The main design principle for rock support is to help
the rock to carry itself. In many cases the adhesion between
the shotcrete and the rock is one of the most important
properties. Fig. 1 shows possible mechanisms
for shotcrete in interaction with the rock. Also the effect
of sealing the joints by shotcrete helps to maintain the
integrity of the rock mass (Morgan and Mowat, 1984),
see Fig. 1(c). Failure normally develops as an adhesion
failure and at a displacement of only a few millimetres
(Stille, 1992). According to Swan et al. (1996) shotcrete
in contrast to mesh can prevent loosening because of
higher stiffness and furthermore it isolates joints from
air and water.
The objective of this study was to: (i) identify typical
failures of shotcrete in a mining environment, (ii)
measure the growth of the adhesion strength of shotcrete
with respect to age, (iii) investigate the influence
of surface treatment (scaling and cleaning) on the adhesion
strength of shotcrete and (iv) investigate the
occurrence of shrinkage cracks in the shotcrete and
failures in the interface between the shotcrete and
the rock.
To achieve the objectives the following activities were
performed:
An extensive failure mapping of shotcrete in the mine.
Field tests in the mine to investigate the compressive
and adhesion strength of shotcrete as a function of
age.
The adhesion strength of shotcrete on water jet-scaled
rock surfaces was compared with shotcrete on rock
surfaces with normal treatment.
Measurements of free shrinkage of shotcrete and failure
mapping of shotcrete subjected to shrinkage.
Because the adhesion strength of shotcrete is a very
important quantity as mentioned earlier some relevant
findings regarding the adhesion strength are presented
in Section 2. Shrinkage is discussed in Section 3. The
failure mapping of shotcrete and the adhesion strength
tests and results are presented in Sections 4–6, shrinkage
tests and results in Section 7 and finally discussion and
suggestions for further research in Section 8.