Shear strengthening of RC short columns with ECC jacket: Cyclic behavior tests

Engineering Structures 160 (2018) 535–545

Contents lists available at ScienceDirect

Engineering Structures journal homepage: www.elsevier.com/locate/engstruct

Shear strengthening of RC short columns with ECC jacket: Cyclic behavior tests

T

⁎

Mingke Deng, Yangxi Zhang , Qiqi Li School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Engineered cementitious composite Short column ECC jacket Cyclic loading Plastic deformation Energy dissipation

This study aims at developing an easy-to-apply strengthening method for reinforced concrete (RC) short columns by using Engineered cementitious composite (ECC), which is a high-performance composite material with tensile strain-hardening behavior and multiple cracking mechanism in tension. Seven identical RC short columns were prepared, and five of them were strengthened with ECC jackets or ferrocement jacket. The main experimental parameters involve the jacketing schemes and the axial load. The failure modes, hysteresis response, deformation capacity, stiffness degradation and energy dissipation capacity of the specimens were studied by the lateral cyclic loading tests. The tests results show that the ECC jacketed columns failed in ductile modes, and exhibited significant improvement in plastic deformation ability and energy dissipation capacity compared with the control specimens. At the ultimate displacement point, the increase of the drift ratio and cumulative energy dissipation of the ECC jacketed columns were 19–111% and 102–549%, respectively, compared with the corresponding control specimens. Under higher axial loads, the ECC jacketing was still certified to be effectively in enhancing the seismic behavior of the original columns. The shear deformation and the shear strength of the original RC columns were effectively improved by the external ECC jacket, and a prediction method for the shear strength of the ECC jacketed columns was given based on the current Chinese codes.

1. Introduction For RC frame structures, the main vulnerable elements in strong earthquake actions are beam-column joints [1–3] and short columns. However, it is difficult to carry out pre-earthquake strengthening for the beam-column joints in existing structures. RC short columns were prone to shear failure and non-ductile bend-shear failure [4,5]. Columns exhibited a drastic decrease of load-carrying capacity after such brittle failure, resulting in poor plastic deformation and insufficient energy dissipation. The fundamental cause of these brittle failures in short RC columns is the brittleness of concrete. When the columns are under high axial load, the brittle behavior of concrete also results in serve spalling and crushing of covers, which aggravates the degradation of the lateral shear strength and the axial load-carrying capacity. This condition increases the risk of collapse of the whole frame structures. Therefore, it is necessary to use practical and effective methods to enhance the shear resistance and plastic deformation capacity of these vulnerable columns before an earthquake. Various strengthening materials such as fiber reinforced polymers (FRP) [6–10], steel jacket [11,12], RC jacket [13] and ferrocement jacket [14–17] have been used to wrap or confine RC columns for

⁎

seismic enhancing. All these strengthening methods could improve the strength and deformation capacity of the RC columns. However, FRP jacketing and steel jacketing are relatively expensive, and the durability and high temperature resistance of these two strengthening schemes are relatively poor. The jacket matrix of ferrocement jacket and RC jacket are brittle materials, thus the ductility improvement of ferrocement or RC jacketed members is small. ECC [18–20] is another group of strengthening materials, which exhibits multiple cracking mechanism and strain-hardening behavior in tension due to the bridging effect of fiber in the matrix of ECC. Based on the excellent tensile properties, ECC has been widely used as enhancement materials in beams [21,22], columns [23–25] and beamcolumn connections [26,27] to replace concrete and part of the transverse reinforcement, finding that the strength and ductility performance of the structural members were significantly enhanced. In addition, the compressive strain capacity of ECC is approximately two times that of the concrete [28,29] and the shear properties of ECC also exhibit strain-hardening behavior [30], these properties of ECC materials contributed to the fiber-bridging effect may effectively resolve the deficiencies of the concrete members associated with the brittleness of concrete. However, there are few reports on the application of ECC in

Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Q. Li).

https://doi.org/10.1016/j.engstruct.2018.01.061 Received 5 November 2017; Received in revised form 16 January 2018; Accepted 22 January 2018 0141-0296/ © 2018 Elsevier Ltd. All rights reserved.

