2. Case Studies in Construction Materials 15 (2021) e00706
2
for shotcrete because of labor-intensive workmanship and time-consuming. Due to the steel mesh difficulties, various kinds of other
fibers such as PP [5], GL [6], polymer [7], amorphous [8], PA [9,10] ST fiber [11,12] have used as alternatives which are incorporating
with steel mesh and separately in shotcrete technology. Nowadays to make shotcrete, steel fiber is often used in mortar and concrete for
improving engineering properties like strength impact and toughness [13]. In addition, steel fiber also increases the ability to with
stand the first cracking [14]. Moreover, shortcrete allow for ductility and reduce the risk of failure during tunnel construction [15]. For
these reasons, flexural properties are crucial in tunneling applications. Fibers are mainly used to improve the flexural strength,
toughness, impact resistance, restrain crack formation, and fracture energy [16]. Besides, for acquiring the rapid hardening and setting
of shotcrete, the initial early strength must be improved using accelerators, as said by many researchers [17]. Song et al. examined
compressive strength, tensile strength and modulus of rupture of the steel fiber reinforced concrete with 0.5,1.0,2.0% volume fraction.
Additionally investigated 2.0% volume fraction achieved the better performance of shotcrete [18]. Wu et al. investigated ultra
high-performance mechanical properties incorporating three types and shapes of steel fiber with definite fiber volume percentage
[19]. Besides, the chloride corrosion effect tremendously decreased by using steel fiber for ultra-performance concrete [20]. Moreover,
accelerating agents mainly used in highway tunnels have high alkali content, which lowers the long-term strength, and this alkaline
component is eluted with groundwater over a long period. For producing high-strength concrete, accelerators used in the construction
field, like mineral-based, are considered [21]. The shotcrete construction performance also depends on the type of accelerator used,
such as alkali-free or aluminate types [22]. Hence, accelerators are used in shotcrete to develop early strength, reduce rebound, and
suppress ground relaxation [23]. The reason is that these accelerators enhance the primary buildup thickness and decrease rebound
and dust, thus, achieving the engineering requirements of tunnel engineering [24]. Moreover, accelerators are also required to avoid
sudden change due to the variation of the amount added, prevent shotcrete surface exfoliation and sagging phenomenon, provide a low
hygroscopic property and good preservability, have less long-term shrinkage and less cracking, and avoid adverse e_ect such as
corrosion of steel [25,26]. Shotcrete accelerators are categorized as aluminate, silicate, alkali-free, and cement mineral accelerators.
According to previous studies, an aluminate accelerator is used to improve the cement hydration and its quick setting [27]. In addition,
the alkali-free accelerator has a less alkaline content, pH of 0–7, and a long-term strength e_ect reduction with a low caustic perfor
mance [28,29]. Moreover, cement mineral accelerators (C12A7) were used to estimate shotcrete performance after 28 days for
compressive and flexural strength showing enhancement and durability [30,31]. In this study, three types of accelerators are used to
evaluate the long-term mechanical performances of shotcrete.
The flexural behavior is usually investigated based on JSCE SF-04 [32] and ASTM C1018–97 [33]. JSCE SF-04 is used to calculate
flexural toughness and flexural toughness factor or equivalent toughness strength by the area under the load-deflection curve up to the
l/150 (l=span) values of deflection. This process is used for the post-crack deflection measure [34]. In contrast, ASTM C1018–97
calculation depends on the first-crack point and uses three enumerated points after the peak load. Thus, it can generate more detailed
information about the specimens and find the accurate value of flexural parameters [35]. Thus, many researchers have investigated the
flexural behavior of fiber concrete. Ulzurrun et al. investigated two volumetric contents of the steel fiber for analyzing the flexural
response under impact loads [36]. Ding et al. explored the mechanical and self-performance applications of steel fiber and carbon fiber
as a smart material and also explained the high toughness and multiple crack behavior of the used concrete [37]. Guler studied the
effects of polyamide micro and macro single and hybrid synthetic fiber for investigating mechanical properties such as flexural
toughness of the structural lightweight aggregate concrete (SLWAC). The authors used different fiber volumes (0.25%, 0.5%, and
0.75%) [38]. Guler et al. investigated the significance of macro steel, forta-ferro, and polyamide synthetic fibers mechanical properties
on the construction technology of wet-mix shotcrete with respect to different types of fiber volume fractions such as 0.25%, 0.5%, and
1% [39]. Moreover, Guler et al. analyzed the prediction of the proposed strength models which are more precise than existing strength
models, and described a more authentic model for stell and synthetic FRC strength prediction [40]. Besides, Guler et al. examined the
capacities of steel and hybrid fiber-reinforced concrete which is filled with square aluminum, CS, and SS tube beams and scrutinized
moment. Ductility and toughness capacity test by four-point plane bending test with different fiber volume fraction ratios (0.5% and
1.5%). Additionally analyzed the pre-peak and post-peak energy absorption capacity [41]. Moreover, the most important properties of
the steel fiber were used to enhance the concrete strength, ductility, elastic modulus, and flexural behavior of concrete and SLWAC
[42–44]. Zeyad investigated the mechanical properties and the workability of the self-compacted concrete with a different type of steel
fibers [45]. They found a negative effect of the fibers on the slump flow, L-box, and V-funnel results; moreover, the compressive
strength and flexural behavior were affected by the use of different fiber types. They described the effect of steel fiber with different
concrete and mainly used 35 mm hooked-type steel fiber with different aspect ratios in the different mixes. Moreover, they used
cement mineral, aluminate, and alkali-free accelerator for concrete and shotcrete specimens.
It is evident that shotcrete applications are well accepted in tunneling applications. Besides, steel fiber and accelerators were also
found suitable to improve the properties of shotcrete. Moreover, it is essential to investigate the long term properties of the shotcrete
properties to gain confidence in the durability of the final product. Three accelerotors were incorporated with steel fiber reinforced
shotcrete specimens designed for tunneling applications. The short and long term flexural performance such as 1,3,6,12 and 24 months
were characterized through flexural toughness.
2. Experimental program
2.1. Materials
Ordinary Portland cement with 61.2% CaO, 20.8% SiO2, and 6.3% Al2O3 was used. According to the Korean Expressway Corpo
ration standard [46] of aggregate for shotcrete, we used crushed coarse aggregate (10 mm maximum size), river sand, and crushed
M.S. Hossain et al.
3. Case Studies in Construction Materials 15 (2021) e00706
3
sand fine aggregate. The physical properties of the fine and coarse aggregate-based accelerator mix are summarized in Table 1.
The initial strength enhancement is mainly characterized by accelerators, which play an important part as a supporting material of
shotcrete. In this study, the Korean made cement mineral, aluminate, and alkali-free accelerators having physical properties shown in
Table 2 were used. Cement mineral accelerators are powder type accelerator that mainly incorporates with 12CaO.7Al2O3 whose
characteristics are increased hardening and accelerated properties of the concrete by constructing the 2CaO.Al2O3.8H2O [47]. The
main criteria of cement mineral accelerators are reaction with water and being cement mineral. NaAlO2 and KAlO2 are the major
ingredients of the aluminate accelerator, which is mainly used for large thicknesses (>15 cm) and are required after excavation and
high early strength support [48]. Based on EN 934–5, a flash setting admixture can be defined as “alkali-free” when its alkali metal
content (sodium and potassium) is lower than 1%, which is expressed as equivalent of Na2O (%Na2O+0658*%K2O), and a pH of 0–7
[49,50]. Alkali-free accelerators improve the working safety by avoiding skin burns, loss of eyesight, and respiratory health problems.
