Recycling value of building materials in building assessment systems

Recycling value of building materials in building assessment systems

Energy and Buildings 43 (2011) 3181–3188 Contents lists available at SciVerse ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/...

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Energy and Buildings 43 (2011) 3181–3188

Contents lists available at SciVerse ScienceDirect

Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

Recycling value of building materials in building assessment systems Mohammad Djavad Saghafi, Zahra Sadat Hosseini Teshnizi ∗ Department of Architecture, Architecture School, College of Fine Arts, University of Tehran, Iran

a r t i c l e

i n f o

Article history: Received 5 July 2011 Accepted 11 August 2011 Keywords: Building materials Environmental assessment systems Recycling Embodied energy Potential recycling energy

a b s t r a c t The selection of green building materials and products is by far the most controversial task in sustainable construction. Determining the merits of building materials and products in terms of their recycling value, which seems to be a simple matter, is a very controversial topic in building assessment systems. This paper suggests a method to assess energy savings by recycling building materials, which can be a potential indicator of recycling worth. The method takes account of material selection, construction and deconstruction technologies, and the frequency of recycling. The result of this study can be used in assessment tools as a factor separate from the embodied energy. Since embodied energy affects the potential recycling energy, another factor is defined, based on these two factors, in order to make it possible to compare and select materials correctly, based on their embodied energy and recycling potential. © 2011 Elsevier B.V. All rights reserved.

1. Introduction More than any other human endeavor the built environment has direct, complex and long-lasting impacts on the biosphere [2]. These impacts (e.g., global warming, fossil fuel and ozone depletion, water use, acidification and toxic release to air, water, and land [This issue]) are significant throughout all the phases of the life span of building materials, yet are often invisible [3]. Each year more than three billion metric tons of raw materials are used to manufacture construction materials and products worldwide [3]. This is about 40–50% of the global economy’s total flow. The inclusion of hidden flows1 is estimated to more than double the consumption of resources for construction materials [3]. Building phases including construction, operation and demolition, use approximately 30–40% of all primary energy utilized worldwide [4] and 15% of the world’s fresh water resources [5] and produce approximately 40–50% of the global output of greenhouse gases [4]. A report by the World Resources Institute projects a 300% rise in energy and material use as the world’s population and economic activity increase over the next 50 years [6]. Consequently, the construction industry has a special obligation to behave proactively

∗ Corresponding author at: No. 44, Shahid Rajayi Alley, Kaveh St., Esfahan 8193636683, Iran. Tel.: +98 311 4592017; fax: +98 311 5275155; mobile: +98 913 1698537. E-mail addresses: [email protected], [email protected] (Z.S. Hosseini Teshnizi). 1 Hidden flows or indirect flows are materials such as: mining overburden, soil erosion, ore waste, effluents and emissions that are released to land, air, or water that never enter the economy as traded commodities. For many products, these indirect flows are substantially larger than the direct flows [3]. 0378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2011.08.016

and shift rapidly from wasteful, harmful practices to a paradigm under which construction and nature work synergistically, rather than antagonistically. This new model of sustainable construction is referred to as high-performance green building [2]. The selection of building materials and products for a highperformance green building project is by far the most difficult and challenging task facing the project team. Determining the merits of building materials and products in terms of how they will affect the environment is the central unresolved problem of the green building movement. There is no clear philosophy that precisely articulates the criteria for this new class of materials and products. A material or product could conceivably appear to be beneficial according to many existing assessment systems, yet not be recyclable and thus subject to disposal after use [2]. Construction and demolition (C&D) waste generation in Iran is much higher than in most other countries, especially developed countries. For example the average C&D waste generation in the United States is 0.77 kg per capita per day2 [7] while this proportion in much higher in Tehran based on reports from Tehran Municipality Waste Management.3 In addition to the high rate of demolition,

