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We have all seen those small letters and numbers that mark practically everything we use in construction. To most, these are meaningless markings that are meant for someone else. However, with shotcrete, nothing could be farther from the truth.

The COVID-19 Pandemic affected our lives in ways none of us have ever experienced in our lifetimes. I’ve been in construction for 45 years and I have never seen our economy shut down, businesses closed, or people required to stay at home. In 2001, the attacks of September 11th temporarily shut down air travel and the stock market, but the American economy remained intact and air travel resumed within a couple of weeks. However, COVID-19, has affected our lives in ways that we could never have imagined. Schools and universities were closed; professional, collegiate and high school sports seasons were suspended and canceled; and restaurants and businesses were closed. We were told to stay home and work remotely, if possible, and businesses across the country followed those directives.

Show me a person who tells you that safety on a mine site is just plain common sense, and I’ll show you someone who doesn’t understand mine safety completely. Mine safety is not simply common sense. It is that, and a whole lot more. Most mines have their own set of mine-specific regulations and rules. Nearly all mines, in Canada, the USA, and Mexico, are required by law, to follow government-mandated requirements such as U.S. Department of Labor- Mining Health & Safety Administration (MSHA) and/or the Occupational Safety & Health Administration (OSHA). Canada and Mexico have their government agencies governing mining as well. Even with strict government regulations, most mines have safety regulations specific to each individual mine. Anyone who desires to visit a mine site needs to accept one thing- whatever the rules and regulations are for a particular mine- these regulations are serious business and are meant to be enforced.

The storm water drains in the city of Mumbai, India, are over 100 years old and constructed with brick arch masonry during the British Era (Fig. 1). The storm water drains (SWD) were prone to frequent cave-ins. To prevent cave-ins, enhance their safety, and maintain the SWD system, the Municipal Corporation of Greater Mumbai (MCGM), under the Central Government of India “BRIMSTOWAD” Scheme, initiated a detailed survey and mapping of the SWD for the City of Mumbai.

This is a revised version of the original article printed in the Summer 2016 of Shotcrete magazine before the OSHA rule was put in place. This revision has added site measured values for air monitoring of crew members on shotcrete projects, as well as ASA’s response to OSHA’s request for information in August 2019. Also, included is a short section on applicable respirators. With this revision our intent is to put the current information you need about the OSHA rule and its impact on shotcrete operations in one place for ready reference.

Because of its durability, strength, and flexibility in application, shotcrete is often used for the construction and stabilization of tunnels and other underground structures. The fact that tunneling involves general construction risks as well as tunnel specific environmental risks, makes this type of application potentially quite dangerous, and must be treated with caution. Risks cannot be eliminated, but we can implement measures to lower the risk.

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
www.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 www.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
www.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.

