Whilst sustainable construction relates to both a building’s structure and the use of proper life-cycle processes, the selection of the most appropriate material/s is deemed a considerable undertaking. The suitability of building materials for various practical uses is determined by their particular properties. Timber, concrete and steel are the most commonly used structural materials in the built environment, and they typically behave quite differently under various loads (Gharehbaghi, 2015).

The following factors (either taken individually or in combination) form the basis for various systems of classification of sustainable materials (Gharehbaghi, 2014):

-The chemical composition of the material, that is their chemical makeup. -The material mode of occurrence in nature, organic and macrobiotic systems. -The refining and manufacturing processes, including costs, to which it must be subjected to, before it gains economic importance. -The material internal (atomic and crystalline) structure. -The industrial uses and purpose of the materials. As already stated, there are three types of materials commonly employed in sustainable construction practices; these are:

-Steel: Conventionally steel is an alloy of iron and other material elements combined. Due to its tensile strength, steel is commonly used in construction. Steel can be molded into different shapes and compositions with sufficient heat. In the construction practice, steel is generally used not only in reinforcing components but also to build beams and other structural components. -Concrete: Concrete, is in essence, a composite material that is extensively used throughout the construction industry around the world. Most concrete mixes use Portland cement and different types of aggregates. Recently, there have been many new developments in reusing recycled concrete as a material aggregate. -Timber: Timber is one of the oldest materials used throughout the construction industry. Although its use has been slowly reducing, in particular for high-rise construction, timber has been processed into beams and other load bearing structural components over the past. In recent times, timber usage has been mainly for fit-outs and non-load bearing structural components. These three materials, whether used individually or in combination still provide the primary source for the sustainable construction of the built environment (Gharehbaghi, 2015). Collectively, they almost provide 90% of all the materials used throughout the construction industry (Gharehbaghi, 2015). Hence, the selection of the most appropriate material/s to use in a particular setting is a considerable undertaking from a sustainable construction standpoint. Although there are various methods available to utilize in material selection, processes are not universal and the importance of factors that are to be accounted for differs according to the country of location. As Allwood et al. (2012) correctly conversed, there are multiple hurdles to incorporate sustainability into material selection practices such as overall cost, material's behavior and various design limitations and requirements.

A main source of environmental impacts associated with greenhouse emissions comes from the mining of raw materials and their production (Goodhew, 2016). However, Gajanan (2012) argued that the embodied energy for production of buildings was still small in comparison to the embodied energy used for the operation during the building's life. Furthermore, Halliday (2012) correctly claimed that whilst approximately 90% of a structure’s CO₂ emissions are a result from the energy consumed during its life, there is much that can be done to reduce that 10% associated with its construction.

Most materials –in particular concrete– is the primary starting point for CO₂ emissions reduction, since it is responsible for approximately 10% of global CO₂ emission's. In addition to concrete, steel and its production processes also produce some CO₂ emission, but not as much as concrete (Woosik et al., 2015). Conceivably, the most emission-less material is timber, since there is not that much pollution produced during its production.

More sustainable forms of materials can exist where recycled aggregates are used in their composition. For example, crushed glass or wood chips can be added, as the basis of concrete aggregates (Khouli et al, 2015). Recently, innovative engineering materials, such as recycled concrete, have emerged with the potential to influence the future of an environmentally sustainable construction product industry.

Steel, concrete and timber are unequivocally the three most commonly used materials in sustainable construction practices. To fully comprehend sustainable construction material selection prcoesses, the commonly used materials need to be thoroughly investigated. For such purpose, three buildings were selected and carefully analyzed. The three buildings were traditional high rise (approximately 18 levels) residential structures in Sydney, Australia. Their age is less than 20 years whereas their material composition consisted of Concrete, Steel and Timber. Nonetheless, the main structural elements of the said buildings were a combination of concrete and steel, with timber being utilized for low load bearing structural elements. All three structures were constructed on deep concrete footing and reinforced foundations of end bearing piles (with some friction piles utilized as well). The analysis of the three buildings was based on the specific materials which were actually used in these three independent sites. Since steel, concrete and timber were the major materials used in these structures, their particular usage and convention require careful analysis. Subsequently, to further demonstrate the utilization of these three materials, tables were developed to exhibit their usage as shown hereinafter. As a basis of comparison, Material Selection Factors [MSF] were utilized.