2 This is the average of reported per capita C&D generation rates in the United States and some of its states based on recent waste characterization studies. There is a considerable difference in the generation rates presented in this report due to a combination of what is and is not reported as C&D; (In some cases total recycled plus disposed materials are included, in other instances C&D materials recovered for recycling are excluded. Moreover, soil generated from land cleaning and excavation is excluded in some databases.); geographic differences (fast and slow growing areas of the country); and differences in the year in which the data were gathered (thus differences in economic conditions) [7]. 3 According to the 2009 annual report of Tehran Municipality Waste Management, the C&D generation rate in Tehran is 46,655 m3 and the average weight of each m3 of

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Nomenclature EE

Eext EP1 EP2 ETi EC Ee EL EO

EEi

EPot ε

n TM

EM

ED Edem ET4 Ew EReci i EDec EProci ET  i

ET5−i

TB Eend

˛

embodied energy: all primary energy consumed in producing a building material/product until product leaves factory gate extraction energy: energy used to acquire raw materials material production energy: energy used to manufacture building material from raw materials product production energy: energy used to manufacture building product from building materials transportation energy: amount of energy needed to transport materials/products construction energy erection energy lifetime energy: energy consumed over building’s life span operating energy: energy required to maintain comfort conditions and day-to-day maintenance of buildings. That is energy needed for heating, ventilation and air conditioning (HVAC), domestic hot water, lighting, and for running appliances. Operational energy energy that is consumed from time recycled material returns to manufacturing phase until recycled product leaves factory gate potential recycling energy: energy that can be saved due to recycling a building material/product extra energy consumed in production phase, in order to make recycling possible or easier in future (design for dismantling) maximum frequency of recycling material’s life span: period of time variations mainly depend on the level of comfort required, climatic conditions, and operating schedules [1] maintenance energy: energy needed for maintenance and repair of building product, during its lifetime destruction energy: end-of-life energy in destruction scenario demolition energy: energy used to demolish building energy consumed in transporting waste to landfill site waste treatment energy: amount of energy needed to remove waste from building industry cycle recycling energy: energy needed for recycling products in each type of end-of-life phases recycling type deconstruction energy: energy used for each material in building instead of demolishing processing energy: energy consumed in processing material in each type of recycling amount of energy needed to transfer disassembled material to processing site amount of energy needed to return recycled materials to building life cycle within which a material can be used in building sector life span of building: average lifetime of a building type (e.g., residential or commercial) amount of energy needed to treat disassembled materials that can no longer be reused in building sector recyclable percentage

m0 mRec P 

primary mass of material recycled mass potential recycling energy per EE unit recycling probability factor: the higher , the greater the probability of recycling that material

there is also a critical demand for new construction, especially in the residential sector. (This is mostly due to population growth, a young population4 and the low average lifespan of buildings.5 ) As a result, in the not too distant future, we will face a high volume of construction debris. In order to overcome this crisis, it is necessary to plan to promote technologies and regulations and to educate the public from now onwards. Studies have been carried out by the Building & Housing Research Center of Iran [13,14], but there are some barriers to achieving a pragmatic approach to environmental issues, especially recycling, in Iran’s construction industry. One such barrier is the absence of an assessment system for materials. Assessment systems are considered to be one of the best methods of promoting the “green buildings” movement by educating the public about building environmental issues and thus increasing market demand for sustainable construction [15]. Competition among owners and developers to achieve high building assessment ratings, in order to obtain a higher market value, will ultimately create a high-quality, high-performance building stock [2]. In this paper a brief overview of energy consumption in the life cycle of a building product is provided and a method is suggested to calculate the part of this energy that can be saved by recycling that building product (potential recycling energy). The result of this study can be used in assessment systems as a recycling worth indicator for building materials and products. Gao et al. [16] have calculated the reduction in the energy and resources consumed by using recycled materials. Thormark [15] has recommended a method to assess the recycling potential of materials, product, and buildings. She also suggested a method to compare buildings, based on their recycling potential. The present study suggests a method to compute the potential recycling energy with regard to: recycling frequency, material discarded during recycling, and technologies applied to construction and demolition. In order to make an accurate comparison between materials and products possible, a factor is suggested which incorporates both embodied energy and potential recycling energy. Since documentation is not available for the embodied energy of common materials in Iran, this study is limited to a theoretical analysis. 2. Recycling in current assessment systems and tools Considering all the environmental aspects of building materials and construction technologies is beyond the time and skill constraints of most designers [3]. Additionally, the environmental

