The replacement of silica fume with hemp flour in shotcrete applications

The replacement of silica fume with hemp flour
in shotcrete applications
Victoria Munro, EIT (1,a), Kris J. Dick, Ph.D., P.Eng (2,b)
1
Department of Biosystems Engineering, University of Manitoba, Canada
2
Associate Professor, Department of Biosystems Engineering,
University of Manitoba, Canada
[email protected]
[email protected]
Keywords: hemp, hemp flour, shotcrete
Abstract: In order to determine the effectiveness of hemp flour in shotcrete, six different shotcrete
mixtures containing hemp were created and underwent initial testing. A control mixture containing
silica fume was used for comparison purposes. Two different hemp flours of different particle size
distributions were utilized. Three different hemp flour doses were used for each hemp flour type
(six total doses).
Testing was performed to determine the density, air content, slump, and compressive
strength (three, seven, and 28-days) of each mixture. Following initial testing, the optimal hemp
dosage and type was selected by comparing the results with that of the control mixture. Both the
control and optimal hemp mixtures were then shot using conventional shotcrete technique, and final
compressive strengths of the both mixtures were then found.
Tests revealed that the hemp-shotcrete had significantly higher air content than the control
silica fume-containing shotcrete. The hemp mixtures also had lower densities. The air content
increased with the hemp dosage, while the compressive strength decreased with the hemp content.
The optimal hemp mixture contained 0.25 kg of hemp per 50-kilogram mixture, with a compressive
strength of 15.65 MPa after 28 days compared to the control mixture strength of 43.70 MPa.
The final compressive strength of the hemp mixture after 28 days was 36.25 MPa, while the
final compressive strength of the silica fume shotcrete was 48.70 MPa. However, the hempshotcrete had a higher continuous strength, increasing 76.4% over 21 days. The hemp-shotcrete
required a longer curing time than the silica fume shotcrete.
1 Introduction
1.1 Problem Scope
Silica fume, used primarily as a cohesive agent in shotcrete, is produced using high-emission
industrial furnaces, while hemp is an organically grown local crop. Several concrete and shotcrete
manufacturers have raised concerns with the price of silica fume, which is considerably higher than
that of Portland cement. The cost of silica fume is approximately $0.50 per kilogram, while hemp
hurd can be purchased in Manitoba for $0.46 per kilogram or less. Hemp is an environmentally
friendly concrete admixture, while remaining at a cost lower than that of silica fume.
1.2 Purpose and Objective
The purpose of the study is to replace silica fume in shotcrete with finely ground hemp flour in
order to eliminate the negative environmental and economic impact of silica fume production in the
shotcrete industry. Hemp is a highly absorbent organic material and this study focused on its
efficiency to act as a cohesive agent and mimic the role of silica fume in shotcrete.
1.3 Measured Parameters
The main aspects this study focused on were: density, air content, slump, and compressive
strength. During initial testing, these parameters were measured immediately after sufficiently
mixing the wet and dry components of each 50-kilogram batch. The compressive strength of all six
hemp mixtures and the control mixture was found after three, seven, and 28-days.
Density is the ratio of a substance’s mass to its total volume. It can be used to predict air
content, as less dense materials indicate high air content. Air content is a measure of the air existing
within a substance and is measured as a percentage. Air content is typically related to shotcrete
strength; high air content will result in low compressive strength.
Compressive strength is the capacity of material to withstand an applied force before
experiencing failure, usually seen in concrete in the form of cracking. The seven and 28-day
strength was measured during final testing for both the optimal hemp-shotcrete and the
conventional silica fume shotcrete.
Slump is used to measure the consistency of shotcrete and is performed using a slump cone.
2. Review of Literature
2.1 Shotcrete
Shotcrete is the application of concrete or mortar using a nozzle and high-pressure air
compressor in order to “shoot” or spray the concrete onto a surface. Shotcrete is used underground
or in areas that are difficult to implement cast-in-place concrete application and can be done using
either a “wet” or “dry-mix”. Beaupré [2] describes shotcrete as “a special process used to place and
compact cementitious materials” most often used for swimming pools, canal and tunnel linings,
rock and slope stabilization, corrosion protection, structural repairs, and underground and surface
construction. It is typically composed of four main elements: cement, water, silica fume, and
fine/coarse aggregates [3].