Engineering Structures 160 (2018) 535–545

M. Deng et al.

Nomenclature

H b h0 λ s ft fy fyv ,fyvj fcu fc ,fcj fcd ,fcdj N nt

nd A,Aj Py ,Δy Pm,Δm Pu,Δu μ θu Ki γ Vmc Vc,Vj ftj Asv ,Asvj sj h 0j

height of the column column width (before, after strengthened) effective depth of RC column shear span ratio spacing of stirrup tensile strength of concrete yield strength of longitudinal bars yield strength of stirrups, mesh bars cube compressive strength of concrete axial compressive strength of concrete, jacket materials design axial compressive strength of concrete, jacket materials axial load of column test axial load ratio

design axial load ratio cross section area of RC column, jacket yield load, yield displacement peak load, peak displacement ultimate load, ultimate displacement displacement ductility factor ultimate drift ratio secant stiffness of column shear strain of plastic hinge region shear strength of jacketed column shear strength of RC column, jacket tensile strength of jacket materials total cross section area of stirrups, horizontal mesh bars spacing of horizontal mesh bar effective depth of jacketed column

strength fcu of the concrete was 34.7 MPa. Table 3 lists the mechanical properties of three types of steel bars used in this test, namely, the longitudinal reinforcing bars, hoops and the mesh bars. fy and fu are the yield strength and the ultimate strength of the steel bars respectively. The mesh bars used in the jackets are regular plain round bars with a diameter of 6 mm.

strengthening or retrofitting of structural members. Deng et al. [31] conducted a study on the seismic performance of masonry walls strengthened with ECC layers, including two control specimens and two strengthened specimens. The test results indicated a remarkable improvement in the seismic response of the strengthened specimens, including higher load-carrying capacity, larger displacement ductility and slower stiffness degradation. Hung and Chen [32] investigated the performance of U-shaped ECC jacketing for retrofitting shear-deficient cantilever RC beams. The results of the study indicated that using ECC to replace the mortar in the ferrocement jacket can reduce the crack width and improve the energy dissipation capacity of the specimens significantly. Based on the above research works, ECC materials were used as the external strengthening jacket to enhance the shear resistance and ductility of the RC short columns. In the present work, the seismic performance of 7 short columns, including 2 control specimens and 5 strengthened specimens, are investigated by reversed cyclic loading tests. The influence of the design variables on the behavior of the test specimens, including the jacketing schemes (i.e., (a) ECC or mortar as the jacket matrix, (b) presence or absence of bar mesh spacing in ECC jacket) and axial load level, are presented and analysed.

3. Experimental program 3.1. Test specimens Seven identical 1:2-scaled RC short columns with a cross section of 250 mm × 250 mm and a height of 600 mm were fabricated. The effective height H, defined as the distance from the top surface of the foundation beams to the lateral loading point of the columns, was 500 mm. Six 16 mm-diameter steel bars (HRB400) were used as the longitudinal reinforcements, and 8 mm-diameter steel bars (HPB335) were used as the hoops spacing at 100 mm. All longitudinal bars were anchored into the foundation beams. The foundation beams had a cross section of 450 mm × 450 mm with a length of 1400 mm. Table 4 lists the experimental parameters and the main information of the specimens. Specimen C-1 and specimen C-5 were employed as control specimens, specimen C-2 was strengthened by the ferrocement jacket, specimen C-3 was strengthened by ECC jacket without bar mesh (0-ECC jacket), and specimens C-4, C-6 and C-7 were strengthened by bar mesh reinforced ECC jackets (B-ECC jacket). The construction steps of the prepared columns to be strengthened were gouging the concrete covers, cleaning the concrete debris, installing the steel mesh (specimens C-2, C-4, C-6 and C-7) and casting the external jackets. All strengthening jackets were four-sided layers with a thickness of 25 mm. The horizontal spacing and the vertical spacing (h × v) of the mesh bars in the jackets are listed in Table 4. For specimen C-2, the templates were erected after the installing of mesh bars, and the mortar was then cast in the templates. The ECC jackets were plastered manually using a trowel, and the jackets were plastered in two layers. The inner layer was plastered with a thickness of approximately 15 mm. After the initial setting of the inner ECC layer, the outer layer with a thickness of approximately 10 mm was plastered. In this tests, putting on a ferrocement jacket consumed approximately 15 h, while the time to install an

