1. INTRODUCTION
⌅Steel-fiber
reinforced concretes (SFRCs) have been widely utilized in deep
underground engineering, tunnels, and pavements due to their improved
strength, toughness, ductility, and ease of construction, etc. (1-61. Belletti, B.; Cerioni, R.; Meda, A.; Plizzari, G.A. (2008) Design aspects on steel fiber-reinforced concrete pavements. J. Mater. Civ. Eng. 20 [9], 599−607. https://doi.org/10.1061/(ASCE)0899-1561(2008)20:9(599).
2.
Kroviakov, S.; Kryzhanovskyi, V.; Zavoloka, M. (2021) Steel fibrous
concrete with high-early strength for rigid pavements repair. IOP Conf. Ser. Mater. Sci. Eng. 1162 [1], 012008. https://doi.org/10.1088/1757-899X/1162/1/012008.
3.
Li, X.; Xue, W.P.; Fu, C.; Yao, Z.S.; Liu, X.H. (2019) Mechanical
properties of high-performance steel-fibre-reinforced concrete and its
application in underground mine engineering. Mater. 12 [15], 2470. https://doi.org/10.3390/ma12152470.
4.
Massone, L.M.; Nazar, F. (2018) Analytical and experimental evaluation
of the use of fibers as partial reinforcement in shotcrete for tunnels
in Chile. Tunn, Undergr, Space Technol. 77, 13−25. https://doi.org/10.1016/j.tust.2018.03.027.
5.
Wang, X.L.; Fan, F.F.; Lai, J.X.; Xie, Y.L. (2021) Steel fiber
reinforced concrete: A review of its material properties and usage in
tunnel lining. Struct. 34 [5], 1080−1098. https://doi.org/10.1016/j.istruc.2021.07.086.
6.
Xu, H.Y.; Wang, Z.J.; Shao, Z.M.; Jin, H.S.; Li, Z.; Jiang, X.Z.; Cai,
L.B. (2020) Experimental study on crack features of steel fiber
reinforced concrete tunnel segments subjected to eccentric compression. Mater. Today Commun. 25, 101349. https://doi.org/10.1016/j.mtcomm.2020.101349.
)
In addition to the fiber volume fraction and aspect ratio of length to
diameter, the longitudinal shape of the fibers affects the bonding
strength between the fibers and concrete, thereby impacting the pullout
behavior of fibers and the mechanical properties of fiber-reinforced
concretes (7-197.
Esmaeili, J.; Andalibi, K.; Gencel, O.; Maleki, F.K.; Maleki, V.A.
(2021) Pull-out and bond-slip performance of steel fibers with various
ends shapes embedded in polymer-modified concrete. Constr. Build. Mater. 271, 121531. https://doi.org/10.1016/j.conbuildmat.2020.121531.
8.
Choi, E.; Mohammadzadeh, B.; Hwang, J.H.; Kim, W.J. (2018) Pullout
behavior of superelastic SMA fibers with various end-shapes embedded in
cement mortar. Constr. Build. Mater. 167, 605−616. https://doi.org/10.1016/j.conbuildmat.2018.02.070.
9. Hao, Y.F.; Hao, H. (2017) Pull-out behaviour of spiral-shaped steel fibres from normal-strength concrete matrix. Constr. Build. Mater. 139, 34−44. https://doi.org/10.1016/j.conbuildmat.2017.02.040.
10. Wille, K.; Kim, D.J.; Naaman, A.E. (2011) Strain-hardening UHP-FRC with low fiber contents. Mater. Struct. 44 [3], 583−598. https://doi.org/10.1617/s11527-010-9650-4.
11.
Wille, K.; Naaman, A.E.; El-Tawil, S.; Parra-Montesinos, G.J. (2012)
Ultra-high performance concrete and fiber reinforced concrete: achieving
strength and ductility without heat curing. Mater. Struct. 45 [3], 309−324. https://doi.org/10.1617/s11527-011-9767-0.
12.
Wu, Z.M.; Shi., C.J.; He., W.; Wu, L.M. (2016) Effects of steel fiber
content and shape on mechanical properties of ultra high performance
concrete. Constr. Build. Mater. 103, 8−14. https://doi.org/10.1016/j.conbuildmat.2015.11.028.
13.
Wu, Z.M.; Khayat, K.H.; Shi, C.J. (2018) How do fiber shape and matrix
composition affect fiber pullout behavior and flexural properties of
UHPC? Cem. Concr. Compos. 90, 193−201. https://doi.org/10.1016/j.cemconcomp.2018.03.021.
14.
Sulthan, F. (2020) Influence of steel fiber shapes on fresh and
hardened properties of steel fiber reinforcement self-compacting
concrete (SFRSCC). IOP Conf. Ser.: Mater. Sci. Eng. 849 [1], 012062. https://doi.org/10.1088/1757-899X/849/1/012062.
15.
Yoo, D.Y.; Kim, S.; Kim, J.J.; Chun, B. (2019) An experimental study on
pullout and tensile behavior of ultra-high-performance concrete
reinforced with various steel fibers. Constr. Build. Mater., 206, 46−61. https://doi.org/10.1016/j.conbuildmat.2019.02.058.
16. Yoo, D.Y.; Park, J.J.; Kim, S.W. (2017) Fiber pullout behavior of HPFRCC: Effects of matrix strength and fiber type. Compos. Struct. 174, 263−276. https://doi.org/10.1016/j.compstruct.2017.04.064.
17.
Liu, Y.W.; Zhang, Z.H.; Shi, C.J.; Zhu, D.J.; Li, N.; Deng, Y.L. (2020)
Development of ultra-high performance geopolymer concrete (UHPGC):
Influence of steel fiber on mechanical properties. Cem. Concr. Compos. 112, 103670. https://doi.org/10.1016/j.cemconcomp.2020.103670.
18.
Tai, Y.S.; El-Tawil, S. (2017) High loading-rate pullout behavior of
inclined deformed steel fibers embedded in ultra-high performance
concrete. Constr. Build. Mater. 148, 204−218. https://doi.org/10.1016/j.conbuildmat.2017.05.018.