C&D waste is about 863 kg [8]. This rate includes soil generated from land cleaning and excavation and recycled materials. Dividing the total daily C&D waste generation (40.26 million tons) by the 2009 Tehran population (8.67 million [9]) yielded a per capita C&D generation rate of 4.64 kg per capita per day in 2009. 4 According to the last census in 2006, Iran’s population was 70.49 million, the growth rate was 1.62 in 1996–2006, the median age was 23.89, and 45% of the population were in the age range 10–29 years. Iran has 17.5 million households and there are 16 million housing units, regardless of ownership, which indicates a severe shortage of residential buildings [10]. 5 The average useful life of a building is 30 years in Iran [11], while it is 60 years in the USA [12]. Explanations for this include: carelessness in complying with building codes and regulations, poor maintenance of buildings, and rapid turnover in building stock [11].

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assessment of materials and buildings is very complex. Owing to the subjectivity and complexity, and the varying needs of different users, numerous tools have been developed, or are under development, in the last 20 years [15]. Material selection criteria are the most notable failures of building assessment systems in maintaining a clear objective function. In fact, defining “sustainable materials” and encouraging their use seems to be one of the biggest challenges facing the developers of green building rating systems [17]. In order to achieve an adequate assessment system, all the environmental impacts during the life cycle of materials and products and their relative magnitude of importance should be considered. Determining the weighting of a given environmental impact is challenging and different weightings can produce very different results [3]. The recycled content and recyclability of the materials and products is one of the factors that affect their evaluation. Here two famous material assessment methods that are used in assessment systems (e.g., LEED [18], BREEAM [19] and GB Tool [20]) are briefly described, with a particular focus on how they handle aspects of recycling. 2.1. Life cycle assessment (LCA) LCA is the most comprehensive method for evaluating the environmental impacts of materials and products [3] and most of the building assessment tools are more or less based on LCA [15]. Typically, the upstream (extraction, production, transportation, and construction) use, and downstream (deconstruction and disposal) flows of a product or service system are inventoried first. Global and/or regional impacts (e.g., global warming, ozone depletion, eutrophication, and acidification) are then calculated on the basis of energy consumption, waste generation, etc. [1]. A limitation of LCA is that it does not adequately address the closed-loop behavior of materials. It neither addresses whether a product or building can be disassembled and recycled, nor their recyclability [2]. In LCA, the effects of recycling are handled through allocation, the process of assigning material and energy flows as well as the associated environmental discharges (e.g., waste generated) of a system to different functions of that system. This process does not work accurately when materials are recycled, because recycling is a system where the “waste” from one function (product) may constitute the “raw material” in a subsequent function. If parts of the total impact are allocated to a subsequent function (recycled product), no product is taking responsibility for these parts if no recycling occurs in future [15]. The effects of allocation can be illustrated by a theoretical example of a 100 kg steel beam. If 60 kg of the mass is allocated to the subsequent recycled beam as raw material and that beam will not be recycled for any reason, then this 60 kg steel is not considered in estimates. As the recycling of building materials will occur in a distant future, if at all, the effect of recycling can only be considered to be a potential effect [15]. Furthermore, available methods make it difficult, and often impossible, to compare the effects of different materials and recycling options [15]. 2.2. Embodied energy (EE) and embodied carbon (EC) The embodied energy (carbon) of a building material can be taken as the total primary energy consumed (carbon released) over its life cycle. This would normally include (at least) excavation, manufacture, and transport. It is known as “cradle-to-gate” and includes all primary energy until the product leaves the factory gate. Ideally the boundaries would be set from the extraction of raw materials (including fuels) until the end of the product’s lifetime (including manufacturing, transportation, heating and lighting of

