Experimental investigation on mechanical properties of Hybrid Fibre Reinforced Concrete

Experimental investigation on mechanical properties of Hybrid Fibre Reinforced Concrete

Construction and Building Materials 157 (2017) 930–942 Contents lists available at ScienceDirect Construction and Building Materials journal homepag...

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Construction and Building Materials 157 (2017) 930–942

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Experimental investigation on mechanical properties of Hybrid Fibre Reinforced Concrete Zhong-Xian Li a, Chang-Hui Li a, Yun-Dong Shi a,⇑, Xiao-Jie Zhou b a b

Key Laboratory of Coast Civil Structure Safety of Ministry of Education, Tianjin University, Tianjin 300350, China School of Civil Engineering, Tianjin Chengjian University, Tianjin 300384, China

h i g h l i g h t s  A new HFRC mixed with steel, basalt and polypropylene fibres is produced.  The mechanical properties of the HFRC were investigated thoroughly.  The correlation between direct shear and splitting tensile of HFRC is suggested.

a r t i c l e

i n f o

Article history: Received 8 April 2017 Received in revised form 11 August 2017 Accepted 18 September 2017

Keywords: Hybrid fibre Shear behaviour Mechanical properties Macro fibre Micro fibre Concrete

a b s t r a c t In order to improve the work behaviour of Plain Concrete (PC) of the shear keys in the immersed tunnel, a series of test programs consisting of direct shear, four points flexure, uniaxial tensile, uniaxial compression and splitting tensile tests were carried out to find the optimized mixture proportion of the fibres in Hybrid Fibre Reinforced Concretes (HFRC). The optimized unit weight of the hybrid fibres was obtained through the parametric studies on the improvements of different fibre contents on the mechanical properties of the HFRC, especially the shear strength and toughness. Additionally, the influences of incorporating hybrid fibres on flexure, direct shear toughness and residual load were also studied. The direct shear strength, shear toughness and residual shear load significantly increased due to the addition of steel fibres and basalt fibres. Through extensive experimental studies, the comparisons of the mechanical properties of the HFRC with different fibre content revealed that group C2 containing 180 kg/m3 steel fibres and 4.5 kg/m3 basalt fibres performed the best in terms of shear strength and toughness. The contributions of different types of fibres to the mechanical properties of HFRC were also investigated. The observations from the tests offer a practical guidance to concrete composites designers on the shear strength and shear deformation ability of different structures, especially for use in constructing the shear keys of an immersed tunnel. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Due to the rapid development in Fibre Reinforced Concrete (FRC) and its extensive applications, FRC is becoming increasingly popular in civil engineering construction. FRC has higher strength, deformation ability and energy dissipation ability than Plain Concrete (PC), it can improve the hysteretic behaviour of columns [1] and decrease the surface cracks in road pavement which consequently improves its service life. FRC can also be used to retrofit the reinforced concrete members after an earthquake or long service life and mitigate the spread of cracks in concrete structures [2]. More recently, FRC has been proposed as the construction ⇑ Corresponding author. E-mail address: [email protected] (Y.-D. Shi). https://doi.org/10.1016/j.conbuildmat.2017.09.098 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

material for the shear keys in the joints of immersed tunnels. In an immersed tunnel, the shear keys are essential for withstanding seismic movements. In engineering construction, concrete with high compressive strength is needed for constructing shear keys. Because the ductility of the shear key is important to the seismic resistance and dynamic vibrations, it is also of interest to improve the ductility property of shear keys in an immersed tunnel. Recently, FRC was used in the joints of an immersed tunnel connecting Hong Kong, Macro and mainland China to improve the mechanical properties of the shear keys. The specific immersed tunnel joint and the shear key are shown in Fig. 1. Previous extensive studies focused on the enhancement of tensile strength and ductility of mono fibre reinforced concrete (MFRC). These studies mainly focused on steel fibres (SF) [3–5] or polypropylene (PF) and polyvinyl alcohol (PVA) fibres [6–10] with

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Fig. 1. Immersed tunnel joint and shear key.

different length, equivalent diameter, aspect ratio and volume of fraction. The advantages of MFRC over PC include improved axial and splitting tensile strengths, and ductile tensile behaviour. SF, PP or PVA fibres are normally used to improve the mechanical properties of MFRC [11]. Previous studies have shown that MFRC with low modulus fibres, such as PP or PVA fibres, usually exhibited lower compressive strength, higher strain capacity and larger crack width properties. On the other hand, MFRC with high modulus fibres, e.g., SF, shows higher ultimate strength and lower workability. Thus, it is of interest to generate a synergistic response through combining the advantages of different types of fibres. The fibres in HFRC can be classified by their modulus (high or low value type) or geometrical size (e.g., macro and micro fibre) [12]. Recently, HFRC [13,14] comprised of two or more different types of fibres has been studied. Compared with the MFRC, HFRC typically exhibits higher/greater compressive and tensile strengths and a stronger energy dissipation ability [15,16]. HFRC with an optimized volume fraction of different fibres can improve the ultimate strength, strain capacity and ultimate crack width [17]. Qian and Stroeven [18] mixed SF and PF in HFRC and noted that a stiffer and stronger SF compared with PF improved the ultimate strength of HFRC, while the more ductile PF compared with SF improved the strain capacity and toughness after post-peak. Park and Kim [19,20] used four types of steel macro fibres and two types of steel micro fibres to investigate the flexural and tensile behaviour of the HFRC. However, the addition of SF at a high volume fraction had some shortcomings, including an increased cost and poor workability. Sivakumaret et al. [21] blended one type of SF and three types of non-metallic fibres in concrete composites to determine the compressive and flexural behaviour of HFRC and found that the SF improved the compressive and flexural strength. Machine et al. [7] used two types of polypropylene fibres in HFRC to investigate the mechanical properties of the HFRC, but the contribution of PP fibres to the post-peak cracking behaviour was not observed to be significant. Yao et al. [22] used SF, PF and Carbon Fibre (CF) at a low fibre volume fraction (0.5%) to investigate the mechanical properties of the HFRC and found that the hybridization containing CF and SF had a better strength and flexure toughness compared with the mixture containing PF and SF. Banthia et al. [23] tested several types of fibres in normal concrete and found that hybridization of PP and CF offered high toughness in HFRC. Dawood et al. [24] tested three types of fibres including SF, palm fibres and Bar-chip fibres to determine the mechanical properties of the HFRC. These previous studies on HFRC mainly focused on compression, flexure and tensile tests. The tests on shear behaviour of HFRC are still limited. Additionally, most studies involving micro fibres in HFRC focused primarily on PP or PVA fibres. Few studies