19.
Krasnovsky, R.; Kapustin, D.; Korotkikh, D.; Efishov, L. (2021)
Complete diagrams of strain under axial tension of steel-fiber
reinforced concrete with different fiber types and content. IOP Conf. Ser.: Mater Sci. Eng. 1030, 012013. https://doi.org/10.1088/1757-899X/1030/1/012013.
).
For example, it has been confirmed experimentally that deformed (i.e.,
hooked and twisted) steel fibers embedded in concrete provide more
effective pullout resistance than straight steel fibers (7-147.
Esmaeili, J.; Andalibi, K.; Gencel, O.; Maleki, F.K.; Maleki, V.A.
(2021) Pull-out and bond-slip performance of steel fibers with various
ends shapes embedded in polymer-modified concrete. Constr. Build. Mater. 271, 121531. https://doi.org/10.1016/j.conbuildmat.2020.121531.
8.
Choi, E.; Mohammadzadeh, B.; Hwang, J.H.; Kim, W.J. (2018) Pullout
behavior of superelastic SMA fibers with various end-shapes embedded in
cement mortar. Constr. Build. Mater. 167, 605−616. https://doi.org/10.1016/j.conbuildmat.2018.02.070.
9. Hao, Y.F.; Hao, H. (2017) Pull-out behaviour of spiral-shaped steel fibres from normal-strength concrete matrix. Constr. Build. Mater. 139, 34−44. https://doi.org/10.1016/j.conbuildmat.2017.02.040.
10. Wille, K.; Kim, D.J.; Naaman, A.E. (2011) Strain-hardening UHP-FRC with low fiber contents. Mater. Struct. 44 [3], 583−598. https://doi.org/10.1617/s11527-010-9650-4.
11.
Wille, K.; Naaman, A.E.; El-Tawil, S.; Parra-Montesinos, G.J. (2012)
Ultra-high performance concrete and fiber reinforced concrete: achieving
strength and ductility without heat curing. Mater. Struct. 45 [3], 309−324. https://doi.org/10.1617/s11527-011-9767-0.
12.
Wu, Z.M.; Shi., C.J.; He., W.; Wu, L.M. (2016) Effects of steel fiber
content and shape on mechanical properties of ultra high performance
concrete. Constr. Build. Mater. 103, 8−14. https://doi.org/10.1016/j.conbuildmat.2015.11.028.
13.
Wu, Z.M.; Khayat, K.H.; Shi, C.J. (2018) How do fiber shape and matrix
composition affect fiber pullout behavior and flexural properties of
UHPC? Cem. Concr. Compos. 90, 193−201. https://doi.org/10.1016/j.cemconcomp.2018.03.021.
14.
Sulthan, F. (2020) Influence of steel fiber shapes on fresh and
hardened properties of steel fiber reinforcement self-compacting
concrete (SFRSCC). IOP Conf. Ser.: Mater. Sci. Eng. 849 [1], 012062. https://doi.org/10.1088/1757-899X/849/1/012062.
).
Furthermore, experimental studies demonstrate that for the same fiber
volume fraction, ordinary concrete reinforced with deformed fibers
performs better in terms of tensile and flexural strengths than those
with straight steel fibers of the same cross-sectional size (7-97.
Esmaeili, J.; Andalibi, K.; Gencel, O.; Maleki, F.K.; Maleki, V.A.
(2021) Pull-out and bond-slip performance of steel fibers with various
ends shapes embedded in polymer-modified concrete. Constr. Build. Mater. 271, 121531. https://doi.org/10.1016/j.conbuildmat.2020.121531.
8.
Choi, E.; Mohammadzadeh, B.; Hwang, J.H.; Kim, W.J. (2018) Pullout
behavior of superelastic SMA fibers with various end-shapes embedded in
cement mortar. Constr. Build. Mater. 167, 605−616. https://doi.org/10.1016/j.conbuildmat.2018.02.070.
9. Hao, Y.F.; Hao, H. (2017) Pull-out behaviour of spiral-shaped steel fibres from normal-strength concrete matrix. Constr. Build. Mater. 139, 34−44. https://doi.org/10.1016/j.conbuildmat.2017.02.040.
), and the same to ultrahigh performance concrete (UHPC) (10-1410. Wille, K.; Kim, D.J.; Naaman, A.E. (2011) Strain-hardening UHP-FRC with low fiber contents. Mater. Struct. 44 [3], 583−598. https://doi.org/10.1617/s11527-010-9650-4.
11.
Wille, K.; Naaman, A.E.; El-Tawil, S.; Parra-Montesinos, G.J. (2012)
Ultra-high performance concrete and fiber reinforced concrete: achieving
strength and ductility without heat curing. Mater. Struct. 45 [3], 309−324. https://doi.org/10.1617/s11527-011-9767-0.
12.
Wu, Z.M.; Shi., C.J.; He., W.; Wu, L.M. (2016) Effects of steel fiber
content and shape on mechanical properties of ultra high performance
concrete. Constr. Build. Mater. 103, 8−14. https://doi.org/10.1016/j.conbuildmat.2015.11.028.
13.
Wu, Z.M.; Khayat, K.H.; Shi, C.J. (2018) How do fiber shape and matrix
composition affect fiber pullout behavior and flexural properties of
UHPC? Cem. Concr. Compos. 90, 193−201. https://doi.org/10.1016/j.cemconcomp.2018.03.021.
14.
Sulthan, F. (2020) Influence of steel fiber shapes on fresh and
hardened properties of steel fiber reinforcement self-compacting
concrete (SFRSCC). IOP Conf. Ser.: Mater. Sci. Eng. 849 [1], 012062. https://doi.org/10.1088/1757-899X/849/1/012062.
).
Moreover, experimental results reveal that a smaller fiber size
provides an evident improvement in the mechanical properties of
concretes (77.
Esmaeili, J.; Andalibi, K.; Gencel, O.; Maleki, F.K.; Maleki, V.A.
(2021) Pull-out and bond-slip performance of steel fibers with various
ends shapes embedded in polymer-modified concrete. Constr. Build. Mater. 271, 121531. https://doi.org/10.1016/j.conbuildmat.2020.121531.