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factory, maintenance, disposal, etc.), known as “cradle-to-grave” [21]. Considering the end of the lifetime of a building in the embodied energy analysis is more difficult than any other industrial product with regard to its complexity and vastness. There is no single, universally accepted method which is, in part, why the subject is so widely debated and methods regularly contested. In existing methods, all the benefits of material recycling (such as energy conservation) are allocated to the primary product or to the recycled product or are divided 50:506 [21]. Since recycling is an indefinite process in the future, these methods have the same weaknesses as was noted in Section 2.1. Since fossil fuels are a primary source of energy during most phases of the material or product’s life cycle, higher EE generally means higher greenhouse gases and therefore higher environmental impacts [3]. (There are some exceptions to this like aluminium, because the power source is not fossil fuels [This issue]. On the other hand, more durable products will have a lower embodied energy per time in use [2].) In this paper, recyclable embodied energy is used as an indicator of the recycling value. Several sources have presented EE figures of different materials [2,3,22,23]. Current interpretations of embodied energy are quite unclear and vary greatly. Thus, embodied energy databases suffer from problems of variability (sometimes by 100% [3]) and incompatibility [23]. Variables include: regional and national conditions, manufacturing processes, recycled content, energy sources, and study parameters [3]. Thus, it follows that these figures should be precisely calculated by the national producers and research centres for the building materials commonly used by existing technologies in Iran. But for the time being, it is acceptable to rely on international databases.

3. Potential recycling energy Potential recycling energy can be briefly described as a measure of the energy which can be saved by recycling a material or product. Thormark describes the “recycling potential” as a way of expressing how much of the embodied energy and natural resources used in a product could, by recycling, be made usable after demolition [15]. 3.1. Application of potential recycling energy Building “waste” can be regarded as potential raw material for the production of new materials. Recycling building materials can considerably reduce the use of energy and natural resources and also reduce the use of land for landfill and resource extraction [15,24]. However, evaluating the relative worth of using recycled versus virgin materials, which should be a relatively simple matter, is controversial. One school of thought, here referred to as the ecological school, maintains that keeping materials in productive use, as in an ecological system, is of primary importance, and that the energy and other resources needed to feed the recycling system is a secondary matter. Another school of thought, the LCA school, suggests that if the energy and the emissions are higher for recycling than for the use of virgin materials, then virgin materials should be used [2]. This school believes that recycling is a means not the ends, but excessive focus on recycling implicitly gives more weight

6 This implementation contains the recycled content and recyclability in just one life cycle of a product and does not consider the entire life cycle. It does not consider how many times a product can be recycled. It simply divides the environmental loadings (in one recycling process) between primary product and recycled product. For instance, metal can be recycled many times and that is an advantage for this material, but this is not take into account in this method (50:50) [21].

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to solid waste and resource depletion issues than to global warming or other measures [17]. On the other hand, in order to foster green building trends in society, it is essential to promote and make possible a better integration of environmental concerns with cost and other traditional decision criteria [This issue]. Hence a method is needed to assess and compare the economic and environmental worth of recycling materials. There is no single universally acceptable method for considering recycling in assessment tools [21]. As was mentioned in Section 1, there is a lack of pragmatic approach to recycling in the construction industry of Iran. The methodology suggested in this paper can be a first step towards paying practical attention to recycling and more generally to environmental issues by achieving the following goals: • Measuring the economic and environmental worth of recycling building materials. • Comparing building materials, construction and demolition methods with regard to the possibility of recycling them. • Identifying materials which have great environmental impact on production, but a small recycling potential. • Utilizing the resulting figures in building labels, codes and guidelines, and defining taxes on materials and building. 3.2. Energy analysis over a building product’s life cycle The energy used in a building component’s life cycle is briefly explained here, in order to explain how “potential recycling energy” is assessed in this paper. However, real numbers should be extracted from existing databases or it should be assessed by precise experiments taking the climate and technological circumstances into account. A building product’s life cycle is divided into three phases (see Fig. 1): manufacture, construction and operation, and demolition. 3.2.1. Manufacturing phase This phase includes extracting raw material, producing the building material/product, and transportation in these procedures. The energy used in this phase is equal to the “embodied energy” in the “cradle-to-gate” method. EE = Eext + EP1 + EP2 +