Moreover, they contribute to environmental protection by reducing the release of harmful components to groundwater from the
shotcrete and its rebound.
Fibers are to strengthen materials, which are much weaker in terms of tension than in compression, and this dates back to ancient
times where straws were used to reinforce clay bricks [51]. Five types of metallic fibers, mainly steel fibers, are discussed in the ASTM
standard based on the product or process used for production, i.e., cold-drawn, cut sheet, melt-extracted, mill cut, and modified
cold-drawn wires. In this study, 35-mm hook type steel fibers were used whose physical properties are shown in Fig. 1 and Table 3.
2.2. Mix design
We followed The Korean Expressway Corporation (2003) guidelines for tunnel shotcrete composition, which is mainly used to
prepare the shotcrete mix design for best quality and standard. Here, 5% cement mineral, 5% aluminate, and 7% alkali-free accelerator
mixing rates were mainly used by the experts at the construction site for high-performance shotcrete. Moreover, for the sustainable
slump and air content, a high-range water reduction agent was used in the mixture design. Table 4 shows the shotcrete performance
test mix design.
2.3. Sample preparation
All concrete and shotcrete specimens were produced in ongoing tunnel construction. Here, concrete was produced by a ready-
mixed plant and transported by a concrete mixer truck. In contrast, the shotcrete test was performed in-situ; a batch plant was
used to mix the ingredients, and the mixed concrete was carried to the shotcrete pumping machine by a ready-mixed concrete truck.
We used the wet shotcrete process for tunneling. The shotcrete test by steel fiber performance and mixing amount was performed using
the mixing equipment available in the laboratory. Concrete reference mixes were prepared for each accelerator mix to compare the
results. In this study, “CO” is used for concrete mold and “SH” is used for shotcrete specimens as the specimen symbols.
CO and SH specimens were manufactured in situ using the reinforced steel, which was transferred from the batching plant by a
ready-mixed concrete truck, and here, SH specimens were incorporated with accelerators. Here, 150 mm × 150 mm x 550 mm
concrete beam specimens of CO and SH were manufactured for the flexural performance test. Moreover, concrete was cast as cylin
drical molds (∅100 mm × 200 mm) for the compressive strength test as per KSF 2405, and SH specimens were fabricated as shotcrete
test panels (250 mm × 600 mm x 500 mm) for core (ø 100 mm × 200 mm) compressive strength. Furthermore, test specimens were
stored for long term of 1, 3, 6, 12, and 24 months. Figs. 2 and 3 show the test specimens preparation for the test.
2.4. Testing methods
KS F 2405 is the standard test method for determining the compressive strength of cylindrical concrete. In this study, this test was
performed using ages of 1, 3, 6, 12, and 24 months with ø 100 × 200 mm cylindrical specimens to determine the compressive strength
of the mix by considering three specimens per mix. For the compressive strength test, the properly cured specimens were prepared for
the test by leveling and smoothing both end surfaces of the specimens for uniform application of the load. The compressive strength of
the specimen was calculated by dividing the maximum load it could carry by the average cross-sectional area. The load was applied at
the rate of 0.02 mm/sec with a preload of about 200 N. The peak load and the load–axial displacement were recorded by an acquisition
system.
Flexural performance tests were performed using 150 × 150 × 550 mm beam specimens based on KSF 2566 (standard test method
Table 1
Physical properties of fine and coarse aggregate.
Test Aggregate Density (g/cm3
) Fineness modulus
Cement mineral mix (CM) Fine 2.61 3.86
Coarse 2.70
Aluminate mix (AM) Fine 2.61 3.84
Coarse 2.70
Alkali-free mix (AF) Fine 2.61 3.78
Coarse 2.70
M.S. Hossain et al.
4. Case Studies in Construction Materials 15 (2021) e00706
4
for flexural performance of fiber reinforced concrete) on the 1, 3, 6, 12, and 24 months-aged specimens to determine the flexural
strength of the mix. This is similar to the JSCE SF-4 where the flexural strength is determined using a beam specimen under a third-
point loading test apparatus. The flexural strength of a specimen can be calculated as follows [52]:
fr =
Pl
bh2
, (1)
where: fr – Flexural strength (MPa).
P – Maximum load obtained (N)
l – Span (mm)
b – Width of the failed cross-section (mm)
h – Height of the failed cross-section (mm)
According to JSCE SF-4, the flexural toughness factor is the main term for expressing the flexural toughness, and this is determined
by the area under the load-deflection curve until the deflection becomes 1/150 of the span (l). This value mainly indicates the energy
absorption capacity of the specimens up to 1/150 x l. The flexural toughness factor is determined as:
Table 2
Physical properties of the accelerators.
Accelerator Type Specific gravity pH Solid content Initial set (hours) Final set (hours)
Cement mineral (CM) Powder 2.76 10–12 99.2 2.0 13.0
Aluminate (AM) Liquid 1.45 13 ± 2 45.7 3.5 13.33
Alkali-free (AF) Liquid 1.36 2.6 42.0 2.75 12.58
Fig. 1. Hooked type steel fiber.
Table 3
Physical and mechanical properties of the steel fiber.
Test Material Length (mm) Diameter (mm) Aspect ratio Tensile strength (N/mm2
)
Cement mineral mix (CM) Steel fiber 35 0.56 62.0 1123
Aluminate mix (AM) Steel fiber 35 0.58 60.1 1043
Alkali-free mix (AF) Steel fiber 35 0.57 60.9 977
Table 4
Shotcrete performance test mix design.
Accelerator Gmax
(mm)
Slump
(mm)
W/
C
S/a
(%)
Unit content (kg/m3
)
Water Cement Sand Gravel High-range water-reducing
agent
Steel fiber (kg/
m3
)
CM 10 100 0.44 65 210 480 1047 568 4.80 (1.0%) 37
AL 0.43 64.7 213 492 1074 608 3.936 (0.8%) 37
AF 0.44 62.1 206 465 1011 622 4.65 (1.0%) 40
M.S. Hossain et al.
5. Case Studies in Construction Materials 15 (2021) e00706
5
Fig. 2. Onsite beam mold shooting.
Fig. 3. Long-term curing.
Fig. 4. Load versus deflection curve for flexural toughness (ASTM C1018–97).
M.S. Hossain et al.
8. Case Studies in Construction Materials 15 (2021) e00706
8
f′
r =
Ab
δtb
×
l
bh2
(2)
where: fr´- Equivalent flexural strength (MPa).
δtb – Deflection at 1/150 of the span (mm)
Ab – Area below the load-deflection curve until the measured deflection becomes 1/150 of the span
l – Span (mm)
b – Width of the failed cross-section (mm)
h – Height of the failed cross-section (mm)
According to ASTM C 1018–97, the toughness test of steel fiber concrete depends on the size of the test specimen, which is
determined according to the length of the steel fiber to be mixed. In the flexural toughness test, the main criteria of ASTM for
determining the toughness index is to calculate the ratio of the elastic energy until the primary crack of concrete occurs, and the plastic
energy is increased by mixing steel fibers. To evaluate the quality of the overall steel fiber concrete, the residual strength coefficient up
to a certain section is calculated using the toughness index [53]. Fig. 4 shows the load versus deflection curve for analyzing the
toughness index based on the first crack, while Eq. (3) shows the formula to calculate the residual strength factor.
R5,10 = 20(I10 − I5) and R10,20 = 10(I20 − I10) (3)
3. Experimental results
3.1. Load versus displacement
For the study objective, the load versus deflection curve plays a vital role in calculating the first crack and post crack [54,55].