2. Materials properties 2.1. ECC and mortar The mixed proportions for the matrix of the ECC and the mortar used in this study are summarized in Table 1. The components of ECC include cement (42.5R Portland Cement), fly ash, river sand (maximum aggregate size < 1.18 mm), water and Polyvinyl alcohol (PVA) fibers. A 1.5% volume incorporation of PVA fibers is used in the ECC, and the mechanical and geometric properties of the fibers are shown in Table 2. The direct tensile stress-strain curve of ECC and the crack pattern of the test specimen are shown in Fig. 1. The tensile strength of ECC used in this study is approximately 5 MPa and the ultimate tensile strain is approximately 2.8%. The compressive strength of ECC and mortar were tested by three cubes (100 mm × 100 mm × 100 mm), respectively, and the average cube compressive strength of ECC and mortar was 59 MPa and 54.9 MPa, respectively. Thus, the strength grade of ECC and mortar are evaluated as C50 according to GB 50010-2010 Code for Design of Concrete Structures [33].

Table 1 Mixed proportion of ECC and mortar (kg/m3).

2.2. Concrete and steel bars The concrete used in this study had a design strength grade of C30, and its compressive strength was tested by three cubes (150 mm × 150 mm × 150 mm). The average cube compressive 536

Materials

Cement

Fly ash

Sand

Water

PVA fiber

ECC Mortar

652 487

534 487

427 780

344 292

26 0

Engineering Structures 160 (2018) 535–545

M. Deng et al.

specimens are shown in Fig. 2. The vertical mesh bars of the jackets are 560 mm in length, and they are terminated at the top of the foundations. N The test axial load ratio of the RC columns is calculated by nt = f A .

Table 2 Performance indicators of PVA.

39

Tensile strength/ MPa

Elastic modulus/ GPa

Elongation/%

1200

32

8

Specific gravity

The design axial load ratio is calculated by nd =

6

Stress (MPa)

5 4 3 2 1 0 0.0

1.25N fcd A

c

according to

the Chinese Code for Seismic Design of Buildings (GB 50011-2010) [34]. Required by GB 50011-2010, the limitation value of the design axial load ratio for the RC frame columns is nd = 0.9. To investigate the effectiveness of ECC jacketing for high axial load ratio RC short columns, the design axial ratios of specimens C-1 and C-5 are taken as nd = 0.8 and nd = 1.0, respectively. According to GB 50010-2010, the relationship between the cubic compressive strength and the axial compressive strength is given by fc = 0.76fcu , and the design axial compressive strength fcd of C30 and C50 concrete are 14.3 MPa and 23.1 MPa, respectively. For the jacketed columns, the test axial load N ratio and design axial load ratio can be calculated by n t = f A + f A and

1.3

c

0.5

1.0

1.5

2.0

2.5

3.0

nd =

3.5

Strain (%)