have investigated the properties of HFRC containing basalt fibre (BF). Therefore, in this paper, BF is adopted as a new micro fibre in HFRC and the shear behaviour of the HFRC is primarily investigated. SF is adopted as the macro fibre and PF is adopted as a secondary micro fibre to find the most appropriate type of micro fibre to mix simultaneously with the macro fibre. In this paper, the mechanical properties of HFRC have been analysed in detail. The correlation between the compressive strength on cubes and compressive strength on prisms, and the correlation between direct shear strength and splitting tensile strength were studied. The specific objectives of this study are to investigate: (1) the influence of different types of fibres on the compression, tension, flexure and direct shear behaviours of HFRC; (2) the influence of different unit weights of fibres on the mechanical properties of HFRC; and (3) the correlations between different aspects of strength (i.e., compression, tension, flexure and shear). This study provides valuable information on the mechanical properties of HFRC and the use of short cut basalt fibres as a micro fibre in HFRC. Useful data on the influence of macro steel fibres blended with different types of micro fibres on the mechanical properties of HFRC are also provided.

2. Materials properties and mix proportions 2.1. Basic constituents of concrete This study used grade 42.5 ordinary Portland cement in all the HFRC mixtures. Dry and clean natural river sand and crushed granite stone were used in the concrete as the fine and coarse aggregate, respectively. Because the workability of the HFRC is a concern, the cement was replaced by fly-ash and mineral powder at approximately 15% of the volume and 4.2 kg/m3 of hyper plasticizer was used in all the concrete mixtures. The specific composites were, water:cement:sand:stone:fly ash:mineral powder:polycarboxylic = 139:290:683:1034:65:65:4.2 kg/m3. 2.2. Fibres This study used milling SF, BF, and PF in concrete cementitious composites as shown in Fig. 1. The main properties of milling steel fibre are given in Table 1. The BF and PF were used as the micro fibres in the mixtures. BF is formed from high-performance volcanic rocks and is a type of silicate. It has excellent physical properties and corrosion resistance, especially alkali resistance. Thus, BF is a good choice for the construction of shear keys of immersed tunnels undersea and it can effectively improve the mechanical

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Table 1 Properties of different types of fibre. Fibre type

Length (mm)

Equivalent diameter (mm)

L/D

Elongation (%)

Tensile Strength (MPa)

Density kg/m3

Steel

30

0.6

50

/

>1000

7850

Basalt

3–12

0.015

/

3.2

3000–3500

2650–3050

Polypropylene

12

/

/

12

>400

910

properties of PC [25]. In this paper, short-cut BF with lengths ranging from 3 mm to 6 mm was utilized. The main properties of BF are detailed in Table 1. To distinguish the physical properties of BF, PF with a length of 12 mm and a diameter of 2 mm was chosen as the secondary micro fibre. PF is a commonly used fibre in HFRC and its geometry and mechanical properties are given in Table 1. 2.3. Specimens preparation and mixing procedure 2.3.1. Specimens preparation The experimental testing included four points flexure tests on prisms, uniaxial tensile tests on specimens, compressive tests on cubes, uniaxial compressive tests on prisms, splitting tensile tests on prisms and direct shear tests on prisms. Nine groups of specimens were prepared for each type of test, and three specimens were prepared in each group. The specific unit weights of the fibres and details of specimens are given in Table 2. The mix proportion of the PC and HFRC was prepared according to the weight method and absolute volume method recommended by the American Concrete Institute (ACI 211.1) [26]. Firstly, the unit weight of PC per unit volume (m3) at 2280 kg/m3 was kept. Secondly, according to the references mentioned in the introduction, the unit weight of SF per unit volume (m3) was adopted at 120 kg/m3 and 180 kg/m3. Thirdly, to determine which type of micro fibre would have a better synergistic effect with SF, the volume fraction of BF and PF were adopted at 0.1% and 0.15%, which converted the unit weights to 3 kg/m3 and 4.5 kg/m3 of BF and 1 kg/m3 and 1.5 kg/m3 of PF, respectively. Additionally, two

comparative groups, B4 and C4, without SF were prepared to determine the mechanical properties of HFRC without a macro fibre. 2.3.2. Mixing procedure The concrete composites were mixed in a 60-liter rotatingmixer. The mixing procedure started with stirring the cement, sand, coarse aggregate, fly-ash and mineral powder for approximately 2 min. Next, approximately 2/3 water was added to the rotating-mixer, and the mixture was mixed for an additional 2 min. Soon afterward, the fibres were manually cast into the rotating-mixer to ensure that the fibres were distributed uniformly in the cementitious composites. Finally, the hyper plasticizer was added to the cementitious composites, and the rest of water was used to wash the residual hyper plasticizer from the test tube. 3. Test setup and instruments 3.1. Direct shear tests According to the Chinese Engineering Constitute Standard (CECS13:2009) [27], a total of 27 specimens and three specimens in each group with size of 100  100  300 mm3 (width  height  length) were adopted to test the shear behaviour of the hybrid fibre concrete. The test setup is shown in Fig. 2(a). This test method is also adopted in JSCE-G 553-1999 standard [28] to offer the pure shear condition which can be easily realized in the laboratory. The stress state of the direct shear specimen is shown in Fig. 2(b). The shear strength can be calculated as follows:

Table 2 Details of proportions in different groups. Specimens Group

Weight of SF in concrete (kg/m3)

Weight of BF in concrete (kg/m3)

Weight of PF in concrete (kg/m3)

Water and Polycarboxylic acid (kg/m3)

Cement (kg/m3)

Sand and Stone (kg/m3)

Fly ash and Mineral powder (kg/m3)

Unit Weight (kg/m3)

A0 B1 B2 B3 B4 C1 C2 C3 C4

0 120 120 120 0 180 180 180 0

0 3 3 0 3 4.5 4.5 0 4.5

0 1 0 1 1 1.5 0 1.5 1.5

139+4.2 139+4.2 139+4.2 139+4.2 139+4.2 139+4.2 139+4.2 139+4.2 139+4.2

290 290 290 290 290 290 290 290 290

683+1034 683+1034 683+1034 683+1034 683+1034 683+1034 683+1034 683+1034 683+1034

65+65 65+65 65+65 65+65 65+65 65+65 65+65 65+65 65+65

2280.2 2404.2 2403.2 2401.2 2284.2 2466.2 2464.7 2461.7 2286.2

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F Upper pressure plate Dial indicator

10/H

a

Dial indicator

10/H H

10/H

a

H

10/H

H+2a Lower pressure plate (a) Direct Shear Test

F/2 V

V

F/2 V

V

Parallel to the shear surface

F/2

Vertical to the shear surface

F/2

(b) The stress state of the direct shear specimen

F Dial indicater Rigid body column

Upper pressure plate

Upper pressure plate Dial indicater

Specimen 300

Rigid body column

Specimen

Lower pressure plate 550 630

Lower pressure plate

(c) Uniaxial Compress Test

(d) Splitting Tensile Test

Upper Plate

100 Upper Plate Rigid Column

Rigid Column

Rigid Column

Rigid Column 1000mm Dial Indicator

Strain gauge *2

Lower Plate

Lower Plate

(e) Uniaxial Tensile Test and Size of Specimen Fig. 2. Test setups.

(f) Four Points Flexure Test

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fs ¼

Z.-X. Li et al. / Construction and Building Materials 157 (2017) 930–942

F max 2bh

ð1Þ

4. Results and discussion 4.1. Compressive, direct shear and splitting tensile strengths

1 b ¼ ðb1 þ b2 þ b3 þ b4 Þ 4

ð2Þ

1 ðh1 þ h2 þ h3 þ h4 Þ 4

ð3Þ



where f s denotes direct shear strength (MPa), F max denotes Max load (kN), b denotes the average width of the specimens (mm), h denotes the average height of the specimens (mm), b1 ; b2 ; b3 ; b4 denote the width of the opposite side belonging to the damaged section (mm), and h1 ; h2 ; h3 ; h4 denote the height of the opposite side belonging to the damaged section (mm).

3.2. Compressive tests The specimens for the cubic compressive strength tests were with the size of 100  100  100 mm3. Each group included three specimens, and the final results were the average obtained from the tests. The compressive tests on cubes followed the guidelines of the Chinese Engineering Constitute (CECS13:2009). In addition, the uniaxial compression prisms were with the size of 100  100  300 mm3 (width  height  length). A total of 27 specimens were prepared for the uniaxial compressive test. The compressive strength and compressive deformation behaviour of the concrete can be improved by mixing the fibres. The testing apparatus had two rigid columns to keep the device steady and could be controlled by displacement. Two dial indicators on the tester were used to measure the displacement and calculate the average value as the deformation of the specimens. The test device is shown in Fig. 2(c).

3.3. Splitting tensile tests The 150  150  550 mm3 (width  height  length) specimens were prepared to obtain the splitting tensile strength. A total of 27 specimens were prepared for the splitting tensile tests. Fig. 2 (d) shows the test setup for the splitting tensile tests.

3.4. Uniaxial tensile tests The size of specimens for the uniaxial tensile tests is shown in Fig. 2(e). The testing apparatus had four rigid columns to keep the device steady and could be controlled by displacement. Three specimens were prepared for each group. The strength and the strain capacity of uniaxial tensile tests could largely influence the other properties of HFRC, especially the flexure behaviour and direct shear behaviour. Two strain gauges were used in each side of the specimen to obtain the elastic strain at elastic stage.

3.5. Four points flexure tests According to the ASTM C1018 1997 [29], the sizes of the specimens for the four points flexure tests were 100  100  400 m3 (width  height  length) and the test setup is shown in Fig. 2(f). This test method can evaluate the flexural behaviour derived from HFRC under four points loading and obtain the toughness parameters from the load–deflection curve.