2 

ETi

(1)

i=0

As an example, in the process of manufacturing a steel beam, mineral ores are the raw materials, the steel bar is the building material, and the steel beam is the building product. For building materials such as cement which are transferred to the site with no extra process, EP2 = 0. 3.2.2. Construction and use phase This phase encompasses all the energy used to transport the material/product to the building site and the energy consumed in the erection, operation, maintenance, and renovation of the building. EC = Ee + ET3

(2)

EL = EO + EM

(3)

3.2.3. Demolition phase Three scenarios are assumed for the end-of-life of buildings: renovation, demolition, and recycling. 3.2.3.1. Renovation. In this strategy the whole building is repaired and reused instead of being demolished. It is the most efficient

method to reduce the environmental impacts of the building industry [25], but will not be discussed in this paper. 3.2.3.2. Destruction (disposal). In this scenario the major part of the materials remain as landfill. Eq. (4) is suggested to assess the energy consumed in this type of end-of-life scenario. ED = Edem + ET4 + Ew

(4)

In order to simplify computations it is assumed that no material is recycled in the destruction strategy. 3.2.3.3. Recycling. If the strategies of recycling used in industrial ecology7 were applied to the built environment, the demolition stage could be replaced by deconstruction. The typical oncethrough life cycle of materials in the built environment could then be altered to accommodate the possible end-of-life scenarios and produce a range of alternative life cycles [25]. Some examples of these scenarios are briefly described below. 3.2.3.3.1. Product recycling. In this method, products (e.g., beams, bricks, glass, etc.) can be reused again without changing their form or nature. This scenario only consumes a small amount of energy [16,25]. 3.2.3.3.2. Material recycling. In this scenario, materials are reprocessed into new components. It involves materials or products that are still in good condition being used in the manufacture of new building components. A good example of this is re-processing a timber beam into smaller members [16,25]. 3.2.3.3.3. Feedstock recycling. This refers to a process where the disassembled material is processed into feedstock as a substitute for natural resources to make a building material. One of the most common current examples of this, is the crushing of reinforced concrete to make aggregate [16,25]. The energy consumed in each type of recycling, which is explained above, can be assessed by the following equation: EReci = EDec + EProci + ET  + ET5−i + EEi i

(5)

EEi is calculated for each type of recycling by the following equations: EE1 = 0 EE2 = EP2 + ET2 EE3 = EP2 + ET2 + EP1 + ET1 During recycling, it is probable that a percentage of raw material will be added to the recycled content. In order to assess the EEi of the recycled material, the total amount of energy should be multiplied by the percentage of recycled material. 4. Assessing potential recycling energy Fig. 2 compares the energy consumed in producing two similar building products utilizing two different strategies (using raw or recycled materials). It indicates that by recycling materials, the energy consumed during manufacturing the new product (EE) as well as energy used during demolition of old product (ED ) can be saved. Potential saved energy (potential recycling energy) can be assessed as follows: EPot = (EE + ED ) − (EReci + ε)

(6)

7 Industrial ecology emerged in the late 1980s. It is the study of the physical, chemical, and biological interactions and interrelationships both within and between industrial and ecological systems [2].