The two most common types of processes for shotcrete are wet-mix and dry-mix [2]. In drymix shotcrete, silica fume is introduced in super sacks with cement, aggregates, and fibers. The
cement and aggregates are batched at the plant while the silica fume and fibers are batched in transit
and mixed on-site. Water is introduced at the nozzle in dry-mix shotcrete. In wet-mix shotcrete, the
silica fume is bulk batched at the plant with water, cement admixtures, and aggregates. It can also
be introduced in slurry form at the nozzle [7].
Air content in fresh wet-mix shotcrete ranges from 3.2 – 5.0%, while the hardened air
content can be in the range of 3.3 – 8.7% [2]. It is possible to produce shotcrete that possesses a
compressive strength of up to 60 MPa using water-reducers and superplastticizers [3].
2.2 Silica Fume
Silica fume is produced during the smelting of silicon and ferrisilicon by collecting the
vapors using large filters in a baghouse [8]. Silica fume is also referred to as micro silica, condensed
silica fume, volatized silica, and silica dust and demonstrates both pozzolanic and cementitious
properties. It is composed primarily of pure silica in non-crystalline form, has a high content of
amorphous silicon dioxide, and the particles are spherical in shape. Silica fume usually contains
more than 90% silicon dioxide with small amounts of iron, magnesium, and alkali oxides [8].
Ferrisilicon and silicon metals are manufactured using electric furnaces, and thus emit high
quantities of carbon dioxide (CO2), a greenhouse gas. Silica fume is the smoke created by the
furnace operation and is collected for use in concrete. It was estimated in 2007 that ferrisilicon
production emitted 10,500 ktCO2 globally, and silicon metal production emitted 3,500 ktCO2
globally [4]. Additionally, silica fume increases the water demand due to its high specific surface.
Morgan and Wolsiefer [7] also found silica fume increased the water demand in shotcrete compared
to a purely Portland cement shotcrete in both wet and dry-mix shotcrete. Silica fume’s high CO2
emissions and high water demand make it an unsustainable product.
Silica fume is far less cost-effective than Portland cement. In 1994, Denis Beaupré reported
that the cost of silica fume was four times the cost of Portland cement[2]. Today silica fume can be
purchased in Manitoba for approximately $0.50 per kilogram, compared to $0.32 per kilogram or
less for Portland cement.
2.3 Structural Properties of Silica Fume
Silica fume is utilized in shotcrete to improve compressive and flexural strength, reduction
of rebound loss in dry-mix shotcrete (up to 50%), achievement of high early strength, synergistic
improvements in crack control, impact resistance, toughness, higher bonding strength, higher
cohesion (resist washout), increased freeze/thaw durability produced by lower permeability,
enhanced resistance to chemical attack, high electrical resistivity, and low permeability to help
resist corrosion of embedded steel [7]. It is a highly pozzolanic mineral admixture used primarily to
improve durability, strength, and to replace Portland cement. In Canada, it is common practice to
replace 7.5 – 12% cement by mass with silica fume [2].
Silica fume has an as-produced bulk density ranging from 130 – 430 kg/m3 and a specific
gravity value of 2.22 [8]. Ninety-five percent of silica fume particles are finer than one micrometer
and have very high surface areas [8]. Morgan and Wolsiefer [7] found that wet-mix silica fume
shotcrete has a higher average compressive strength than dry-mix; however, the flexural strength of
the dry-mix shotcrete is higher than that of the wet-mix.
2.4 Hemp
Hemp is an organic material that is both economically and environmentally sustainable.
Hemp does not require pesticides or herbicides to grow [10] and is typically produced as fibers or
hurds. Both the hurds and fibres are currently utilized in precast concrete blocks and panels as
insulating materials, known as “hempcrete”. Hemp is grown locally on farms in Manitoba, thus
reducing or eliminating transportation emissions. Hemp’s carbon footprint is much lower than that
of silica fume.
Hemp fibres have a high tensile strength, which increases with compaction [1]. Hempcrete
is able to undergo significant deformation before failure, reaching deformation values of above 50%
before collapse [1]. In addition to being extremely ductile, hemp has a high thermal conductivity,
ranging from 0.13 – 0.19 W/m˚C [5]. Similar to silica fume, hemp is an extremely absorbent
material. The hurds can absorb three to four times their weight in water within five minutes (Dick
and Pinkos, 2014).
