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Bonding behaviour of arch-type steel fibres in a cementitious composite
Composite Structures 133 (2015) 117–123
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Bonding behaviour of arch-type steel fibres in a cementitious composite Jong-Pil Won ⇑, Jae-Ho Lee, Su-Jin Lee Department of Civil & Environmental System Engineering, Konkuk University, Seoul 143-701, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 23 July 2015 Keywords: Arch type Bond behaviour Pull-out Steel fibre Toughness
a b s t r a c t The bond properties of arch-type steel fibres were evaluated in a cementitious composite according to the JCI SF-8 standard as a function of the arch characteristics of curvature radius (20, 25, 30 and 35 mm) and bend length (0, 1.5, 2.5 and 3.5 mm). Fibre pull-out was observed during the pull-out test for all specimens regardless of the curvature radius for bend lengths of 0 and 1.5 mm, but pull-out or fracture of fibre was observed at bend lengths of 2.5 and 3.5 mm. The maximum bond strength of all arch-type steel fibres, except that with a bend length of 0 mm, was at least 1.5-times higher than for hooked-end-type steel fibres. The interfacial toughnesses of all specimens except those made with arch-type fibres having a bend length of 0 mm did not exhibit fracture of the fibres; improved bond properties were demonstrated by toughnesses that were over double that with hooked-end type steel fibres. Excellent pull-out resistance of the arch-type steel fibres was maintained until the completion of pull-out after debonding, even after the peak load. This behaviour was attributed to frictional sliding and mechanical anchorage forces within the cement matrix that simultaneously operated over the entire fibre surface. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction A cementitious composite is a brittle material that fails under applied tension or dynamic load. The generation and growth of fractures under impact are difficult to control in such materials. New reinforcing materials such as steel fibres have been developed to change the characteristics of the matrix to a more ductile nature while controlling fracture growth; these are currently used as reinforcing materials in various structures [1]. Incorporating steel fibres can increase the tensile strength and the energy absorption capability of a cementitious composite. Hence, it is used as common reinforcing material for shotcrete or roads, airport runways, maritime structures and slab-on-grade members, for example [2–4]. The bond properties between the steel fibre and the cement matrix are key to secure improved mechanical properties of the cementitious composite. Fibre shape, dimensions, and embedded length, and the embedded angle of the fibre, density of the interface between the fibre and the matrix, load characteristics, and loading speed are all elements that affect bond properties [5,6]. Quantitative bond properties of fibre-cementitious composites are typically determined using a pull-out test. In this test, single
⇑ Corresponding author. Tel.: +82 2 450 3750; fax: +82 2 2201 0907. E-mail address: [email protected] (J.-P. Won). http://dx.doi.org/10.1016/j.compstruct.2015.07.074 0263-8223/Ó 2015 Elsevier Ltd. All rights reserved.
and multiple fibres embedded in a cementitious composite are simultaneously pulled-out from the matrix. Load–slip behaviour can be evaluated in this manner [7]. Shannag et al. used a pull-out test with a straight type of steel fibre that had a tensile strength of 2950 MPa, in which the embedded length within their cementitious composite was a variable [8]. They found that the pull-out load and interfacial toughness values were maximised when the matrix strength was high and the embedded length in the cement matrix was also high [8]. The improvement of bond properties with increasing strength of the cement matrix was confirmed by Naaman [9]. Numerous preliminary studies have been reported that used pull-out tests with various shapes of steel fibres [10–14]. Naaman developed twisted steel fibres with triangular and rectangular cross-sections and reported improved bond properties and mechanical properties via pull-out and flexural tests [13]. A high pull-out resistance strength over the entire length of the fibre was achieved with a steel fibre having a tensile strength of 1700 MPa [13]. Zile characterised the pull-out of hooked-ends-type steel fibres within a cement matrix; the behaviour was explained in terms of five stages, or two stages in the case of a crimped type of steel fibre [14]. Fig. 1 shows how the ends of the hooked-ends type of steel fibres were divided into sections that were subject to frictional sliding and plastic bending as they passed through three straight-line sections and two curved sections. Additionally, in the case of a crimped type of steel fibre, pull-out behaviour was
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Fig. 1. Schematic diagram of hooked fibre pull-out [15].
characterised by dividing the fibre into straight sections separated by curved sections (Fig. 2). The steel fibres used as reinforcement materials can have different specifications and are used in various mixing quantities according to the application. However, the hooked-ends type of steel fibre with each end bent is commonly used worldwide. There is a limit to the improvement of mechanical properties: Pull-out strength is drastically reduced when fractures are generated in the matrix as the bent fibres are withdrawn. In this study, a pull-out test was used to evaluate the performance of a new shape of arch-type steel fibre used as reinforcement in a cementitious composite with a view to improving bond properties relative to the current hooked-ends type of steel fibre. 2. Materials and mix proportions 2.1. Arch-type steel fibre Steel fibre with an arched shape (Fig. 3) can improve the mechanical properties of a cementitious composite by augmenting the anchorage behaviour of the two ends of the fibre. Forming the fibre in an arch shape with curvature over most of its length provides greater pull-out resistance strength than is obtained with the current hooked-ends type of steel fibre. For the hooked-ends type, the pull-out load results only from the friction resistance between the steel fibre and the cementitious matrix; the ends become bent during the debonding and pull-out. With an arch type of steel fibre, bond properties are improved because the mechanical anchorage and frictional resistance act concurrently against the pull-out load; the steel fibre retains its arch shape even after bending of the ends. To determine the optimal design for such an arch type of steel fibre, the curvature radius and the length of the bent end anchors were set as variables. The pull-out test was carried out in parallel with hooked-ends fibres
Fig. 3. Arch-type steel fibre.
to gauge the changes in bond properties; the arched and hooked-ends types of fibres were made with wire of the same tensile strength, diameter and embedded length. Table 1 summarizes the test variables. Specimens were identified as follows: A20_1.5 refers to a steel fibre having a curvature radius of 20 mm and a bend length of 1.5 mm. A separate nomenclature was not required for the hooked-ends type of steel fibre because only one design was evaluated. 2.2. Mix proportion Specimens for the pull-out tests were prepared from Type I Portland cement having a specific gravity of 3.15 and fine aggregate having a specific gravity of 2.58. Mixes were made to a target compressive strength of 30 MPa (Table 2). 3. Experimental 3.1. Compressive strength of the cementitious composites Three specimens of dimensions 50 50 50 mm were tested in duplicate. The specimens were cured at a temperature of
Fig. 2. Geometry of crimped steel fibre [15].
