Precast segments with sulphoaluminate cement and GFRP reinforcement

During tunnel excavation by means of a tunnel boring machine (TBM), the lining consists of precast structural elements, placed by the TBM during the excavation process and used as support elements during the advancing phase. The use of concrete based on sulphoaluminate binders allows the production of the elements to be speeded up, a reduction in the number of segments stacked waiting for the required strengths to be achieved and provides a more eco-sustainable process avoiding the use of steam curing. In order to verify the feasibility of the proposed solution, two full-scale tests (bending test and TBM jacks thrust test) on metro tunnel precast segments were carried out. The internal reinforcement consisted of a next-gen Glass Fibre Reinforced Polymer (GFRP) cage. The use of GFRP reinforcement, to replace traditional steel, in tunnel segments provides several advantages mainly related to durability aspects or when the use of a provisional lining is foreseen

Two-step nucleation mechanism in solid–solid phase transitions

The microscopic kinetics of ubiquitous solid-solid phase transitions remain poorly understood. Here, by using single-particle-resolution video microscopy of colloidal films of diameter-tunable microspheres, we show that transitions between square and triangular lattices occur via a two-step diffusive nucleation pathway involving liquid nuclei. The nucleation pathway is favoured over the direct one-step nucleation because the energy of the solid/liquid interface is lower than that between solid phases. We also observed that nucleation precursors are particle-swapping loops rather than newly generated structural defects, and that coherent and incoherent facets of the evolving nuclei exhibit different energies and growth rates that can markedly alter the nucleation kinetics. Our findings suggest that an intermediate liquid should exist in the nucleation processes of solid-solid transitions of most metals and alloys, and provide guidance for better control of the kinetics of the transition and for future refinements of solid-solid transition theory