LPT2: more on reinforced concrete

The tunnels at London Power Tunnels 2 use a spray of reinforced concrete to stabilise the lining whilst thick concrete rings are inserted.

The material chosen consists of cement, sand, and steel fibre reinforcements. . I imagined these to be thin and easily bent, but on inspection of the sprayed wall in the tunnel I saw they were firm: of about 1mm diameter, hard and spiky. Although you might expect the spray to form a smooth-ish façade on the inside of the tunnel, in fact it is very rough-looking, with spikes of steel fibre.