Abstract:
Construction and demolition (C&D) waste are of significant importance due to the generation of the waste in immense quantity. The inert nature and availability throughout the world desire it’s recycling in various concrete and infrastructure applications. It was estimated that C&D waste generation rate crossed the 3 billion tonnes annually until 2012. The major part of the waste is inert and consists of concrete, asphalt, dredging spoils, ceramics, glass and hazardous waste. The treatment of these materials costs a significant amount of energy and resources. The recycling provides the aspect of conservation of materials and resources in the form of new products and introduces the sustainability towards concrete materials incorporation. Recycled concrete aggregates from this waste can be incorporated into the production of new concrete or road sub-base material. Recycled aggregates are of inferior quality due to adhered mortar onto the surface of the aggregates producing concrete of lower strength. In recent times, significant attention is being paid to the removal and strengthening of the adhered mortar to produce concrete of approximate standard quality. The major objective of the thesis is to introduce sustainable and environment-friendly materials in concrete production, which lead to reduce cement utilisation without compromising the concrete properties. Therefore, the use of organic waste is chosen, which is available abundantly and can be processed to produce stable char product through pyrolysis. The preparation and detailed testing of char concrete composites were part of the research. Char was tested for several design strengths and later tested for mechanical strength and water absorption potential in natural and recycled aggregate concrete. These analyses were accompanied by comprehensive characterisation, economic, environmental and carbon footprint investigation. The use of biochar (biomass-derived carbonaceous material, also known as ‘char’) in concrete composites, has been thoroughly investigated in natural aggregate concrete and recycled aggregate concrete. The use of char according to the volume and binder content explored in detail and recommendations were made for the use of char in different scenarios. Char incorporation into concrete according to the volume from 0.1 to 1% of the total volume of concrete showed a decrease in compressive and splitting tensile strength at first, however, modifying the char through bentonite clay, and builders lime treatment improved the properties and produced results significantly higher than the preliminary investigation. Wood char and rice husk char both created favourable strength characteristics and reduced permeable voids which further enhanced durability properties. The reduction in workability was noticed with the increment of char, and a further reduction in workability was observed with the replacement of natural aggregates altogether with recycled aggregates. Engineered char was recommended to use at 0.5% of the total volume of concrete for both natural and recycled aggregate concrete. Char incorporation into concrete according to binder content, also showed an optimistic trend in different strength designs. Char use in recycled aggregate concrete is particularly recommended with superplasticiser to decrease water consumption. Recycled aggregates and char both intensified the water consumption, which in turn decreased the strength performance and led to higher permeable voids. The use of char up to 3% of total binder content is recommended to produce effective results with superior strength and durability properties. Calcium and silica-rich char both performed satisfactorily at several design strengths up to 3% of total binder content. Char concrete composites contain the potential to absorb carbon dioxide further to increase the reactivity in concrete during its lifespan. Polypropylene fibre reinforced concrete composites contains significant disadvantage in terms of reduction of compressive strength. The bending strength is usually increased, prolonging the complete fracture of concrete; however, introduce incompatibility issues. The use of char in polypropylene (PP) fibre reinforced concrete composites produced interesting results and coped with the compressive strength decrease in PP fibre reinforced concrete. After casting the concrete, the remarkable increase in strength was observed within two days in which concrete surpassed the 28 days strength of non-char added reinforced concrete. The extraordinary performance in polypropylene fibre reinforced concrete reaffirms the idea of char’s affinity towards polypropylene in concrete. The strong cohesion and adhesion forces played a significant role in obtaining the 90% increase in compressive strength. A substantial decrease in permeable voids indicates the char’s potential to produce compact structure, improving the potential for high resistance towards water-related durability issues. Nanomechanical properties were also significantly affected by the char’s inclusion in recycled aggregate concrete composites. Phase distribution showed the production of high-density C-SH in old interfacial transition zones (ITZ) with char addition than plain recycled aggregate concrete. The hardness data points showed a significant shift towards higher hardness values with the increase in char addition and a decrease in contact depth, indicating a maturity of concrete in the ITZ region. The addition of rice husk char at 3% significantly increased the modulus across ITZ than plain recycled aggregate concrete. Char addition improved the overall properties of fibre reinforced recycled aggregate concrete with a significant edge on mechanical strength. Compressive strength was found to increase in the range of 30 – 40 % with the addition of rice husk and wood char. Similarly, flexural strength has also shown an increasing trend by up to 30% than non-char added counterparts. Engineered poultry litter char, on the other hand, showed comparable properties to control samples and did not show positive behaviour in recycled aggregate concrete as compared to other types of char samples. Higher carbon dioxide capture was noticeable in char added cementitious composites with a significantly higher reactivity. Furthermore, rice husk char tends to change the morphology of the hydration products at higher additions ≥ 5% of binder content which ultimately shows its limitations in cementitious composites. It can be concluded that char improved the fibre reinforced recycled aggregate concrete properties from mechanical and durability perspectives, however, might compromise the properties of concrete at ≥ 5% replacement of binder on the long-term scenario. We can conclude that char influenced the properties of concrete substantially from macro to nanoscale and thus contributed towards strength enhancement; however, there is a cautionary measure that needs to be considered while adding these materials in concrete. For instance, char properties can vary widely depending on the feedstock and production process which is sometimes detrimental to the concrete microstructure leading to a high amount of variation in concrete maturity. The minor addition, such as in the range of 2 – 3 % of total binder content is recommended and should not be added further due to change in morphology of hydration products. This affects the packing density in concrete structure resulting in a substantial reduction in strength performance for both natural and recycled aggregate concrete and therefore must be used for low-risk structural applications. Char addition improved the overall properties of fibre reinforced recycled aggregate concrete with a significant edge on mechanical strength. Compressive strength was found to increase in the range of 30 – 40 % with the addition of rice husk and wood char. Similarly, flexural strength has also shown an increasing trend by up to 30% than non-char added counterparts. Engineered poultry litter char, on the other hand, showed comparable properties to control samples and did not show positive behaviour in recycled aggregate concrete as compared to other types of char samples. Higher carbon dioxide capture was noticeable in char added cementitious composites with a significantly higher reactivity. Furthermore, rice husk char tends to change the morphology of the hydration products at higher additions ≥ 5% of binder content which ultimately shows its limitations in cementitious composites. It can be concluded that char improved the fibre reinforced recycled aggregate concrete properties from mechanical and durability perspectives, however, might compromise the properties of concrete at ≥ 5% replacement of binder on the long-term scenario. We can conclude that char influenced the properties of concrete substantially from macro to nanoscale and thus contributed towards strength enhancement; however, there is a cautionary measure that needs to be considered while adding these materials in concrete. For instance, char properties can vary widely depending on the feedstock and production process which is sometimes detrimental to the concrete microstructure leading to a high amount of variation in concrete maturity. The minor addition, such as in the range of 2 – 3 % of total binder content is recommended and should not be added further due to change in morphology of hydration products. This affects the packing density in concrete structure resulting in a substantial reduction in strength performance for both natural and recycled aggregate concrete and therefore must be used for low-risk structural applications.