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Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Yunpei Liang
Professor, State Key Laboratory of Coal Mine Disaster Dynamics and
Control, Chongqing University, Chongqing, China
Yunhai Cheng
Professor, The Key Laboratory of Safe and High-efficiency,
Coal Mining, Mining of Ministry of Education of Anhui,
Anhui University of Science and Technology, Huainan
Anhui, China
Houli Fu
Professor, School of Civil Engineering & Architecture, Linyi University, Linyi,
Shandong, China
Xuelong Li
Lecturer, State Key Laboratory of Coal Mine Disaster Dynamics and Control,
Chongqing University, Chongqing, China; State Key Laboratory of Coal
Resources and Safe Mining, China University of Mining and Technology,
Xuzhou, Jiangsu, China (corresponding author: lixlcumt@126.com)
Bo Li
Post-doctoral student, State Key Laboratory of Coal Mine Disaster Dynamics
and Control, Chongqing University, Chongqing, China
Yulong Chen
Professor, State Key Laboratory of Hydroscience and Engineering,
Tsinghua University, Beijing, China
As the depth of mines increases, the in situ stress increases gradually, resulting in increasingly demanding
requirements for the support strength of coal-mine roadways and making it more difficult to provide this support.
For this reason, more effective forms of support are needed. Concrete is a typical support form, but ordinary
concrete has low support strength, cracks easily and is prone to brittle failure, whereas polypropylene fibre-
reinforced concrete has a good inhibition effect on crack propagation and can remarkably improve the post-cracking
performance and toughness of concrete. This paper studies the mechanical properties and application effects of wet-
sprayed polypropylene fibre concrete in the mining context. A final mix proportion is obtained, a flexural toughness
test on the round slabs of the concrete is conducted, and the corresponding initial cracking strength, peak strength,
fracture energy and toughness are studied. This wet-sprayed polypropylene fibre concrete was applied and its
application effect investigated. The results show that wet-sprayed polypropylene fibre concrete has advantages such
as enhanced integrity and more uniform stress distribution. The initial cracking strength, peak load and energy
absorbed of wet-sprayed polypropylene fibre concrete specimens are higher than those of dry-sprayed reinforcing
mesh concrete specimens. An engineering practice application indicates that the resilience rate and dust
concentration were significantly reduced during wet-sprayed polypropylene fibre concrete construction, and
construction efficiency and economic benefits clearly increased. The research achievements are of great theoretical
and practical significance in developing a further understanding of the fracture instability mechanism and mechanical
properties of wet-sprayed polypropylene fibre concrete. This method may be applied widely in sinking and driving
engineering to improve the support effect and ensure mine safety.
Notation
R
d
resilience rate of dry-sprayed concrete, taken as 30%
R
w
resilience rate of wet-sprayed concrete, taken as 10%
S
d
actual material cost of dry-sprayed concrete
S
w
actual material cost of wet-sprayed concrete
T
d
ratio of actual spraying thickness of dry-sprayed
concrete to the designed spraying thickness
T
w
ratio of actual spraying thickness of wet-sprayed
concrete to the designed spraying thickness
Z
d
theoretical material cost of dry-sprayed concrete,
US$75·30/m
3
Z
w
theoretical material cost of wet-sprayed concrete,
US$69·25/m
3
α
0
1
original crack length in district I
Δαfracture increases length at district II
σ
0
principal stress perpendicular to the direction of
the fracture
Introduction
Coal is an important source of energy and accounts for a
large proportion of China’s primary energy consumption
(Li et al., 2016; Liu et al., 2019a, 2019b; Wang and Zhang,
2018). In recent years, because of the transformation of
China’s economic structure, increasing attention has been paid
to coal-induced environmental problems. The proportion of
coal in energy consumption has declined somewhat, but it is
predicted that the proportion of coal in energy sources may
still reach 50% by 2050 (Tzampoglou and Loupasakis, 2018;
Zou and Lin, 2017, 2018). This increase is mainly due to the
conditions in China, such as being rich in coal and poor in oil
and gas. China’s economic focus is mainly on the central
East region, which has the largest coal demand. In this region,
the coal-mining depth has increased at a speed of 10–20 m/a
and many mines have reached depths of more than 600 m.
When coal mining occurs at deep strata, the in situ stress
1
Cite this article
Liang Y, Cheng Y, Fu H et al.
Research into the mechanical properties of wet-sprayed polypropylene
fibre-reinforced concrete.
Magazine of Concrete Research,
https://doi.org/10.1680/jmacr.18.00025
Magazine of Concrete Research
Research Article
Paper 1800025
Received 11/01/2018; Revised 27/03/2019;
Accepted 29/03/2019
ICE Publishing: All rights reserved
Keywords: durability-related properties/
fibre-reinforced concrete/
testing, structural elements
increases and coal-mine roadway deformations become more
serious, which are significant contributing factors in coal and
rock dynamic disasters. Reasonable and effective supports are
fundamental measures when attempting to control coal-mine
roadway deformations and to ensure roadway stability (Li
et al., 2018; Lu et al., 2017; Song et al., 2016).
At present, typical support forms include ashlar lining, bolt-
shotcrete, bolt mesh shotcrete, anchor cable, grouting reinforce-
ment, reinforced concrete and steel frames. For any support
form, concrete must be sprayed on to the surface of the
surrounding rocks to maintain the long-term stability of these
rocks and to prevent the coal from further weathering
and spontaneous combustion. Therefore, concrete properties,
including mechanical properties, deformation and toughness,
play a crucial role in the stability of surrounding rocks.
