (Cement Concrete and Aggregates Australia, 2010). There have been new emerging solutions to minimize the energy consumption which will be discussed in the later chapters. Recyclability & Carbon absorption Another sustainable practice is that concrete can also be reused for secondary applications. Crushed concrete is now being used as sub-base in roadways or construction drives, or as aggregate in both new concrete construction and asphalt pavement. Each of these secondary uses may involve carbon absorption cycle, especially if they expose more concrete surfaces to carbon dioxide in the atmosphere, underground or underwater. This process of reabsorbing CO2 is also known as Carbonation.
Figure 9. Crush concrete being used in road construction
Social, Environmental and Economic impact ‘Concrete construction has and will continue to have, a great social impact on the world.’ (Cement Concrete and Aggregates Australia, 2010). Concrete is the main provider of durable infrastructure such as roads, highways, rail networks, wharf and port facilities. Without concrete the world’s economics would grind to a halt. Moreover, concrete allows these infrastructures to be built economically, which has an inherent social dimension as well. As concrete is essentially a simple, natural material, it is widely available in practically all parts of the world. Therefore, poorer nations can be empowered by building social infrastructure that is affordable and produced from locallysourced materials, thus providing employment in process. Also concrete industry is one of the major sources of employment in developed countries and largely contributes to the economy. ‘In the United States alone, concrete construction accounted for 2 million jobs in 2002. About 2.7 billion m3 of concrete were produced per person annually’ (Naik, 2008). Therefore concrete is necessary not only to provide social development, but to sustain employment such as batch plant operator, truck drivers, equipment operators as well as professional engineers and architects.
Figure 10. Reinforced concrete project, South Africa
Figure 11. Loading wire mesh into trucks
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Environmental sustainability is an area where concrete has progressed well. Life cycle assessment evaluates the environmental impact of a structure from its ‘inception to its demolition’ (Jacobs, 2009) .They includes extraction, manufacture, construction, use, maintenance, demolition and recycling. Concrete performs well when accurate and holistic comparisons are made with other building materials. In terms of energy efficiency field, for instance, the energy saving of concrete structures (5-15%) in use/operational phase easily offset the amount of energy consumed in their manufacture and installation (4-5%).”Usually, some 80-90% of the energy used during a building’s life-cycle is consumed during the in-use phase” (Jacobs J. P., 2009). Therefore, significant amount of energy can be saved during this period.
Figure 12. UK concrete environmental impact
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3. Utilization of the thermal mass As with all the environmental benefits of the concrete, the thermal mass of the concrete helps to reduce down the building carbon emission. Moreover, it provides architects and engineers with an important design tool that influences their environmental design strategy. Therefore, the extent to which the thermal properties of concrete benefit the environment will be discussed below.
Figure 13. Comparison of changes in seasonal mean temperature, summer and winter, by the 2080s under High emissions scenarios, from the UKCIP02 report as projected in UKCP09 (10, 50 and 90% probability level)
Our climate is changing. ‘In the UK, building accounts for 40-50% of all carbon emissions’ (RIBA journal, 2007). As temperature increases there will be growing need to maintain the temperature of the building that is suitable for its occupants. This results in high costs in heating and cooling of the building which results in high level of carbon emission. As a designer we all have a ‘responsibility to minimise the environmental impact of our design’ (Bennett, 2010) by addressing a solution towards the problem. Concrete in this case can provide a “concrete” solution towards the problem. This chapter investigates how the use of thermal mass of the concrete for the heating and cooling of the building can be useful for designers to achieve a sustainable design What is Thermal mass? “Thermal mass is the ability of a material to absorb heat” (RIBA journal, 2007). In construction, material with a high thermal mass work like thermal batteries absorbing heat during certain times of the day and releasing it at others. Concrete has a high density which means it can store heat. Furthermore, concrete is also a good heat conductor which allows the material to conduct heat that is neither too low nor too high. The principal of thermal mass technology are explained in (Figure 13) below.
Figure 14. Principals of thermal mass technology
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Thermal mass in summer for cooling of the building Concrete works as a sustainable alternative to conventional air-conditioning which can be found in the combined use of high thermal mass, night cooling, insulation and shading. These technologies are collectively known as fabric energy storage (FES). FES works by reducing the peak temperature within a building, but also at the same time delaying its onset by feature of the slow thermal response of the structure (Figure14). For instance, it has been estimated that in a heavy thermal mass building, this thermal lag can be up to 6 hours. Therefore maximum temperature will be reached when the building is not in full occupation. As the evening progresses the external air temperature drops. This makes night ventilation an effective means of removing accumulated heat from the concrete and lowering the temperature in preparation for the next day. This heat cycle is known as ‘thermal flywheel effect’ (RIBA journal, 2007).