Construction of sprayed concrete lining (SCL) ground support across the world utilizes the construct, verify and rework cycle. This methodology typically requires survey verification of the as-built result against design for each stage of the ground support installation. However, processing and analyzing the measurement data is a time consuming and often intensive manual process. Often once the survey information is available the construction crew will have already left, this will require rework on the next cycle.
Leveraging the latest in high-density LiDAR and
high-speed computing technologies, provides the
ability for construction crews to receive near realtime feedback of their SCL construction against
design. This potentially can significantly improve the
efficiency and quality of SCL reinforcement, while
reducing waste in construction.
CONSTRUCTION CHALLENGES
In a typical shotcrete application stage, the thickness of shotcrete applied is highly dependent on the
skill and experience of the nozzlemen. Upon completion of the shotcrete placement the compliance
of the sprayed concrete thickness with the design
requirements is not known until after a survey is
completed. The survey results highlight areas of over
spray (excessive thickness) or under spray (deficient
thickness), resulting in shotcrete wastage or costly
rework.
For example, during application of shotcrete
nozzlemen often use bolt tips as guidance to allow
them to gauge the approximate depth of their placement. The nozzleman’s experience plays a large role
in ensuring that the correct thickness is achieved.
However, to reduce the amount of under spray sections and prevent rework the nozzleman may choose
to place more shotcrete than required.
In many tunnel and cavern projects, the design
profile of the tunnel or cavern is critical and requires
strict thickness tolerances during SCL construction
to ensure the as-built sections fall within the design
profile specifications. In these types of construction,
both under spray and over spray could result in
non-compliance which then requires very costly and
time-consuming rework. Often depth pins and string
lines are installed in the area to provide guidance to
the nozzlemen, allowing them to visually gauge when
they have achieved profile. The process of installing
depth pins and string lines are time consuming
and labor intensive. This significantly increases the
time and cost of construction. Once installed, the
nozzlemen will have to estimate the placement
thickness between the string lines, which once again
is heavily dependent on the skill and experience of
the nozzlemen.
www.shotcrete.org Spring 2020 | Shotcrete 25
STATE OF THE ART CONSTRUCTION
TECHNOLOGY
Since the introduction of the Building Information
Modeling (BIM) standards, governments around the
world are rapidly adapting BIM for their construction
projects. This is evident in countries including Hong
Kong, Singapore, Norway and Sweden. Figure 1
show the core BIM construction workflow where
design and authoring of the architectural and tunnel
designs are done in 3D CAD software, such as Revit
and Civil3D, where a full 3D model of the completed
section is created. This is followed by the Virtual Design
and Construction (VDC) process where a complete
construction simulation is run using the CAD models.
This helps validate the both the construction process
and the schedule. Clash detection is also accomplished
with the design model to detect any potential design
conflicts before the construction plan is approved.
By using FPGA based System-on-Chip (SoC) technology, similar to the Zynq-mp processor core technology that can deliver teraflops of computing performance,
computationally intensive signal processing algorithms
can be implemented in the hardware using the
Programmable Logic portion of the system (see Figure
2). The Programmable Logic area, in yellow, allows the
computer designer to create custom digital signal processing cores, like GPUs, and execute them in parallel,
allowing high speed processing of large datasets. The
Programmable Logic area also has the benefit of having
dedicated memory banks that support concurrent
access, unlike conventional computer memory access,
via a common bus architecture. The architecture above
makes it possible to process high-density data, like
the 3D point cloud data produced by laser scanners in
real-time.
REAL-TIME IN-SITU MEASUREMENT
TECHNOLOGY
Real-time in-situ measurement technology refers to a
portable measurement device equipped with onboard
high-speed computing capabilities to deliver live or near
real-time high-resolution information. One example is the
production of information such as deformation or shotcrete thickness results in 3D. Figure 3 shows the comparison between using a conventional LiDAR against an
in-situ LiDAR technology, in this case the Geotechnical
Monitoring LiDAR (GML).
BIM construction is one of the key reasons for the
use of laser scanner technology. 3D Laser scanners
are used to scan as-built construction elements.
Scanning is typically carried out by a survey team
where the laser scanner is deployed in the excavation
heading to collect the as-built scan data. This scan
data is then brought up to the project office, where
a powerful desktop computer analyses the data in a
georeferenced coordinate system. This point cloud data
set often requires some manual processing to correct
for measurement errors and to correlate with the design
data. The entire process is required before producing a
report that can be used for analysis and then feedback
to the construction crew. This process typically takes
between 2 to 4 hours per station. Hence, the use of
such technology in civil constructions are limited.
Since 2016 the rapid adoption of high-speed
embedded computing platforms, like FieldProgrammable Gate Array (FPGA) and Graphics
Processing Unit (GPU) processor cores for embedded
systems, advanced data processing has become
prevalent. These technologies allow battery-powered
devices to achieve computing performance of one trillion