M.D. Saghafi, Z.S. Hosseini Teshnizi / Energy and Buildings 43 (2011) 3181–3188

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Fig. 1. Life cycle of a product and energy consumption in this cycle. Recycling a product prevents energy consumption in both the manufacturing (for next product) and destruction (for existing product) phases. In demolition phase material disposal would be replaced with recycling, as shown in diagram.

cycles of nature, in which there is no waste .Not less waste and fewer negative effects, but more positive effects .One organism’s waste is food for another, and nutrients and energy flow perpetually in closed-loop cycles of growth. No waste is released in nature and waste equals food [6]. However, with regard to common materials and technologies, the quality of most of the materials decreases through the recycling and during their lifespan; hence, the possible frequency of recycling is limited. Eq. (7) suggests a way to assess how many times a material can be recycled: n=

Fig. 2. Energy consumption in two recycling and disposal scenarios. Potential recycling energy for a product is the energy that can be saved by recycling it instead of remanufacturing. EPot can be obtained by subtracting the energy used in the recycling scenario from the energy used in destruction (disposal) scenario (see Eq. (6)). EPot = (EE + ED ) − (ERec + ε).

All the energies in all the equations in this paper are assessed for one measurement unit of a building material/product. It is essential to note that material measurement units vary according to various functions in the building industry. It can be 1 kg, 1 m, 1 m2 , or 1 m3 in the SI units for the mass, length, area, or volume of that material. For example, the square meter is used for floor covering and the meter is used for beams. Using standard measurement units makes it possible to compare the different materials that can be utilized in a function. If the design for deconstruction concept has not been adopted during the design phase, it is assumed that ε = 0. Eq. (6) is valid for the materials which will not be replaced during the building’s lifespan, i.e. materials that have a lifetime longer than a building (e.g., those materials that are used in the structure of a building), because it is assumed that each time the material is recycled, it prevents consuming EE and ED . 4.1. Frequency of recycling As noted in Section 2.2, the durability of a material is very important in terms of the amount of EE. Thus, the higher recycling frequency, the higher the value of EPot a material would have. The ideal form of using building materials is the closed-loop nutrient

TM TB

(7)

Processing of the material during recycling procedures increases its life span and should be taken into account in assessing the life span of the material. In order to estimate the maximum frequency of recycling for a material, it is assumed that the recycled material is utilized in the same type of building with the same lifespan as the primary material was used (e.g., primary and recycled products are both used in residential buildings with TB = 35 years). In order to obtain the value of TM for common building materials, producers and standard organizations should be obliged to assess the materials experimentally. If n not an integer, the recycled material needs to be replaced before the building’s life has ended. In some building sectors, such as structure, the integer part of n is acceptable, because it is not possible to replace these sectors during the period of operation. Materials that are finally released in the environment and have limited recycling frequency should be recycled naturally, meaning that the nature should be able to recycle them and turn them back into nutrients for ecosystems. If a material is recycled n times, the potential recycling energy is calculated as follows (see Fig. 3): EPot = [nEE + (n + 1)ED ] = (nERec + ε + Eend )

(8)

Eend = EDec + ET4 + Ew

(9)

In Eq. (9) it is assumed that all the primary material can be recycled, regardless of any damage that may have occurred during the recycling process. Moreover, it is assumed that each time the material is recycled, the recycling type and energy is the same. Recycling is an uncertain event in the future. Thus, if for any reason the material is recycled less frequently than is possible, the value of EPot will be less than is assumed in Eq. (8). So this quantity

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Fig. 3. Energy consumption in two recycling and disposal scenarios if material is recycled twice. When a product is recycled twice, manufacturing energy would be saved twice and destruction energy would be saved three times and recycling energy would be replaced twice (see Eq. (8)). EPot = (2EE + 3ED ) − (2ERec + ε + Eend ).

