02 Nov 2017
Edeoghon Otibho Stephanie
Life Cycle Assessment of Aggregates
Aggregates are the most widely excavated and used building materials in the construction industry. This statement is supported by the European standards definition which defines aggregates as "coarse materials used in structural constructionsâ€™â€™ (EN12620: 2002; BGS, 2007). Aggregate is a word used to describe processed or naturally occurring crushed rock, gravel and sand which are used to provide wear resistance and strength to architectural structures ( Barksdale, 2000). Aggregates are classified into three groups on the basis of the production process they undergo. These include virgin, recycled and manufactured aggregates. Virgin aggregate otherwise called naturally occurring or primary aggregates are construction aggregates produced from the physical extraction of naturally occurring earth minerals through a process of screening and crushing to produce sand and gravel. Recycled aggregates are construction aggregates that are produced from the recycling of inorganic construction materials that have previously been used in construction processes. Example includes recycled construction and demolition waste (BGS, 2007; Richardson et al., 2004). Manufactured aggregates consist of slag produced from the combined industrial effect of thermal and additional processing criteria applied to mineral aggregates (BGS, 2007). Aggregates either in their natural form or in combination with other materials to form asphalt and concretes are used in the construction of residential and commercial building or in road and bridges construction (Langer, 1988).
The uncertainty of the future availability of virgin aggregates to meet the growing demand by both the concrete producing and the construction industry gives cause for concern as the annual European Union production of aggregate stands at 3 billion tonnes (EEA, 2008). This increase in demand and the resultant effects is seen in countries like the United Kingdom where sand and gravel reserves have dropped from a figure of 907 million tonnes in 1995 ( Brown, 2006; Fisher et al., 2008 ) to 544 million tonnes in 2009 (BGS, 2011). With this projected continued rise in aggregate demand strategies need to be put in place to address the downward supply trend (Brown and Highley, 2006) and also assess the environmental impacts associated with the constituent make up and use of the aggregates (Marinkovic et al., 2010). While it is recognisable that virgin aggregates are invaluable to the successful economic and social development of humanity it is also important to take into consideration that its production and use should be in accordance with environmental sustainable development practices (Blengini and Garbarino, 2010).
On the other hand, it is important to take into consideration that with an increase in the demand and use of aggregates come a corresponding increase in the production of construction and demolition waste. The environmental impact of construction and demolition waste is of great concern as about 900 million of the 3 billion annually generated waste in the European Union countries are from construction and demolition waste ( EC, 2010) of which the UK contributes 77.4 million (DEFRA, 2010). The conservation and protection of the natural environment and the environmental impact associated with the creation and disposal of construction and demolition waste can be achieved by incorporating a construction and demolition waste recycling process that invariably produces useful recycled aggregates, preserve virgin aggregates and minimize the quantity of construction and demolition waste deposited in landfills (Blengini and Garbarino, 2010; Marinkovic, 2010; Wilburn and Goonan, 1998; Sagoe-Crentsil et al., 2000). The production of recycled aggregates from construction and demolition waste follows a two-staged process of screening of waste to remove contaminants and materials not suitable for the construction process and a crushing process where waste are further reduced to the particulate size needed for further construction use. The quality of the recycled aggregate produced from this process is largely determined by the quality of the parent construction and demolition waste, the screening and crushing process and any other processing criteria the material undergoes (Padmini et al., 2009; Wrap 2010; Marie and Quiasrawi, 2012).
