Two significant improvements in the development of shotcrete were the introduction of silica fume in the dry mix method – a biproduct of silicone metals – and steel or polypropylene fiber reinforcement.
Silica fume, also known as micro silica, reacts with the calcium hydroxide produced during cement hydration. This additive enables shotcrete to achieve twice, or three times the compressive strength of plain shotcrete mixes resulting in an extremely strong, impermeable and durable shotcrete.
Similarly, the addition of steel fiber into shotcrete, which also acts as a reinforcing agent, dramatically increases the tensile strength of the shotcrete and enables miners and tunnelers to reduce the effort of installing wire mesh, thus saving a considerable amount of time and money.
The thickness of the shotcrete layers varies, depending on the mix type and the project requirements, but this is normally up to 50 mm for wet mix and 30 mm for dry mix in one path. In many cases though, thicker application is required which means that multiple layers have to be applied.
A great many parameters are taken into account when matching shotcrete to different applications. These include sand/aggregate grading, cement type and amount, hydration control of admixtures, type of plastiscizers/superplasticizers, workability, accelerator type, temperature, pulsation, nozzle systems, to name a few.
Safety is obviously a major consideration in tunneling but so is speed and cost. Each operation in the tunnel has to be carried out as quickly and as efficiently as possible. The faster the contractor can apply the shotcrete the better it is for the overall economy of the project, and the more likely that the contractor will meet the contracted completion date. Similarly, the more shotcreting time can be reduced, the faster the advance, thereby reducing the costs for tunneling.