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J.-P. Won et al. / Composite Structures 133 (2015) 117–123 Table 1 Pull-out test variables. Steel fibre type
Tensile strength (MPa)
Embedded length (mm)
Curvature, R (mm)
Bend length, le (mm)
Arch Hook-end
1300
25
20, 25, 30, 35 –
0, 1.5, 2.5, 3.5 –
23 ± 2 °C and a relative humidity of 50% for 24 h, and then removed from the mould. The compressive strength was measured after curing for 28 days in a water tank maintained at 23 ± 2 °C. 3.2. Bond properties Bond properties were evaluated according to the JCI SF-8 standard [16]. Fig. 4 shows how the steel fibre was centrally anchored after dividing a dog-bone-shaped specimen into two parts [15]. Test specimens were initially cured at 23 ± 2 °C and a relative humidity of 50% for 24 h and then immersed for 28 days in a water tank maintained at 23 ± 2 °C. Three specimens were then tested in duplicate. Fig. 5 shows the experimental test set-up. Specimens were tested at a crosshead speed of 0.5 mm min1 using a universal testing machine equipped with a 5-kN load cell (Instron Model 3369) and operating under displacement control. The pull-out test was used to identify the maximum bond strength and interfacial toughness. The former was calculated according to Eq. (1).
smax ¼
P max
ð1Þ
p df l
where smax is the maximum bond strength, P max is the maximum pull-out load, l is the embedded length and df is the fibre diameter. The interfacial toughness was calculated as the area under the pull-out load–slip curve. This property indicates the energy absorption capability against a tensile force operating on the cementitious composite. The interfacial toughnesses for the arch-type of steel fibre were determined for fibre lengths of 5, 10, 15 and 20 mm. The toughness was set at zero if the failure mode was fibre fracture rather than pull-out. 4. Results and discussion 4.1. Compressive strength of the cementitious composites Composites aged for 28 days had an average compressive strength of 33.5 MPa. This satisfied the design requirement. 4.2. Pull-out load–slip behaviour Fig. 6 shows a typical pull-out load–slip graph for a cementitious composite made with the hooked-ends type of steel fibre. Using this type of steel fibre, the entire embedded length always pulled out completely. Fig. 7 shows the pull-out load–slip behaviours for the various arch-type steel fibre designs. Figs. 8 and 9 show the maximum bond strength and interfacial toughness, respectively, for the various designs of steel fibres. They show that the embedded lengths of the arch type of steel fibre with bend lengths of 0 and 1.5 mm pulled out completely regardless of the curvature radius. The cases
Table 2 Mixture proportions of the cementitious composites. fck (MPa)
Water–cement ratio
Cement:sand ratio (wt.)
30
0.6
1:2.5
having a 1.5 mm bend length in which both bent ends served as anchoring points had higher pull-out strengths and interfacial toughnesses than those with a 0 mm bend length that lacked anchoring points. More fibre fractures were observed for the cases having longer bend lengths, i.e., 2.5 and 3.5 mm bend lengths, when the curvature radius was smaller. For the same bend length, the interfacial toughness increased as the curvature radius decreased. 4.3. Bond strength and interfacial toughness Figs. 10 and 11 show how the bond properties changed as a function of curvature radius and bend length; comparative data are provided for the hooked-ends fibre. The maximum bond strength was higher for specimens containing an arch type of steel fibre with anchorage segments than for those containing the hooked-ends fibre. The case of A20_0 lacking a bend length gave a maximum bond strength that was 9.0% lower and an interfacial toughness to 20 mm that was 57.5% higher than values for the hooked-ends type. Fig. 12(a) shows that the pull-out of A20_0 straightened the arched portion because there was no anchorage part within the cement matrix. The hooked-ends type of steel fibre having an anchorage part (Fig. 6) also pulled-out in its entirety; its greater maximum bond strength is attributed to frictional resistance by the anchorage part. However, Fig. 7(a) reveals that A20_0 had excellent interfacial toughness because the entire embedded length was pulled out without a significant decrease in the load after the maximum pull-out load even though there were no length anchorage parts. The A20_1.5, A20_2.5 and A20_3.5 arch type of steel fibre having a curvature radius of 20 mm gave maximum bond strengths that were 114.5, 201.5 and 208.0% higher, respectively, compared with the hooked-ends type of fibre. The maximum bond strength increased with increasing bend length because the increasing anchorage length within the cement matrix increased the resistance against the pull-out load. The calculated interfacial toughness at a slip of 20 mm for A20_1.5 showed a 209.4% improvement over the hooked-ends type of steel fibre. Fig. 12(a) shows that the anchorage part straightened during pull-out. Fig. 7(a) can then be interpreted as follows: The maximum pull-out load was reached via additional resistance of the anchorage points as their bend length unfolded after the pull-out load of the initial linear span had reached its maximum value. Fig. 12(a) also reveals that the arch types of steel fibre in A20_2.5 and A20_3.5 were not pulled out but rather were fractured in all specimens. The pull-out load–slip curve of Fig. 7(a) is then explained as follows: In the A20_2.5 and A20_3.5 cases, fibre fracture occurred when the pull-out load of the initial linear span quickly reached its maximum value. Some debonding and pull-out from the cement matrix likely occurred but the interfacial toughness could not be calculated because the fractured fibre did not slip sufficiently for the calculation. For the arch type of steel fibre with a 25-mm curvature radius and bent ends, the maximum bond strength was greater than for the hooked-ends type and the magnitude of the increase increased with increasing bend length. In contrast to the case of A25_0, Fig. 12(b) shows that the maximum bond strength was 12.7% lower
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Fig. 4. Arrangement of the partitioning board and fibres, setting in the mould and a single fibre [16].
