Sprayed concrete for repairing concrete structures - Concrete Repair - page 223

Layers are built up by making several passes of the nozzle over the area,
generally in an overlapping oval configuration. The nozzle should be at right
angles to the substrate and at the correct distance from it. This would be
between 600 mm and 1 m for the dry process and between 250 and 500 mm
for the wet process. The nozzleman will recognise the correct distance by
the amount of rebound and the effectiveness of compaction ( Figure 13.6).
Thickness of the section to be sprayed can be indicated by stretched
'piano' wires located at the required finished surface. Application should
start at the bottom of a vertical or sloping surface and from the shoulder
to the crown for overhead working. As repairs usually contain corners and
sharp angles, care should be taken to avoid trapping rebound material at
these critical locations.
The nozzleman's assistant should remove any rebound that might become
encased in the works and this rebound should be discarded, not fed back into
the system, as its grading has changed. He may also be required to trim back
the sprayed material to the line indicated by the stretched wires, although
it is better practice to spray to a thickness less than that indicated and to
finish off with a flash coat, which is more like a render than a concrete.
Trowelling or screeding is best avoided as this action creates tension cracks
in the sprayed concrete's surface.
Once a section is complete, it must receive curing, which can be a
sprayed-on membrane (if no further coating is to be applied) or any of the
other conventional curing techniques. Unfortunately, where large areas are
Figure 13.6 Concrete spraying.