According to JSCE SF-4 and ASTM C1018–97, the flexural toughness depends on the load versus deflection curve, which is shown in
Fig. 5.
3.2. Effect of accelerator types on compressive strength
The change in compressive strength from the 1- to 24-month-old specimens showed that, for all accelerators, the CO specimen gains
strength until 6 months and a relatively small strength reduction of 3–5% in 24 months. Moreover, the CM and AL mix SH specimens
compressive strength increased in the 12 months and reduced in the 24 months. The AL mix showed the lowest strength reduction of
approximately 7%. In contrast, there was approximately a 19% reduction in the CM mix. The strength change in the AF mix was not
steady; however, for the 24-month-old specimens, the compressive strength decreased by 7 MPa in comparison with that for the 12-
Fig. 6. Long term compressive strength of the CM, AL, and AF accelerator mix.
M.S. Hossain et al.
9. Case Studies in Construction Materials 15 (2021) e00706
9
month-old specimens. Fig. 6 shows the change in compressive strength by age in each accelerator mix. Based on these results, and
considering the advantages of AF such as durability, environmental safety, and workers’ safety, this accelerator is more useful than AL.
3.3. Effect of accelerator types on flexural strength
In the flexural performance test, flexural strengths of both CO and SH specimens increased from 3 to 12 months and decreased after
24 months except for the CM mix, which showed high flexural strength after 24 months in both the CO and SH specimens. The flexural
strength reduction in SH was higher than that of CO; however, the reduction was not substantial in comparison with the compressive
strength change. The changes in both compressive and flexural strengths indicate that the accelerating agents reduce the long-term
performance of CO. Fig. 7 shows the change in flexural strength according to different ages for each accelerator mix. Moreover, as
the age of the specimens increased, their self-CO specimens decreased.
3.4. Effect of accelerator types on flexural toughness JSCE SF-4
3.4.1. 1 month flexural toughness
All accelerator mixes shown in Figs. 8 and 9 show high equivalent flexural strengths, above the standard equivalent strength 3 MPa
stated by the Korean Expressway Corporation. Based on the JSCE SF-4 and the Korean Expressway Corporation, flexural toughness was
calculated by the area under the load-deflection curve when the deflection becomes l/150 of the span. Toughness is the important
variable for constitute the energy absorption capacities of the specimens until collapse under the flexural load and notifying us of the
inelastic behavior. If the toughness area is increased, then the equivalent flexural toughness also increases. In this study, the analyzed
flexural toughness of CO specimens which were higher than that of SH specimens. The flexural toughness of CM accelerator-based SH
specimens was 87.13 kN mm, which was lower than that of CO specimens (128.420 kN mm). Moreover, AL accelerator mixes
conveyed the same behavior as CM accelerator mixes. In contrast, AF accelerators showed the opposite behavior. Here, the flexural
toughness area of the SH specimens (80.9 kN mm) was higher than that of the CO specimens (76.32 kN mm). According to the formula,
SH specimens with AF accelerators showed an equivalent flexural strength of 3.59 MPa, which was higher than that of CO specimen,
3.39 MPa. The other two showed a behavior similar to the CO specimens, which are 5.71 MPa and 4.89 MPa; thus, they were higher
than that of SH specimens, 3.88 MPa, and 4.17 MPa, respectively. In consequence, the shotcrete tunnel engineering using AF is more
effective than that using CM or alkaline accelerators because of showing more energy bearing capacity of the shotcrete specimens
rather than concrete specimens.
3.4.2. 3 months flexural toughness
In this study, the flexural toughness of 3 months-age specimens was scrutinized in Figs. 10 and 11. It was seen that the flexural
toughness of SH specimens was 76,649.359 N mm, which was higher than that of CO specimens (71,060.538 N mm). Besides, AL
accelerator mixes indicated the same behavior as that of CM accelerator mixes. In contrast, AF accelerators showed the opposite
behavior. Here, the flexural toughness area of the SH specimens (50,499.507 N mm) was lower than that of the CO specimens
Fig. 7. Long term flexural strength of the CM, AL, and AF accelerator mix.
M.S. Hossain et al.
10. Case Studies in Construction Materials 15 (2021) e00706
10
Fig. 8. Flexural toughness area after 1 month.
Fig. 9. Equivalent flexural strength after 1 month.
Fig. 10. Flexural toughness area after 3 months.
M.S. Hossain et al.
11. Case Studies in Construction Materials 15 (2021) e00706
11
(59,257.973 N mm). This indicates that in that period CM and AL ae showing the better performance. Thus, the accelerating effect of
the CO and SH specimens is indicated. According to JSCE SF-4, SH specimens with CM accelerators showed an equivalent flexural
strength of 3.41 MPa, which was higher than that of CO specimen (3.15 MPa). AL accelerators showed the same behavior, however, AF
accelerators mixes in SH specimens showed equivalent flexural strength of 2.24 MPa, which is lower than that of CO specimens
(2.63 MPa). Besides all, the energy absorption capacities of all specimens were drastically deceased compared to 1 month-specimens.
3.4.3. 6 months flexural toughness
6 months specimens were explored that all accelerator mixes flexural toughness area and equivalent flexural strength value of CO
specimens were higher than SH specimens value as like as normal behavior. However, all accelerator mixes CO and SH specimens value
slightly increased then 3months result. These results are shown in the Figs. 12 and 13. So, 6months specimens had more energy ab
sorption and large deformation occurred to protect the failure that 3 months results. Moreover, these results less than 1month results
according to the accelerators criteria. In this study mentioned, according to the long-term concrete energy absorption capacity
decreased in mixtures. Moreover, CM and AL mixtures behavior drastically changing than AF mixture specimens. So, AF mixtures with
fibers are more constructive to prevent cracks in the concrete bridging and detain the development of the cracks in concrete matrix.
3.4.4. 12 months flexural toughness
The flexural toughness of 12 months aged-specimens were analyzed, and the AL accelerator-based SH specimens exhibited a
flexural toughness higher than that of CO specimens. Besides, AF accelerator mixes indicated the same behavior as AL accelerator
mixes with almost equal values (51,836.797 and 511,627.747 N mm) for SH and CO specimens, respectively. However, CM accel
erators showed the opposite behavior. Moreover, AF mixtures results were more consistent than other mixtures. Based on JSCE SF-4,
SH specimens with AL accelerators showed an equivalent flexural strength of 2.91 MPa, which was higher than that of CO specimen
Fig. 11. Equivalent flexural strength after 3 months.
Fig. 12. 6months flexural toughness area.
M.S. Hossain et al.
12. Case Studies in Construction Materials 15 (2021) e00706
12
(2.59 MPa). AF accelerators showed the same behavior, however, CM accelerators mixes with SH specimens showed an equivalent
flexural strength of 2.31 MPa, which was lower than that of CO specimens (2.92 MPa). Thus, the accelerating effect of the CO and SH
specimens is indicated. However, all equivalent flexural strength was less than 3 MPa; so, not much effect of steel fiber of the CO and
SH specimens is seen in Figs. 14 and 15.
3.4.5. 24 months flexural toughness
Flexural toughness of 24 months-aged specimens showed that all accelerators results are slightly decreased compared to 12 months-
aged specimens except AF accelerators flexural toughness are and equivalent flexural strength of SH specimens. The equivalent flexural
strength of SH specimens was 2.42 MPa, which was higher than that of 12 months-ages specimens (2.30 MPa). Thus, it is indicated that
AF accelerators incorporated with steel fiber possess a long-term energy absorption capacity that is higher than other mixes. Moreover,
hooks on the two ends of the steel fiber acknowledge them to comprehend more energy while stripping from the concrete matrix.
