Abstract
Concrete as a building material has revolutionised the construction industry in modern civilization. It’s become one of the most widely used construction material with the highest global acceptance and provides strength, durability and versatility to infrastructure projects. Ordinary Portland Cement (OPC) Concrete as a construction material has become a complex issue globally due to the existential climate change caused by rising carbon emission levels hence, mitigating concrete's carbon footprint is essential to human existence.A holistic way to approach this is to design a more effective concrete that includes reducing cement content in mix proportion by incorporating water-reducing admixtures or using supplementary cementitious materials. Nonetheless, an absolute sustainable concrete must exhibit minimal embodied carbon and maintain long-term durability. In this present study, A Factorial Experimental Design was used to assess the simultaneous combined effects of two or more variables and their interactions. Cement reduction is the primary approach adopted to mitigate the embodied carbon footprint of concrete. Forty concrete mixes were developed to achieve low-carbon concrete while preserving required mechanical properties and durability. To attain this goal, producing concrete mixtures with different cement compositions designed specifically to alter key factors that influence embodied carbon was essential. The study was divided into three primary supplementary sections of the work programme as follows.
The first part outlines the mix composition of concrete materials, Ordinary Portland cement (OPC), at varying composition levels of 100%, 90%, 85%, 80% and 75% OPC i.e. 100% OPC is the quantity of cement obtained from standard mix design process. A polycarboxylate superplasticizer, Fosroc Auracast 200, was used at either 0.5%, 0.75%, 1.0% or 1.25% of cement mass. An uncrushed aggregate of a maximum size of 10 and 20mm while the uncrushed fine aggregate of 2mm maximum size was used. Four grades of concrete C20/25, C28/35, C35/45 & C45/55 at 28 days based on cube strengths were selected for this study. The concrete samples were produced with a mix design complying with British standard specifications (BS EN 1992-1-1: 2004) and the British DoE method.
The second segment of the work schedule emphasises the performance of hardened state concrete, including its mechanical properties and durability assessment. The performance parameters have been investigated by testing 100 x 100 mm cubes for compressive strength, 150 x 150 x 150 mm cubes for transport properties (water permeability), and cylinders with a diameter of 150 mm and a height of 300 mm for modulus of elasticity. Durability was assessed by measuring the penetration depth after 150mm cubes were placed under accelerated water pressure of 500 KPa for 72 hours.
In the final section, an embodied carbon analysis was conducted on samples of concrete composed of 100% OPC and 75% OPC. The concrete mixture was analysed using the embodied carbon factors from the Inventory of Carbon and Energy (ICE) database. A set of concrete beams was designed to resist moments of 40, 60, 80, and 100 KN.m with 0.75 OPC concrete, after which a carbon savings assessment and life cycle cost analysis were conducted. Test results demonstrated that some mix design factors significantly improve performance, such as increasing strength, enhancing durability, and meeting the objectives to reduce carbon dioxide emissions.
This research has examined various concrete mix designs that are known to produce concrete with a reduced carbon footprint. It has examined the impact of mix design on mechanical performance, transport characteristics, and resistance to deformations. Forty concrete mixtures have been developed adopting binder reduction, with cement content decreased by 10%, 15%, 20%, and 25%, complying with the specifications of BS 8500-1:2006 for designated strength grades (C20/25, C28/35, C35/45, and C45/55) and durability criteria. A concrete mix of 75% Ordinary Portland Cement (OPC) with a low water-to-cement (w/c) ratio of 0.35 and the maximum dose of polycarboxylate superplasticizer (PCE) at 1.25% was determined to satisfy engineering performance and durability criteria while exhibiting the lowest embodied carbon for environmental sustainability. The study results show polycarboxylate superplasticizers added to the concrete not only improve the mechanical performance and durability but also enhance workability, as not all the water present during mixing is involved in the hydration process. It also reduced the expansion of the concrete capillaries, contributing to reducing carbon production and subsequently lowering the embodied carbon, as per Nukah et al., 2022.
The findings from this study emphasize the importance of carefully considering cement content and incorporating additives like PCE in concrete mix designs to achieve desired performance outcomes while minimizing environmental impact. The substantial carbon savings of 21.69% to 22.41% across the four concrete strength grades underscore the environmental advantages of adopting reduced cement mixtures. This consistent reduction in carbon emissions showcases the broad applicability of such sustainable practices in mitigating the environmental footprint of concrete production
| Date of Award | 25 Jul 2025 |
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| Original language | English |
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| Supervisor | Stephanie Barnett (Supervisor), Laurie Clough (Supervisor) & Keiron Roberts (Supervisor) |