The form of hemp used in this study is hemp flour, collected as a byproduct from hemp hurd
production. The hemp dust generated in hurd processing was collected and bagged for use in the
experimental shotcrete.
3 Materials and Methods
All experimental research was performed at the Multcrete facility located at 555 Hervo
Street, Winnipeg, Manitoba. Testing was done according to specifications set forth by Canadian
Standards Association (CSA) A23.1-09/A23.2-09[6]: Concrete materials and methods of concrete
construction/Test methods and standard practices for concrete. All equipment was provided by
Multicrete. The hemp flour was provided by Plains Hemp, located at Hwy #5 - Municipal Road
130, 1/2 mile west of Gilbert Plains, MB.
No special design considerations are required for shotcrete and is therefore subject to the
CSA standards specified above.
3.1 Initial Testing
In order to determine the effectiveness of hemp flour in shotcrete, six different mixtures
containing hemp were created and underwent initial testing. A control mixture containing silica
fume was used for comparison purposes. Each mixture was a total of 50 kilograms containing
water, Type 10 (GU) shotcrete cement provided by Multicrete Systems Inc. (Multicrete), an
aggregate mixture provided by Multicrete, and hemp flour or silica fume. One hemp dosage,
hereafter referred to as the “low” dose, contained 0.25 kilograms of hemp flour per 50-kilogram
mixture. The “medium” dose contained 0.5 kilograms per 50-kilogram mixture, and the “high”
hemp dose contained 1.0 kilogram per 50-kilogram mixture. The control mixture contained 1.0
kilogram of silica fume per 50-kilogram mixture. The detailed mixture dry component breakdown
can be seen in the following table:
Table 1. Control, low hemp dose, medium hemp dose, and high hemp dose dry component
composition
Component
Type 10 (GU) cement
Silica fume
Dry sand
Hemp flour
Control
9.15 kg
1.00 kg
39.85 kg
N/A
Low dose
9.15 kg
N/A
40.60 kg
0.25 kg
Medium dose
9.15 kg
N/A
40.35 kg
0.50 kg
High dose
9.15 kg
N/A
39.85 kg
1.00 kg
A detailed chemical laboratory analysis on Type 10 (GU) cement can be seen in Appendix
A. The “dry sand” component consisted of both fine and coarse aggregates.
Two different hemp flours of different particle size distributions (Grade 1 with a slightly
large particle size and Grade 2 having a slightly finer particle size) were utilized with three different
hemp flour doses will be used for each hemp flour type (six total doses). The mixtures were coded
as G1L, G1M, G1H, G2L, G2M, and G2H, depending on the hemp flour type and hemp dosage.
This has been summarized in Table 2:
Table 2. Hemp-shotcrete mixture coding
Name
G1L
G1M
G1H
G2L
G2M
G2H
Hemp dosage
Low
Medium
High
Low
Medium
High
Hemp flour type
Grade 1
Grade 1
Grade 1
Grade 2
Grade 2
Grade 2
Initial testing was performed to determine the density, air content, slump, and compressive
strength of each mixture. This was done to determine which hemp-shotcrete mixture had the most
desirable properties and will be selected for use in the final testing stage.
The dry components (cement, sand, and silica fume or hemp flour) were first weighed on a
scale sensitive to within 0.05 kg for each mixture. The dry components were first added to the
plastic drum concrete mixer rotating at a speed of 26 revolutions-per-minute (rpm) and water was
added, as needed, during mixing to achieve the desired texture. The Multicrete expertise supervised
the water addition to determine when the desired texture was achieved (see Results). The water and
dry components were allowed to mix until appropriate intermingling of the ingredients was
achieved.
Once the desired texture was achieved, each shotcrete mixture underwent testing for air
content, slump, and were cored for compressive strength testing. The temperature was recorded
before commencing testing for each mixture to ensure consistent conditions. The air, dry
component, water, and wet mixture temperature can be seen in Table 3:
Table 3. Air, water, dry component, and wet mixture temperature during initial testing
Tair (˚C)
Tmix (˚C)
Twater (˚C)
Tdry component (˚C)
Control
16.9
15.8
23.9
17.0
G1L
20.7
22.5
25.0
20.3
G1M
22.8
22.7
26.9
20.7
G1H
23.2
23.9
26.1
22.6
G2L
19.4
23.4
27.0
20.3
G2M
19.4
22.0
23.9
19.5
G2H
21.5
21.9
22.8
20.4
The wet density was calculated by placing the mixture in a measuring bowl of known
weight (3.88 kg) and volume (0.00704 m3). The shotcrete was then placed in the bowl and weighed.