However, Figs. 16 and 17 indicates CM and AL accelerator mixes had a higher value than that of AF accelerator mixes specimens,
2.72 MPa and 2.67 MPa, of CM-SH and AL-SH, respectively. In addition, AF-SH specimens were more consistent than other values, and
for the safety of the workers, AF accelerators should be used in construction sites.
3.5. Flexural toughness results according to ASTM C1018-97
3.5.1. Effect of accelerator types on long term deflection results
According to ASTM C1018–97, a three-point bending test was performed for the flexural toughness and first crack strength for the
fiber reinforced concrete. The flexural strength and toughness increased with the amount of steel fiber. Although the single point
deflection is used in JSCE SF-4 for measuring the flexural behavior, ASTM C1018–97 is used to analyze the four specific deflections
under the load-deflection curve. Here, the first deflection δ is mainly used for considering the first-crack toughness, and then, 3 δ, 5.5 δ,
Fig. 13. 6months equivalent flexural strength.
Fig. 14. Flexural toughness area after 12 months.
M.S. Hossain et al.
13. Case Studies in Construction Materials 15 (2021) e00706
13
and 10.5 δ times of the first-crack mainly indicated the post crack behavior under the load-deflection curve. So, ASTM C1018–97 is
more useful to investigate the part-by-part defection under the load. Specimens demonstrated a more ductility behavior and reached
deflection value from the first crack load. Appendix 1 showed the beam cracking pattern of the specimens.The crack pattern exhibited
the load-bearing capacity and bridging inside the concrete matrix with both end hooks type stell fiber. Besides, cracking patterns were
different in time duration. So, in Appendix-1 showed the 1month and 24 months test specimens beam crack pattern. Table 5 manifested
Fig. 15. Equivalent flexural strength after 12 months.
Fig. 16. 24months flexural toughness area.
Fig. 17. 24months equivalent flexural strength.
M.S. Hossain et al.
16. Case Studies in Construction Materials 15 (2021) e00706
16
the δ, 3δ, 5.5δ and 10.5δ deflections for three samples of each mixing which are observe the actual performance of each specimen. The
appeared CM CO02 specimens had large deflection after 1 month age, and AF SH01 also had a large load caring capacity. On the
contrary, 3 months age results were smooth and sharp for CM and AL mixes, but AF indicated the changing and highest value of the
specimens with different deflection. We observed enormous changes in the specimens at different ages with several samples.
Fig. 18. Toughness indices of CM, AL, and AF accelerator mix.
M.S. Hossain et al.
17. Case Studies in Construction Materials 15 (2021) e00706
17
3.5.2. Effect of accelerator types on long-term toughness area
In this study, another important parameter, the first crack toughness, was studied according to ASTM C1018–97. Toughness mainly
depends on the energy equal to the area under the load-deflection curve. The first crack toughness indicates the energy absorption
capacity and load-carrying capacity of the first crack points of the specimens. After that, other toughness areas are also indicated based
Fig. 18. (continued).
M.S. Hossain et al.
18. Case Studies in Construction Materials 15 (2021) e00706
18
on the part-by-part deflections. These areas are calculated using the trapezoidal rule in CM, AL, and AF accelerator mixes specimens.
Table 6 shows the toughness area of the δ, 3δ, 5.5δ, and 10.5δ deflections under various loads. Thus, these areas are mainly used to
investigate the post load-bearing capacity of the specimens. Moreover, accelerators which are incorporated in the CO and SH speci
mens are also seen.
3.5.3. Effect of accelerator types on long term toughness index
The toughness index acquired by the specific deflection area by the first-crack area is defined by ASTM C1018–97 based on which,
there are three types of toughness index: I5, I10, and I20. Values of the toughness index are mainly used to investigate the comparison
of the performance of the various kinds of steel fiber concrete throughout the construction and development work. These also explain
the effectiveness of fibers in enhancing the post-cracking behavior. This study focuses on the use of CM, AL, and AF accelerators
incorporated with steel fiber in SH specimens and, in contrast, CO specimens incorporated with steel fiber excluding accelerators. The
main part of this study was investigating the long-term performance of the CO and SH specimens for 1, 3, 6, 12, and 24 months. Fig. 18
shows the toughness index of all mixes of all specimens. Here, we explain the 3 specimens of all mixes over time to scrutinize the
performance of all specimens individually. The CM CO01 specimen after 1 month showed the highest value of the toughness index such
as 5.94, 15.60, and 30.25 for I5, I10, and I20 respectively. Moreover, AF SH 03 showed a slightly higher value of 5.90,11.94 and 18.24
for I5, I10, and I20 respectively. This discernment occurred due to the casting and proper temping of the specimens, and it also depends
on the proper finishing of the bottom part of the specimens. According to ASTM C1018–97, I5 showed the initial energy absorption
Fig. 18. (continued).
M.S. Hossain et al.
19. Case Studies in Construction Materials 15 (2021) e00706
19
based on the first crack. So, if this value is high, it means it possesses a great energy absorption capacity, and I10 and I20 mean the same
thing. I5, I10, and I20 after 3 months aging in specimens were similar to the 1 month results barring some exceptional values. So, we
can ignore those values because the 3 specimen values were shown separately. Results of 6 months-aged specimens displayed slightly
higher I5 values than that of 3 months; on the contrary, I10 and I20 results were similar. So, results of 6 months-aged specimens
explained that the initial energy absorption capacity and bending resistance were increased due to the accelerators incorporated with
steel fiber. Besides that, results of 12 and 24 months-aged specimens were more or less same with respect to the toughness index. So,
accelerators incorporated with steel fiber mixes in CO and SH specimens had minimal effect of long term duration. As JSCE SF-4 could
not analyze that way, we must analyze part by part for better understanding.
3.5.4. Effect of accelerator types on long-term residual factor
Residual strength factors based on ASTM C1018–97 are mainly used to examine the comparison of the performance of various kinds
of steel fiber concrete throughout the construction and development work. These also explain the effectiveness of fibers in enhancing
the post-cracking behavior. For analyzing the residual strength factor, we used R5,10 [20(I10-I5)] and R10,20[10(I20-I10]] terms ac
cording to ASTM C1018–97. The main part of this study was investigating the long-term performance of CO and SH specimens after 1,
3, 6, 12, and 24 months using the residual strength factor. Table 7 shows the residual strength factor of all mixes of all specimens. Here,
CM CO01 specimen after 1 month aging showed the highest value of the residual strength factor such as 193.29 and 146.48 for R5,10
and R10,20respectively. The values of R5,10 are bigger than R10,20 because I5 and I10 values are higher than I10 and I20 values. It indicates
the post cracking values at 5.5 times of the first –crack different from 3 times of the first-crack is bigger than the difference between
10.5 times to 5.5 times of the first crack. In this study, we can see that the post-cracking bending resistance increased due to the use of
steel fiber and accelerators. Moreover, CM and AL accelerator mixes showed slightly different results from CO and SH specimens.
Besides that, AF mixes showed the opposite behavior, which is displayed in Table 7. R5,10, and R10,20 of the 3 month-aged specimens
showed that SH specimens’ values were higher than that of CO specimens. However, AL mixes values were higher than the AF values.
Results of 6 months-aged specimens suggested that most of the results of AF mixes were higher than 3 months results of AF-SH mixes.
