- Email: [email protected]
Combined effects of steel fiber and coarse aggregate size on the compressive and flexural toughness of high-strength concrete
Accepted Manuscript Combined effects of steel fiber and coarse aggregate size on the compressive and flexural toughness of high-strength concrete Seok-Joon Jang, Hyun-Do Yun PII: DOI: Reference:
S0263-8223(17)33127-6 https://doi.org/10.1016/j.compstruct.2017.11.009 COST 9075
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
4 September 2017 29 October 2017 6 November 2017
Please cite this article as: Jang, S-J., Yun, H-D., Combined effects of steel fiber and coarse aggregate size on the compressive and flexural toughness of high-strength concrete, Composite Structures (2017), doi: https://doi.org/ 10.1016/j.compstruct.2017.11.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Combined effects of steel fiber and coarse aggregate size on the compressive and flexural toughness of high-strength concrete Seok-Joon Jang1 and Hyun-Do Yun2* 1
Graduate Student, Department of Architectural Engineering, Chungnam National University, Daejeon, 305-764, Republic of Korea 2
Professor, Department of Architectural Engineering, Chungnam National University, Daejeon, 305-764, Republic of Korea *
Corresponding Author: (E-mail: [email protected], Tel. +82-42-821-5622)
Abstract This paper investigates the effects of steel fiber content and coarse aggregate size on the mechanical properties of high-strength concrete with a specified compressive strength value of 60 MPa. The paper also explores the correlation between the compressive and flexural toughness of high-strength steel fiber-reinforced concrete (SFRC). For this purpose, twelve high-strength SFRC mixtures with four fiber volume fraction of steel fiber ( = 0.5%, 1.0%, 1.5%, and 2.0%) and different aggregate sizes were designed and fabricated. Compressive and flexural tests for each concrete mixture were conducted, and the test results were used to investigate the effects of steel fiber volume fraction and aggregate size on the compressive and flexural toughness of high-strength SFRC prims. The results indicate that the mechanical properties of SFRC are related more closely to volume fraction than to aggregate size. The compressive and flexural toughness ratios of the SFRC significantly improved with an increase in fiber content. Also, equations that are suggested to determine the compressive toughness ratio based on the equivalent flexural strength ratio were used to predict the mechanical properties of the SFRC in this study.
Keywords: Steel fiber, toughness, flexural testing, high-strength concrete
1. Introduction Recently, demands for taller buildings and longer bridges have increased, thus accelerating the use of high-strength concrete in construction to reduce cross-sections and the self-weight of concrete structures. High-strength concrete offers several benefits in terms of structural performance and durability compared to normal-strength concrete. However, high-strength concrete is characterized by its brittleness. Therefore, one of the main concerns with regard to high-strength concrete is the need to develop ways to convert its brittle behavior to ductile behavior. It is generally known that the addition of short synthetic or metallic fibers can improve the post-cracking behavior of concrete by bridging cracks and increasing the ductility and energy dissipation capacity of concrete members and structures [1-3]. Fiber reinforced concrete (FRC) to enhance the mechanical properties of concrete are manufactured with various types of metallic [4-7], polymeric [8-10], and hybrid fibers [11-13]. Various types of steel fiber that differ in tensile strength as well as shape and size have been developed and used for SFRC. Soroushian and Bayasi [14] investigated the relative effectiveness of different types of steel fiber in normal-strength concrete. The types of steel fiber used in their study are straight-round, crimped-round, crimped-rectangular, hookedsingle, and hooked-collated fibers. They found that hooked-end fiber is more effective than straight or crimped fiber in improving both the flexural and compressive behavior of concrete. However, Yakoub [15] concluded from analyzing 281 SFRC beams that hooked steel fiber is not as efficient as crimped fiber in terms of providing a capacity for shear resistance for reinforced concrete (RC) beams. Through statistical analysis of fibers produced all over the world, Katzer [16] reported that 67.1% are the hooked type and the other most popular fibers are straight fiber (9.1%), mechanically deformed fiber (9.1%), crimped fiber (7.9%), and other fiber types with different ends (6.6%). The Katzer study [16] clearly indicates that hooked-end steel fiber is the most popular and effective type of reinforcing fiber for concrete. Over the last four decades, extensive studies on SFRC have confirmed the effectiveness of deformed steel fibers as additional reinforcement in structural elements such as beam [17-23], wall [24, 25], column [26-28], coupling and joint members [29,30]. The utilization of deformed steel fiber instead of vertical stirrups in RC beams was first investigated by Batson et al. [17] in the early 1970s. To determine the effectiveness of steel
fiber as web reinforcement, they carried out shear tests on 72 RC beam specimens with different shear span ratios, fiber volume fractions, and fiber shapes. Batson et al. [17] concluded that the replacement of vertical stirrups by round, flat, or crimped steel fiber provides effective reinforcement against shear failure. Based on Parra-Montesinos’ (2006) database [20] that contains information about the shear strength of beams fabricated with deformed steel fiber, the American Concrete Institute (ACI) building code, ACI 318-08 [31], introduced the use of deformed steel fiber as minimum shear reinforcement for normalstrength concrete beams. Yoo et al. [23] investigated the feasibility of eliminating the minimum shear reinforcement in reinforced high-strength concrete beams that have a compressive strength value of 67 MPa by adding 0.75% hooked steel fiber. They concluded that the minimum shear reinforcement for reinforced high-strength concrete beams indeed could be eliminated by the inclusion of 0.75% hooked steel fiber. However, only limited data are available regarding high-strength concrete in test results for SFRC beams. To mitigate damage in RC columns during severe earthquakes, the traditional method is to set closely-spaced transverse hoops to improve the confinement of the concrete at plastic hinge regions. However, a relatively large number of transverse hoops may result in a congestion of the reinforcement as well as an increase in the cost and time for construction [27]. Recently, the use of SFRC has been regarded as an alternative reinforcement solution to the use of transverse hoops. Germano et al. [28] conducted an experimental study of the structural performance of SFRC columns subjected to uniaxial and biaxial cyclic loads. Their test results indicate that the using steel fiber has a positive effect on mitigation of columns damage and energy dissipation. Bayasi and Gebman [30] conducted a literature review and reported an experimental study of the effects of steel fiber on seismic beam-column connections. Their results show that the confining effect of steel fibers results in a reduction of the lateral reinforcement in seismic beam-column joints. Thus, major efforts have been made in recent years to eliminate the shear or confinement reinforcement in RC members by adding steel fiber to save costs and time that are needed for cutting and placing rebar. The effects of fiber on the mechanical properties of concrete are governed by the fiber volume fraction, fiber type, and cementitious matrix properties. As specified in ACI 318-08 [31] for the use of steel fiber as an alternative to minimum shear reinforcement, the flexural toughness of SFRC can be a significant index to determine the number of transverse hoops that can be eliminated when steel fiber is included in the concrete.
The compressive toughness also can be used as index to determine the confinement ability of SFRC. However, special equipment and loading methods are required to measure the descending curve of SFRC in compression. Furthermore, obtaining data about the descending curve for high-strength concrete in compression compared to in flexure is difficult. Although numerous studies have investigated the effects of hooked-end steel fiber on the mechanical properties of normal-strength concrete, test data for high-strength concrete are limited. Therefore, this paper first aims to examine the effects of hooked-end steel fiber volume fraction and coarse aggregate size on the compressive and flexural behavior of high-strength concrete. Then, this paper explores the relationship between compressive toughness and flexural toughness of high-strength concrete with hooked-end steel fiber based on this study’s flexural and compressive test results.
2. Experimental Program 2.1 Mixture Proportions and Materials An experimental program was designed for this study to examine the effects of fiber volume fraction and coarse aggregate size, also referred to as nominal maximum aggregate size (NMAS), on the mechanical properties of high-strength concrete. Table 1 presents the twelve series of SFRC mixtures used in the study. The mixture codes reflect the NMAS ranges and volume fractions, as follows. The crushed coarse aggregate was divided into three NMAS categories: small coarse aggregate ranging from 5 mm to 8 mm NMAS, medium coarse aggregate ranging from 5 mm to 13 mm NMAS, and large coarse aggregate ranging from 13 mm to 19 mm NMAS. The four volume fractions of the added hooked-end steel fiber were 0.5%, 1.0%, 1.5%, and 2.0%. Also, the specified compressive strength of all the SFRC mixtures was 60 MPa. The SFRC mixtures with a 0.3 water-to-cement ratio were used to cast specimens for compressive and flexural testing.
Table 1 Mix proportions of SFRC Mixture code Coarse aggregate (mm)
Fiber volume fraction (%)
Unit weight (kg/m3)
Water
Cement
Fine aggregate
Coarse aggregate
Steel fiber
SFRC8-0.5 SFRC8-1.0 SFRC8-1.5 SFRC8-2.0 SFRC13-0.5 SFRC13-1.0 SFRC13-1.5 SFRC13-2.0 SFRC19-0.5 SFRC19-1.0 SFRC19-1.5 SFRC19-2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
200 200 200 200 200 200 200 200 200 200 200 200
666 666 666 666 666 666 666 666 666 666 666 666
900 900 900 900 900 900 900 900 900 900 900 900
540 540 540 540 540 540 540 540 540 540 540 540
39.2 78.5 117.7 157.0 39.2 78.5 117.7 157.0 39.2 78.5 117.7 157.0
5-8 5-8 5-8 5-8 5-13 5-13 5-13 5-13 13-19 13-19 13-19 13-19
For the fabrication of the SFRC specimens, the cement used in all the mixtures was ordinary Portland cement (ASTM Type 1) with a specific gravity of 3.14 g/cm3. The fine aggregate was local natural sand with specific gravity and water absorption values of 2.62 and 1.91%, respectively. The coarse aggregate was crushed limestone. Figs. 1 and 2 show the size distribution and size ranges of the coarse aggregate used in this study, respectively. A superplasticizer was used to improve the workability of the SFRC mixtures and distribute the fibers uniformly in the fresh concrete. Hooked-end steel fiber with a 30 mm length and an aspect ratio of 60 was used for reinforcing the concrete. Table 2 provides a summary of the mechanical properties of the hooked-end steel fibers used in this study.