Any air within the mixture was removed by striking the side of the bowl with a rubber mallet. The
density was calculated using:
ρ wet =
mbowl +shotcrete − mbowl
Vbowl
(1)
A conventional slump test was used to find the slump of the wet shotcrete mix, with a slump
in the range of 4.5” – 6.5” considered satisfactory. A conical testing specimen with a 200 mm base
€
diameter and 100 mm top diameter
of 300 mm height was
used as a mold, with the top and bottom open and parallel
to one another oriented perpendicular to the axis of the
cone, as seen in Figure 1. The mold was placed upon a flat
level wooden board and was filled in three layers, each
approximately one-third the volume of the mold. Each
layer was rodded using 25 strokes of a 16 mm diametertamping rod immediately after being placed within the
mold. Once the third layer had been added to the mold and
rodded, the top of the mold was leveled by placing the rod
parallel to the ground and rolling it atop the mold several
times. The mold was then carefully raised in a vertical
Figure 1. Slump test mold
motion.
The slump was determined by measuring the difference in height of the mold and the
average height of the top of the settled wet shotcrete that had been within the mold using a 12”
ruler. If a slump of 4.5” – 6.5” was not achieved, the shotcrete was returned to the drum mixer and
more water was added conservatively. If the slump exceeded 6.5”, the mixture was discarded and
remade with less water.
A calibrated, pressure-tight air entrainment meter was used to measure the air content of the
wet shotcrete mix (Fig. 2), consisting of the same measuring bowl used in density testing and a
cover. The cover of the air entrainment meter contained an air chamber, hand pump, a valve to
bring the air chamber to atmospheric pressure, an operating valve to allow air in the chamber to
enter the measuring bowl, two bypass valves that release air from the container directly to the
atmosphere, flanges to create an air tight seal between the cover and measuring bowl, and a
pressure gauge to display the air content.
The wet shotcrete mixture was placed within the
measuring bowl in three layers, each one-third of the volume
of the measuring bowl. After placing each layer within the
bowl, a 16 mm-diameter tamping rod was used to rod the
layer by stroking it 25 times. Once the last layer was added
and rodded, the surface was leveled by placing the rod
parallel to the ground and rolling it atop the bowl several
times. The cover was then placed on the bowl with the
operating valve closed and petcocks open, and the flanges
were shut to create an airtight seal. All the air above the
concrete was removed by injecting water into the petcock
using a small syringe until water flowing from the opposite
petcock contains no air bubbles. The hand pump was then
used to pump air into the chamber until a slightly higher
pressure than the starting point is attained, and the two
petcocks were then closed. The pressure gauge needle
returned to the initial starting point from air escaping
through the air bleed valve while lightly tapping the pressure
gauge. The main valve connecting the air chamber to the
measuring bowl was then opened quickly and the measuring
bowl was hit using a rubber mallet. The gauge was then
Figure 2. Air entrainment meter to
lightly tapped again. Once coming to rest, the air content
measure wet shotcrete air content
percentage was read from the display.
To find the compressive strengths of all seven mixtures, the wet shotcrete was placed in
identical plastic cylindrical
molds (three per mixture, 21
specimens total). The molds
were first sprayed with
lubrication oil to ensure the
shotcrete specimen could be
removed from the mold.
Caps were placed on each
mold filled with shotcrete
and placed in a curing box
with temperature sensors for
three, seven, and 28 days.
The curing boxes reached a
maximum temperature of
24.0˚C and had a minimum
temperature of 22.3˚C
(Fig.3).
Figure 3. Cylindrical shotcrete specimens placed inside curing
boxes
Once the cylindrical test cores had been cured for the designated number of days, they were
removed from the mold and placed in the concrete compression machine. The specimen was
aligned with the bottom and upper bearing block and a constant loading rate was applied to the
specimen until reaching failure. In this case, failure was considered to be the point where the
cylindrical shotcrete specimen began showing visible cracks. The applied load when reaching
failure was read from the apparatus display. The applied load in kN was normalized to a stress in
MPa, by dividing by the specimen cross sectional area:
P=
F
Acs
(2)
This process was done after three, seven, and 28 days for each mixture. The hemp-shotcrete
mixture with the most desirable properties and matched closest with those of the control mixture
was then selected and used, along with the control silica fume-containing mixture, in final testing.