Fig. 5. Pull-out test set-up.
Fig. 6. Pull-out load–slip curve of hooked-end-type steel fibre.
even though it pulled out in a similar manner as the hooked-ends type. Fig. 7(b) shows that a certain amount of residual load was maintained until the entire embedded length was pulled out
because the bending part of the arch type of fibre created friction within the cement matrix after the maximum pull-out load was reached. Sample A25_0 had 71.3% greater interfacial toughness even though it lacked an anchorage ending. Samples A25_1.5, A25_2.5 and A25_3.5 contained the arch type of steel fibre with a 25 mm curvature radius but variable bent length. The maximum bond strengths were 100.9, 172.8 and 198.5% better, respectively, than with the hooked-ends type and, similar to the arch type of fibres having a 20-mm curvature radius, the bond strength increased as the bend length increased. The A25_1.5 fibre pulled out like the A25_0 one without a bend length (Fig. 12(b)). However, Fig. 7(b) shows that A25_1.5 had a greater maximum pull-out load value. This was because it had an anchorage ending part. The interfacial toughness improved by 193.8% because of a smaller reduction in the magnitude of the residual strength after the maximum load compared with the hooked-ends type of steel fibre having the same anchorage ending part. The A25_2.5 and A25_3.5 fibres pulled out or fractured during testing (Fig. 12(b)), indicating that the anchorage strength within the cement matrix was borderline. Fibres A20_2.5 and A20_3.5 had the smallest curvature radius and the same bend length, but all specimens fractured at this small radius of curvature. This confirms that pull-out resistance is affected concurrently by the curvature radius of the arch and by the length of the anchorage bend length within the cement matrix. The pull-out load–slip curve of Fig. 7(b) shows that the slope of the initial linear region is lower for A25_2.5 and A25_3.5 than for A20_2.5 and A20_3.5. The calculated interfacial toughnesses of the non-fractured fibres of A25_2.5 and A25_3.5 were 129.4% and 177.2% higher, respectively, than that for the hooked-ends type of fibre. Inspection of all of the specimens containing the A30_0 fibre revealed that the initially curved fibres straightened during pull-out (Fig. 12(c)). Because there was no anchorage part, the maximum bond strength was 19.2% lower than that for the hooked-ends type but the interfacial toughness up to 20 mm was 11.9% higher because of the frictional resistance of the arched portion of the fibre. The pull-out load–slip curve for the hooked-ends type of steel fibre (Fig. 6) shows that the load steadily decreased with pull-out of the fibre. This was because the curved part of the fibre debonded from the matrix after the maximum pull-out load was reached. In comparison, the maximum bond strength with A30_0 occurred during the pull-out process rather than at the maximum value (Fig. 7(c)). This was because the frictional resistance against the pull-out load occurred over the entire length of the arch type of steel fibre. The influence of bend length was also studied with an arch type of steel fibre having a 30-mm curvature radius. The maximum
J.-P. Won et al. / Composite Structures 133 (2015) 117–123
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Fig. 8. Bond strength of steel fibres.
Fig. 9. Energy to 20 mm of steel fibres.
Fig. 10. Comparison of bond strengths for arch- and hooked-ends types of fibres.
Fig. 7. Pull-out load–slip curve of arch-type steel fibres.
bond strengths of A30_1.5, A30_2.5 and A30_3.5 were 114.9, 193.2 and 205.3% improved, respectively, over that for the hooked-ends type of fibre. Similarly to the fibres having 20- and 25-mm curvature radii, the bond strength for the 30-mm case also increased with increasing bend length. The A30_1.5 always pulled out
without fracture (Fig. 12(c)). Fig. 6(c) was used to study the pull-out load–slip behaviour. The initial linear region of the curve had a lower slope than for the arch type of steel fibres with smaller curvature radii. Consequently, the interfacial toughness was 150.4% greater than that for the hooked-ends type of fibre. This was lower than that found with the arch-type steel fibres with smaller curvature radii even for the same bend length. A30_2.5 showed pull-out or fracture but the A30_3.5 fibres only fractured (Fig. 12(c)). The interfacial toughness of the unfractured A30_2.5 specimens was 176.8% higher than for the hooked-ends type. This was a similar level to that found for A25_3.5. It appears
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Fig. 11. Comparison of energies to 20 mm for arch- and hooked-ends types of fibres.
that the entire arch-type steel fibre was gently pulled out without causing a drastic decrease in the load as the curvature radius increased, despite the short anchorage length. Fig. 7(c) shows a steeper slope of the initial linear portion of the curve for A30_3.5 than for A30_2.5. The fibres fractured at the maximum pull-out
load as the anchorage and the bend lengths increased and also because of better adhesion with the cement matrix. A35_0, which had no anchorage end parts, had a maximum bond strength and interfacial toughness up to 20 mm that were 62.2 and 43.1% lower, respectively, than those for the hooked-ends type of fibre. Fig. 12(d) shows that A35_0 pulled out similarly to arch-type steel fibres having smaller curvature radii, and Fig. 7(d) indicates that it was pulled out with no residual bond strength after the maximum pull-out load. This behaviour is attributed to insufficient frictional resistance within the cement matrix during pull-out, as found for the straight parts of hooked-ends fibres having very large curvature radii. All of the fibres pulled out because of insufficient anchoring within the cement matrix, despite the increased bend length (Fig. 12(d)). The maximum bond strengths of A35_1.5, A35_2.5 and A35_3.5 were 64.4, 146.1 and 164.4% higher, respectively, than for the hooked-ends type of fibre. As found with the arch-type steel fibres having smaller curvature radii, the bond strength of the A35 fibres increased with increasing bend length. However, the increase in the maximum bond strength was lower than that found with the arch-type steel fibres having smaller curvature radii. This is attributed to the lower frictional resistance of the arch part as the curvature radius increased, as described previously. The pull-out load– slip curve of Fig. 7(d) supports this reasoning. The slope of the
Fig. 12. Specimens after the pull-out test for the arch-type steel fibres.