(a) Stress-strain curve

The specimens were tested under combined constant axial load and reversed-cyclic lateral load. The test setup is shown in Fig. 3. The constant axial load was applied first on the top surface of the column by a hydraulic jack, and the lateral load imposed on the specimens was applied through a (MTS) 1000 kN hydraulic actuator. The foundation beams were fixed on the ground with four anchor bolts. The loading procedure involved a force-controlled stage and a displacement-controlled stage following the Specification of Testing Methods for Earthquake Resistance Building [35] protocol, as shown in Fig. 4. In the force-controlled stage of the loading step, the increment of the lateral cycle load was 20 kN until the columns yielded, which is defined by the yielding of the longitudinal steel bars during the tests. Subsequently, the loading procedure was changed into the displacement-controlled load. In these tests, the yield points of the specimens are different, thus the increment of the displacement was taken as 2 mm and each displacement load cycle was repeated three times. For normal RC members, the ultimate state is defined at the point where the postpeak strength is degraded to 85% of the peak strength. For ECC members, the ultimate state is defined differently in different research papers. Parra-Montesinos [36] and Fischer and Li [37] considered the ultimate state for ECC components as a 20–50% loss in strength. Xu et al. [38] and Yuan et al. [39] terminated the test for ECC members when 85% of the post-peak load was reached. Considering the laboratory safety, and in order to compare the behavior enhancement after strengthening, the test was terminated when the strength of the specimen dropped below its 85% peak value. As shown in Fig. 5, the lateral displacement of the loading point of

Fig. 1. Tensile test of ECC.

Table 3 Mechanical properties of steel bars. Properties

Type

Diameter (mm)

fy (MPa)

fu (MPa)

Longitudinal bars Stirrups Mesh bars

HRB400 HPB335 HPB300

16 8 6

410 424 341

570 578 530

Table 4 Main parameters of specimens. Specimen

Jacket matrix

Mesh spacing h × v (mm)

Jacket scheme

Axial load N (kN)

nt

nd

C-1 C-2 C-3 C-4 C-5 C-6 C-7

– Mortar ECC ECC – ECC ECC

– 80 × 80 – 80 × 80 – 80 × 80 40 × 80

– Ferrocement 0-ECC B-ECC – B-ECC B-ECC

572 572 572 572 715 715 715

0.35 0.22 0.21 0.21 0.43 0.26 0.26

0.80 0.47 0.47 0.47 1.00 0.58 0.58

ECC jacket was 2–5 h. After the casting of the cement jackets, the specimens were water cured for 28 days, and then natural cured until the tests. The geometric sizes and the reinforcement details of the

8@100 hoops 25

Jacket Foundation 1

6@80/40

long.bars 6 16

1 2

300

100 500

Load point

250 1-1

Jacket

25

Vertical bar mesh

8 20 8@100 450 2-2

2

Fig. 2. Dimensions of specimens and reinforcement details.

537

100 100

450

6@80 Bar mesh spacing

8@100

1400

cj j

respectively.

3.2. Test devices, loading procedure, and instrumentation

(b) Crack pattern

25 250 25

1.25N , fcd A + fcjd Aj

80 80 80

12

Diameter/mm

450

Length/mm

270

Horizontal bar mesh

Engineering Structures 160 (2018) 535–545

M. Deng et al.

Fig. 3. Test set-up.

Hydraulic jack

Roller

Steel hinge

MTS Actuator

Reaction wall

Reaction rack

Loading point Specimen

Ground

Anchor bolt

Foundation beam

Displacement

Force

the foundation beam, and the strain gauges were fixed on the first three stirrups above the top surface of the foundation beam.

ǻy+2n

4. Experimental results and analysis

ǻy

4.1. Test observations and failure modes

Pcr 0

The crack patterns for all the specimens at the failure stage are shown in Fig. 6. The control specimens (C-1 and C-5) exhibited similar failure process. Some tiny flexural cracks were initially observed on the tension side of the column foots. Subsequently, diagonal cracks occurred and the initial flexural cracks widened and spread with the increase of lateral displacement. Finally, columns failed in shear due to the widening of major diagonal cracks and crushing of concrete. Specimen C-5 showed considerably more severe crushing of concrete and faster strength degradation than specimen C-1 because of the increased axial load. Although flexural-shear cracks were observed on the ferrocement jacketed column (C-2), it failed in brittle flexural-shear mode due to the brittleness of the mortar. Apparent diagonal cracks band were observed in specimens C-3, C-4 and C-6, but the specimens all failed in flexural-shear mode with ductility. The ECC materials exhibited strain-hardening behavior in shear stress, thus premature failure of the core RC columns was avoided by the confinement provided by the external ECC jackets. Compared with specimen C-6, more flexural cracks were observed on specimen C-7, and the number of diagonal cracks decreased, indicating that the behavior of specimen C-7 was