Figure 15. Stabilising effect of thermal mass on internal temperature
Thermal mass in winter for heating of the building FES benefits is not limited to the summer months as buildings can also be designed to capture solar gains during the winter, storing them using thermal mass. This technique has been used ‘to good effect at the BEDZED [Figure 15] housing development in South London, where it is estimated that heating energy usage is reduced up to 30%’ (The Concrete Centre, 2005). During winter months, excess heat can also be captured from occupants, lighting, computers and other equipment. This stored heat is slowly released later in the day, helping to keep the building warm and reduce heating costs. In order to take the advantage of the winter sun, large areas of south facing glazing is required which allows the low winter sun to heat the concrete surface. However, this technique is beyond the scope of this dissertation, therefore, shall not be discussed further.
Figure 16. Bedzed housing development, South London showing sunspaces
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Case Study The Jubilee Library, Brighton Bennetts Associates Architects
Figure 17. The Jubilee Library Brighton
The Jubilee Library in Brighton (Figure 16) is the most popular and significant new building in the City of Brighton for some years. It is the ‘centrepiece of the Jubilee Street development and the catalyst for regeneration of Brighton’s cultural quarter; it is a highly innovative design with sustainability at its heart’ (Bennett, 2010). Brighton & Hove City Council described the library as “one of the most energy efficient public buildings in the country” The Jubilee library integrates the building structure, fabric and environmental strategy to achieve a sustainable design. The physical form of the library is designed to minimise the demand on services through the strategy of ‘exposed thermal mass’ and the concrete plank structure, commonly known by its trade name as “TermoDeck”(Figure 17). ‘TermoDeck’ is a “fan assisted, heating, cooling and ventilation system that uses the high thermal mass of structural, hollow core floor slabs through which warmed or cooled fresh air is distributed” (Hollowcore Concrete Pty.Ltd) In the case of Jubilee Library, the use of concrete (TermoDeck) is the major heating and cooling strategy of the building as it creates the basis of the servicing strategy for the whole building. The floor plank concrete structures are supported off a steel frame and provide heating, cooling and ventilation to building spaces by channelling warmed or cooled air through integral hollow cores, which passes through the centre of each plank. The air supply duct is used to carry air from the central air handling unit which plugs into the concrete floor plank. A series of diffusers are then
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Figure 18. The TermoDeck System
located adjacent to the external walls to deliver air into the space below. Once the tempered air is delivered to the perimeter spaces, it is then drawn into the three story central library hall by the stack effect of the three wind tower which directs the air out of the building. During this, the air that passes through the concrete at low velocities results in ‘passive exchange’. This creates the thermal flywheel effect; therefore fluctuating temperature outside the building is less experienced when the occupants enter the library. Therefore, high external temperatures are not reflected internally. This is because, the thermal mass and the high insulation value of the external concrete delay the heat transfer. Spaces with the high demand of cooling, heating and ventilation are dealt with passing the air though a greater number of cores and increasing the number of diffusers to a particular space. Spaces with particularly high demand such as the IT suite were “provided with local chiller units to mitigate any peak temperature” (Bennett, 2010). In this manner, ‘TermoDeck’ system provides “100 per cent fresh, filtered air which significantly improves the occupant comfort and far exceeds CIBSE minimum fresh air requirements” (Bennett, 2010). During the occupied hours for instance, during night, the air is pushed through the concrete cores to cool the structure, making it ready for morning use. The series of “BMS controlled roof vents and the
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wind towers remove the excess heat from the building” (Bennett, 2010). Therefore, this reduces energy demand for the following day by reducing the ‘cooling load’. Consequently, the use of thermal mass of the concrete in this way is shown to be an efficient means of maintaining the stable temperature and occupant comfort inside the building. Another strategy that library uses in utilizing the thermal mass, is through the use of the in-situ concrete structure which adds to the thermal mass of the building. In situ means “in the original place”, therefore the finished concrete form is achieved by pouring the concrete into a formwork, where it remains. In an environmental perspective, in-situ concrete has the advantages associated with all concrete, such as thermal mass, durability, and use of recycled materials. Additionally, in-situ concrete is an excellent solution for free forming concrete into a variety of shapes, spans and forms.
Figure 19. Reinforced in-situ concrete columns
Similarly, reinforced in-situ concrete is used to create the main hall space. The space also creates the enclosed “U shaped strip” (Bennett, 2010) which supports ancillary accommodation and offices on the three sides. The fourth side is bounded only by a glazed façade. Within this enclosure, reinforced in-situ concretes ‘table structures’ are stacked on top of each other which creates the two double height space in the building. This space houses the main library book stacks and reading areas. Reinforced in-situ concrete was also used to construct “eight freestanding concrete columns [Figure 18] which were arranged in pairs, running the central volume” (Bennett, 2010). The cylindrical column rise from each floors and “splay out to form column heads” supporting the slab above. In this case, in-situ concrete provides the ‘durability’ that the structure needs. Furthermore, the central concrete structure is physically separated from the ancillary spaces by a void. This void is connected with central space at the first floor thorough series of bridges (Figure 19). The bridges provide access from the perimeter spaces through the surrounding timberpanelled wall. This enables the voids to allow natural daylight to enter the hall reading areas and provide paths for air circulation.