3.2 Final Testing
€
The optimal hemp-shotcrete mixture was selected from the six original compositions based
on initial testing results. The optimal mixture was shot alongside the control mixture using
conventional, dry-mix shotcrete technique onto two-feet by two-feet panels approximately six
inches deep. The dry components were added to a bag placed above a shotcrete pot connected to
two Doosan Portable P185WJD Tier 4i compliant air compressors. The bag deposited the dry
components into the shotcrete pot, which contained a rotary blade to mix the dry ingredients. One
Doosan air compressor projected the shotcrete onto the panels while another air compressor turned
the rotary blade. A nozzle with a hose connected to the shotcrete pot containing mixed dry
components, and water was added to the nozzle before projection via the water supply connection.
Once each mixture sufficiently filled their respective panels, the panels were allowed to cure
for three, seven, and 28 days. The hemp-shotcrete was too wet for testing after three days, and thus
testing was only possible after seven and 28 days.
Following the designated curing time, 50 mm cubes were cut from each panel. These cubes
were then weighed using a scale sensitive to within 0.05 kg and divided by the known specimen
volume (0.000125 m3) to calculate the density:
ρ cube =
mcube
Vcube
€
Figure 4. Shotcrete pot setup with hanging bag containing dry
ingredients
(3)
The shotcrete cubes underwent the same testing
procedure outlined in section 3.1 to determine the
compressive strength, replacing the cylindrical test
specimens with 50 mm cubes. The compressive strength
was converted to MPa using formula (2). The effectiveness
of the hemp-shotcrete was determined by its similarity to the
control silica fume shotcrete in terms of compressive
strength.
3.3 Particle Size Distribution Determination
Figure 5. Cured hemp-shotcrete
In order to find the particle size distribution of the
cubes (left) and control shotcrete
cubes (right)
two supplied hemp flours, a mechanical sieve analysis was
performed using a stack of sieves ranging from 0.0075 –
1.180 mm square openings. A sample of each hemp flour grade was placed on a scale sensitive to
0.05 kg and weighed to find the total mass. The sample was then placed atop the sieve stack
arranged in decreasing size with the largest sieve at the top and the smallest at the bottom. A lid was
placed atop the stack to prevent any flour from
escaping and was placed in a mechanical sieve
shaker for ten minutes. The sieves were
secured within the apparatus by bolting the lid
to a tightening plate.
Once the hemp flour had been
separated using the mechanical shaker, the
stack was removed. The amount of hemp flour
remaining atop each sieve was adequately
removed from the sieve and weighed using the
same scale. The sum of the mass of hemp flour
Figure 6. Sieve stack placed in mechanical sieve
on each sieve had to match the initial mass to
ensure no flour had been lost in the separation shaker
process. The percent composition of the total particle size distribution was calculated using:
% distribution =
€
msieve
× 100%
mtotal
(4)
4. Results
4.1 Water Addition
The following table shows the amount of water manually added for each shotcrete mixture
prepared for initial testing:
Table 4. Water additions for initial testing shotcrete mixtures
Control
G1L
G1M
G1H
G2L
G2M
G2H
Water added
(L)
6.67
6.67
7.02
8.36
6.33
6.92
8.13
4.2 Initial Testing
Initial testing yielded the following measured results for slump, density, and air content:
Table 5. Slump, density, and air content for the initial wet shotcrete mixtures
Control
G1L
G1M
G1H
G2L
G2M
G2H
Slump
4.5"
6.5"
4.0"
4.5"
5.0"
5.0"
4.0"
Density (kg/m3)
2232.95
1906.25
1877.84
1678.98
2005.68
1877.84
1835.23
Air content
3.3%
7.4%
17.0%
24.0%
8.0%
16.0%
18.0%
Compression testing performed on the initial shotcrete mixtures after three, seven, and 28
days of curing resulted in the data displayed in Figure 1:
50.00
45.00
Compressive Strength (MPa)
40.00
35.00
Control
30.00
G1L
G1M
25.00
G1H
20.00
G2L
15.00
G2M
G2H
10.00
5.00
0.00
0
5
10
15
20
25
30
Time (days)
Figure 7. Compressive strength of the initial shotcrete mixtures after three, seven, and 28 days
As can be seen from the above figure, the G2L hemp-shotcrete had slightly higher
compressive strength than the other hemp-shotcrete mixtures, and was thus selected as the optimal
hemp-shotcrete mixture carried forward to final testing.
