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Improvement effect of steel fiber orientation control on mechanical performance of UHPC
Construction and Building Materials 188 (2018) 709–721
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Improvement effect of steel fiber orientation control on mechanical performance of UHPC Huanghuang Huang a, Xiaojian Gao a,b,c,⇑, Linshan Li a, Hui Wang d a
School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, China c Key Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China d Faculty of Architectural, Civil Engineering and Environment, Ningbo University, 315000 Ningbo, China b
h i g h l i g h t s One flow controlling method was developed to manufacture UHPC specimen. Fiber orientation number and coefficient in UHPC were significantly improved. Flexural strength, toughness and sMOR were increased by 64.3%, 65.1% and 77.1%.
a r t i c l e
i n f o
Article history: Received 22 May 2018 Received in revised form 22 August 2018 Accepted 22 August 2018
Keywords: Ultra-high performance concrete (UHPC) Flow control Fiber orientation Mechanical properties
a b s t r a c t In order to improve mechanical properties of ultra-high performance concrete (UHPC), one L-shape device with a narrow horizontal channel was developed to control the flow of fresh mixture and then the orientation of steel fibers was significantly improved. Mixtures with water-to-binder ratios of 0.20, 0.22 and 0.24 were prepared by introducing steel fibers with 1%, 2%, and 2.5% of the total volume respectively. Three fiber orientation parameters (fiber orientation angle, fiber orientation number and fiber orientation coefficient) were quantitatively evaluated by the image analysis method. Based on experimental results, mechanical properties (compressive strength, flexural strength, toughness, deflection at modulus of rupture and maximum deflection) were notably increased with the higher dosage of steel fibers. Compared with specimens prepared by a direct cast, this flow control method improved fiber orientation number and fiber orientation coefficient from 0.6 to 0.7 and 0.4–0.6 to 0.7–0.9 and 0.6–0.8 respectively. The best improvement on flexural strength, toughness, deflection at modulus of rupture and maximum deflection reached 64.3%, 65.1%, 77.1% and 14.9% respectively. Based on experimental results, linear relationships were regressed for fiber orientation parameters and mechanical properties of UHPC. Therefore, a potential method was developed to improve the reinforcement effect of steel fibers in UHPC. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Ultra-high performance concrete (UHPC) is generally made of large quantities of Portland cement, silica fume (SF) and fine aggregates with steel fibers for reinforcement and a high dosage of superplasticizer for satisfactory fluidity at a very low water to binder ratio (w/b) [1,2]. Due to its very high compressive strength (>150 MPa), flexural strength (>8 MPa), excellent durability and
⇑ Corresponding author at: School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China. E-mail address: [email protected] (X. Gao). https://doi.org/10.1016/j.conbuildmat.2018.08.146 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
toughness [3,4], UHPC has been developed into many attractive applications to real structures in Europe, Asia and North America in recent years [5–7]. These remarkable properties are mainly attributed to the very low w/b, well-graded aggregates and cementitious materials based on the packing density theory. Additionally, short steel fibers are employed to enhance the flexural strength and toughness due to the stress transfer from the matrix to the fibers, the fibers bridging at crack surfaces and resisting against crack propagation [8]. In general, steel fibers are randomly dispersed in UHPC when being prepared by a direct cast method. It is beneficial to obtain a more isotropic mechanical behavior. However, for uniaxial
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Table 1 Chemical and physical properties of cementing materials. Cement
SF
Chemical analysis (wt%) CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3
66.45 17.84 4.26 3.58 2.14 0.16 0.96 4.10
0.63 87.67 0.28 0.60 3.41 1.30 4.12 0.84
Physical properties Apparent density (g/m3) Blaine (BET) SSA (m2/g) Pozzolanic activity index
3.10 0.36 –
2.25 17.30 108.6
Table 2 Sieve analysis of quartz sand. Sieve size (mm)
Cumulative percentage retained (%)
2.5 1.25 0.63 0.315 0.16 <0.16
Fine sand
Coarse sand
0 0.16 0.36 28.30 81.16 100
0 0.10 12.14 87.26 97.74 100
Table 3 Basic properties of steel fibers.
2. Materials and methods
Diameter, df Length, Lf Aspect ratio Density Tensile strength Elastic modulus (mm) (mm) (Lf/df) (g/cm3) (MPa) (GPa) 0.2
13.0
65.0
materials [14]. Therefore, the usage of a lower amount of steel fibers is one of the most promising methods to reduce the production cost of this material. To achieve this purpose, hooked or twisted steel fibers and fibers with a higher ratio of length to diameter were introduced without degradation of mechanical properties [15–17]. In addition, the steel fibers can also be significantly reduced by improving the fiber orientation in UHPC to enhance the mechanical performance. The rheology and flowability properties of fresh mixtures [18–20], mixing and placing method [21–23], extrusion process and nozzle injection technique [24,25], and induced rotation of steel fiber in electromagnetic field [13,26,27] have been utilized to modify the fiber orientation in cementitious materials, generating the enhancement up to 117.3% for flexural/tensile strength. In addition, the wall effect was also used to improve the fiber orientation. Roy et al. utilized one chute to align the fibers in one direction by pouring UHPC on the upper end of the device and specimens with fibers orientated perpendicular to the load direction presented the better pullout load [28]. This paper presented one L-shape device to improve fiber orientation by controlling the flow of fresh UHPC mixtures. Three fiber orientation parameters, i.e. fiber orientation angle, fiber orientation number and fiber orientation coefficient were evaluated by image analysis. The improvement effect of this manufacture method on mechanical properties including compressive strength, flexural strength, toughness, deflection at modulus of rupture and maximum deflection were experimentally evaluated. Finally, the relationships between fiber orientation parameters and mechanical properties were regressed.
