Performance optimisation of alkali-activated slag ultra-low carbon concrete (AAS-ULCC) for shield tunnel segments by steel fibres

Using alkali-activated slag ultra-low carbon concrete (AAS-ULCC) for manufacturing segments offers a novel approach to reducing carbon emissions in shield tunnels. However, the inherent high shrinkage and brittleness of AAS-ULCC could potentially impact the safety and durability of tunnel structures. This study focusses on optimising AAS-ULCC to enhance its suitability for segment structures in shield tunnels. Steel fibres were incorporated to prepare steel fibre-reinforced alkali-activated slag ultra-low carbon concrete (SF-AAS-ULCC), and the impact of varying the length and volume fraction (V_f) of steel fibres on the long-term mechanical properties, shrinkage, and durability of AAS-ULCC was systematically investigated. The findings indicate that adding 1.5% V_f of steel fibres significantly reduced the slump of AAS-ULCC. As the V_f of steel fibres increased, there was a corresponding increase in compressive strength, axial compressive strength, splitting tensile strength, flexural strength, and resistance to sulfate attack in SF-AAS-ULCC. Additionally, the failure mode of SF-AAS-ULCC shifted from brittle to multiple-crack ductile failure. Both autogenous and drying shrinkage of SF-AAS-ULCC were reduced. Analysis using X-ray computed tomography (X-CT) and scanning electron microscopy (SEM) revealed that the steel fibres are tightly bonded with the matrix, and the inclusion of steel fibres decreased the porosity of the matrix and effectively inhibited the development of micro-cracks. The research outcomes offer reliable experimental data for promoting the application of AAS-ULCC in segment production, thereby contributing to the reduction of carbon emissions in the tunnel construction sector.