Bond behavior of deformed bar embedded in Engineered Cementitious Composites under cyclic loading

Construction and Building Materials 197 (2019) 164–174

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Bond behavior of deformed bar embedded in Engineered Cementitious Composites under cyclic loading Mingke Deng, Jiaojiao Pan ⇑, Hongzhe Sun School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China

h i g h l i g h t s
Bond behavior of deformed bar in ECC under cyclic loading were investigated.
Effects of various parameters on energy dissipation capacity were evaluated.
Degradation laws of bond strength under cyclic loading were discussed.
A calculating equation of bond strength was proposed.

a r t i c l e

i n f o

Article history: Received 25 September 2018 Received in revised form 15 November 2018 Accepted 23 November 2018

Keywords: Engineered Cementitious Composites Bond strength Cyclic loading Degradation Energy dissipation capacity

a b s t r a c t The bond behavior between deformed bar and ECC has a significant effect on the mechanical performance of reinforced ECC structures when subjected to dynamic loading. In this paper, 12 groups of ECC specimens and one group of concrete specimen as control were prepared to investigate the bond behavior of deformed bar under cyclic loading. Designed parameters included compressive strength, flexural toughness, cover thickness and anchorage length. Experimental results showed that, owing to the fiber bridging effect preventing the opening and propagation of internal cracks, failure modes of ECC specimens demonstrated an obvious ductility whereas the brittle splitting failure occurred for concrete specimen. The bond strength of steel bar in ECC had an extreme enhancement as the compressive strength of matrix increased. In comparison with the monotonic specimen, the cyclic specimen undergoes an obvious degradation in terms of bond strength as well as stiffness, and the degradation coefficient of bond strength ranged from 0.6 to 0.9. It was noticed that the increase in compressive strength and cover thickness can reduce the degradation degree of bond strength. Furthermore, the energy dissipation capacity of cyclic specimens rose first and then fell since the toughness of matrix increased. Finally, a calculating equation of bond strength under cyclic loading was proposed and the predicted results had a great agreement with the test results. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Engineered Cementitious Composites (ECC) [1,2] is a class of High Performance Fiber Reinforced Cementitious Composites (HPFRCC) [3] and consists mainly of cement, fine aggregates, fibers and admixture, which shows remarkable properties in terms of high strength, high ductility and durability [4]. This material also is characterized by an excellent strain hardening behavior with multiple fine cracks when subjected to tensile loading [5,6]. Due to the outstanding deformation capacity and damage tolerance, ECC has been widely utilized in engineering structures [7,8]. Results indicated that the reinforced ECC members exhibited the ⇑ Corresponding author. E-mail address: [email protected] (J. Pan). https://doi.org/10.1016/j.conbuildmat.2018.11.200 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

improved structural performances in terms of deformability, ductility and energy dissipation capacity [9,10]. Besides, the brittle failure mode of shear-critical components such as beam-column joints, coupling beams as well as low-rise walls also was changed into ductile shear failure [11]. To understand the interaction between ECC and steel bar, the tension stiffening experiments of reinforced ECC specimens were conducted. Results showed that the good strain compatibility between ECC and steel reinforcement was achieved and excellent bond behavior in ductile materials caused smaller splitting crack than reinforced concrete specimens [12,13]. In recent years, the bond behavior between deformed steel bar and ECC under monotonic loading has been researched. By pullout tests comprising various types of fibers, Chao [14] observed that the confinement and bridging effects provided by fibers after cracking supplied

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M. Deng et al. / Construction and Building Materials 197 (2019) 164–174 Table 1 Mixture proportions of ECC by weight. Mixture