7.9
2850.0
2.1. Materials
200.0
mechanical tests, the efficiency of fibers in accordance to the principle tensile stress is low, only 30% as reported [9], and the improvement of mechanical performance is very limited. It was reported that the mechanical properties of UHPC are affected by the fiber characteristics (e.g. geometry, aspect ratio and volume fraction) [8], fiber-to-matrix bond properties [10] and fiber orientation [11–13]. The former two aspects can be tailored by the design of mixing proportion and the fiber orientation is difficult to control during the manufacture of UHPC. The high cost of UHPC at present is mainly attributed to the higher price of steel fibers and superplasticizer than other raw
Ordinary Portland cement with strength grade of 52.5 was used in this study. Silica fume consisting of spherical particles with size from 0.1 lm to 1 lm was utilized as supplementary cementitious material due to its pozzolanic reactivity and filling effect [29]. The chemical components and physical properties of these two cementing materials are given in Table 1. Two kinds of quartz sand were used as aggregates and the sieve analysis results of them are shown in Table 2. A type of copper coated steel fiber with tensile strength of 2850 MPa was incorporated in mixtures, more details about the geometrical and physical properties of the used steel fibers are summarized in Table 3. A polycarboxylate-based superplasticizer (SP) with water-reducing range of more than 30% and solid content of about 40% by weight was added to improve the fluidity of fresh mixture.
Table 4 Mix proportions of UHPC. w/b ratio
Cement (kg/m3)
SF (kg/m3)
Fine sand (kg/m3)
Coarse sand (kg/m3)
Steel fibers (% by volume of mixture)
SP (% by weight of binder)
0.20 0.20 0.20 0.22 0.22 0.22 0.22 0.24 0.24 0.24 0.24
920 920 920 920 920 920 920 920 920 920 920
276 276 276 276 276 276 276 276 276 276 276
202 202 202 202 202 202 202 202 202 202 202
810 810 810 810 810 810 810 810 810 810 810
0 1 2 0 1 2 2.5 0 1 2 2.5
2.0 2.0 2.0 1.8 1.8 1.8 1.8 1.6 1.6 1.6 1.6
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2.2. UHPC preparation The mix proportions are shown in Table 4. To investigate the improvement effect of fiber orientation control on mechanical properties of UHPC with different matrix strength, eleven mixtures were prepared by changing w/b ratio from 0.20 to 0.24. 1%, 2% and 2.5% volume fraction of steel fibers were incorporated. For every mixture, the ratio of cement: SF: fine sand: coarse sand was kept at the same value. To prevent steel fibers sedimentation and improve stability, the dosage of SP was decreased from 2.0% to 1.6% with the increasing w/b ratio from 0.20 to 0.24. All the raw materials were blended by using a Hobart mixer. Firstly, the solid materials except for steel fibers were premixed at a low speed (198 rpm) for 4 min to obtain a homogeneous mix-
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ture. Then, the SP was premixed with water and added in two halves in 6 min. When the mixture had a good flowability, the steel fibers were slowly added into the mixer in 10 min to achieve a uniform dispersion. 2.3. Experimental methods As given in Fig. 1, the L-shape device includes two parts. The vertical container was designed with size of 100 mm 100 mm 300 mm. On the one hand, this size is big enough for free flow of UHPC and rotation of steel fibers with length of 13 mm and fibers are randomly distributed in this zone. On the other hand, this size is also easily to operate at the lab. The horizontal channel was designed with height of 10 mm and length
Fig. 1. (a) Geometry of the L-shape device and (b) preparation of UHPC specimens by using this device.
Fig. 2. Image analysis procedure for determining fiber orientation.
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of 50 mm. The height of 10 mm is smaller than fiber length of 13 mm and thus all fibers tend to rotate to a horizontal state when flowing through this channel. Fresh mixture was poured into the apparatus and subsequently flowed out from the narrow channel into the mold. Specimens with sizes of 70 mm 70 mm 230 mm (b h l) were fabricated by seven layers without vibration. For every layer, the device was slowly moved from one end to the other end of the mold. After that, all the specimens were covered with plastic sheet to reduce moisture evaporation and placed in a chamber with temperature of 20 ± 2 °C and relative humidity of more than 95% for 48 h. After the first two days curing, the specimens were demolded and kept in a steam curing chamber at 90 °C for 72 h. To test flexural performance of UHPC specimens, three-point bending test with a span of 175 mm was conducted by a universal testing machine (UTM) with a maximum load capacity of 100 kN at a rate of 0.4 mm/min [8]. A 500 N preload was applied to allow the accommodation of loading apparatus. The flexural strength fs can be calculated by:
fs ¼
3FL
ð1Þ
2
2bh
where F is the maximum load obtained from the load-deflection curve, L is the span of the test, b and h are the width and height of the specimen respectively. The toughness T is defined as the area up to a certain deflection under the load-deflection curve. The obtained values for the deflection at modulus of rupture (MOR), L/150, L/100, L/75 and L/50 are usually used to evaluate the energy absorption capacity of UHPC [30,31]. In this study, the toughness was determined as the area under the load-deflection curve for the deflection at L/75 as described below.
Z
s¼L=75
T¼
ð2Þ
FðsÞds s¼0
where s is the deflection of specimen. For quantitatively evaluating the fiber orientation, slices selected near the localized crack (to reduce the effects of fiber pull-out) were processed by coarse grinding, fine grinding and ultrasonic wave cleaning successively to obtain smooth surfaces, and increase color contrast between fibers and matrix. After that, one microscopy digital camera, Olympus DSX500 with 5/0.15BD lens was employed to aquire the high resolution images of specimen cross sections. Every side of image was removed
(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 3. Compressive strength of UHPC specimens with different fiber volumes.
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6.5 mm to eliminate the effects of mold. The size of view filed was 3.5 mm 3.5 mm with resolution of 500 500 for every cross section, and a total of 256 pictures were taken and these pictures were then assembled into one integrated image with 10% of overlapping zone. The acquired image (called RGB image) was disposed by an Image Pro Plus software and converted into a binary image by setting a threshold value for increasing color contrast further to separate the steel fibers from the surrounding cementitious matrix [32]. The whole image analysis procedure is given in Fig. 2. After obtaining the binary image, the inclined angle of single fiber to the cutting plane h can be computed as:
Z
gh ¼
hmax
713
ð5Þ
pðhÞcos2 hdh
hmin
where pðhÞ is the probability density function (PDF) of fiber orientation. gh ¼ 1 indicates that every fiber is aligned parallel to the tensile direction, and gh ¼ 0 indicates that every fiber is aligned perpendicular to the tensile direction.