J.-P. Won et al. / Composite Structures 133 (2015) 117–123
initial linear portion was very mild compared with that for the arch-type steel fibres with smaller curvature radii. The maximum pull-out load was lower, despite the longer anchorage length. Anchoring of the arch-type steel fibre improved with increasing bend length, but the larger curvature arches were easily pulled out without frictional resistance. The interfacial toughnesses for A35_1.5, A35_2.5 and A35_3.5 were 107.5, 139.6 and 139.4% greater, respectively, than for the hooked-ends types. These increases were less than those obtained for the arch-type steel fibres having smaller curvature radii. In summary, arch-type steel fibres without anchorage parts showed little reduction in bond strength compared with the hooked-ends type, except A35_0 which had the greatest curvature radius but had increased interfacial toughness. We established that the frictional resistance of the curved arch part improved the bond properties with the cement matrix. Additionally, the bond strength and interfacial toughness increased with increasing bend length regardless of the curvature radius. However, at a very long bend length, the interfacial toughness diminished because improved anchorage caused fibre fracture to occur rather than pull-out. There was a pronounced effect of bend length for a given curvature radius for the arch-type fibres. This difference in behaviour from that of the hooked-ends type of fibre is attributed to pull-out resistance that is enhanced over the arched portions and not solely located at the anchorage points. 5. Conclusions The bond properties of arch-type steel fibres in a cementitious composite were evaluated. A single-fibre, double-sided pull-out test was used to evaluate the effect of changing curvature radius and bend length. Comparisons were made to the hooked-ends type of steel fibre that is most commonly used. Bond strength and interfacial toughness increased with increasing bend length regardless of the curvature radius. Unlike the hooked-ends type of steel fibre, the arch-type fibres had pull-out resistance at the anchorage parts and also over the curved portions of the arches. With the exception of A35_0, which had the largest curvature radius, the arch-type steel fibres lacking an anchorage part showed increased interfacial toughness compared with the hooked-ends types of fibres. The frictional resistance of the curved portion of the arch improved the bond properties within the cement matrix. Bent arch-type steel fibres provided a maximum bond strength that was 1.5 times higher than with the hooked-ends type of steel fibre.
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The interfacial toughness up to 20 mm of the arch-type steel fibres, except those with 0 mm bend length, was twice that of the hooked-ends type of steel fibres. This study has demonstrated that arch-type steel fibres can provide higher maximum bond strengths than the hooked-ends type of steel fibre after debonding. This is attributed to frictional sliding that occurs within the matrix duct and to plastic bending of the entire steel fibre. Additionally, constant pull-out resistance was observed after the peak load and continued until the completion of the pull-out. This was because of the concurrent operation of frictional sliding and mechanical anchorage forces. Such behaviour is quite unlike that found with the hooked-ends type of fibres. Acknowledgements This paper was supported by Konkuk University in 2014 and steel fibres were provided by KOSTEEL Co. from South Korea. References [1] Bentur A, Mindess S. Fibre reinforced cementitious composites. Elsevier Appl Sci 1990. [2] Shotcrete for Underground Support IV. Proceedings of the Engineering Foundation Conference Tlfs. Austria. 1995. [3] Beckett D. Comparative tests on plain fabric reinforced & steel fiber reinforced concrete ground slabs. Concrete 1989:43–5. [4] Lee C. Verification of load-carrying capacity of steel fiber reinforced concrete slab-on-grade. Korea Concr Inst 2007:337–40. [5] Cao HC, Beschoff E, Ruhle M, Evans AG, Marshall DB, Brennan JJ. Effect of interfaces on the properties of fiber-reinforced ceramics. J Am Ceram Soc 1990;73(6):1692–9. [6] Evans AG, He MY, Hutchinson JW. Interface debonding and fiber cracking in brittle matrix composites. J Am Ceram Soc 1989;72(12):2300–3. [7] Balaguru PN, Shah SP. Fiber-reinforced cement composite. New York: McGrawHill Inc; 1992. [8] Shannag MJ, Brincker R, Hansen W. Pullout behavior of steel fibers from cement-based composites. Cem Concr Res 1997;27(6):925–36. [9] Naaman AE, Najm H. Bond-slip mechanisms of steel fibers in concrete. ACI Mater J 1991;88(2):135–45. [10] Banthia N, Trottier JF. Concrete reinforced with deformed steel fibres Part I : bond-displacement mechanisms. ACI Mater J 1994:435–45. Sept-Oct. [11] Song F. Effect of fibre properties and embedment conditions on fibre pullout behaviour from concrete matrix. In: Proceedings of the 9th fib International PhD Symposium in Civil. 2012. p. 597–601. [12] Abu-Lebdeh T, Hamoush S, Heard W, Zornig B. Effect of matrix strength on pullout behavior of steel fiber reinforced very-high strength concrete composites. Constr Building Mater 2011;25:39–46. [13] Naaman AE. Engineered steel fibers with optimal properties for reinforcement of cement composites. J Adv Concr Technol 2003;1(3):241–52. [14] Zile E, Zile O. Effect of the fiber geometry on the pullout response of mechanically deformed steel fibers. Cem Concr Res 2013;44:18–24. [15] Won JP, Hong BT, Lee SJ, Choi SJ. Bonding properties of amorphous micro-steel fibre-reinforced cementitious composites. Compos Struct 2013;102:101–9. [16] JCI SF-8, Method of test for bond of fibers, Japan Concrete Institute, 2002.