A1 A2 A3 B1 B2 B3

Binder cement

Fly ash

Mineral powder

Silica fume

45% 50% 55% 40% 40% 40%

55% 30% 20% 55% 55% 55%

– 20% 20% – – –

– – 5% 5% 5% 5%

post-cracking tensile capacity and effectively limited crack width, thereby leading to improved bond behavior. Based on the fourpoint bending experiment of beam specimens with lap splices, Bandelt [15] found that the multiple cracking occurred at splice region and the bond-slip behavior of steel reinforcement in HPFRCC exhibited a higher toughness. Li [16] investigated the bond performance of steel bar in HPFRCC before and after exposure to elevated temperatures. Test results proved that the bond strength decreased as the heating temperature increased and was reduced further when the heated specimens were cooled in water instead of air. Deng [17] concluded that increasing the fiber volume content was effective to lower the width of splitting crack on the surface of specimens. Toshiyuki [18] noted that the cover thickness and bar spacing of the main bars in ECC elements was reduced due to the enhanced bond performance. Lee [19] and Choi [20] evaluated the effect of anchorage length and put forward to the design suggestion of anchorage length of steel reinforcement in ECC members. Due to the application of ECC material in seismic structures, a fundamental understanding of bond behavior under earthquake loading is essential to structural design and finite element analysis. Existing researches showed that cyclic loading would destroy the concrete keys between rebar lugs as a result of the extruding deformation of matrix and the accumulation of cracks and therefore bond strength deteriorated [14,21]. However, effects of mechanical properties of ECC materials on deterioration law of bond slip behavior have not been evaluated and a suitable calculating method of bond strength for steel bar in ECC under cyclic loading is not proposed yet. The main objective of this research is to investigate the bond behavior of deformed bar in ECC under cyclic loading by testing 12 groups of ECC specimens and one group of concrete specimen. Design parameters contain compressive strength and flexural toughness of ECC material, cover thickness and anchorage length. Furthermore, effects of parameters on bond strength, deterioration law of bond strength and energy dissipation capacity are discussed. Finally, a calculating model was proposed to predict the bond strength under cyclic loading, which had an important influence on structural design and finite element analysis of reinforced ECC structures under earthquake actions. 2. Experimental program 2.1. Material properties To investigate the effects of mechanical properties of materials on the bond behavior, six mixtures ECC and a normal concrete were prepared in this research. The Ordinary Portland Cement (P
O 42.5R), Class F fly-ash, mineral powder, silica fume, fine river sand with a 1.18 mm maximum aggregate size, high range water reducing admixture (HRWR) and fibers were used in ECC. The proportions of ECC mixture are shown in Table 1. The series A represents the different compressive strength grades of ECC and the series B represents the different flexural toughness grades of ECC.

Water binder ratio

Sand binder ratio

Fibers/% vol PVA

PE

0.31 0.27 0.24 0.29 0.29 0.29

0.36 0.36 0.36 0.36 0.36 0.36

1.5 1.5 1.5 1.5 – –

– – – – 1.5 2.0

Table 2 Performance indicators of fiber. Fiber

Length (mm)

Diameter (mm)

Tension strength (MPa)

Elastic modulus (GPa)

Density (g/cm3)

PVA PE

12 12

35 43

1500 1900

36 39

1.29 0.97

Table 3 Mechanical properties of ECC and concrete. Mixture

fcu (MPa)

T (MPa)

ft (MPa)

A1 A2 A3 B1 B2 B3 N1

51.7 71.0 81.3 51.9 51.8 51.4 63.9

33.4 26.8 28.7 34.3 105.5 250.8 –

4.1 3.7 5.1 3.9 3.1 3.7 –

fcu is compressive strength; T is flexural toughness; ft is tensile strength.

The performance indicators of polyvinyl alcohol (PVA) fiber and polyethylene (PE) fiber are listed in Table 2. The normal concrete is represented by the letter N and the mixture proportion of concrete by weight was cement:fly ash:sand:coarse aggregate:water = 1.00:0.25:1.75:2.70:0.45. Mechanical properties of ECC and normal concrete are given in Table 3. The average compressive strength for each mixture was obtained by testing three cube specimens having a size of 100 mm at the age of 56 days. The flexural behavior of each mixture was investigated by testing three beams, which had a 40 mm square cross section with a span length of 150 mm and were loaded in four-point bending. The flexural toughness T of ECC materials was calculated according to the Eq. (1). As seen in Table 3, the flexural toughness of B2 is 3.1 times higher than that of B1. It also was noticed that the flexural toughness was increased by 2.4 times as the PE fiber volume content increased from 1.5% to 2%.

T¼

Xu 2

bh

ð1Þ

where Xu is the area of the load-deflection curve when the load decreases to 85 percent of peak load (see Fig. 1). b and h are the width and height of cross section of beam specimen, respectively. The dog-bone specimens with a tested cross section of 15 mm  50 mm and a length of 150 mm were designed to evaluate the tensile properties of ECC. Uniaxial tension tests are carried out and the typical tensile stress-strain curves are given in Fig. 2. The specimen B3 with 2% PE fiber volume content showed a superior tensile strain-hardening response up to 2.9% strain and had a maximum tensile stress of 3.7 MPa. As the fiber volume fraction decreased to 1.5%, specimen B2 showed a reduction in deformability and tensile strength. The specimen B1 with 1.5% volume content of PVA fibers had a peak tensile stress of 3.9 MPa and an

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bond length la

PVC tube

d

c

F(N)

c

rebar 100

Ωu

120

150

Fig. 3. Details of pullout specimens (mm).

Table 4 Specimen design.

0

(mm)

0.85 Fig. 1. Flexural toughness calculation.

Tensile stress (MPa)

5 4

Code

Matrix

Section size (mm)

c/d

la/d

dA1 dA2 dA3 dB1 dB2 dB3 dD1 dD2 dD3 dE1 dE2 dE3 dN1

A1 A2 A3 B1 B2 B3 A1 A1 A1 A1 A1 A1 N1

120  120 120  120 120  120 120  120 120  120 120  120 100  100 80  80 60  60 120  120 120  120 120  120 120  120

3.3 3.3 3.3 3.3 3.3 3.3 2.5 1.9 1.4 3.3 3.3 3.3 3.3

5 5 5 5 5 5 5 5 5 3 4 7 5

c is the cover thickness; d is the bar diameter, la is the anchorage length.