, 15-1715.
Yoo, D.Y.; Kim, S.; Kim, J.J.; Chun, B. (2019) An experimental study on
pullout and tensile behavior of ultra-high-performance concrete
reinforced with various steel fibers. Constr. Build. Mater., 206, 46−61. https://doi.org/10.1016/j.conbuildmat.2019.02.058.
16. Yoo, D.Y.; Park, J.J.; Kim, S.W. (2017) Fiber pullout behavior of HPFRCC: Effects of matrix strength and fiber type. Compos. Struct. 174, 263−276. https://doi.org/10.1016/j.compstruct.2017.04.064.
17.
Liu, Y.W.; Zhang, Z.H.; Shi, C.J.; Zhu, D.J.; Li, N.; Deng, Y.L. (2020)
Development of ultra-high performance geopolymer concrete (UHPGC):
Influence of steel fiber on mechanical properties. Cem. Concr. Compos. 112, 103670. https://doi.org/10.1016/j.cemconcomp.2020.103670.
).
Besides, concretes reinforced with straight steel fibers with smaller
cross-sectional dimensions outperforms those with deformed steel fibers
in terms of tensile and flexural strengths (77.
Esmaeili, J.; Andalibi, K.; Gencel, O.; Maleki, F.K.; Maleki, V.A.
(2021) Pull-out and bond-slip performance of steel fibers with various
ends shapes embedded in polymer-modified concrete. Constr. Build. Mater. 271, 121531. https://doi.org/10.1016/j.conbuildmat.2020.121531.
, 1414.
Sulthan, F. (2020) Influence of steel fiber shapes on fresh and
hardened properties of steel fiber reinforcement self-compacting
concrete (SFRSCC). IOP Conf. Ser.: Mater. Sci. Eng. 849 [1], 012062. https://doi.org/10.1088/1757-899X/849/1/012062.
, 1616. Yoo, D.Y.; Park, J.J.; Kim, S.W. (2017) Fiber pullout behavior of HPFRCC: Effects of matrix strength and fiber type. Compos. Struct. 174, 263−276. https://doi.org/10.1016/j.compstruct.2017.04.064.
). It should be noted that the fibers involved aforementioned studies have a circular shaped cross section.
Despite
the limited number of reported studies,the cross-sectional shape of the
fibers is revealed to impact the performance of fiber-reinforced
concretes (2020.
Rezakhani, R.; Scott, D.A.; Bousikhane, F.; Pathirage, M.; Cusatis, G.
(2021) Influence of steel fiber size, shape, and strength on the
quasi-static properties of ultra-high performance concrete: Experimental
investigation and numerical modeling. Constr. Build. Mater. 296 [1], 123532. https://doi.org/10.1016/j.conbuildmat.2021.123532.
, 2121.
Ushida, K.; Nasir, S.; Uehara, T.; Umehara, H. (2004) Effects of fiber
shapes and contents on steel fiber reinforcement in high-strength
concrete. Concr. Res. Technol. 15 [2], 13−23. https://doi.org/10.3151/crt1990.15.2_13.
).
According to Rezakhani et al., for instance, steel fibers with a
square-shaped cross-section of 0.91 × 0.91 mm contribute less toward
enhancing the compressive strength of concrete than those with
circular-shaped cross-sections with diameters of 0.5, 0.2, and 0.16 mm (2020.
Rezakhani, R.; Scott, D.A.; Bousikhane, F.; Pathirage, M.; Cusatis, G.
(2021) Influence of steel fiber size, shape, and strength on the
quasi-static properties of ultra-high performance concrete: Experimental
investigation and numerical modeling. Constr. Build. Mater. 296 [1], 123532. https://doi.org/10.1016/j.conbuildmat.2021.123532.
).
Furthermore, the fiber with a smaller diameter is more favorable for
improving the tensile strength and toughness of concrete than that with a
larger diameter (2020.
Rezakhani, R.; Scott, D.A.; Bousikhane, F.; Pathirage, M.; Cusatis, G.
(2021) Influence of steel fiber size, shape, and strength on the
quasi-static properties of ultra-high performance concrete: Experimental
investigation and numerical modeling. Constr. Build. Mater. 296 [1], 123532. https://doi.org/10.1016/j.conbuildmat.2021.123532.
).
According to Ushida et al., the compressive and flexural strength of
UHPC reinforced with straight steel fibers with a crescent moon-shaped
section having an area of 0.13 mm2 is higher than those reinforced with wavy steel fibers with a square-shaped cross-section with an area of 0.25 mm2 and lower than those reinforced with hooked steel fibers having a rectangular cross-sectional shape with an area of 0.28 mm2 (2121.
Ushida, K.; Nasir, S.; Uehara, T.; Umehara, H. (2004) Effects of fiber
shapes and contents on steel fiber reinforcement in high-strength
concrete. Concr. Res. Technol. 15 [2], 13−23. https://doi.org/10.3151/crt1990.15.2_13.
).
Therefore, to analyze the impact of the cross-sectional shape of the
fiber on the performance of fiber-reinforced concretes, the fibers
concerned must have equivalent section sizes. However, studies on the
comparison of the mechanical properties of SFRCs using fibers with
equivalent section sizes and various section shapes are lacking. The
steel fibers used in steel fiber reinforced concrete in reported studies
are mostly with circular or rectangular-shaped section. In China, steel
fibers adopted in refractory ceramic castables for industrial kilns are
typically with sector annular-shaped section (22-2422.
Feng, H.X.; Jiao, Y.J.; Cao, X.Y.; Liu, J.; Han, Y.H. (2023) Effect of
steel fiber on thermal shock resistance of mullite castable. J. Chin. Ceram. Soc. 51 [6], 1565−1571. Retrieved from https://qikan.cqvip.com/Qikan/Article/Detail?id=7110021145.
23.
Ni, K.X.; Zhang, M.J.; Gu, H.Z.; Huang, A.; Li, H.M.; Shao, Z.J. (2017)
Steel fiber toughening mullite-SiC castables for coke dry quenching
furnace corbel pillar. China’s Refractory 26 [1], 24−30. Retrieved fromhttp://www.cnref.cn/EN/Y2017/V26/I1/24.