The results for the cube and prism compressive strength, direct shear strength and splitting tensile strength for all HFRC mixtures are presented in Table 3 and their variation laws for different groups are shown in Fig. 3. From the cube and prism compressive strength results, it is observed that a strength enhancement occurred for the HFRC containing SF compared with the PC. This result indicates that BF and PF had little influence on the compressive strength. Meanwhile, the maximum increase in cube and prism compressive strengths were 22% and 17%, respectively, compared with PC, which occurred in group B2 containing 120 kg/m3 SF+3 kg/m3 (0.1%) BF. Next, comparing the compressive strength of B3 containing 120 kg/m3 SF +1 kg/m3 (0.1%) PF with B2, it is evident that with the same volume fraction of micro fibres of BF and PF, BF had a better synergistic effect than PF combined with the macro fibre, SF. However, the results indicate that the difference in the performance of the HFRC with respect to cube and prism compressive strength was not significant. The direct shear strengths of all Hybrid Fibre Reinforced Concretes were observed to be much higher compared with the Plain Concrete A0 from Table 3. Unlike the small increase in compressive strength, the maximum increase in direct shear strength was approximately 116%. The test results indicate that the group C2, containing 180 kg/m3 SF and 4.5 kg/m3 BF, showed the highest shear strength among all HFRCs. The increase of macro and micro fibres resulted in this significant enhancement of the direct shear strength. Comparing HFRC B4 containing 3 kg/m3 BF+1 kg/m3 PF with A0, the shear strength of B4 was improved from 5.5 MPa to 7.1 MPa. However, comparing HFRC C4 containing 4.5 kg/m3 BF +1.5 kg/m3 PF with A0, the increased unit weights of the micro fibres did not increase its shear strength. This result indicates that there is a reasonable range of unit weight of micro fibres. Within the reasonable range of micro fibres, the shear strength will be enhanced; otherwise, the shear strength may decrease. This is due to the volume expansion of PF when it meets water. If the unit weight of PF exceeds a reasonable range, the extra PF will cause a nonuniform distribution of the fibres, and thus leads to the decrease in the direct shear strength. Additionally, the result that SF made the primary contribution to the direct shear strength of HFRC indicates that BF and PF enhanced the shear strength to a lesser extent. Splitting tensile tests results are also presented in Table 3. All HFRCs had an appreciable increase in splitting tensile strengths. Comparing the splitting tensile strengths of HFRCs with PC, the maximum increase was 100%. Similar to the variation law of direct shear strength, the splitting tensile strength of all groups increased when the unit weight of the fibres in HFRC was increased, except C4, which contained BF and PF. This phenomenon indicates that splitting tensile tests have a similar failure mechanism to the direct shear tests. The ultimate compressive load, direct shear load and residual direct shear load are presented in Table 4. The residual shear load indicates the load carrying capacities of the specimens after failure. The residual shear loads were provided by the macro fibres, SF, in the HFRC. The group C, with higher unit weight of SF, showed higher residual direct shear loads than group B by about 15%. Additionally, C2 containing 180 kg/m3 SF and 4.5 kg/m3 BF showed the maximum residual direct shear load, while the residual shear load of PC, B4 and C4 without SF was zero.

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Z.-X. Li et al. / Construction and Building Materials 157 (2017) 930–942 Table 3 Compressive, direct shear and splitting tensile tests results. Groups

A0 B1 B2 B3 B4 C1 C2 C3 C4 Groups

Compressive Strength of Cubic 2 f cc

3 f cc

f cc

SD

Cov

f cp

f cp

f cp

f cp

SD

Cov

66.7 74.9 77.9 69.2 66.2 70.0 73.0 71.0 61.8

63.8 72.5 76.0 66.4 63.9 69.2 77.5 67.8 59.0

61.4 73.8 79.6 73.1 61.8 74.2 75.6 65.9 57.0

63.9 73.7 77.8 69.6 64.0 71.1 75.4 68.2 59.3

2.7 1.2 1.8 3.4 2.2 2.7 2.3 2.6 2.4

0.04 0.02 0.02 0.05 0.03 0.04 0.03 0.04 0.04

44.9 50.4 50.49 48.0 44.8 48.1 52.5 49.1 43.2

42.9 51.3 52.0 49.2 43.3 50.5 48.9 46.7 42.3

47.0 53.0 54.3 50.1 45.8 52.0 51.5 50.2 44.8

44.9 51.5 52.3 49.1 44.6 50.2 51.0 48.7 43.4

2.0 1.3 1.9 1.0 1.3 1.9 1.9 1.8 1.3

0.04 0.03 0.04 0.02 0.03 0.04 0.04 0.04 0.03

1

Shear Strength of Prism

2

3

Splitting Tensile Strength

fs

fs

fs

f s

SD

Cov

f st

f st

f st

f st

SD

Cov

6.1 9.3 10.7 10.5 7.3 9.8 11.8 10.3 5.8

5.4 9.9 11.1 10.2 6.9 10.5 12.6 11.4 5.4

5.1 9.2 11.3 9.9 7.2 10.1 11.4 11.2 5.7

5.5 9.4 11.0 10.2 7.1 10.1 11.9 11.0 5.6

0.54 0.37 0.27 0.33 0.22 0.34 0.62 0.57 0.17

0.10 0.04 0.02 0.03 0.03 0.03 0.05 0.05 0.03

2.6 4.7 4.8 4.5 3.4 4.9 5.2 4.6 2.7

2.4 4.4 4.5 4.2 3.3 5.0 5.4 5.0 3.0

2.7 4.6 5.1 4.6 3.6 4.5 5.1 5.5 2.8

2.6 4.6 4.8 4.4 3.4 4.8 5.2 5.0 2.8

0.16 0.16 0.30 0.22 0.15 0.29 0.16 0.46 0.15

0.06 0.03 0.06 0.05 0.04 0.06 0.03 0.09 0.05

1

A0 B1 B2 B3 B4 C1 C2 C3 C4

Compressive Strength of Prism

1 f cc

2

3

1

2

3

75 70

B1 S120B3P1 B2 S120B3 B3 S120P1 B4 B3P1 C1 S180B4.5P1.5 C2 S180B4.5 C3 S180P1.5 C4 B4.5P1.5

65 60

56

B C

52

B1 S120B3P1 B2 S120B3 B3 S120P1 B4 B3P1 C1 S180B4.5P1.5 C2 S180B4.5 C3 S180P1.5 C4 B4.5P1.5

48 44

A0 B1/C1 B2/C2 B3/C3 B4/C4

A0 B1/C1 B2/C2 B3/C3 B4/C4

Groups (a)Variation of compressive strengths on cube

Groups (b)Variation of compressive strengths on prism

15 Shear Strenght(MPa)

B C

Compressive Strength(MPa)

