Concrete with fast setting and hardening process for use in SFRC precast tunnel segments

Segmental lining gained popularity over the years due to the possibility of operating in difficult excavation conditions, high production rates during digging process and high standards of quality. In segmental lining, the tunnel is made by a sequence of rings placed side-by-side, each one composed by precast tunnel segments (from 4 to 10, depending on the tunnel geometry). This tunnel type can be used in different underground applications (Subways, Highways, Railways, Water Supply etc.). As far as tunnel segments production is concerned, it generally requires long curing time (>24h for demoulding a single segment or 8h by adding a specific treatment) and a wide area to allow primary and secondary storage of segments. However, the availability of large storage areas near the building site is often a constraint, which causes high transport costs for the segments and, in the worst cases, hinders the tunnel construction. Speed-up the production of segments is very often essential and to do this a steam curing process have to be adopted. However, the latter is extremely expensive and requires an additional dedicated production line. These criticalities can be overcome by the development of a concrete with a fast setting and hardening process; in fact, it could allow a quick segment demoulding (about 3h) without the use of additional expensive steam curing process and reducing segments storage area (due to the enhanced production rate). The aim of this study is twofold: the first is to develop a concrete with a rapid setting and hardening process adequate for being used in precast tunnel segment industry. The second is to include steel fibres as reinforcement to obtain a Fibre Reinforced Concrete (FRC) having an adequate post-cracking performance to totally substitute the traditional reinforcement (steel bars). These goals were successfully reached by the optimization of a compound based on Portland cement and Calcium Sulfoaluminate Cement (CSA) which presents favourable early strengths development and whose compressive and fracture properties were compared to that of concrete with Ordinary Portland Cement (OPC). The results obtained are very promising making feasible to reduce segments production times as well as corresponding storage areas. Moreover, it is expected to obtain segments less prone to impact damage, lower CO2 emissions and manufacturing costs.