So, the performance increased with time. Then, 12 months result also decreased as compared to the 6 months result, but values
changing the behavior of AF mixes of SH were unstable. Moreover, 24 months results showed that the values of AL and AF mixes
increased slightly as compared to 12 months result. So, accelerators incorporated with steel fiber mixes in CO and SH specimens had
minimal effect on long-term performance.
Table 7
Long term residual factor results of CM, AL, and AF accelerator mix.
1 month 3 months 6 months 12 months 24 months
R5,10 R10,20 R5,10 R10,20 R5,10 R10,20 R5,10 R10,20 R5,10 R10,20
CM CO1 193.3 146.5 71.6 60.9 64.5 58.9 65.9 48.6 61.8 60.1
CO2 75.1 73.0 72.0 77.3 56.6 58.3 71.3 67.2 53.2 36.1
CO3 71.1 89.9 68.5 57.3 89.7 92.3 106.9 70.7 58.3 51.5
Avg. 113.2 103.1 70.7 65.2 70.3 69.8 81.3 62.2 57.8 49.2
SH1 55.4 57.8 72.4 59.6 67.3 56.3 60.6 43.8 50.9 37.4
SH2 52.4 66.8 62.0 62.1 68.1 58.8 99.5 60.2 47.6 34.9
SH3 93.8 73.7 61.5 53.6 58.2 53.3 58.4 45.4 53.6 43.7
Avg. 67.2 66.1 65.3 58.4 64.5 56.1 72.9 49.8 50.7 38.7
AL CO1 93.9 72.4 112.6 48.9 64.6 56.2 77.3 63.5 60.3 50.3
CO2 47.7 39.9 62.5 59.8 59.7 55.5 81.3 67.7 61.3 45.2
CO3 62.9 58.5 51.2 48.6 70.9 64.0 64.8 45.6 49.3 37.1
Avg. 68.2 56.9 75.4 52.4 65.0 58.6 74.5 58.9 57.0 44.2
SH1 63.6 57.9 62.0 56.6 63.3 53.2 66.3 54.2 62.9 49.8
SH2 66.0 69.5 60.7 53.2 77.8 70.3 106.9 70.7 58.2 49.9
SH3 72.9 69.3 62.8 59.6 61.4 53.3 61.6 40.6 70.1 59.2
Avg. 67.5 65.5 61.8 56.4 67.5 58.9 78.2 55.1 63.7 53.0
AF CO1 64.5 58.5 35.6 15.3 57.4 53.5 46.4 30.9 51.9 34.8
CO2 67.6 59.2 49.2 29.9 74.0 65.4 63.9 37.7 52.1 34.9
CO3 63.2 47.9 30.6 16.8 65.1 50.1 85.9 59.4 47.3 30.8
Avg. 65.1 55.2 38.5 20.7 65.5 56.3 65.4 42.7 50.4 33.5
SH1 83.5 41.9 48.0 29.8 60.9 38.9 92.9 21.1 54.4 43.9
SH2 77.0 73.3 65.2 49.7 77.7 64.2 63.5 42.0 64.7 49.5
SH3 120.9 63.0 36.7 24.0 76.4 54.9 60.3 41.5 63.5 45.2
Avg. 93.8 59.4 50.0 34.5 71.7 52.7 72.2 34.9 60.9 46.2
M.S. Hossain et al.
20. Case Studies in Construction Materials 15 (2021) e00706
20
4. Conclusion
In this study, the flexural toughness of 1, 3, 6, 12, and 24 months old specimens was measured using JSCE SF-04 and ASTM
C1018–97. Shotcrete long-term performance tests were executed according to different types of accelerating agent and mixing ratio of
steel fiber. The main findings are reported below:
1) Compressive and flexural strength test results showed that the concrete specimens have higher strength than that of shotcrete
specimens in all types of accelerators, and both specimens showed a significant strength reduction after 12 months. Compressive
strength test of 1 month old specimens showed that the Korea Expressway Corporation standard 21 MPa was satisfied in all test
variables. The low compressive and flexural strength after 24 months indicate that accelerating agents reduce the long-term
performance. But based on the result, compressive and flexural strength were not steady in AF. However, AL mixtures were
steadier. It also showed the highest compressive strength of the SH was 47.5 MPa. Also, in the flexural strength, AF mixtures
shotcrete was very unsteady. According to the 12 months results, AL shotcrete flexural strength result was 7.15 MPa and 24 months
result was 5.44 MPa. So, we examined that the drastic change occurring in AL flexural strength was not for the tunnel shotcrete. On
the contrary, AF shotcrete shows a slightly lower flexural strength (6.82 MPa) than AL shotcrete. But at 24 months, the flexural
strength was a little bit high (5.47 MPa), which indicated a higher load-carrying capacity in the long-term. Also, AF accelerator is
safer than the AL accelerator. For the no alkali content, AF is safe for the environment and workers. So, based on all criteria, AF is
more useful than AL and CM for tunnel shotcrete.
2) According to JSCE SF-04, the flexural toughness depends on the load-deflection curve up to the deflection = span/150. Moreover,
the Korean expressway corporation adopts JSCE SF-04 for evaluating the flexural toughness. Based on the Korean expressway
corporation, the flexural toughness is explained by the equivalent flexural strength or toughness factor, which is more than 3 MPa.
For that reason, 1 month results satisfied the criteria. CM and AL accelerator mixes of CO and SH specimens result showed SH
specimens result was lower than CO specimens result. On the contrary, AF accelerator showed a different behavior. The equivalent
flexural strength after 3 months decreased as compared to that after 1 month; however, SH specimens strength of 3.41 MPa was
higher than CO specimens result (3.35 MPa). Moreover, 6 months results were slightly increased than 3 months results. However,
12 months results again decreased based on the 6 months result, and without CM accelerator AL and AF accelerator mix SH
specimens had higher strengths than CO specimens. At 24 months, AF-SH specimens result was increased again based on the 12
months results, and this gap between CO and SH specimens was slightly bigger than others. So, with respect to long term and
consistency, AF accelerator is a better choice in tunnel construction sites.
3) ASTM C1018-97 defines the investigation of the first crack and post-peak load specific deflections for calculating the toughness
index and residual factor. These results are mainly used to analyze the specific part under the initial crack. So, this process is very
sensitive to determine the initial crack of the specimens, otherwise, this test is not performed well. Here, we analyzed all three
specimens to analyze the actual behavior. The AF specimens also showed consistent results for the shotcrete. Moreover, the ASTM
C1018-97 result showed the part-by-part deflection and toughness area of all specimens for better understanding.
In conclusion, based on the results analysis suggest that it is necessary to promote the use of AF accelerators considering the long-
term performance of tunnels and the safety of the workers. Moreover, the use of high-performance steel fibers should also be
considered for better performance of tunnels. Appendix A shows the crack behavior of the samples. Also, in future, more tests for
chemical composition changing into concrete by hydration will be done to determine the appropriate combination.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Data Availability Statement
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon
reasonable request.
Acknowledgments
This research was supported by the National Research Foundation (NRF) of Korea (grant No. NRF- 2020R1A2C3009894).
Appendix 1
1 month Concrete Beam Cracking Pattern.
M.S. Hossain et al.
21. Case Studies in Construction Materials 15 (2021) e00706
21
M.S. Hossain et al.
22. Case Studies in Construction Materials 15 (2021) e00706
22
24 months Concrete Beam Cracking Pattern.
(continued on next page)
M.S. Hossain et al.
23. Case Studies in Construction Materials 15 (2021) e00706
23
(continued)
(continued on next page)
M.S. Hossain et al.
