Civil Engineering Reference
In-Depth Information
Mix design.
Shotcrete mixes comprise cement
and aggregate (10-2.5 mm aggregate and sand),
together with admixtures (superplasticizers) to
provide high early strengths. The properties of
shotcrete are enhanced by the use of micro-silica
that is added to the mix as a partial replacement
for cement (USBM, 1984). Silica fume is an ultra
fine powder with a particle size approximately
equal to that of smoke. When added to shotcrete,
silica fume reduces rebound, allows thicknesses of
up to 500 mm to be applied in a single pass, and
covers surfaces on which there is running water.
There is also an increase in the long-term strength
in most cases.
Shotcrete can be applied as either a wet-mix
or a dry-mix. For wet-mix shotcrete the compon-
ents, including water, are mixed at a ready-mix
concrete plant and the shotcrete is delivered to
the site by ready-mix truck. This approach is
suitable for sites with good road access and the
need for large quantities. For dry-mix shotcrete
the dry components are mixed at the plant and
then placed in 1 m
3
bags that have a valve in the
bottom (Figure 12.14). At the site, the bags are
discharged into the hopper on the pump and
a pre-moisturizer adds 4% water to the mix. The
mix is then pumped to the face where additional
water is added through a ring valve at the nozzle.
The advantages of the dry-mix process are its use
in locations with difficult access, and where small
quantities are being applied at a time. It is also
useful to be able to adjust the quantity of water
in areas where there is varying amounts of seepage
on the face.
Typical mixes for dry-mix and wet-mix silica
fume, steel fiber reinforced shotcrete are shown
in Table 12.9 (Morgan
et al
., 1989).
Shotcrete strength.
The strength of shotcrete
is defined by three parameters that correspond
to the types of loading conditions to which
shotcrete may be subjected when applied to a
slope. Typical values for these parameters are as
follows:
(a)
Compressive strength of 20 MPa at 3 days
and 30 MPa at 7 days;
(b)
First crack flexural strength of 4.5 MPa at 7
days; and
(c)
Toughness indices of
I
5
=
4 and
I
10
=
6.
Figure 12.14
Dry-mix shotcrete process
using bagged mix feeding a pump and
pre-moisturizer.