24. Wang, D.J.; Yu, S.Z.; Hu, Z.Y.; Duan, C.Y. (2021) Development and application of ρ-Al2O3 bonded mullite-corundum castables. Refrac. Lime. 46 [2], 26−28. Retrieved from http://qikan.cqvip.com/Qikan/Article/Detail?id=7104489221.
).
It can be expected, for the same cross-sectional area, the surface area
per unit length of fibers with different cross-sectional shapes may
also be different, and thus lead to the load transfer capacity during
pulling been different. However, there are no studies on the mechanical
properties of SFRCs using fibers with sector annular-shaped section.
In this study, steel-fiber reinforced ordinary concrete samples are prepared using straight navicular steel fibers with sector annular-shaped section and corrugated steel fiber with a rectangular-shaped section of equivalent section size. Moreover, a comparative study is done to analyze the effects of the fiber section shape and its volume fractions on the compressive strength, splitting tensile strength, and flexural strength of the concretes. A major part of this analysis is focused on exploring the differences in the mechanical properties based on the differences in the two fibers in terms of interface bonding, interface load transfer capacity, and the load-bearing capacity of fibers.
2. RAW MATERIALS AND EXPERIMENTAL PROCEDURE
⌅In the experimental procedure, P.O. 42.5 R ordinary Portland cement (Jidong Cement Co., Ltd., China) and polycarboxylic acid water reducer (Sobute New Materials Co., Ltd., China) are used. As coarse aggregates, gravel with a particle size of 5-20 mm, which has apparent, bulk and stacking densities of 2720 kg/m3, 1520 kg/m3, and 1650 kg/m3, respectively, and water absorption of 0.22%, is used. As the fine aggregate, sand with an apparent density of 2640 kg/m3 is adopted. Table 1 lists the aggregate gradation obtained by the screening test.
| Fine | Sieve size /mm | 4.75 | 2.36 | 1.18 | 0.6 | 0.3 | 0.15 | Sieve bottom |
| Cumulative sieve residue /% | 2.45 | 9.99 | 18.18 | 39.33 | 68.86 | 88.29 | 100.00 | |
| Coarse | Sieve size /mm | - | 26.5 | 19 | 16 | 9.5 | 4.75 | Sieve bottom |
| Cumulative sieve residue /% | - | 0.00 | 2.70 | 22.16 | 85.41 | 99.36 | 100.00 |
In this study, two types of steel fibers are used as reinforcement: the straight navicular steel fiber with an annular-sector-shaped section and the corrugated steel fiber with a rectangular-shaped section. The straight navicular steel fiber with annular-sector-shaped section (446, Zhengzhou Xuanhua Steel Fiber Co., Ltd. China) is produced by extracting the heat-resistant stainless steel melt followed by the rapid quenching method. The corrugated steel fiber with a rectangular-shaped section (CF-35, Beijing Sino-sina Building Technology Co., Ltd. China) is cut from a cold-rolled galvanized iron sheet. Table 2 lists the composition of the fibers (only steel). Before mixing, the navicular fiber is ultrasonically cleaned in a solution of citric acid. Figure 1 shows the appearance and section shape of the two steel fibers, respectively.
| C | Si | Mn | P | S | Ni | Cr | Fe | |
|---|---|---|---|---|---|---|---|---|
| Corrugated | 0.14~0.22 | 0.3 | 0.3~0.65 | 0.045 | 0.05 | - | ≤0.030 | Balance |
| Navicular | ≤0.30 | ≤3.00 | ≤1.50 | ≤0.04 | ≤0.03 | ≤0.45 | 24.00~27.00 | Balance |
The sectional shapes of the two fibers are simplified for further analysis into ideal rectangular and sector annular. Figure 2 depicts a schematic representation of the cross-sectional size parameters of the two fibers. As indicated in the figure, point C represents the section centroid. The x c -axis and the y c -axis represent the centroidal axes. The center of the circle corresponding to the annular sector is represented by point O. Table 3 displays the length and the geometric parameters of the fiber sections observed through a microscope (Axio Scope A1, Carl Zeiss Microscopy GmbH, Germany) based on 11 groups of randomly chosen fibers. The diameter of a solid circular section fiber with the same cross-sectional area to a non-circular section fiber is defined as the equivalent diameter of a non-circular section fiber.
| fiber | Length l/mm | Width b/mm | Thickness δ/mm | Height h /mm | Sectorial angle 2θ/rad | Sectionarea A/mm2 | Section circumference p/mm | Equivalent diameter de/mm | Equivalent aspect ratio |
|---|---|---|---|---|---|---|---|---|---|
| Corrugated | 42.76 | 1.85 | 0.34 | 0.34 | - | 0.648 | 4.39 | 0.91 | 47.09 |
| Navicular | 26.79 | 1.89 | 0.30 | 0.62 | 2×0.734 | 0.818 | 5.61 | 1.02 | 26.26 |
The water used for mixing is municipal water from the city of Yangling. Concrete samples with different fiber volume fractions of 0%, 0.5%, 1.0%, 1.5%, and 2.0% were prepared to analyze their impact on the mechanical properties of concrete. The water-to-cement ratio of 0.5 is adopted for each mixture. Table 4 lists the sample numbers and their respective mix proportions.