Many researchers have studied the engineering of concrete
support tunnels around the world. Tests conducted by Zou
et al. (2008) and Marzouk and Marzouk (1995) indicate that
as the freeze–thaw cycles increase, the degradation degree of
concrete mechanical properties, such as axial compressive
strength, splitting tensile strength and repeated load resistance,
increases gradually. Soroushian and Elzafraney (2004) studied
the damage caused by compression, impact, fatigue and the
freeze–thaw cycle on ordinary concrete and high-performance
concrete as well as the damage effects on concrete properties,
such as bending strength, impact strength, permeability
and crack propagation degree. Hasan and Kabir (2011) and
Sukumar et al. (2008) proposed the corresponding prediction
models based on the mechanical properties of early-age con-
crete and predicted the mechanical properties of late-age con-
crete. Beushausen et al. (2012) studied the impact of different
water/cement ratios and different slag substitutes on strength
development of early-age concrete. Kaszyn
́ska (2002) studied
the relationship between the hydration heat and dynamic force
and the strength of early-age high-strength concrete. Zreiki
et al. (2010) studied the mechanical properties of early-age
concrete to predict crack propagation and residual stress and
evaluate the strain, residual strength and crack propagation
risk of early-age concrete (also investigated by Chen et al.,
2018; Kong et al., 2016; Liu et al., 2019a, 2019b). The con-
crete-filled steel tube support developed by Gao et al. (2010) is
characterised by a large counterforce and high-cost perform-
ance and has found extensive field application. Han et al.
(2010) explored the use of wet shotcrete and resin bolt support
technology in loose rock masses and found that the average
strength of wet shotcrete is 45–85% higher than that of dry
shotcrete. Feng et al. (2015) studied the effect of gangue par-
ticle size on the properties of gangue concrete and demon-
strated that, when the maximum particle size of aggregates is
no more than 12 mm, the gangue concrete has high early
strength, clearly developed late strength, and good imperme-
ability (as discussed by Cheng et al., 2017a, 2017b; Kong
et al., 2018; Nie et al., 2018; Wang et al., 2017).
Ordinary concrete has low support strength, cracks easily and
is prone to brittle failure, whereas polypropylene fibre-
reinforced concrete has a good inhibiting effect on crack propa-
gation and can remarkably improve post-cracking performance
and concrete toughness (Banthia and Sappakittipakorn, 2007;
Cai et al., 2018; Corinaldesi et al., 2015; Liu et al., 2017).
Polypropylene fibre is an organic synthetic material (Figure 1),
and the main material is modified polypropylene resin.
Polypropylene fibre has features of both synthetic fibre and
steel fibre. The main features of polypropylene fibre include
(a) strong toughness and very small and even negligible
damage to mixing tools and transport tools
(b) acid and alkali resistance up to more than 98% and good
rust prevention
(c) very light and fine make-up, a diameter of 0·8–1·0 mm,
and easy transportation and mixing with concrete
(d) fine appearance, similar colour to cement, and subtle
colour when mixed with concrete
(e) a high safety factor without hazards of any kind.
Some scholars studied polypropylene fibre concrete in the
1980s, 1990s and early 2000s. Polypropylene fibres can effec-
tively limit the early cracking of concrete, so as to enhance
the durability of concrete (Mohammadhosseini et al., 2017;
Toutanji, 1999). Research by Swamy and Jojagha (1982) and
Balendran et al. (2002a, 2002b) indicates that steel fibres can
increase the concrete compressive capacity and play an active
role in lightweight aggregate concrete. Borri et al. (2011) studied
the effect of steel fibres on the mechanical properties of light-
weight aggregate concrete and the effect of different fibres on
the properties of concrete matrix materials, such as coal ash.
The research conducted by Balendran et al. (2002a, 2002b) indi-
cates that, in comparison with high-strength concrete, light-
weight aggregate concrete has better bending resistance and
splitting resistance. Chen et al. (2006) found that polypropylene
fibre can increase the toughness and fracture resistance of con-
crete and that polypropylene fibre has a positive correlation
with concrete durability. Hannawi et al. (2016) added different
high-performance shaped fibres to the concrete and found that
Figure 1. Polypropylene fibres
2
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
Offprint provided courtesy of www.icevirtuallibrary.com
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the apparent density and compressive capacity changed little,
but the tensile capacity improved greatly. Research by Song
et al. (2005) indicates that polypropylene fibre can reinforce con-
crete strength, reduce and even avoid cracks formed by dry shot-
crete, and offer such advantages as good acid and alkali
resistance (as discussed by Shen et al., 2018). The continuous
development of new materials and continuous improvement of
the structural constituents has provided a theoretical foundation
for wet-sprayed polypropylene fibre concrete, which is better
than reinforcing mesh concrete in many respects (Minelli and
Plizzari, 2015; Tiberti et al., 2014; Yan et al., 2013).
The methods of concrete spraying applied in sinking and
driving engineering mainly include wet spraying, dry spraying
and tidal spraying (Zhang and Bi, 2013). The dry-spraying tech-
nology does not require many facilities and the process flow is
simple and easily operated and controlled, but it generates a lot
of high-concentration dust and is thus extremely unfavorable to
workers’health. In addition, dry-spraying technology has a high
resilience rate and low strength, requires a thicker shotcrete
layer, is prone to cracking, delamination and peeling, and leads
to potential safety hazards (Puppala et al., 2010; Río et al.,
2015; Wubbolts et al., 1999). In comparison with dry-spraying
technology, the tidal-spraying technology is characterised as
follows: a small amount of water is added to fine and coarse
aggregates, such as sand and stones, and then these aggregates
are mixed with the cement; additional water needs to be added
to the sprayer. As a transitional technology, tidal spraying
cannot radically overcome the shortcomings of dry-spraying
technology, including dust pollution, low strength and easy dela-
mination and peeling (Hou et al., 2013; Li et al., 2013;
Takemoto, 2000). To overcome the shortcomings of these two
technologies, wet-sprayed concrete technology has been tried.
