https://doi.org/10.14311/APP.2022.33.0181
Acta Polytechnica CTU Proceedings 33:181–187, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence

Published by the Czech Technical University in Prague

FUNDAMENTAL PROPERTIES AND DURABILITY OF SLOPE
PROTECTION SPRAY MORTAR REINFORCED WITH BAMBOO

FIBERS

Kazuo Fujiyoshia,∗, Takao Uedab, Masayuki Tsukagoshic

a Kankyo Bosai Co., Ltd., 1-57 Akui-cho, Tokushima 770-0046, Japan
b Inst. of Science and Engineering, Tokushima University, 2-1 Minamijyosanjima-cho, Tokushima 770-8506,

Japan
c Faculty of Engineering, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
∗ corresponding author: k-fujiyoshi@kan-bou.jp

Abstract. In order to enhance resistance against cracking and durability, short fibers such as steel
and organic fibers have been conventionally mixed into spray mortar used for slope protection. In this
research, fundamental properties of bamboo-fiber-mixed spray mortar were examined by laboratory
tests. The mechanical properties of spray mortar containing bamboo fibers were examined under
cyclic wet and dry conditions along with its resistance against freezing and thawing by a spray test.
It was confirmed that 0.75% mixture of bamboo fibers in spray mortar most successfully improved
mechanical properties and durability. These include adhesion strength to the base surface following
exposure to cyclic wet/dry conditions and overall resistance against freezing and thawing. In addition,
higher compressive strength and adhesion strength to the base surface were achieved by further mixing
in vinylon fibers or fly ash in combination with bamboo fibers.

Keywords: Adhesion strength, bamboo fiber, cyclic wet and dry condition, fly ash, resistance against
freezing and thawing, spray mortar, vinylon fiber.

1. Introduction
The spray application method of mortar for slope pro-
tection has been widely used because it can help pre-
serve bedrock conditions from weathering and ero-
sion. However, it has been observed at many sites
that sprayed mortar cracks, peels and flakes due to
aging. Sprayed mortar is directly influenced by the
external environment immediately after construction
and initial cracking occurs in certain weather condi-
tions. Therefore, in recent years, the use of spray
mortar mixed with short fibers such as organic and
inorganic fibers has been increasing for the purpose
of reducing initial cracking and improving flexural
toughness [1].

Bamboo forests are abundant in Japan and have
been a familiar source of natural materials. How-
ever, proper forest management is somewhat being
neglected due partly to the lack of successors who
take care of the forests recently. When proper man-
agement is neglected, the bamboo forests may trig-
ger landslides and other disasters caused by crowded
bamboo growth density and shallow rooting system.
As part of the efforts to widening the scope of bamboo
material applications, experiments have taken place
in the use of short bamboo fibers as a reinforcement
medium in concrete and mortar. Past experiments
[2] have found that bamboo fibers mixed in concrete
improve flexural toughness and resistance to crack
development.

Based on these findings, the fundamental proper-

ties of bamboo-fiber-mixed-mortar for slope protec-
tion were tested in laboratory. Then, the durabil-
ity of bamboo fiber reinforced mortar was tested by
spraying on formworks in the actual working spray
facilities. The durability examination included two
mechanical property tests (for compressive and ad-
hesion strength) under Cyclic Wet/Dry Conditions
(hereinafter referred to as "CWDC"), and a resistance
test in freezing/thawing conditions.

2. Experimental program
2.1. Properties of fibers
The bamboo fibers used in this study were prepared
using fibers collected from a bamboo tree with a
unique rotating knife. Bamboo trees used were any-
where from three to five years old and the culti-
var is called Moso bamboo grown in Awaji, Japan.
Prepared bamboo fibers were then classified by fiber
length to L = 20 ± 10 mm using 1.2 − 5.0 mm sieves.
The classified bamboo fibers were air dried indoors for
about three weeks prior to actual experiments. Ad-
ditionally, commercially available vinylon fibers were
mixed into spray mortar to facilitate comparison with
spray mortar reinforced with bamboo fibers. The ma-
terial properties of the bamboo and vinylon fibers are
shown in table 1. The dry surface state was defined as
a state where the fibers absorbed water for 24 hours
and then dehydrated until the fiber surface was dry.
To achieve this dry surface state, the fibers that ab-
sorbed water were dehydrated for three minutes using

181

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K. Fujiyoshi, T. Ueda, M. Tsukagoshi Acta Polytechnica CTU Proceedings

a centrifugal dehydrator.

