Civil Engineering Reference
In-Depth Information
The nozzleman controls the water addition and whilst this at first seems
somewhat unprofessional, if too little water is added the amount of rebound
increases, and if too much water is added, the concrete will not have
sufficient rigidity to stay in place. The angle of spraying is also critical to
avoid excessive rebound (
Figure 13.3).
When using a basic sand/cement mixture, rebound is higher than for the
wet process; typically 10-15% when spraying downwards and as much as
50% when spraying overhead. Because this rebound is mainly the larger
aggregate particles, the mix as-placed will be richer than the as-mixed
materials, which would have an aggregate/cement ratio in the range 3.5 - 4.5
by weight. The resulting 28-day strength is typically 40-50 MPa. Pre-bagged
materials, which contain cohesion-promoting admixtures, do not produce
as much rebound.
Figure 13.4
shows the spraying technique to achieve best
quality, so it can be seen that a combination of spraying at approximately
right angles and with a circular motion is preferred. However, to avoid
shadow zones, it is also necessary to spray behind reinforcement, at an angle,
when some rebound is unavoidable.
Concrete produced by the dry process can be the more variable. For example,
because it is built up in layers and rebound is high, it can have variable density,
especially at the interface between two layers. Rebound can also become
trapped in corners, reducing the density. However, shrinkage is generally lower
than for other similar but wetter concretes because of its low w/c ratio.
Equipment
The twin-chamber machines referred to earlier have now virtually ceased
to be used and have been replaced by the rotor type, shown in
Figure 13.5,
where the dry materials are fed into an open hopper, which feeds a rotating
barrel. As the barrel rotates, the pockets are brought under a compressed