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Bonding behaviour of arch-type steel fibres in a cementitious composite Jong-Pil Won ⇑, Jae-Ho Lee, Su-Jin Lee Department of Civil & Environmental System Engineering, Konkuk University, Seoul 143-701, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 23 July 2015 Keywords: Arch type Bond behaviour Pull-out Steel fibre Toughness
a b s t r a c t The bond properties of arch-type steel fibres were evaluated in a cementitious composite according to the JCI SF-8 standard as a function of the arch characteristics of curvature radius (20, 25, 30 and 35 mm) and bend length (0, 1.5, 2.5 and 3.5 mm). Fibre pull-out was observed during the pull-out test for all specimens regardless of the curvature radius for bend lengths of 0 and 1.5 mm, but pull-out or fracture of fibre was observed at bend lengths of 2.5 and 3.5 mm. The maximum bond strength of all arch-type steel fibres, except that with a bend length of 0 mm, was at least 1.5-times higher than for hooked-end-type steel fibres. The interfacial toughnesses of all specimens except those made with arch-type fibres having a bend length of 0 mm did not exhibit fracture of the fibres; improved bond properties were demonstrated by toughnesses that were over double that with hooked-end type steel fibres. Excellent pull-out resistance of the arch-type steel fibres was maintained until the completion of pull-out after debonding, even after the peak load. This behaviour was attributed to frictional sliding and mechanical anchorage forces within the cement matrix that simultaneously operated over the entire fibre surface. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction A cementitious composite is a brittle material that fails under applied tension or dynamic load. The generation and growth of fractures under impact are difficult to control in such materials. New reinforcing materials such as steel fibres have been developed to change the characteristics of the matrix to a more ductile nature while controlling fracture growth; these are currently used as reinforcing materials in various structures [1]. Incorporating steel fibres can increase the tensile strength and the energy absorption capability of a cementitious composite. Hence, it is used as common reinforcing material for shotcrete or roads, airport runways, maritime structures and slab-on-grade members, for example [2–4]. The bond properties between the steel fibre and the cement matrix are key to secure improved mechanical properties of the cementitious composite. Fibre shape, dimensions, and embedded length, and the embedded angle of the fibre, density of the interface between the fibre and the matrix, load characteristics, and loading speed are all elements that affect bond properties [5,6]. Quantitative bond properties of fibre-cementitious composites are typically determined using a pull-out test. In this test, single
⇑ Corresponding author. Tel.: +82 2 450 3750; fax: +82 2 2201 0907. E-mail address: [email protected] (J.-P. Won). http://dx.doi.org/10.1016/j.compstruct.2015.07.074 0263-8223/Ó 2015 Elsevier Ltd. All rights reserved.
and multiple fibres embedded in a cementitious composite are simultaneously pulled-out from the matrix. Load–slip behaviour can be evaluated in this manner [7]. Shannag et al. used a pull-out test with a straight type of steel fibre that had a tensile strength of 2950 MPa, in which the embedded length within their cementitious composite was a variable [8]. They found that the pull-out load and interfacial toughness values were maximised when the matrix strength was high and the embedded length in the cement matrix was also high [8]. The improvement of bond properties with increasing strength of the cement matrix was confirmed by Naaman [9]. Numerous preliminary studies have been reported that used pull-out tests with various shapes of steel fibres [10–14]. Naaman developed twisted steel fibres with triangular and rectangular cross-sections and reported improved bond properties and mechanical properties via pull-out and flexural tests [13]. A high pull-out resistance strength over the entire length of the fibre was achieved with a steel fibre having a tensile strength of 1700 MPa [13]. Zile characterised the pull-out of hooked-ends-type steel fibres within a cement matrix; the behaviour was explained in terms of five stages, or two stages in the case of a crimped type of steel fibre [14]. Fig. 1 shows how the ends of the hooked-ends type of steel fibres were divided into sections that were subject to frictional sliding and plastic bending as they passed through three straight-line sections and two curved sections. Additionally, in the case of a crimped type of steel fibre, pull-out behaviour was
118
J.-P. Won et al. / Composite Structures 133 (2015) 117–123
Fig. 1. Schematic diagram of hooked fibre pull-out [15].
characterised by dividing the fibre into straight sections separated by curved sections (Fig. 2). The steel fibres used as reinforcement materials can have different specifications and are used in various mixing quantities according to the application. However, the hooked-ends type of steel fibre with each end bent is commonly used worldwide. There is a limit to the improvement of mechanical properties: Pull-out strength is drastically reduced when fractures are generated in the matrix as the bent fibres are withdrawn. In this study, a pull-out test was used to evaluate the performance of a new shape of arch-type steel fibre used as reinforcement in a cementitious composite with a view to improving bond properties relative to the current hooked-ends type of steel fibre. 2. Materials and mix proportions 2.1. Arch-type steel fibre Steel fibre with an arched shape (Fig. 3) can improve the mechanical properties of a cementitious composite by augmenting the anchorage behaviour of the two ends of the fibre. Forming the fibre in an arch shape with curvature over most of its length provides greater pull-out resistance strength than is obtained with the current hooked-ends type of steel fibre. For the hooked-ends type, the pull-out load results only from the friction resistance between the steel fibre and the cementitious matrix; the ends become bent during the debonding and pull-out. With an arch type of steel fibre, bond properties are improved because the mechanical anchorage and frictional resistance act concurrently against the pull-out load; the steel fibre retains its arch shape even after bending of the ends. To determine the optimal design for such an arch type of steel fibre, the curvature radius and the length of the bent end anchors were set as variables. The pull-out test was carried out in parallel with hooked-ends fibres
Fig. 3. Arch-type steel fibre.
to gauge the changes in bond properties; the arched and hooked-ends types of fibres were made with wire of the same tensile strength, diameter and embedded length. Table 1 summarizes the test variables. Specimens were identified as follows: A20_1.5 refers to a steel fibre having a curvature radius of 20 mm and a bend length of 1.5 mm. A separate nomenclature was not required for the hooked-ends type of steel fibre because only one design was evaluated. 2.2. Mix proportion Specimens for the pull-out tests were prepared from Type I Portland cement having a specific gravity of 3.15 and fine aggregate having a specific gravity of 2.58. Mixes were made to a target compressive strength of 30 MPa (Table 2). 3. Experimental 3.1. Compressive strength of the cementitious composites Three specimens of dimensions 50 50 50 mm were tested in duplicate. The specimens were cured at a temperature of
Fig. 2. Geometry of crimped steel fibre [15].
