NATM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 625

The required early strength for the shotcrete can be achieved by the use of additives
such as so-called spray bonding agents (SBM) or spray cements that obviate the need
for setting accelerators. Also alkali-free accelerating admixtures in powder or fluid form
may be added (Wittke et al. 2002). In special cases, such as for high water inflow, spray
bonding agents and alkali-free accelerating admixtures may also be applied in combi-
nation (ÖBV 1989).
Early strength
With respect to the requirements of strength development, shotcrete up to an age of
24 hours (green shotcrete) according to ÖBV (1989) is distinguished into three early
strength classes J 1 , J 2 and J 3 (Fig. 20.8).
Figure 20.8 Early strength classes of green shotcrete (ÖBV 1989)
Class J 1 shotcrete is suitable for the application of thin layers on dry surfaces with-
out specific static requirements. It has the advantage of little dust development and
rebound. Class J 2 shotcrete is generally used if static requirements exist with respect to
the green shotcrete, such as when shotcrete is provided for the external lining of a traffic
tunnel. Class J 3 shotcrete is required if rapidly developing loads from such as water
pressure and rock pressure are to be expected.