3. Results and discussion 1
h ¼ cos
df a
ð3Þ
where df and a are the diameter of fiber and the major axis length of the fiber image respectively. The fiber orientation number ah is computed as [13]: T
ah ¼
Nf 1 X
NTf
ð4Þ
coshn
n¼1
where N Tf is the total number of fibers crossing the analyzed section, hn is the inclined angle of the nth fiber. Furthermore, the fiber orientation coefficient gh can be expressed as [33]:
3.1. Compressive strength Fig. 3 presents compressive strength results of UHPC specimens with different fiber contents at w/b = 0.20, 0.22 and 0.24. The load direction was perpendicular to the fiber orientation. The compressive strength is enhanced by the steel fiber incorporation as expected. The best improvement reaches 29.6% when steel fibers are incorporated by the volume fraction of 2.5% for mixtures with w/b ratio of 0.24. It can be found that there is a slight increase of compressive strength induced by the orientation of steel fibers. It can be concluded that compressive strength is mainly influenced by the fiber content and w/b ratios rather than the fiber orientation.
(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 4. Load-deflection curves of UHPC specimens with different fiber volumes.
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(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 5. Flexural strength and toughness of UHPC specimens with different fiber volumes.
3.2. Flexural load-deflection curve The flexural load-deflection curves of UHPC specimens with different fiber volumes at w/b = 0.20, 0.22 and 0.24 are presented in Fig. 4. It is obviously seen that the mechanical properties are significantly enhanced by the increasing fiber volume and orientation at all the three w/b ratios. The limit of proportionality (LOP) and modulus of rupture (MOR) were utilized to describe the first cracking point and post-cracking peak point respectively [31]. It can be easily observed that the LOPs for all tested specimens of at a given w/b approximate to that of the blank samples without fibers. This is reasonable that the first cracking behavior largely depends on the matrix strength rather than the fiber volume fraction or orientation. Therefore, the load, toughness and deflection at LOP show no significant difference as described by other researchers [34]. 3.3. Flexural strength and toughness Flexural strength and toughness of UHPC specimens with different fiber contents at three w/b ratios are shown in Fig. 5. Flexural strength and toughness of UHPC are remarkably improved by the addition of steel fibers and the more enhancement is induced by the higher addition level of steel fibers. The best improvement for flexural strength reaches 118.1% when the steel fiber addition
increases from 1% to 2.5% at w/b ratio of 0.22. With respect to the toughness, the best enhancement is 112.6% when the steel fiber addition increases from 1% to 2.5% at w/b ratio of 0.24. It was reported that flexural strength is strongly affected by fiber bridging strength, the fiber bridging strength is improved with the increasing fiber volume due to more fibers at crack surfaces, leading to a larger bonding area between fibers and matrix [34]. In terms of the fiber spacing theory, flexural strength rises with the decreasing fiber distance due to the larger fiber volume [35]. On the other hand, the toughness of FRC is closely related to the fiber pullout surface energy, given as: fsVfL2f /df [36], where fs is a parameter taking the fiber orientation factor, group reduction factor and bond strength of fiber–matrix interface, etc. into account. Thus, the toughness enhances with the larger fiber volume Vf. As given in Fig. 5, both flexural strength and toughness are dramatically increased by the modified orientation of steel fibers. It is also observed that the best improvement effect for flexural strength and toughness occurred for 1% fiber volume content. The flexural strength is enhanced by 50.9%, 64.3% and 55.0% when being compared with ones prepared by the direct cast method at w/b ratio of 0.20, 0.22 and 0.24 respectively. The toughness is improved by 56.7%, 52.9%, 65.1% at these three w/b ratios respectively. Due to the flexural strength is strongly affected by fiber bridging strength rb ðdÞ, which is generally given as [37]:
H. Huang et al. / Construction and Building Materials 188 (2018) 709–721
(a) w/b = 0.20
715
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 6. sMOR and smax of UHPC specimens with different fiber volumes.
rb ðdÞ ¼
4V f
p
2 df
Z 0
p 2
Z
l 2
Pðh; le ; dÞpðle ÞpðhÞcoshdle dh
ð6Þ
0
where le is the embedded length of the fiber, d is the crack opening displacement, Pðh; le ; dÞ is the pullout resistance force of a single fiber at the crack surface, pðle Þ is the probability density function for le . Thus, the fiber bridging strength increases with the better fiber orientation because of a larger value of pðhÞcosh is obtained, meaning a greater number of fibers orienting towards to the tensile direction and leading to a higher flexural strength. With respect to the toughness, on the one hand, a higher flexural strength is advantageous to improve the toughness, on the other hand, the toughness is enhanced because of the improved fiber orientation considering the parameter fs according to other researchers [36]. 3.4. Deflection at MOR and maximum deflection The deflection at MOR, sMOR, and maximum deflection smax of UHPC specimens with different fiber volumes at three w/b ratios are given in Fig. 6. The sMOR equals to the smax for the specimen without steel fibers, and the sMOR (smax) is notably increased by the addition of steel fibers. With the fiber content increasing from 1% to 2.5%, sMOR and smax are also evidently enhanced. The best improvement for sMOR and smax reached 118.8% and 8.7% respec-
tively when 2.5% steel fiber was added to mixture with w/b ratio of 0.22. This is attributed to the improved fiber bridging capacity and fiber bridging strength as well as the reduced fiber distance as a result of higher fiber volume fraction [34,35]. This tendency was also reported by other researchers [33]. Both sMOR and smax are dramatically increased by the improved fiber orientation. It can be also concluded that the best modification for sMOR is obtained at 1% fiber addition, being 36.1%, 77.1% and 58.0% at w/b ratio of 0.20, 0.22 and 0.24 respectively when compared with those of specimens prepared by the direct cast. With regard to the smax, the largest improvement are 14.9%, 13.4% and 9.8% which are obtained at 2% fiber incorporation for w/b ratio of 0.20, 0.22 and 0.24 respectively. As discussed above, the fiber bridging strength is enhanced by a better fiber orientation because more fibers are aligned to the tensile direction and a larger stress can be transferred from the matrix to the fibers for resisting against cracks extension. 3.5. Evaluation of fiber orientation Fig. 7 shows the probability density distribution of fiber orientation angle for each specimen from (a) to (p). It is obviously seen that the histograms of specimens with oriented steel fibers are leftskewed, indicating that most of fibers in the specimens have a lower inclined angle and behave a higher tendency to be parallel
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Fig. 7. Probability density distribution of fiber orientation angle.