floating-point operations per second (1teraFLOPS). Part
of this rapid adoption is due to the global development
of algorithms and processor cores for machine learning
platforms and real-time autonomous vehicle projects.
Fig. 1: BIM Construction Workflow
Fig. 2: Xilinx FPGA Architecture
26 Shotcrete | Spring 2020 www.shotcrete.org
In the above comparison, the ability to analyze and
report the desired construction results automatically and
in minutes has the potential to significantly change SCL
construction processes.
GEOTECHNICAL MONITORING LIDAR
(GML) TECHNOLOGY
The GML technology was designed and developed by
GroundProbe, a mining technology company that supplies slope stability monitoring radar systems such as
the SSR-XT for the mining industry. Figure 4 shows the
GML system as a complete standalone battery-operated
LiDAR solution, that was designed to be a one person
operation. This technology is equipped with an onboard
high-speed computing device and signal processing
software, that can produce high density point cloud
information in real-time.
MEASUREMENT ACCURACY
The first proof of suitability for the new technology was
to verify the measurement accuracy for shotcrete thickness measurements, against the existing total station
pick up by survey control.
The GML was deployed in various control environments in a tunnel project to verify the thickness measurements. The first method was to compare the results
of existing shotcrete thickness reports produced by
the survey pick-ups against the thickness reported by
the GML scanner. In this process, the GML was setup
next to the total station during conventional pickup to
scan the excavated sections before any shotcrete was
installed. Upon installation of the bolts and shotcrete,
both the GML and the total station were redeployed to
complete the final as-placed scan. These results were
tabulated in typical profile section views as per Figure 5.
Fig. 3: LiDAR vs In-situ LiDAR Workflow Comparison
In Figure 5, the GML results are in blue and the
results of the total station survey are in pink. Results can
be seen in all four scans, the GML results were almost
identical to the total station measurements.
In another verification test, core samples were drilled
to check the thickness against the GML measurements.
In this test, the GML was deployed to scan the section
before any shotcrete was installed. Once the shotcrete
had been placed and cured the drill rig was deployed to
drill four test holes, where the depth of the cores were
measured. These four holes were marked on the tunnel
surface to allow the user to locate the holes in the GML
data.
Figure 6 shows the drill results in the GML SSR-Viewer software. The image was produced by the GML data
and clearly shows the marked holes. For each marked
hole a group of points were selected, creating the annotated figures shown in the figure. An average thickness
measurement was computed for each of the annotated
group of points and displayed in the charts.
Fig. 5: GML & Total Station Comparison Results
Fig. 4: Geotechnical Monitoring LiDAR (GML)
www.shotcrete.org Spring 2020 | Shotcrete 27
CHECK AGAINST DESIGN PROFILE
When operating in Profile Mode, the GML can import
BIM CAD models into the device and automatically
calculate the deviations against the design model. This
allowed the construction crew to have real-time feedback of their work while in the tunnel. Figure 7 shows
an example of the software operating in Construction
Guidance Mode in a tunnel excavation operation.
In Figure 7, the grey point cloud is the scan data and
it overlays the different design profile data, shown in purple and blue. The software automatically calculates the
Fig. 6: GML & Drill Test Comparison Results
Fig. 7: GML Profile Mode
Table 1: GML & Drill Test Comparison Results Table
Table 1 shows the core sample measurements,
against the GML measured results, for the placed
shotcrete thickness. The GML thickness measurements
were accurate to within 1-2 mm (0.04 to 0.08 in.). However, there was a large discrepancy with the results for
Fig 10373RH, as notated in the table below. This was
subsequently investigated, and it was found the core
samples had not been measured correctly.
Figure Name Drilled Results GML Results
Fig 10373 C 110mm 109.3mm
Fig 10372 75mm 75.4mm
Fig 10372 RH 120mm 118.2mm
Fig 10373 RH 100mm 52mm
distance (in millimeters) to and from the selected profile
lines, to produce the deviations in a hot-cold heat map.
LIVE SHOTCRETE SPRAY GUIDANCE
The following case study was based on data from an
Australian tunneling project in 2018. The project used
GML for managing the shotcrete thickness for the primary lining ground support in a road header cut tunnel.
The typical construction issue faced in this project
was the amount of shotcrete being ordered for each cut.
Quantities were based on a calculated estimation, often
with large amounts of excess material being ordered
to accommodate rebound and the spraying skill of the
nozzlemen. During the spraying stage, the nozzlemen
have depth pins to gauge the spray thickness, hence
the final sprayed thickness varies widely depending on
the skill of the nozzleman.
In this project, the GML was used to provide in-situ
feedback to the nozzlemen to guide them to spray to
the required design thickness. Since the GML was introduced in a later stage of the project it was challenging
to change the operating procedure. This required
significant planning to be able to train and guide the
nozzlemen to spray to the correct thickness. Using
the GML system helped to reduce the overall shotcrete
usage for the project.
In the early stages of implementation, the GML was
used to characterize the quality of shotcrete application
by the different nozzlemen. Figure 8 shows the typical
spray quality before use of the GML to guide the
nozzlemen. The shotcrete thickness in the images were
represented with red for areas under design thickness,
purple for areas over design thickness and green for
areas with the desired design thickness. As illustrated
in in Figure 8a, the sprayer was able to cover the bolts