24. Case Studies in Construction Materials 15 (2021) e00706
24
References
[1] L. Malmgren, E. Nordlund, S. Rolund, Adhesion strength and shrinkage of shotcrete, Tunn. Undergr. Space Technol. 20 (1) (2005) 33–48.
[2] L. Malmgren, E. Nordlund, Interaction of shotcrete with rock and rock bolts—a numerical study, Int. J. Rock Mech. Min. Sci. 45 (4) (2008) 538–553.
[3] G. Liu, X. Guo, W. Cheng, L. Chen, X. Cui, Investigating the migration law of aggregates during concrete flowing in pipe, Constr. Build. Mater. 251 (2020),
119065, https://doi.org/10.1016/j.conbuildmat.2020.119065.
[4] L. Chen, X. Zhang, G. Liu, Analysis of dynamic mechanical properties of sprayed fiber-reinforced concrete based on the energy conversion principle, Constr.
Build. Mater. 254 (2020), 119167, https://doi.org/10.1016/j.conbuildmat.2020.119167.
[5] G. Liu, W. Cheng, L. Chen, Investigating and optimizing the mix proportion of pumping wet-mix shotcrete with polypropylene fiber, Constr. Build. Mater. 150
(2017) 14–23.
[6] M. Khooshechin, J. Tanzadeh, Experimental and mechanical performance of shotcrete made with nanomaterials and fiber reinforcement, Constr. Build. Mater.
165 (2018) 199–205.
[7] M. Baricevic, M. Pezer, M. Rukavina, Serdar N. Stirmer, Effect of polymer fibers recycled from waste tires on properties of wet-sprayed concrete, Constr. Build.
Mater. 176 (2018) 135–144.
[8] Jun-Mo Yang, Jin-Kook Kim, Doo-Yeol Yoo, Performance of shotcrete containing amorphous fibers for tunnel applications, Tunn. Undergr. Space Technol. 64
(2017) 85–94.
(continued)
M.S. Hossain et al.
25. Case Studies in Construction Materials 15 (2021) e00706
25
[9] Yoon J.H., Jeon J.K., Kim Y.K., Chung J.M., A feasibility study on polyamide fiber reinforced shotcrete for rock support in a tunnel. Proceedings, The World of
Tunnel Conference 2013; 1: 1–8. 10.Jeon Joong, Kim WooSeok, Kim Gyu, Jeon Chan. Polyamide Fiber Reinforced Shotcrete for Tunnel Application. Materials
2016;9(3):163. https://doi.org/10.3390/ma9030163.
[10] X. Yan, L. Liu, J. Zhang, Y. Li, H. Wang, Experimental study on basic mechanical properties of steel fiber-reinforced siliceous wet shotcrete, Adv. Mater. Sci. Eng.
4 (2018) 1–8.
[11] S.J. Lee, A.H. Eom, S.J. Ryu, J.P. Won, Optimal dimension of arch-type steel fibre-reinforced cementitious composite for shotcrete, Compos. Struct. 152 (2016)
600–606.
[12] S.P. Shah, J.I. Daniel, S.H. Ahmad, M. Arockiasamy, P.N. Balaguru, C.G. Ball, H.P. Ball Jr., N. Banthia, G.B. Batson, A. Bentur, D. Casamatta, Guide for
specifying, proportioning, mixing, placing, and finishing steel fiber reinforced concrete, ACI Mater. J. 90 (1) (1993) 94–101.
[13] S. Saatci, F.J. Vecchio, Effects of Shear Mechanisms on Impact Behavior of Reinforced Concrete Beams, American Concrete Institute,, 2009.
[14] B. Maidl, M. Thewes, U. Maidl, Handbook of Tunnel Engineering ii. Basics and Additional Services for Design and Construction, Ernst & Sohn,, Berlin, 2014.
[15] T.M. Nahhas, Flexural behavior and ductility of reinforced lightweight concrete beams with polypropylene fiber, J. Constr. Eng. Manag. 1 (1) (2013) 4–10.
[16] R.P. Salvador, S.H. Cavalaro, M.A. Cincotto, A.D. de Figueiredo, Parameters controlling early age hydration of cement pastes containing accelerators for sprayed
concrete, Cem. Concr. Res. 89 (2016) 230–248.
[17] P.S. Song, S. Hwang, Mechanical properties of high-strength steel fiber-reinforced concrete, Constr. Build. Mater. 18 (2004) 669–673.
[18] Z. Wu, C. Shi, W. He, L. Wu, Effects of steel fiber content and shape on mechanical properties of ultra high performance concrete, Constr. Build. Mater. 103
(2016) 8–14.
[19] J. Chang, K. Cui, Y. Zhang, Effect of hybrid steel fibers on the mechanical performances and microstructure of sulphoaluminate cement-based reactive powder
concrete, Constr. Build. Mater. 261 (2020), 120502.
[20] J.P. Won, U.J. Hwang, C.K. Kim, S.J. Lee, Mechanical performance of shotcrete made with a high-strength cement-based mineral accelerator, Constr. Build.
Mater. 49 (2013) 175–183.
[21] Zhang, X., Zeng, L., Qiao, M., Wang, W., Chen, J., Hong, J. and Ran, Q., 2020. Optimising the setting behaviour of accelerated cement paste containing poly
(AANa-co-DMAA). Advances in Cement Research, pp.1–8.
[22] S. Pyo, T. Koh, M. Tafesse, H.K. Kim, Chloride-induced corrosion of steel fiber near the surface of ultra-high performance concrete and its effect on flexural
behavior with various thickness, Constr. Build. Mater. 224 (2019) 206–213.
[23] I. Sadrinejad, R. Madandoust, M.M. Ranjbar, The mechanical and durability properties of concrete containing hybrid synthetic fibers, Constr. Build. Mater. 178
(2018) 72–82.
[24] Nordström, E. Durability of Sprayed Concrete: Steel Fiber Corrosion in Cracks. Ph.D. Thesis, Luleå Tekniska Universitet, Luleå, Sweden, 2005.
[25] Won, J.P.; Hwang, U.J.; Kim, C.K.; Lee, S.J. Mechanical performance of shotcrete made with a high-strength cement-based mineral accelerator. Constr. Build.
Mater. 2013, 49, 175–183.
[26] Galobardes, I.; Picolo, R.; de Figueiredo, A.D.; Cavalaro, S.H.; Rueda, Á. Wet mix sprayed concrete set accelerating admixtures: Comparison between traditional
aluminate based accelerators and new formulations of alkali free accelerators, in: Proceedings of the 56 Congresso Brasileiro do Concreto, Natal, Brasil, 7–10
October 2014.
[27] C. Paglia, F. Wombacher, H. Böhni, The influence of alkali-free and alkaline shotcrete accelerators within cement systems: Influence of the temperature on the
sulfate attack mechanisms and damage, Cem. Concr. Compos. 33 (2003) 387–395.
[28] J.P. Won, U.J. Hwang, S.J. Lee, Enhanced long-term strength and durability of shotcrete with high-strength C12A7 mineral-based accelerator, Cem. Concr.
Compos. 76 (2015) 121–129.
[29] Wang, J.; Xie, Y.; Zhong, X.; Li, L. Test and simulation of cement hydration degree for shotcrete with alkaline and alkali-free accelerators. Cem. Concr. Compos.
2020, 112, 103684.
[30] A. Bentur, S. Mindess, Fiber Reinforced Cementitious Composites, CRC Press,, Boca Raton, FL, USA, 2006.
[31] J. SF, Method of test for flexural strength and flexural toughness of fiber reinforced concrete, Japan Concrete Institute,, 1984.