H. Huang et al. / Construction and Building Materials 188 (2018) 709–721
Fig. 7 (continued)
717
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Table 5 Summary of fiber orientation number and fiber orientation coefficient of UHPC. w/b ratio
Steel fiber volume (%)
Orientation
ah
gh
0.20
1
Not orient Orient Not orient Orient
0.625 0.754 0.621 0.728
0.525 0.695 0.555 0.605
Not orient Orient Not orient Orient Not orient Orient
0.685 0.849 0.664 0.780 0.650 0.796
0.520 0.735 0.475 0.630 0.470 0.655
Not orient Orient Not orient Orient Not orient Orient
0.660 0.703 0.608 0.757 0.662 0.731
0.475 0.640 0.410 0.600 0.470 0.635
2 0.22
1 2 2.5
0.24
1 2 2.5
to the tensile direction. On the other hand, the histograms of specimens prepared by the direct cast are right-skewed, tending to be perpendicular to the tensile direction. Similar distribution was also reported for the specimens casted in an electromagnetic field or under other flow controlling condition [13,38]. The fiber orientation parameters, i.e. fiber orientation number and fiber orientation coefficient can be calculated by Eqs. (4) and (5) as summarized in Table 5. It can be easily found that specimens prepared by this flow control method present the higher fiber orientation number (ah) and fiber orientation coefficient (gh). The ah and gh of specimens with a modified fiber orientation reach as high as 0.7–0.9 and 0.6–0.8 respectively in this paper, while those of specimens prepared by the direct cast are 0.6–0.7 and 0.4–0.6 respectively. 3.6. Relations between fiber orientation parameters and mechanical properties From the above experimental results, UHPC specimen with better fiber orientation (larger ah and gh) presents higher mechanical properties including flexural strength, toughness, sMOR and smax when the steel fiber volume and w/b ratio is kept the same. On the other hand, the mechanical performance is notably influenced by fiber content and w/b ratio when UHPC specimens are prepared by the same method [39]. The parameter k = ahghVf can be thought as a scalar descriptor of the fiber structure of the composite combining the effects of fiber orientation number, fiber orientation coefficient and fiber volume. Fig. 8 presents the relation between the parameter k and mechanical properties (i.e. flexural strength, toughness, sMOR and smax) at w/b = 0.20, w/b = 0.22 and w/b = 0.24. It can be found that the mechanical properties increase linearly more or less with the increase of the parameter k . Flexural strength and toughness exhibit the best linear relations with the parameter k with the coefficients of determination (R2) of higher than 0.93 at the w/b ratio of 0.22 and 0.24, while the sMOR has a little weaker linear relation with the parameter k with R2 of more than 0.81. Values of R2 become lower when the w/b ratio is reduced to 0.20, being attributed to the lack of data support used for fitting (only 1% and 2% of steel fiber volume). When taking the effects of w/b ratios into consideration, the parameter k was multiplied with the reciprocal of the w/b ratio
due to the improving effect on mechanical performance of lower w/b ratio. The relations between the parameter k b/w and mechanical properties are presented in Fig. 9. As can be seen, the mathematical relations between the parameter k b/w and flexural strength, toughness, sMOR, and smax are given as follows.
fs ¼ 7:576 þ 6:244kb=w
ð7Þ
T ¼ 16:982 þ 13:866kb=w
ð8Þ
sMOR ¼ 0:524 þ 0:243kb=w
ð9Þ
smax ¼ 3:081 þ 0:160kb=w
ð10Þ
Eqs. (7)–(9) show a strong coefficient of determination. In another word, flexural strength, toughness and sMOR tends to linearly increase with the higher value of the parameter k b/w. These three equations are potential to be used to evaluate flexural strength, toughness and sMOR of UHPC specimens for a given fiber orientation number, fiber orientation coefficient, fiber volume and w/b ratio. It needs more efforts to determine the feasibility of these equations for UHPC with different mixtures and raw materials. 4. Conclusions Based on the above experimental results, the following conclusions can be drawn: One novel method for controlling flow of fresh mixture was developed. It can improve the fiber orientation number and fiber orientation coefficient from 0.6 to 0.7 and 0.4–0.6 to 0.7– 0.9 and 0.6–0.8 respectively. UHPC specimens prepared by this flow control method exhibited much better mechanical performance than those prepared by the direct cast. The maximum improvement on flexural strength, toughness, sMOR and smax reached 64.3%, 65.1%, 77.1% and 14.9% respectively. Linear relations can be obtained by a regression between mechanical properties and fiber orientation parameters. These relations can be used to evaluate flexural strength, toughness
H. Huang et al. / Construction and Building Materials 188 (2018) 709–721
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(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 8. Relations between the parameter k and mechanical properties (i.e. flexural strength, toughness, sMOR and smax) at (a) w/b = 0.20, (b) w/b = 0.22 and (c) w/b = 0.24.
and sMOR of UHPC specimens when the fiber orientation number (ah), fiber orientation coefficient (gh), fiber volume (Vf) and water-to-binder ratio (w/b) are provided. Conflict of interest The authors declare no conflict of interest.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51578192), the National Key R&D Program of China (No. 2017YFB0309901) and the Initial Fund of Ningbo University of China (No. 011-421805030).
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Fig. 9. Relations between the parameter k b/w and (a) flexural strength, (b) toughness, (c) sMOR, (d) smax.