80

12 9

B C

B1 S120B3P1 B2 S120B3 B3 S120P1 B4 B3P1 C1 S180B4.5P1.5 C2 S180B4.5 C3 S180P1.5 C4 B4.5P1.5

6 3

A0 B1/C1 B2/C2 B3/C3 B4/C4

Groups (c)Variation of shear strengths on prism

Splitting Tensile Strenght(MPa)

Compressive Strength(MPa)

n f cc , compressive strength of cube, and n equals 1–3. f cc , average compressive strength of cube. n f cp , compressive strength of prism, and n equals 1–3. f cp , average compressive strength of prism. n f s , direct shear strength of cube, and n equals 1–3, f s , average direct shear strength. n f st , splitting tensile strength of prism, and n equals 1–3, f st , average splitting tensile strength. SD, standard deviation. Cov, coefficient of variance.

6

B C

4

2

B1 S120B3P1 B2 S120B3 B3 S120P1 B4 B3P1 C1 S180B4.5P1.5 C2 S180B4.5 C3 S180P1.5 C4 B4.5P1.5

A0 B1/C1B2/C2B3/C3 B4/C4

Groups (d)Variation of splitting tensile strengths on prism

S120B3P1 denotes SF120 kg/m3+BF3 kg/m3+PF1 kg/m3 in the HFRC. Fig. 3. Variation of strengths in groups B and C.

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Table 4 Compressive and direct shear loads. Groups

Ultimate compressive load (kN)

Ultimate direct shear load (kN)

Residual direct shear load (kN)

Compressive toughness (up to 1 mm deflection)

Direct shear toughness (up to 3 mm deflection)

A0 B1 B2 B3 B4 C1 C2 C3 C4

448 515 523 491 446 502 510 487 434

110 188 220 204 142 202 238 220 112

0 86 92 77 0 99 106 93 0

0.98 1.16 1.22 1.20 1.04 1.20 1.22 1.20 1.00

2.16 19.25 21.48 20.52 3.94 22.86 24.65 23.98 3.41

Compressive Stress-Strain Curve Sample Energy Dissipation peak post-peak stage

f cp max 45

pre-peak stage

0.5 f cp max

30 15

εf

0 0.000

ε 0.5 f

cp max

0.002

0.004

Mpa 12 f 10 Shear Stregnth

Compressive Stregnth

Mpa 60

8 6

Shear Dflection-Strain Curve Sample s max peak Energy Dissipation post-peak stage f ε3 pre-peak stage

4

εf

2 cp max

0.006

0

0.008

0

s max

ε3

2

4

mm 6

Shear Deflection

Compressive Strain

(a)Representative curve of compress

(b) Representative curve of direct shear

A0 B1 B2 B3 B4

45 30 15 0 0.000

0.002

0.004

0.006

30 15

A0 B2 C2

45 30 15 0 0.000

0.002

0.004

0.006

Compressive Strain

0.008

60 Compressive Strenght (MPa)

Compressive Strenght (MPa)

(d)

0.002

0.004

0.006

(e)

30 15

30 15 0 0.000

0.002

0.004

0.006

0.002

0.004

0.006

0.008

Compressive Strain A0 B3 C3

45

A0 B1 C1

45

0 0.000

0.008

(c)

60

Compressive Strain

Compressive Strain 60

A0 C1 C2 C3 C4

45

0 0.000

0.008

(b)

60

Compressive Strenght (MPa)

(a)

0.008

Compressive Strain

60 Compressive Strenght (MPa)

60

Compressive Strenght (MPa)

Compressive Strenght (MPa)

Fig. 4. Representative curves of compressive and direct shear.

(f)

A0 B4 C4

45 30 15 0 0.000

0.002

0.004

0.006

0.008

Compressive Strain

Fig. 5. Compressive stress–strain curves.

4.2. Shear and compressive toughness Fig. 4 shows the representative compressive stress–strain and shear load–deflection curves. The curves are divided into two stages, i.e., the pre-peak stage and the post-peak stage. f cpmax , 0:5f cpmax , ef cpmax , and e0:5f cpmax in compression and f smax , f e3 ; ef smax , and e3 in direct shear are specified in Fig. 4 to define the areas

under the curves. The areas are used to evaluate the compressive and shear toughness, and the values of toughness are presented in Table 2. The toughness of the HFRC is a measurement of the energy dissipation ability of the fibres. Fig. 5 illustrates the average compressive Stress-Strain curves of each group. Compared with HFRC, PC had a relatively low modulus which is obtained from the slope of the curves and toughness

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Z.-X. Li et al. / Construction and Building Materials 157 (2017) 930–942

0

0

2 4 Shear Deflection (mm)

(d)

240 Shear Load (kN)

2

4

Shear Deflection (mm)

0

2 4 Shear Deflection (mm)

(e)

240

A0 B2 C2

80

0

80

0

6

160

0

160

6

80

0

2

4

80

0

2 4 Shear Deflection (mm)

(f)

240

A0 B3 C3

6

A0 B1 C1

160

0

6

160

0

(c)

240

A0 C1 C2 C3 C4

Shear Load (kN)

Shear Load (kN)

80

Shear Load (kN)

Shear Load (kN)

160

(b)

240

A0 B1 B2 B3 B4

Shear Load (kN)

(a)

240

6

A0 B4 C4

160

80

0

Shear Deflection (mm)

0

2

4

6

Shear Deflection (mm)

Fig. 6. Direct shear deflection-load curves.