Generally, buildings expend great quantities of materials, energy and other resources. This produces momentous environmental impacts during construction, operation and the eventual demolition. The main aim of environmentally sustainable and responsible materials is to consume fewer resources, particularly less energy, and to diminish unfavorable environmental impacts (Miller and Kenneth, 2013). At present, the construction and building industry is seeking and moving towards sustainable materials. Utilizing sustainable materials is not only due to the market demands, but also because of potential cost savings (Montoya, 2010). Trying to define sustainability is a difficult task, and one will find as many varied classifications as possible. There are some key points that tend to overlap to provide a workable definition from which to start, and balancing these factors is the key to a sustainable approach. Traditionally, operation cost reduction and energy-saving benefits are important considerations for sustainable construction. Accordingly, sustainable materials should consider an environmental aspect, such as, CO2 production, environmental deterioration and so on. In addition, sustainable materials should also consider economic issues, such as, the overall costs, long-term sustainability/longevity, productivity and economic growth, and so on. The latter factors are quite important for the construction of buildings, particularly in developing countries. Collectively, key factors in selecting suitable sustainable materials (i.e. Material Selection Factors [MSF]) can include (Natee et al., 2016; Tucker, 2015): 1.Environmental Impact. Traditionally, to analyze the environmental impact of building materials, the life-cycle of the different materials should be considered. This life-cycle consists of the extraction of the raw materials, the manufacturing, construction, use and demolition. 2.Fire Performance. How a material performs during fires can be assessed through various building codes and test standards. Various Australian codes and standards exist such as AS1684 Residential Timber-Framed Construction. The categories for a material’s fire performance typically include: •Combustible materials, for example explosive and burnable materials such as steel and timber. •Non-combustible materials, for example non-explosive and non-burnable materials such as concrete. •Fire-resistant materials. •Ignition-resistant materials. 3.Structural Performance (Strength and Durability). The strength of a structure/material is simply the load which will break the structure and/or materials (Gharehbaghi, 2014). The durability, for example the life span of a material, is its resistance to deterioration. Deterioration may occur due to physical, chemical or biological factors. Durability is important for structural materials because they are expected to last a long time. Sometimes materials need to be treated in some way to improve their durability. The suitability of building materials for various structure uses is determined by their particular properties. Timber, concrete and steel are known to behave quite differently under load. 4.Functioning Capabilities. The material’s functioning capabilities not only include improved Load Bearing Capacity (LBC), but also enhanced stress and strain potential (Gharehbaghi, 2014). Stress strain curve is a behavior of material when it is subjected to load. Structural failure refers to loss of the load carrying capacity of a component or member within a structure or of the structure itself. Structural failure is initiated when the material is stressed to its strength limit, thus causing fracture or excessive deformation. The ultimate failure strength of the material, component or system is its maximum load-bearing capacity. In addition to these four key factors, other areas, e.g. economics, ease of construction, etc., can also be considered to ensure informed decisions are made when selecting suitable sustainable materials. Different decision making models have been used in construction-related problems in the past, e.g. Georgy and Barsoum (2005), Georgy et al. (2005), and others. In the herein study, to account for the different factors of concern, e.g. environmental impact, fire performance, etc., a simple additive scoring method is employed to estimate the score of a construction material (MSFi) based on a number of factors (j).

As described above, 4 factors are taken into account. The score or rating of a material with respect to a particular factor (s_ij) is contingenct upon how such material fares in this particualr domain. For instance, concrete is known to perform better than steel in case of fire, while both concrete and steel perform significantly better than timber. This means that the individual score (s_ij) for concrete will be greater than steel whereas the lowest score will be given to timber. The weight (w_j) reflects the relative importance of a certain factor comapred to others. When the individual scores/ratings are added up using the given weights, materials can be compared to one another based on the estimated MSF_i scores.

There are two types of additive scoring methods; they are: (a) the weighted scoring models, and (b) the unweighted scorning models. In the latter case, which admittedly is much less common, the different factors are considered to be equally important. For illustration purposes, this will be assumed in this paper.

Below table provides a comparison amongst the three key construction materials while accounting for the Material Selection Factors (MSF) discussed above. This MSF setting is a vital component in selecting the suitable material for sustainable construction.

As noticed, concrete has the highest MSF rating/score, which is slightly better than steel. On the other hand, timber has a significantly lower MSF, and accordingly it is less likely to be used in demanding areas such as high seismic activity zones. This ranking is based upon an assumption that all the four MSF have equal relative importance. If the relative weights amongst the 4 MSF change, a different ranking may emerge. For instance if environmental impacts are highly regarded due to the sensitivity of area where structure is to be constructed, timber may come at the top due to its excellent environmental impacts rating.

Although sustainable construction typically relates to the building’s structure and the use of proper life-cycle processes; the selection of the most appropriate material/s to employ is also a significant consideration. This paper focused on the three main conventional materials for general construction: Steel, Concrete and Timber. As discussed, whether used individually or in combination, these three materials provide a foundation for a sustainable built environment. Also, they collectively constitute almost 90% of all the materials used throughout the construction industry. Therefore, the selection of any of these three materials is one of the most important factors that impact the sustainability of buildings. As a part of the material selection process, there are many factors that require careful deliberation. These factors were referred to as Material Selection Factors (MSF). Paper particularly focused on four specific factors, which could affect the material selection. These factors included, Environmental Impact, Fire Performance, Structural Performance (Strength and Durability), and finally Functioning Capabilities. In addition, while the four factors were carefully considered, the three main materials (Steel, Concrete and Timber) were also compared via the MSF equation. Subsequently, it was demonstrated that the concrete and steel are the most sustainable materials for demanding situations, where, structural performance and functional capabilities are of essence. While timber was not collectively highly ranked, it still performed admirably with respect to environmental impacts (higher than steel and concrete). Conversely, timber performed poorly with respect to fire performance, while in such situation concrete performed as the best sustainable material. In conclusion, the MSF ranking was based upon an assumption that the four factors had the same level of importance. Nonetheless, if a different weighting scale is used, the MSF ranking will ultimately shift. Therefore, additional research may be required with different MSF rankings and values to further examine the overall performance between steel, concrete and timber.