Portal: General

This year, 2020, we have been challenged in a manner unlike anything we have experienced in America since the 1918 influenza pandemic that killed millions worldwide. The coronavirus (COVID-19) was identified in Wuhan, China, after initially being reported as a cluster of pneumonia cases in December 2019. Despite efforts to contain the virus, it rapidly spread to Italy and the rest of Europe and eventually the United States. As a result, on February 26, the first cases of COVID-19 began to appear in the Seattle,
Wash. area. Acting rapidly, many state governors issued
emergency restrictions and stay-at-home orders. Across
America we all watched the coronavirus task force briefings and New York Governor Andrew Cuomo’s morning
broadcasts, as the response unfolded in especially
hard-hit New York. In Pennsylvania we were issued
stay-at-home orders by our Governor Tom Wolf for all
but life-sustaining businesses. Which resulted in PennDOT closing down construction projects. In the months
that followed, state governors began to reopen their
economies. With the return to work there are protocols
and procedures we will need to follow to keep our crews
safe and well. Below are some guidelines and information that I hope you will find helpful.
INTENT OF PROGRAM
COVID-19 is an easily transmitted disease, especially
in group settings. It is essential that positive actions be
implemented to slow or stop the spread of the virus to
safeguard the public safety. Construction is vital to our
nation’s economy. Recognizing this, we need to find
ways to accomplish our work while not endangering
our employees. This requires several precautions to
protect our workers, their families, and members of
the community. It is necessary that all businesses in
the construction industry conducting person-to-person
activities follow the directives and requirements of their
state governments. Other local political units or privately
owned companies may elect to impose more stringent
requirements. In such cases we must adhere to the
client or owner’s requirements.
Recommended Guidelines
• Follow all applicable provisions that your State
government has provided for business safety
measures.
• Requiring that every person present at a work
site wear a face mask or face coverings.
Dealing with COVID-19
in Construction
By Ted W. Sofis
Fig. 1: Social distancing with mask inside Fig. 2: Hand Sanitizer and Disinfectant Wipes
shotcrete.org Spring 2020 | Shotcrete 35
• Establish protocols for action to be taken when
cases or probable cases of COVID-19 are
discovered or whenever people in the company
could have possibly been exposed to someone
who may have the virus.
• Require social distancing with a 6 ft (2 m) minimum distance between workers, unless the safety of the workers makes an exception necessary,
(for example, team lifting.)
• Follow other Department of Health (DOH) and
Centers for Disease Control and Prevention
(CDC) guidance.
• Provide hand washing stations at appropriate
locations on your job sites, such as building entrances, break areas, offices, trailers and job site
egress areas.
• Implement cleaning or sanitizing protocols at all
construction sites and projects. Identify and regularly clean and disinfect areas that are high risk
for transmission. Establish requirements to clean
common areas and regularly trafficked areas
periodically.
• Ensure all gatherings are limited to no more than
the maximum gathering size mandated by the
state where your project is located. Maintain 6 ft
(2 m) social distancing at all times when required
to meet, even when meeting outside.
• Use virtual meetings and distribute information
electronically whenever possible.
• Stagger shifts, work breaks, work areas, and
different trades, wherever feasible, to minimize
the number of workers on site.
• Limit tool sharing and sanitize tools or equipment
if they must be shared.
• Employ jobsite screening based on CDC guidance to determine if employees should be
working. Prohibit employees with any symptoms
of Covid-19 from working. Encourage any sick
employees to stay at home.
• Prohibit unnecessary visitors to any project
or work site and limit the number of supplier
deliveries.
• Limit access to enclosed spaces as much as
possible.
• Ensure workers are traveling to and from the
jobsite in separate vehicles. Whenever possible,
make sure that they do not share a vehicle.
• Identify a Coronavirus Safety Officer for each
project or work site, or (on a large-scale project)
for each contractor or subcontractor at the site.
The primary responsibility of the Safety Officer is
to convey, implement and enforce the social distancing and other requirements of the company
program for the protection of employees, suppliers, and other personnel at the site.
Fig. 3: Social distancing with mask outside
36 Shotcrete | Spring 2020 shotcrete.org
Ultimately, as a contractor, I realize the difficulties
involved in implementing and following coronavirus
guidelines, while at the same time trying to efficiently
perform and complete our projects. The reality is
that we have no other option. This pandemic is so
widespread and highly contagious that it forced
the shutdown of our nation’s economy for months.
