5
– Nozzle distance: typically 1–2m between nozzle
tip and surface.
– Nozzle angle: should always point 90° or perpen-
dicular to the surface. Note that incorrect nozzle
angle and distance significantly negatively influ-
ence concrete quality, like poor compaction, and
will dramatically increase the rebound.
– Spraying sequence: the area is clear and safe to
begin spraying. Start with the nozzle pointing
downward, turn on the air, and subsequently, the
accelerator. Turn on the shotcrete pump before
spraying the substrate.
– Spraying direction: to avoid trapping rebound,
spray the bottom of the rib and move higher up to
the shoulder and crown.
– Thickness: following the criteria of the ground
support plan, typically 75–150 mm thick.
– Boom handling
– Personal protective equipment (PPE)
The checklist was quantified based on qualitative rating
(i.e., poor—0, fair—2.5, fair to good— 5.0, good—7.5,
and very good—10), which later averaged to get the score
per each aspect. These values were recorded and tracked
continually for different sprayers, machines, and contrac-
tors, and they were reported to the area supervisor to see the
improvement progress and provide feedback if something
needed to be fixed.
As one quantitative parameter to be assessed for shot-
crete quality, shotcrete rebound was measured manually
during spray. Two spraying zones, namely the back and rib
area, were distinguished since it was observed that rebound
from the back appeared to be a lot more than spraying to
the rib, potentially due to gravity (Figure 5a). The rebound
percentage was measured in volumetric calculations in this
following formula:
%Vsprayed
V
rebound
rebound bulk =-(1)
The sprayed material leaving the nozzle Vsprayed is estimated
based on the shotcrete pump capacity, the average dura-
tion of each pump swing, and the total spraying time. The
rebound material was collected on the floor using a spread-
reused ventilation duct during the shotcrete application.
After spraying for the designated section (rib/back), the
shotcrete application was paused briefly to move the ven-
tilation duct somewhere safer for further measurements. A
simple calibrated bucket measurement was used to estimate
the bulk volume of rebound material Vrebound
–bulk .Aside
from the rebound data, the nozzleman name and accelera-
tor dosage are other parameters noted. Here, each aver-
aged nozzleman rating becomes the independent factor to
compare with the rebound percentage. The data taken with
the accelerator dosage below the standard application was
removed since it showed more rebound, as expected.
Nozzlemen evaluated through rebound percentage (rib
vs. rebound, calculation method)
RESULTS AND DISCUSSION
Most of the nozzlemen improved in terms of theo-
retical knowledge. Interactive dashboards using Microsoft
Excel Pivot Chart were developed to track nozzleman rating
and to filter the rating during particular months. Figure 6
shows the dashboard example to observe each nozzleman’s
progress.
For the assessment involving rebound measurements
conducted in April 2018, the nozzleman ratings were aver-
aged exclusively for that month to enhance relevancy and
Figure 5. (a) Typical spraying observation area and rebound tarp preparation and (b)
collection of rebound material and volumetric calculation
– Nozzle distance: typically 1–2m between nozzle
tip and surface.
– Nozzle angle: should always point 90° or perpen-
dicular to the surface. Note that incorrect nozzle
angle and distance significantly negatively influ-
ence concrete quality, like poor compaction, and
will dramatically increase the rebound.
– Spraying sequence: the area is clear and safe to
begin spraying. Start with the nozzle pointing
downward, turn on the air, and subsequently, the
accelerator. Turn on the shotcrete pump before
spraying the substrate.
– Spraying direction: to avoid trapping rebound,
spray the bottom of the rib and move higher up to
the shoulder and crown.
– Thickness: following the criteria of the ground
support plan, typically 75–150 mm thick.
– Boom handling
– Personal protective equipment (PPE)
The checklist was quantified based on qualitative rating
(i.e., poor—0, fair—2.5, fair to good— 5.0, good—7.5,
and very good—10), which later averaged to get the score
per each aspect. These values were recorded and tracked
continually for different sprayers, machines, and contrac-
tors, and they were reported to the area supervisor to see the
improvement progress and provide feedback if something
needed to be fixed.
As one quantitative parameter to be assessed for shot-
crete quality, shotcrete rebound was measured manually
during spray. Two spraying zones, namely the back and rib
area, were distinguished since it was observed that rebound
from the back appeared to be a lot more than spraying to
the rib, potentially due to gravity (Figure 5a). The rebound
percentage was measured in volumetric calculations in this
following formula:
%Vsprayed
V
rebound
rebound bulk =-(1)
The sprayed material leaving the nozzle Vsprayed is estimated
based on the shotcrete pump capacity, the average dura-
tion of each pump swing, and the total spraying time. The
rebound material was collected on the floor using a spread-
reused ventilation duct during the shotcrete application.
After spraying for the designated section (rib/back), the
shotcrete application was paused briefly to move the ven-
tilation duct somewhere safer for further measurements. A
simple calibrated bucket measurement was used to estimate
the bulk volume of rebound material Vrebound
–bulk .Aside
from the rebound data, the nozzleman name and accelera-
tor dosage are other parameters noted. Here, each aver-
aged nozzleman rating becomes the independent factor to
compare with the rebound percentage. The data taken with
the accelerator dosage below the standard application was
removed since it showed more rebound, as expected.
Nozzlemen evaluated through rebound percentage (rib
vs. rebound, calculation method)
RESULTS AND DISCUSSION
Most of the nozzlemen improved in terms of theo-
retical knowledge. Interactive dashboards using Microsoft
Excel Pivot Chart were developed to track nozzleman rating
and to filter the rating during particular months. Figure 6
shows the dashboard example to observe each nozzleman’s
progress.
For the assessment involving rebound measurements
conducted in April 2018, the nozzleman ratings were aver-
aged exclusively for that month to enhance relevancy and
Figure 5. (a) Typical spraying observation area and rebound tarp preparation and (b)
collection of rebound material and volumetric calculation