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[32] R. Wang, X.J. Gao, H.H. Huang, G.S. Han, Influence of rheological properties of cement mortar on steel fiber distribution in UHPC, Constr. Build. Mater. 144 (2017) 65–73. [33] D.Y. Yoo, N. Banthia, Y.S. Song, Predicting the flexural behavior of ultra-highperformance fiber-reinforced concrete, Cem. Concr. Compos. 74 (2016) 71–87. [34] D.Y. Yoo, S.T. Kang, N. Banthia, Y.S. Song, Nonlinear finite element analysis of ultra-high-performance fiber-reinforced concrete beams, Int. J. Damage Mech. 26 (5) (2017) 735–757. [35] J.P. Romualdi, J.A. Mandel, Tensile strength of concrete affected by uniformly distributed and closely spaced short lengths of wire reinforcement, ACI Struct. J. 61 (6) (1964) 27–37. [36] A.E. Naaman, Toughness, ductility, surface energy and deflection-hardening FRC composites, in: Proceedings of JCI International Workshop on Ductile Fiber Reinforced Cementitious Composites (DFRCC) – Application and Evaluation (DFRCC-02), Takayama, Japan, 2002, pp. 33–57 [37] V.C. Li, Y. Wang, S. Backer, A micromechanical model of tension-softening and bridging toughness of short random fiber reinforced brittle matrix composites, J. Mech. Phys. Solids. 39 (5) (1991) 607–625. [38] S.T. Kang, J.K. Kim, The relation between fiber orientation and tensile behavior in an Ultra High Performance Fiber Reinforced Cementitious Composites (UHPFRCC), Cem. Concr. Res. 41 (2011) 1001–1014. [39] W. Abbass, M.L. Khan, S. Mourad, Evaluation of mechanical properties of steel fibers reinforced concrete with different strength of concrete, Constr. Build. Mater. 168 (2018) 556–569.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Improvement effect of steel fiber orientation control on mechanical performance of UHPC Huanghuang Huang a, Xiaojian Gao a,b,c,⇑, Linshan Li a, Hui Wang d a
School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, China c Key Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China d Faculty of Architectural, Civil Engineering and Environment, Ningbo University, 315000 Ningbo, China b
h i g h l i g h t s One flow controlling method was developed to manufacture UHPC specimen. Fiber orientation number and coefficient in UHPC were significantly improved. Flexural strength, toughness and sMOR were increased by 64.3%, 65.1% and 77.1%.
a r t i c l e
i n f o
Article history: Received 22 May 2018 Received in revised form 22 August 2018 Accepted 22 August 2018
Keywords: Ultra-high performance concrete (UHPC) Flow control Fiber orientation Mechanical properties
a b s t r a c t In order to improve mechanical properties of ultra-high performance concrete (UHPC), one L-shape device with a narrow horizontal channel was developed to control the flow of fresh mixture and then the orientation of steel fibers was significantly improved. Mixtures with water-to-binder ratios of 0.20, 0.22 and 0.24 were prepared by introducing steel fibers with 1%, 2%, and 2.5% of the total volume respectively. Three fiber orientation parameters (fiber orientation angle, fiber orientation number and fiber orientation coefficient) were quantitatively evaluated by the image analysis method. Based on experimental results, mechanical properties (compressive strength, flexural strength, toughness, deflection at modulus of rupture and maximum deflection) were notably increased with the higher dosage of steel fibers. Compared with specimens prepared by a direct cast, this flow control method improved fiber orientation number and fiber orientation coefficient from 0.6 to 0.7 and 0.4–0.6 to 0.7–0.9 and 0.6–0.8 respectively. The best improvement on flexural strength, toughness, deflection at modulus of rupture and maximum deflection reached 64.3%, 65.1%, 77.1% and 14.9% respectively. Based on experimental results, linear relationships were regressed for fiber orientation parameters and mechanical properties of UHPC. Therefore, a potential method was developed to improve the reinforcement effect of steel fibers in UHPC. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Ultra-high performance concrete (UHPC) is generally made of large quantities of Portland cement, silica fume (SF) and fine aggregates with steel fibers for reinforcement and a high dosage of superplasticizer for satisfactory fluidity at a very low water to binder ratio (w/b) [1,2]. Due to its very high compressive strength (>150 MPa), flexural strength (>8 MPa), excellent durability and
⇑ Corresponding author at: School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China. E-mail address: [email protected] (X. Gao). https://doi.org/10.1016/j.conbuildmat.2018.08.146 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
toughness [3,4], UHPC has been developed into many attractive applications to real structures in Europe, Asia and North America in recent years [5–7]. These remarkable properties are mainly attributed to the very low w/b, well-graded aggregates and cementitious materials based on the packing density theory. Additionally, short steel fibers are employed to enhance the flexural strength and toughness due to the stress transfer from the matrix to the fibers, the fibers bridging at crack surfaces and resisting against crack propagation [8]. In general, steel fibers are randomly dispersed in UHPC when being prepared by a direct cast method. It is beneficial to obtain a more isotropic mechanical behavior. However, for uniaxial
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Table 1 Chemical and physical properties of cementing materials. Cement
SF
Chemical analysis (wt%) CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3
66.45 17.84 4.26 3.58 2.14 0.16 0.96 4.10
0.63 87.67 0.28 0.60 3.41 1.30 4.12 0.84
Physical properties Apparent density (g/m3) Blaine (BET) SSA (m2/g) Pozzolanic activity index
3.10 0.36 –
2.25 17.30 108.6
Table 2 Sieve analysis of quartz sand. Sieve size (mm)
Cumulative percentage retained (%)
2.5 1.25 0.63 0.315 0.16 <0.16
Fine sand
Coarse sand
0 0.16 0.36 28.30 81.16 100
0 0.10 12.14 87.26 97.74 100
Table 3 Basic properties of steel fibers.
2. Materials and methods
Diameter, df Length, Lf Aspect ratio Density Tensile strength Elastic modulus (mm) (mm) (Lf/df) (g/cm3) (MPa) (GPa) 0.2
13.0
65.0
materials [14]. Therefore, the usage of a lower amount of steel fibers is one of the most promising methods to reduce the production cost of this material. To achieve this purpose, hooked or twisted steel fibers and fibers with a higher ratio of length to diameter were introduced without degradation of mechanical properties [15–17]. In addition, the steel fibers can also be significantly reduced by improving the fiber orientation in UHPC to enhance the mechanical performance. The rheology and flowability properties of fresh mixtures [18–20], mixing and placing method [21–23], extrusion process and nozzle injection technique [24,25], and induced rotation of steel fiber in electromagnetic field [13,26,27] have been utilized to modify the fiber orientation in cementitious materials, generating the enhancement up to 117.3% for flexural/tensile strength. In addition, the wall effect was also used to improve the fiber orientation. Roy et al. utilized one chute to align the fibers in one direction by pouring UHPC on the upper end of the device and specimens with fibers orientated perpendicular to the load direction presented the better pullout load [28]. This paper presented one L-shape device to improve fiber orientation by controlling the flow of fresh UHPC mixtures. Three fiber orientation parameters, i.e. fiber orientation angle, fiber orientation number and fiber orientation coefficient were evaluated by image analysis. The improvement effect of this manufacture method on mechanical properties including compressive strength, flexural strength, toughness, deflection at modulus of rupture and maximum deflection were experimentally evaluated. Finally, the relationships between fiber orientation parameters and mechanical properties were regressed.