The guiding principle of wet shotcrete is as follows: after pre-
mixing all of the concrete ingredients, they are quickly sprayed
out of the machine sprayer. Meanwhile, the sprayer’s accelerator
is uniformly mixed with the concrete to set and lay the concrete
on to the receptor surface to form a covering layer (Galobardes,
2013; Yang and Zeng, 2011). In this process, water is fully
mixed with the aggregates from the start, thus greatly reducing
dust concentration, improving the operational environment, and
reducing the incidence rate of occupational diseases. In addition,
the accelerator can effectively reduce the resilience ratio and
prevent the concrete from cracking and peeling-off (Goodier
et al., 2008; Kaufmann et al., 2013). This technology, however,
still has some disadvantages, such as low strength and a higher
degree of roughness (Benaissa et al., 2015; Liu et al., 2004).
The reinforcing mesh shotcrete technology has been widely
applied in coal mines (Jeon et al., 2016; Kirsten, 1993; Li et al.,
2017), but it has several disadvantages, including high time con-
sumption, high cost, large dust concentration, easy cracking and
peeling of the supporting layer, unevenness of the shotcrete layer
and non-uniform stress. Moreover, because of the porous surface
of the plain concrete and its softness, this technology also has
many shortcomings, such as easy deformation, low tensile capa-
city, easy wear, easy percolation and poor durability (Cengiz and
Turanli, 2004; Lee et al., 2016; Sun and Xu, 2009). With the
wet-sprayed concrete method, a reasonable proportion of poly-
propylene fibres are uniformly mixed in the concrete, and this can
overcome the shortcomings of the concrete and increase elong-
ation, tensile capacity and compressive capacity. Furthermore,
the mixed material has high hardness and high toughness
(Banthia et al., 1994; Jeng et al., 2002). Compared with plain
concrete, polypropylene fibres have strong bending resistance, are
not easily fractured and can be uniformly mixed with concrete.
After combining polypropylene fibres with concrete, themain fea-
tures are as follows (Corinaldesi et al., 2015; Yuan et al., 2011).
(a) The surface of the polypropylene fibres is uneven, thus
increasing the friction and bonding force with concrete,
so that polypropylene fibres are not easily separated from
concrete, and the concrete is not easily cracked.
(b) The concrete mixture has better durability and toughness,
and it can be securely bonded on the receptor surface,
thus greatly increasing the affinity to the matrix.
(c) The polypropylene fibres have been stretched during
production, thus increasing the concrete tensile capacity.
(d) The polypropylene fibres can be mixed with the concrete
uniformly and will not coagulate into blocks, thus
increasing the impermeability.
(e) The appearance is fine and the surface is even (Tong and
Qin-Yong, 2011).
After the polypropylenefibres havebeen added to wet-sprayed con-
crete, the latter is characterised by high toughness and low dust
concentration, so it is of great potential application as a support
form for coal-mine roadways. When the traditional reinforcing
mesh bolt-shotcrete support is used, gaps are easily formed after
the mesh is reinforced, such that the reinforcing mesh cannot be
bonded completely. The wet concrete sprayer has a large volume
and the equipment is complicated. Thus, the application of the
wet-sprayed concrete technology in coal mines is limited, and, to
date, this technology has not been comprehensively developed
and applied in Chinese coal mines. Therefore, the study of wet-
sprayed polypropylene fibre technology that is applicable to coal
mines is of great theoretical and practical significance for perfect-
ing this spraying technology, increasing work efficiency and redu-
cing dust emission. These improvements would provide a more
secure and healthy working environment; increase the strength,
hardness and flexibility of the spraying surface; and further
improve the safety and quality of coal-mine roadway support.
Round slab experiment on wet-sprayed
polypropylene fibre concrete
Preparation of the round slab specimen of wet-
sprayed polypropylene fibre concrete
To conduct tests on a round slab specimen of wet-sprayed
polypropylene fibre concrete, the wet-sprayed polypropylene
3
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution
fibre concrete was first prepared, and the specimen prepared
for testing using a tailor-made mould. Finally, the tests were
conducted to obtain the relevant parameters.
Basic raw materials
All of the raw materials for the test came from the concrete
spraying site. These comprised Portland 42.5 silicate composite
cement; crushed stones with diameters <10 mm as coarse
aggregate; and medium sand with modulus 3·1 as fine aggre-
gate, which has the characteristics of hardness and strong dura-
bility. The high-strength polypropylene fibres were provided by
a new material technology company. The reinforcing mesh was
obtained from the mine. The parameters of the reinforcement
are shown in Table 1.
Accelerator
To congeal the concrete quickly, different accelerators
(Galobardes, 2013) were added for the different concrete oper-
ation processes. The two main types of accelerator on the
market are alkali-free liquid accelerators and aluminate accel-
erators. Their characteristics are shown in Table 2.
According to Table 2, the overall effect of alkali-free liquid accel-
erators is better than that of aluminate accelerators. Field investi-
gations indicate that the powdery aluminate accelerator is widely
used in dry-sprayed concrete and that the alkali-free liquid accel-
erator is applied in wet-sprayed concrete. In the test under dis-
cussion here, the accelerator also complied with regulations.
Taking the accelerator content (relative to cement mass) as the
dependent variable –that is, 5, 6, 7 and 8% –the 2 h early
strength of concrete was tested using the penetration method.
The accelerator content could be determined from the test
results (Figure 2).
As shown in Figure 2, the accelerator content had an obvious
effect on the 2 h strength of concrete, which basically increased
as the accelerator content increased. When the accelerator
content was 5%, the 2 h strength of concrete was 0·91 MPa.
When the accelerator content was 6%, the 2 h strength of con-
crete increased substantially to 1·91 MPa and the correspond-
ing growth rate was 30·77%. When the accelerator content was
7%, the 2 h strength of concrete was significantly increased to
1·23 MPa and the corresponding growth rate was 3·36%.
When the accelerator content was 8%, the 2 h strength of con-
crete increased substantially to 1·26 MPa, and the variation of
the growth rate remained small, only 2·44%. This rate indi-
cated that, when the accelerator content was more than 6%,
the early strength of the concrete variation was small. In view
of saving on the cost of raw materials, the authors set the
accelerator content at 6%.