Nothing like this has ever happened before in our
lifetimes. To ignore this, we jeopardize the lives of
our employees and their families. For the safety of
everyone involved and the communities where we
live, we need to get through this together. A vaccine
for COVID-19 will be developed and therapeutic
medications for treating the virus will become
available. In the interim, we need to find ways to
be safe and productive. I hope that you find these
guidelines helpful. More importantly, it is my hope that
all of you and all of your employees make it through
this construction season safely and in good health.
Ted Sofis and his brother, William J.
Sofis Jr., are the Principal Owners of
Sofis Company, Inc. After he received
his BA in 1975 from Muskingum College,
New Concord, OH, Ted began working
full time as a shotcrete nozzleman and
operator servicing the steel industry. He
began managing Sofis Company, Inc., in
1984 and has over 40 years of experience in the shotcrete
industry. He is a member of various ASA committees and an
ACI Shotcrete Nozzleman Examiner for shotcrete certification. Over the years, Sofis Company, Inc., has been involved
in bridge, dam, and slope projects using shotcrete and
refractory installations in power plants and steel mills. Sofis
Company, Inc., is a member of the Pittsburgh Section of the
American Society of Highway Engineers (ASHE) and ASA.
Fig. 4: Face mask protection in close proximity Fig. 5: The respiratory protection worn during gunning operations
fulfills the COVID-19 face mask directive.

The decline in the number of workers in the construction industry is a severe problem in Japan. Formwork is indispensable for concrete structures, but due to the shortage of carpenters, formwork assembly tends to be slow and can cause project delays. To deal with this problem, new workers are being hired and formwork carpenters are being trained, but the payoff is not immediate because the acquisition of the required skills takes several years of education and experience. The “formless construction method” that eschews
formwork, might be a possible solution (Fig. 1), substituting an outer shell formed with sprayed mortar. The
reinforced concrete structure is then created by placing
reinforcing bar and casting self-compacting concrete
(SCC) inside the outer shell.
In this approach, the lateral pressure of the fresh concrete stresses the outer shell during concrete casting, so
the shell must have high tensile strength. Since Japan is
an earthquake-prone country, structures are required to
have strong deformation performance and must resist
the large bending and compressive stresses generated during earthquakes. Furthermore, to reduce the life
cycle cost of the structure and increase its sustainability,
both high durability and maintenance-free design of the
structure are required. Thus, the outer shell must have
a high resistance to chloride ion penetration and other
aggressive exposures.
To satisfy the performance requirements, we decided
to use Ultra High Strength Fiber Reinforced Concrete
New Application Method of
Sprayed UHPFRC
By Satoru Kobayashi
Fig. 1: Concept of formless construction method
(UHPFRC) as the sprayed material. UHPFRC is a highstrength and high-ductility material with compressive
strength of 22,000 to 36,000 lb/in2
(150 to 250 MPa)
and tensile strength not lower than 1200 lb/in2
(8 MPa).
It is also characterized by a highly dense concrete matrix
with very low water and air permeability and thus high
chloride resistance. In Japan, UHPFRC has been used
mainly for factory produced precast products. Recently,
with the advent of mass manufacturing of precast members with large sections, the application of UHPFRC
to civil engineering structures has been increasing. The
largest such project to date is the application of UHPFRC for the floor slabs of Runway D at Tokyo International Airport with a UHPFRC volume of approx. 26,000
yd3
(20,000 m3
).
Thus far, UHPFR has rarely been used with sprayed
placement, and forming the outer shell of a structure
with sprayed UHPFRC is a novel challenge. This article
outlines the experimental method developed and used
to form the outer shell of a structure with sprayed
UHPFRC.
We focused on columns as the target structure. This
formless construction method requires a core material
that is easy to install and remove. Air tubes were adopted as the core material. Figure 2 shows the installation
of the air tubes. The thickness of the member was 16 x
16 in. (400 × 400 mm), and the thickness of UHPFRC
was 1.6 in. (40 mm). The height of the columns was 59
in. (1500 mm).
In this experiment, a mortar pump (squeeze type),
maximum discharge rate 8 yd3
/hr (100 L/min) and a
delivery hose with a diameter of 2 in. (55 mm) were used.