correctly but left large areas under sprayed between
the bolts. Figure 8b, shows the opposite. Often the
nozzlemen would overspray the entire area just to
ensure there were no under spray areas resulting in
using significantly more shotcrete than necessary.
28 Shotcrete | Spring 2020 www.shotcrete.org
The GML was used in the project for a total of eight
months and after the first month using the GML the project was able to reduce the shotcrete material orders by
30% for the remaining seven months of operation.
SHOTCRETE FINAL LINING
CONSTRUCTION
The following case study was based on data from another Australian tunneling project between March and May
of 2019. The project utilized GML for controlling the shotcrete spraying process for the tunnel-wide, final lining, to
reduce or eliminate rework due to shotcrete not meeting
the required minimum thickness.
In this final lining shotcrete application, shotcrete was
sprayed continuously between cross passage (CP) to
cross passage, completing a 390 ft (120 m) section at a
time. This required the shotcrete rig and crew to move
13 to 20 ft (4 to 6 m) each time, to complete shotcreting
the entire section. Given the design requirements for
thickness and the tunnel design profile, depth pins and
string lines were installed as guides prior to the spraying
of the final shotcrete lining. The string lines were installed
by a crew of two operators, one surveyor and the use of
Mobile Elevated Work Platform (MEWP). The installation
took the crews approximately two shifts to complete
each section. Figure 11 shows a tunnel section that has
the depth-pins and string-lines installed.
Fig. 8: Detection of overspray and under spray of shotcrete
During the first four weeks of the project, after buy-in
from the site engineers, foremen and nozzlemen, the
project started seeing improvement in the spray quality.
The sprayer was able to use the GML guidance to cover
up the thin spots as seen in Figure 8b. However, there
were still some amount of overspray. Figure 9 shows an
example where the nozzlemen was able to detect thin
areas and rectify them immediately using the GML.
Shortly after the first month, the majority of the nozzlemen were able to use the GML to guide their shotcrete placement to reduce the amount of over spray.
Figure 10 shows the reduction of overspray areas. In
this example, the nozzlemen were able to reduce 33%
of shotcrete usage by using the GML. More importantly,
this was achieved within two weeks.
Fig. 9: Nozzlemen guided to fix up thin spots in shotcrete application
Fig. 10: Reduction of shotcrete usage
www.shotcrete.org Spring 2020 | Shotcrete 29
Fig. 11: Depth-pin and string line installation in tunnel section
Fig. 12: GML scan showing under spray sections in Red
Fig. 13: GML Scan Color threshold for spray thickness
Prior to the use of GML, the nozzlemen were using
the string lines as a guide to spray the desired thickness
and profile. Once the spray was completed, the section
was surveyed to verify the shotcrete placement against
the design profile and required thickness. The project
ran for months and found there were too many thin
spots that required rework, despite the installation of
depth pins and string lines. The GML was then deployed
simply as a verification tool to capture the construction
baseline. Figure 12 shows a typical rework issue on the
project.
Figure 12 shows thin spots represented in the hot
color palette (red) and over sprayed sections in the cold
color palette (purple). The thin spots were spread across
the entire sections requiring significant amounts of
rework and verification by dedicated rework crews.
GML DEPLOYMENT CHALLENGES
The first key question regarding deployment of the
technology was whether it can operate in cycle, particularly during the SCL stage of the tunnel construction
process, to monitor spray conformance.
During spray conformance monitoring, the GML is
typically deployed next to the front stabilization jack
of the shotcrete rig and remains in place during the
entire spray sequence. This position allows a wide scan
area to be captured with minimal obstruction from the
shotcrete rig. The GML completes a baseline scan
under two minutes before the shotcrete operator starts
to spray. Once the shotcrete operator is satisfied with
the first pass, the boom is lowered, and a second scan
is captured. As shown in Figure 13, the results are then
presented to the shotcrete operator on a tablet to indicate any areas that have not reached the required thickness. The scanner operator then uses a laser pointer
or cap lamp to guide the shotcrete operator to respray
the thin areas. Once both operators are satisfied, a final
scan is taken to confirm the results.
The other key issue was the nature of the final lining
spraying process, were the shotcrete rig and crew
needed to advance at every 13 to 20 ft section. This
required the GML to be relocated with the rig to operate
30 Shotcrete | Spring 2020 www.shotcrete.org
in cycle. The GroundProbe team worked closely with
the shotcrete crew along with engineers to develop a
shotcreting sequence that allows the GML to operate in
cycle. Leveraging the rapid processing capabilities of the
GML, the system was able to operate without delay for
the majority of the construction.
Given the support of the engineering and final lining
team, and lots of teamwork, the project was able to
begin to achieve the desired results within the second
week of deployment. Figure 14 shows the desired spray
result. In Figure 14, it can clearly be seen that there were
no thin spots and the reduction of over sprayed sections
was significant.
This was a considerable improvement on the project
and the technology was introduced to cover more areas
of final lining construction within the project. Inside the
initial two months of deployment, the final lining teams
were able to complete 2.6 miles (4.2km) of final lining
construction without rework, significantly reducing the
amount of shotcrete material used for the project.
CONCLUSION
The rapid progression of powerful embedded computing