3 2

B1 B2 B3

1 0 0

1

2

3

4

Strain (%) Fig. 2. Typical tensile stress-strain curves.

ultimate tensile strain of 1.0%. Similar tensile strain ability was observed in specimens A1, A2 and A3. To avoid the yield failure of rebar before pullout, the high strength steel bar with a diameter of 16 mm was used in the tested specimens. The tensile yield strength and ultimate strength for steel bars were 712 MPa and 900 MPa, respectively. 2.2. Test specimens Pullout specimens are adopted in this paper to investigate the interfacial bond performance between reinforcing bar and matrix as a result of their simple fabrication and test [22,23]. The details of pullout specimens are illustrated in Fig. 3. A single steel bar was embedded centrically in a concrete prism that had a length of 120 mm and a varying cube cross section for the different cover thicknesses. It should be pointed out that the stress fields in pullout specimen don’t accurately simulate bond conditions in actual structures. The compression force exerted on the matrix induced by the bearing plate would create a favorable stress condition in the rebar perimeter and increase the bond strength. Contact between steel reinforcement and the surrounding matrix at the unbonded region was broken by sheathing deformed bar in PVC tubes. In this way, the required anchorage length was obtained

and the local stress concentration at the end of specimen was avoided. 12 groups of ECC specimens and one group of concrete specimen were designed in this paper. Each group contained six identical specimens, three for monotonic loading and others for cyclic loading. As shown in Table 4, the dN1 was prepared as the control specimen. The series dA, dB, dD and dE were designed to discuss the influence of compressive strength, flexural toughness, cover thickness and anchorage length on the bond behavior of steel bar in ECC. All specimens were cast in customized plywood molds and demoulded after 24 h. They were covered with wet cloth for 7 days and then cured outdoors until tested. 2.3. Test procedure Fig. 4 gives the schematic diagram of test setup. The specimens were loaded using a specially fabricated mild steel frame rigidly connected to the test machine. The load was applied by a MTS testing machine with the capacity of 250 kN. For monotonic specimens, the load was applied by displacement control at a rate of 1 mm/min. For the cyclic loading, all specimens were tested using an identical loading scheme as shown in Fig. 5. The loading rate was 1 mm/min at the initial loading stage and was changed into 2 mm/min after the control displacement increased to 3 mm. The relative slip between steel bar and matrix was measured by installing two linear variable differential transducers (LVDTs) at the free end. The loading and slippage were monitored by an automatic data acquisition system and the test was stopped when a slippage of at least 10 mm was recorded. 3. Experimental results and discussion 3.1. Failure modes Fig. 6 shows the failure mode of normal concrete specimen and the interface between rebar and matrix. As seen in Fig. 6(a), when

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M. Deng et al. / Construction and Building Materials 197 (2019) 164–174

8

P

7

6

5.5

loading cell

spherical LVDTs fixture

Displacement (MPa)

4.5

4 2 0

0.25

0.75 0.5

-2

1.25

1

1.75

1.5

2.25

2

2.75

2.5

-4

3.5

3 4 5

-6

1mm/min

2mm/min

6

-8 0

2

4

6

8

10

12

14

16

18

20

No. of Cycle Fig. 5. Cyclic loading scheme.

specimen

steel frame

rebar

P Fig.4. Schematic diagram of test setup.

internal cracks extend to the surface of specimen, the concrete splits into two or three prisms and the brittle splitting failure occurs. From Fig. 6(b) and (c), it also can be found that the concrete keys between rebar lugs remain intact and no crushed matrix is observed. This phenomenon suggested that the splitting failure of concrete specimens was caused by the inadequate tensile strength of matrix. Due to fiber bridging effect preventing the opening and propagation of cracks, the ECC specimens still maintained intact after failure and showed outstanding damage tolerance. Two ductile failure modes were observed for ECC specimens: pullout failure and splitting-pullout failure. Pullout failure means that no crack is observed on the surface of specimens (Fig. 7(a)). Splittingpullout failure means that the internal cracks extend to the surface of specimens when the steel bar is pulled out (Fig. 7(b) and (c)). Besides, it was worth noticing that multiple fine cracks were observed only for specimen dB2 and dB3 whereas the number of splitting cracks of dB1 and other specimens failed in splittingpullout was not more than four. This phenomenon indicated that

Fig. 6. Failure mode of concrete specimen: (a) splitting failure; (b) interface of rebar; (c) interface of matrix.

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M. Deng et al. / Construction and Building Materials 197 (2019) 164–174

Fig. 7. Failure modes of ECC specimens: (a) pullout failure; (b) splitting-pullout failure; (c) splitting-pullout failure with multiple fine cracks; (d) interface of rebar; (e) interface of matrix.