which is obtained from areas under curves. It can be observed from the figures that the compressive strains ef cpmax corresponding to the maximum f cpmax in each group are very close to each other, with gaps no larger than 10%. However, the compressive strains e0:5f cpmax corresponding to 0:5f cpmax in each group are significantly different. This result indicates that the hybrid fibres in HFRC play their roles mainly at the post-peak stage in the compressive situation. The average direct shear load–deflection curves are shown in Fig. 6 and the toughness indices are shown in Table 4. Compared with PC, the shear energy dissipation ability of HFRC had an appreciable increase, especially in the HFRC containing SF. As in the case of direct shear, SF made the primary contribution to enhance the shear strength and toughness. Comparing the HFRCs group B2 and C2 containing SF and BF with the HFRCs group B3 and C3 containing SF and PF, the toughness of B2 and C2 were higher than B3 and C3 by about 5.1% and 3%, respectively. This indicates that with the same volume fraction of micro fibres, BF has a better synergistic effect with SF in shear toughness than PF does. It can be observed from Fig. 6(c)–(e) that increasing the unit weight of SF by 60 kg/m3 in group C, shear strength and shear toughness increased approximately 10% and 15%, respectively. It is noted from Fig. 6(f) that increasing in the unit weight of micro fibres BF and PF in C4 by 1.5 kg/m3 and 0.5 kg/m3 respectively led to a decrease in the shear toughness of approximately 13%, comparing with B4. This indicates that the unit weight of micro fibres within a certain range will enhance the shear behaviour of HFRC. However, if they are beyond the certain range, micro fibres do not contribute to the improvement of the shear toughness. Meanwhile, it is recommended to mix the micro fibres with the macro fibres in the HFRC simultaneously. In both the compressive and shear tests, macro cracks first appeared on the surface of the specimens at the maximum load. However, micro cracks already existed inside the concrete cementitious composites before the peak value was reached. When the load kept increasing, micro cracks expanded from inside the cement to the outside and gradually formed the macro cracks.

Micro fibres inside the cementitious composites could bridge the micro cracks and delay the expansion of the micro cracks at the pre-peak stage. At the post-peak stage, when the micro cracks expanded into macro cracks, micro fibres gradually stopped working, while macro fibres continued to work. Macro fibres could absorb the energy that was released by the macro cracks and bridge the macro cracks. SF as the macro fibre bore the majority of shear force and prevented the macro cracks from spreading on the surface. Thus, the HFRCs containing macro fibres performed much better than the HFRCs without macro fibres at the postpeak stage. From the direct shear tests results, the failure mode of the HFRCs containing macro fibres was ductile failures; meanwhile, the PC and HFRCs containing only micro fibres still failed via brittle failures. The failure mode of the HFRCs in different groups is shown in Fig. 7(a)–(f). The specimens in groups B1 and C2 containing macro fibres and micro fibres simultaneously were still via ductile failure. Brittle failure occurred for the specimens in groups A0 containing no fibres and B4 and C4 containing micro fibres only. Therefore, the ductility of HFRC depends primarily on the macro fibres, SF. Micro fibres only can impart ductility and toughness support to a small extent. This is because different types of hybrid fibres with different geometrical sizes will offer different connections or restraint conditions between the fibres and the concrete cementitious composites. This connection or restraint condition will affect the mechanical bond strength. Macro fibres with higher stiffness and larger geometrical sizes play a leading role in determining the strength and toughness. Micro fibres with lower stiffness and smaller size are more sufficiently mixed in the cementitious composites. 4.3. Four points flexure tests The results of four points flexure tests are shown in Table 5. f cr and f eq are the initial crack strength and equivalent flexure strength respectively, and f p is the peak load strength. d is the initial creak deflection of the specimen. The Xd , X3d , X5:5d and X10:5d

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Fig. 7. Different direct shear failure modes of specimens in different groups.

Table 5 Four points flexure tests results. Groups

A0 B1 B2 B3 B4 C1 C2 C3 C4

Strength

Max Load (kN)

f cr

f eq

fp

5.53 5.86 5.99 5.78 5.64 6.36 6.54 6.13 5.82

/ 4.76 4.49 3.96 / 5.78 5.99 5.40 /

6.18 7.05 7.38 7.02 6.57 7.53 8.79 7.77 6.93

20.6 23.5 24.6 23.4 21.9 25.1 29.3 25.9 23.1

Residual Load (kN)

0 5.57 6.9 4.87 0 7.1 7.83 6.78 0

are calculated according to the areas under deflection-load curves and d is the initial crack deflection of the specimen. 3d, 5.5d and 10.5d represent 3, 5.5 and 10.5 times of the d respectively. I5 , I10 , and I20 are calculated by X3d /Xd , X5:5d /Xd and X10:5d /Xd respectively. The flexure strength of PC was 6.18 MPa and the flexure strengths of HFRCs varied between 6.93 and 8.79 MPa, which were improved by 12.1% and 42.2% respectively compared with PC. The HFRCs specimens with higher unit weight of SF possessed the higher flexure strength generally. As expected, different types of micro fibres had different influences on the HFRCs. The HFRCs with BF fibres exhibited higher strength and toughness than the HFRCs with PF fibres. The toughness indices I5 , I10 , and I20 of group C2 were close

Flexure Index f e /f cr

0 0.81 0.75 0.69 0 0.91 0.92 0.88 0

Toughness (N  m)

Toughness index

Xd

X3d

X5:5d

X10:5d

I5

I10

I20

3.5 4.3 4.3 3.8 3.9 4.4 4.6 4.3 4.0

0 18.1 17.1 14.5 0 20.7 22.8 17.9 0

0 35.2 31.3 26.9 0 40.2 46.7 34.3 0

0 55.9 46.8 39.7 0 60.7 79.9 54.0 0

0 4.2 4.0 3.9 0 4.7 4.9 4.2 0

0 8.2 7.3 7.1 0 9.1 10.1 8.1 0

0 13.1 10.8 10.5 0 13.7 17.3 12.7 0

to the referenced toughness indices (ASTM C1018 1997) of ideal elastic–plastic materials, which are 5, 10 and 20 respectively. Furthermore, the toughness indices and residual loads of group A0, B4 and C4 were zero. This result indicates that the HFRCs without SF could not resist the macro cracks spreading after peak load that resulted in the failure of the specimens. The four points flexure response can be inferred according to the uniaxial tensile response [30]. Fig. 8 shows the average curves calculated from three specimens. The deflection-hardening response was observed in group B2 and C2 containing SF and BF, and the deflection-softening response was observed in B3 and C3 containing SF and PF.