merely represents a potential capability of materials and in order to prevent mistakes in assessments, it should be considered separate from the embodied energy figures in assessment systems. Eq. (10) assesses the potential recycling energy, considering possible differences in recycling types each time:



EPot = [nEE + (n + 1)ED ] −





n

EReci + Eend + ε

(10)

i=1

4.2. Recycling disposal Material discarded during the recycling processes, affects the value of EPot . The ˛-factor is suggested as follows, in order to calculate EPot including this effect: ˛=

mRec m0

(11)

The ˛-factor is dependent on the material type and technologies utilized in a project and should be assessed by the manufacturers and standard organizations on the basis of practical experience. It may be a characteristic of the material and its construction technique. Eq. (8) for calculating EPot can be developed by considering the ˛-factor in the assessments (see Fig. 4):



EPot =

EE

 −

n 

i

˛ + ED

i=1

ERec

n 



i

˛

i=0 n  i=1

1

˛ + Eend

 n 

 i

n

(1 − ˛) + ˛

 +ε

(low EE), and much of this energy is recyclable (high Epot ). Thus, for a meaningful comparison between different alternatives, these two quantities should be measured together: p=

EPot EE

(13)

The higher the p-factor, the more energy per embodied energy unit is recyclable. Consequently, that material is preferable in terms of both the Epot and EE factors. The probability of recycling, and the scope for dismantling and separating materials from each other, are other important factors that affect the recycling worth assessment. Despite a need for recycling in the future, whether or not a material or component will actually be recycled depends on many factors which influence or contradict each other. In addition, the probability of each factor is very different. Together the factors make up a complicated system [15]. This quality could be made visible by introducing a probability factor () as follows: p=

EPot EE

(14)

The higher the value of , the greater the probability of recycling the material.  should be quantified by analysing factors such as: the time requirement, risk of working in the area, and variety of possible uses of the material [15] and their degree of importance and their interactions. This procedure may need many practical experiments. We now give an imaginary example for a steel beam: EE = 30 MJ/kg;

(12)

i=1

It is assumed that ERec is the same in all the recycling steps. Since Eq. (12) aims to assess the potential recycling energy for one measurement unit of a material, it does not consider the energy consumption for primary materials that substitute for discarded materials.

ED = 5 MJ/kg;

Eend = 7 MJ/kg;

ε = 5 MJ/kg

If the beam material is recycled three times and 90% of the mass is recyclable each time, then EPot = [30(0.9 + 0.92 + 0.93 ) + 5(1 + 0.9 + 0.92 + 0.93 )] −[10(0.9 + 0.92 + 0.93 ) + 5 + 7[(0.1 + 0.192 + 0.273 ) +0.66]] = 52.34 Mj/kg

5. Comparing materials and products A problem that arises with treating EPot and EE as separate entities is whether or not they should be weighted together. In other words, how do we compare the materials and then determine which is the best on the basis on their recycling value [15]? In comparing two materials, on the basis of their recycling value, it is probable that EPot be higher, simply because of the higher rate of EE. That cannot be regarded as an advantage. Indeed, an ideal building product only spends a little energy in the production phase

ERec3 = 10 MJ/kg;

p=

52.34 = 1.74 30

6. Discussion In this paper a methodology is suggested to consider recycling benefits (EPot ) separate from the production (EE) and demolition

M.D. Saghafi, Z.S. Hosseini Teshnizi / Energy and Buildings 43 (2011) 3181–3188

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 2

2



Fig. 4. Energy consumption in two scenarios of recycling and disposal when material is recycled twice in terms of recycling damage. ˛ is undamaged percentage of primary material that is not discarded and can be recycled each time and does not need to be replaced with raw material (see Eq. (12)). EPot = EE



ERec

2

i=1

˛i + Eend

2

i=1

3

(1 − ˛i ) + ˛



i=1

˛i + ED

i=0

˛i −

+ε .