Mix proportion
According to the mix proportion requirements in JGJ 55-2011
(CNS, 2011) and GB 50086-2001 (CNS, 2001), the ratio of
cement to sand and stone in wet-sprayed imitation steel concrete
Table 1. Parameters of reinforcement
Type Length: mm Diameter: mm Density: g/cm
3
Content: kg/m
3
Polypropylene fibre 38 0·8 1·10 1
Reinforcing mesh 1000 5·5 7·85 40
Table 2. Comparison between alkali-free liquid accelerators and powdery aluminate accelerators
Parameter Alkali-free liquid accelerators Aluminate accelerators
Characteristic Weak acidity, no alkali ion content Strong basicity, high alkali ion content
Safety of use Safe Unsafe, intense assimilation through
respiratory tract and eyes
Alkali–aggregate reaction risk Low High
Long-term strength comparison 95–110% 60–80%
Concrete durability Increasing compactness and reduced cracking risk Poor compactness and high cracking risk
Resilience rate <10% >20%
1·2
1·0
0·8
0·6
0·4
0·2
Concrete strength: MPa
Acceterator content (%)
12345678
Figure 2. Concrete strength after 2 h using different accelerator
contents
4
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution
must be kept at 1·0 : 3·4–4·5, the water/cement ratio at
0·42–0·50, and the sand content at 50–60%. Dry-sprayed rein-
forcing mesh concrete is different from wet-sprayed polypropy-
lene fibre concrete, and the mix proportion of their raw
materials is somewhat different as well, so the previous mix pro-
portion must be adjusted appropriately (Deng and Xia, 2006).
Slump can reflect concrete flowability. The flowability of sprayed
concrete is crucial in construction. Large slump leads to easy
bleeding, whereas relatively small slump results in pipe blocking.
According to GB 50086-2001 (CNS, 2001), slump must be kept
at 8–12 cm after the preparation of hybrid materials for wet-
sprayed concrete is completed. If the slump does not meet the
requirements, it must be adjusted according to the following
standard: in the case of low slump, the water/cement ratio must
be kept at the original value and the cement slurry mass must be
increased; in the case of high slump, the sand content should
be kept at the original value and the quantity of stones must be
increased. According to GB/T 50080-2002 (CNS, 2002), the
slump test (Figure 3) may be conducted in the mine scrapyard.
For the present study, the mass ratio of cement, sand and
stone was determined to be 1:2:1·8 and the water/cement ratio
was 0·45.
Water reducer
One type of concrete additive is a water reducer. The advantages
of using a water reducer are (a) it can maintain the proportion
of all constituents in concrete and can improve the dispersivity
and flowability of the cement particles, and (b)whenother
working conditions are invariable, the water consumption is
reduced and the proportion of water and cement is effectively
reduced, thereby saving materials and increasing the strength
and durability of concrete (Sujjavanich, 2005).
According to the authors’experience, the optimum water
reducer content is 0·8%. The water reducer used in this paper
is a high-efficiency naphthalene water reducer, which is yellow-
ish-brown and powdery and has a water-reducing rate up to
more than 20% (Pang et al., 2008).
Reinforcement parameters
Ordinary concrete is a brittle material, and its tensile strength
is far lower than its compressive strength. To increase concrete
toughness, the common practice is to mix concrete with
metal materials, organic materials and inorganic materials of
high strength and ductibility to produce composite materials
and thus increase the durability and reliability of concrete
(Mckee et al., 2018). The concrete used in this paper was pre-
pared by adding high-strength polypropylene fibres to ordinary
concrete while the reinforcing mesh was obtained from the
mine. The parameters of reinforcement are shown in Table 1.
Final mix proportion
As a result of the previous test, a final economic and practical
mass proportion of concrete materials was obtained as follows:
the weight ratio of cement, sand and stone was 1:2:1·8, and the
water/cement ratio was 0·45; the alkali-free accelerator content
reached 6%, the water reducer content reached 0·8%, and the
polypropylene fibre content was 1 kg/m
3
.
Test specimen preparation
To fully reflect the actual mechanical properties of concrete,
the round slab test specimens of wet-sprayed polypropylene
fibre concrete and dry-sprayed reinforcing mesh concrete were
prepared at the concrete spraying site. In accordance with
ASTM C 1550-03a (ASTM, 2003), a new mould with a dia-
meter of 800 mm and a height of 750 mm was developed to
prepare the large concrete slab. The design and appearance of
the mould are shown in Figures 4 and 5.
The mould for preparing round slab specimens of concrete has
the following advantages.
(a) It has a composite structure: the side groove of the mould
is divided into four equal segments, thereby facilitating
splicing, use, maintenance and transportation of the
mould as well as complete removal of the specimens in
the late stage.
(b) It has multiple functions: the mould can be used to
prepare various specimens, such as common casting-
moulded or spraying-moulded round concrete specimens
and round fibre-reinforced concrete specimens as well as
round reinforcing mesh (or machine-woven wire mesh)
concrete specimens. When a round reinforcing mesh (or
machine-woven wire mesh) concrete specimen is prepared,
the reinforcing mesh can be suspended well in the central
part of the specimen through a hole in the middle of the
arc groove, so that the mechanical properties of the
prepared reinforcing mesh concrete specimen remain
consistent, to the greatest extent possible, with those of
the reinforcing mesh concrete structure applied in actual
engineering.
(c) It has a side groove fixing mode: the distance between
the arc gasket and the splicing position of every two
Figure 3. The scene of the slump test
5
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
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segments is adjusted by means of the swivel bolt, thus
facilitating the assembling or disassembling of the four
arc grooves.
In accordance with the relevant requirements of the flexural
toughness test on round concrete slabs, the specimens of poly-
propylene fibre concrete and ordinary concrete were prepared
using the previous method. Three specimens per group and
no fewer than two effective specimens were included. The
diameter and thickness of the spraying-moulded specimen
were 800 mm ± 10 mm and 75 mm ± 15 mm, respectively. The
specimens were demoulded 24 h after pouring, cured in a stan-
dard curing chamber for 28 d, and taken out of the curing
chamber for drying 3 h before the test.