and LiDAR technology enables the development of near
real-time in-situ scanning solutions. These advances
allowed application of the new technology into SCL
construction operations and enabled modifying current
practices to achieve improved safety, quality and cost
savings.
The rapid advancements in computing technology is
certainly a key enabler, allowing technology designers to
develop tools that could significantly evolve the mining
and construction industry. However, the success of such
technology relies heavily on the people operating in the
industry. Our experience shows that the success of the
case studies referenced in this paper, depended heavily
on the engagement of the engineering and construction
crews, especially the nozzlemen. One of the biggest
challenges faced was the management of change in the
construction processes. Effective communication and a
collaborative approach, to reach a desired solution, was
critical to the success of introducing the changes. Leveraging the experience of the foreman and nozzlemen also
was a key element in the development of the technology
and the construction process.
Fig. 14: GML guided spray with zero non-compliant thin spots
Finally, project management teams also play an
important role in fostering the development of innovative technologies by adopting them at an early stage of
a project. This is critical for any emerging technology
to achieve the desired potential, and to incrementally
change the current state of the art.
ACKNOWLEDGEMENT
We would like to thank the excavation and final lining
teams mentioned in this article, especially the nozzlemen
and site engineers for their contribution to the technology.
We would also like to thank the monitoring team from
21MT, led by Christian Reich, for their highly skilled operators that helped to train and integrate the GML technologies into the construction projects.
References
1. Chen, B.; Mares, D.; Carter, N.; Ayres, P.; Voysey, G., 2020,
“Leveraging Super-Computing and High-Density Lidar Technologies
for Real-Time Verification and Rectification of Tunnel Construction
Against Design,” ITA–AITES World Tunnel Congress AND 46th
General Assembly.
2. Bergmann, N. W., 2005, FPGA-Based reconfigurable system-onchip. Very Large-Scale Integration System-on-Chip (VLSI-SoC2005),
Perth, Australia, 17-19 October, 2005. Perth, Australia: International
Federation of Information.
3. D. Lv, X. Ying, Y. Cui, J. Song, K. Qian and M. Li, “Research on
the technology of LIDAR data processing,” 2017 First International
Conference on Electronics Instrumentation & Information Systems
(EIIS), Harbin, 2017, pp. 1-5. doi: 10.1109/EIIS.2017.8298694.
4. Processing. Altameemi, A. A., & Bergmann, N. W. (2016).
Enhancing FPGA softcore processors for digital signal processing
applications. 2016 Sixth International Symposium on Embedded
Computing and System Design (ISED), 294–298.
https://doi.org/10.1109/ISED.2016.7977100
5. Zynq-mp-core-dual.png (800×900). (n.d.). Retrieved 18 September 2019, from https://www.xilinx.com/content/dam/xilinx/imgs/
block-diagrams/zynq-mp-core-dual.png.
www.shotcrete.org Spring 2020 | Shotcrete 31
HULL .125 AD HORIZONTAL PLACEHOLDER.
Did not see in the folder.
For the last 11 years, Ben Chen has played a lead technical and commercial role in delivering the
technology roadmap and his team was able to deliver an array of technologies to significantly diversify and grow the company’s brand, market share and revenue. More recently, his team has delivered
the Geotechnical Monitoring Lidar (GML) technology that won the Financial Review 2018 Most Innovative Products and Most Innovative Company awards. In the same year they also delivered the Geotechical Monitoring Station (GMS) technology that won the 2018 Good Product Design Awards.
Peter Ayres is the Lead – Tunnelling Solutions for GroundProbe and a former Technical Services Manager for
Orica. Currently he is working with the GroundProbe’s Product Development team in the development and
implementation of the GML system globally to both Mining and Civil industry. Over the past 12 years, he has
worked as a Tunnel Designer with Arup in New York, USA, followed by 6 years with Leighton Contractors (Asia)
Ltd. in Hong Kong as a Tunnelling Engineer and Blasting Engineer. Projects have included the 7 Line Extension, NY; West Kowloon Terminus & XRL822, HK; Harbour Area Treatment Scheme, HK; and the Tseung Kwan
O – Lam Tin Tunnel, HK. Peter has an M.Eng. in Mining Engineer from Camborne School of Mines – University
of Exeter and is currently studying for an LLM in Construction Law and Arbitration at Robert Gordon University.
Christian Reich is the Founder and Managing Director of 21MT, an Australian company committed
to implementing innovative technologies and solutions in the mining and tunneling industries. Since
2018, Christian has led 21MT in developing extensive experience using Groundprobe’s real-time GML
scan technology in Australian tunneling projects and has established a proven track record of efficiency improvements by integrating real-time scanning into the excavation cycle. Previously Christian
worked at Atlas Copco in Germany as the Product Manager for underground rock excavation equipment.
He graduated from Technische Universität Clausthal in mining engineering and business studies.
Nick Carter is the Lead – Technical Solutions at GroundProbe and has been involved in the innovation and
development of GroundProbe’s emerging technologies since 2011. He has travelled to the farthest and deepest
expanses of the earth to provide mining and civil markets with these technologies and globalized understanding
of the application requirements. He currently works on the Geotechnical Monitoring Lidar (GML) technology
with a small, core team of talented people who regularly solve challenging problems with creative solutions.
The GML has received innovation awards from the Australian Financial Review and most recently won the
Technology Transfer Award at the 2019 Institution of Engineering Technology (IET) Innovation Awards in London.