Fig. 1 Grading curves of aggregate
(a) 5 mm - 8 mm
(b) 5 mm - 13 mm
(c) 13 mm - 19 mm
Fig. 2 NMAS ranges of coarse aggregate particles Table 2 Mechanical properties of hooked-end steel fiber Hooked-end steel fiber
Tensile strength (MPa) 1,100
Length (mm) 30
Diameter (mm) 0.5
Aspect ratio (length/diameter) 60
2.2 Specimen Manufacture and Test Procedure Each SFRC mixture was mixed using a double axial mixer. First, the cement, fine aggregate, and coarse aggregate were dry-mixed for one minute, followed by the addition of water with a superplasticizer. Then, mixing was continued for three minutes. After the steel fiber was added, mixing was continued for an additional two minutes to dissolve the glue in the steel fiber bundles. Cylindrical and prismatic specimens were cast in steel molds and then compacted via
vibration. The specimens were demolded after 24 hours and then cured in 20℃ water for 28 days. The specimens were air-dried in the laboratory 12 hours before testing. The slump and air content of all the study mixtures were measured using fresh SFRC mixtures according to ASTM C143 [32] and ASTM C231 [33], respectively. To investigate the effects of steel fiber volume fraction and NMAS on the compressive behavior of highstrength concrete, three cylindrical specimens were cast, each with a diameter of 100 mm and height of 200 mm. The cylinders were tested to measure the compressive strength and elastic modulus according to ASTM C39 [34]. Fig. 3(a) shows a specimen in the compressive test setup. The load was applied using a universal testing machine (UTM) with the capacity of 3,000 kN. The tests were carried out with a displacement ratio of 10 µm/s to obtain the postpeak behavior of the SFRC. During the test, two linear variable differential transformers (LVDTs) were installed to measure the axial strain. Three prismatic specimens with a 100 × 100 mm cross-section and 300 mm span were fabricated using each SFRC mixture according to ASTM C1609 [35]. Fig. 3(b) shows the flexural test setup. Third-point bending loading was used for the flexural tests in conjunction with a 200 kN UTM. The mid-span deflection measurement of the prism was obtained using two LVDTs placed on a steel yoke. A yoke can measure the relative deflections of the support and central part of a prismatic specimen, thereby improving the precision of first-crack deflection measurements.
(a) Compression test setup
(b) Flexural test setup
Fig. 3 Test set-ups to measure SFRC mechanical properties
3. Test Results and Discussion 3.1 Fresh Properties
The SFRC slump and air content tests were conducted immediately after mixing. As clearly noted from Table 3, the SFRC mixtures with fiber volume fractions of 1.5% and 2.0% have less workability than those with 0.5% and 1.0% fiber. In contrast, the size of the coarse aggregate does not significantly affect the workability of SFRC. Due to the blocked pores created by the distributed steel fiber, the air content in fresh mixtures increases with an increase in the steel fiber content. The SFRC8-0.5 mixture, on the other hand, has the highest air content of 6.4% due to the mixing condition. Table 3 Fresh properties of SFRC Mixture code
Slump (mm)
Air content (%)
SFRC8-0.5 SFRC8-1.0 SFRC8-1.5 SFRC8-2.0 SFRC13-0.5 SFRC13-1.0 SFRC13-1.5 SFRC13-2.0 SFRC19-0.5 SFRC19-1.0 SFRC19-1.5 SFRC19-2.0
265 210 130 105 240 243 185 105 245 245 175 85
6.4 3.9 4.5 2.0 2.7 3.5 3.8 5.5 1.9 2.6 3.5 2.4
3.2 Compressive Strength The experimental results for the cylindrical specimens under compression indicate no significant changes in compressive strength for SFRC with various coarse aggregate sizes and fiber volume fractions. For the SFRC with an NMAS of 8 mm and volume fraction of 0.5%, however, the compressive strength decreased compared to that of other mixtures due to its higher air content, as noted in Table 3. Table 4 presents the compressive properties of the SFRC specimens, i.e., compressive strength, strain at peak stress, and the modulus of elasticity, as obtained from compression testing. The peak strain is shown to increase in accordance with an increase in the confined effect caused by the hooked-end steel fiber.
Table 4 Compressive properties of SFRC Mixture code
′ (MPa)
(10-6 )
Ε (GPa)
SFRC8-0.5 SFRC8-1.0 SFRC8-1.5 SFRC8-2.0 SFRC13-0.5 SFRC13-1.0 SFRC13-1.5 SFRC13-2.0 SFRC19-0.5 SFRC19-1.0 SFRC19-1.5 SFRC19-2.0
58.2(±1.0) 68.6(±0.7) 63.6(±0.6) 68.2(±0.5) 64.5(±0.5) 63.2(±0.8) 64.3(±0.2) 65.4(±1.1) 66.6(±2.0) 67.1(±0.6) 67.6(±1.7) 65.5(±1.5)
3300(±95) 3432(±297) 3463(±85) 3591(±158) 3227(±303) 3320(±78) 3424(±64) 3394(±66) 3244(±234) 3148(±84) 3278(±276) 3322(±344)
25.5(±0.5) 28.7(±0.8) 27.9(±0.5) 29.7(±0.7) 30.6(±2.1) 28.7(±1.6) 28.5(±0.8) 29.2(±0.3) 30.0(±0.3) 30.2(±0.9) 30.9(±0.3) 29.0(±0.8)
Fig. 4 shows typical compressive stress-strain relationships for the SFRC in terms of fiber volume fraction. The compressive stress and strain were normalized by compressive strength and strain at peak stress, respectively. The effects of the steel fiber volume fraction on the initial behavior appear to be insignificant, whereas the post-peak ductility of the SFRC increases with an increase in fiber content. The use of steel fiber as transverse reinforcement for compression RC members is shown to improve the confinement and ductility.
(a) SFRC8 series
(b) SFRC13 series
(c) SFRC19 series
Fig. 4 Effects of fiber volume fraction on normalized stress-strain curves of SFRC
3.3 Compressive Toughness The empirical equations used for the compressive behavior of SFRC found in the literature
employ the expression proposed by Carreira and Chu [36] for plain concrete, as given by Eq. (1).
fc β (ε c /ε 0 ) = (1) f'c β − 1 + (ε c /ε 0 )β where εc is the uniaxial strain; fc is the compressive stress; f’c is the compressive strength; β is the material parameter; and ε0 is the strain at the compressive strength. Ezdeldin and Balaguru [37] and Nataraja et al. [38] employed the same expression to suggest strain-stress curves for SFRC using crimped steel fiber; they obtained the value of β from the fiber volume fraction, aspect ratio, and reinforcing index. Lee et al. [39] reported compressive behavior of SFRC that contained hooked-end steel fiber and suggested a modified expression, presented here as Eqs. (2.1) through (2.4).
fc A (ε c /ε 0 ) = (2.1) f'c A − 1 + (ε c /ε 0 )B A =B =
1
f' 1− c ε 0E c
A = 1 + 0 .723 (V f
,ε c /ε 0 ≤ 1 .0 (2.2)
lf −0 .957 ) ,εc /ε 0 > 1 .0 (2.3) df
f' l B = ( c )0 .064 [1 + 0 .882 (V f f )−0 .882 ,ε c /ε 0 > 1 .0 (2.4) 50 df where A and B are the material parameters; Vf is the fiber volume fraction; and lf and df represent the fiber length and diameter, respectively. Fig. 5 presents comparisons of typical actual behavior under compression and behavior obtained from prediction models using Eqs. (2.1) through (2.4) for SFRC with 13 mm NMAS. As shown in the figures, the model suggested by Lee et al. [39] can represent the compressive behavior of SFRC with fiber volume fractions of 1.0%, 1.5%, and 2.0%. The post-peak behavior of the SFRC specimen with 0.5% fiber could not be measured due to the rapid decrease in loading after the peak stress was reached. Thus, the model was used to predict that specimen’s post-peak behavior.
(a) Fiber volume fractions of 0.5% and 1.0%
(b) Fiber volume fractions of 1.5% and 2.0%
Fig. 5 Test and analytical stress-strain curves of SFRC under compression The compressive toughness ratio can be calculated from the strain-stress curves of cylindrical specimens. In this study, the compressive toughness ratio was calculated as the ratio of the area of square made by compressive strength and specified strain to that of the compressive stress-strain curve up to the specified strain, as proposed by Nataraja et al. [38]. As noted from Eqs. (3.1) and (3.2), the specified strain values are set at 0.009 and 0.015, which are three and five times, respectively, the concrete ultimate strain of 0.003 provided in the ACI 318-14 building code [40].