[32] American Society for Testing and Materials, Standard Test Method for Flexural Toughness and First-crack Strength of Fiber-reinforced Concrete (using Beam
with Third-point Loading), ASTM,, 1998.
[33] M.C. Nataraja, N. Dhang, A.P. Gupta, Toughness characterization of steel fiber-reinforced concrete by JSCE approach, Cem. Concr. Res. 30 (4) (2000) 593–597.
[34] Y. Ding, Y. Zhang, A. Thomas, The investigation on strength and flexural toughness of fibre cocktail reinforced self-compacting high performance concrete,
Constr. Build. Mater. 23 (1) (2009) 448–452.
[35] G. Ulzurrun, C. Zanuy, Flexural response of SFRC under impact loading, Constr. Build. Mater. 134 (2017) 397–411.
[36] Y. Ding, G. Liu, A. Hussain, F. Pacheco-Torgal, Y. Zhang, Effect of steel fiber and carbon black on the self-sensing ability of concrete cracks under bending,
Constr. Build. Mater. 207 (2019) 630–639.
[37] S. Guler, The effect of polyamide fibers on the strength and toughness properties of structural lightweight aggregate concrete, Constr. Build. Mater. 173 (2018)
394–402.
[38] S. Guler, B. Oker, Z.F. Akbulut, Workability, strength and toughness properties of different types of fiber-reinforced wet-mix shotcrete, Structures 31 (2021)
781–791.
[39] S. Guler, D. Yavuz, F. Korkut, A. Ashour, Erratum: Borderud SP, Li Y, Burkhalter JE, Sheffer CE and Ostroff JS. Electronic cigarette use among patients with
cancer: Characteristics of electronic cigarette users and their smoking cessation outcomes. Cancer. doi: 10.1002/ cncr.28811, Cancer 121 (2015) 800, https://
doi.org/10.1002/suco.201800088.
[40] S. Guler, D. Yavuz, Post-cracking behavior of hybrid fiber-reinforced concrete-filled steel tube beams, Constr. Build. Mater. 205 (2019) 285–305.
[41] P. Shafigh, H. Mahmud, M.Z. Jumaat, Effect of steel fiber on the mechanical properties of oil palm shell lightweight concrete, Mater. Des. 32 (7) (2011)
3926–3932.
[42] V.C. Khed, B.S. Mohammed, M.S. Liew, N.A.W.A. Zawawi, Development of response surface models for self-compacting hybrid fibre reinforced rubberized
cementitious composite, Constr. Build. Mater. 232 (2020), 117191.
[43] J. Jun Li, C. jun Wan, J. gang Niu, L. feng Wu, Y. chao Wu, Investigation on flexural toughness evaluation method of steel fiber reinforced lightweight aggregate
concrete, Constr. Build. Mater. 131 (2017) 449–458.
[44] A.M. Zeyad, Effect of fibers types on fresh properties and flexural toughness of self-compacting concrete, J. Mater. Res. Technol. 9 (3) (2020) 4147–4158.
[45] Korea Highway Corporation Road research institute (2000), Enhancement and quality control of steel fiber reinforced shotcrete (II).
[46] J.P. Won, U.J. Hwang, S.J. Lee, Enhanced long-term strength and durability of shotcrete with high-strength C12A7 mineral-based accelerator, Cem. Concr. Res.
76 (2015) 121–129.
[47] C. Maltese, C. Pistolesi, A. Bravo, F. Cella, T. Cerulli, D. Salvioni, A case history: Effect of moisture on the setting behaviour of a Portland cement reacting with an
alkali-free accelerator, Cem. Concr. Res. 37 (6) (2007) 856–865.
[48] C. Maltese, C. Pistolesi, A. Bravo, F. Cella, T. Cerulli, D. Salvioni, Effects of setting regulators on the efficiency of an inorganic acid based alkali-free accelerator
reacting with a Portland cement, Cem. Concr. Res. 37 (4) (2007) 528–536.
[49] R.P. Salvador, S.H. Cavalaro, I. Segura, A.D. Figueiredo, J. Pérez, Early age hydration of cement pastes with alkaline and alkali-free accelerators for sprayed
concrete, Constr. Build. Mater. 111 (2016) 386–398.
[50] KS F 2781(2007), Steel Fiber Reinforced Wet-mix Shotcrete.
[51] KS F 2566 (2014), Standard Test Method For Flexural Performance of Fiber Reinforced Concrete.
M.S. Hossain et al.
26. Case Studies in Construction Materials 15 (2021) e00706
26
[52] F. Jeng, M.L. Lin, S.C. Yuan, Performance of toughness indices for steel fiber reinforced shotcrete, Tunn. Undergr. Space Technol. 17 (1) (2002) 69–82.
[53] G.Y. Cui, J.S. Qi, D.Y. Wang, Experimental Study on Load Bearing Characteristics of Steel Fiber Reinforced Concrete Lining in the Soft Surrounding Rock Tunnel,
Adv. Civ. Eng. (2020) 2020.
[54] Carmona, S. and Molins, C., 2020, September. Equivalence Between Flexural Toughness and Energy Absorption Capacity of FRC. In RILEM-fib International
Symposium on Fibre Reinforced Concrete (pp. 253–261). Springer, Cham.
M.S. Hossain et al.
![Case Studies in Construction Materials 15 (2021) e00706
Available online 24 September 2021
2214-5095/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Case study
Long-term effect of accelerator content on flexural toughness of
steel fiber reinforced shotcrete for tunnel construction
Mohammad Shakhawat Hossain a
, Seungyeon Han b
, Seong Kwon Kim c
,
Kyong-Ku Yun d,*
a
Department of Civil Engineering, Kangwon National University, 1 Gangwondaegil, Chuncheon 24341, Korea
b
KIIT(Kangwon Institute of Inclusive Technology), Kangwon National University, 1 Gangwondaegil, Chuncheon 24341, Korea
c
Institute for Advanced Construction Materials, Kangwon National University, 1 Gangwondaegil, Chuncheon 24341, Korea
d
Department of Civil Engineering, Kangwon National University, 1 Gangwondaegil, Chuncheon 24341, Korea
A R T I C L E I N F O
Keywords:
Steel fiber shotcrete
Long-term performance
Cement mineral accelerator
Aluminate accelerator
Alkali-free accelerators
Flexural toughness
Tunnel
A B S T R A C T
Shotcrete is widely used in underground and tunneling applications due to their superior flexural
properties and impact resistance. There is a higher need in improving these properties by adding
various admixtures and fiber reinforcements to achieve superior performance to accomplish faster
turnout in tunneling constructions. In this study, the effect of three accelerators on the long-term
flexural performance of steel-fiber reinforced shotcrete were investigated. The types of acceler
ators used were aluminate, cement mineral, and alkali-free accelerators, whose effects on the
flextural properties were investigated over the periods of 1, 3, 6, 12, and 24 months. The flexural
toughness was first estimated in accordance with JSCE SF-04, where up to span/150 deflection
area under the load-deflection curve was used for calculating the flexural toughness. Whereas
ASTM C1018–97 was used to evaluate the first-crack and post-peak load toughness behavior with
three specified points for better understanding the flexural behavior of all mixtures. Moreover, it
is important to use alkali-free accelerators in terms of both the long-term performance of the
tunnels and the safety of the workers. The long-term performances of the specimens showed by
the compressive strength and flexural strength were comparable. Finally, according to the long-
term effectiveness and performance, the best mix design for the tunnel construction sites is also
proposed.