Fig. 2: Installation of air tubes
shotcrete.org Spring 2020 | Shotcrete 33
The diameter of the tip nozzle was
0.6 in. (15 mm). The sprayed material was required to stick on vertical
surfaces without sagging. To this
end, a non-alkali hardening accelerator was added at the nozzle.
Figure 3 shows the spraying of
UHPFRC. The material adhered to
the vertical surfaces of the air tubes
without sagging, and coverage of
the sprayed material to the required
thickness and height of 5 ft (1.5 m)
was achieved without problem.
Figure 4 shows the removal of
the air tubes that was easily accomplished.
As the next steps, reinforcing
bars will be set and SCC will be
cast inside the outer shell. The
structural performance will be evaluated by flexural strength testing.
Fig. 3: Spraying of UHPFRC on air tubes Fig. 4: Removal of the air tubes
Satoru Kobayashi
is a senior
researcher
for Kajima
Technical Research
Institute based
in Japan. He
graduated from
Hiroshima University where he studied
the durability of concrete. He is highly
skilled in concrete, for example, selfcompacting concrete, anti-washout
underwater concrete, dam concrete,
and shotcrete. Recently his research
project focuses on the new application
method of UHPFRC and various
ways to use it at the construction site
effectively in order to improve the
durability of the structure and the
productivity of the construction process.

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.
WHAT WE KNOW
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
angle.
RESEARCH DYNAMIC
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)
shotcrete.org 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
shotcrete.
METHODOLOGY
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 shotcrete.org
PHASE 1
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.
PHASE 2
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
configuration.
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)
150
ft3
/min
200
ft3
/min
ACME
Long 24.0 23.5
Short 18.5 23.0
1978
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
DISCUSSION
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
/min
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
airflow.
INFLUENCE OF THE NOZZLE BODY
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.
CONCLUSION
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.
References
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.
shotcrete.org 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.

Although dry-mix shotcrete has been used extensively in both mining, tunneling and concrete
repair projects, it suffers from the reputation that it will produce more dust than other repair methods or
even wet-mix shotcrete. Due to the fact that dry-mix shotcrete is most often applied using pre-packaged
bagged material, the act of emptying the shotcrete itself into the shotcrete spraying equipment inherently
generates dust. In contrast, the vast majority of wet-mix shotcrete is sprayed using shotcrete supplied via readymix trucks which introduces little to no dust generation at the jobsite itself. Dust is generated at the ready-mix
plant during batching and controlled by the ready-mix shotcrete supplier via engineering controls such as dust
collection. By the time the contractor receives the material it is fully mixed and does not emit any dust on-site.
There are options available to reduce the amount of dust
generated while spraying dry-mix shotcrete but using
dust-reducing additives to the pre-packaged dry-mix
shotcrete is still an area of interest for development. This
article explores the results of testing several dust-reducing additives, how dust generation can be evaluated,
and how the inclusion of these additives can affect the
mechanical properties of dry-mix shotcrete
Previous work has shown that by modifying the
mixture design of conventional dry-mix shotcrete, the
cracking resistance can be greatly increased in laboratory conditions (Clements & Robertson, 2019). Although it
was found that removing silica fume from the formulation
greatly increased the cracking resistance, it also drastically increases the rebound observed during shotcrete
placement. As a result, King – A Sika Company (KING)
developed a testing program to evaluate four prototype
mixture designs to select the mixture with the lowest
cracking potential for further development. The selected
candidate mixture was sprayed following an intensive
testing protocol to characterize the desired mechanical
and durability properties of the mixture. The initial results
of the testing program and the preliminary results of the
full characterization testing program are contained in this
article.
DRY-MIX SHOTCRETE DUST
GENERATION
Workplace dust is an unavoidable risk in many construction-related occupations and especially true for dry-mix
shotcrete. Prolonged exposure of workers to elevated
concentrations of silica dust can lead to irreversible
physical damage such as silicosis. Currently, the only
reliable, proactive defense is the use of proper engineering controls such as suitable ventilation, appropriate
dust respirators and appropriate PPE. However, the best
form of risk management is to eliminate or reduce the
potential of the risk itself.
When observing dry-mix shotcrete placement, it is
first important to identify the regions where dust can
potentially be generated. Dust is generated at high
concentrations in two specific regions: at the discharge
from the nozzle; and feeding material into the dry-mix
shotcrete machine (Figure 1 and 2).