Table 5 Test results. Code

sm u (MPa)

SD (MPa)

sþu (MPa)

su (MPa)

su (MPa)

SD (MPa)

k

su;pre (MPa)

Failure mode

dA1 dA2 dA3 dB1 dB2 dB3 dD1 dD2 dD3 dE1 dE2 dE3 dN1

6.879 17.018 28.231 8.713 11.976 10.267 7.402 5.922 4.860 7.063 5.805 11.368 19.034

1.214 0.563 2.246 1.585 1.213 1.511 0.053 0.598 0.948 1.226 0.171 0.957 2.048

4.188 9.715 20.680 7.348 9.480 8.388 4.064 3.429 3.626 5.401 4.729 6.296 18.845

4.989 16.427 21.686 8.431 12.084 9.683 5.290 3.716 5.085 5.797 5.392 8.034 18.002

4.589 13.071 21.183 7.889 10.782 9.036 4.677 3.572 2.921 5.599 5.060 7.165 18.423

0.685 1.501 1.642 1.339 1.431 1.517 1.400 1.003 0.375 1.320 0.247 0.321 2.438

0.667 0.768 0.750 0.905 0.900 0.880 0.632 0.603 0.601 0.793 0.872 0.630 –

5.2 11.4 16.8 5.4 8.4 6.2 5.2 4.3 3.5 6.7 5.8 5.2 –

P SP SP SP SP SP P P SP P P SP S

sm u is the bond strength under monotonic loading. sþu andsu are the positive and negative bond stress peak under cyclic loading, respectively. su is the bond strength under cyclic loading (su ¼ ðsþu þ su Þ=2). k is the degradation coefficient of bond strength (k ¼ su =sm u ). su;pre is the predicted bond strength. SD is the standard deviation. S represents the splitting failure; P represents the pullout failure; SP represents the splitting-pullout failure.

the number of splitting cracks was in proportion to the toughness of material when the splitting-pullout failure occurred. The ECC specimens were cut in half after test in order to observe the contact interface between steel bar and matrix. From Fig. 7(d) and (e), it can be seen that the ECC keys between steel ribs are sheared off and the steel bar is pulled out with the crushed ECC powder. This observation indicated that the failure of ECC specimens was related to the shear strength of matrix. Failure types of all specimens are summarized in Table 5.

3.2. Bond stress-slip behavior The distribution of bond stress along the embedded length was assumed to be uniform since the anchorage length was relatively small. Then the average bond stress was calculated by

s¼

P

pdla

ð2Þ

where s is the average bond stress (MPa), P is the applied load (N).

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M. Deng et al. / Construction and Building Materials 197 (2019) 164–174

demonstrate an obvious degradation in strength and stiffness when compared with monotonic specimens. It was caused by the degradation in bond performance and energy dissipation capacity of the interface between steel bar and matrix. On the other hand, the bond strength and bond stiffness for ECC specimens under cyclic loading also severely degraded as the cyclic number increased after the peak point. The larger the controlling displacement was,

Fig. 8 depicts the typical bond stress-slip curves of cyclic specimens and monotonic specimens. The control specimen dN1 failed after only a few cycles and showed a poor hysteretic response. In contrast, the bond stress-slip hysteretic curve of ECC specimens was characterized by more numbers and larger area of hysteretic loops, which exhibited great ductility and energy dissipation capacity. From Fig. 8, it is observed that the cyclic specimens

30

30

dA1

30

dA2

24

18

12

12

12

0 -6 -12 -18

cyclic loading monotonic loading

-24 -30 -10

-8

-6

-4

-2

0

2

4

6

8

Bond stress (MPa)

18

6

6 0 -6 -12 -18

cyclic loading monotonic loading

-24 -30 -10

10

-8

-6

-4

Slip (mm)

4

6

8

-12 -18

-30 -10

10

6

6

6

3 0 -3 -6 -9

Bond stress (MPa)

9

3 0 -3 -6 -9

cyclic loading monotonic loading -8

-6

-4

-2

0

2

4

6

8

-12 -15 -10

10

-8

-6

-4

-2

0

2

4

6

8

-15 -10

10

Bond stress (MPa)

Bond stress (MPa)

3 0 -3 -6 -9

-6

-4

-2

0

2

4

6

8

-15 -10

10

-8

-6

-4

Slip (mm)

-2

0

2

4

6

8

-15 -10

10

dE2

Bond stress (MPa)

Bond stress (MPa)

3 0 -3 -6 -9

-4

-2

0

2

-8

-6

-4

4

6

8

-15 -10

10

-8

-6

-4

Slip (mm)

0

2

4

6

8

10

dE3

3 0 -3 -6

-2

0

2

4

6

8

cyclic loading monotonic loading

-12 10

-15 -10

-8

-6

Slip (mm)

dN1

24 18 12 6 0 -6 -12 -18

cyclic loading monotonic loading

-24 -8

-6

-4

-2

0

2

-4

-2

0

2

Slip (mm)