TR
3
=
TR
5
=
TF 3 0 .009 f'c
TF 5 0 .015 f'c
(3.1)
(3.2)
where TR3 and TR5 are the compressive toughness ratios up to the strain values of 0.009 and 0.015, respectively. Also, TF3 and TF5 are the areas under the stress-strain curve up to those strain levels. Fig. 6 presents the compressive toughness ratios calculated via the measured data and via the prediction model using Eqs. (2.1) through (2.4). As shown, the compressive toughness ratios using these two methods are similar to each other. Therefore, the equation can be used to calculate the toughness ratio instead of incomplete post-peak behavior of SFRC based on the experimental results. Table 4 also shows the required parameters based on the compressive test results.
(a) Up to strain of 0.009
(b) Up to strain of 0.0015
Fig. 6 Comparisons of compressive toughness ratios based on measured data and analytical results Fig. 7 shows the effects of both volume fraction and aggregate size on the compressive toughness ratios of the SFRC mixtures. As shown, the compressive toughness ratios increase with an increase in fiber content of the SFRC mixtures for all the study aggregate sizes. The SFRC specimens with 8 mm NMAS show greater compressive toughness than those of other aggregate sizes. However, compared to steel fiber volume fraction, aggregate size has an insignificant effect on the post-peak compressive behavior and toughness of the SFRC mixtures.
(a) Up to strain of 0.009
(b) Up to strain of 0.015
Fig. 7 Effects of fiber volume fraction and aggregate size on compressive toughness ratios of SFRC
3.4 Flexural Strength of SFRC Fig. 8 illustrates the method used to define the first crack and ultimate load stage of SFRC under flexural loading. Benthia and Trottir [41] distinguished the initial ascending parts of the SFRC curve as three stages: first nonlinearity, significant nonlinearity, and the end of the matrix contribution. They also suggested that the end of the matrix contribution is the first crack stage of SFRC. In this study, the initial point was set based on the initial stiffness of the specimen, as found in ASTM C1018 [42] and shown in Fig. 8. This method can compensate for the error that occurs at the initial state of a prismatic specimen under small loads. The addition of steel fiber has more of an effect on the behavior after the first crack than on the initial behavior. The post-cracking behavior of SFRC can be divided into deflection-softening and deflection-hardening behavior. Table 5 lists the flexural test results based on these definitions.
Fig. 8 Characteristics of load-deflection curve of SFRC [35,41,42]
Table 5 Flexural properties of SFRC Mixture code
Pc (kN)
δc (mm)
Pu (kN)
δu (mm)
SFRC8-0.5
35.0(±0.4)
0.066(±0.003)
35.0(±0.4)
0.066(±0.003)
SFRC8-1.0
36.4(±5.6)
0.063(±0.003)
41.1(±7.5)
0.233(±0.148)
SFRC8-1.5
36.1(±7.6)
0.067(±0.004)
48.0(±2.0)
0.406(±0.004)
SFRC8-2.0
37.3(±3.0)
0.061(±0.010)
53.9(±2.3)
0.321(±0.090)
SFRC13-0.5
36.5(±2.9)
0.066(±0.003)
36.5(±2.9)
0.066(±0.003)
SFRC13-1.0
36.2(±0.9)
0.066(±0.004)
37.3(±1.0)
0.151(±0.146)
SFRC13-1.5
35.9(±2.8)
0.059(±0.006)
45.7(±6.6)
0.314(±0.015)
SFRC13-2.0
39.7(±2.2)
0.069(±0.008)
51.4(±7.3)
0.536(±0.190)
SFRC19-0.5
37.3(±4.8)
0.065(±0.004)
37.3(±4.8)
0.065(±0.004)
SFRC19-1.0
41.7(±1.4)
0.073(±0.006)
42.7(±3.0)
0.107(±0.054)
SFRC19-1.5 SFRC19-2.0
40.3(±4.9) 37.3(±4.9)
0.068(±0.008) 0.062(±0.003)
52.5(±1.5) 48.0(±4.7)
0.452(±0.116) 0.876(±0.608)
Fig. 9 illustrates the effects of fiber volume fraction and aggregate size on typical flexural responses for the three SFRC mixture series. As expected, the initial behavior of all the specimens is similar until the cracking stage, because all the mixtures have similar compressive strength and modulus of elasticity values. On the other hand, the effects of steel fiber volume fraction and NMAS on the behavior after cracking are significant. The SFRC specimens with a fiber volume fraction of 0.5% exhibit deflection-softening behavior after the initial crack for all aggregate sizes. In the case of the SFRC specimens with 1.0% fiber, both deflection-softening and deflection-hardening are evident. The hardening behavior led to an increase in peak strength, as shown in Table 5. The SFRC8-1.0 mixture shows the highest average increasing ratio between the first crack and ultimate load of 13% compared with the SFRC mixtures with the same steel fiber volume fraction with 13 mm and 19 mm NMAS. SFRC specimens that contained 1.5% and 2.0% fiber also exhibited deflection-hardening behavior with an increase in the ultimate load and deflection values. With an increase in the steel fiber volume fraction, the ultimate load value of the SFRC increased correspondingly. For the SFRC19-2.0 mixture, however, the ultimate strength decreased more than that of the SFRC19-1.5 mixture due to the fiber ball that was caused by the higher content of steel fiber and larger aggregate particles in the SFRC19-2.0 mixture.
(a) SFRC8 series
(b) SFRC13 series
(c) SFRC19 series
Fig. 9 Typical flexural behavior of SFRC In this study, the ultimate strength of the SFRC increased with an increase in fiber volume fraction, which is closely related to the number of steel fibers that provide a bridging action. The fiber distribution in terms of the casting direction is also an important factor for determining the flexural performance of SFRC [43]. Fig. 10 illustrates the typical failure mode after flexural testing of the SFRC with the bridging action of the hooked-end steel fibers. The surface indicates that failure was attributable mostly to pull-out rather than rupture of the steel fibers. Fig. 11 shows the effect of the number of steel fibers for the tensile zone (shown in Fig. 10) on the ultimate load of the SFRC prismatic specimens. As shown in Fig. 11, the SFRC specimens with 8 mm and 13 mm NMAS have more bridging steel fibers compared to specimens with the NMAS of 20 mm. It is concluded that the ultimate strength of SFRC tends to increase in accordance with the number of steel fibers with bridging action.
Fig. 10 Fiber distribution at failure face
Fig. 11 Effect of number of fibers on ultimate load of SFRC
3.5 Equivalent Flexural Strength Ratio The equivalent flexural strength ratio was characterized in this study as per ASTM C1609 [27] to investigate the flexural toughness of SFRC. Eqs. (4.1) and (4.2) were used to calculate the equivalent flexural strength ratios up to deflections of 1/300 and l/150 of the span length, respectively.
R T ,300 =
R T ,150 =
150T 300
f1bh 2 150T 150
f1bh 2
(4.1)
(4.2)
where RT,300 and RT,150 are equivalent flexural strength ratios up to the deflections of L/300 and L/150, respectively; T300 and T150 are the areas under load-net deflection curves 0 to L/300 and L/150, respectively; f1 is the first-peak strength; b is the width of the specimen; h is the height of the specimen; and L is the span length. Fig. 12 shows the effects of both fiber volume fraction and aggregate size on the equivalent flexural strength ratios of the SFRC. ACI building code [40] allows the use of at least 60 kg of deformed steel fiber per cubic meter of concrete (fiber volume fraction of approximately 0.75%) as a design alternative to the minimum shear reinforcement for 0.5 in normal-strength concrete beams with compressive strength values less than or equal to 40 MPa. In addition to the specified minimum content of deformed steel fiber, ACI building
code also specifies that the SFRC used for shear resistance must satisfy three requirements: (1) the compressive strength acceptance criteria for standard-cured specimens, (2) the residual strength obtained from flexural testing in accordance with ASTM C1609 [35] at a midspan deflection of 1/300 of the span length that is at least the greater of 90% of the measured firstpeak strength obtained from a flexural test or 90% of the strength that corresponds to 0.62√f ’c (MPa), and (3) the residual strength obtained from flexural testing in accordance with ASTM C1609 at a midspan deflection of 1/150 of the span length that is at least the greater of 75% of the measured first-peak strength obtained from a flexural test or 75% of the strength that corresponds to 0.62√f’c (MPa). Fig. 12 also shows the acceptance criteria for SFRC as the minimum shear transverse reinforcement provided in ACI 318-14 [40]. As expected, an increase in flexural toughness that corresponds to an increase in steel fiber content was observed; flexural toughness increased linearly to 1.5% fiber volume fraction. On the other hand, for SFRC specimens with over 2% fiber, similar flexural toughness was observed. According to the flexural test results, excessive amounts of steel fiber contribute to a reduction in the effectiveness of the steel fiber due to the inhomogeneous distribution of the fibers. In other words, SFRC specimens with a small NMAS exhibit greater toughness than those with a large NMAS. Fig. 13 shows the correlation between the number of steel fibers and the equivalent flexural strength ratio. The flexural toughness ratio increases with an increase in the number of steel fibers for both cases of deflections to L/300 and L/150.