Bamboo Vinylon
Density: Air-dry (g/cm3) 0.77 1.09
Density: Dried surface (g/cm3) 1.27 1.17
Water absorption rate (%) 87.2 12.1
Fiber length (mm) 19.8 (Av.) 18.0
Fiber diameter (mm) 0.59 (Av.) 0.2
Aspect ratio (−) 34 90
Tensile strength (MPa) 154 490

Table 1. Material properties of bamboo fiber and
vinylon fiber.

2.2. Laboratory test for fundamental
properties

2.2.1. Mix proportions of mortar and
specimen preparation

Table 2 shows the mix proportions of mortar for fun-
damental property tests in laboratory, normal mortar
with C = 400 kg/m3, W/C = 55% and S/C = 4.2 was
named "N" as a standard composition of spray mor-
tar. Based on N, compositions with bamboo fibers
added with mixing ratios of 0.25%, 0.75% and 1.25%
in volume were each named "B0.25," "B0.75" and
"B1.25." In addition, 20% in fine aggregate of B0.75
was replaced with fly ash, which was named "B+FA."
In B+FA, W/B was 32% when the binder included
fly ash. Furthermore, a composition N with 0.75%
vinylon fibers was named "V," and the N mortar with
0.375% each of bamboo fibers and vinylon fibers was
named "B+V."

Readily available Portland cement (density: 3.16
g/cm3) was used. Crushed sand manufactured in
Naruto, Japan was used as fine aggregate (density:
2.57 g/cm3, fineness modulus: 2.63). Moreover, type
II fly ash (density: 2.33 g/cm3, specific surface area:
3240 cm2/g, ignition loss: 2.8%) specified under the
Japanese Industrial Standard (JIS A 6201) was used.

The mortar was mixed in accordance with the ce-
ment physical test method (JIS R 5201), and the
fibers were immersed in the mixing water. All compo-
sitions in this experiment were stiff consistency mor-
tar with low fluidity close to zero slump. In mortar
spraying operations on the field, mortar is sprayed on
the bedrock by compressed air generated by a com-
pressor. Thus, the sprayed mortar not only adheres
to the bedrock but is also compacted. Since fun-
damental property tests are conducted indoors, the
specimens were prepared by an elaborate compaction
to obtain effects similar to spray compaction with-
out spraying. Cylindrical specimens (! 50×100 mm)
for the compression test were compacted every three
layers by tamping down with a thrust bar.

2.2.2. Curing and various tests
All specimens went through indoor air curing for 28
days (room temperature: 20 ◦C , 75% R.H.), being

sealed except for the upper surfaces of the specimens
to simulate site conditions.

Two cases of compression tests were conducted. In
the first case, the compressive strength was measured
after air curing for 28 days. In the second case, the
specimens were subjected to CWDC for 12 days af-
ter air curing for 28 days. CWDC were based on
the method for the accelerated rock slaking test (JGS
2125) to simulate the environment of natural weath-
ering after spraying on slope surfaces. The specimens
were dried at 40 ◦C for 48 hours, then immersed in
water at 20 ◦C for 24 hours, and then dried at 110 ◦C
for 24 hours. This cycle was repeated three times.

A porosity test was performed using cylindrical
specimens (! 5 × 10 cm) with a material age of 28
days. The mortar porosity was determined by equa-
tion 1, which was consisted of the mass of the spec-
imen dehydrated in a drying furnace at 80 ◦C for 24
hours (Wd) and the mass of its surface-dried state
immersed in water at 20 ◦C for 48 hours (Ws).

ε = Ws − Wd

ρ × V
× 100 (%) (1)

where ε is porosity (%), ρ is density of water and
V is volume of specimen.

2.3. Spray test using actual plant
2.3.1. Mix proportions of mortar and spray

method
For the spray test, the amount of mixed fibers was
determined to be 0.75% as found to perform the best
by the laboratory test. Five kinds of mortar compo-
sitions, N, B, V, B+V and B+FA were selected. It is
presumed that the viscosity gets too high due to the
influence of the fly ash that replaced fine aggregate
in B+FA, and there is a risk of clogging the material
hose and nozzle. Consequently, the replacement rate
of fly ash was adjusted to be 7%, and W/B was 45%
when fly ash was used as the binder.