30

-30 -10

-2

-9

cyclic loading monotonic loading

-12

Bond stress (MPa)

-6

cyclic loading monotonic loading

12

6

cyclic loading monotonic loading

10

Slip (mm)

12

-9

8

15

6

-6

6

-6

6

-3

4

0

9

0

2

-3

9

3

0

dD3

Slip (mm) dE1

-2

3

9

-8

-4

-12

15

12

-15 -10

-6

-9

cyclic loading monotonic loading

-12

15

-12

-8

12

6

-8

cyclic loading monotonic loading

15

dD2

6

cyclic loading monotonic loading

10

-6

6

-9

8

0

9

-6

6

-3

9

-3

4

3

9

0

2

Slip (mm)

12

3

0

-12

15

dD1

12

-2

dB3

Slip (mm)

15

-15 -10

-4

-9

cyclic loading monotonic loading

Slip (mm)

-12

-6

12

9

-15 -10

-8

15

dB2

9

-12

cyclic loading monotonic loading

Slip (mm)

12

Bond stress (MPa)

Bond stress (MPa)

2

0 -6

-24

15

dB1

12

Bond stress (MPa)

0

6

Slip (mm)

15

Bond stress (MPa)

-2

dA3

24

18

Bond stress (MPa)

Bond stress (MPa)

24

4

6

8

10

Slip (mm)

Fig. 8. Bond stress-slip curves of specimens under monotonic and cyclic loading.

4

6

8

10

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M. Deng et al. / Construction and Building Materials 197 (2019) 164–174

the quicker the degradation speed was. This could be briefly interpreted that the high bearing force exerted on the matrix induced by steel lugs leaded to the formation of inclined cracks and the matrix crushing [14]. The simplified diagram extracted from the tested curves is depicted in Fig. 9(a), which is thought to be suitable to reflect the actual bond stress-slip relationship of pullout specimens at a given loading cycle. The displacement of rebar rib and the opening and closure of internal inclined cracks during the loading process are shown in Fig. 9(b). As seen in Fig. 9, a whole cyclic process includes six stages as follows.

B

G F

A D

C

3.2.2. Descending stage (unloading stage) BC in the positive direction The applied load dropped immediately once the control displacement was reached. When the bond stress was unloaded to zero (point C), the inclined cracks were closed and the elastic deformation of matrix vanished entirely. The left interval had a slight reduction but still existed because of the unrecoverable plastic deformation.

s

E

3.2.3. Horizontal stage (frictional stage) CD Subsequently, the load was applied in the opposite direction and the rebar started to move to the left. At the horizontal stage, the slip resistance was quite weak and mainly governed by the friction action between steel reinforcement and matrix. The slip in the positive direction decreased sharply with the increase in reversed load. In contrast, the bond stress had a stagnated period and the bond stiffness was close to zero. At the point D, the rib of reinforcing bar contacted with the matrix again in the opposite direction. At the same time, an interval was observed at the right of steel rib because of the considerable unrecoverable residual slip caused by the inelastic deformation and local damage of matrix.

(a) A

Pull p

e

p

e

B

P+ =P+max interval

p

e

C

3.2.1. Ascending stage AB in the positive direction de and dp are the elastic deformation and plastic deformation respectively during the loading process. At the initial loading stage, nearly by the point A, the bond force was dependent on the chemical adhesion between steel bar and cement gels. However, this bond ingredient was rather small and failed completely once the slip at the free end occurred. With the further increase in pullout force, the friction and mechanical interlocking provided the major bond resistance. The compressive force induced by mechanical interlocking would lead to initial inclined cracks gradually formed in matrix. In this case, the fiber bridging was effective in preventing the cracks from widening and thus the bearing force was redistributed to the whole matrix. As a result, the pullout load continually rose which was accompanied by multiple fine cracks appeared. When the displacement reached to the controlling value, i.e. the peak point B, an interval was found at the left of rebar rib (Fig. 9(b)). It was induced by the reason that the move of rebar to the right leaded to the separation of rib and matrix at the left.

+

Unload P =0 interval

p

e

D

Push interval

E

P - =Pmax e p

interval

F

-

Unload P =0 e

interval

G

Pull interval

(b) Fig. 9. Bond behavior at a given cycle. (a) Simplified diagram of bond stress-slip curve; (b) displacement of rebar rib.

3.2.4. Ascending stage DE in the negative direction When the slip decreased to zero, the steel rib and matrix were fully in contact again. Then the bond stress and stiffness went up quickly. The bond response at the ascending stage in the negative direction was quite similar to that in the positive direction. An internal inclined crack finally formed at the top of rib and a larger interval appeared at the right of rebar lug. 3.2.5. Descending stage EF in the negative direction When the reversed load was removed, bond stress had a dramatic decrease accompanied by the closure of cracks. However, the residual slip could not be restored completely. This was attributed to the inelastic deformation and the local damage caused by the matrix crushing. At the point F, the positive loading was applied again. 3.2.6. Horizontal stage FG in the opposite direction At this stage, the bond stress remained stable and bond stiffness was rather small, similar to the response of stage CD. The slip increased rapidly after the applied load exceeded the frictional resistance. With this, a reverse horizontal stage of bond stressslip curve formed. At the point G, steel lug and matrix were fully in contact once more in the pullout direction and so far a full loading cycle finished.

