Civil Engineering Reference
In-Depth Information
Preliminary tests are normally carried out to establish the suitability of the
constitutent materials and the final product. The term 'suitability' covers all
aspects of the process as well as the final product's physical characteristics.
Cement
Most types of cement are used, as in any concrete work, depending on the
requirement of the finished product. The cement to be used should be fresh
and free from lumps.
Aggregates
Most sprayed concrete uses sand but aggregates up to 10 mm in size may be
used where the section thickness allows, and provided the machinery will
accommodate them.
The grading of the aggregate is important and should be within the limits
shown in
Table 13.1.
The finer region is more suitable for dry mixes, although
a high percentage of particles < 0.25 mm can lead to dust problems. Sand
for finishing coats (sometimes referred to as flash coats as they are flashed
over the surface as a sort of render) may be finer than the grading limits
shown. Finer sands will generally produce a concrete with greater drying
shrinkage while coarser sands will give more rebound. The aggregate size
will also be restricted by the thickness of the repair section or coating.
In the dry process, the aggregate and sand need to be slightly damp to
help suppress dust and to guard against segregation of the cement and sand
in the hoses. The moisture content needs to be constant at around 5-6%.
Too much moisture promotes clogging or blocking of the machine.
Sand
Gravel
Fine
Medium
Coarse
Fine
Medium
Coarse
100
90
80
70
60
50
40
30
Wet-mix process
grading envelope
Dry-mix process
grading envelope
20
10
0
100
0.1
1
10
Particle size (mm)