4.3 Final Testing
Following initial testing, final compression tests were performed on the shot optimal hempshotcrete mixture (G2L) and a shot control mixture containing silica fume. The final densities of the
optimal hemp-shotcrete and control mixture were found to be 2321.60 kg/m3 and 2636.00 kg/m3,
respectively. The final compression strengths seven and 28 days after shooting both mixtures can be
seen in Figure 2.
60
Compressive Strength (MPa)
50
40
30
Control
G2L
20
10
0
0
5
10
15
20
25
30
Time (days)
Figure 8. Final compressive strength of the optimal hemp-shotcrete mixture and the control silica fumecontaining shotcrete
4.4 Particle Size Distribution of the Hemp Flours
The following figure details the particle size distribution of both hemp flours obtained by
mechanical sieve analysis:
50.00%
45.00%
Percent composition
40.00%
35.00%
30.00%
25.00%
G1 hemp flour
20.00%
G2 hemp flour
15.00%
10.00%
5.00%
0.00%
0 - 0.075
0.075 - 0.150 0.150 - 0.300 0.300 - 0.600 0.600 - 1.180
Particle size (mm)
Figure 9. Particle size distribution of G1 and G2 hemp flours
5. Discussion
5.1 Density and Particle Size Distribution
The control mixture was found to have a higher density than the hemp-shotcrete both as wet
mixtures in initial testing and the shot, cured cubes in final testing. The Grade 1 hemp had a lower
density than the Grade 2 hemp. This may have been due to a slightly higher composition of particle
sizes less than 0.0075 mm, as a smaller particle size creates a higher porosity and lower density.
The medium hemp dose resulted in identical densities, while the high and low densities varied
significantly between the two hemp flour types.
The density values increased from the wet shotcrete mixtures calculated in initial testing to
the cured specimens in final testing. The control density increased from 2232.95 kg/m3 to 2636.00
kg/m3. The optimal hemp-shotcrete mixture, G2L, increased from 2005.68 kg/m3 to 2321.60 kg/m3,
meaning both shotcrete densities increased by over 300 kg/m3 once shot and cured. This was likely
caused by the high-velocity projection of the shotcrete onto the panels, compressing the mixtures,
decreasing the pore space, and therefore increasing the density.
The low-density values are related to the high air content values measured in all the hempshotcrete mixtures.
The particle size distribution of the two hemp grades were fairly similar, with the Grade 1
hemp having more particles smaller than 0.0075 mm and between 0.600 – 1.180 mm than the Grade
2 flour. The Grade 2 had a larger proportion of particles in the 0.075 – 0.300 mm “middle” range.
5.2 Water Addition
Due to the highly absorbent properties of hemp, it was anticipated that the hemp-shotcrete
mixtures would have a much higher water demand than the control mixture. However, while the
water requirement did increase with higher hemp content, it was not a significant increase. In fact,
the G2L mixture had a lower water requirement (6.33 L per 50 kg batch) than the control mixture
(6.67 L per 50 kg batch) with the slump only differing by 12mm. Overall, the hemp-shotcrete
utilizing the Grade 2 hemp had a lower water demand than that containing the Grade 1 hemp.
The high doses of hemp flour for each grade had less than a two-litre water requirement
difference than the control silica fume shotcrete. Due to the time constraints and limited scope of
the project, it cannot be determined why the water demand was lower than anticipated in the hempshotcrete.
5.3 Air Content
As can be seen from Table 5, all six hemp-shotcretes had a significantly higher air content
than the control shotcrete; the lowest being over double that of the control air content. In the case of
the G1H hemp-shotcrete, the air content was nearly 25%. The G2L shotcrete had a slightly higher
air content than the lose hemp dose utilizing the Grade 1 hemp flour, with all other Grade 2 air
contents being lower than the Grade 1 air contents. Additionally, air content increased significantly
with hemp content.
