An investigation into the use of waste materials for concrete applications in the South African construction sector.
Concrete is one of the most common construction material in the world, which contributes approximately 5% of man-made CO2 globally (C&CI, 2011). An increase in consumer resource consumption and waste production creates an opportunity for research into concrete which uses less virgin minerals, waste materials and has a lower cement content. Such prior research into alternative aggregates has not only delivered innovative high-end concrete adaptations for high performance and specialist applications, but also economical greener alternative concrete solutions which provide a means for developing nations to improve their standard of living by incorporating materials which are cheaper or more environmentally friendly and still yield adequate performance. This study is aimed at the latter, by investigating the potential viability of waste concrete mixes that involved the partial substitution of cement and aggregates with locally sourced waste products. Cement was substituted with coal bottom-ash (BA), stone aggregate with sugarcane bagasse fibre (SCBF) and recycled high density polyethylene (HPDE) plastic pellets. In this study, an investigation into the potential viability of waste modified concrete in a local context was carried out by comparing strength (compressive, flexural, and splitting), durability (Oxygen Permeability Index, Chloride Conductivity, and Water Sorptivity), workability and economics (R/m3 concrete cost & scenario analyses) with conventional concrete. The critical volume substitution for each waste material was found after evaluating the compressive strength of each waste and this was used to develop mixed waste specimens to investigate combinations of waste in concrete. To further investigate the behaviour of waste concrete mixes and fulfil knowledge gaps identified in literature, this study also investigated the effect of varying volume substitutions (2.5%, 5%, 10%, 20%, 40%) of waste materials on strength, workability, density and specific heat as well as properties such as elastic modulus, SEM analysis, and the effect of moisture state, using the optimum/critical volume substitutions identified. The effect of varying substitutions of waste was shown to have a noticeable effect on concrete properties and the optimum/critical volume percentages were found to be 2,5% for HDPE, 5% for BA and 10% for SCBF. Waste mixes also performed best when used on their own as the mixed combinations generally were inferior in performance. The SCBF and BA mixes at 10% and 5% substitution respectively, did have potential viability for structural applications when using dry waste materials as workability was acceptable, mixes were 4 above the target strength and were relatively durable based on this study. HDPE (2,5%) however, only had the potential for non-structural applications that support passive heating and cooling measures. However, with further research into developing a waste concrete mix design specification to achieve a pre-determined compressive strength, HDPE and the natural moisture state mixes that yielded compressive strengths above 30 MPa (general structural use), still had the future potential to be viable conventional concrete alternatives. Economically, if the project was profit-driven, none of the waste mixes were viable in terms of maximising profit as they cost more than the conventional mix for both the small scale precast and larger scale cast in-situ scenarios. The waste mixes also did not offer any significant enhanced concrete properties to validate the higher costs. However, if corporate social responsibility (CSR) or government environmental incentives were factors, then by reducing virgin mineral consumption and utilizing waste materials, this promotes environmental preservation. Hence, the reduction in profits may therefore be the warranted, thus making the waste mixes economically viable in the context of CSR projects or with the potential for government environmental incentives.
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