Test method for round concrete slabs
The loading test apparatus used in this test was a custom
microcomputer-controlled compression testing machine
(Figure 6). The testing machine can be used to conduct
loading and unloading tests on size specimens. The specific
test steps were as follows (ASTM C 1550 (ASTM, 2003)).
(a) A prepared concrete specimen with good flatness was
selected, the specimen placed on to the test stand
(Figure 7), and the test stand height adjusted to keep the
specimen horizontal.
(b) The compression testing machine was started, and the
loading mode of the loading system set. The loading
mode used in this paper was displacement loading.
According to the relevant specifications and the actual
effect of testing, the determined loading rate for the test
was 4 mm/min. A small prestress was set at 200 N.
(c) The controller was adjusted to achieve complete contact
between the pressure head and the specimen to reach the
set prestress value, and then the test was conducted
according to the set loading mode. After complete
unstable failure of the specimen, the loading was stopped
and the crack shape observed.
(d) The pressure head was raised, the specimen replaced, and
the first three steps were repeated until testing of all the
specimens was completed.
1
2
3
4
5
6
7
4
3
2
56
2
3
4
5
6
4
2
3
56
Figure 4. New design structure of the test mould: 1, mould base
plate; 2, bolt; 3, square groove; 4, arc gasket; 5, arc groove; 6, arc
groove; 7, reinforcing mesh (or machine-woven wire mesh)
Figure 5. The new test mould
Figure 6. Microcomputer control system
Figure 7. Concrete specimen layout
6
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
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Experimental results
Concrete specimen failure mode
Figures 8(a) and 8(b) show the results of the test on multiple
specimens, which indicates the failure mode of wet-sprayed
polypropylene fibre concrete and reinforcing mesh concrete on
completion of loading.
According to the concrete failure mode in Figure 8, the breakage
of polypropylene fibre steel concrete was thorough and the speci-
men was finally broken into three blocks. Because of stress con-
centration, multiple visible fine cracks were first generated at the
specimen stress concentration position (i.e. the press head pos-
ition) during initial loading. As the test proceeded, the pressure
increased gradually and cracks propagated gradually from the
loading centre point to the periphery. After reaching the peak
stress, the bending capacity decreased slowly and cracks quickly
propagated to the specimen’s edge at nearly the same time,
breaking the specimen into three large blocks (Deng et al., 2015;
Huang et al., 2018; Liu et al., 2018). This reaction indicates the
uniform stress on the specimen and occurrence of its final plastic
failure (Figure 8(a)). Cracks in the ordinary reinforcing mesh
concrete specimen were generated at only the pressure head pos-
ition during loading. Because no polypropylene fibres were
added to the specimen, its overall harmony was poor. A hole
formed only around the pressure head because of stress concen-
tration and because pressure head extrusion caused the concrete
to fail. Several cracks propagated through the specimen to its
edge, but the concrete kept its overall integrity and was not com-
pletely unstable and broken into several blocks (Figure 8(b)).
Ultimately, brittle failure of the specimen occurred.
Initial cracking load and peak load
Initial cracking load refers to the load on the material’sinitial
cracking point, while peak load refers to the maximum load.
The initial cracking point denotes the conversion from the
linear elastic stage to the non-linear elastic-plastic stage, and this
point is the one at which the linear part of the load–deflection
curve intersects with its non-linear part. Figure 9 shows the
(a) (b)
Figure 8. Concrete failure mode: (a) wet-sprayed polypropylene fibre concrete; (b) reinforcing mesh concrete
0
5
10
15
20
25
Load: kN
30
0 1020304050
Deflection: mm
0
5
10
15
20
25
Load: kN
30
0 1020304050
Deflection: mm
(a) (b)
Figure 9. Load–deflection curve of concrete: (a) wet-sprayed polypropylene fibre concrete; (b) dry-sprayed reinforcing mesh concrete
7
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
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typical load–deflection curve of wet-sprayed polypropylene fibre
concrete and dry-sprayed reinforcing mesh concrete.
As shown in Figure 9(a), the load–deflection curve of poly-
propylene fibre steel concrete is gentle. The initial cracking
load was 9·6 kN, and then, as the deflection increased, the
load variation was relatively small. That is, polypropylene fibre
concrete has a large plastic variation. When the deflection was
25 mm, the load slowly increased to the peak load of 28·4 kN.
Afterwards, as loading was performed, the deflection increased
and the load decreased slowly. The deflection reached 50 mm,
and the load decreased to 10 kN until it brought about com-
plete instability of the specimen and test completion.
Before reaching the peak load of the dry-sprayed reinforcing
mesh concrete, the load increased sharply as the deflection
varied slightly (Figure 9(b)). The initial cracking load was
8·2 kN, which is 14·58% lower than that of the wet-sprayed
polypropylene fibre concrete. In addition, when the deflection
was 5 mm, the load increased to a peak value of 26·2 kN,
which is 7·75% lower than that of the wet-sprayed poly-
propylene fibre concrete. Afterwards, brittle failure of the
specimen occurred, and the load decreased sharply to 13 kN.
During the test, the load declined almost linearly as
deflection increased. This process occurs mainly because the
reinforcing mesh cannot couple effectively with the concrete
matrix after the concrete breaks, such that the specimen loses
its load-bearing function and cannot continuously bear the
load.
Energy absorption
The energy absorbed during concrete breakage mainly refers
to fracture energy. According to the determination method
in ASTM C 1550 (ASTM, 2003), for energy absorbed in the
fracture during a flexural toughness test, the area obtained
from the integration of the load to deflection denotes fracture
energy, thus obtaining the energy–deflection curve of concrete
failure, as shown in Figure 10.