Fig.1: Case 1-High concentration of dust emitted from the nozzle.
Fig. 2: Case 2: High concentration of dust emitted when pre-packaged dry-mix is emptied into the material hopper of a dry-mix
shotcrete machine.
shotcrete.org Spring 2020 | Shotcrete 11
Fig. 3: CIPAC MT 171 table for evaluating the level of optical dust
emissions for DustView II (CIPAC, 2015).
Fig. 4: An example of the results which can be obtained from the
DustView II.
Fig. 5: Locations A, B & C for DustTrak II Aerosol Monitor 8530
dust level monitoring.
QUANTIFYING DUST
When it comes to quantifying dust levels at the hopper for dry-mix shotcrete, two methodologies can be
proposed. These methodologies shall be referred to as
the “static” and “dynamic” method of testing.
A static dust emission test involves measuring the
dust levels with the DustView II from Palas based on
the standard CIPAC MT 171. (CIPAC, 2015). This device functions by dropping a powder sample of 0.035
ounces (30g) down a cylindrical tube. As the powder
descends, dust particles are measured through extinction measurement with a laser beam. The results are
then summarized with an optical dust value, referred to
as the “Dust Number”. The Dust Number can be calculated using the software offered by the device and
serves as a manner to interpret dust emission activity.
Dust Number = Maximum Dust Value + Dust Value
30-seconds after the Maximum Dust Value. (Palas, n.d)
As per CIPAC MT 171, and as seen in Figure 3, in
the event where the Dust Number is lower than 25, it
shall be considered as being “essential non dusty”. An
“essential non dusty” material is one where dust levels
are lowered but can still be seen by the naked eye.
Figure 4 is an example of how DustView II records
the activity of dust particles during static testing. The
graph showcases how the dust activity peaks in the
initial moments and then decreases gradually as the
dust settles. The Dust Number for this particular test
was 18.63 which would mean that the product falls in
second category from Figure 3.
The dynamic method takes measurements during
a live test with real equipment and external activities. During a real dry-mix shotcrete test, there are
many variables which can generate additional dust:
compressed air, ambient wind pressures and currents, movement from equipment and personnel,
entrapped-air, etc. This manner of testing uses the
DustTrak II Aerosol Monitor 8530 by TSI. The DustTrak
II performs readings with gravimetric sampling. It is
capable of measuring aerosol concentrations ranging
from 0.001 to 400 mg/m3
. (TSI, n.d).
In KING’s shotcrete laboratory, a series of tests
were conducted to see where the dust should be measured. Three locations for the monitors were established for the testing (Figure 5):
• Location A – situated 1 ft (0.3 m) from the hopper of the dry-mix shotcrete machine;
• Location B – set 10 ft (3 m) from Location A
and;
• Location C – located in the shotcrete shooting
chamber.
The chamber was rectangular in shape and the
entrance was sealed with rubber lathes to contain dust
from the nozzle. Through trial and error, it was deemed
too turbulent to take accurate readings when the DustTrak II was placed in the shooting chamber at Location
C. For Location B the aerosol recordings did not depict any significant differences between shooting and
not shooting. It was only at Location A when the monitor was placed 1 ft (0.3 m) from the dry-mix shotcrete
machine , that significant fluctuations were recorded,
and corresponded with the shotcreting activities.
12 Shotcrete | Spring 2020 shotcrete.org
DEVELOPMENT OF A DUST REDUCED
SHOTCRETE
Trials to date for reducing the amount of dust generated during dry-mix shotcreting for Case 2 are still in
the preliminary stages. General findings have been
positive for reducing emissions. However, dust reducing additives used thus far were shown to influence
two major components of dry-mix shotcreting: 1) The
amount of water required at the nozzle to properly hydrate the mix; and 2) A reduction in early and later-age
strength gain.
In Figure 6, there is an example of how DustTrak II
records the activity of dust particles during dynamic
testing. The orange curve is the control mixture and
the green and yellow are with two different types of
dust reducing admixtures. Everything in gray represents dust generated by other equipment.
Dust reducing additives have been effective at
lowering dust emissions by up to 43% in dry-mix shotcrete mixes when using the dynamic method. When
comparing the compressive strengths for the different
formulations as seen in Figure 7, the strength development is slower. However, the differences in strength
can be associated with the fact that more water was
required at the nozzle to produce a cohesive spray.