(a) Up to deflection of L/300
(b) Up to deflection of L/150
Fig. 12 Effects of fiber volume fraction and aggregate size on equivalent flexural strength ratio of SFRC
(a) Up to deflection of L/300
(b) Up to deflection of L/150
Fig. 13 Effects of number of fibers on equivalent flexural strength ratio of SFRC
3.6 Correlation between Compressive and Flexural Toughness Ratios The toughness ratio represents the energy absorbance capacity and ductility of a material. In this study, the toughness ratio of SFRC was investigated under compression and bending loads. Fig. 14 shows the relationship between the compressive toughness ratio and the equivalent flexural strength ratio of the SFRC. The test results suggest that the proposal to use Eqs. (5.1) and (5.2) for the calculation of the compressive toughness ratio based on the equivalent flexural strength. Fig. 14 shows that the suggested models provide a good estimation compared to the test results. The models also can be used effectively to better understand the mechanical properties of SFRC. TR
3
= 0 .717 (R T ,300 )0 .358 (5.1)
TR
5
= 0 .6431 (R T ,150 )0 .5725
(5.2)
(a) Up to strain of 0.009 and deflection of L/300
(a) Up to strain of 0.015 and deflection of L/150
Fig. 14 Correlations between compressive toughness ratio and equivalent flexural strength ratio of SFRC
4
Conclusions
The compressive, flexural, and toughness ratios of SFRC were investigated through experimental research in this study. The test results have led to the following main findings. 1) Both steel fiber volume fraction and aggregate size do not have a significant effect on the compressive strength and modulus of elasticity of SFRC. However, the peak strain, post-peak behavior, and toughness of SFRC under compression improve with an increase in fiber volume fraction. 2) Flexural strength, toughness and the equivalent flexural strength ratio significantly increase with an increase in fiber content. The improvement of the flexural performance of SFRC is related mainly to the number of steel fibers in the tensile zone of a prismatic specimen. The limited hooked-end steel fiber volume fraction of 1.5% is suggested for the economical and efficient use of steel fiber based on this study’s flexural test results. 3) The NMAS has less effect on the mechanical properties of SFRC than the fiber volume fraction. For SFRC specimens with a higher percentage of fiber, on the other hand, using small aggregate can aid the homogeneous dispersion of steel fiber. Mathematical expressions are suggested in this paper for the compressive toughness ratio with regard to the equivalent flexural strength ratio of SFRC.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(NRF2016R1D1A3B02008179)
Reference [1] ACI Committee 544, Design Considerations for Steel Fiber Reinforced Concrete (ACI 544.4R-88 (Reapproved 2009)), American Concrete Institute 1988 [2] Zollo RF. Fiber-reinforced concrete: an overview after 30 years of development. Cement and Concrete Composites 1997;19(2):107-22. [3] Di Prisco M, Plizzari G, Vandewalle L. Fibre reinforced concrete: new design perspectives. Materials and Structures 2009;42(9):1261-81. [4] Soroushian P, Bayasi Z. Fiber type effects on the performance of steel fiber reinforced concrete. Materials Journal 1991;88(2):129-34. [5] Song PS, Hwang S. Mechanical properties of high-strength steel fiber-reinforced concrete. Construction and Building Materials 2004;18(9):669-73. [6] Olivito RS, Zuccarello FA. An experimental study on the tensile strength of steel fiber reinforced concrete. Composites Part B: Engineering 2010;41(3):246-55. [7] Orbe A, Rojí E, Losada R, Cuadrado J. Calibration patterns for predicting residual strengths of steel fibre reinforced concrete (SFRC). Composites Part B: Engineering 2014;58:408-17. [8] Li VC, Wang S, Wu C. Tensile strain-hardening behavior of polyvinyl alcohol engineered cementitious composite (PVA-ECC). ACI Materials Journal 2001;98(6):483-92. [9] Torigoe SI, Horikoshi T, Ogawa A, Saito T, Hamada T. Study on evaluation method for PVA fiber distribution in engineered cementitious composite. Journal of Advanced Concrete Technology 2003;1(3):265-8. [10] Yun HD. Effect of accelerated freeze–thaw cycling on mechanical properties of hybrid PVA and PE fiber-reinforced strain-hardening cement-based composites (SHCCs).
Composites Part B: Engineering 2013;52:11-20. [11] Ahmed SFU, Maalej M, Paramasivam P. Flexural responses of hybrid steel–polyethylene fiber reinforced cement composites containing high volume fly ash. Construction and building materials 2007;21(5):1088-97. [12] Banthia N, Majdzadeh F, Wu J, Bindiganavile V. Fiber synergy in Hybrid Fiber Reinforced Concrete (HyFRC) in flexure and direct shear. Cement and Concrete Composites 2014;48:91-7. [13] Shu X, Graham RK, Huang B, Burdette EG. Hybrid effects of carbon fibers on mechanical properties of Portland cement mortar. Materials & Design (1980-2015) 2015;65:1222-28. [14] Soroushian P, Bayasi Z. Fiber type effects on the performance of steel fiber reinforced concrete. Materials Journal, 1991;88(2):129-34. [15] Yakoub HE. Shear stress prediction: Steel fiber-reinforced concrete beams without stirrups. ACI Structural Journal 2011;108(3):304-14. [16] Katzer J. Steel fibers and steel fiber reinforced concrete in civil engineering. Pacific Journal of Science and Technology 2006;7(1):53-8. [17] Batson G, Jenkins E, Spatney R. Steel fibers as shear reinforcement in beams. Am Concrete Inst Journal & Proceedings 1972;69(10)640-4. [18] Swamy RN, Bahia HM. The effectiveness of steel fibers as shear reinforcement. Concrete International 1985;7(3):35-40. [19] Narayanan R. Darwish IYS. Use of steel fibers as shear reinforcement. ACI Structural Journal 1987;84(3):216-227. [20] Parra-Montesinos GJ. Shear strength of beams with deformed steel fibers. Concrete International 2006;28(11):57-66. [21] Kim KS, Lee DH, Hwang JH, Kuchma DA. Shear behavior model for steel fiberreinforced concrete members without transverse reinforcements. Composites Part B: Engineering 2012;43(5):2324-34. [22] Cuenca E, Echegaray-Oviedo J, Serna P. Influence of concrete matrix and type of fiber
on the shear behavior of self-compacting fiber reinforced concrete beams. Composites Part B: Engineering 2015;75:135-47. [23] Yoo DY, Yuan T, Yang JM, Yoon YS. Feasibility of replacing minimum shear reinforcement with steel fibers for sustainable high-strength concrete beams. Engineering Structures 2017;147:207-22. [24] Lim WG, Kang SW, Yun HD. Shear Behavior of Squat Steel Fiber Reinforced Concrete (SFRC) Shear Walls with Vertical Slits. In Applied Mechanics and Materials 2013;372:20710. [25] Carrillo J, Pincheira JA, Alcocer SM. Behavior of low-rise, steel fiber-reinforced concrete thin walls under shake table excitations. Engineering Structures 2017;138:146-58. [26] Ganesan N, M0urthy JR. Strength and behavior of confined steel fiber reinforced concrete columns. ACI Materials Journal 1990;87(3):221-7. [27] Huang L, Xu L, Chi Y, Xu H. Experimental investigation on the seismic performance of steel–polypropylene hybrid fiber reinforced concrete columns. Construction and Building Materials 2015;87:16-27. [28] Germano F, Tiberti G, Plizzari G. Experimental behavior of SFRC columns under uniaxial and biaxial cyclic loads. Composites Part B: Engineering 2016;85:76-92. [29] Khalifa ES. Analytical model for steel fiber concrete composite short-coupling beam. Composites Part B: Engineering 2014;56:318-29. [30] Bayasi Z, Gebman M. Reduction of lateral reinforcement in seismic beam-column connection via application of steel fibers. ACI Structural Journal 2002;99(6):772-80. [31] ACI Committee 318. Building Code Requirements for Structural Concrete (ACI 318-08): An ACI Standard: Commentary on Building Code Requirements for Structural Concrete (ACI 318R-08). American Concrete Institute 2008. [32] ASTM C143/C143M-15. Standard test method for slump of hydraulic-cement concrete. American Society of Testing and Materials 2015:1–4 [33] ASTM C 231/C231M-17. Standard test method for air content of freshly mixed concrete by the pressure method. American Society of Testing and Materials 2017:1–10
[34] ASTM C39/C39M-17. Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. American Society of Testing and Materials 2017: 1–8. [35] ASTM C1609/C1609M-12, Standard test method for flexural performance of fiberreinforced concrete (using beam with third-point loading). American Society of Testing and Materials 2012:1–8. [36] Carreira DJ, Chu KH. Stress-strain relationship for plain concrete in compression. In Journal Proceedings 1985;82(6):797-804. [37] Ezeldin AS, Balaguru PN. Normal-and high-strength fiber-reinforced concrete under compression. Journal of materials in civil engineering 1992;4(4):415-29. [38] Nataraja MC, Dhang N, Gupta AP. Stress–strain curves for steel-fiber reinforced concrete under compression. Cement and concrete composites 1999;21(5):383-90. [39] Lee SC, Oh JH, Cho JY. Compressive behavior of fiber-reinforced concrete with endhooked steel fibers. Materials 2015;8(4):1442-58. [40] ACI Committee 318. Building Code Requirements for Structural Concrete (ACI 318-14): An ACI Standard: Commentary on Building Code Requirements for Structural Concrete (ACI 318R-14). American Concrete Institute 2015. [41] Banthia N, Trottier JF. Test methods for flexural toughness characterization of fiber reinforced concrete: some concerns and a proposition. ACI Materials Journal 199;92:48-57. [42] ASTM C1018-97, Standard test method for flexural toughness and first-crack strength of fiber-reinforced concrete (using beam with third-point loading). American Society of Testing and Materials 2012:1–8. [43] Holschemacher K, Mueller T, Ribakov Y. Effect of steel fibres on mechanical properties of high-strength concrete. Materials & Design (1980-2015) 2010;31(5):2604-15.
