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Relationship between mix design, concrete performance and carbon footprint

Al-Naqkeeb, Ahmad Shamil (2020) Relationship between mix design, concrete performance and carbon footprint. PhD thesis, University of Leeds.

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Abstract

Reducing the carbon footprint of concrete is an imperative. Judicious concrete mix design is one means of achieving this, including the use of supplementary cementitious materials, water reducing admixtures and reduced concrete workability. However, a truly sustainable concrete should also, in addition to having a low embodied carbon, provide long-term durability. In the present work, many factors have been adopted to reduce the embodied carbon dioxide in concrete. Eighteen concrete mixes were designed to provide low-carbon concrete while maintaining durability. To achieve these requirements, it was necessary to cast a series of concrete samples with compositions designed to vary key components known to control embodied carbon dioxide. Based on the statistical approach, 18 concrete mixtures were produced and designed in L18 orthogonal array with six design variables. The work is divided into three complementary parts as follows: Part one characterizes of the constituent concrete materials, including Portland cement and three different supplementary cementitious materials (pulverized fuel ash, plus course and fine ground granulated blast furnace slag) at different replacement levels (0%, 15%, 30% of the total binder content by mass). A superplasticizer, Sika Viscocrete 25 MP, was used at either 0.2%, 0.6% or 1.0% binder weight. Crushed aggregate of maximum size of 10 and 20mm while the crushed fine aggregate were of 5mm maximum size were used. Part two of the works highlights the performance of hardened state concrete (mechanical properties) including the time-dependent deformations and durability. These properties have been investigated by testing 100mm cubes for compressive strength, 75 x 75 x 200 mm prisms for shrinkage, and transport properties (sorptivity and permeability on 100mm cubes and 40mm thick x 50 mm Ø cylinders. Durability was assessed through measuring the depth of carbonation following accelerated carbonation of 100 mm cube samples. Finally, in the third part, microstructural investigations were undertaken using pastes that were prepared with the same composition as the concrete, but without any aggregates. The pastes were investigated using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Moreover, hydration was investigated using isothermal conduction calorimetry. Test results proved that some mix design variables have a significant positive impact on the performance; for example strength enhancement, improving carbonation resistance meanwhile, satisfying the requirement of reducing carbon dioxide emissions. Numerically speaking a concrete containing fine slag at 15 or 30% replacement, low w/b ratio (0.35) above and high dosage of superplasticizer (1.0%) was found to meet the engineering performance, carbonation resistance, and environmental sustainability. It was also found that coarse slag might be another choice of cement replacement but with slightly decreased carbonation resistance. Slag showed better performance than PFA. This can be attributed to the role of slag in refining the microstructure by: (i) leading to finer porosity, increased degree of hydration, (iii) by inference, changing the C-S-H from fibrillary to foil-like (iiii) improving carbonation resistance. The above significant conclusions are the result of the high fineness of slag which in turn reduced the pore structure, high reactivity of slag which in turn accelerated the hydration at early age and the pozzolanic reaction which improved the microstructure at long age. This study has considered a range of concrete mix designs known to yield concrete with a reduced carbon footprint. It has looked at the effects of mix design on mechanical performance, transport properties and resistance to carbonation. Using the Taguchi method, 18 concrete mixtures are produced and designed in L18 orthogonal array with six design variables (binder content, superplasticizer, supplementary cementitious materials, percentage of SCMs, aggregate Size (10- 20) mm and water-to-binder ratio) set at one of three levels have been prepared.

Item Type: Thesis (PhD)
Keywords: Low carbon concrete, durability, carbonation resistance, mix design, carbon footprint.
Academic Units: The University of Leeds > Faculty of Engineering (Leeds) > School of Civil Engineering (Leeds)
Depositing User: Mr. Ahmad Al-Naqkeeb
Date Deposited: 06 Aug 2020 07:10
Last Modified: 06 Aug 2020 07:10
URI: http://etheses.whiterose.ac.uk/id/eprint/27244

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