Development of a Novel Self-Healing Composite for Sustainable and Resilient Concrete Infrastructure

Concrete is the most widely used construction material in the world. The construction industry annually uses 4.3 billion tons of ordinary Portland cement (OPC) as binder for concrete, accounting for around 7% of global CO2 emissions. To reduce the environmental impact of concrete industry in the UK, industrial by-products, such as pulverised fuel ash (PFA) and ground granulated blast-furnace slag (GGBS), are usually used for partial replacement of OPC. Although partial replacement of OPC can reach up to 50%, the total replacement of OPC in concrete with these wastes is not feasible without the addition of alkaline activating agents.

Geopolymers, also called "alkali-activated materials", that are cement-free eco-friendly materials synthesized at ambient or elevated temperature by alkali activation of aluminosilicate source materials such as low-calcium PFA and GGBS, have been drawing a lot of attention as a promising alternative to OPC. GPC has many advantages over OPC concrete (OPCC), such as light weight, good fire resistance, low alkali-aggregate expansion, and good resistance to corrosion, acid attack and freeze-thaw cycles. Using geopolymer as the binder in concrete can help reduce embodied energy and carbon footprint by up to 80%. However, GPC is inherently brittle similar to OPCC and susceptible to cracking that would facilitate corrosion of reinforcing steel and impair durability of reinforced concrete (RC) structures, and thus hinder its widespread application. In addition, the resilience of concrete infrastructure that associates with the usability of RC structures is a major concern. It is essential for GPC to possess the capability to recover permanent deformation upon yielding (i.e., re-centring) or the ability to reduce residual crack sizes (i.e., crack closure) when subjected to cyclic loads in order to maintain the functionality and serviceability of a structure over its service life. As such, it is vital to develop strain hardening fibre reinforced GPC, also known as engineered geopolymer composite (EGC) to suppress the brittleness of GPC and improve its durability through multiple crack propagation with controlled crack widths.

In this project, for the first time, a novel self-healing EGC that integrates the greenness potential of GPC and the energy absorption capacity of shape memory alloy (SMA) fibres without permanent deformation will be developed. The project involves the development of a novel mix design methodology that integrates micromechanical modelling, design of experiment and life cycle analysis. A range of advanced experimental techniques (e.g., in-situ X-ray computed tomography imaging, image volume correlation, and scanning electron microscope) and modelling approaches (e.g., multiscale lattice Boltzmann-finite element method, and multiscale fracture model) will be used to characterise microstructure and simulate engineering properties of EGC respectively, which will provide insight into the overall performance of EGC and its self-healing efficiency.

This research will make it possible to develop a novel EGC with eminent mechanical properties and desired crack-healing capacity. It would expedite the use of GPC and SMA fibres in civil infrastructure applications, particularly for concrete structures subjected to dynamic loads and aggressive environments, which will help greatly enhance resilience, sustainability and durability of concrete infrastructure. The outcomes of this project are expected to result in direct benefits to society by extending the lifetime and by reducing the environmental impact, and repair and maintenance costs of RC structures.

This project has great potential to make a major impact on all stakeholders involved in sustainable materials for infrastructures (e.g. material suppliers, consultants and contractors), as well as policy makers at government and local level that have the mission to reduce energy use and carbon emissions in the built environment. The specific impacts can be summarised as follows:

Environmental impact: A comprehensive understanding of microstructure-property relationships in cement-free geopolymer concrete provided by this work will promote the replacement of Portland cement with industrial by-products, e.g. fly ash and slag, which has obvious environmental benefits and will positively contribute to tackling climate change and achieving the strict greenhouse gas emission reduction targets for the UK in the built environment. The reuse of these by-products will reduce the amount of waste going to landfill and the landfill taxes, which will benefit the environment and UK population.

Economic impact: An unprecedented insight into self-healing engineered geopolymer composites will contribute to the design and construction of durable and resilient concrete structures or elements, which will have a major benefit to the economy in terms of significant reduction in maintenance costs of infrastructures. The reuse of industrial by-products will be of great benefit to cement manufacturers, coal power plants and iron and steel plants that coordinate with each other to produce geopolymers for infrastructures. The use of sustainable concrete will raise the public image of the construction companies and allows contractors to win the competitive bids on construction projects because of higher BREEAM and CEEQUAL rating. Moreover, results of this project will facilitate innovation for the industry and thus provide an international technological lead and enhance the competitiveness of the UK construction industry. This will lead to economic success in markets and create new jobs.

Societal impact: In the longer term, this project will have a major impact on society in terms of the greater sustainability and higher resilience of concrete infrastructures which will reduce maintenance time and travel delays, and improve the quality of life.

Impacts on government and policy makers: Results of the project will facilitate new recommendations on design of engineered geopolymer concrete structures and innovation for the industry with a greater sustainability. This will have a major impact on policy makers, construction industry, material suppliers and consultants by allowing standards of practice and issuing guidance for material specifications and optimisation to consider both sustainability and resilience.

People impact: Research associate, PhD, MSc and UG students engaged in the project will acquire valuable experience of the advanced experimental and modelling techniques in addition to experience in analysing experimental data and writing technical reports or scientific publications. This will help to develop the UK's expertise and the output will be highly-skilled individuals suitable for employment in industry or academia. The public including school pupils will also engage with and benefit from public outreach activities through the project, which will contribute to the EPSRC goals of inspiring future engineers.

The research has a strong multidisciplinary nature covering mainly three different aspects from academic and industrial perspectives including understanding of behaviour of sustainable fibre reinforced concrete and composites, microstructural characterisation using X-ray imaging and transport phenomena in porous media. Thus, this project falls into four broad prioritized EPSRC's strategic themes of Structural Engineering, Materials Engineering - Composites, Image and Vision Computing, and Fluid Dynamics and Aerodynamics (all maintain), and is aligned with the EPSRC's theme of Energy (materials engineering).