Sprayed concrete for primary tunnel lining

The process of adding accelerators to sprayed concrete of the primary lining applied by Metrostav during the course of the Vitkov tunnels excavation has been monitored in a systematic manner. In addition to the customary check testing carried out to verify whether the set/hardness build-up curves prescribed are maintained, also other relevant quantities were monitored and recorded, e.g. the concrete mixture temperature, ambient temperature, consistency of the mixture and dosage of additives. Also the concrete application by the nozzle operator was monitored, with a view to the correctness of the procedure. The results are currently being assessed and should be used not only as a basis for the development of modified sprayed concrete formulas to be applied in the conditions of production in a certified concrete mixing plant but also for the determination of the minimum theoretical and practical requirements for qualification testing of nozzle operators.