| Samples number | water/cement ratio | Water /(kg/m3) | Cement /(kg/m3) | Fine aggregate /(kg/m3) | Coarse aggregate /(kg/m3) | Water reduce agent /(kg/m3) | Navicular fiber /(kg/m3) | Corrugated fiber /(kg/m3) |
|---|---|---|---|---|---|---|---|---|
| F0J0 | 0.5 | 180 | 360 | 805 | 1105 | 2.16 | 0 | 0 |
| F5J0 | 0.5 | 180 | 360 | 805 | 1067 | 2.88 | 38 | 0 |
| F10J0 | 0.5 | 180 | 360 | 805 | 1029 | 3.60 | 76 | 0 |
| F15J0 | 0.5 | 180 | 360 | 805 | 991 | 4.32 | 114 | 0 |
| F20J0 | 0.5 | 180 | 360 | 805 | 953 | 5.04 | 152 | 0 |
| F0J5 | 0.5 | 180 | 360 | 805 | 1067 | 2.88 | 0 | 38 |
| F0J10 | 0.5 | 180 | 360 | 805 | 1029 | 3.60 | 0 | 76 |
| F0J15 | 0.5 | 180 | 360 | 805 | 991 | 4.32 | 0 | 114 |
| F0J20 | 0.5 | 180 | 360 | 805 | 953 | 5.04 | 0 | 152 |
The compressive and splitting tensile strength testing was conducted on cubes molded with a nominal size of 100 mm×100 mm×100 mm, whereas the three-point bending strength testing was conducted on unnotched prisms that were molded with a nominal size of 100 mm×100 mm×400 mm. The pullout behavior of the steel fibers was measured using 8-shaped specimens with four fibers embedded within the concrete matrix. As depicted in Figure 3 (a), the fiber embedment lengths are fixed at half of the fiber length from the center of the 8-shaped sample. The samples were demolded after casting for 24 h and subsequently cured in a chamber with relative humidity (R.H.) of 95% at 20 °C for 28 days.
The testing of the mechanical properties of the samples is performed by an Instron 1195 machine at room temperature (25 °C and 65% R.H.). As displayed in Figure 3 (b), a uniaxial pullout load is applied on the 8-shaped sample at a loading rate of 0.4 mm/min. The fiber pullout testing was conducted on five samples in each group. The loading rate for the compressive strength test and splitting strength test is 0.4 MPa s-1, while that for the flexural strength test is 0.2 mm min-1. Every test involves the testing of three samples in each configuration. The average of the tested values that were chosen with an error range within 15% is used to calculate the strength under consideration.
The bonding strength of the steel fibers to the concrete is calculated by (1313.
Wu, Z.M.; Khayat, K.H.; Shi, C.J. (2018) How do fiber shape and matrix
composition affect fiber pullout behavior and flexural properties of
UHPC? Cem. Concr. Compos. 90, 193−201. https://doi.org/10.1016/j.cemconcomp.2018.03.021.
):
Equations [2] and [3] are used to determine the splitting strength, σ ts, and flexural strength, σ f respectively (2525.
Ministry of Water Resources, People’s Republic of China. SL352—2020.
(2021) Test code for hydraulic concrete. Beijing: China Water and Power
Press.
, 2626.
EN 14651: 2005+A1: 2007, (2008) Test method for metallic fibre
concrete—measuring the flexural tensile strength. British Standards
Institution, London, UK.
).
where P max denotes the maximum load. p represents the cross-sectional circumference of the fiber; L E represents the embedment length of the fiber; and n denotes the number of embedded fibers, which, in this study, equals 4. A represents the bearing area of the cubic specimen. L denotes the span during the bending test and is equal to 300 mm. b and h are the width and height of the cross-section of the bending samples, respectively.
3. RESULTS
⌅Figure 4 displays the compressive strength of the SFRCs with different fiber volume fractions. It is observed that when the fiber volume fraction is 0.5%, the compressive strength of both SFRCs increases. This strength decreases as the fiber volume fraction rises to 1.5%, after which it increases again. Moreover, the concrete reinforced with the navicular fiber has a higher compressive strength than that with the corrugated fiber. The compressive strength of plain concrete is 39.5 MPa. However, when the fiber volume fraction is 0.5%, the compressive strengths of the navicular steel-fiber reinforced concrete and that of the corrugated steel-fiber reinforced concrete are 53.8 MPa and 52.6 MPa, respectively. The compressive strengths of navicular steel-fiber reinforced concrete and that of corrugated steel-fiber reinforced concrete, respectively, are 51.9 MPa (enhanced 33.92%) and 44.7 MPa (enhanced 9.11%) when the fiber volume fraction is 2.0%.
Figure 5 shows the flexural strength of the SFRCs with different fiber volume fractions. It is noted that with increasing fiber volume fraction, the flexural strength of both SFRCs increases. Furthermore, for the same fiber volume fraction, the concrete reinforced with navicular fibers has a higher flexural strength than that reinforced with corrugated fibers. Moreover, the flexural strength of the plain concrete is 6.08 MPa. When the fiber volume fraction is 2.0%, the navicular steel-fiber reinforced concrete and the corrugated steel-fiber reinforced concrete have flexural strengths of 7.90 MPa and 7.23 MPa, respectively, which are 29.93% and 18.91% higher than the plain concrete.
Figure 6 shows several typical load-displacement curves of plain and steel-fiber reinforced concretes during bending. The load-displacement curves of these two SFRCs are significantly different from those of plain concrete. The load-displacement curve of the plain concrete during bending demonstrates a typical brittle failure mode that is characterized by the monotonic and rapid increase of the load to the maximum value and then sharply drops off a cliff. The load-displacement curves of both types of SFRC demonstrate a bimodal ductile failure mode. During the entire loading process, the load initially rises monotonically and rapidly to the maximum value and then gradually decreases to a peak valley value. Subsequently, it slowly rises again to a certain peak value and then slowly decreases again until failure. Besides, the maximum load during the bending process and the displacement corresponding to the load dropping to 20% of the maximum load of the fiber reinforced concretes are considerably larger than those of plain concrete. Thus, the incorporation of steel fibers can significantly improve the strength and ductility of concrete during bending. Notably, the load decrement from the maximum value to the peak valley value of the descending stage for the navicular steel-fiber reinforced concretes is less than that for the corrugated steel-fiber reinforced concretes, whereas the load increment from the peak valley value to the peak value of the descending stage is larger. For instance, the maximum load, the peak valley value, and the peak value of descending stage for the navicular steel-fiber reinforced concrete F20J0 are respectively 18.02 kN, 13.84 kN, and 15.35 kN, whereas the respective values are 16.63 kN, 11.53 kN, and 12.23 kN for the corrugated steel-fiber reinforced concrete F0J20. Particularly, the load decrement from the maximum value to the peak valley value of the descending stage for the navicular steel-fiber reinforced concrete F20J0 is 4.18 kN, which is less than the 5.10 kN of the corrugated steel-fiber reinforced concrete F0J20. Meanwhile, the load increment from the peak valley value to the peak value of the descending stage for the navicular steel-fiber reinforced concrete F20J0 is 1.51 kN, which is greater than that for the corrugated steel-fiber reinforced concrete F0J20, which has a value of 0.70 kN. Furthermore, for the same fiber volume fraction, the displacements that correspond to the peak load and the load falling to 20% of the maximum load of the navicular steel-fiber reinforced concretes are larger than those of the corrugated steel-fiber reinforced concretes.