171

30

30

25

25

Bond strength (MPa)

Bond strength (MPa)

M. Deng et al. / Construction and Building Materials 197 (2019) 164–174

20 15 10 5

20 15 10 5 0

0 50

60

70

80

0

90

50

100

Compressive strength (MPa)

(a)

250

300

(b) 25

Bond strength (MPa)

25

Bond strength (MPa)

200

30

30

20 15 10

20 15 10 5

5 0 1.0

150

Toughness (MPa)

0 1.5

2.0

2.5

3.0

3.5

3

4

5

(c)

6

7

la/d

c/d

(d)

Fig. 10. Effects of parameters on the bond strength. (a) Compressive strength; (b) toughness; (c) cover thickness; (d) anchorage length.

3.3. Bond strength Test results are given in Table 5. The effects of parameters on the bond strength of cyclic specimens are illustrated in Fig. 10. The following investigation was carried out based on the average value of bond strengths obtained from both pushing and pulling actions.

3.3.1. Effects of compressive strength As shown in Table 3, the compressive strengths of dA1, dA2 and dA3 are 51.7, 71.0 and 81.3 MPa, respectively. The bond strength of ECC specimen has a dramatic enhancement with the increase in compressive strength as presented in Fig. 10(a). In the case of small difference in flexural toughness, the bond strengths of dA2 and dA3 were 2.8 and 4.6 times respectively higher than that of specimen dA1. It was due to the fact that increasing compressive strength improved the shear resistance of ECC keys between the rebar lugs and thus the mechanical interlocking between reinforcing bar and matrix was enhanced significantly.

3.3.2. Effects of toughness Referring to Table 5, the bond strengths of dB2 and dB3 increase by 37% and 15% respectively in comparison with specimen dB1 when their compressive strengths were quite close. This phenomenon occurred because the crack resistance capacity of PE fibers was superior to that of PVA fibers. However, the bond strength of specimens had a slight reduction as the PE fiber content increased from 1.5% to 2%. In view of this, it was true to some

extent that increasing toughness was expected to yield a preferable bond behavior. It was worth noting that, the bond strength of dB1 was 1.7 times that of dA1 although the compressive strength and flexural toughness of both of them were almost same. Referring to Table 1, it was found that the ingredient of B1 included 5% silica fume. Therefore, it could be inferred that the addition of silica fume had a high efficiency in improving the interfacial performance and consequently the bond behavior between matrix and steel bar was enhanced. Furthermore, mixture A3, B2 and B3 also contain silica fume (Table 1) and the bond strengths of specimen dA3, dB2 and dB3 are 4.6, 2.3 and 2.0 times higher than that of dA1, respectively (Table 5). This fact further proved that adding silica fume was of great benefit to improving the bond strength between steel bar and ECC.

3.3.3. Effects of cover thickness The radial components of bearing force induced by mechanical interlocking between rebar lugs and matrix usually lead to the circumferential tensile stress in the matrix. Once this tensile stress exceeded the tensile strength of matrix, internal cracks appeared. The lateral confinement effect of cover and fiber bridging stress can prevent the extension of these cracks to the surface of specimens and then ensure enough bond strength. It was well agreed that the larger cover thickness, the stronger resistance ability to split. Consequently, the bond strengths of dC2 and dC1 increase by 27% and 67% respectively in comparison with dC3 as shown in Fig. 10(c). However, when the cover thickness was adequate, the bond strength of specimen relied on the shear resistance of

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Table 6 Calculated results of cumulative dissipated energy. Code

E0 (MPamm)

E0.85 (MPamm)

E0.5 (MPamm)

