Theoretical background for pressure moulding of concrete tunnel lining
Concrete mix is a coherent mass the physical nature of which is determined by properties of its solid components, water and air. It is the air component which ensures further compaction of the mix under external loading, since solid ingredients and water are practically incompressible. The forces acting on the concrete mix, obtained by blending solid components, water and air, are those due to gravity (weight), inertia and friction. The choice of concrete mix in terms of its fluidity depends on the type of the structure to be concreted, and type of mix supply and placement. Void ratio of concrete mix can be determined with an apparatus which would measure in each test the amount of water squeezed out in the graduated cylinder. The compressibility of concrete mix is a characteristic property, dependent on porosity changes resulting from rearrangement of particles as water flows out under external action.