Aggregates obtained from the recycling of construction and demolition waste (recycled aggregates) are for the most part used as sub-base for pavement construction (Al-Ali et al., 2001; Xu-ping, 2009) and in sand lime brick construction (El-Hawary, 2005; Al-Otaibi, 2007). This lower quality construction application of recycled aggregate is attributed to the fact that recycled aggregates have lower quality when placed in comparison with virgin aggregates (Marinkovic et al., 2010; Marie and Quiasrawi, 2012). The difference in the quality of recycled and virgin aggregates is linked to the presence of mortar and cement paste from parent construction and demolition waste which remains as part of the recycled aggregate produced from the recycling process ( De Juan and Gutierrez , 2009; Marinkovic et al., 2010; Sagoe and Brown, 2002). The presence of the mortar affects the physical characteristics of the recycled aggregate which when compared with virgin aggregates gives the following results; an abrasion resistance decrease of up to 70% for recycled aggregate ( Poon et al., 2004; Lopez-Gayarre et al., 2009 ); a coarse recycled aggregate water absorption of 3.5% (Rahal, 2007; Lopez-Gayarre et al., 2009) to 9.2% (Xiao et al., 2009) as against that of virgin aggregate which falls in a range of 0.5-5% ( Qasrawi et al., 2012) and a decrease in density of up to 10% for recycled aggregate (Hasen 1992; Poon et al., 2004; Dejuan and Guitierrez 2009).
In addition to the above highlighted differences between recycled aggregates and virgin aggregates, other factors an organisation needs to take into consideration as to the choice of aggregate type to use in construction includes the cost- benefit analysis (capital and operational cost), environmental impacts (carbon dioxide emission and energy consumption) , the availability and accessibility of aggregates.
The main purpose of this study is to identify:
The influence of capital and operating cost in decision making between the use of recycled aggregates and virgin aggregates.
The environmental impacts associated with the use of recycled aggregates vis a vis virgin aggregates.
The role of the relative location of virgin and recycled materials in dictating relative transport cost.
To assist top organisational management in coming to a decision on what aggregate choice to use, a Life Cycle Assessment (LCA) Model that incorporates a framework for the analysis, evaluation of economic and environmental data is used. The LCA model used follows the ISO 14040:2006 standard (ISO, 2006a) and ISO 14044:2006 standard (ISO, 2006b). The LCA model is used in the evaluation of the environmental impacts of construction materials and buildings (Wu et al., 2005 and Ortiz et al., 2009).
The LCA is an objective process used in evaluating the environmental impact associated with a product, process or activity throughout its lifetime by identifying its input energy and material and its output waste and emissions released to the environment (Liu et al., 2010; Madival et al., 2009; Fisher et al., 2008). The result gotten from an LCA is used in the detection of fractions within the process lifecycle where environmental improvements can be made (Graedel and Allenby, 2010). The LCA model incorporates four fundamental steps namely; the goal and scope definition step, life cycle inventory analysis (LCI) step, life cycle environmental impact assessment (LCIA) step and finally the interpretation step (ISO 2006 a and b; Georgakellos, 2006; Curran, 2006)
The objective of this study is to develop a life cycle assessment and life cycle inventory model that will help the management of Enterprise limited identify, analyse and evaluate the economic and environmental impacts of the use of recycled aggregate in relation to virgin aggregates from their point of excavation to the point of disposal i.e. cradle to the grave analysis of aggregate.
The LCI phase of LCA involves the process of measuring and collecting data of all product system input such as energy and raw materials and all outputs such as emissions and waste. Examples of collected data include: energy consumption, transport, carbon dioxide emission.
The life cycle impact assessment phase is the stage where life cycle inventory data and information are classified, characterised, normalised and valuated using common functional units that can be compared with each other.
The classification process involves the identification of the environmental impacts associated with the use of recycled and virgin aggregates in this study. E.g. General environmental impacts associated with the release of carbon dioxide in the environment which is seen in the context of climate change.
The characterization step involves a quantitative modelling of all emissions (e.g. carbon dioxide emission) expressed as an impact score in a common functional unit to all contributions within that impact category according to the environmental mechanism. This common unit allows the easy summing of all emission contributions and material extraction process within each environmental impact category thus converting the inventory data to an environmental impact score.
The normalisation phase of life cycle impact assessment involves reflecting the relative magnitude of the environmental impact score on a scale that is common to all categories of the same impact to make the interpretation process easy.
The valuation process is the grading and grouping of different environmental impacts and resource consumption process according to the scale of preference of this study. This grading system is needed when a trade-off is needed to be made e.g. a trade-off of whether to use recycled aggregates, virgin aggregate or a combination of both in a construction process.
Evaluate result with goals
Meet clients and negotiate project remit
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