Loading cycles

Pcr ǻy ǻy+2n

Fig. 4. Loading procedure.

each specimen was measured by a linear variable differential transducer (LVDT-1). The shear deformation of the plastic hinge region was measured by two cross dial gauges. LVDT-3 and LVDT-4 were used to measure the flexural deformation of the plastic hinge region. LVDT-2 was installed to monitor the slip of the foundation beams. Each RC column had a total of 12 strain gauges installed on the longitudinal bars and hoops to collect the strain of the reinforcements. The strain gauges on the longitudinal bars were located 50 mm above the top surface of

100

Lateral loading

LVDT-1 strain gauges for stirrups

450

LVDT-4

LVDT-3

50

25 275

500

Dial gauges

LVDT-2

Fig. 5. Measurement position.

538

strain gauges for long. bars

Engineering Structures 160 (2018) 535–545

M. Deng et al.

(a) Specimen C-1

(b) Specimen C-2

(e) Specimen C-5

(c) Specimen C-3

(f) Specimen C-6

(d) Specimen C-4

(g) Specimen C-7

Fig. 6. Failure patterns of specimens.

dominated by flexural. Further, no bond failures were found on the interface between the ECC jackets and the core concrete, indicating that the manually plastered ECC jackets had good synergy performance with the core concrete.

The hysteresis loops were much plumper and the post-peak strength degradation was slower. As a result, the ECC jacketed columns had more hysteresis loops, higher ductility and higher energy dissipation capacity, which shows excellent seismic performance. In addition, even the 0-ECC jacket (C-2), the ECC jacket without bar mesh, was more effective in improving the hysteresis behavior and deformation of the original column than the ferrocement jacket (C-3). Thus, the 0-ECC jacketing can be used as a quick strengthening method for RC columns. Under higher axial load ratios, the peak load of specimen C-5 was slightly (7.9%) higher than specimen C-1, but the ductility obviously decreased and the post-peak strength degenerated faster, as shown in Fig. 7(a and e), and specimen C-5 lost its axial load-carrying capacity soon after the peak load, which exhibited poor collapse-resistance capacity. For the B-ECC jacketed columns, specimen C-6 had a similar seismic response to specimen C-4, with high ductility and high energy dissipation capacity. This result was due to the high axial compressive strain ability of ECC, which effectively avoided the core concrete crushing. The post-peak strength degradation of specimen C-7 was even slower than that of specimen C-4, because the behavior of specimen C-7 was dominated by flexural. Further, the axial load-carrying capacity of the B-ECC jacketed columns (C6 and C-7) stayed steady when the specimens lost 15% of their lateral shear strength (peak load), indicating that the B-ECC jacket can be used to improve the seismic

4.2. Hysteresis behavior The experimental lateral load-displacement (P-Δ) hysteresis loops of all columns are shown in Fig. 7. The control specimens exhibited poor hysteresis response, which was characterized by thin hysteresis loops and rapid decrease in the load-carrying capacity after the peak load. By comparing the hysteresis loops of specimen C-1 and specimen C-5, it is evident that specimen C-5 exhibited more rapid strength degradation after the peak load than specimen C-1, and this was largely attributable to the increased axial load ratio. Concrete is a brittle material with crack-softening behavior, thus specimens C-1 and C-5 exhibited poor behavior. On the other hand, ECC is a ductile material with strain-hardening behavior and multiple crack mechanism in tension, thus the ECC jackets could provide effective confinement to the core columns. As a result, the cracking and softening of the core concrete were delayed, and the ultimate compressive strain of the concrete was improved. Compared with the control specimens, the ECC jacketed columns exhibited a much better hysteresis response. 539