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)
www.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 www.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.
www.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.
www.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 www.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
www.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 www.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.

My name is Brian Lywandowsky and I work for a large concrete construction company in the San Francisco Bay Area. I’ve worked in the concrete pumping business since I was sixteen. Since then, I have owned a small pumping business
with my father and in 2007, I moved onto the company where I work today. I currently manage the Concrete Pumping and Lightweight Cellular Concrete divisions.
In my years as an owner-operator of boom and line
pumps, I had some experience with shotcrete, with the
majority of it being in the pool industry. Most of the shotcrete
work I witness now is commercial work in the Bay
Area and Northern California that consists of perimeter
walls in subterranean parking garages, large shear walls,
columns, retaining walls, sculpted walls, and repairs on
bridges, just to name a few.
After spending time with the shotcrete crews in the
field, I was able to learn more about the industry from a
different point of view than just the pumping side of the
business. I was very impressed with how well the
shotcrete crews would perform their jobs on a day-today
basis. The crews would often arrive on a job that
was not set up or properly prepared. Many times, they
needed to set up the delivery line comprised of steel
pipe and rubber hose using unique techniques and with
a limited amount of time. Once the job was set up and
they could begin shooting, the crew would face less
than ideal conditions, such as improper tied reinforcing
bars, inadequately supported forms, and other obstacles.
This made it tough to shoot properly, not to mention
having to remove rebound from the work area to a
designated disposal location. However, the crews would
just put their heads down and go to work, producing a
fantastic end product.
The shotcrete crews have also done a great job
understanding what it takes to get the most out of the
pumping equipment. They’ve figured out that using
steel pipe on any pour that required more than 100 ft
(30 m) of delivery line would keep pumping pressures to
a minimum, create less breakdowns of the equipment,
and help get more volume of concrete placed per hour.
Using steel pipe from the back of the pump and limiting
the rubber hose is a huge help to the overall daily
production. It also helps eliminate premature wear of
the outside of the rubber hoses caused by the sliding
or sawing action produced by long runs and high line
pressure in a rubber-only delivery system.
Using the concepts I learned while owning and operating
concrete boom pumps, my experience could help
crews optimize the set up and cleanup of the pumping
system portions of the job. When starting a new project
with difficult conditions, we now show up with a truck
either a day or two before the job begins with all the
necessary pumping components. We’ll go ahead and
install the delivery system so we are ready to shotcrete
on shoot day. We designed specialized mobile parts that
clamp to solider beams, standpipe 90-degree elbows,
and brackets that can be bolted or welded to the existing
buildings or structures.
COMPRESSED AIR METHOD
When on the job, I’ve seen many crews cleaning both
rubber and steel delivery systems with compressed air.
I believe this is the norm in the industry. The blowout
process crews used had likely evolved over time after
many near misses and hose whippings. Crews would
start by using air with no type of dart or plug to force the
concrete out of the system. This would serve to limit, but
not prevent, the amount of hose whippings at the end of
Fig. 1: Long vertical run of shotcrete line, “a shotcrete standpipe”
www.shotcrete.org Spring 2020 | Shotcrete 21
Fig. 2: Steel delivery line exiting pump
the line caused by the compressed air moving through
the line. The pump operator would unhook the system
from the pump and then use a blowout cap to push air
through the system, starting at the pump and blowing
the concrete out the end of the hose. Typically, the largest
guy on the crew would try to control the hose end
as the air was being applied to the delivery line being
cleaned out. He would hold the hose for dear life and
get a series of small but somewhat controlled explosions
of concrete at the end of the rubber hose. This went on
until more air than concrete was being expelled from
the end hose indicating the majority of the concrete was
cleared out of the system.
In the next step, the pump operator removed the
blowout cap and placed a small sponge in the line and
reattached the blowout cap to the pump end of the
hose. Once again, the man on the discharge end would
kneel on the ground with the hose running between his
legs or alternately trying to tie down the hose in some
way. He would then signal the operator to start the air
flow and hold on with a grip that was second to none.
By leaving a minimal amount of the concrete in the system,
it becomes much more difficult to be able to judge
where the sponge is within the system and how fast it
is traveling. As it travels closer to the end hose, it can
create hose whipping.
The use of compressed air to clear concrete from
the system is extremely dangerous. However, with the
proper training and correct parts, this process can be
safe, fast, and clean. The American Concrete Pumping
Association (ACPA) has created rules for cleaning pipes
with compressed air, that I believe need to be implemented
into the American Shotcrete Association (ASA)
safety guidelines. The shotcrete system is different
from placing boom type work when it comes to using
diverter valves and designated pumping stations. While
shotcrete locations typically change from day to day and
are not typically using a diverter valve, the rest of the
ACPA rules still apply.
Be extremely careful when using compressed air
to clean out the placing line.
1. Cleaning with air requires two trained people.
2. No person is allowed to be near the discharge
end of delivery line.
3. A dart catcher must be used and the outlet must
be controlled.
4. A proper blowout cap must be used.
5. The discharge end of the delivery line should
be in a position to permit easy discharge of
concrete.
6. The dart or plug used must not be able to let
compressed air pass by and into the concrete.
7. No rubber hose can be cleaned with air unless
using an attachment specifically used for
clean-out into a designated box or mixer truck.
8. Work on the delivery line is allowed only after line
has been relieved of compressed air.
9. A good, reliable method of communication
between the operator and crew at the end of the
delivery line is needed.
10. All PPE must be worn when cleaning out the
delivery line, including gloves, safety goggles,
ear plugs, respirator, long sleeve shirt, work
boots, and vest.
22 Shotcrete | Spring 2020 www.shotcrete.org
Compressed air can only be used to clean out a
steel delivery line. It must never be used on rubber hose
as the hose whipping effect at the discharge end can
be extremely dangerous. When using compressed air,
one must be able to control and catch the object that
is used to clear the line. It is also very important to only
have trained people doing the cleanout. A blowout cap
must have the proper distance between the air inlet and
dump valve. The catcher must be properly sized to not
allow the dart to escape but allow the exiting concrete
to easily flow through it. A proper plug or dart must be
used to push concrete and it must not let air bypass
directly into the concrete. Allowing air to bypass can
create a blockage by separating the concrete.
When the shotcrete placement is finished, the trained
crew members will remove the rubber hose and connect
a dart catcher to the end of the steel line. Once this is
done, the operator will breach the line at the pump and
insert a rubber dart on the pump end of the line. The
operator and designated crew member must be able
to communicate, generally by radio. The pump operator
will begin to insert air into the delivery line and once
concrete begins to move, he will begin to control the
amount of air being added to the system. As the concrete
begins to move and clear the line, it will take less
pressure to move the concrete. The existing air in the
system will begin to decompress, accelerating the plug
or dart. It is important to feather the air into the system
and open the dump valve at the blowout cap to relieve
air and keep a slow and steady flow through the delivery
line. It is important to have a trained crew member communicate
with the operator when the dart is speeding
up, and how close the dart is from exiting the system to
keep a controlled blowout.
Cleaning the rubber hose can be done very easily
by using a garden hose with 50 psi of water pressure.
Once the rubber hose has been disconnected from the
steel delivery line, a clump of wet paper is forced into
the hose. A water cap is clamped onto the hose and a
standard water hose is hooked up and used to clear the
concrete from the hose. The hose is cleared when the
paper exits the other end of the hose.
WATER WASHOUT
The water washout is by far the safest and most practical
means to clean both the steel and rubber delivery
lines. By using water, one has a material that doesn’t
compress and have the potential for an explosive discharge
that has unfortunately become the norm in the
shotcrete industry.
Water cleanout only works when one has access
to a good water supply from a high flow water source
such as a fire hydrant, water buffalo, or water truck. At
the end of the shooting, the operator cleans the hopper
and valve by doing a quick washout of the pump and
inserts a plug or dart into the delivery line at the back of
the pump. They should then fill the hopper and, once
full, begin to pump water through the system until the
plug or dart is pumped out of the end of the system.
The water method is far safer than using compressed
air because it eliminates the potential for violent hose
whippings. Concrete pumps are capable of producing
Fig. 3: Shotcrete standpipe clamp and support
Fig. 4: Blowout cap Fig. 5: Dart catcher
www.shotcrete.org Spring 2020 | Shotcrete 23
much greater line pressure than even high-pressure air
compressors. However, the challenges are often the
availability of water and a place to put the water once
the system has been cleared.
SUMMARY
Either one of these cleanout processes when properly
executed will decrease cleanup time, create a cleaner
steel and rubber system, and make priming the system
for the next day’s shooting much more successful. The
safety aspect is the most important consideration for
using proper techniques in the cleaning of the delivery
line. Using trained crew members and proven techniques
will keep everybody safe and give them the best
possible process so they can safely and efficiently place
shotcrete each and every day they go to work.
Brian Lywandowsky has been in the
concrete pumping business for 31 years. He
started out as a partner at Eagle Concrete
Pumping with his parents. The business was
small and started out with two ball valve
pea gravel pumps then steadily grew the
company into a fleet of five boom pumps
and four-line pumps. In 2007, Lywandowsky
sold Eagle Concrete Pumping to The Conco Companies
and went to work for Conco as an Area Manager in Redding,
CA. After seven years with Conco in Redding, Lywandowsky
made the move to the Bay Area and has worked his way to
now managing the Northern California pumping operation. In
addition to concrete pumping, Lywandowsky provides support
and leadership in the development and production of a
new Lightweight Cellular Concrete operation known as Con-
Foam along with his work in Conco’s shotcrete business.
Fig. 6: Steel line transitioning into rubber hose for final discharge Fig. 7: Shotcrete standpipe