When observing results from the Rapid Chloride Ion
Penetration (RCP) ASTM C 1202, the relationship
between additional water can be seen. Referring to
Table 1, the early RCP values are elevated for one of
the dust additives but at 28 days all formulations reach
relatively low coulomb ratings. Figures 8, 9 and 10
show the visible reduction of dust at the hopper when
using the dust reduction additives.
Fig. 6: Dynamic test results for dry-mix shotcretes with and without
dust reducing additives.
Fig. 8: Dry-mix shotcrete without any additives.
Fig. 9: Dust Reduction Additive No.1.
Fig. 7: Compressive Strength development for dry-mix shotcretes
with and without dust reducing additives.
Table 1: Chloride Ion Penetration results for dry-mix shotcretes with
and without dust reducing additives.
Mix Design
Chloride Ion
Penetration
(7 Days)s
Chloride Ion
Penetration
(28 Days)
Control 1500 coulombs 650 coulombs
Dust Reduction
Additive No. 1 3600 coulombs 750 coulombs
Dust Reduction
Additive No. 1 1400 coulombs 500 coulombs
shotcrete.org Spring 2020 | Shotcrete 13
Fig. 10: Dust Reduction Additive No.2.
LOW CRACKING POTENTIAL DRY-MIX
SHOTCRETE
When repairing concrete structures, best practice is
replacing any deteriorated concrete, with a material
that closely matches the mechanical properties of the
substrate when possible. Even though shotcrete can
be very similar to cast-in-place concrete when shot,
the shotcrete process and mixture design can invariably lead to increased shrinkage and volume change.
This volume change becomes very important for a
shotcreted concrete repair, as the substrate restrains
the shotcrete from shrinking after placement. If the tensile stress developed in the patch or resurfaced area
exceeds the tensile strength of the shotcrete it will lead
to cracking or de-bonding.
To characterize the volume change of shotcrete
AASHTO T 344 standard test method (ring test) was
adapted to the shotcrete process at Laval University
(Girard, Jolin, Bissonnette & Lemay, 2017). Using this
method, KING was able to screen several prototype
mixture designs for a low cracking potential dry-mix
shotcrete. During this testing program the ring specimens (Figure 11) were wet cured for a period of 3
days, followed by being placed in a controlled environment at 50% (±5%) relative humidity and a temperature of 70 ± 2°F (21 ± 1°C). The results of this testing
program are presented in Table 2.
Mix
No.
Compressive
Strength ASTM C
1604 (7 Days)
Compressive
Strength ASTM C
1604 (28 Days)
Age of
Cracking
AASHTO T
344 (Days)
1 5800 psi (40 MPa) 7100 psi (49 MPa) 15
2 6235 psi (43 MPa) 6815 psi (47 MPa) 25
3 6380 psi (44 MPa) 6525 psi (45 MPa) 45
4 4640 psi (32 MPa) 5510 psi (38 MPa) 38
Table 2: Age of cracking for different prototype dry-mix shotcrete
formulas.
Based on the results of the initial screening tests
Mix No. 3 was selected for the next phase, which
included a testing program to assess many mechanical and durability properties. This testing program
also included the spraying of AASHTO T 344 rings
which were then cured using three different curing
regimes. The three curing regimes included exposure
to 50% (±5%) relative humidity for the entire age of the
specimen (Dry), three days of wet curing followed by
exposure to 50% (±5%) relative humidity (Wet) and
curing compound being applied to the exposed surfaces of the ring after spraying and demoulding then
exposure to 50% (±5%) relative humidity for the entire
age of the specimen (Curing Compound). All the ring
specimens in each curing regime were maintained at
a temperature of 70 ± 2°F (21 ± 1°C). The preliminary
results of the ring tests performed in the second phase
of the testing program for the candidate low cracking
potential dry-mix shotcrete are presented in Table 3.
It can be seen that exposing the rings of this candidate mixture to 50% (±5%) relative humidity without
any curing, can still perform better than typical silica
fume enhanced dry-mix shotcrete with three days of
wet curing which would normally crack near six to seven days (Menu, Pépin Beaudet, Jolin, Bissonnette &
Molez, 2018). However, in comparison to exposing the
rings to either three days of wet curing or using curing
compound has extended the age of cracking, to such
an extent that the rings had not cracked prior to the
publication this article and continue to be monitored.
