الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 127 |  |  |
| | | from | 127 | | |
Abstract
The World’s construction industry is faced with new challenges. Legislation, environmental levies, voluntary agreements like Kyoto Protocol, which commits industrial countries in 1997 to reduce the emission of greenhouse gases between the years 2008 and 2012 by 5% compared to 1990 levels, and Due to demand increase of customers on the construction industry which means that the industry must continuously improve its environmental performance by developing and implementing cleaner technology. There is often an economic benefit from these activities.
Carbon dioxide is the main indicator used in the assessment of concrete constructions and other facilities compared to non-concrete constructions, and although the concrete is known by the massive quantities of carbon dioxide emissions during the process of production, it is important to include the term of building processing in this type of assessments.
The thermal mass of concrete helps to improve the energy performance of a building which again will reduce the effect of a high initial CO2 footprint. A slight difference in the energy performance of a building design may tip the balance from an environmentally sound design to the direct opposite in terms of energy performance. After end of service life concrete is suitable for recycling back into construction applications. Furthermore, the concrete rubble will carbonate and absorb CO2 from the atmosphere.
A “holistic” approach is needed to achieve real environmental improvements in the construction sector. A building or any other structure has to be considered as a product, consequently the total environmental impact associated with the “product” during the entire life cycle can be considered. Thus the life cycle of concrete structures has been divided into five different phases such as:
1. Phase I : Manufacturing of raw materials.
This part involves some information of the use of materials in the construction industry. It relates to the environmental impacts of construction materials to the total human activities and Emission of carbon dioxide during manufacturing the raw materials of concrete, specially manufacturing of ordinary Portland cement which the following reaction takes place in the first part of the clinker kiln:
CaCO3 → CaO + CO2
Calcium carbonate Calcium oxide Carbon dioxide
Emission of carbon dioxide is primarily related to consumption of energy used for processes and transportation related to the raw materials production.
2. Phase II: Mixing of raw materials.
This section presents how to optimizing the use of Portland cement while maintaining required performance is a crucial step towards decreasing the carbon footprint of concrete. Life cycle assessment reveals that cement content is the most important factor in determining a concrete mix’s embodied energy and carbon footprint. The production of Portland cement emits approximately 0.927 tons of carbon dioxide for each ton of Portland cement produced. On average, the cement used in a concrete mix represents over 85% of embodied energy and up to 96% of greenhouse gas (GHG) emissions per unit volume of concrete produced.
3. Phase III: Construction phase.
When designing and constructing a concrete structure the mass of the concrete construction elements can account for around 50 per cent of their overall environmental impact. Therefore, where there is potential to design out the mass whilst maintaining structural performance, this can have a significant positive impact on the rating.
4. Phase IV: Service life phase.
The operation of buildings during their service life is the main contributor to the CO2 balance over its full life cycle. Heating and ventilation and cooling of buildings are responsible for about 80 % of the total energy consumption including the embodied energy corresponding with the production of the building. Since the service life is often 50-70 years or even more the annual energy consumption has very large impact on the total carbon footprint of a building. For this reason alone it makes good sense to design the building with minimum energy consumption in mind.
5. Phase V: Demolition and recycling.
When producing clinker, limestone is heated and carbon dioxide is released. In concrete, the reverse reaction takes place. Carbon dioxide is absorbed and the calcium oxide is transformed back to calcium carbonate. Tests have shown that at least 75 per cent of the calcium oxide of the cementitious part of concrete can be transformed to calcium carbonate, if enough time is given.
So, a general methodology for CO2 emission calculations has been given. Examples of such calculations are presented based on inventory data from the literature. Furthermore, the need for including the full life cycle is demonstrated. It has also been demonstrated how the accuracy of life cycle inventory data may influence the outcome of such calculations. The following general conclusions are drawn from the calculations:
• Concrete is a building material that will increase the carbon footprint in terms of embodied CO2 during the production phases, compared with lightweight materials.
• The main contributor to the concrete carbon footprint is the cement manufacturing process and application of supplementary cementitious materials has great potential for emission savings. However, steel reinforcement is also a significant contributor.
• Transportation of raw materials and finished products has only minor influence on the carbon footprint. Only in case of extreme transportation distances it may play a significant role. The inventory data for transportation is very dependent on local conditions on the place of production.
• The high thermal mass of concrete should be utilized to improve the energy performance of buildings. Since the energy consumption for building operation is much higher than the embodied energy in the building materials the service life period is very important to include when different structural designs are compared.
• Heavy building materials with high thermal mass mean less annual energy consumption for heating/cooling/ventilation which again means less carbon emissions. Even a small annual difference will add up to a significant amount over a service life of say 70 years.
• After ended service life concrete should be demolished and crushed down to small fractions suitable for applications in road construction, back-filling material, etc. This reduces the need for land filling and the need for natural aggregates. CO2 emissions coming from demolishing and crushing of concrete are balanced out by the CO2 uptake from the carbonation process in the concrete rubble.