Figure 20. Central concrete structure and the light well
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At the roof level, three voids within the centre of the slab provide the “bases for the tall wind towers, which extend above the library roofline” (Bennett, 2010). On the first floor level the voids directly replicate below to provide light wells and provide an efficient air path from the ground floor. The roof glazing in conjunction with the glazed south façade also allows natural daylight to enter the library hall and reading spaces.
Figure 21, Section sketch, clearly showing the wind towers
Furthermore, complex concrete ‘tapered column heads’ are cast using fibreglass moulds. This feature of the concrete directly refers to the ‘versatility’ feature of a concrete. In this case, the ‘fluid’ nature of the concrete allows the architect and builder to achieve a complex form which otherwise could not have been possible to achieve with alternate materials e.g. steel, timber.
Figure 22. Concrete column structure during construction
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4. Arising difficulties & solutions with the concrete consumption The previous two chapters discussed about the sustainability of the concrete through identifying sustainable features of the material. Also, the case study on the use of concrete in the Jubilee library was carried out to display the sustainable features of concrete. From the case study it was evident that concrete, favoured for its excellent thermal mass and durability was extensively used for the library to meet it’s the environmental strategy. However, there still remains a significant area of concern that needs to be further explored. For example, large amount of CO2 emission during the production of the cement is one of these major areas of concern. Large amount of aggregates required in producing the concrete for the increasing population and the large water requirement for the curing of the concrete are other identified concerns.
Figure 23. Concrete construction in Dubai, Abu Dhabi
It might be appropriate by stressing the fact that the concrete is one of the most important, “versatile and widely used building around the world” (Meyer C. ). It has achieved this predominance because of a number of decisive advantages, some which have already been highlighted in the previous chapter. “As a result of this popularity, the concrete production has had an enormous impact on the environment”. Some of these major impacts are highlighted below: 1. “Worldwide, more than 10 billion tons of concrete are produced each year” (Meyer C. , 2008). Therefore vast amounts of natural resources will be needed to produce those billon tons of concrete each year. 2. “The production of each ton of Portland cement releases almost one ton of carbon dioxide into the atmosphere” (Meyer C. , 2008) “Worldwide, the cement industry is estimated to be responsible for about 7% of all carbon dioxide generated”. 3. Third, “the production of the concrete requires vast amount of water, which is particularly burdensome in those regions of the earth that are not blessed with an abundance of fresh water”. Worldwide, “the concrete industry uses about one billion cubic meter of water each year, and this does not even include wash water and curing water”. 18
4. Finally, the demolition debris and the need of disposal of concrete structures create another environmental burden.
Figure 24. Limestone quarrying
Sustainable solutions towards to the impact: 1. Use lower percentage of Portland cement and replace it by natural supplementary ‘cementious’ materials such as fly ash or ground granulated blast-furnace slag. 2. Improving the long term durability of the concrete; by doubling the life span of the concrete structure, less amount of material will be need for their replacement. 3. Increase the reliance on recycled materials to minimise the demand of virgin materials. 4. Minimise dependency on main water source and opt to use recycled water where possible. “Cement conservation is the first step in reducing the energy consumption and greenhouse-gas emissions” (Mehta, 2001). This allows less percentage of Portland cement to be used in the mixture of concrete which means the ‘environmental burden’ associated with the production of the cement is minimized. The lower percentage can be replaced with a substitute which has similar cementious properties of Portland cement, but derived from more environmentally friendly manner. One of such best known replacements of is fly ash, which is “the by-product of coal combustion, which otherwise would be a waste product to be disposed of at great cost” (Meyer C. , 2008).
Figure 25. Fly Ash Bricks
Figure 26. Similarities between Cement and Fly Ash
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Through the use of Fly Ash there have been many environmental advantages over the conventional Portland cement. Christian Meyer of Columbia University quotes that “it is theoretically possible to replace 100% of Portland cement by Fly Ash” (Meyer C. ). Fly Ash can have better strength and durability properties than concrete. This is because it generates “less heat hydration and it is particularly suited for mass concrete applications” (Meyer C. ). It is also widely available is most parts of the world where the coal burning is prevalent. Finally, as a bonus to all the above advantages, Fly Ash is generally less expensive than Portland cement.