Figure 7 demonstrates the splitting tensile strength of the SFRCs with different fiber volume fractions. The figure reveals that the splitting tensile strength of both types of SFRCs increases with increasing fiber volume fraction. The navicular steel-fiber reinforced concretes have a slightly higher splitting tensile strength than the corrugated steel-fiber reinforced concretes. Moreover, it is observed that as the fiber volume fraction increases from 0% to 2.0%, the average splitting tensile strength of the concrete reinforced with the navicular steel fiber and the corrugated steel fiber increases from 5.58 MPa to 7.06 MPa and 7.04 MPa, which signifies an increase of nearly 26.52% and 26.16%, respectively.
Based on the aforementioned findings, it is observed that, though the addition of 2.0% corrugated steel fiber improves the compressive strength of plain concrete only slightly, there is a substantial improvement in both splitting tensile strength and flexural strength. The addition of 2.0% navicular steel fiber can significantly enhance the compressive strength, splitting tensile strength, flexural strength, and ductility of plain concrete. Generally, fiber-reinforced concretes are subjected to bending and compression. Furthermore, the compressive strength of steel-fiber reinforced concretes is significantly higher than the tensile strength. Hence, improving the tensile and flexural properties is of great significance, and the addition of the 2.0% steel fiber is conducive to comprehensively improving the mechanical properties of concrete.
4. ANALYSIS AND DISCUSSION
⌅During compressive loading, the aggregate inside the samples is typically subjected to a compressing effect, while some of the steel fiber could also be subjected to a bending effect in addition to the pressure. The elastic modulus and compressive strength of steel fiber are considerably higher than those of concrete. Hence, the steel fiber is conducive to improving compressive strength. However, as indicated in Figure 8, increasing the fiber volume fraction increases the air content of the concrete, which reduces the compactness of the concrete matrix and consequently weakens the compressive strength. The strengthening and weakening effects will increase with increasing steel fiber content. The compressive strength of SFRC depends on the competition of these two positive and negative effects.
Based on the simplified cross-section shapes presented in Figure 2 and the size parameters of the two fiber sections listed in Table 3, the computed values of the moment of inertia of the cross-sectional area about the centroidal axes x c and y c for the navicular steel fiber, I Fxc and I Fyc , respectively, are:
The calculated values of the moment of inertia of the cross-sectional area about the centroidal axes x c and y c for the corrugated steel fiber, I Jxc and I Jyc , respectively, are:
Thus, the moment of inertia of the navicular fiber on its centroidal axis is nearly 2.0 times that of the corrugated fiber on the same centroidal axis. This implies that the bending stiffness of the navicular steel fiber is 2.0 times that of the corrugated fiber. When the fiber is subjected to a bending load, the navicular fiber has a larger bearing capacity than the corrugated fiber. Additionally, as presented in Table 3, since the cross-sectional area of the navicular fiber is slightly larger than that of the corrugated fiber, it can withstand a greater axial load (tension and compression). Thus, it can be inferred that for the same fiber volume fraction, the navicular steel-fiber reinforced concretes can withstand a higher compressive load than the corrugated steel-fiber reinforced concretes and demonstrate superior compressive strength.
On the contrary, as indicated in Figure 8, the air content of both the fiber-reinforced concretes is extremely close to that of the plain concrete at a volume fraction of 0.5%. In this case, the fibers are largely subjected to compression, and due to the random distribution of fibers, relatively few fibers are subjected to bending. Fiber-reinforced concretes exhibit a significant increase in compressive strength since the reinforcement effect of fibers on concrete is greater than the weakening effect. As the fiber volume fraction exceeds 0.5%, a substantial increase is observed in the air content of the navicular fiber-reinforced concrete, from 2.4% to 3.5%, and in the air content of the corrugated fiber-reinforced concrete from 2.7% to 4.9%, respectively. Consequently, the increase in the compactness decrement of the fiber-reinforced concrete is more evident, which implies an enhanced weakening effect. Furthermore, the compressive strength of the fiber-reinforced concretes decreases slightly as the steel fiber volume fraction increases further. When the fiber volume fraction exceeds 1.5%, the air content decreases slightly with steel fiber volume fraction increasing. Therefore, the reinforcement effect of fibers on concrete enhances, whereas the weakening effect diminishes. Hence, the compressive strength of fiber-reinforced concrete increases again.
The tensile and bending strengths of fiber-reinforced
composites are mainly attributed to the fibers acting as a bridge across
cracks, followed by the pullout process (2727.
Xiong, B.W.; Wang, C.W.; Liu, K.; Wang, Z.J.; Xie, Z.Z.; Zhang, T.; Li,
X.T. (2020) Interfacial phase induced load transfer in short carbon
fiber reinforced Nb/Nb5Si3 composites. Mater. Sci. Eng. A. 799, 140156. https://doi.org/10.1016/j.msea.2020.140156.
, 2828.
Sunaga, D.; Koba, T.; Kanakubo, T. (2021) Modeling of bridging law for
bundled aramid fiber-reinforced cementitious composite and its
adaptability in crack width evaluation. Mater. 14 [1], 179. https://doi.org/10.3390/ma14010179.