S0263-8223(17)33127-6 https://doi.org/10.1016/j.compstruct.2017.11.009 COST 9075
To appear in:
Composite Structures
Received Date: Revised Date: Accepted Date:
4 September 2017 29 October 2017 6 November 2017
Please cite this article as: Jang, S-J., Yun, H-D., Combined effects of steel fiber and coarse aggregate size on the compressive and flexural toughness of high-strength concrete, Composite Structures (2017), doi: https://doi.org/ 10.1016/j.compstruct.2017.11.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Combined effects of steel fiber and coarse aggregate size on the compressive and flexural toughness of high-strength concrete Seok-Joon Jang1 and Hyun-Do Yun2* 1
Graduate Student, Department of Architectural Engineering, Chungnam National University, Daejeon, 305-764, Republic of Korea 2
Professor, Department of Architectural Engineering, Chungnam National University, Daejeon, 305-764, Republic of Korea *
Corresponding Author: (E-mail: [email protected], Tel. +82-42-821-5622)
Abstract This paper investigates the effects of steel fiber content and coarse aggregate size on the mechanical properties of high-strength concrete with a specified compressive strength value of 60 MPa. The paper also explores the correlation between the compressive and flexural toughness of high-strength steel fiber-reinforced concrete (SFRC). For this purpose, twelve high-strength SFRC mixtures with four fiber volume fraction of steel fiber ( = 0.5%, 1.0%, 1.5%, and 2.0%) and different aggregate sizes were designed and fabricated. Compressive and flexural tests for each concrete mixture were conducted, and the test results were used to investigate the effects of steel fiber volume fraction and aggregate size on the compressive and flexural toughness of high-strength SFRC prims. The results indicate that the mechanical properties of SFRC are related more closely to volume fraction than to aggregate size. The compressive and flexural toughness ratios of the SFRC significantly improved with an increase in fiber content. Also, equations that are suggested to determine the compressive toughness ratio based on the equivalent flexural strength ratio were used to predict the mechanical properties of the SFRC in this study.
Keywords: Steel fiber, toughness, flexural testing, high-strength concrete
1. Introduction Recently, demands for taller buildings and longer bridges have increased, thus accelerating the use of high-strength concrete in construction to reduce cross-sections and the self-weight of concrete structures. High-strength concrete offers several benefits in terms of structural performance and durability compared to normal-strength concrete. However, high-strength concrete is characterized by its brittleness. Therefore, one of the main concerns with regard to high-strength concrete is the need to develop ways to convert its brittle behavior to ductile behavior. It is generally known that the addition of short synthetic or metallic fibers can improve the post-cracking behavior of concrete by bridging cracks and increasing the ductility and energy dissipation capacity of concrete members and structures [1-3]. Fiber reinforced concrete (FRC) to enhance the mechanical properties of concrete are manufactured with various types of metallic [4-7], polymeric [8-10], and hybrid fibers [11-13]. Various types of steel fiber that differ in tensile strength as well as shape and size have been developed and used for SFRC. Soroushian and Bayasi [14] investigated the relative effectiveness of different types of steel fiber in normal-strength concrete. The types of steel fiber used in their study are straight-round, crimped-round, crimped-rectangular, hookedsingle, and hooked-collated fibers. They found that hooked-end fiber is more effective than straight or crimped fiber in improving both the flexural and compressive behavior of concrete. However, Yakoub [15] concluded from analyzing 281 SFRC beams that hooked steel fiber is not as efficient as crimped fiber in terms of providing a capacity for shear resistance for reinforced concrete (RC) beams. Through statistical analysis of fibers produced all over the world, Katzer [16] reported that 67.1% are the hooked type and the other most popular fibers are straight fiber (9.1%), mechanically deformed fiber (9.1%), crimped fiber (7.9%), and other fiber types with different ends (6.6%). The Katzer study [16] clearly indicates that hooked-end steel fiber is the most popular and effective type of reinforcing fiber for concrete. Over the last four decades, extensive studies on SFRC have confirmed the effectiveness of deformed steel fibers as additional reinforcement in structural elements such as beam [17-23], wall [24, 25], column [26-28], coupling and joint members [29,30]. The utilization of deformed steel fiber instead of vertical stirrups in RC beams was first investigated by Batson et al. [17] in the early 1970s. To determine the effectiveness of steel
fiber as web reinforcement, they carried out shear tests on 72 RC beam specimens with different shear span ratios, fiber volume fractions, and fiber shapes. Batson et al. [17] concluded that the replacement of vertical stirrups by round, flat, or crimped steel fiber provides effective reinforcement against shear failure. Based on Parra-Montesinos’ (2006) database [20] that contains information about the shear strength of beams fabricated with deformed steel fiber, the American Concrete Institute (ACI) building code, ACI 318-08 [31], introduced the use of deformed steel fiber as minimum shear reinforcement for normalstrength concrete beams. Yoo et al. [23] investigated the feasibility of eliminating the minimum shear reinforcement in reinforced high-strength concrete beams that have a compressive strength value of 67 MPa by adding 0.75% hooked steel fiber. They concluded that the minimum shear reinforcement for reinforced high-strength concrete beams indeed could be eliminated by the inclusion of 0.75% hooked steel fiber. However, only limited data are available regarding high-strength concrete in test results for SFRC beams. To mitigate damage in RC columns during severe earthquakes, the traditional method is to set closely-spaced transverse hoops to improve the confinement of the concrete at plastic hinge regions. However, a relatively large number of transverse hoops may result in a congestion of the reinforcement as well as an increase in the cost and time for construction [27]. Recently, the use of SFRC has been regarded as an alternative reinforcement solution to the use of transverse hoops. Germano et al. [28] conducted an experimental study of the structural performance of SFRC columns subjected to uniaxial and biaxial cyclic loads. Their test results indicate that the using steel fiber has a positive effect on mitigation of columns damage and energy dissipation. Bayasi and Gebman [30] conducted a literature review and reported an experimental study of the effects of steel fiber on seismic beam-column connections. Their results show that the confining effect of steel fibers results in a reduction of the lateral reinforcement in seismic beam-column joints. Thus, major efforts have been made in recent years to eliminate the shear or confinement reinforcement in RC members by adding steel fiber to save costs and time that are needed for cutting and placing rebar. The effects of fiber on the mechanical properties of concrete are governed by the fiber volume fraction, fiber type, and cementitious matrix properties. As specified in ACI 318-08 [31] for the use of steel fiber as an alternative to minimum shear reinforcement, the flexural toughness of SFRC can be a significant index to determine the number of transverse hoops that can be eliminated when steel fiber is included in the concrete.
The compressive toughness also can be used as index to determine the confinement ability of SFRC. However, special equipment and loading methods are required to measure the descending curve of SFRC in compression. Furthermore, obtaining data about the descending curve for high-strength concrete in compression compared to in flexure is difficult. Although numerous studies have investigated the effects of hooked-end steel fiber on the mechanical properties of normal-strength concrete, test data for high-strength concrete are limited. Therefore, this paper first aims to examine the effects of hooked-end steel fiber volume fraction and coarse aggregate size on the compressive and flexural behavior of high-strength concrete. Then, this paper explores the relationship between compressive toughness and flexural toughness of high-strength concrete with hooked-end steel fiber based on this study’s flexural and compressive test results.
2. Experimental Program 2.1 Mixture Proportions and Materials An experimental program was designed for this study to examine the effects of fiber volume fraction and coarse aggregate size, also referred to as nominal maximum aggregate size (NMAS), on the mechanical properties of high-strength concrete. Table 1 presents the twelve series of SFRC mixtures used in the study. The mixture codes reflect the NMAS ranges and volume fractions, as follows. The crushed coarse aggregate was divided into three NMAS categories: small coarse aggregate ranging from 5 mm to 8 mm NMAS, medium coarse aggregate ranging from 5 mm to 13 mm NMAS, and large coarse aggregate ranging from 13 mm to 19 mm NMAS. The four volume fractions of the added hooked-end steel fiber were 0.5%, 1.0%, 1.5%, and 2.0%. Also, the specified compressive strength of all the SFRC mixtures was 60 MPa. The SFRC mixtures with a 0.3 water-to-cement ratio were used to cast specimens for compressive and flexural testing.
Table 1 Mix proportions of SFRC Mixture code Coarse aggregate (mm)
Fiber volume fraction (%)
Unit weight (kg/m3)
Water
Cement
Fine aggregate
Coarse aggregate
Steel fiber
SFRC8-0.5 SFRC8-1.0 SFRC8-1.5 SFRC8-2.0 SFRC13-0.5 SFRC13-1.0 SFRC13-1.5 SFRC13-2.0 SFRC19-0.5 SFRC19-1.0 SFRC19-1.5 SFRC19-2.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
200 200 200 200 200 200 200 200 200 200 200 200
666 666 666 666 666 666 666 666 666 666 666 666
900 900 900 900 900 900 900 900 900 900 900 900
540 540 540 540 540 540 540 540 540 540 540 540
39.2 78.5 117.7 157.0 39.2 78.5 117.7 157.0 39.2 78.5 117.7 157.0
5-8 5-8 5-8 5-8 5-13 5-13 5-13 5-13 13-19 13-19 13-19 13-19
For the fabrication of the SFRC specimens, the cement used in all the mixtures was ordinary Portland cement (ASTM Type 1) with a specific gravity of 3.14 g/cm3. The fine aggregate was local natural sand with specific gravity and water absorption values of 2.62 and 1.91%, respectively. The coarse aggregate was crushed limestone. Figs. 1 and 2 show the size distribution and size ranges of the coarse aggregate used in this study, respectively. A superplasticizer was used to improve the workability of the SFRC mixtures and distribute the fibers uniformly in the fresh concrete. Hooked-end steel fiber with a 30 mm length and an aspect ratio of 60 was used for reinforcing the concrete. Table 2 provides a summary of the mechanical properties of the hooked-end steel fibers used in this study.
