Shotcrete placement is by definition driven by the particles’ high velocities. The kinetic energy provided by the velocity is how we obtain the desired consolidation of the in-place material upon impact to achieve good performances. Thus, it is important to look at the velocities found in the shotcrete spray using a rigorous approach to compare different nozzles.
In the past, some researchers have tried to explore
the rebound phase of shotcrete placement. The most
advanced work, made by Armelin (1997), led to a model
of a single particle impact on an elasto-plastic substrate.
His work especially outlines the importance of the velocity on the particle impact energy, and more widely, on
overall rebound.
To have a better understanding of what is going
on during the spraying, the placement and rebound
phases, this theory had to be extended from a single
particle to the entire spray stream. Past research at
the Université Laval Shotcrete Laboratory discovered
specific patterns in the shotcrete spray for each process
and equipment employed. Nicolas Ginouse was the
first to develop a method to properly measure particle
velocities from the nozzle tip to the receiving surface. By
filming the spray with a high-speed camera and tracking
the particles frame by frame, he was able to evaluate the
particles’ velocities in the entire spray stream. Noteworthy, he found that particles kept accelerating after exiting
the nozzle as the maximal velocities measured are greater at 1.0 m (3.2 ft) than at 0.5 m (1.6 ft) from the nozzle
tip (Fig. 1).
Also, velocities are not uniformly distributed around
the central axis of the spray stream. The wet-mix process produces more uniform velocities than the dry-mix
process. This means that a higher proportion of particles
are travelling at a faster speed in wet-mix (Ginouse &
Jolin, 2014).
Finally, one can observe that in dry-mix the exact
velocity pattern changes with the type of nozzle tip.
The speed reduction at the edges is more important
with the double-bubble nozzle-type (in green) than the
spirolet nozzle (in red).
These discoveries have provided a major step forward in our understanding of the shotcrete placement
process. According to Armelin (1997), rebound is linked
with the ratio between the kinetic energy of a particle
and the debonding energy. Given that kinetic energy depends on the square of the speed, it is logical to believe
that velocity spray patterns play a key role in rebound.
Thus, the hypothesis that the lower value of rebound
produced with wet-mix compared to dry-mix is partly
due to their very different velocity profile within the spray
stream. Thus, efficiency of the nozzle can be evaluated
through analysis of the velocity patterns.
One key observation of Ginouse’s study is that a
shotcrete spray can be simply characterized by two parameters: the maximum velocity and the spray opening
The results of Ginouse’s research have opened many
R&D topics. Thus, a series of projects have emerged
to extend the research effort on the study of the shotFig. 1: Fitted velocity profiles at 0.5m and 1.0 m for the wet-mix
(in blue) and dry-mix (in green and red) Spring 2020 | Shotcrete 17
Fig. 2: Experimental imaging device (Bérubé, 2017)
Fig. 4: Close-up of the nozzle tips
Fig. 3: Tested nozzles during experiments – ACME (top) and 1978
(bottom) (Bérubé, 2017)
crete spray. One of these research topics was about the
influence of the equipment (nozzle) on the performance,
especially concerning rebound, in each process.
Simon Bérubé has been a part of this momentum
established at Université Laval Shotcrete Laboratory. His
research (Bérubé, 2017) has focused on the influence of
the nozzle on the particle velocities in wet-mix shotcrete,
and on the mass distribution in the dry-mix spray stream
of concrete.
This article develops one specific aspect of Bérubé’s
project. By using the same setup as Ginouse, this
research evaluated the influence of the nozzle body and
the nozzle tip shape on the spray pattern in wet-mix
This study took place in a controlled laboratory environment with conventional industrial shotcrete equipment.
A shotcrete hydraulic cylinder pump, an Allentown
Powercreter 10, was used to pump a modern shotcrete
mixture designed for wet-mix placement. To lubricate
the 50 mm (2 in.) 20 m (65 ft) long delivery hose, a
cement grout with the same water/binder ratio as the
concrete, was pumped before the spraying.
Fig. 2 shows the experimental set-up for the imaging
device used in our facility. The 1250 frames per second capacity camera is positioned perpendicular to the
screen and the shotcrete spray. The white screen helps
to ensure a adequate contrast to discern particles in the
spray when the processing on captured images is done.
When shotcreting, the nozzle is held in a static support
and kept motionless to avoid the effects due to movement of the nozzle and the material stream.
Images are then post-processed with specialized
software to track the particles image by image. With the
data acquired, particle velocity profile and spray limits
can be defined.
Two conventional nozzles, the so-called ACME
Nozzle and the 1978 Nozzle (Fig. 3), were put to the
test. Those nozzles present some interesting differences:
while both air rings are similar (8 holes), the air plenum of
the 1978 Nozzle is clearly thinner and narrower than the
one found on the ACME Nozzle, and the ACME Nozzle
has a 19.1 mm (0.75 in.) air inlet whereas the 1978
Nozzle has a 12.7 mm (0.5 in.).
Moreover, the nozzle tips have noticeable differences
(Fig. 4). The ACME Nozzle tip (referred to as long nozzle
tip) is 193 mm (7.6 in.) long and has a 31 mm (1.2 in.)
diameter outlet, whereas the 1978 Nozzle tip (referred
to as short nozzle tip) is 130 mm (5.1 in.) long and has a
36 mm (1.4 in.) diameter outlet. The ACME Nozzle tip is
therefore longer and more tapered than the 1978 Nozzle
tip. Furthermore, these two nozzle tips have different
rigidity due to their thicknesses and the rubber used.