7.9
2850.0
2.1. Materials
200.0
mechanical tests, the efficiency of fibers in accordance to the principle tensile stress is low, only 30% as reported [9], and the improvement of mechanical performance is very limited. It was reported that the mechanical properties of UHPC are affected by the fiber characteristics (e.g. geometry, aspect ratio and volume fraction) [8], fiber-to-matrix bond properties [10] and fiber orientation [11–13]. The former two aspects can be tailored by the design of mixing proportion and the fiber orientation is difficult to control during the manufacture of UHPC. The high cost of UHPC at present is mainly attributed to the higher price of steel fibers and superplasticizer than other raw
Ordinary Portland cement with strength grade of 52.5 was used in this study. Silica fume consisting of spherical particles with size from 0.1 lm to 1 lm was utilized as supplementary cementitious material due to its pozzolanic reactivity and filling effect [29]. The chemical components and physical properties of these two cementing materials are given in Table 1. Two kinds of quartz sand were used as aggregates and the sieve analysis results of them are shown in Table 2. A type of copper coated steel fiber with tensile strength of 2850 MPa was incorporated in mixtures, more details about the geometrical and physical properties of the used steel fibers are summarized in Table 3. A polycarboxylate-based superplasticizer (SP) with water-reducing range of more than 30% and solid content of about 40% by weight was added to improve the fluidity of fresh mixture.
Table 4 Mix proportions of UHPC. w/b ratio
Cement (kg/m3)
SF (kg/m3)
Fine sand (kg/m3)
Coarse sand (kg/m3)
Steel fibers (% by volume of mixture)
SP (% by weight of binder)
0.20 0.20 0.20 0.22 0.22 0.22 0.22 0.24 0.24 0.24 0.24
920 920 920 920 920 920 920 920 920 920 920
276 276 276 276 276 276 276 276 276 276 276
202 202 202 202 202 202 202 202 202 202 202
810 810 810 810 810 810 810 810 810 810 810
0 1 2 0 1 2 2.5 0 1 2 2.5
2.0 2.0 2.0 1.8 1.8 1.8 1.8 1.6 1.6 1.6 1.6
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2.2. UHPC preparation The mix proportions are shown in Table 4. To investigate the improvement effect of fiber orientation control on mechanical properties of UHPC with different matrix strength, eleven mixtures were prepared by changing w/b ratio from 0.20 to 0.24. 1%, 2% and 2.5% volume fraction of steel fibers were incorporated. For every mixture, the ratio of cement: SF: fine sand: coarse sand was kept at the same value. To prevent steel fibers sedimentation and improve stability, the dosage of SP was decreased from 2.0% to 1.6% with the increasing w/b ratio from 0.20 to 0.24. All the raw materials were blended by using a Hobart mixer. Firstly, the solid materials except for steel fibers were premixed at a low speed (198 rpm) for 4 min to obtain a homogeneous mix-
711
ture. Then, the SP was premixed with water and added in two halves in 6 min. When the mixture had a good flowability, the steel fibers were slowly added into the mixer in 10 min to achieve a uniform dispersion. 2.3. Experimental methods As given in Fig. 1, the L-shape device includes two parts. The vertical container was designed with size of 100 mm 100 mm 300 mm. On the one hand, this size is big enough for free flow of UHPC and rotation of steel fibers with length of 13 mm and fibers are randomly distributed in this zone. On the other hand, this size is also easily to operate at the lab. The horizontal channel was designed with height of 10 mm and length
Fig. 1. (a) Geometry of the L-shape device and (b) preparation of UHPC specimens by using this device.
Fig. 2. Image analysis procedure for determining fiber orientation.
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of 50 mm. The height of 10 mm is smaller than fiber length of 13 mm and thus all fibers tend to rotate to a horizontal state when flowing through this channel. Fresh mixture was poured into the apparatus and subsequently flowed out from the narrow channel into the mold. Specimens with sizes of 70 mm 70 mm 230 mm (b h l) were fabricated by seven layers without vibration. For every layer, the device was slowly moved from one end to the other end of the mold. After that, all the specimens were covered with plastic sheet to reduce moisture evaporation and placed in a chamber with temperature of 20 ± 2 °C and relative humidity of more than 95% for 48 h. After the first two days curing, the specimens were demolded and kept in a steam curing chamber at 90 °C for 72 h. To test flexural performance of UHPC specimens, three-point bending test with a span of 175 mm was conducted by a universal testing machine (UTM) with a maximum load capacity of 100 kN at a rate of 0.4 mm/min [8]. A 500 N preload was applied to allow the accommodation of loading apparatus. The flexural strength fs can be calculated by:
fs ¼
3FL
ð1Þ
2
2bh
where F is the maximum load obtained from the load-deflection curve, L is the span of the test, b and h are the width and height of the specimen respectively. The toughness T is defined as the area up to a certain deflection under the load-deflection curve. The obtained values for the deflection at modulus of rupture (MOR), L/150, L/100, L/75 and L/50 are usually used to evaluate the energy absorption capacity of UHPC [30,31]. In this study, the toughness was determined as the area under the load-deflection curve for the deflection at L/75 as described below.
Z
s¼L=75
T¼
ð2Þ
FðsÞds s¼0
where s is the deflection of specimen. For quantitatively evaluating the fiber orientation, slices selected near the localized crack (to reduce the effects of fiber pull-out) were processed by coarse grinding, fine grinding and ultrasonic wave cleaning successively to obtain smooth surfaces, and increase color contrast between fibers and matrix. After that, one microscopy digital camera, Olympus DSX500 with 5/0.15BD lens was employed to aquire the high resolution images of specimen cross sections. Every side of image was removed
(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 3. Compressive strength of UHPC specimens with different fiber volumes.