Fig. 1 Grading curves of aggregate
(a) 5 mm - 8 mm
(b) 5 mm - 13 mm
(c) 13 mm - 19 mm
Fig. 2 NMAS ranges of coarse aggregate particles Table 2 Mechanical properties of hooked-end steel fiber Hooked-end steel fiber
Tensile strength (MPa) 1,100
Length (mm) 30
Diameter (mm) 0.5
Aspect ratio (length/diameter) 60
2.2 Specimen Manufacture and Test Procedure Each SFRC mixture was mixed using a double axial mixer. First, the cement, fine aggregate, and coarse aggregate were dry-mixed for one minute, followed by the addition of water with a superplasticizer. Then, mixing was continued for three minutes. After the steel fiber was added, mixing was continued for an additional two minutes to dissolve the glue in the steel fiber bundles. Cylindrical and prismatic specimens were cast in steel molds and then compacted via
vibration. The specimens were demolded after 24 hours and then cured in 20℃ water for 28 days. The specimens were air-dried in the laboratory 12 hours before testing. The slump and air content of all the study mixtures were measured using fresh SFRC mixtures according to ASTM C143 [32] and ASTM C231 [33], respectively. To investigate the effects of steel fiber volume fraction and NMAS on the compressive behavior of highstrength concrete, three cylindrical specimens were cast, each with a diameter of 100 mm and height of 200 mm. The cylinders were tested to measure the compressive strength and elastic modulus according to ASTM C39 [34]. Fig. 3(a) shows a specimen in the compressive test setup. The load was applied using a universal testing machine (UTM) with the capacity of 3,000 kN. The tests were carried out with a displacement ratio of 10 µm/s to obtain the postpeak behavior of the SFRC. During the test, two linear variable differential transformers (LVDTs) were installed to measure the axial strain. Three prismatic specimens with a 100 × 100 mm cross-section and 300 mm span were fabricated using each SFRC mixture according to ASTM C1609 [35]. Fig. 3(b) shows the flexural test setup. Third-point bending loading was used for the flexural tests in conjunction with a 200 kN UTM. The mid-span deflection measurement of the prism was obtained using two LVDTs placed on a steel yoke. A yoke can measure the relative deflections of the support and central part of a prismatic specimen, thereby improving the precision of first-crack deflection measurements.
(a) Compression test setup
(b) Flexural test setup
Fig. 3 Test set-ups to measure SFRC mechanical properties
3. Test Results and Discussion 3.1 Fresh Properties
The SFRC slump and air content tests were conducted immediately after mixing. As clearly noted from Table 3, the SFRC mixtures with fiber volume fractions of 1.5% and 2.0% have less workability than those with 0.5% and 1.0% fiber. In contrast, the size of the coarse aggregate does not significantly affect the workability of SFRC. Due to the blocked pores created by the distributed steel fiber, the air content in fresh mixtures increases with an increase in the steel fiber content. The SFRC8-0.5 mixture, on the other hand, has the highest air content of 6.4% due to the mixing condition. Table 3 Fresh properties of SFRC Mixture code
Slump (mm)
Air content (%)
SFRC8-0.5 SFRC8-1.0 SFRC8-1.5 SFRC8-2.0 SFRC13-0.5 SFRC13-1.0 SFRC13-1.5 SFRC13-2.0 SFRC19-0.5 SFRC19-1.0 SFRC19-1.5 SFRC19-2.0
265 210 130 105 240 243 185 105 245 245 175 85
6.4 3.9 4.5 2.0 2.7 3.5 3.8 5.5 1.9 2.6 3.5 2.4
3.2 Compressive Strength The experimental results for the cylindrical specimens under compression indicate no significant changes in compressive strength for SFRC with various coarse aggregate sizes and fiber volume fractions. For the SFRC with an NMAS of 8 mm and volume fraction of 0.5%, however, the compressive strength decreased compared to that of other mixtures due to its higher air content, as noted in Table 3. Table 4 presents the compressive properties of the SFRC specimens, i.e., compressive strength, strain at peak stress, and the modulus of elasticity, as obtained from compression testing. The peak strain is shown to increase in accordance with an increase in the confined effect caused by the hooked-end steel fiber.
Table 4 Compressive properties of SFRC Mixture code
′ (MPa)
(10-6 )
Ε (GPa)
SFRC8-0.5 SFRC8-1.0 SFRC8-1.5 SFRC8-2.0 SFRC13-0.5 SFRC13-1.0 SFRC13-1.5 SFRC13-2.0 SFRC19-0.5 SFRC19-1.0 SFRC19-1.5 SFRC19-2.0
58.2(±1.0) 68.6(±0.7) 63.6(±0.6) 68.2(±0.5) 64.5(±0.5) 63.2(±0.8) 64.3(±0.2) 65.4(±1.1) 66.6(±2.0) 67.1(±0.6) 67.6(±1.7) 65.5(±1.5)
3300(±95) 3432(±297) 3463(±85) 3591(±158) 3227(±303) 3320(±78) 3424(±64) 3394(±66) 3244(±234) 3148(±84) 3278(±276) 3322(±344)
25.5(±0.5) 28.7(±0.8) 27.9(±0.5) 29.7(±0.7) 30.6(±2.1) 28.7(±1.6) 28.5(±0.8) 29.2(±0.3) 30.0(±0.3) 30.2(±0.9) 30.9(±0.3) 29.0(±0.8)
Fig. 4 shows typical compressive stress-strain relationships for the SFRC in terms of fiber volume fraction. The compressive stress and strain were normalized by compressive strength and strain at peak stress, respectively. The effects of the steel fiber volume fraction on the initial behavior appear to be insignificant, whereas the post-peak ductility of the SFRC increases with an increase in fiber content. The use of steel fiber as transverse reinforcement for compression RC members is shown to improve the confinement and ductility.
(a) SFRC8 series
(b) SFRC13 series
(c) SFRC19 series
Fig. 4 Effects of fiber volume fraction on normalized stress-strain curves of SFRC
3.3 Compressive Toughness The empirical equations used for the compressive behavior of SFRC found in the literature
employ the expression proposed by Carreira and Chu [36] for plain concrete, as given by Eq. (1).
fc β (ε c /ε 0 ) = (1) f'c β − 1 + (ε c /ε 0 )β where εc is the uniaxial strain; fc is the compressive stress; f’c is the compressive strength; β is the material parameter; and ε0 is the strain at the compressive strength. Ezdeldin and Balaguru [37] and Nataraja et al. [38] employed the same expression to suggest strain-stress curves for SFRC using crimped steel fiber; they obtained the value of β from the fiber volume fraction, aspect ratio, and reinforcing index. Lee et al. [39] reported compressive behavior of SFRC that contained hooked-end steel fiber and suggested a modified expression, presented here as Eqs. (2.1) through (2.4).
fc A (ε c /ε 0 ) = (2.1) f'c A − 1 + (ε c /ε 0 )B A =B =
1
f' 1− c ε 0E c
A = 1 + 0 .723 (V f
,ε c /ε 0 ≤ 1 .0 (2.2)
lf −0 .957 ) ,εc /ε 0 > 1 .0 (2.3) df
f' l B = ( c )0 .064 [1 + 0 .882 (V f f )−0 .882 ,ε c /ε 0 > 1 .0 (2.4) 50 df where A and B are the material parameters; Vf is the fiber volume fraction; and lf and df represent the fiber length and diameter, respectively. Fig. 5 presents comparisons of typical actual behavior under compression and behavior obtained from prediction models using Eqs. (2.1) through (2.4) for SFRC with 13 mm NMAS. As shown in the figures, the model suggested by Lee et al. [39] can represent the compressive behavior of SFRC with fiber volume fractions of 1.0%, 1.5%, and 2.0%. The post-peak behavior of the SFRC specimen with 0.5% fiber could not be measured due to the rapid decrease in loading after the peak stress was reached. Thus, the model was used to predict that specimen’s post-peak behavior.
(a) Fiber volume fractions of 0.5% and 1.0%
(b) Fiber volume fractions of 1.5% and 2.0%
Fig. 5 Test and analytical stress-strain curves of SFRC under compression The compressive toughness ratio can be calculated from the strain-stress curves of cylindrical specimens. In this study, the compressive toughness ratio was calculated as the ratio of the area of square made by compressive strength and specified strain to that of the compressive stress-strain curve up to the specified strain, as proposed by Nataraja et al. [38]. As noted from Eqs. (3.1) and (3.2), the specified strain values are set at 0.009 and 0.015, which are three and five times, respectively, the concrete ultimate strain of 0.003 provided in the ACI 318-14 building code [40].