Mortar spraying was conducted in the facilities nor-
mally used for wet spraying. The fibers were mixed
in in a small portion at a time to prevent unevenness,
and the mixing duration was set to be two minutes
per batch.

2.3.2. Specimen preparation and various
tests

Figure 1 shows an outline of the formworks sprayed
with mortar and the specimens cut out. Two types
of wooden formworks in the figure were prepared for
each mortar composition. Three cylindrical speci-
mens (! 50 × 100 mm) for compression test and four
specimens for adhesion test were made in a form-
work with dimensions of 850 × 300 × 150 mm. In
the attempt to take actual spraying on bedrock or
existing concrete surfaces into account, two kinds of
specimens for the adhesion test were prepared by
spraying on rock pieces (! 75 × 25 mm, compres-
sive strength: 207 N/mm2) and on a flat concrete

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vol. 33/2022 Spay Mortar Reinforced with Bamboo Fibers

Fiber type Fiber addition rate W/B Unit amount
[−] [vol. %] [%]

!
kg/m3"

C W S Fiber PA
N − − 55 400 220 1679 0 0
B0.25 Bamboo 0.25 55 400 220 1676 1.93 0
B0.75 Bamboo 0.75 55 400 220 1670 5.78 0
B1.25 Bamboo 1.25 55 400 220 1663 9.63 0
V Vinylon 0.75 55 400 220 1662 8.14 0
B+V Bamboo, Vinylon 0.375, 0.375 55 400 220 1674 2.89, 4.07 0
B+FA Bamboo 0.75 32 400 220 1356 5.78 280

Table 2. Mix proportions of mortar in laboratory test.

 

850 

30
0 

300 400 150

Specimen for 
compression test 
(ij50×100) 25

0 

550 

Specimen for freeze/thaw 
test (ij100×200)  

Specimen for adhesion test 
to rock piece (ij75×25) 

Specimen for adhesion test 
to flat concrete plate 

Unit: mm

Figure 1. Top view of formworks for spraying and sampling of specimens.

plate (300 × 300 × 60 mm for interlocking block, flex-
ural strength: 5.5 N/mm2). Rock pieces were cut
out from sandstone with a stone cutter. The cross
section of the stones for bonding with mortar be-
came smoother than the surface of flat concrete plate.
Moreover, two cylindrical specimens for freeze/thaw
test (! 100×200 mm) were cut out of mortar sprayed
on a wooden formwork with the size of 550×250×250
mm.

The sprayed formworks were cured outdoors ex-
posed to natural weather for 28 days from 11/24 to
12/22. During this period, the average temperature
was 8.3 ◦C, and it rained for seven days. Each spec-
imen except for freeze/thaw test was cut out at a
material age of 25 days. When the age reached 28
days, the compression strength test (JIS A 1108) and
the first case of the adhesion strength test (JSCE-
K 561-2013) were conducted. An adhesion strength
testing device of Building Research Institute method
was used for the adhesion test. After outdoor ex-
posure for 28 days, two specimens for the adhesion
strength test were exposed to CWDC for 12 days.
Then the second case of the adhesion test was per-
formed. Three cylindrical specimens (! 50×100 mm)
were cut out of mortar after the second adhesion test,
and used for the second compression test.

56-day-old specimens were cut out and used for the
freeze/thaw test. In this test, 300 cycles of freezing
and thawing in water were conducted according to
the freeze/thaw test method of concrete (JIS A 1148,
procedure A), and the relative dynamic modulus of
elasticity and the mass reduction rate were obtained.
The relative dynamic modulus of elasticity was de-

termined by the ultrasonic propagation velocity with
reference to the previous study [3].