Curing Regime for Rings Age of Cracking AASHTO
T 344 (Days)
Dry (50% RH) 20
Wet (3 Days Wet, 50% RH) 42+*
Curing Compound (50%
RH) 42+*
4640 psi (32 MPa) 5510 psi (38 MPa)
Table 3: Age of cracking for low cracking potential dry-mix shotcrete
using different curing methods. *Rings had not cracked at the time of
publishing this article.
14 Shotcrete | Spring 2020 shotcrete.org
CONCLUSIONS
Dust reduction technology for shotcrete is an area
that needs further research. Improving the health and
safety of the workers who are exposed to dust daily
will benefit these individuals and the entire shotcrete
industry. Dry-mix shotcrete with reduced dust emissions are currently achievable, but the effect of the
additives on the physical properties and durability of
shotcrete must be explored further.
It has been shown that by modifying the mixture
design of dry-mix shotcrete, the cracking potential can
be greatly reduced. Upon selecting the best performing mix design, it can also be seen that using no curing with the low cracking potential dry-mix shotcrete is
better than current dry-mix shotcrete technology with
three days of wet curing. In a laboratory environment it
was observed that the use of three days of wet curing
or the use of curing compound with this new technology can drastically reduce the potential for cracking.
Low cracking potential dry-mix shotcrete continues
to be evaluated to assess the appropriate durability
parameters.
References
1. Title 29. Labor Subtitle B. Regulations Relating to Labor
Chapter XVII. “Occupational Safety and Health Administration,
Department of Labor Part 1926. Safety and Health Regulations for
Construction Subpart Z.” Toxic and Hazardous Substances Section
1926.1153. Respirable crystalline silica.
2. AASHTO T 344. (2008). “Standard Method for Estimating the
Cracking Tendency of Concrete.” Washington, DC, USA: American
Association of State and Highway Transportation Officials.
3. ASTM C1202 (2018). “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.” West
Conshohocken, PA, USA: ASTM International.
4. ASTM C1604 (2012). “Standard Test Method for Obtaining and
Testing Drilled Cores of Shotcrete.” West Conshohocken, PA, USA:
ASTM International.
5. MT 171.1, 2015, CIPAC 5003/m MT 171.1 “Dustiness of
Granular Products, Collaborative International Pesticides Analytical
Council Limited.”
6. Clements, W.; Robertson, K. (2019) “Compatible Shotcrete
Specifications and Repair Materials.” Shotcrete Magazine, 20-25.
7. Côté R., Ferland H., & Robertson K. (2014). “McCormick Dam &
Power Station: Submerged Concrete Repairs.” Shotcrete Magazine,
36-39.
8.“DustTrak II Aerosol Monitor 8530,” TSI Incorporated, Shoreview, MN, 2020, https://tsi.com/products/aerosol-and-dust-monitors/
dust-monitors/dusttrak-ii-aerosol-monitor-8530/. (last accessed May
19, 2020)
9. “DustView II,” Palas, Germany, 2020, https://www.palas.de/en/
product/dustview2. (last accessed May 19, 2020)
10. Girard, S.; Jolin, M.; Bissonnette, B.; and Lemay, J-D. (2017)
“Measuring the Cracking Potential of Shotcrete.” Concrete International, V. 39, No. 8, 44-48.
11. Menu, B.; Pépin Beaudet, A.; Jolin, M.; Bissonnette, B.; Molez,
L. (2018) “Évaluation quantitative de la sensibilité à la fissuration du
béton projeté au moyen de l’essai de retrait restreint annulaire.”
William Clements, MASc., P. Eng., is
Engineering Services Manager for King – A
Sika Company, where he is responsible
for all mixture design development, quality
control and technical support. He received
his bachelor’s and master’s degrees in civil
engineering from the University of Windsor,
Windsor, ON, Canada. He is a member of
the American Concrete Institute (ACI); a member of ACI Committee 506 and 239; and Subcommittees 239-D and 546-D. .
Cody Fournier, Jr. Eng, M. Eng, is
an Engineering Services Representative for King – A Sika Company, where
he engages in product development,
quality control and technical support for
Eastern Canada. He received his bachelor’s and master’s in civil engineering
from Concordia University, Montreal,
QC, Canada. He is a member of the American Concrete Institute (ACI) and Subcommittee 506.1R.