1. Introduction
Shotcrete, also known as sprayed concrete, is used for slope stability of construction or initial protection in tunnel engineering [1,
2]. Dry-mix and wet-mix shotcrete systems are the major procedures for applying the shotcrete. However, among of these wet-mix
shotcrete is more popular due to the less hazardous to human health [3,4]. Shotcrete is mainly used onto the surface through a
nozzle by compressed air, pneumatically projected, and dynamically compacted under high velocity. Moreover, shotcrete is used for
controlling low tensile strength and vulnerability to cracking because of numerous movements on the coating surfaces. Previously,
steel mesh and different types of fibers have been used to control the performance of shotcrete. However, steel wire mesh is not suitable
* Corresponding author.
E-mail addresses: shakhawat@kangwon.ac.kr (M.S. Hossain), syhan8704@kangwon.ac.kr (S. Han), kimskwon@kangwon.ac.kr (S.K. Kim),
kkyun@kangwon.ac.kr (K.-K. Yun).
Contents lists available at ScienceDirect
Case Studies in Construction Materials
journal homepage: www.elsevier.com/locate/cscm
https://doi.org/10.1016/j.cscm.2021.e00706
Received 22 August 2021; Received in revised form 15 September 2021; Accepted 19 September 2021](https://image.slidesharecdn.com/05-230217220012-8f49ff55/85/05-pdf-1-320.jpg)
![Case Studies in Construction Materials 15 (2021) e00706
2
for shotcrete because of labor-intensive workmanship and time-consuming. Due to the steel mesh difficulties, various kinds of other
fibers such as PP [5], GL [6], polymer [7], amorphous [8], PA [9,10] ST fiber [11,12] have used as alternatives which are incorporating
with steel mesh and separately in shotcrete technology. Nowadays to make shotcrete, steel fiber is often used in mortar and concrete for
improving engineering properties like strength impact and toughness [13]. In addition, steel fiber also increases the ability to with
stand the first cracking [14]. Moreover, shortcrete allow for ductility and reduce the risk of failure during tunnel construction [15]. For
these reasons, flexural properties are crucial in tunneling applications. Fibers are mainly used to improve the flexural strength,
toughness, impact resistance, restrain crack formation, and fracture energy [16]. Besides, for acquiring the rapid hardening and setting
of shotcrete, the initial early strength must be improved using accelerators, as said by many researchers [17]. Song et al. examined
compressive strength, tensile strength and modulus of rupture of the steel fiber reinforced concrete with 0.5,1.0,2.0% volume fraction.
Additionally investigated 2.0% volume fraction achieved the better performance of shotcrete [18]. Wu et al. investigated ultra
high-performance mechanical properties incorporating three types and shapes of steel fiber with definite fiber volume percentage
[19]. Besides, the chloride corrosion effect tremendously decreased by using steel fiber for ultra-performance concrete [20]. Moreover,
accelerating agents mainly used in highway tunnels have high alkali content, which lowers the long-term strength, and this alkaline
component is eluted with groundwater over a long period. For producing high-strength concrete, accelerators used in the construction
field, like mineral-based, are considered [21]. The shotcrete construction performance also depends on the type of accelerator used,
such as alkali-free or aluminate types [22]. Hence, accelerators are used in shotcrete to develop early strength, reduce rebound, and
suppress ground relaxation [23]. The reason is that these accelerators enhance the primary buildup thickness and decrease rebound
and dust, thus, achieving the engineering requirements of tunnel engineering [24]. Moreover, accelerators are also required to avoid
sudden change due to the variation of the amount added, prevent shotcrete surface exfoliation and sagging phenomenon, provide a low
hygroscopic property and good preservability, have less long-term shrinkage and less cracking, and avoid adverse e_ect such as
corrosion of steel [25,26]. Shotcrete accelerators are categorized as aluminate, silicate, alkali-free, and cement mineral accelerators.
According to previous studies, an aluminate accelerator is used to improve the cement hydration and its quick setting [27]. In addition,
the alkali-free accelerator has a less alkaline content, pH of 0–7, and a long-term strength e_ect reduction with a low caustic perfor
mance [28,29]. Moreover, cement mineral accelerators (C12A7) were used to estimate shotcrete performance after 28 days for
compressive and flexural strength showing enhancement and durability [30,31]. In this study, three types of accelerators are used to
evaluate the long-term mechanical performances of shotcrete.
The flexural behavior is usually investigated based on JSCE SF-04 [32] and ASTM C1018–97 [33]. JSCE SF-04 is used to calculate
flexural toughness and flexural toughness factor or equivalent toughness strength by the area under the load-deflection curve up to the
l/150 (l=span) values of deflection. This process is used for the post-crack deflection measure [34]. In contrast, ASTM C1018–97
calculation depends on the first-crack point and uses three enumerated points after the peak load. Thus, it can generate more detailed
information about the specimens and find the accurate value of flexural parameters [35]. Thus, many researchers have investigated the
flexural behavior of fiber concrete. Ulzurrun et al. investigated two volumetric contents of the steel fiber for analyzing the flexural
response under impact loads [36]. Ding et al. explored the mechanical and self-performance applications of steel fiber and carbon fiber
as a smart material and also explained the high toughness and multiple crack behavior of the used concrete [37]. Guler studied the
effects of polyamide micro and macro single and hybrid synthetic fiber for investigating mechanical properties such as flexural
toughness of the structural lightweight aggregate concrete (SLWAC). The authors used different fiber volumes (0.25%, 0.5%, and
0.75%) [38]. Guler et al. investigated the significance of macro steel, forta-ferro, and polyamide synthetic fibers mechanical properties
on the construction technology of wet-mix shotcrete with respect to different types of fiber volume fractions such as 0.25%, 0.5%, and
1% [39]. Moreover, Guler et al. analyzed the prediction of the proposed strength models which are more precise than existing strength
models, and described a more authentic model for stell and synthetic FRC strength prediction [40]. Besides, Guler et al. examined the
capacities of steel and hybrid fiber-reinforced concrete which is filled with square aluminum, CS, and SS tube beams and scrutinized
moment. Ductility and toughness capacity test by four-point plane bending test with different fiber volume fraction ratios (0.5% and
1.5%). Additionally analyzed the pre-peak and post-peak energy absorption capacity [41]. Moreover, the most important properties of
the steel fiber were used to enhance the concrete strength, ductility, elastic modulus, and flexural behavior of concrete and SLWAC
[42–44]. Zeyad investigated the mechanical properties and the workability of the self-compacted concrete with a different type of steel
fibers [45]. They found a negative effect of the fibers on the slump flow, L-box, and V-funnel results; moreover, the compressive
strength and flexural behavior were affected by the use of different fiber types. They described the effect of steel fiber with different
concrete and mainly used 35 mm hooked-type steel fiber with different aspect ratios in the different mixes. Moreover, they used
cement mineral, aluminate, and alkali-free accelerator for concrete and shotcrete specimens.
It is evident that shotcrete applications are well accepted in tunneling applications. Besides, steel fiber and accelerators were also
found suitable to improve the properties of shotcrete. Moreover, it is essential to investigate the long term properties of the shotcrete
properties to gain confidence in the durability of the final product. Three accelerotors were incorporated with steel fiber reinforced
shotcrete specimens designed for tunneling applications. The short and long term flexural performance such as 1,3,6,12 and 24 months
were characterized through flexural toughness.
2. Experimental program
2.1. Materials
Ordinary Portland cement with 61.2% CaO, 20.8% SiO2, and 6.3% Al2O3 was used. According to the Korean Expressway Corpo
ration standard [46] of aggregate for shotcrete, we used crushed coarse aggregate (10 mm maximum size), river sand, and crushed
M.S. Hossain et al.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)