3. Fundamental property
laboratory test results

3.1. Porosity

Figure 2 shows the results of porosity test at a ma-
terial age of 28 days. According to this figure, when
the mixing amount of bamboo fibers increases from
0% (N) to 1.25% (B1.25), the porosity also increases.
This increase is caused by fine bubbles introduced in
the mortar with the fibers. The porosity of V mixed
with vinylon fibers is the largest. One of the rea-
sons is because the number of vinylon fibers per mor-
tar volume is greater than the number of bamboo
fibers. The diameter of vinylon fiber is about 1/3
that of bamboo fiber as shown in table 1. When the
same volume of fibers (0.75%) is mixed, the number
of vinylon fibers is about 10 times that of bamboo
fibers. The porosity of B+V mixed with composite
fibers (bamboo/vinylon), is almost the same as B0.75
mixture of bamboo fibers solely. Moreover, the poros-
ity of B+FA with the combination of bamboo fibers
and fly ash is the smallest because mortar voids are
reduced by fly ash. Since fine aggregate was replaced
with fly ash, voids were filled by the filler effect of
fly ash with extra fine particles. In the long term, as
the pozzolanic reaction of the fly ash proceeds, the
porosity of B+FA will decrease further.

183



K. Fujiyoshi, T. Ueda, M. Tsukagoshi Acta Polytechnica CTU Proceedings

 

10

12

14

16
Po

ro
si

ty
(%

)

Figure 2. Porosity of mortar measured in laboratory
test.

3.2. Compressive strength
The results of compressive strength test of 28-day-old
specimens are shown in figure 3 "Pre wet/dry expo-
sure." The compression strength increased when the
addition rate of bamboo fibers increased from 0% (N)
to 0.75% (B0.75), but it decreased when the rate fur-
ther increased from 0.75% (B0.75) to 1.25% (B1.25).
In general, the higher the porosity and organic con-
tent, the lower the compressive strength. However,
when bamboo fiber content increased from 0% to
0.75%, not only the porosity, but also the compres-
sive strength increased. The cause of the increase in
compressive strength is inferred to be influenced by
the internal curing of bamboo fibers. Since the spec-
imens were cured in air, the moisture in the mortar
(needed for the hydration reaction) tended to evap-
orate easily. Because moisture contained in bamboo
fibers with high water absorption rate (as shown in
table 1) is replenished inside the mortar as it dries,
the strength of B0.25 and B0.75 is considered to in-
crease from that of N. When bamboo fiber mixing ra-
tio increases to 1.25%, the increased voids (as shown
in figure 2) have a more negative effect on the mortar
strength. It also becomes difficult to sufficiently dis-
perse the fibers while mixing, so that the compressive
strength of B1.25 seemingly decreased.

V in which vinylon fibers solely was mixed had a
lower compressive strength than N due to its large
porosity, and B+V in which bamboo and vinylon
fibers were combined had comparable strength to
N. B+FA in which bamboo fibers and fly ash were
used in combination had the smallest porosity and
the highest compressive strength. B0.75 also shows a
similarly high-level of strength to B+FA. In terms of
strength, approximately 0.75% of bamboo fiber mix
ratio proved to be most desirable.

When the specimens were exposed to CWDC for
12 days after 28 days of outdoor air curing, the
strength of the bamboo-fiber-mixed compositions ex-
cept B+FA shows a relatively large decrease (see fig-
ure 3 "Post wet/dry exposure"). The decrease is as-
sumed to be caused by the hollow cells of bamboo

20

30

40

50

C
om

pr
es

si
ve

 st
re

ng
th

 
(N

/m
m

2 )

Pre wet/dry exposure
Post wet/dry exposure

Figure 3. Compressive strength of mortar measured
in laboratory test.

fibers [4]. It is presumed that shrinkage stress is gen-
erated in the mortar around the hollow cells while
drying in wet/dry cycles, and defects have occurred
inside the mortar. The strength of B+FA mixed with
fly ash largely increases. Since fly ash used, over-
all amount of the binder increased. This resulted
in increased amount of unreacted binder. It is pos-
sible that the hydration reaction of the unreacted
binder progressed rapidly during water immersion in
wet/dry cycles.

4. The results of spray test using
actual plant

4.1. Porosity
Figure 4 shows the results of porosity test of 256-day-
old specimens (! 50 × 100) cut out in the spray test,
together with the porosity of 28-day-old specimens in
the laboratory test. According to figure 4, the values
of B and V in the spray test are less than those in
the laboratory test, and are similar to the value of N.
The reason is because the spraying pressure purged
the fine bubbles which were entrained while mixing
in the fibers [5]. The value of B+FA in the spray test
is drastically lower than that in the laboratory test.
The decrease was caused by densification of mortar,
in addition to bubbles vanished while spraying. Since
the specimen material age reached as old as 256 days,
the pozzolanic reaction of the fly ash progressed.