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30

20

10

0

0

1 2 3 4 Mid-span Deflection(mm)

20

10

0

5

0

Group A0 30

10

2 4 6 Mid-span Deflection(mm)

20

0

8

0

10

2 4 6 Mid-span Deflection(mm)

Average Area of Group C1 Average Curve of Group C1

10

0

2 4 6 Mid-span Deflection(mm)

Group C2

30

Average Area of Group C3 Average Curve of Group C3

10

0

0

8

Group C1

20

8

8

20

0

5

Load(kN)

Load(kN)

Load(kN)

1 2 3 4 Mid-span Deflection(mm)

30

Average Area of Group C2 Average Curve of Group C2

0

2 4 6 Mid-span Deflection(mm)

Group B4

20

0

0

30

Average Area of Group B4 Average Curve of Group B4

Group B3

30

10

Group B2

10

0

20

0

8

Load(kN)

Average Area of Group B3 Average Curve of Group B3

20

0

2 4 6 Mid-span Deflection(mm)

Average Area of Group B2 Average Curve of Group B2

Group B1

Load(kN)

Load(kN)

30

30

Average Area of Group B1 Average Curve of Group B1 Load(kN)

Average Curve of GroupA0

Load(kN)

Load(kN)

30

2 4 6 Mid-span Deflection(mm)

8

Average Area of Group C4 Average Curve of Group C4

20

10

0

0

2 4 6 Mid-span Deflection(mm)

Group C3

8

Group C4

Fig. 8. The deflection-load curves of four points flexure tests.

Table 6 Uniaxial tensile tests results. Groups

ft

STDEV

f st

STDEV

W0:05 (N  m)

W0:2 (N  m)

Re0:2 ¼ f

A0 B1 B2 B3 B4 C1 C2 C3 C4

2.31 3.81 4.13 3.79 2.47 3.97 4.33 3.91 2.52

0.08 0.10 0.20 0.18 0.12 0.08 0.11 0.24 0.09

2.6 4.6 4.8 4.4 3.4 4.8 5.2 5.0 2.8

0.16 0.16 0.30 0.22 0.15 0.29 0.16 0.46 0.15

1.09 2.47 2.27 2.27 1.15 3.08 3.27 2.85 1.17

/ 6.16 6.91 5.56 / 9.06 9.68 8.29 /

/ 0.404 0.418 0.367 / 0.571 0.559 0.530 /

4.4. Uniaxial tensile tests The results of uniaxial tensile tests are shown in Table 6. The uniaxial tensile strength of PC A0 was 2.31 MPa, and the highest tensile strength of HFRC was 4.33 MPa, which was improved by 87.4% compared with PC. The maximum uniaxial tensile strength of the other HFRCs containing SF was improved by 78.8% compared with PC. Compared with PC, the uniaxial tensile strength of HFRCs

W 0:2

t AL0 0:2%

containing BF and PF was only improved slightly. The uniaxial tensile strength of group C4 was slightly higher than that of group B4 by about 2% illustrating that increasing the volume fraction of micro fibres did not improve the tensile strength significantly. On the other hand, groups C1, C2 and C3 with higher unit weight of SF had an obvious higher tensile strength than those of groups B1, B2 and B3. This phenomenon indicates that the unit weight of SF has a significantly influence on uniaxial tensile strength.

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Stress(MPa)

4

C2 C1

3

C3 B2

2 A0 B1 B4

1 0 0.000

are known, the direct shear strength and compressive strength of prisms can be estimated and calculated by Eq. (4). The error is within ±10%.

A0 B1 B2 B3 B4 C1 C2 C3 C4

5

4.6. Overall effect of fibres to the mechanical properties of HFRC

B3

C4

0.001

0.002

0.003

Strain Fig. 9. Uniaxial tensile stress–strain curves.

Fig. 9 shows the stress–strain curves of specimens in uniaxial tensile tests. This figure shows that the elastic stage of the specimens in uniaxial tensile tests was identical. However, the plastic stage after peak load was various among each other. Specifically, the strain capacity of the specimens in groups C1, C2 and C3 with higher unit weight of SF was found to be larger than that of groups B1, B2 and B3. The strain capacity is measured by the areas under stress–strain curves, which are shown by W0:05 and W0:2 in Table 6. The group C2 with 180 kg/m3 SF + 4.5 kg/m3 BF had the strongest tensile strength and the highest strain capacity. There were multiple cracks existing in the HFRCs except the specimens in group B4 and C4 without SF. The test setup and parts of specimens under uniaxial tensile are shown in Fig. 10. 4.5. Correlations among different aspects of strength Fig. 11(a) and (b) show the correlations between different aspects of strength. According to the test data, Eq. (4) is obtained by nonlinear regression,