TR
3
=
TR
5
=
TF 3 0 .009 f'c
TF 5 0 .015 f'c
(3.1)
(3.2)
where TR3 and TR5 are the compressive toughness ratios up to the strain values of 0.009 and 0.015, respectively. Also, TF3 and TF5 are the areas under the stress-strain curve up to those strain levels. Fig. 6 presents the compressive toughness ratios calculated via the measured data and via the prediction model using Eqs. (2.1) through (2.4). As shown, the compressive toughness ratios using these two methods are similar to each other. Therefore, the equation can be used to calculate the toughness ratio instead of incomplete post-peak behavior of SFRC based on the experimental results. Table 4 also shows the required parameters based on the compressive test results.
(a) Up to strain of 0.009
(b) Up to strain of 0.0015
Fig. 6 Comparisons of compressive toughness ratios based on measured data and analytical results Fig. 7 shows the effects of both volume fraction and aggregate size on the compressive toughness ratios of the SFRC mixtures. As shown, the compressive toughness ratios increase with an increase in fiber content of the SFRC mixtures for all the study aggregate sizes. The SFRC specimens with 8 mm NMAS show greater compressive toughness than those of other aggregate sizes. However, compared to steel fiber volume fraction, aggregate size has an insignificant effect on the post-peak compressive behavior and toughness of the SFRC mixtures.
(a) Up to strain of 0.009
(b) Up to strain of 0.015
Fig. 7 Effects of fiber volume fraction and aggregate size on compressive toughness ratios of SFRC
3.4 Flexural Strength of SFRC Fig. 8 illustrates the method used to define the first crack and ultimate load stage of SFRC under flexural loading. Benthia and Trottir [41] distinguished the initial ascending parts of the SFRC curve as three stages: first nonlinearity, significant nonlinearity, and the end of the matrix contribution. They also suggested that the end of the matrix contribution is the first crack stage of SFRC. In this study, the initial point was set based on the initial stiffness of the specimen, as found in ASTM C1018 [42] and shown in Fig. 8. This method can compensate for the error that occurs at the initial state of a prismatic specimen under small loads. The addition of steel fiber has more of an effect on the behavior after the first crack than on the initial behavior. The post-cracking behavior of SFRC can be divided into deflection-softening and deflection-hardening behavior. Table 5 lists the flexural test results based on these definitions.
Fig. 8 Characteristics of load-deflection curve of SFRC [35,41,42]
Table 5 Flexural properties of SFRC Mixture code
Pc (kN)
δc (mm)
Pu (kN)
δu (mm)
SFRC8-0.5
35.0(±0.4)
0.066(±0.003)
35.0(±0.4)
0.066(±0.003)
SFRC8-1.0
36.4(±5.6)
0.063(±0.003)
41.1(±7.5)
0.233(±0.148)
SFRC8-1.5
36.1(±7.6)
0.067(±0.004)
48.0(±2.0)
0.406(±0.004)
SFRC8-2.0
37.3(±3.0)
0.061(±0.010)
53.9(±2.3)
0.321(±0.090)
SFRC13-0.5
36.5(±2.9)
0.066(±0.003)
36.5(±2.9)
0.066(±0.003)
SFRC13-1.0
36.2(±0.9)
0.066(±0.004)
37.3(±1.0)
0.151(±0.146)
SFRC13-1.5
35.9(±2.8)
0.059(±0.006)
45.7(±6.6)
0.314(±0.015)
SFRC13-2.0
39.7(±2.2)
0.069(±0.008)
51.4(±7.3)
0.536(±0.190)
SFRC19-0.5
37.3(±4.8)
0.065(±0.004)
37.3(±4.8)
0.065(±0.004)
SFRC19-1.0
41.7(±1.4)
0.073(±0.006)
42.7(±3.0)
0.107(±0.054)
SFRC19-1.5 SFRC19-2.0
40.3(±4.9) 37.3(±4.9)
0.068(±0.008) 0.062(±0.003)
52.5(±1.5) 48.0(±4.7)
0.452(±0.116) 0.876(±0.608)
Fig. 9 illustrates the effects of fiber volume fraction and aggregate size on typical flexural responses for the three SFRC mixture series. As expected, the initial behavior of all the specimens is similar until the cracking stage, because all the mixtures have similar compressive strength and modulus of elasticity values. On the other hand, the effects of steel fiber volume fraction and NMAS on the behavior after cracking are significant. The SFRC specimens with a fiber volume fraction of 0.5% exhibit deflection-softening behavior after the initial crack for all aggregate sizes. In the case of the SFRC specimens with 1.0% fiber, both deflection-softening and deflection-hardening are evident. The hardening behavior led to an increase in peak strength, as shown in Table 5. The SFRC8-1.0 mixture shows the highest average increasing ratio between the first crack and ultimate load of 13% compared with the SFRC mixtures with the same steel fiber volume fraction with 13 mm and 19 mm NMAS. SFRC specimens that contained 1.5% and 2.0% fiber also exhibited deflection-hardening behavior with an increase in the ultimate load and deflection values. With an increase in the steel fiber volume fraction, the ultimate load value of the SFRC increased correspondingly. For the SFRC19-2.0 mixture, however, the ultimate strength decreased more than that of the SFRC19-1.5 mixture due to the fiber ball that was caused by the higher content of steel fiber and larger aggregate particles in the SFRC19-2.0 mixture.
(a) SFRC8 series
(b) SFRC13 series
(c) SFRC19 series
Fig. 9 Typical flexural behavior of SFRC In this study, the ultimate strength of the SFRC increased with an increase in fiber volume fraction, which is closely related to the number of steel fibers that provide a bridging action. The fiber distribution in terms of the casting direction is also an important factor for determining the flexural performance of SFRC [43]. Fig. 10 illustrates the typical failure mode after flexural testing of the SFRC with the bridging action of the hooked-end steel fibers. The surface indicates that failure was attributable mostly to pull-out rather than rupture of the steel fibers. Fig. 11 shows the effect of the number of steel fibers for the tensile zone (shown in Fig. 10) on the ultimate load of the SFRC prismatic specimens. As shown in Fig. 11, the SFRC specimens with 8 mm and 13 mm NMAS have more bridging steel fibers compared to specimens with the NMAS of 20 mm. It is concluded that the ultimate strength of SFRC tends to increase in accordance with the number of steel fibers with bridging action.
Fig. 10 Fiber distribution at failure face
Fig. 11 Effect of number of fibers on ultimate load of SFRC
3.5 Equivalent Flexural Strength Ratio The equivalent flexural strength ratio was characterized in this study as per ASTM C1609 [27] to investigate the flexural toughness of SFRC. Eqs. (4.1) and (4.2) were used to calculate the equivalent flexural strength ratios up to deflections of 1/300 and l/150 of the span length, respectively.
R T ,300 =
R T ,150 =
150T 300
f1bh 2 150T 150
f1bh 2
(4.1)
(4.2)
where RT,300 and RT,150 are equivalent flexural strength ratios up to the deflections of L/300 and L/150, respectively; T300 and T150 are the areas under load-net deflection curves 0 to L/300 and L/150, respectively; f1 is the first-peak strength; b is the width of the specimen; h is the height of the specimen; and L is the span length. Fig. 12 shows the effects of both fiber volume fraction and aggregate size on the equivalent flexural strength ratios of the SFRC. ACI building code [40] allows the use of at least 60 kg of deformed steel fiber per cubic meter of concrete (fiber volume fraction of approximately 0.75%) as a design alternative to the minimum shear reinforcement for 0.5 in normal-strength concrete beams with compressive strength values less than or equal to 40 MPa. In addition to the specified minimum content of deformed steel fiber, ACI building
code also specifies that the SFRC used for shear resistance must satisfy three requirements: (1) the compressive strength acceptance criteria for standard-cured specimens, (2) the residual strength obtained from flexural testing in accordance with ASTM C1609 [35] at a midspan deflection of 1/300 of the span length that is at least the greater of 90% of the measured firstpeak strength obtained from a flexural test or 90% of the strength that corresponds to 0.62√f ’c (MPa), and (3) the residual strength obtained from flexural testing in accordance with ASTM C1609 at a midspan deflection of 1/150 of the span length that is at least the greater of 75% of the measured first-peak strength obtained from a flexural test or 75% of the strength that corresponds to 0.62√f’c (MPa). Fig. 12 also shows the acceptance criteria for SFRC as the minimum shear transverse reinforcement provided in ACI 318-14 [40]. As expected, an increase in flexural toughness that corresponds to an increase in steel fiber content was observed; flexural toughness increased linearly to 1.5% fiber volume fraction. On the other hand, for SFRC specimens with over 2% fiber, similar flexural toughness was observed. According to the flexural test results, excessive amounts of steel fiber contribute to a reduction in the effectiveness of the steel fiber due to the inhomogeneous distribution of the fibers. In other words, SFRC specimens with a small NMAS exhibit greater toughness than those with a large NMAS. Fig. 13 shows the correlation between the number of steel fibers and the equivalent flexural strength ratio. The flexural toughness ratio increases with an increase in the number of steel fibers for both cases of deflections to L/300 and L/150.