4.2. Compressive strength
Figure 5 shows the measurement results of com-
pressive strength and density of cylindrical speci-
mens (! 50 × 100 mm) before and after Exposure
to Cyclic Wet/Dry Conditions (hereinafter referred
to as "ECWDC"). Before ECWDC, the compressive
strength of B was similar to that of N and V, while
the density of B mixed with bamboo fibers was the
smallest. The moisture within the mortar easily evap-
orates during outdoor curing. In this situation, it is
assumed that the bamboo fibers that have absorbed
water gradually supplied moisture (the internal cur-
ing effect [6]) and the hydration reaction of B further

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vol. 33/2022 Spay Mortar Reinforced with Bamboo Fibers

10

12

14

16

N B V B+V B+FA

Po
ro

si
ty

 (%
)

Laboratory test
Spray test

Figure 4. Porosity of mortar measured in spray test.

developed. That is why the compressive strength of
B+V is greater than that of V. The reason why B+FA
had the largest strength was that the dense mortar
matrix was formed by the lower W/B, the filler ef-
fect of fly ash and the internal curing of the bamboo
fibers.

After ECWD, the density of all compositions in-
creased, but the compressive strength of N and B
decreased. The decrease was especially drastic in B
than in N. The air-dry density of the bamboo fibers is
small as shown in Table ??, and the inside of the bam-
boo fiber cells becomes hollow when drying [4]. Ac-
cordingly, it is considered that the internal defects of
the mortar were caused by the stress of drying shrink-
age. The compressive strength of V, B+V, and B+FA
increased slightly. These increases are attributed to
the hydration reaction of the unreacted binder that
was promoted during the wet period of CWDC. V
and B+V were also mixed with vinylon fibers which
do not have a hollow structure like bamboo fibers. As
for B+FA, the addition of fly ash resulted in increased
amount of powder element that acted as a binding
agent, which led to increased compressive strength.
The increased compressive strength is assumed to be
further improved by the pozzolanic reaction of fly ash
over a long period.

4.3. Adhesion strength
Figure 6 shows the measurement results of adhesion
strength between spray mortar and rock/concrete
surfaces before and after ECWDC. Before ECWDC
(material age of 28 days), since the rock surfaces
cut with a rock cutter were smooth, the adhesion
strength with the rock in all compositions was as
small as approximately 1.0 N/mm2. Peeling frac-
ture also occurred at the adhered interfaces. As for
concrete surfaces, the base concrete or spray mortar
was destroyed, showing values exceeding 2.0 N/mm2.
These values can be regarded as close to the tensile
strength of mortar and concrete. Regardless of the

 
2.1

2.12

2.14

2.16

2.18

2.2

20

30

40

50

N B V B+V B+FA

D
ensity (g/cm

3)

C
om

pr
es

si
ve

 st
re

ng
th

 (N
/m

m
2 )

Compressive strength: Pre wet/dry exposure
Compressive strength: Post wet/dry exposure
Density: Pre wet/dry exposure
Density: Post wet/dry exposure

Figure 5. Compressive strength of mortar measured
in spray test.

base type, the value of B+V using composite fibers
was the largest.

Adhesion strength after ECWDC decreased on
rock/concrete surfaces for all compositions. The de-
crease was caused by the changes in the mortar vol-
ume following ECWDC, increasing the shear strain
on the adhered interfaces. For the rock surfaces,
peeling fracture occurred at the interfaces. The val-
ues of B and B+V were about twice the value of N,
and greater than V. For the concrete surfaces, the
base concrete or spray mortar was destroyed. The
values of B+V and B+FA were larger than other
compositions. These results show that compositions
with bamboo fibers mixed in have excellent adhesion
strength. Bamboo fibers inside the mortar absorb
water during the wet period of CWDC, and gradu-
ally supply moisture in the dry period. It means that
drying shrinkage strain on the adhered interfaces are
suppressed.