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J.-P. Won et al. / Composite Structures 133 (2015) 117–123 Table 1 Pull-out test variables. Steel fibre type
Tensile strength (MPa)
Embedded length (mm)
Curvature, R (mm)
Bend length, le (mm)
Arch Hook-end
1300
25
20, 25, 30, 35 –
0, 1.5, 2.5, 3.5 –
23 ± 2 °C and a relative humidity of 50% for 24 h, and then removed from the mould. The compressive strength was measured after curing for 28 days in a water tank maintained at 23 ± 2 °C. 3.2. Bond properties Bond properties were evaluated according to the JCI SF-8 standard [16]. Fig. 4 shows how the steel fibre was centrally anchored after dividing a dog-bone-shaped specimen into two parts [15]. Test specimens were initially cured at 23 ± 2 °C and a relative humidity of 50% for 24 h and then immersed for 28 days in a water tank maintained at 23 ± 2 °C. Three specimens were then tested in duplicate. Fig. 5 shows the experimental test set-up. Specimens were tested at a crosshead speed of 0.5 mm min1 using a universal testing machine equipped with a 5-kN load cell (Instron Model 3369) and operating under displacement control. The pull-out test was used to identify the maximum bond strength and interfacial toughness. The former was calculated according to Eq. (1).
smax ¼
P max
ð1Þ
p df l
where smax is the maximum bond strength, P max is the maximum pull-out load, l is the embedded length and df is the fibre diameter. The interfacial toughness was calculated as the area under the pull-out load–slip curve. This property indicates the energy absorption capability against a tensile force operating on the cementitious composite. The interfacial toughnesses for the arch-type of steel fibre were determined for fibre lengths of 5, 10, 15 and 20 mm. The toughness was set at zero if the failure mode was fibre fracture rather than pull-out. 4. Results and discussion 4.1. Compressive strength of the cementitious composites Composites aged for 28 days had an average compressive strength of 33.5 MPa. This satisfied the design requirement. 4.2. Pull-out load–slip behaviour Fig. 6 shows a typical pull-out load–slip graph for a cementitious composite made with the hooked-ends type of steel fibre. Using this type of steel fibre, the entire embedded length always pulled out completely. Fig. 7 shows the pull-out load–slip behaviours for the various arch-type steel fibre designs. Figs. 8 and 9 show the maximum bond strength and interfacial toughness, respectively, for the various designs of steel fibres. They show that the embedded lengths of the arch type of steel fibre with bend lengths of 0 and 1.5 mm pulled out completely regardless of the curvature radius. The cases
Table 2 Mixture proportions of the cementitious composites. fck (MPa)
Water–cement ratio
Cement:sand ratio (wt.)
30
0.6
1:2.5
having a 1.5 mm bend length in which both bent ends served as anchoring points had higher pull-out strengths and interfacial toughnesses than those with a 0 mm bend length that lacked anchoring points. More fibre fractures were observed for the cases having longer bend lengths, i.e., 2.5 and 3.5 mm bend lengths, when the curvature radius was smaller. For the same bend length, the interfacial toughness increased as the curvature radius decreased. 4.3. Bond strength and interfacial toughness Figs. 10 and 11 show how the bond properties changed as a function of curvature radius and bend length; comparative data are provided for the hooked-ends fibre. The maximum bond strength was higher for specimens containing an arch type of steel fibre with anchorage segments than for those containing the hooked-ends fibre. The case of A20_0 lacking a bend length gave a maximum bond strength that was 9.0% lower and an interfacial toughness to 20 mm that was 57.5% higher than values for the hooked-ends type. Fig. 12(a) shows that the pull-out of A20_0 straightened the arched portion because there was no anchorage part within the cement matrix. The hooked-ends type of steel fibre having an anchorage part (Fig. 6) also pulled-out in its entirety; its greater maximum bond strength is attributed to frictional resistance by the anchorage part. However, Fig. 7(a) reveals that A20_0 had excellent interfacial toughness because the entire embedded length was pulled out without a significant decrease in the load after the maximum pull-out load even though there were no length anchorage parts. The A20_1.5, A20_2.5 and A20_3.5 arch type of steel fibre having a curvature radius of 20 mm gave maximum bond strengths that were 114.5, 201.5 and 208.0% higher, respectively, compared with the hooked-ends type of fibre. The maximum bond strength increased with increasing bend length because the increasing anchorage length within the cement matrix increased the resistance against the pull-out load. The calculated interfacial toughness at a slip of 20 mm for A20_1.5 showed a 209.4% improvement over the hooked-ends type of steel fibre. Fig. 12(a) shows that the anchorage part straightened during pull-out. Fig. 7(a) can then be interpreted as follows: The maximum pull-out load was reached via additional resistance of the anchorage points as their bend length unfolded after the pull-out load of the initial linear span had reached its maximum value. Fig. 12(a) also reveals that the arch types of steel fibre in A20_2.5 and A20_3.5 were not pulled out but rather were fractured in all specimens. The pull-out load–slip curve of Fig. 7(a) is then explained as follows: In the A20_2.5 and A20_3.5 cases, fibre fracture occurred when the pull-out load of the initial linear span quickly reached its maximum value. Some debonding and pull-out from the cement matrix likely occurred but the interfacial toughness could not be calculated because the fractured fibre did not slip sufficiently for the calculation. For the arch type of steel fibre with a 25-mm curvature radius and bent ends, the maximum bond strength was greater than for the hooked-ends type and the magnitude of the increase increased with increasing bend length. In contrast to the case of A25_0, Fig. 12(b) shows that the maximum bond strength was 12.7% lower
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Fig. 4. Arrangement of the partitioning board and fibres, setting in the mould and a single fibre [16].
