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
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. 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.
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
• Location C – located in the shotcrete shooting
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
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
(7 Days)s
Chloride Ion
(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 Spring 2020 | Shotcrete 13
Fig. 10: Dust Reduction Additive No.2.
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.
Strength ASTM C
1604 (7 Days)
Strength ASTM C
1604 (28 Days)
Age of
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
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
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
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,
8.“DustTrak II Aerosol Monitor 8530,” TSI Incorporated, Shoreview, MN, 2020,
dust-monitors/dusttrak-ii-aerosol-monitor-8530/. (last accessed May
19, 2020)
9. “DustView II,” Palas, Germany, 2020,
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.