Figure 27. RockTorn cement recycling centre, Chepstow, UK
However, the disadvantage of Fly Ash is that it has “relatively slow rate of strength development” (Meyer C. , 2008). This can have disadvantage in application where an early strength is required such as bridges and dams. When this is the case, a chemical accelerator is available to speed up the hydration rates of Fly Ash in concrete. Moreover, “properties of Fly Ash vary considerably power plant to power plant, primarily because of the differences in the source of coal” (Meyer C. , 2008). Therefore, quality control in this case is vital to ensure that higher quality properties of Fly Ash are maintained throughout the industries. Furthermore, increasing the reliance on recycled materials minimizes the demand for aggregates. “In North America, Europe and Japan, about two thirds of the construction and demolition waste consists of masonry and old concrete rubble” (Mehta, 2001). This can therefore be a great opportunity for the concrete industry to improve its resource productivity output by using ‘recycled aggregate’ derived from construction and demolition wastes. For instance, in the UK, concrete debris was extensively used for the improvement of the A34/M4 roadways. In this case, the “requirement for imported primary aggregates was reduced by over
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50,000 tonnes by using locally sourced recycled aggregate from demolition rubble and recycled asphalt form the redundant carriage way” (The Waste and Resources Action Programme).
Figure 28. Water, curing off the concrete surface
Conservation of the water on the other hand poses another challenge to the concrete industry. “Due to growing agricultural, urban, and industrial needs, water tables in every continent are falling” (Mehta, 2001). Concrete is “one of the largest industrial consumers of fresh water” (Mehta, 2001), therefore it is imperative for the concrete industry to use water more efficiently. Mehta of University of California of Berkley suggests that “yearly global mixing water requirement of 1 trillion L can be cut in half by better and by greatly expanding the use of mineral admixtures and superplasticizers” (Mehta, 2001).
Moreover, industries should be minimizing their dependency on mains water supplies and adopt recycling systems and alternative water sources, such as rain water harvesting. For instance, in the UK “180 litres of water are used per tonne of precast concrete product; 38 percent of which is from licensed non-mains sources” (The Concrete Centre, 2010). This suggests that making an effort to minimize the water consumption do intern reduce the dependency of water on main sources. Likewise, in addition to above solutions, durability is the key in improving the sustainability of the concrete. In most case, a sustainable concrete will have all above listed aspects, however from a design point of view, the emphasis also needs to be given to the durability of the concrete. “Reducing the cement levels can actually improve the durability of the final concrete” as Ravindra Dhir, director of the concrete technology group of the University of Dundee, UK points out. He also claims that “it is the cement that provides a route by which elements of exposure can go in and out. So in theory, the less you use, the better the concrete should be” (Crow, 2008). This means there will be less pores present in the concrete which prevents corrosive materials such as chlorides and sulphates to penetrate the structure and attack the metal reinforcement – “the cause of well over 90 percent of problems of concrete durability” (Crow, 2008). However, achieving durability in concrete not only means selection of concrete materials or right mix proportions, but it also requires a good craftsmanship which ensures “construction detailing, temperature control, adequate compaction and protection of the fresh concrete”. It might be worth asking ourselves “why do modern reinforced concrete structures sometimes begin to deteriorate in 20 years or less, whereas buildings and seawalls made of unreinforced Roman concrete continue to be in good condition after almost 2000 years?”
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Conclusion In the past and present, concrete has always found itself at the forefront when it comes to the material that is durable, versatile and environmentally friendly. In regards to this, real examples and the case study on the Jubilee library on the ‘thermal mass’ properties of the material support the sustainable qualities of concrete. It still remains the case that ‘thermal mass’ of the concrete is a major tool in cutting down on building carbon emission: one of the major contributors of greenhouse gases. Similarly, new technology such as TermoDeck for heating and cooling of the building has allowed concrete to exploit its thermal properties in most efficient ways. However, as well as having large environmental benefits, concrete also finds itself less immune from overexploitation of natural resources. This is because of the “huge quantities [that] are used” (Crow, 2008). Moreover, production of the Portland cement is highly energy intensive and has the highest embodied energy percentage compared to other aggregates. Therefore, a simple solution to this is to decrease the usage of concrete and opt for similar, alternative cementitious material which are recycled or produced as waste products from industrial plant e.g. Flyash. Examples have also shown that recycling concrete at the end of its life, and using recycled water when possible reduces the strain on natural resources and minimises the carbon footprint associated with concrete production. All things considered, improving durability of the concrete is the long term solution for making concrete use sustainable. Lastly, there have been many other examples regarding the sustainable use of concrete which gives reason to believe that concrete is well positioned for success, despite existing difficulties. Moreover, tool and sustainable solutions are in place to deal with these difficulties.
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