),
which is directly influenced by the bonding between the fiber and
concrete matrix. A better frictional and stronger adhesive bond along
the fiber interface improves the overall mechanical properties of the
fiber-reinforced concrete (2929. Naaman, A.E. (2003) Engineered steel fibers with optimal properties for reinforcement of cement composites. J. Adv. Concr. Technol. 1 [3], 241−252. https://doi.org/10.3151/jact.1.241
). Based on the morphologies of the two kinds of fibers depicted in Figure 1, the surface of the navicular steel fiber is noted to be rougher than the corrugated fiber. Figure 9 illustrates the load-displacement curves of the pullout test of the
navicular and corrugated steel fibers. The peak load for corrugated
fiber (about 0.978 kN) is observed to be slightly higher than that for
navicular fiber (0.877 kN). However, the two types of fibers achieve
their maximum load at a displacement of nearly 1.0 mm. The
load-displacement curve of the corrugated fibers sharply drops off a
cliff with a displacement of nearly 2 mm after peak load. The
load-displacement curve of the navicular fibers demonstrates a clear,
gradually decreasing segment following the peak load until failure with a
considerably increased displacement of approximately 6 mm, which is
indicative of an evident sliding and pulling-off behavior. Notably, the
bonding strength of the navicular steel fibers to the concrete (2.92
MPa) is slightly higher than that of the corrugated fiber (2.61 MPa).
Furthermore, as depicted in Figure 10,
all of the navicular steel fibers are pulled out and adhered with
cement paste, while all of the corrugated fibers are broken. The broken
section of the bent samples in Figure 11 reveals that the pulled-out navicular steel fibers are also wrapped
with a minimal amount of cement paste, whereas traces of cement paste
were observed on the surface of the corrugated steel fiber. As listed in Table 3,
the navicular fiber has a larger cross-sectional circumference than the
corrugated fiber. Therefore, the navicular fiber can withstand a higher
friction load than the corrugated fiber. Consequently, the navicular
steel fiber is anticipated to have greater friction resistance to fiber
sliding and subsequent pullout than the corrugated steel fiber.
Despite the random orientation of steel fibers in concrete, it can be described as parallel loading direction, perpendicular loading direction, and other directions. The other directions can be viewed as the superposition of the parallel and perpendicular loading directions. The orientation of steel fibers parallel to the loading direction and perpendicular to the loading direction is analysed and numbered as 1 and 2, respectively, for ease of reference. The schematic view presented in Figure 12 reveals that near the concrete crack, fiber 1 primarily bears the tensile action (the bridge across cracks), while fiber 2 predominantly bears the tearing action of the concrete separating from the fiber surface. Near the support, fiber 1 largely bears the bending action caused by the shear force perpendicular to the direction of fiber length. Meanwhile, fiber 2 mainly bears the shear action parallel to the direction of fiber length. The aforementioned actions are applied to the fibers by the load transfer at the interface between the fiber and concrete. As compared to the corrugated steel fiber, the navicular steel fiber has a higher load transfer capacity and a higher bearing capacity for the tearing action of the concrete separating from the fiber surface. Furthermore, based on the size parameters of the cross-section of the fibers listed in Table 3, the volume of the individual navicular fiber (21.91 mm3) is inferred to be considerably smaller than that of the individual corrugated fiber (27.69 mm3). Therefore, for the same volume fraction, the number of navicular fibers must be higher. Given that the navicular steel fiber has a higher bearing capacity for bending and tensile loading, the navicular steel-fiber reinforced concrete has a higher bearing capacity for bending loads for the same fiber volume fraction. Consequently, for the same volume fraction, the navicular steel fiber has superior strengthening effects than the corrugated steel fiber.
However, as indicated in Figure 6,
the gradual decrease in the load after the peak load implies that the
load is transferred from the concrete matrix to the steel fibers
following the formation of cracks in the concrete matrix. Additionally,
the sliding between the fibers and the concrete matrix impedes the rapid
expansion and propagation of cracks in the concrete matrix. Moreover,
the smaller value of load decrease indicates a greater sliding
resistance between the steel fibers and the concrete matrix. Thus, the
load transfer effect is better, which implies that the bonding between
the steel fibers and the concrete matrix is stronger. Similar to the
dislocation movement in crystal materials, when the sliding reaches a
certain degree, the sliding will be blocked, causing a gradual increase
in the external load (3030.
Zhang, N.; Carrez., P.; Shahsavari, R. (2017) Screw-dislocation-induced
strengthening-toughening mechanisms in complex layered materials: the
case study of tobermorite. ACS Appl. Mater. Interf. 9 [2], 1496−1506. https://doi.org/10.1021/acsami.6b13107.
, 3131.
Shuang, F.; Dai, Z.; Aifantis, K.E. (2021) Strengthening in
metal/graphene composites: capturing the transition from interface to
precipitate hardening. ACS Appl. Mater. Interf. 13 [22], 26610−26620. https://doi.org/10.1021/acsami.1c05129.
).
The bond between the steel fibers and the concrete matrix will be
destroyed as the load increases to a particular extent, resulting in the
pullout of fibers and load reduction. Consequently, the pullout of the
fibers at different positions decreases the load gradually (3232.
Cheng, L.F.; Wu, S.J.; Zhang, L.T.; Xu, Y.D. (2009) Mechanical
self-adaptability of a SiC/PyC/SiC composite during oxidation in air. J. Compos. Mater. 43 [4], 305−313. https://doi.org/10.1177/0021998308098240.
, 3333.
Pereira, E.B.; Fischer, G.; Barros, J. (2012) Direct assessment of
tensile stress-crack opening behavior of strain hardening cementitious
composites (SHCC). Cem. Concr. Res. 42 [6], 834−846. https://doi.org/10.1016/j.cemconres.2012.03.006.
).
Therefore, the load increment from the peak valley value to the peak
value of the descending stage represents the difficulty of fibers
pulling out and the ultimate overall cracking. Furthermore, based on the
images taken after three-point bending (see Figure 13),
it is noted that the navicular steel-fiber reinforced concretes
demonstrate more evident multi-deflection crack propagation than the
corrugated steel-fiber reinforced concretes. The cracks will deflect
when their propagation is hindered by the steel fibers during loading
after the formation of the initial cracks in the concrete matrix. The
more visible the crack deflection, the more obvious the inhibition
effect of fibers on the crack propagation of the concrete matrix, and
the greater the energy (load) required for the cracking of the concrete
matrix; hence, a better strengthening and ductility-enhancing effect on
concrete (34-3734.