Fig. 5. Pull-out test set-up.
Fig. 6. Pull-out load–slip curve of hooked-end-type steel fibre.
even though it pulled out in a similar manner as the hooked-ends type. Fig. 7(b) shows that a certain amount of residual load was maintained until the entire embedded length was pulled out
because the bending part of the arch type of fibre created friction within the cement matrix after the maximum pull-out load was reached. Sample A25_0 had 71.3% greater interfacial toughness even though it lacked an anchorage ending. Samples A25_1.5, A25_2.5 and A25_3.5 contained the arch type of steel fibre with a 25 mm curvature radius but variable bent length. The maximum bond strengths were 100.9, 172.8 and 198.5% better, respectively, than with the hooked-ends type and, similar to the arch type of fibres having a 20-mm curvature radius, the bond strength increased as the bend length increased. The A25_1.5 fibre pulled out like the A25_0 one without a bend length (Fig. 12(b)). However, Fig. 7(b) shows that A25_1.5 had a greater maximum pull-out load value. This was because it had an anchorage ending part. The interfacial toughness improved by 193.8% because of a smaller reduction in the magnitude of the residual strength after the maximum load compared with the hooked-ends type of steel fibre having the same anchorage ending part. The A25_2.5 and A25_3.5 fibres pulled out or fractured during testing (Fig. 12(b)), indicating that the anchorage strength within the cement matrix was borderline. Fibres A20_2.5 and A20_3.5 had the smallest curvature radius and the same bend length, but all specimens fractured at this small radius of curvature. This confirms that pull-out resistance is affected concurrently by the curvature radius of the arch and by the length of the anchorage bend length within the cement matrix. The pull-out load–slip curve of Fig. 7(b) shows that the slope of the initial linear region is lower for A25_2.5 and A25_3.5 than for A20_2.5 and A20_3.5. The calculated interfacial toughnesses of the non-fractured fibres of A25_2.5 and A25_3.5 were 129.4% and 177.2% higher, respectively, than that for the hooked-ends type of fibre. Inspection of all of the specimens containing the A30_0 fibre revealed that the initially curved fibres straightened during pull-out (Fig. 12(c)). Because there was no anchorage part, the maximum bond strength was 19.2% lower than that for the hooked-ends type but the interfacial toughness up to 20 mm was 11.9% higher because of the frictional resistance of the arched portion of the fibre. The pull-out load–slip curve for the hooked-ends type of steel fibre (Fig. 6) shows that the load steadily decreased with pull-out of the fibre. This was because the curved part of the fibre debonded from the matrix after the maximum pull-out load was reached. In comparison, the maximum bond strength with A30_0 occurred during the pull-out process rather than at the maximum value (Fig. 7(c)). This was because the frictional resistance against the pull-out load occurred over the entire length of the arch type of steel fibre. The influence of bend length was also studied with an arch type of steel fibre having a 30-mm curvature radius. The maximum
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Fig. 8. Bond strength of steel fibres.
Fig. 9. Energy to 20 mm of steel fibres.
Fig. 10. Comparison of bond strengths for arch- and hooked-ends types of fibres.
Fig. 7. Pull-out load–slip curve of arch-type steel fibres.
bond strengths of A30_1.5, A30_2.5 and A30_3.5 were 114.9, 193.2 and 205.3% improved, respectively, over that for the hooked-ends type of fibre. Similarly to the fibres having 20- and 25-mm curvature radii, the bond strength for the 30-mm case also increased with increasing bend length. The A30_1.5 always pulled out
without fracture (Fig. 12(c)). Fig. 6(c) was used to study the pull-out load–slip behaviour. The initial linear region of the curve had a lower slope than for the arch type of steel fibres with smaller curvature radii. Consequently, the interfacial toughness was 150.4% greater than that for the hooked-ends type of fibre. This was lower than that found with the arch-type steel fibres with smaller curvature radii even for the same bend length. A30_2.5 showed pull-out or fracture but the A30_3.5 fibres only fractured (Fig. 12(c)). The interfacial toughness of the unfractured A30_2.5 specimens was 176.8% higher than for the hooked-ends type. This was a similar level to that found for A25_3.5. It appears
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Fig. 11. Comparison of energies to 20 mm for arch- and hooked-ends types of fibres.
that the entire arch-type steel fibre was gently pulled out without causing a drastic decrease in the load as the curvature radius increased, despite the short anchorage length. Fig. 7(c) shows a steeper slope of the initial linear portion of the curve for A30_3.5 than for A30_2.5. The fibres fractured at the maximum pull-out
load as the anchorage and the bend lengths increased and also because of better adhesion with the cement matrix. A35_0, which had no anchorage end parts, had a maximum bond strength and interfacial toughness up to 20 mm that were 62.2 and 43.1% lower, respectively, than those for the hooked-ends type of fibre. Fig. 12(d) shows that A35_0 pulled out similarly to arch-type steel fibres having smaller curvature radii, and Fig. 7(d) indicates that it was pulled out with no residual bond strength after the maximum pull-out load. This behaviour is attributed to insufficient frictional resistance within the cement matrix during pull-out, as found for the straight parts of hooked-ends fibres having very large curvature radii. All of the fibres pulled out because of insufficient anchoring within the cement matrix, despite the increased bend length (Fig. 12(d)). The maximum bond strengths of A35_1.5, A35_2.5 and A35_3.5 were 64.4, 146.1 and 164.4% higher, respectively, than for the hooked-ends type of fibre. As found with the arch-type steel fibres having smaller curvature radii, the bond strength of the A35 fibres increased with increasing bend length. However, the increase in the maximum bond strength was lower than that found with the arch-type steel fibres having smaller curvature radii. This is attributed to the lower frictional resistance of the arch part as the curvature radius increased, as described previously. The pull-out load– slip curve of Fig. 7(d) supports this reasoning. The slope of the
Fig. 12. Specimens after the pull-out test for the arch-type steel fibres.