y ¼ 1=ða  b 

pffiffiffi xÞ

Fig. 12 describes the overall evaluation of the mechanical properties of HFRC. These figures show every aspect of mechanical properties of HFRC and can guide engineers to choose the most appropriate mixture proportion according to different situations in practical engineering. The values shown in the figures are the strengths and toughness indices of HFRC normalized by the indices of PC. It is clear that in the aspect of cubic and prism compressive strength, the variation range of the ratios is 0.95–1.22. Meanwhile in the aspect of direct shear strength and splitting tensile strength, the variation range of ratios is 1.02–2.16 and 1.07–2.0, respectively. Furthermore, in the aspect of four points flexure strength and uniaxial tensile strength, the variation range of ratios is 1.06–1.42 and 1.07–1.87, respectively. The values indicate that fibres used in this study improve the strengths significantly, especially in the aspect of shear strength, splitting tensile strength, flexure strength and uniaxial tensile strength which are related to the material failure mechanism of tensile. It can be observed from Fig. 12(c) and (d) that in the aspect of compressive and direct shear toughness, the variation range of the ratios is 1.06–1.25 and 1.58–11.56, respectively. In the aspect of four points flexure and uniaxial tensile toughness, the variation range of the ratios is 1.14–22.8 and 1.06–8.88, respectively. The toughness indices are improved significantly in shear, flexure and tensile tests which indicate that the energy dissipation capacity been improved significantly because of the mixed fibres. From these figures in Fig. 12, it is obvious that group C2 mixed with SF and BF has the highest mechanical strengths and the best energy dissipation ability at the same time. This result indicates that with the same volume fraction of BF and PF, BF has a better synergistic effect with SF in strength and toughness. It is useful to employ the results in designing the shear keys in an immersed tunnel, e.g., if the tunnel is built in an earthquake zone, SF and BF are the best options for the shear keys because the released energy by the earthquake needed to be dissipated is considerable.

ð4Þ

The correlation coefficient R of compressive strength on cubes versus compressive strength of prisms and splitting tensile strength versus direct shear strength are 0.967 and 0.973, respectively. It is obvious that in the mechanical properties of HFRC, if the splitting tensile strength and compressive strength of cubes

5. Conclusions This paper describes the HFRC which was made by 42.5 Portland cement and different types of fibres with a density of 2280– 2466 kg/m3, a shear strength of 5.5  11.9 MPa, a uniaxial com-

Fig. 10. Specimens under uniaxial tensile.

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54

Tests Values Calculated Values

51 48 45 42

y=1/(0.051-0.0036x0.5) R=0.967 60 65 70 75 80 Compressive strength on cubic(MPa) (a) Correlation between compressive strength on cubes and prisms

Direct shear strength(MPa)

Compressive strength on prism(MPa)

Z.-X. Li et al. / Construction and Building Materials 157 (2017) 930–942

Tests Values Calculated Values y=1/(0.39-0.134x0.5) R=0.973

12 10 8 6

3 4 5 Splitting tensile strength(MPa) (b) Correlation between splitting tensile strength and direct shear strength

Fig. 11. Correlations among different aspects of strength.

Compress on cubic 2.0

Uniaxial tensile 20

1.5

Splitting tensile

1.0

A0 B1 B2 B3 B4

15

Compress on prism

10 5

0.5

A0 B1 B2 B3 B4

0

0.0

Direct shear

-5

Four points flexure

Uniaxial tensile

Four points flexure

Uniaxial Compress

Direct Shear

(a)Strengths of group B compared to A0

(c)Toughness of group B compared to A0

Compress on cubic 2.5 2.0

Splitting tensile

1.5

A0 C1 C2 C3 C4

1.0

Compress on prism

0.5

Direct shear

Uniaxial tensile 25 20 15 10 5 0 -5

A0 C1 C2 C3 C4

Four points flexure

0.0

Uniaxial tensile

Four points flexure

Uniaxial Compress Direct Shear

(b) Strengths of group C compared to A0

(d)Toughness of group C compared to A0

Fig. 12. Overall assessment of mechanical properties of HFRC.

pressive strength of 43.4  52.3 MPa, a uniaxial tensile strength of 2.5  4.3 MPa, a four points flexure strength of 6.6  8.8 MPa, and a splitting tensile strength of 2.6  5.2 MPa. The mechanical properties of the HFRC were studied through experimental research and main conclusions are as follows:

(1) SF can improve the strength and toughness of HFRCs significantly, especially in shear, tensile and flexure. Nevertheless, the compressive strength and toughness were improved slightly only with the appropriate unit weight of 120 kg/m3. BF and PF could improve the pull-out strength of the macro

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fibre and bridge the micro cracks developed inside the concrete cementitious composites, thus, improving the toughness of the HFRC. A better synergistic effect was obtained when mixed with BF and SF simultaneously than mixed with PF and SF. (2) The shear strength and toughness of HFRC would be improved about 10% by increasing the unit weight of SF from 120 kg/m3 to 180 kg/m3. However, the micro fibres themselves barely improved the mechanical properties. Increasing the unit weight of micro fibres in C4 compared with B4 would lead to a decrease in the shear behaviour of HFRC. Thus, the unit weight of the micro fibres should be controlled within a certain range. The recommended unit weights of micro fibres BF and PF are 4.5 kg/m3 and 1 kg/m3, respectively. (3) Keeping the unit weight of SF and BF at 180 kg/m3 and 4.5 kg/m3 in C2, respectively, the hybrid fibres could offer the strongest shear strength, uniaxial tensile strength, four points flexure strength, and splitting tensile strength and the best energy dissipation ability. According to different requirements of different engineering projects, different proportions of fibres in the HFRC can be chosen. (4) It is obvious that there is a certain correlations existing between the mechanical properties of HFRC, splitting tensile strength versus direct shear strength and compressive strength on cubes versus compressive strength on prisms.

Acknowledgements The authors gratefully acknowledge the financial supports for this research by the National Natural Science Foundation of China (NSFC) under grant number 51427901, 51238007, 51621092 and 51408410, the National Key R & D Program of China under grant number 2016YFC0701100 and 2016YFC0701105, and the Natural Science Foundation of Tianjin, China under grant number 15JCQNJC07200.

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