CanCrete Equipment Ltd. (CanCrete) is a Canadian family-run business based out of Mississauga, ON, Canada, with a long history of supporting the shotcrete industry in the Greater Toronto Area. CanCrete focuses on anything that is required to move cementitious material from point A to point B, including concrete, shotcrete mixtures, grout, self-leveling, fireproofing, and epoxy. The company serves customers in markets from small-line pumping to high-rise placing equipment with equipment sales and rentals, parts and accessories sales, technical and engineering support, and equipment servicing. CanCrete has the equipment that contractors need to pump or spray mixtures, such as concrete, epoxy coatings, fireproofing and insulation materials, and shotcrete. For these industries, CanCrete stocks hoses, pipes, clamps, elbows, reducers, reducing elbows, sponge balls, concrete cleaners, slick pack, pole guns, and much more at the Toronto warehouse. For customers preferring Original Equipment Manufacturer (OEM) parts, those are readily available for a variety of manufacturers.

Revolution Gunite provides services in the Southeast using the dry-mix shotcrete process for the swimming pool industry as well as for infrastructure and architectural work. Its current service area includes North Carolina, South Carolina, Virginia, West Virginia, and eastern Tennessee.

Since the shotcrete process originated well over 100 years ago, improvements in materials, equipment, and placement techniques have enabled it to become a well-proven method for structural concrete placement. The efficiency and flexibility of shotcrete have been used to great advantage in sizable structural projects, as the high-velocity impact inherent in the process provides the compaction needed to turn low-slump concrete into freestanding vertical and overhead placements with minimal formwork.