Xue, R.; Liu, P.; Zhang, Z.J.; Zhang, N.L.; Zhang, Y.H.; Wang, J.P.
(2021) Improvement of toughness of reaction bonded silicon carbide
composites reinforced by surface-modified SiC whiskers. Ceram. Int. 47 [13], 18150−18156. https://doi.org/10.1016/j.ceramint.2021.03.133.
35.
Zando, R.B.; Mesgarnejad, A.; Pan, C.; Shefelbine, S.J.; Erb, R.M.
(2020) Enhanced toughness in ceramic-reinforced polymer composites with
herringbone architectures. Compos. Sci. Technol. 204 [1], 108513. https://doi.org/10.1016/j.compscitech.2020.108513.
36. Zhang, K.; Gao, B.Z.; Gong, M.; Tong, Z.Y.; Fan, J.P. (2022) Design of crack deflection induced high toughness laminated Si3N4 ceramics based on hollow oriented one-dimensional microstructure. Ceram. Int. 48 [15], 21370−21377. https://doi.org/10.1016/j.ceramint.2022.04.103.
37.
Jang, S.J.; Yun, H.D. (2017) Combined effects of steel fiber and coarse
aggregate size on the compressive and flexural toughness of
high-strength concrete. Compos. Struct. 185, 203−211. https://doi.org/10.1016/j.compstruct.2017.11.009.
).
Based on the aforementioned discussion, it can be concluded that, as
compared to corrugated steel fiber, navicular steel fiber has stronger
interface bonding to concrete, better resistance to cracking and pullout
damage, and consequently a better ductility-enhancing effect on
concrete (see Figure 6).
The
flexural strength and ductility of SFRC are verified to be dependent on
the number of steel fibers bearing the tensile action (3737.
Jang, S.J.; Yun, H.D. (2017) Combined effects of steel fiber and coarse
aggregate size on the compressive and flexural toughness of
high-strength concrete. Compos. Struct. 185, 203−211. https://doi.org/10.1016/j.compstruct.2017.11.009.
).
Fiber 1, bearing the tensile action and acting as crack bridges,
increases with the increasing fiber volume fraction. Although similar to
the compressive strength, an increase in the air content is unfavorable
to bonding between the fibers and concrete, which consequently reduces
the strength. However, the bonding and shear strengths between the
fibers and concrete and the tensile strength of concrete are
significantly lower than the strength of the steel fibers. The
strengthening effect of fiber is greater than the weakening effect of
tensile. Consequently, the flexural strength and ductility of both SFRCs
are enhanced with increasing fiber volume fractions.
Bridging
across cracks from the fibers inhibits the formation and growth of
initial cracks formed in the concrete matrix and is hence beneficial to
tensile strength (1717.
Liu, Y.W.; Zhang, Z.H.; Shi, C.J.; Zhu, D.J.; Li, N.; Deng, Y.L. (2020)
Development of ultra-high performance geopolymer concrete (UHPGC):
Influence of steel fiber on mechanical properties. Cem. Concr. Compos. 112, 103670. https://doi.org/10.1016/j.cemconcomp.2020.103670.
, 38-4038.
Liu, W.; Luo, L.; Xu, S.L.; Zhao, H.H. (2014) Effect of fiber volume
fraction on crack propagation rate of ultra-high toughness cementitious
composites. Eng. Fract. Mech. 124-125, 52−63. https://doi.org/10.1016/j.engfracmech.2014.03.007.
39.
Michels, J.; Christen, R.;Waldmann, D. (2013) Experimental and
numerical investigation on postcracking behavior of steel fiber
reinforced concrete. Eng. Fract. Mech. 98, 326−349. https://doi.org/10.1016/j.engfracmech.2012.11.004.
40. Wang, Z.L.; Shi, Z.M.; Wang, J.G. (2011) On the strength and toughness properties of SFRC under static-dynamic compression. Compos. Part B-Eng. 42 [5], 1285-1290. https://doi.org/10.1016/j.compositesb.2011.01.027.
). As displayed in Figure 11 (a),
the corrugated fibers are first straightened in the process of being
pulled out and subsequently hinder the expansion of the cracks in the
concrete matrix. Therefore, there is retardation for the corrugated
fibers to hinder the expansion of the cracks. The navicular steel fibers
are straight in the longitudinal direction, which makes them free of
the straightening process and thus quickly inhibits the expansion of the
cracks in the concrete matrix. However, the larger aspect ratio of
corrugated fiber is conducive to the consequent bearing during the
propagation of cracks. Consequently, the splitting tensile strength of
the navicular steel-fiber reinforced concretes is roughly comparable to
that of the corrugated steel-fiber reinforced concretes. Additionally,
with an increase in the fiber volume fraction, the fibers bearing the
tensile action in the concrete increase. Consequently, the splitting
tensile strength of the concrete increases.
5. CONCLUSIONS
⌅The introduction of steel fibers improves the mechanical properties of plain concrete. With increasing fiber volume fraction, the compressive strength of both SFRCs exhibits the same pattern of initially increasing, then decreasing, and then increasing again, and displays the maximum value when 0.5% of steel fiber is added. The flexural strength and splitting tensile strength of both SFRCs increase with increasing fiber volume fraction. For the same fiber volume fraction, the concrete reinforced with navicular steel fibers has superior mechanical properties than those reinforced with corrugated steel fibers.
During bending, plain concrete presents a typical brittle failure mode, while both SFRCs present a bimodal ductile failure mode.
The addition of 2.0% of steel fiber is conducive to comprehensively improving the mechanical properties of concrete.
The difference in improvement in the mechanical properties of the two types of SFRCs arises from the difference in the characteristics of their cross-section. As compared to the corrugated steel fiber, the rougher surface and the larger section circumference make the navicular steel fiber have a stronger interface bonding to concrete and consequently possess a higher friction resistance to fiber sliding and subsequent pullout. The larger bending stiffness and cross-sectional area contribute to the higher load-bearing capacity of the navicular steel fiber.