The long nozzle tip is stiffer.
18 Shotcrete | Spring 2020
The first trials of the project brought to light interesting
differences in the concrete spray produced by the two
nozzles presented before.
Table 1 presents the spray characteristics at 1.0 m
from the nozzle outlet for the two nozzles and for two
different airflows.
As shown in Table 1, particles in the spray produced
by the ACME Nozzle travels around 50% faster than
the one in the 1978 spray for both airflows. Moreover,
the ACME spray is half narrower than the 1978 spray.
The first interesting observation is the nozzle (body
and nozzle tip) has a noticeable effect on the particle
velocities and spray limits. To investigate the origin of
those differences, the second step of this project was
to focus more on this piece of equipment.
To explore further, the nozzle tips were switched. This
way, the long nozzle tip was put on the 1978 Nozzle
body, and the short nozzle tip likewise switched to the
ACME Nozzle body. Table 2 presents the experimental
program and the results obtained with each
Characteristics ACME 1978
Airflow (ft3
/min) 150 200 150 200
Vmax (m/s) 24.0 23.5 16.0 16.9
Angle (°) 28.2 27.2 49.6 52.0
Body Tip
Vmax (m/s)
Long 24.0 23.5
Short 18.5 23.0
Long 19.5 22.4
Short 16.0 16.9
Table 1: Spray Characteristics for the two nozzles
Table 2: Maximum velocity for each nozzle configuration
From a theoretical rebound point of view, the ACME
Nozzle body combined with the long nozzle tip proved
to be the best configuration achievable, considering
the particles velocities. For both 150 and 200 ft3
air flow, particles traveled around 24 m/s (79 ft/s). The
worst configuration would be the 1978 Nozzle body
combined with the short nozzle tip. Particles travel
around 16 to 17 m/s (52 to 56 ft/s) regardless of the
To evaluate the nozzle tip efficiency in future research,
velocities will be compared using the 150 ft3
/min air flow
since that is the more critical case for spray velocities.
Using the same nozzle body, the short nozzle tip always
produced lower particle velocities compared to the long
nozzle tip.
It is interesting to mention that, at 200 ft3
/min, a good
nozzle body (ACME) combined with the extra airflow
helped to reduce the “bad” effect of the short nozzle tip.
This brief study showed the importance of choosing
both the right nozzle body and the right nozzle tip to ensure optimal placement conditions. Moreover, it seems
that increasing the airflow will not always increase particle velocities. Cutting the end of the nozzle tip reduces
back thrust and may facilitate nozzle movement for the
nozzleman and is sometimes seen on construction sites.
However, this practice will lead to a reduction in the
shotcrete spray velocity and in turn reduce the shotcrete
placement quality and overall performance.
The authors would like to acknowledge Andy
Kultgen with ConForms for his help and suggestions in
this project, as well as supplying the different nozzles
used in this study.
1. Armelin, H. S. (1997). Rebound and toughening mechanisms in
steel fiber reinforced dry-mix shotcrete. (PhD. Thesis), University of
British Columbia, Vancouver.
2. Bérubé, S. (2017). Effect of the projection nozzle on the projected concrete bounce. (M.Sc.), Université Laval, Quebec.
3. Ginouse, N. & Jolin, M. (2014) Effect of Equipment on Spray
Velocity Distribution in Shotcrete Applications. Construction and
Building Materials. Spring 2020 | Shotcrete 19
Following a bachelor’s degree in mechanical engineering from the Arts et Métiers
in France, Pierre Siccardi pursued his
studies at Université Laval, Québec,
Canada where he obtained his master’s degree. His research project on
shotcrete nozzles led to the filing of a
patent. He is currently pursuing his PhD
under the supervision of Prof. Marc Jolin. His project now
focuses on the homogeneity and the adjustment of concrete mixes in a mixer truck using an on-board system.
Simon Bérubé earned his civil engineering
bachelor’s degree from Université Laval
in 2014. Following that, he completed his
master’s degree in the same field in 2018
under Dr. Marc Jolin, during which he
undertook a research project on shotcrete
equipment modelization. Simon Bérubé
now works for CIMA+ since 2016, a private
engineering firm located in Quebec City.
Achraf Laradh is a master’s student in the
Department of Civil and Water Engineering at
Université Laval, Québec City, QC, Canada.
The focus of his graduate research project is
to model the spray of concrete to evaluate the
placement quality considering the rebound
of the shotcrete. He received his engineering
degree at Arts et Métiers Paristech, France.
Marc Jolin, FACI, is a Full Professor
in the Department of Civil and Water
Engineering at Université Laval. He
received his PhD from the University
of British Columbia, Vancouver, BC,
Canada, in 1999. An active member of
Centre de Recherche sur les Infrastructures en Béton (CRIB), he is involved in
projects on service life, reinforcement encasement quality,
fibers, admixtures and rheology of shotcrete. He is Past
Chair of the ACI Comittee 506 Shotcreting, and secretary
of ACI Subcommitee C601-I, Shotcrete Inspector, Shotcrete Inspector, and is a member and Past Chair of ACI
committee C660, Shotcrete Nozzleman Certification.