From the energy–deflection curve of reinforced polypropylene
fibre steel concrete shown in Figure 10(a), it is evident that the
energy sharply increased in a linear fashion at the initial break-
age stage, when the load was borne by the polypropylene fibres
and concrete, and the cracking started. When the
polypropylene fibres began to transmit interfacial stress, the
energy tended to increase slowly as deflection increased, and
the specimen demonstrated a plastic variation. As shown in
Figure 9(a), the energy was 100 J at the load peak. After
reaching this peak load, the polypropylene fibres that bonded
along the two sides of the concrete fracture surface played a
role in anti-cracking, which is reflected in the increasing resist-
ance of the concrete deformation, and thereby in the increasing
toughness of the polypropylene fibre concrete (Banthia and
Sappakittipakorn, 2007; Hu et al., 2018). The energy reached
150 J as the specimen reached complete failure.
The energy-deflection variation law of dry-sprayed reinforcing
mesh concrete is similar to that of wet-sprayed polypropylene
fibre concrete. That is, the energy growth rate is at its largest
during the initial loading stage, but the energy is only 40 J as
the specimen quickly reaches its load peak. Afterwards, the
reinforcing mesh concrete suffered from brittle failure because
of this poor deformation resistance, and the energy growth rate
also decreased. After reaching the load peak, however, the
deformation increased, the load decreased linearly, and the
energy growth rate also decreased. At the end of the test,
the specimen energy reached 80 J, which is nearly 50% lower
than that of the wet-sprayed polypropylene fibre concrete
specimen. This difference occurs mainly because the reinfor-
cing mesh function in the reinforcing mesh concrete is not
stronger than that of the fibres. Therefore, the dry-sprayed rein-
forcing mesh concrete has low deformation resistance and thus
poor toughness and low energy storage capacity.
Flexural toughness index
The toughness index quantitatively characterises the fibre anti-
cracking capacity, energy absorption capacity, and overall
–20
0 10 20 30 40 50
160
140
120
Energy: J
100
80
60
40
20
0
Deflection: mm
(a)
0 10 20 30 40 50
80
70
60
Energy: J
50
40
30
10
20
0
Deflection: mm
(b)
Figure 10. Energy–deflection curve of concrete: (a) wet-sprayed polypropylene fibre concrete; (b) dry-sprayed reinforcing mesh concrete
8
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution
structural deformation resistance of the polypropylene fibre
concrete material.
According to the flexural load–deflection relation chart of con-
crete, the flexural toughness of various concretes can be
measured with a toughness index. The flexural toughness of
round concrete slabs is evaluated according to ASTM C 1550
(ASTM, 2003). The flexural toughness indexes I
5
,I
10
,I
20
and
I
40
are the ratio of energy value at 5, 10, 20 and 40 mm, respect-
ively. The energy value is the deflection to that of the initial
cracking deflection position. The energy absorption value of the
wet-sprayed polypropylene fibre concrete and dry-sprayed rein-
forcing mesh concrete at the initial cracking position in this
paper is 32·7 J and 28·6 J, respectively. The flexural toughness
index at each specified deflection is shown in Table 3.
As shown in Table 3, the bending resistance of wet-sprayed
polypropylene fibre concrete is better than that of dry-sprayed
reinforcing mesh concrete: the range of the flexural toughness of
wet-sprayed polypropylene fibre concrete varies from 1·52 to
4·15 and that of dry-sprayed reinforcing mesh concrete varies
from 1·36 to 2·53. The bending resistance of wet-sprayed poly-
propylene fibre concrete is 11·8, 30·7, 47·8 and 64·0%, respect-
ively. This resistance is higher than that of reinforcing mesh
concrete at the same specified deflections. Thus, as the load
increases, the degree of concrete failure also increases, and the
toughness of polypropylene fibre concrete is better reflected.
Discussion
Behaviour of fibres in wet-sprayed polypropylene
fibre concrete
Polypropylene fibre is a composite material. The term ‘com-
posite material’(Borri et al., 2011; Liu et al., 2013) refers to a
solid material artificially synthesised from two or more kinds
of raw material with different physical and chemical properties
in a variety of possible ways. A composite material covers one
or several disperse phases and one continuous phase (Cheng
et al., 2017a, 2017b; Moore et al., 2004). The continuous
phase is the matrix of the material, and the disperse phase is
the continuous phase. Disperse phases can greatly improve
various functions of composite materials, which is why the dis-
perse phase is called an enhanced phase. During the trans-
mission process of the load on a fibre composite, round cracks
are generally formed on the fibre’s two sides. In addition, the
fibre’s disorder distribution can increase the composite’s hard-
ness, compressive capacity and durability.
Polypropylene fibre concrete is within the elastic deformation
range in the initial failure stage. As the load force increases, it
enters the elastic-plastic deformation stage, cracks propagate
stably, and fibres play a role in anti-cracking and tension resist-
ance. The bonding action of fibres will cause fibres and the
concrete fracture surface to bear the load jointly. Thus, the
fibres can effectively increase the strength and deformation
resistance of the matrix. Continuous increases in stress will
lead to crack development and finally cause these cracks to be
interconnected or penetrated. Thus, the specimen experiences
plastic failure (He and Song, 2010; Ozguven and Ozcelik,
2013).
The composite material has a concrete matrix, which consists
of various substances in different forms and has weak com-
pressive capacity and ductility. When fibres are added to the
concrete matrix, its toughness and ductility can be effectively
enhanced to prevent it from being fractured during use.
Various institutions have developed corresponding fracture
models. The most extensive fracture model was built according
to the relationship of stress with post-fracture displacement
(Naaman et al., 1992). The stress form is shown in Figure 11.
This model powerfully illuminates the fact that the fibre struc-
ture can enhance concrete toughness. The model divides a
crack into three zones.
&Zone I refers to the concrete crack zone. There are no fibre
structures in the crack and the fibre is connected in the two
fracture surfaces. There is no stress on the crack position.
&Zone II is in the fracture occurrence stage. There are some
fibres on the fracture surface sides, and these fibres will
inhibit the fracture’s continuous development. This zone
plays a plastic role in the structure integrity. There are
cracks in this zone, but they are not completely initiated.
