|Title:||Reusing recycled aggregates in structural concrete|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
Concrete -- Recycling.
Aggregates (Building materials) -- Recycling.
|Department:||Department of Civil and Structural Engineering|
|Pages:||xxxii, 278 p. : ill. ; 30 cm.|
|Abstract:||The construction activities in Hong Kong generate about 19 million tones of construction and demolition (C&D) materials each year. The disposal of waste has become a severe social and environmental problem in the territory. The possibility of recycling of waste from the construction industry is thus of increasing importance. In addition to the environmental benefits in reducing the demand on land for disposing the waste, the recycling C&D wastes can also help to conserve natural materials and to reduce the cast of waste treatment prior to disposal. Research studies on the reuse of recycled aggregates in concrete have continuously been conducted since the end of the 2nd World War. The findings indicated that recycled aggregate derived from demolished concrete or masonry can be extensively used in civil infrastructure projects including the production of Portland cement concrete, sub-base materials in road construction projects and the production of masonry units. But in practice, recycled aggregate is not commonly used in the production of concrete. One of the reasons is that Portland cement concrete is produced to form structural elements which have to meet strict strength and durability requirements. Extensive research is required to verify the properties of recycled aggregate concrete before it can be confidently adopted by the concrete industry. The aim of the thesis is to provide a scientific bases for the possible use of recycled aggregates in structure concrete by conducting a comprehensive laboratory programme to gain a better understanding of the mechanical, microstructure and durability properties of concrete produced with recycled aggregates. The characteristics of the recycled aggregates produced both from the laboratory and sourced from a commercially operated pilot C&D material recycling plant was first studied. A mix proportioning procedure was then established to produce six series of concrete mixtures using different percentages of recycled coarse aggregates with and without the use of fly ash. The water-to-cement (binder) ratios of 0.55, 0.50, 0.45 and 0.40 were used. The fresh properties of recycled aggregate concrete (RAC) were first quantified. The influences of recycled aggregate on the slump, air content and bleeding of the fresh concretes were also investigated. The results confirmed that use of recycled aggregates at an air-dried state in concrete resulted in higher initial slumps which took longer to decrease to zero when compared with the concrete with natural aggregates. The use of recycled aggregates also resulted in a higher rate of bleeding and bleeding capacity. Delaying the starting of bleeding tests reduced the bleeding rate and bleeding capacity of recycled and conventional aggregate concrete. The replacement of cement by 25% fly ash increased the slump of RAC mixtures and reduced the bleeding rate and bleeding capacity. The effect of fly ash on the hardened properties of RAC was then studied and compared with those RAC prepared with no fly ash addition. The test results showed that the use of fly ash as a partial replacement of cement decreased the compressive strength, tensile splitting strength and static modulus of elasticity. However, the use of fly ash as an additional mineral admixture in RAC increased the compressive strength, tensile splitting strength and static modulus of elasticity. Also, the use of fly ash both as a partial replacement of cement and as an additional mineral admixture in RAC was able to reduce the drying shrinkage and creep and increased the resistance to chloride-ion penetration of the RAC. By adjusting the W/C ratio it was possible to match the designed compressive strength of the RAC containing 100% recycled aggregate with that of the corresponding natural aggregate concrete. Furthermore, the effects of steam curing on the hardened properties of RAC were investigated. The results showed that steam curing at 65oC increased the early ages (1, 4, and 7-day) strength of all concrete mixtures. However the 28 and 90-day strengths and Young's modulus of the steam cured concrete were lower than those of the water cured concrete. Steam curing reduced the drying shrinkage and creep and increased the resistance to chloride-ion penetration of RAC and fly ash RAC. In terms of micro-structural properties, the interfacial transition zones of the original aggregates and the old mortar/cement paste of the recycled aggregates and the interfacial transition zones between natural and recycled aggregates and the new cement pastes were analyzed by SEM and EDX-mapping. The effect of recycled aggregate on the pore size distributions of the RAC was also studied and found to be dependent on the percentage of coarse aggregate substituted and whether or not fly ash was used. The experimental results indicated that steam cured conventional aggregate concrete was more porous with distinct cracks formed when compared with the steam cured RAC. The total porosity and the average pore size of the RAC increased with an increase in the recycled aggregate content. The replacement of cement by fly ash reduced the total porosity and the average pore size of both the conventional aggregate concrete and RAC. Moreover, a detailed set of results on the fracture properties for RAC were obtained. It was found that recycled aggregates increased the matrix-aggregate interfacial bond strength and fracture energy. Fly ash replacement at a level of 25% also increased the bond strength and fracture energy of RAC. The experimental results showed a substantial improvement in the post-peak ductility for the RAC when fly ash was used. Based on the detailed experimental results, a number of recommendations were made on how to optimize the use of recycled aggregates for structural concrete production. Also, suggestions were made on improving the production process of concrete using recycled aggregate.|
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