(a) Up to deflection of L/300
(b) Up to deflection of L/150
Fig. 12 Effects of fiber volume fraction and aggregate size on equivalent flexural strength ratio of SFRC
(a) Up to deflection of L/300
(b) Up to deflection of L/150
Fig. 13 Effects of number of fibers on equivalent flexural strength ratio of SFRC
3.6 Correlation between Compressive and Flexural Toughness Ratios The toughness ratio represents the energy absorbance capacity and ductility of a material. In this study, the toughness ratio of SFRC was investigated under compression and bending loads. Fig. 14 shows the relationship between the compressive toughness ratio and the equivalent flexural strength ratio of the SFRC. The test results suggest that the proposal to use Eqs. (5.1) and (5.2) for the calculation of the compressive toughness ratio based on the equivalent flexural strength. Fig. 14 shows that the suggested models provide a good estimation compared to the test results. The models also can be used effectively to better understand the mechanical properties of SFRC. TR
3
= 0 .717 (R T ,300 )0 .358 (5.1)
TR
5
= 0 .6431 (R T ,150 )0 .5725
(5.2)
(a) Up to strain of 0.009 and deflection of L/300
(a) Up to strain of 0.015 and deflection of L/150
Fig. 14 Correlations between compressive toughness ratio and equivalent flexural strength ratio of SFRC
4
Conclusions
The compressive, flexural, and toughness ratios of SFRC were investigated through experimental research in this study. The test results have led to the following main findings. 1) Both steel fiber volume fraction and aggregate size do not have a significant effect on the compressive strength and modulus of elasticity of SFRC. However, the peak strain, post-peak behavior, and toughness of SFRC under compression improve with an increase in fiber volume fraction. 2) Flexural strength, toughness and the equivalent flexural strength ratio significantly increase with an increase in fiber content. The improvement of the flexural performance of SFRC is related mainly to the number of steel fibers in the tensile zone of a prismatic specimen. The limited hooked-end steel fiber volume fraction of 1.5% is suggested for the economical and efficient use of steel fiber based on this study’s flexural test results. 3) The NMAS has less effect on the mechanical properties of SFRC than the fiber volume fraction. For SFRC specimens with a higher percentage of fiber, on the other hand, using small aggregate can aid the homogeneous dispersion of steel fiber. Mathematical expressions are suggested in this paper for the compressive toughness ratio with regard to the equivalent flexural strength ratio of SFRC.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(NRF2016R1D1A3B02008179)
Reference [1] ACI Committee 544, Design Considerations for Steel Fiber Reinforced Concrete (ACI 544.4R-88 (Reapproved 2009)), American Concrete Institute 1988 [2] Zollo RF. Fiber-reinforced concrete: an overview after 30 years of development. Cement and Concrete Composites 1997;19(2):107-22. [3] Di Prisco M, Plizzari G, Vandewalle L. Fibre reinforced concrete: new design perspectives. Materials and Structures 2009;42(9):1261-81. [4] Soroushian P, Bayasi Z. Fiber type effects on the performance of steel fiber reinforced concrete. Materials Journal 1991;88(2):129-34. [5] Song PS, Hwang S. Mechanical properties of high-strength steel fiber-reinforced concrete. Construction and Building Materials 2004;18(9):669-73. [6] Olivito RS, Zuccarello FA. An experimental study on the tensile strength of steel fiber reinforced concrete. Composites Part B: Engineering 2010;41(3):246-55. [7] Orbe A, Rojí E, Losada R, Cuadrado J. Calibration patterns for predicting residual strengths of steel fibre reinforced concrete (SFRC). Composites Part B: Engineering 2014;58:408-17. [8] Li VC, Wang S, Wu C. Tensile strain-hardening behavior of polyvinyl alcohol engineered cementitious composite (PVA-ECC). ACI Materials Journal 2001;98(6):483-92. [9] Torigoe SI, Horikoshi T, Ogawa A, Saito T, Hamada T. Study on evaluation method for PVA fiber distribution in engineered cementitious composite. Journal of Advanced Concrete Technology 2003;1(3):265-8. [10] Yun HD. Effect of accelerated freeze–thaw cycling on mechanical properties of hybrid PVA and PE fiber-reinforced strain-hardening cement-based composites (SHCCs).
Composites Part B: Engineering 2013;52:11-20. [11] Ahmed SFU, Maalej M, Paramasivam P. Flexural responses of hybrid steel–polyethylene fiber reinforced cement composites containing high volume fly ash. Construction and building materials 2007;21(5):1088-97. [12] Banthia N, Majdzadeh F, Wu J, Bindiganavile V. Fiber synergy in Hybrid Fiber Reinforced Concrete (HyFRC) in flexure and direct shear. Cement and Concrete Composites 2014;48:91-7. [13] Shu X, Graham RK, Huang B, Burdette EG. Hybrid effects of carbon fibers on mechanical properties of Portland cement mortar. Materials & Design (1980-2015) 2015;65:1222-28. [14] Soroushian P, Bayasi Z. Fiber type effects on the performance of steel fiber reinforced concrete. Materials Journal, 1991;88(2):129-34. [15] Yakoub HE. Shear stress prediction: Steel fiber-reinforced concrete beams without stirrups. ACI Structural Journal 2011;108(3):304-14. [16] Katzer J. Steel fibers and steel fiber reinforced concrete in civil engineering. Pacific Journal of Science and Technology 2006;7(1):53-8. [17] Batson G, Jenkins E, Spatney R. Steel fibers as shear reinforcement in beams. Am Concrete Inst Journal & Proceedings 1972;69(10)640-4. [18] Swamy RN, Bahia HM. The effectiveness of steel fibers as shear reinforcement. Concrete International 1985;7(3):35-40. [19] Narayanan R. Darwish IYS. Use of steel fibers as shear reinforcement. ACI Structural Journal 1987;84(3):216-227. [20] Parra-Montesinos GJ. Shear strength of beams with deformed steel fibers. Concrete International 2006;28(11):57-66. [21] Kim KS, Lee DH, Hwang JH, Kuchma DA. Shear behavior model for steel fiberreinforced concrete members without transverse reinforcements. Composites Part B: Engineering 2012;43(5):2324-34. [22] Cuenca E, Echegaray-Oviedo J, Serna P. Influence of concrete matrix and type of fiber
on the shear behavior of self-compacting fiber reinforced concrete beams. Composites Part B: Engineering 2015;75:135-47. [23] Yoo DY, Yuan T, Yang JM, Yoon YS. Feasibility of replacing minimum shear reinforcement with steel fibers for sustainable high-strength concrete beams. Engineering Structures 2017;147:207-22. [24] Lim WG, Kang SW, Yun HD. Shear Behavior of Squat Steel Fiber Reinforced Concrete (SFRC) Shear Walls with Vertical Slits. In Applied Mechanics and Materials 2013;372:20710. [25] Carrillo J, Pincheira JA, Alcocer SM. Behavior of low-rise, steel fiber-reinforced concrete thin walls under shake table excitations. Engineering Structures 2017;138:146-58. [26] Ganesan N, M0urthy JR. Strength and behavior of confined steel fiber reinforced concrete columns. ACI Materials Journal 1990;87(3):221-7. [27] Huang L, Xu L, Chi Y, Xu H. Experimental investigation on the seismic performance of steel–polypropylene hybrid fiber reinforced concrete columns. Construction and Building Materials 2015;87:16-27. [28] Germano F, Tiberti G, Plizzari G. Experimental behavior of SFRC columns under uniaxial and biaxial cyclic loads. Composites Part B: Engineering 2016;85:76-92. [29] Khalifa ES. Analytical model for steel fiber concrete composite short-coupling beam. Composites Part B: Engineering 2014;56:318-29. [30] Bayasi Z, Gebman M. Reduction of lateral reinforcement in seismic beam-column connection via application of steel fibers. ACI Structural Journal 2002;99(6):772-80. [31] ACI Committee 318. Building Code Requirements for Structural Concrete (ACI 318-08): An ACI Standard: Commentary on Building Code Requirements for Structural Concrete (ACI 318R-08). American Concrete Institute 2008. [32] ASTM C143/C143M-15. Standard test method for slump of hydraulic-cement concrete. American Society of Testing and Materials 2015:1–4 [33] ASTM C 231/C231M-17. Standard test method for air content of freshly mixed concrete by the pressure method. American Society of Testing and Materials 2017:1–10
[34] ASTM C39/C39M-17. Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. American Society of Testing and Materials 2017: 1–8. [35] ASTM C1609/C1609M-12, Standard test method for flexural performance of fiberreinforced concrete (using beam with third-point loading). American Society of Testing and Materials 2012:1–8. [36] Carreira DJ, Chu KH. Stress-strain relationship for plain concrete in compression. In Journal Proceedings 1985;82(6):797-804. [37] Ezeldin AS, Balaguru PN. Normal-and high-strength fiber-reinforced concrete under compression. Journal of materials in civil engineering 1992;4(4):415-29. [38] Nataraja MC, Dhang N, Gupta AP. Stress–strain curves for steel-fiber reinforced concrete under compression. Cement and concrete composites 1999;21(5):383-90. [39] Lee SC, Oh JH, Cho JY. Compressive behavior of fiber-reinforced concrete with endhooked steel fibers. Materials 2015;8(4):1442-58. [40] ACI Committee 318. Building Code Requirements for Structural Concrete (ACI 318-14): An ACI Standard: Commentary on Building Code Requirements for Structural Concrete (ACI 318R-14). American Concrete Institute 2015. [41] Banthia N, Trottier JF. Test methods for flexural toughness characterization of fiber reinforced concrete: some concerns and a proposition. ACI Materials Journal 199;92:48-57. [42] ASTM C1018-97, Standard test method for flexural toughness and first-crack strength of fiber-reinforced concrete (using beam with third-point loading). American Society of Testing and Materials 2012:1–8. [43] Holschemacher K, Mueller T, Ribakov Y. Effect of steel fibres on mechanical properties of high-strength concrete. Materials & Design (1980-2015) 2010;31(5):2604-15.