Engineering Structures 160 (2018) 535–545

400

300

300

200

200

200

100

100

100

0

0

-100

-200

-300

-300

-400 -20 -15 -10

-5

0

5

10

15

-400 -20 -15 -10

20

0 -100 -200 -300

Specimen C-2

-5

0

5

10

15

20

-400 -20 -15 -10

/mm

/mm

(b)

(c)

400

400

300

300

300

200

200

200

100

100

100

0

0

-100

-100

-200

-200 -300 0

5

10

15

-400 -20 -15 -10

20

5

10

15

20

0 -100 -200

-300

Specimen C-4

-5

0

(a) 400

-400 -20 -15 -10

Specimen C-3

-5

/mm

P/kN

P/kN

-100

-200 Specimen C-1

P/kN

400

300

P/kN

400

P/kN

/kN

M. Deng et al.

-300

Specimen C-5

-5

/mm

0

5

10

15

20

/mm

(d)

-400 -20 -15 -10

Specimen C-6

-5

0

5

10

15

20

/mm

(e)

(f)

400 300

P/kN

200 100 0 -100 -200 -300

Specimen C-7

-400 -25 -20 -15 -10 -5

0

5 10 15 20 25

/mm

(g) Fig. 7. Hysteretic curves of specimens.

horizontal bar mesh is used in the jacket (Fig. 8b). Under a higher axial load level, the post-peak strength degradation of specimen C-5 was much more rapid than specimen C-1, but the skeleton curves of specimen C-4 and specimen C-6 were almost overlapping, indicating that the B-ECC jacketing is an effective method for improving the deformation capacity of the RC columns with exceed axial load ratio. For the jacketed columns, the external jackets provided part of the shear strength, and the confinement effect provided by the jackets effectively delayed the widening of the diagonal cracks and the spalling of concrete covers in the core columns. As a result, the degradation of the aggregate interlock forces of the core columns was delayed. It is evident from Table 5 that the jacketed columns exhibited obvious increase in peak load in comparison to the control specimens ranging between 44.6% and 48.7%. This result indicates that the jacketing schemes had little influence on the shear capacity of the jacketed columns. The ductility factor μ and ultimate drift ratio θu directly reflect the plastic deformation capacity of the specimens. Larger ductility factor and ultimate drift ratio means larger plastic deformation capacity and higher energy dissipation capacity. As shown in Table 5, the ductility factors of the jacketed columns were significantly increased up to 102.1% compared with the control specimens. In particular, the ductility factors of specimens C-6 and C-7 were increased by 41.7% and

performance and the collapse-resistance of high axial load ratio RC columns. 4.3. Skeleton curves and performance indexes The skeleton curves of all specimens obtained from the hysteresis loops are presented in Fig. 8. To quantify the improvement in the seismic performance of the jacketed columns, the displacement and the corresponding load at the yield (i.e., Δy and Py), peak (i.e., Δm and Pm), and ultimate (i.e., Δu and Pu) points obtained from the skeleton curves were listed in Table 5. The yield point was determined by the energy method proposed by Mahin and Bertero [40], and the ultimate load was defined to the point where the load was reduced to 85% of the peak load. The displacement ductility factor μ = Δu/Δy and the ultimate drift ratio θu = Δu/H summarized in Table 5 are used to evaluate the ductility and the plastic deformation capacity of the specimens. It should be noted that the yield load, peak load, ultimate load and the corresponding displacements in Table 5 are the average values in pushing and pulling. Compared with the corresponding control specimens, the ECC jacketed columns all had obviously higher peak loads, higher ductility and slower post-peak strength degradation. The ductility of the B-ECC jacketed columns can be further improved when a larger amount of 540

Engineering Structures 160 (2018) 535–545

400

400

300

300

200

200

100

100 P(kN)

P(kN)

M. Deng et al.

0 C-1 C-2 C-3 C-4

-100 -200 -300 -400 -20 -15 -10

-5

0 5 Δ(mm)

10

15

Fig. 8. Skeleton curves.

