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s3597637 Minkyung Christy Kang Use of recycled materials in road pavements; subgrades, subbases and stabilisation of highly compressible and low strength soils. Road pavements consist of multiple layers; pavement, subbase and subgrade. Strength of subbase and subgrade affects pavement performance hence impacting pavement design (Moffatt, 2017). For use in pavement sublayers, mechanical and strength deficiencies of unsuitable soil need be improved through soil stabilisation; addition of binder materials (Jameson, 2019). Lime and cement are traditionally used binder materials for subgrade, and virgin aggregates are used for subbase layers. However increasing costs and scarcity of natural resources accentuated the need for alternative materials (Bandara et al., 2015). Pollution due to demolition wastes and industrial by-products is also an increasing global concern (Mroueh et al., 2001, Arulrajah et al., 2013, Tam et al., 2018). Use of recycled materials conserves natural resources, reduces production costs and pollution, therefore demonstrating sustainable engineering practice. This review will discuss three recycled materials; fly ash, recycled concrete aggregate and recycled asphalt pavement. Fly ash (FA) is a by-product from coal fired power stations which enhances interlocking mechanism in soil particles and reduce porosity, consequently increasing strength and durability (Mohammadinia et al., 2017). Kolias et al. (2005) asserts addition of cement to FA achieves greater strength due to enhanced particle distribution allowing better binding effect. Bandara et al. (2015) also claims FA itself obtains insufficient uniaxial compressive strength (UCS) to be used as a subgrade stabilising agent. However, such claims contradict the primary purpose of using recycled materials. In contrast, other researches prove clear improvement in UCS, Californian bearing ratio (CBR) and resilient modulus (Mr) in FA stabilised soil (Li et al., 2008, Mohammadinia et al., 2017) but Mohammadinia et al. (2017) suggests excessive FA content results in discontinuity in interlocking mechanism and undergoing numerous freezethaw (F-T) cycles may impact the strength. Recent studies discovered Mr of FA stabilised materials decreased consistently and the trend plateaued after 5 F-T cycles, and FA containing high calcium oxide (CaO) exhibited the least reduction in Mr (Rosa et al., 2017). This finding is significant as it explores the limitations on previous studies and demonstrates long term suitability of FA as stabilising agent. Additional study is required on percolation (Li et al., 2008), cracking potential of stabilised subgrade (Kolias et al., 2005, Bandara et al., 2015) and possibility of soil and groundwater contamination from hazardous substances leaching from FA stabilised materials. Recycled concrete aggregate (RCA) is concrete debris from demolition waste. It is widely recommended as a substitute for virgin aggregates in subbases for its superior performance. It has been reported RCA improves Mr, shear strength and saturated hydraulic conductivity compared to natural materials (Arulrajah et al., 2013, Kang et al., 2011), which indicates RCA subbases have better drainage than natural aggregates. Such improved strength is due to high cohesion achieved from residual cementing when contacted with water, but also has risks of causing shrinkage and reflective cracking although unlikely to affect the pavement performance (Arulrajah et al., 2013). Contrastingly, a recent research states only certain RCA subbase mixes achieved desirable rutting resistance and deformation (Saberian et al., 2018). This disagreement may be related to another concern; consistency of performance due to varying RCA properties depending on source. Low quality RCA usually ends up in landfill due to high porosity, and application in pavement still requires improvement (Tavakol, 2019). Tam et al. (2018) mentions that quality can be controlled following institutional classifications in waste processing plants. However, these classifications are too broad for precise quality control. Replacing natural aggregates with RCA reduces environmental loading but the extent varies on each construction (Mroueh et al., 2001). As with FA, no study is conducted on possibility of degrading over time and leaching of hazardous substances.

s3597637 Minkyung Christy Kang Recycled asphalt pavement (RAP) is a demolition waste from road pavements, which consists of bitumen and aggregates. It has high permeability and hydraulic conductivity than RCA (Arulrajah et al., 2013). Like RCA, RAP showed similar if not greater drainage characteristics than 100% virgin aggregates (Kang et al., 2011). Despite its high draining quality, it has been commonly reported RAP needs to be mixed with virgin materials for subbase application due to its low CBR achievement (Arulrajah et al., 2013, Puppala et al., 2012, Taha et al., 2004). Contrasting its poor suitability, Bocci et al. (2010) states that pavement construction utilising RAP generated only 80 tons of noxious gas while traditional method generated 1000 tons. Research proves significant reduction in energy and water consumption and chemical pollution when using RAP in subbase, negligible when compared to using virgin materials (Celauro et al., 2015, Bocci et al., 2010). However, it is critical to note these studies are based on reconstruction of existing roads and such environmental benefits develop from reduced transportation by sourcing RAP from the existing roads, as mentioned by the authors. Moreover, substitutability and mechanical quality improvement of RAP requires further research for the material to be entirely utilised. Unlike FA and RCA, RAP is found to be safe from leaching hazardous substances when applied in pavement (Kang et al., 2011). Limitations include: •

There is a cracking potential of FA stabilised subgrades.

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The possibility of soil and groundwater contamination from hazardous substances leaching from FA stabilised materials and RCA containing subbases remains unstudied.

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Unprecise quality control and there are no findings on degradability of RCA over time.

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RAP still requires virgin materials to be applied in subbases for high strength achievement.

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