Zone II is a debonding zone. There are mutually contained
structures between Zones I and II.
&Zone III is referred to as a transition zone in the concrete
material structure. There are only fine cracks in this zone.
As the stress increases gradually, this zone will become
Zone II.
According to this model, fibres can inhibit the occurrence of
fracture, can make the concrete materials form a transition zone
at the fracture position, and can create a fine fracture zone
before complete damage occurs, thus preventing the fracture
from extending. When cement materials bear stress, the fibres at
the cracks will inhibit the crack from extending continuously.
Tortuous cracks form inside the materials when stress increases.
The fibre slows down the crack initiation velocity, and the
toughness and surface integrity of materials can be maintained
for a certain amount of time. In this process, the crack’sfrac-
ture energy will gradually increase in the concrete materials; in
the late fracture stage, only high stress can provide the energy
Table 3. Flexural toughness index
Concrete type
Flexural toughness index
I
5
I
10
I
20
I
40
Polypropylene fibre 1·52 2·17 3·09 4·15
Reinforcing mesh 1·36 1·66 2·09 2·53
9
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution
of the crack’s continuous extension (Aslantas
¸, 2003; Kamiya
and Sekine, 1996). This results in an increase in the strength of
polypropylene fibre concrete.
Engineering application
According to experimental studies, wet-sprayed polypropylene
fibre concrete has several advantages, including excellent per-
formance, high strength, good toughness and high absorbed
energy. This paper studied the effect of wet-sprayed polypropy-
lene fibre concrete in a coal mine. Wet-sprayed concrete con-
struction can be divided into two steps: compounding and wet
spraying. Coarse and fine aggregates are screened and com-
pounded at a certain proportion on the ground. They are then
transported by explosion-proof tank trucks to the coal faces.
After the mixing and adding of additives, the aggregates are
sprayed by manipulators. Polypropylene fibres can be added
during compounding or mixing. The main equipment is shown
in Figure 12. Manipulators are used to spray concrete and
thus not only reduce labour costs but also ensure, to a great
extent, the safety of construction personnel.
Using this process, the wet-sprayed polypropylene fibre concrete
was applied in a coal mine in Shanxi Province, China. The
application effect was investigated and analysed, mainly in terms
of concrete resilience rate, dust concentration and economic
benefit.
Concrete resilience rate
Resilience rate can be defined as the ratio of the amount of
concrete that falls from the sprayed face to the total amount of
concrete that is used during spraying. This ratio is generally
expressed as the mean of side and top resilience rates. Tests
were conducted at three different operation sites in a coal-mine
roadway (i.e. a,band c) and the concrete resilience rate at
each location is shown in Table 4.
As shown in Table 4, the resilience rate of wet-sprayed poly-
propylene fibre concrete is far lower than that of dry-sprayed
reinforcing mesh concrete. The wet-sprayed rate at the three
spraying sites is 9·7, 10·1 and 8·5%, respectively, whereas the
rate of the dry-sprayed reinforcing mesh concrete is 32·4, 34·1
and 30·2%, respectively. The mean resilience rate of wet-
sprayed polypropylene fibre concrete and dry-sprayed reinfor-
cing mesh concrete is 9·4% and 32·4%, respectively; thus the
former is only 29·01% of the latter. The resilience rate
reduction not only saves raw materials but also enhances
worker safety and efficiency.
Dust concentration
Dust concentration is an important index for evaluating a con-
crete spraying environment, and the dust generated by wet
spraying results mainly from the coal face and the feeding
position (i.e. the hopper). For this reason, the dust concen-
tration at these two sites was monitored using a CCHZ-1000
full automatic dust tester. The dust concentration at the two
sites was tested 20 min after the completion of the spraying
operation. The results are shown in Tables 5 and 6.
As shown in Tables 5 and 6, the dust concentration near
the coal face during wet-sprayed polypropylene fibre concrete
construction is remarkably lower than that during dry-
sprayed reinforcing mesh concrete construction, and, to some
extent, it is also lower near the hopper. Taking a 5 m distance
to the coal face as an example, the total dust concentration
generated by wet-sprayed polypropylene fibre concrete
construction and dry-sprayed reinforcing mesh concrete con-
struction is 3·14 and 48·35 mg/m
3
, respectively; the respirable
dust concentration is 0·63 and 3·35 mg/m
3
, respectively. Thus,
σ0
α01Δα
I II III
Figure 11. Fracture zone division of wet-sprayed polypropylene
fibre concrete
Spray
surface
Figure 12. Layout of wet-spraying machine; left to right: concrete mixer used in mining; wet concrete sprayer; auxiliary
concrete-spraying vehicle used in mining (manipulator)
10
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution
it can be concluded that the wet-sprayed polypropylene fibre
concrete is more favorable for creating a good working environ-
ment and protecting workers’health.
Economic benefit
The one-time input cost of the wet-sprayed concrete technol-
ogy is high because of the high price of wet sprayers and
liquid accelerators, but the economic benefits resulting from
the reduced resilience rate, materials saved and increased
working efficiency are enough to make up for the additional
costs. Therefore, in addition to playing a role in the support of
coal-mine roadways, wet-sprayed concrete technology can gen-
erate economic benefits for the mine. Table 7 shows the typical
cost of 1 m
3
of concrete material.