According to CSA A23.1-09 [6], the highest air content requirement for any hydraulic
concrete with any sized aggregate is 9% (CSA page 124), in which only the G1L and G2L hempshotcrete mixtures do not exceed. However, this is the acceptable air content for coarse aggregates
of maximum size 10 mm. As there were coarse aggregates larger than 10 mm used in the hempshotcrete mixture, the hemp mixture does not fall within the 10 mm aggregate size category and
thus the air content of all hemp mixtures exceeds the air content requirement specified. It is more
likely that the maximum aggregate size used falls within the “28 – 40 mm” category, and thus the
air content must fall within 4 - 7%. As the lowest achieved air content for the hemp-shotcrete is
7.4%, all hemp-containing shotcretes tested exceeded allowable air contents.
The control mixture (3.3%) falls slightly beneath the range of 4 - 7% air content. This slight
difference can be considered negligible, as shotcrete is typically applied in underground or
inaccessible areas and have stable conditions with mild exposure to freeze-thaw cycles. Since air
content is required in concrete to reduce damage due to these cycles, the minimum required air
content can be reduced to 3% [9]. Considering the minimal exposure to freeze-thaw cycles, the
control mixture air content falls within the required range.
From the obtained data, it is unclear what created the high air content values when hemp is
added to shotcrete. Further testing is required to determine the cause of the large amount of air
within the hemp-shotcrete.
5.4 Initial Compressive Strength
The hemp-shotcrete initial compressive strengths were all found to be significantly lower
than that of the silica fume shotcrete (43.70 MPa after 28 days). All of the hemp mixtures
containing Grade 2 hemp had higher strength values than the Grade 1-containing shotcretes with the
same hemp flour content. All seven shotcrete mixtures, regardless of admixture, had increasing
strength with time. The silica fume shotcrete compressive strength nearly doubled from day three to
day 28, while the maximum hemp-shotcrete increase was only 5 MPa in the G1L and 4.5 MPa in
the G2L mixtures in the same time frame.
As the hemp content increased, the compressive strength decreased. The low strength values
can therefore be linked directly to the hemp flour. The increasingly high air contents (and low
density) associated with the hemp flour content are likely the cause of the low strength of the hempshotcrete, however the exact reactions that take place and conditions that create the high air content
and low compressive strength are not determinable from the available data.
Shotcrete would likely be classified as having a CSA “C-4” class of exposure, defined as
“non-structurally reinforced concrete exposed to chlorides, but not to freezing and thawing” [6]. As
shotcrete is typically used in underground construction and within the mining industry, exposure to
chlorides is expected. Underground temperature conditions typically remain stable and thus
freezing/thawing cycles are minimal. The minimum specified compressive strength by CSA for
concretes of type C-4 is 25 MPa after 28 days [6]. While the silica fume shotcrete exceeds this
value, all six hemp-shotcretes do not. The hemp-shotcrete that comes closest to this value is G2L,
with a compressive strength of 15.65 MPa after 28 days. As this is the hemp mixture closest to the
requirement, the G2L mixture was selected as the optimal hemp-shotcrete mixture to use in final
testing.
5.5 Final Compressive Strength
Similar to the initial compressive strength, the shot hemp-shotcrete had a much lower
compressive strength than the control shotcrete. The final compressive strength of the silica fume
shotcrete was 48.70 MPa and 36.25 MPa for the G2L hemp-shotcrete. However, the difference of
12.45 MPa is significantly less than 28.05 MPa difference between the mixtures observed in initial
testing.
Although the seven-day compressive strength of the hemp-shotcrete was only 20.55 MPa
compared to 40.45 MPa for the silica fume shotcrete, the hemp-shotcrete had a higher continuous
strength (76.4%) than the control mixture (20.4%). It is unclear why the hemp-shotcrete had such a
drastic continuous strength increase in comparison to the control shotcrete, but it may be related to
the longer curing time required to the hemp-containing mixture. The hemp-shotcrete required more
time to dry and thus more time to reach its compressive strength capacity.