H. Huang et al. / Construction and Building Materials 188 (2018) 709–721
6.5 mm to eliminate the effects of mold. The size of view filed was 3.5 mm 3.5 mm with resolution of 500 500 for every cross section, and a total of 256 pictures were taken and these pictures were then assembled into one integrated image with 10% of overlapping zone. The acquired image (called RGB image) was disposed by an Image Pro Plus software and converted into a binary image by setting a threshold value for increasing color contrast further to separate the steel fibers from the surrounding cementitious matrix [32]. The whole image analysis procedure is given in Fig. 2. After obtaining the binary image, the inclined angle of single fiber to the cutting plane h can be computed as:
Z
gh ¼
hmax
713
ð5Þ
pðhÞcos2 hdh
hmin
where pðhÞ is the probability density function (PDF) of fiber orientation. gh ¼ 1 indicates that every fiber is aligned parallel to the tensile direction, and gh ¼ 0 indicates that every fiber is aligned perpendicular to the tensile direction.
3. Results and discussion 1
h ¼ cos
df a
ð3Þ
where df and a are the diameter of fiber and the major axis length of the fiber image respectively. The fiber orientation number ah is computed as [13]: T
ah ¼
Nf 1 X
NTf
ð4Þ
coshn
n¼1
where N Tf is the total number of fibers crossing the analyzed section, hn is the inclined angle of the nth fiber. Furthermore, the fiber orientation coefficient gh can be expressed as [33]:
3.1. Compressive strength Fig. 3 presents compressive strength results of UHPC specimens with different fiber contents at w/b = 0.20, 0.22 and 0.24. The load direction was perpendicular to the fiber orientation. The compressive strength is enhanced by the steel fiber incorporation as expected. The best improvement reaches 29.6% when steel fibers are incorporated by the volume fraction of 2.5% for mixtures with w/b ratio of 0.24. It can be found that there is a slight increase of compressive strength induced by the orientation of steel fibers. It can be concluded that compressive strength is mainly influenced by the fiber content and w/b ratios rather than the fiber orientation.
(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 4. Load-deflection curves of UHPC specimens with different fiber volumes.
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(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 5. Flexural strength and toughness of UHPC specimens with different fiber volumes.
3.2. Flexural load-deflection curve The flexural load-deflection curves of UHPC specimens with different fiber volumes at w/b = 0.20, 0.22 and 0.24 are presented in Fig. 4. It is obviously seen that the mechanical properties are significantly enhanced by the increasing fiber volume and orientation at all the three w/b ratios. The limit of proportionality (LOP) and modulus of rupture (MOR) were utilized to describe the first cracking point and post-cracking peak point respectively [31]. It can be easily observed that the LOPs for all tested specimens of at a given w/b approximate to that of the blank samples without fibers. This is reasonable that the first cracking behavior largely depends on the matrix strength rather than the fiber volume fraction or orientation. Therefore, the load, toughness and deflection at LOP show no significant difference as described by other researchers [34]. 3.3. Flexural strength and toughness Flexural strength and toughness of UHPC specimens with different fiber contents at three w/b ratios are shown in Fig. 5. Flexural strength and toughness of UHPC are remarkably improved by the addition of steel fibers and the more enhancement is induced by the higher addition level of steel fibers. The best improvement for flexural strength reaches 118.1% when the steel fiber addition
increases from 1% to 2.5% at w/b ratio of 0.22. With respect to the toughness, the best enhancement is 112.6% when the steel fiber addition increases from 1% to 2.5% at w/b ratio of 0.24. It was reported that flexural strength is strongly affected by fiber bridging strength, the fiber bridging strength is improved with the increasing fiber volume due to more fibers at crack surfaces, leading to a larger bonding area between fibers and matrix [34]. In terms of the fiber spacing theory, flexural strength rises with the decreasing fiber distance due to the larger fiber volume [35]. On the other hand, the toughness of FRC is closely related to the fiber pullout surface energy, given as: fsVfL2f /df [36], where fs is a parameter taking the fiber orientation factor, group reduction factor and bond strength of fiber–matrix interface, etc. into account. Thus, the toughness enhances with the larger fiber volume Vf. As given in Fig. 5, both flexural strength and toughness are dramatically increased by the modified orientation of steel fibers. It is also observed that the best improvement effect for flexural strength and toughness occurred for 1% fiber volume content. The flexural strength is enhanced by 50.9%, 64.3% and 55.0% when being compared with ones prepared by the direct cast method at w/b ratio of 0.20, 0.22 and 0.24 respectively. The toughness is improved by 56.7%, 52.9%, 65.1% at these three w/b ratios respectively. Due to the flexural strength is strongly affected by fiber bridging strength rb ðdÞ, which is generally given as [37]:
H. Huang et al. / Construction and Building Materials 188 (2018) 709–721
(a) w/b = 0.20
715
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 6. sMOR and smax of UHPC specimens with different fiber volumes.
rb ðdÞ ¼
4V f
p
2 df
Z 0
p 2
Z
l 2
Pðh; le ; dÞpðle ÞpðhÞcoshdle dh
ð6Þ
0
where le is the embedded length of the fiber, d is the crack opening displacement, Pðh; le ; dÞ is the pullout resistance force of a single fiber at the crack surface, pðle Þ is the probability density function for le . Thus, the fiber bridging strength increases with the better fiber orientation because of a larger value of pðhÞcosh is obtained, meaning a greater number of fibers orienting towards to the tensile direction and leading to a higher flexural strength. With respect to the toughness, on the one hand, a higher flexural strength is advantageous to improve the toughness, on the other hand, the toughness is enhanced because of the improved fiber orientation considering the parameter fs according to other researchers [36]. 3.4. Deflection at MOR and maximum deflection The deflection at MOR, sMOR, and maximum deflection smax of UHPC specimens with different fiber volumes at three w/b ratios are given in Fig. 6. The sMOR equals to the smax for the specimen without steel fibers, and the sMOR (smax) is notably increased by the addition of steel fibers. With the fiber content increasing from 1% to 2.5%, sMOR and smax are also evidently enhanced. The best improvement for sMOR and smax reached 118.8% and 8.7% respec-
tively when 2.5% steel fiber was added to mixture with w/b ratio of 0.22. This is attributed to the improved fiber bridging capacity and fiber bridging strength as well as the reduced fiber distance as a result of higher fiber volume fraction [34,35]. This tendency was also reported by other researchers [33]. Both sMOR and smax are dramatically increased by the improved fiber orientation. It can be also concluded that the best modification for sMOR is obtained at 1% fiber addition, being 36.1%, 77.1% and 58.0% at w/b ratio of 0.20, 0.22 and 0.24 respectively when compared with those of specimens prepared by the direct cast. With regard to the smax, the largest improvement are 14.9%, 13.4% and 9.8% which are obtained at 2% fiber incorporation for w/b ratio of 0.20, 0.22 and 0.24 respectively. As discussed above, the fiber bridging strength is enhanced by a better fiber orientation because more fibers are aligned to the tensile direction and a larger stress can be transferred from the matrix to the fibers for resisting against cracks extension. 3.5. Evaluation of fiber orientation Fig. 7 shows the probability density distribution of fiber orientation angle for each specimen from (a) to (p). It is obviously seen that the histograms of specimens with oriented steel fibers are leftskewed, indicating that most of fibers in the specimens have a lower inclined angle and behave a higher tendency to be parallel
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Fig. 7. Probability density distribution of fiber orientation angle.