As shown in Table 7, the material cost of 1 m
3
wet-spraying
concrete is US$69·25, which is about 11·8% lower than that
of dry-spraying concrete (US$75·30). The resilience rate of dry-
sprayed concrete is generally 30%, whereas that of wet-sprayed
concrete is roughly 10%. When comprehensively considering the
material savings (calculated as a 20% reduction in spraying
thickness) resulting from the increased strength of wet-sprayed
concrete and the reduced resilience rate, the actual material cost
can be calculated using the following formulas
1:Sw¼Zw1þRw
ðÞTw
and
2:Sd¼Zd1þRd
ðÞTd
Table 4. Concrete resilience rates
Construction technology
Resilience rate: %
abcMean
Wet-sprayed polypropylene fibre concrete 9·7 10·1 8·5 9·4
Dry-sprayed reinforcing mesh concrete 32·4 34·1 30·2 32·4
Table 5. Dust concentration near the coal face
Distance to the
coal face: m
Wet-sprayed polypropylene fibre Dry-sprayed reinforcing mesh
Total dust
concentration: mg/m
3
Respirable dust
concentration: mg/m
3
Total dust
concentration: mg/m
3
Respirable dust
concentration: mg/m
3
1 6·36 1·45 80·44 9·65
5 3·14 0·63 48·35 3·35
10 1·78 0·24 19·86 0·82
Table 6. Dust concentration near the hopper
Distance to the
hopper: m
Wet-sprayed polypropylene fibre Dry-sprayed reinforcing mesh
Total dust
concentration: mg/m
3
Respirable dust
concentration: mg/m
3
Total dust
concentration: mg/m
3
Respirable dust
concentration: mg/m
3
1 9·68 1·06 73·24 7·54
5 4·56 0·42 37·61 2·61
10 2·21 0·17 15·68 0·58
Table 7. Typical cost of 1 m
3
of concrete material
Item
Wet-spraying technology Dry-spraying technology
Consumption: kg Unit price: US$/t Cost: US$ Consumption: kg Unit price: US$/t Cost: US$
Cement 350 61·40 24·40 450 61·40 24·60
Sand 950 5·10 4·85 900 5·10 4·60
Stone 855 5·80 5·00 900 5·80 5·30
High-efficiency water reducer 3·5 1169·30 4·10 ———
Common accelerator 17·5 847·70 14·80 18 153·50 2·80
Polypropylene fibre 1 1607·70 16·10 ———
Reinforcing mesh ———40 950·00 38·00
Material cost ——69·25 ——75·30
11
Magazine of Concrete Research Research into the mechanical properties
of wet-sprayed polypropylene
fibre-reinforced concrete
Liang, Cheng, Fu et al.
Offprint provided courtesy of www.icevirtuallibrary.com
Author copy for personal use, not for distribution
where S
w
is the actual material cost of wet-sprayed concrete;
S
d
is the actual material cost of dry-sprayed concrete; Z
w
is the theoretical material cost of wet-sprayed concrete,
US$69·25/m
3
;Z
d
is the theoretical material cost of dry-sprayed
concrete, US$75·30/m
3
;R
w
is the resilience rate of wet-sprayed
concrete, taken as 10%; R
d
is the resilience rate of dry-
sprayed concrete, taken as 30%; and T
d
is the ratio of actual
spraying thickness of dry-sprayed concrete to the designed
spraying thickness, taken as 1.
According to the calculation results from these formulas, the
actual material cost S
d
is US$97·70 and the actual material
cost S
w
is US$58·40, and thus the material cost actually saves
about 40% per tonne. If a thickness reduction of 33% per
spraying is calculated, then the actual material cost of wet-
sprayed concrete is only US$48·90, and thus the material cost
saved is 50%.
In addition, machines can perform wet-sprayed polypropylene
fibre concrete construction, thereby effectively reducing
manpower consumption, achieving higher working effi-
ciency, shortening the construction period and improving
productivity.
Conclusions
Through experiments, a wet-sprayed polypropylene fibre con-
crete for mining was developed. Its mechanical properties were
tested and the behaviour discussed, and it was found to have a
very good application in the mining context. On the basis of
these findings, the following conclusions may be made.
(a) The flexural toughness and absorbed energy of
wet-sprayed polypropylene fibre concrete are obviously
higher than those of dry-sprayed reinforcing mesh
concrete; the initial load and peak load of wet-sprayed
polypropylene fibre concrete are respectively 17·1 and
8·4% higher than those of dry-sprayed reinforcing mesh
concrete; and the flexural toughness of wet-sprayed
polypropylene fibre concrete and dry-sprayed reinforcing
mesh concrete ranges from 1·52 to 4·15 and from 1·36
to 2·53, respectively. As the concrete failure degree
increases, the toughness of polypropylene fibre concrete
is better reflected.
(b) The length, diameter and number of fibres affect the
mechanical properties of concrete. Fibres can bond with
concrete quite well, thus inhibiting the occurrence of
fractures. Furthermore, fibres can form a transition zone
at the fracture position and a fine fracture zone before the
concrete completely fractures, thus effectively alleviating
fracture dispersion. When these cracks continue to extend,
tortuous cracks will form and the initiation velocity of the
crack will slow down.
(c) Field practice indicates that the resilience rate of
wet-sprayed concrete has been reduced and is only 29%
that of dry-sprayed concrete. The dust concentration
generated by wet-sprayed concrete construction is also
greatly reduced. In comparison with dry-sprayed concrete,
wet-sprayed concrete saves on actual material cost by no
less than 40–50% at the same required strength. This
resilience rate indicates that wet-sprayed polypropylene
fibre concrete can obviously improve the economics,
effectiveness and safety.
Only two indices cannot completely reflect the fracture mech-
anics characteristics and interfacial stress transmission action of
polypropylene fibres in concrete. As a brittle multiphase compo-
site material, polypropylene fibre concrete has a complex mech-
anism. In future studies, a constitutive model based on material
mechanics and fracture mechanics, conforming to its load vari-
ation law, is needed for polypropylene fibre concrete.
Acknowledgements
Xuelong Li and Yunhai Cheng contributed equally to this
paper. This work was financially supported by the National
Science and Technology Major Project of China (Grant No.
2016ZX05045004), the State Key Research Development
Programme of China (Grant No. 2016YFC0801404 and
2016YFC0801402), the National Natural Science Foundation
of China (51674050) and the Research Fund of the State Key
Laboratory of Coal Resources and Safe Mining, CUMT
(SKLCRSM19KF008), whose contributions are gratefully
acknowledged. The authors also thank the editor and anon-
ymous reviewers very much for their valuable advice.
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