The hemp-shotcrete panels required more curing time than the silica fume shotcrete. While
the silica fume shotcrete was sufficiently cured for the three-day compressive strength tests, the
hemp-shotcrete was still very wet and testing could not be performed. The hemp-shotcrete cubes
were ready to be tested after seven days, but the continuous strength increase could be related to the
hemp-shotcrete requiring more time to sufficiently set and cure. Although it was observed that the
cubes were sufficiently dry by seven days, 28 days (or slightly less) may be the optimal curing time
for hemp-shotcrete. All final compressive strengths met the C-4 class of exposure compressive
strength requirement specified in CSA A23.1-09.
While the hemp-shotcrete is not likely to meet the high-strength concrete requirement of 70
MPa at 91 days outlined in CSA A23.1-09 (CSA 2009), it does meet the general requirements for
normal concrete of class C-4 exposure.
6. Conclusion
All six hemp-shotcrete mixtures had extremely high air content with low compressive
strength values. The final hemp-shotcrete required over three days of curing time, but had a high
continuous strength value of 76.4%, increasing from 20.55 MPa after seven days to 36.25 MPa after
28 days. The tested conventional shotcrete had a final compressive strength of 48.70 MPa, 12.45
MPa higher than the hemp-shotcrete. The air content of the conventional shotcrete was 3.3%, over
4% lower than the lowest measured hemp-shotcrete air content. The silica fume-containing
shotcrete had a higher density throughout testing than all hemp-shotcrete mixtures.
While the hemp-shotcrete did meet the general requirements for normal concrete of class C4 exposure defined in CSA A23.1-09, the hemp-shotcrete did not match the measured parameters
and performance of the silica fume shotcrete. Thus, the hemp flour failed to sufficiently replace
silica fume as a shotcrete admixture.
7. Recommendations
Due to the time constraints and scope of the project, further testing to determine the cause of
the high air content was not possible and should be performed in a separate project. It is
recommended that a controlled laboratory test be performed on the hemp-shotcrete to observe the
reactions that occur within the mixture. This would help explain the undesirable properties of the
organic shotcrete and possibly result in a solution to the low strength properties. Knowing the origin
of the high air content may improve the performance of the hemp-shotcrete.
It may be beneficial to test the hemp-shotcrete utilizing homogenous mixtures to determine
an optimal particle size, possibly using particles solely less than 0.075 mm in diameter in
comparison with a 0.600 mm particle size. This may help reduce air content and improve upon the
compressive strength.
Testing to determine the ductile and thermal properties of the hemp-shotcrete may provide
evidence for a wide variety of uses. Research performed by Camoes et al. [5] found that the
implementation of hemp in concrete creates an extremely ductile material. This discovery was
supported by the research done by [1] et al. in 2009. The low thermal conductivity of hemp may
allow hemp-shotcrete to be used as a spray insulation product while also providing secondary
strength to the structure.
It is not recommended that the hemp-shotcrete be used in any high-strength applications.
8. Acknowledgements:
The authors wish to thank Multicrete Systems Inc of Winnipeg for their time and support, Plains
Hemp for the hemp materials, Peter Hildebrand and the Alternative Village staff for their
assistance.
9. References
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characteristics on the mechanical properties of lime and hemp concrete. European Journal
of Environmental and Civil Engineering 13: 1039-1050.
[2]Beaupré, D. 1994. Rheology of high performance shotcrete. Published Ph.D. thesis. Vancouver,
British Columbia: Department of Civil Engineering, The University of British Columbia.
[3]Beaupré D. and M. Jolin. 2003. Understanding wet-mix shotcrete: mix design, specifications,
and placement. Shotcrete Summer 2003 [Internet]: 6-12. Available from:
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[5]Camoes, A., R. Eires, R. Fangueiro, S. Jalali,, and J.P. Nunes. 2004. New eco-friendly hybrid
composite materials for civil construction. Project No. POCI/ECM/55889/2004. University
of Minho. Guimaraes, Portugal.
[6]CSA Group (CSA). 2009. A23.1-09/A23.2-09 (R2014). Concrete materials and methods of
concrete construction/Test methods and standard practices for concrete.
[7]Morgan, D.R. and J.T. Wolsiefer. 1991. Silica fume in shotcrete. In CANMET/ACI International
Workshop on this use of Silica in Concrete, 1-11. Washington, D.C. April 1991.
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67-119. Berlin, Germany: Springer.
[9]Portland Cement Association (PCA). 1998. Control of air content in concrete. Concrete
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[10] Dick K.J. and Pinkos, J. 2014 Thermal, Moisture and Energy Performance of a Hempcrete
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