(ED ) energy. This is important because the recycling benefits and the effect of material selection and the technologies that were applied then become visible. For this purpose, as Thormark suggested [15], all impacts from the production and waste treatment of a material (EE, ED ) are treated as a separate quality allocated to the original product and the recycled product takes responsibility for the recycling processes (ERec ) in assessment tools. The potential benefits of recycling are treated as a separate capability of a product (ERec ). In this case, if recycling does not occur in the future, no errors occur in the primary assessments. In previous studies material lifetime, energy consumption, and the probability of recycling were considered. In this paper a new approach is presented which considers the lifetime of the material, based on the building’s life span. Material discarded during recycling is taken into account. A new method is suggested for comparing materials/products based on both their recycling potential and embodied energy, and their recycling probability. The present authors acknowledge that this methodology has limitations in tracing material in the processing procedures. The primary material would combine with some new material during the recycling. Furthermore, predicting recycling types would be difficult or impossible. The assumptions that are implied for defining these equations reduce the comprehensiveness and the accuracy of the recycling potential energy. An accurate calculation of this recycling energy is possible at the end of a material’s life cycle. However, these uncertain figures can represent a general view of the recycling worth of building materials/products and construction/demolition techniques that can be utilized in assessment tools. Lack of information about a material’s lifetime, the amount of energy consumed in each phase, and material discarded during recycling, for common materials in Iran, is not a weakness of the suggested method. With a greater focus on recycling, better data is likely to be provided. Energy is the only analysed factor in this study and other parameters such as the type of energy, emissions, economic factors, and other impacts are not discussed. As a rule of thumb, energy is a reasonable indicator of the overall environmental impact of building materials. However, in order to obtain a comprehensive assessment of a material’s recyclability, it should not be the only factor. The equations could be extended for a building or its assemblies by summing the values of the different materials used in them. Materials or products that use nonrenewable resources or produce toxic releases in their life cycle, or they are not naturally

recyclable regardless of EE and EPot figures, are not environmentally preferred materials. Decreasing the operating energy (by promoting quality and standards) in high-performance green building projects, increases the importance of the energy used to produce materials. (Hernandez and Kenny [26] have provided a model to account for embodied energy together with energy use during operation.) A comprehensive assessment should consider an optimal balance between EE, OE, and EPot . The effect of contemplating the recyclability in the design phase (design for deconstruction) can be calculated with the suggested equations. In general, this strategy decreases ERec , increases ˛factor and thus increases EPot . In other words, it increases the quality and quantity of recycling. 7. Conclusion Recycling and closing the material loop are efficient strategies for reducing the environmental impacts of the building industry. However, there is no internationally accepted, comprehensive and pragmatic method for assessing and comparing the recycling potential of materials. In this paper, the potential recycling energy has been suggested as a factor for assessing the recycling value of materials. This quantity can be applied in assessment systems, distinct from the embodied energy. This strategy prevents errors in the primary assessments, if recycling does not occur in the future. Regarding the interactions and interrelations between the potential recycling energy and the embodied energy, the p-factor is suggested to represent the potential recycling energy, per embodied energy unit that makes comparisons between materials and products possible. These factors would help the designers and contractors to compare and select green building materials. They can also be used in building regulations and standards to define taxes and discounts based on the recyclability of materials. References [1] T. Ramesh, R. Parakash, K.K. Shukla, Life cycle energy analysis of buildings: an overview, Energy and Buildings 42 (10) (2010) 1592–1600. [2] C.J. Kibert, Sustainable Construction: Green Building Design and Delivery, John Wiley, Hoboken, New Jersey, 2005. [3] M. Calkins, Materials for Sustainable Sites: A Complete Guide to the Evaluation, Selection, and Use of Sustainable Construction Materials, John Wiley, Hoboken, New Jersey, 2009. [4] M. Asif, T. Muneer, R. Kelley, Life cycle assessment: a case study of a dwellinghome in Scotland, Building and Environment 42 (3) (2007) 1391–1394.

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