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Title: Effect of Partial Replacement of Cement by Fly Ash and Silica Fume on Strength Characteristics of Recycled Coarse Aggregate Concrete
Authors: Sidhu, Ajayveer Singh
Supervisor: Sharma, Shruti
Keywords: Recycled;Aggregate;Concrete;Compressive strength
Issue Date: 27-Oct-2016
Abstract: Construction activity leads to generation of wastes, which include sand, gravel, concrete, stone, bricks, wood, metal, glass, plastic, paper etc. The management of construction and demolition waste is a major concern due to the increasing quantum of demolition’s rubble, continuing shortage of dumping sites, increase in transportation and disposal cost and above all growing concern about pollution and environmental deterioration. To make the concrete industry more sustainable and environment friendly, researchers are working continuously and they came up with the idea of green concrete (recycled concrete). 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 the recycled aggregate concrete before It can be confidently be adopted by the concrete industry. The aim of the thesis is to provide a base for extensive scientific study for the possible use of recycled aggregates in structural concrete by conducting comprehensive laboratory studies to gain a better understanding of the mechanical, durability and workability properties of concrete produced with the recycled aggregates. The characteristics of the recycled aggregates produced from the laboratory and sourced from a commercially operated pilot C&D material recycling plant was first studied. A mix proportioning was then established to produce a concrete mix of M30 grade with 0.45 water cement ratio. Then a trial study was run by replacing natural coarse aggregate by recycled coarse aggregate at replacement level of 50% and 100%. The fresh and hardened properties of recycled coarse aggregate were first quantified. The influences of recycled aggregate on the slump and compressive strength were investigated. The results confirmed that the use of recycled coarse aggregates contributed to low slump values and very low degree of compressive strength. In order to overcome problems during simple replacement of natural coarse aggregate by recycled coarse aggregate, some part of cement was replaced by supplementary materials. Two supplementary materials, i.e. fly ash and silica fume were selected for the study. The replacement level of cement by fly ash was selected at 20%, 35% and 55% while that of silica fume was selected at 5%, 10% and 15% by weight of cement. The hardened concrete properties were firstly quantified. The influence of recycled aggregate along with the fly ash and silica fume on the compressive strength (3 days, 7 days and 28 days), split tensile strength and flexural strength (both at 28 days) were studied. The test results confirmed that the use of supplementary materials increased the strength of the concrete. From all the mixes, fly ash and silica fume with replacement level at 35% and 15% respectively showed optimum results even with recycle coarse aggregate upto 50% and 100% replacement level. So these optimum mixes were selected for further testing. The fresh concrete properties were quantified by conducting tests for air content and bleeding test for optimum mix design. The test results showed that by increasing the content of recycled coarse aggregate resulted in increased air content. When fly ash and silica fume is added the air content decreases. In case of bleeding test same behaviour was experienced. With increase in the content of recycled coarse aggregate, the bleeding rate was increased. But when fly ash and silica fume were added, it resulted in decreased bleeding rate. When bleeding test was started after some time of mixing, it resulted in decreased bleeding rate. Furthermore, the testing for durability properties were carried out for optimum level of concrete mix. The tests conducted were rapid chloride permeability, carbonation test and water penetration test. In all the tests, concrete showed decreased level of resistance with an increase in level of recycled coarse aggregate. When fly ash and silica fume is added, the resistance was increased marginally. One non destructive test was also performed by using ultrasonic guided waves. It was also clear from the test that with substitution of recycled aggregate along with silica fume and fly ash resulted in more fast setting time period than that of control sample. Based upon the test results a number of recommendations were made on how to optimize the use of recycled coarse aggregates for structural concrete production.
Appears in Collections:Masters Theses@CED

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