Page 73 - Numerical Analysis and Modelling in Geomechanics

54 A.A.JAVADI

Figure 2.18 Variation of air permeability of shotcrete with time.
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of cement paste and concrete, as one would expect. Illston argues that in the
curing process of cement, the hydration products infill the skeleton structure,
blocking the flow channels and so reduce the permeability. During the early age,
the hydration proceeds rapidly and so the reduction in permeability is higher.
The rate of reduction in permeability reduces by several orders of magnitude in
the first few weeks for cement paste (see Figure 2.19).
Mehta and Monterio 21 state that with increasing age of cement as hydration
proceeds, the pore diameter is reduced and the rate of reduction in pore diameter
is much higher at the early age. Illston 20 explains that similar factors, as with
cement paste, control the permeability and strength of concrete and therefore,
similar changes in permeability can be expected for concrete and shotcrete.
22
Nyame states that the permeability of concrete is reduced with time.
This shows that the predicted behaviour of permeability of shotcrete is
reasonable and comparable to similar materials, like cement paste and concrete,
which follow similar procedures during the curing process.
This can be further justified by the evidence from the Feldmoching tunnel
project. When the tunnel moved into the clay layer, although the exposed area of
shotcrete lining increased continuously with increasing tunnel length, a
significant reduction occurred in the measured total air loss and air loss from the
tunnel walls. This can only be explained by the continuous and rapid reduction in
the air permeability of shotcrete with time.