The recent Position Statement #2, “Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes,” published by the ASA Underground Committee, pointed out many aspects critical to successful performance and raised some potential issues affecting the placement.1 In the position statement, specific techniques are presented to prevent problems such as delamination, voids, or fallouts. In the discussion, the potential issue of steel fiber-reinforced shotcrete (FRS) causing damage and potentially puncturing the membrane was raised. From the experience of the committee and the available information, it was concluded that: • The forces acting on the fiber are not strong enough to push the fiber into the membrane; and • The fibers tend to orient parallel to the membrane on impact, thus reducing the risk of damage. In parallel, a research project on this subject had been undertaken at Université Laval’s Shotcrete Laboratory, with the results only recently available. This article presents the results of this investigation.2 It is intended to support ideas presented in the ASA position paper and to help in the decision-making process when dealing with waterproofing membranes and FRS in underground projects.

The addition of fibers to concrete and mortars as reinforcement is not a new concept. The ancient Egyptians used straw to reinforce mud bricks for use in structures like the core walls of the pyramids. During the first century AD, the Romans incorporated horsehair fibers in the construction of structures like the Coliseum to help prevent drying shrinkage cracking of the concrete. In the modern era, the first scientific studies on the use of steel fibers to reinforce concrete date back to the 1960s and 1970s.1,2 The use of steel fiber-reinforced shotcrete (FRS) was first introduced in the 1970s.3 The first documented use of FRS was in 1973 by the U.S. Army Corps of Engineers in a tunnel adit project at the Ryrie Reservoir in Idaho. Soon thereafter it became well recognized that soil and rock excavations could effectively be stabilized with steel FRS and its use and acceptance increased globally. In the mid-1990s, the use of macrosynthetic fibers in shotcrete was developed and has increased with particular success in temporary support in underground mines where large deformation capacity is desired. Since the 1970s, thousands of projects have been successfully completed using fibers as reinforcement, including shotcrete, slabson-ground, composite steel decks, slabs-on-pile, and precast elements.

The Shotcrete Inspector Certification program began out of the need in the industry to have knowledgeable individuals inspecting shotcrete projects. Shotcrete contractors often find themselves having to educate project inspectors on the details needed for quality shotcrete placement. Shotcrete is a superior placement option in many cases, yet unfortunately, the expertise in evaluating the work is often non-existent. Many contractors and specifiers are faced with situations on jobsites where the lack of shotcrete-specific knowledge from the inspector has either limited the use of shotcrete or created potential litigious situations. The industry needs a knowledgeable on-site inspector, not necessarily an expert in contract documents or even a laboratory tester. As is the case with form-and-pour concrete, an inspector who has demonstrated basic shotcrete knowledge will

Shotcrete is a placement method for concrete. Even though the shotcrete process is over 100 years old, it constantly evolves and today, it is a modern, costeffective, and more sustainable method of concrete placement. Those of us who regularly shotcrete a variety of work will at times have inspections and therefore have to work with inspectors. Understanding the inspector’s role as a safeguard or agent for the owner is important. However, we often find dealing with inspectors can be a cumbersome part of our job. Knowing how to do something properly and being able to fully explain the means and methods of a specialized process (shotcrete) to someone who has little to no firsthand experience are two completely different animals

Frequently, shotcrete contractors are required to shoot test panels at the beginning as well as throughout projects. Several tests are performed from these panels; however, compressive strength testing is the most common. This article will primarily address compressive strength testing. At some point, every contractor has missed a required break strength on a core extracted from one of their panels. Frequently, those bad results are not caused by bad material or poor workmanship. They are caused by poor test panel handling and improperly performed test standards. Often, the causes of those bad test results could have been easily avoided. Let’s discuss some of the more common test panel mistakes witnessed in the field