H. Huang et al. / Construction and Building Materials 188 (2018) 709–721
Fig. 7 (continued)
717
718
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Table 5 Summary of fiber orientation number and fiber orientation coefficient of UHPC. w/b ratio
Steel fiber volume (%)
Orientation
ah
gh
0.20
1
Not orient Orient Not orient Orient
0.625 0.754 0.621 0.728
0.525 0.695 0.555 0.605
Not orient Orient Not orient Orient Not orient Orient
0.685 0.849 0.664 0.780 0.650 0.796
0.520 0.735 0.475 0.630 0.470 0.655
Not orient Orient Not orient Orient Not orient Orient
0.660 0.703 0.608 0.757 0.662 0.731
0.475 0.640 0.410 0.600 0.470 0.635
2 0.22
1 2 2.5
0.24
1 2 2.5
to the tensile direction. On the other hand, the histograms of specimens prepared by the direct cast are right-skewed, tending to be perpendicular to the tensile direction. Similar distribution was also reported for the specimens casted in an electromagnetic field or under other flow controlling condition [13,38]. The fiber orientation parameters, i.e. fiber orientation number and fiber orientation coefficient can be calculated by Eqs. (4) and (5) as summarized in Table 5. It can be easily found that specimens prepared by this flow control method present the higher fiber orientation number (ah) and fiber orientation coefficient (gh). The ah and gh of specimens with a modified fiber orientation reach as high as 0.7–0.9 and 0.6–0.8 respectively in this paper, while those of specimens prepared by the direct cast are 0.6–0.7 and 0.4–0.6 respectively. 3.6. Relations between fiber orientation parameters and mechanical properties From the above experimental results, UHPC specimen with better fiber orientation (larger ah and gh) presents higher mechanical properties including flexural strength, toughness, sMOR and smax when the steel fiber volume and w/b ratio is kept the same. On the other hand, the mechanical performance is notably influenced by fiber content and w/b ratio when UHPC specimens are prepared by the same method [39]. The parameter k = ahghVf can be thought as a scalar descriptor of the fiber structure of the composite combining the effects of fiber orientation number, fiber orientation coefficient and fiber volume. Fig. 8 presents the relation between the parameter k and mechanical properties (i.e. flexural strength, toughness, sMOR and smax) at w/b = 0.20, w/b = 0.22 and w/b = 0.24. It can be found that the mechanical properties increase linearly more or less with the increase of the parameter k . Flexural strength and toughness exhibit the best linear relations with the parameter k with the coefficients of determination (R2) of higher than 0.93 at the w/b ratio of 0.22 and 0.24, while the sMOR has a little weaker linear relation with the parameter k with R2 of more than 0.81. Values of R2 become lower when the w/b ratio is reduced to 0.20, being attributed to the lack of data support used for fitting (only 1% and 2% of steel fiber volume). When taking the effects of w/b ratios into consideration, the parameter k was multiplied with the reciprocal of the w/b ratio
due to the improving effect on mechanical performance of lower w/b ratio. The relations between the parameter k b/w and mechanical properties are presented in Fig. 9. As can be seen, the mathematical relations between the parameter k b/w and flexural strength, toughness, sMOR, and smax are given as follows.
fs ¼ 7:576 þ 6:244kb=w
ð7Þ
T ¼ 16:982 þ 13:866kb=w
ð8Þ
sMOR ¼ 0:524 þ 0:243kb=w
ð9Þ
smax ¼ 3:081 þ 0:160kb=w
ð10Þ
Eqs. (7)–(9) show a strong coefficient of determination. In another word, flexural strength, toughness and sMOR tends to linearly increase with the higher value of the parameter k b/w. These three equations are potential to be used to evaluate flexural strength, toughness and sMOR of UHPC specimens for a given fiber orientation number, fiber orientation coefficient, fiber volume and w/b ratio. It needs more efforts to determine the feasibility of these equations for UHPC with different mixtures and raw materials. 4. Conclusions Based on the above experimental results, the following conclusions can be drawn: One novel method for controlling flow of fresh mixture was developed. It can improve the fiber orientation number and fiber orientation coefficient from 0.6 to 0.7 and 0.4–0.6 to 0.7– 0.9 and 0.6–0.8 respectively. UHPC specimens prepared by this flow control method exhibited much better mechanical performance than those prepared by the direct cast. The maximum improvement on flexural strength, toughness, sMOR and smax reached 64.3%, 65.1%, 77.1% and 14.9% respectively. Linear relations can be obtained by a regression between mechanical properties and fiber orientation parameters. These relations can be used to evaluate flexural strength, toughness
H. Huang et al. / Construction and Building Materials 188 (2018) 709–721
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(a) w/b = 0.20
(b) w/b = 0.22
(c) w/b = 0.24 Fig. 8. Relations between the parameter k and mechanical properties (i.e. flexural strength, toughness, sMOR and smax) at (a) w/b = 0.20, (b) w/b = 0.22 and (c) w/b = 0.24.
and sMOR of UHPC specimens when the fiber orientation number (ah), fiber orientation coefficient (gh), fiber volume (Vf) and water-to-binder ratio (w/b) are provided. Conflict of interest The authors declare no conflict of interest.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51578192), the National Key R&D Program of China (No. 2017YFB0309901) and the Initial Fund of Ningbo University of China (No. 011-421805030).
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Fig. 9. Relations between the parameter k b/w and (a) flexural strength, (b) toughness, (c) sMOR, (d) smax.
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