4.4. Resistance against
freezing/thawing

Figure 7 shows the mass reduction rate and the rela-
tive dynamic modulus of elasticity in the freeze/thaw
test results. With respect to the three compositions
N, V and B+V, relative dynamic elastic modulus
dropped sharply before reaching 200 cycles. These re-
ductions were attributed to the peeling fracture of the
mortar pieces at the corner portions of the cylindrical
specimens. It is possible that the cracks progressed
from the partial sand streaks generated during spray-
ing and led to the peeling fracture. The sand streaks
also existed on the specimens of B and B+FA mixed
with bamboo fibers, but the relative dynamic elastic
modulus showed a gentle decrease, with 89% and 57%
at 300 cycles, respectively.

After the freeze/thaw test, deterioration due to
scaling was observed on the specimen surfaces in all

185



K. Fujiyoshi, T. Ueda, M. Tsukagoshi Acta Polytechnica CTU Proceedings

0

1

2

3

N B V B+V B+FA

A
dh

es
io

n 
st

re
ng

th
(N

/m
m

2 )
Rock: Pre wet/dry exposure
Rock: Post wet/dry exposure
Concrete: Pre wet/dry exposure
Concrete: Post wet/dry exposure

Figure 6. Adhesion strength between sprayed mor-
tar and rock/concrete surfaces measured in spray
test.

compositions. As shown in figure 7, the mass reduc-
tion rate moved momentarily to the negative side in
all compositions, but then shifted to the positive side.
The cause of the negative shift was that fine cracks
occurred at the early stage of the freeze/thaw action,
then the mass increased due to the penetration of
moisture into the cracks. As the number of cycles
increased, scaling or peeling fracture occurred and fi-
nally the mass decreased.

In the previous study on sprayed concrete [5], it has
been reported that helpful air bubbles for freeze/thaw
resistance are lost by spraying pressure. When alter-
native measures were taken to obtain the benefits of
bubbles (such as adding hollow microspheres to spray
concrete mixed with vinylon fibers), fine cracks were
suppressed and reduction in relative dynamic elastic
modulus was hardly observed. In this freeze/thaw
test, no special measures were taken to compensate
for the bubbles lost by spraying pressure. Thus, V
and B+V mixed with vinylon fibers are considered
to have shown small resistance against freezing and
thawing.

In contrast to N, V and B+V, the freeze/thaw re-
sistance of B and B+FA mixed with bamboo fibers
was greatly improved. It can be presumed that bub-
bles remaining in hollow parenchyma cells of bamboo
fibers absorbed and relaxed the expansion pressure
during freezing.

5. Conclusion
In this study, fundamental properties and durability
of slope protection spray mortar reinforced with bam-
boo fibers were experimentally examined. The results
and findings are summarized as follows.

 

-1.5

1.5

� ��� ��� ���

m
as

s 
re

du
ct

io
n

ra
te

 (㸣
)

0

20

40

60

80

100

0 100 200 300
R

el
at

iv
e 

dy
na

m
ic

 e
la

st
ic

 
m

od
ul

us
  (
㸣

)

Number of cycles

N

B

V

B+V

B+FA

Figure 7. Variation curves of mass reduction rate
and relative dynamic elastic modulus measured dur-
ing freeze/thaw test in spray test.

1. As far as strength is concerned, 0.75% mixing ratio
of bamboo fibers proved to perform the best. Com-
pressive strength decreased following exposure to
cyclic wet/dry conditions when only bamboo fivers
were mixed. However, when bamboo fibers were
mixed with vinylon fibers or fly ash, the decreased
strength was improved.

2. Concerning spray mortar with 0.75% bamboo
fibers mixed in, adhesion strength between the base
rock/concrete and spray mortar after exposure to
cyclic wet/dry conditions was increased by 180%
for the base rock and by 120% for the base con-
crete, compared to mortar without fibers. The per-
formance improvement was more remarkable when
vinylon fibers or fly ash was mixed in in addition
to bamboo fibers.

3. Spray mortar with 0.75% bamboo fibers had great
resistance against deterioration caused by freezing
and thawing, and relative dynamic elastic modulus
remained at 89% after 300 cycles in the freeze/thaw
test (mortar without fibers: 0% at 120 cycles). The
mortar combining 0.75% bamboo fibers with fly ash
remained at 59% at 300 cycles.

Acknowledgements
We would like to express our gratitude to Sugimoto-
Shoten Co., Ltd. for providing bamboo fiber materials
for the research and Green System Co., Ltd. for cooper-
ating in spray test.

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