dA1 dA2 dA3 dB1 dB2 dB3 dD1 dD2 dD3 dE1 dE2 dE3

5.72 24.53 59.93 5.86 17.47 13.53 11.69 6.55 1.94 2.24 14.21 18.85

24.84 86.91 107.71 24.67 49.33 30.20 22.42 29.89 17.93 5.80 46.25 40.25

53.55 181.45 136.55 75.18 118.24 87.59 49.91 53.90 54.02 72.45 117.74 83.20

stress area in the whole length decreased and thus the average bond stress reduced. But this reduction would cease when the embedded length was beyond 5 times diameter. It was worthwhile to note that the bond strength had an obvious increase as the anchorage length further increased to 7 times diameter. This phenomenon indicated that the larger anchorage length was beneficial to the bond behavior between steel bar and HDC. 3.4. Energy dissipation capacity Energy dissipation capacity is usually considered as an essential index to investigate the seismic behavior of specimens under cyclic loading [11]. Three characteristic points were chosen to calculate the cumulative dissipated energy in this paper. E0, E0.85 and E0.5 represented cumulative dissipated energy at the peak point and two ultimate points corresponding to 85% and 50% of the peak load, respectively. The calculated results of cumulative dissipated energy of all cyclic specimens are listed in Table 6. The variation trends of energy dissipation capacity with design parameters are depicted in Fig. 11. It can be visibly seen from Fig. 11 that the cumulative dissipated energies of all specimens still have a large promotion when the bond stress decreases to the 85% and 50% of peak. It was a pretty good proof that the ECC specimens had an excellent ductility and damage resistance capacity. The effects of parameters on the energy dissipation capacity were analyzed as follows.

matrix and increasing cover thickness did not benefit further in terms of bond behavior. This conclusion was well verified by the fact that the difference of bond strength between dA1 and dC1 was relative small although the cover thickness of former was larger than that of latter.

3.4.1. Effects of compressive strength The increase in compressive strength has a positive effect on the energy dissipation capacity of ECC specimens as shown in Fig. 11

200

Cumulative dissipated energy (MPa.mm)

Cumulative dissipated energy (MPa.mm)

3.3.4. Effects of anchorage length As illustrated in Fig. 10(d), the bond strength has a slight decrease with the increased anchorage length from 3d to 5d. This result can be explained by the following reason. The bond stress calculated by Eq. (2) reflected the average bond stress along the embedded length. In fact, the distribution of bond stress is nonuniform along the whole embedded length [18]. When the anchorage length was shorter, the high stress area accounted for a large proportion and therefore the average bond stress was relatively higher. As the anchorage length increased, the percentage of high

E0 E0.85

160

E0.5 120

80

40

0 50

60

70

80

90

200

E0 E0.85

160

E0.5 120

80

40

0 0

50

100

Compressive strength (MPa)

E0 E0.85 E0.5 120

80

40

0 1.0

1.5

2.0

250

2.5

3.0

3.5

200

E0 E0.85

160

E0.5 120

80

40

0 3

4

5

6

7

la/d

c/d

(c)

200

(b) Cumulative dissipated energy (MPa.mm)

Cumulative dissipated energy (MPa.mm)

(a) 200

160

150

Toughness (MPa)

(d)

Fig. 11. Energy dissipation capacity. (a) Compressive strength; (b) toughness; (c) cover thickness; (d) anchorage length.

300

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(a). At the peak point, the cumulative dissipated energy of dA2 and dA3 were 4.3 and 10.5 times greater than that of dA1, respectively. When the load decreased to 85%, the cumulative dissipated energy of dA2 and dA3 were 3.5 and 4.3 times that of dA1, respectively. This case can be attributed to the improved bond behavior induced by the increase in shear strength of matrix. However, the cumulative dissipated energy of dA2 was higher than that of dA3 when the bond stress decreased to half. It suggested that the damage of latter was more serious than that of former and the ductility of latter was inferior to that of former. 3.4.2. Effects of toughness From Fig. 11(b), it can be seen that the energy dissipation capacity of specimens rises first and then falls as the flexural toughness of material increases. The cumulative dissipated energies E0, E0.85 and E0.5 of specimen dB2 were superior to those of dB1 and dB3. It was caused by the fact that the increased toughness could improve the bond strength while the excessive toughness was adverse to the bond strength. In addition, for specimen dB2, the cumulative dissipated energy at 85% and 50% of peak increased by 1.8 and 5.8 times respectively in comparison to that at peak. It confirmed that specimen dB2 showed a significant ductility due to the higher toughness and damage resistance capacity of matrix. 3.4.3. Effects of cover thickness As shown in Fig. 11(c), the energy dissipation capacity of all specimens shows smaller amplitude of fluctuation with the increase in cover thickness. This phenomenon can be attributed to the fact that the tensile strain hardening behavior of ECC materials effectively prevented the opening and extension of splitting cracks and thus the requirement of ECC specimens to cover thickness was lowered. The failure of ECC specimens were due to the shear off of ECC keys between rebar lugs and the energy dissipation was mainly induced by the relative slip between rebar and matrix. Therefore, increasing cover thickness did not benefit further in terms of improving energy dissipation capacity. 3.4.4. Effects of anchorage length The influence of anchorage length on energy dissipation capacity did not show an obvious rule. As seen from Fig. 11(d), the cumulative dissipated energies of dE2 are higher than those of dE1, dA1 and dE3. It just indicated that the anchorage length as 4 times diameter was most favorable to enhancing the energy dissipation capacity.