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initial linear portion was very mild compared with that for the arch-type steel fibres with smaller curvature radii. The maximum pull-out load was lower, despite the longer anchorage length. Anchoring of the arch-type steel fibre improved with increasing bend length, but the larger curvature arches were easily pulled out without frictional resistance. The interfacial toughnesses for A35_1.5, A35_2.5 and A35_3.5 were 107.5, 139.6 and 139.4% greater, respectively, than for the hooked-ends types. These increases were less than those obtained for the arch-type steel fibres having smaller curvature radii. In summary, arch-type steel fibres without anchorage parts showed little reduction in bond strength compared with the hooked-ends type, except A35_0 which had the greatest curvature radius but had increased interfacial toughness. We established that the frictional resistance of the curved arch part improved the bond properties with the cement matrix. Additionally, the bond strength and interfacial toughness increased with increasing bend length regardless of the curvature radius. However, at a very long bend length, the interfacial toughness diminished because improved anchorage caused fibre fracture to occur rather than pull-out. There was a pronounced effect of bend length for a given curvature radius for the arch-type fibres. This difference in behaviour from that of the hooked-ends type of fibre is attributed to pull-out resistance that is enhanced over the arched portions and not solely located at the anchorage points. 5. Conclusions The bond properties of arch-type steel fibres in a cementitious composite were evaluated. A single-fibre, double-sided pull-out test was used to evaluate the effect of changing curvature radius and bend length. Comparisons were made to the hooked-ends type of steel fibre that is most commonly used. Bond strength and interfacial toughness increased with increasing bend length regardless of the curvature radius. Unlike the hooked-ends type of steel fibre, the arch-type fibres had pull-out resistance at the anchorage parts and also over the curved portions of the arches. With the exception of A35_0, which had the largest curvature radius, the arch-type steel fibres lacking an anchorage part showed increased interfacial toughness compared with the hooked-ends types of fibres. The frictional resistance of the curved portion of the arch improved the bond properties within the cement matrix. Bent arch-type steel fibres provided a maximum bond strength that was 1.5 times higher than with the hooked-ends type of steel fibre.
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The interfacial toughness up to 20 mm of the arch-type steel fibres, except those with 0 mm bend length, was twice that of the hooked-ends type of steel fibres. This study has demonstrated that arch-type steel fibres can provide higher maximum bond strengths than the hooked-ends type of steel fibre after debonding. This is attributed to frictional sliding that occurs within the matrix duct and to plastic bending of the entire steel fibre. Additionally, constant pull-out resistance was observed after the peak load and continued until the completion of the pull-out. This was because of the concurrent operation of frictional sliding and mechanical anchorage forces. Such behaviour is quite unlike that found with the hooked-ends type of fibres. Acknowledgements This paper was supported by Konkuk University in 2014 and steel fibres were provided by KOSTEEL Co. from South Korea. References [1] Bentur A, Mindess S. Fibre reinforced cementitious composites. Elsevier Appl Sci 1990. [2] Shotcrete for Underground Support IV. Proceedings of the Engineering Foundation Conference Tlfs. Austria. 1995. [3] Beckett D. Comparative tests on plain fabric reinforced & steel fiber reinforced concrete ground slabs. Concrete 1989:43–5. [4] Lee C. Verification of load-carrying capacity of steel fiber reinforced concrete slab-on-grade. Korea Concr Inst 2007:337–40. [5] Cao HC, Beschoff E, Ruhle M, Evans AG, Marshall DB, Brennan JJ. Effect of interfaces on the properties of fiber-reinforced ceramics. J Am Ceram Soc 1990;73(6):1692–9. [6] Evans AG, He MY, Hutchinson JW. Interface debonding and fiber cracking in brittle matrix composites. J Am Ceram Soc 1989;72(12):2300–3. [7] Balaguru PN, Shah SP. Fiber-reinforced cement composite. New York: McGrawHill Inc; 1992. [8] Shannag MJ, Brincker R, Hansen W. Pullout behavior of steel fibers from cement-based composites. Cem Concr Res 1997;27(6):925–36. [9] Naaman AE, Najm H. Bond-slip mechanisms of steel fibers in concrete. ACI Mater J 1991;88(2):135–45. [10] Banthia N, Trottier JF. Concrete reinforced with deformed steel fibres Part I : bond-displacement mechanisms. ACI Mater J 1994:435–45. Sept-Oct. [11] Song F. Effect of fibre properties and embedment conditions on fibre pullout behaviour from concrete matrix. In: Proceedings of the 9th fib International PhD Symposium in Civil. 2012. p. 597–601. [12] Abu-Lebdeh T, Hamoush S, Heard W, Zornig B. Effect of matrix strength on pullout behavior of steel fiber reinforced very-high strength concrete composites. Constr Building Mater 2011;25:39–46. [13] Naaman AE. Engineered steel fibers with optimal properties for reinforcement of cement composites. J Adv Concr Technol 2003;1(3):241–52. [14] Zile E, Zile O. Effect of the fiber geometry on the pullout response of mechanically deformed steel fibers. Cem Concr Res 2013;44:18–24. [15] Won JP, Hong BT, Lee SJ, Choi SJ. Bonding properties of amorphous micro-steel fibre-reinforced cementitious composites. Compos Struct 2013;102:101–9. [16] JCI SF-8, Method of test for bond of fibers, Japan Concrete Institute, 2002.