gel, consequently increasing the compactness of matrix. As a consequence, the bond performance of ECC specimens under cyclic loading was improved and the degradation of bond strength was relieved. 4. Prediction of bond strength Test results in Section 3.3 suggested that the bond strength was significantly influenced by the compressive strength, flexural toughness, cover thickness and anchorage length. As illustrated in Fig. 10(a), the bond strength increases exponentially as the compressive strength of materials increases. Thus the relationship between bond strength and compressive strength was expressed using the following equation: 2:7 su ¼ 3:4  105 f cu

ð3Þ

The bond strength for specimens with similar compressive strength rose first and descended later along with the increase in toughness, and therefore the influence of flexural toughness was reflected by a quadratic function as follows:

su ¼ 3:3  104 T 2 þ 0:1T þ 3:6

ð4Þ

The study by Orangun [24] suggested a basic form for the bond strength equation considering the cover thickness and anchorage length. Almost the same format was adopted by Darwin [25] and Harajli [26]. Therefore, this research used the form proposed by Orangun [24] to reflect the effects of cover thickness and anchorage length. Finally, a combined effect of compressive strength, flexural toughness, cover thickness and anchorage length was taken into account in this paper and then the calculating equation of bond strength was proposed as expressed in Eq. (5) through the regression analysis of experimental data.



su ¼ 4  106 3:3  104 T 2 þ 0:1T þ 3:6

  c d 2:7 0:7 þ 1:4 þ 9:6 f cu d la ð5Þ

when c=d  3, c=d is taken equal to 3; when la =d  5, la =d is taken equal to 5. The calculated values of bond strength are listed in Table 5. The correlation coefficient between the tested and predicted bond strength is 0.964. Comparison between experimental and predicted results is depicted in Fig. 12. From Fig. 12, it can be found that the predicted results agree well with the experimental results,

3.5. Degradation of bond strength

25

20 u,pre (MPa)

The ratio of bond strength under cyclic loading to that under monotonic loading was defined as the degradation coefficient of bond strength k. It reflected the degradation degree of bond strength under the earthquake action and the smaller degradation coefficient reflected more severe degradation. Referring to the Table 5, it can be seen that the degradation coefficient of bond strength ranges from 0.6 to 0.9. The increase in compressive strength and cover thickness could reduce the degradation degree of bond strength. However, the degradation of bond strength became more serious with the larger anchorage length. The degradation coefficients of bond strength of dB1, dB2 and dB3 were very close, suggesting that the increased flexural toughness had a negligible effect on the degradation of bond strength. Furthermore, it should be pointed out that the degradation coefficients of bond strength of dB1, dB2 and dB3 were close to 0.9, significantly higher than other specimens. This result would be attributed to the addition of silica fume in ECC mixture. Silica fume can fill pores between cement particles and react with hydrates to produce a

15

10

5

0 0

5

10

15

20

25

u,exp (MPa) Fig. 12. Comparison between experimental results and predicted results.

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suggesting that the proposed model in this paper can be used to predict the bond strength between the deformed bar and ECC. 5. Conclusions In this paper, 12 groups of ECC specimens and one group of concrete specimen were tested in order to investigate the bond behavior of the deformed bar in ECC under cyclic loading. Effects of parameters, including compressive strength, flexural toughness, cover thickness and anchorage length, on the bond strength and energy dissipation capacity were discussed. The following conclusions were drawn from experimental results. (1). The concrete specimen failed in the brittle splitting of cover due to the inadequate tensile strength of matrix. In contrast, the pullout or splitting-pullout failure occurred for ECC specimens because the fiber bridging effect effectively prevented the opening and propagation of splitting cracks. These two failure modes showed an obvious ductility and were associated with the shear strength of ECC. (2). The bond strength of ECC specimen had a dramatic enhancement with the increase in compressive strength of matrix. The bond strengths of dA2 and dA3 were 2.8 and 4.6 times higher than that of specimen dA1, respectively. Besides, the flexural toughness of ECC materials was beneficial to the bond performance to some extent. (3). The increase in compressive strength had a positive effect on the energy dissipation capacity of ECC specimens. The cumulative dissipated energy at peak point of dA2 and dA3 at the peak point were 4.3 and 10.5 times greater than that of dA1, respectively. The energy dissipation capacity of specimens rose first and then fell with the increased toughness of materials. (4). Increasing compressive strength and cover thickness can reduce the degradation degree of bond strength. However, the degradation of bond strength became more serious with the larger anchorage length. In addition, the degradation coefficients of bond strength of dB1, dB2 and dB3 were significantly higher than other specimens, indicating that the addition of silica fume in ECC mixture could relieve the degradation of bond strength under cyclic loading. (5). A calculating equation of bond strength for ECC specimen under cyclic loading was proposed in this paper which took the combined effect of compressive strength, flexural toughness, cover thickness and anchorage length into account. The predicted results matched well with experimental results. Conflict of interest The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgements This research is funded by the National Natural Science Foundation of China (51578445, 51708445). The authors would like to thank the laboratory teachers from School of Civil Engineering for their help with specimen fabrication and test.

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