0 -100

Z-5 Z-6 Z-7

-200 -300 -400 -20 -15 -10

20

-5

0

5

10

15

20

Δ(mm)

˄a˅effect of jacketing schemes

˄b˅effect of mesh ratio

400 300

P(kN)

200 100 0 C-1 C-4 C-5 C-6

-100 -200 -300 -400 -20 -15 -10

-5

0

5

10

15

20

Δ(mm)

˄c˅effect of axial load

85.6%, respectively, above that of specimen C-5, demonstrating that the effectiveness of the B-ECC jacketing for the high axial load ratio RC columns. The ultimate drift ratio of the ECC jacketed columns was between 2.33% and 3.14%, which was well above the requirement of 2% in the Chinese Code for Seismic Design of Buildings [GB 500112010]. The ultimate drift ratio of specimen C-2 was 1.96%, which indicates that the ferrocement jacket is not an effective method for improving the deformation capacity of the RC short column with relative high axial load ratio. In addition, the ultimate drift ratio of specimens C-6 (2.35%) and C-7 (3.14%) was found to be 60.9% and 110.9% higher than specimen C-5, respectively. This again verified the high efficiency of B-ECC jacketing under high axial load ratio.

Ki =

| + Pi | + |−Pi | | + Δi | + |−Δi |

(1)

where +Pi and −Pi are the peak load of the ith cycle in pushing and pulling, respectively, and + Δi and −Δi are the corresponding displac. Compared with the control specimens (Fig. 9a), the initial stiffness of the jacketed columns was much higher and the stiffness decreased faster before the peak load. After the peak loads, the stiffness degradation rates of the jacketed columns were lower than the control specimens, and the stiffness degradation rate of the column strengthened with B-ECC jacket was the slowest. With the increase of the axial load (Fig. 9b), the stiffness degradation rate of the control specimen increased remarkably, however the stiffness degradation curves of specimen C-4 and specimen C-6 were almost overlapping, indicating that B-ECC jacketing can effectively reduce the stiffness degradation rate of high axial load ratio columns. The reason for this phenomenon is the strain-hardening behavior and high compressive strain ability of ECC, which significantly enhanced the damage tolerance of the specimens. Further, increasing the transverse mesh ratio of the B-ECC jacket contributed to a slower stiffness

4.4. Stiffness degradation The stiffness degradation curves of the columns are shown in Fig. 9. The secant stiffness Ki is defined as the slope of the line from the origin of coordinates to the peak load of each (ith) cycle, is used to analyse the stiffness degradation behavior: Table 5 Comparisons of characteristic load and displacement. Specimens

C-1 C-2 C-3 C-4 C-5 C-6 C-7

Yield point

Peak point

Ultimate point

Deformation

Py/kN

Δy/mm

Pm/kN

Δm/mm

Pu/kN

Δu/mm

μ

θu/%

176.32 259.81 253.24 264.54 193.87 268.30 260.68

3.41 2.49 2.78 2.17 2.63 3.03 2.59

212.34 317.3 309.11 318.55 229.32 319.43 316.3

6.65 5.36 7.43 5.51 4.91 6.31 5.69

180.49 269.71 262.74 270.77 194.92 256.19 268.86

9.77 9.81 13.22 11.64 7.45 11.76 15.68

2.87 3.94 4.76 5.36 2.83 3.88 6.05

1.95 1.96 2.64 2.33 1.49 2.35 3.14

541

Engineering Structures 160 (2018) 535–545

M. Deng et al.

Fig. 9. Curves of stiffness degradation of specimens.

degradation rate (Fig. 9c).

Table 6 Energy dissipations of specimens.

4.5. Energy dissipation capacity

Specimens

The energy dissipation capacity is one of the most important indexes to evaluate the seismic performance of structural members. The cumulative energy dissipation for each specimen was defined as the sum of the area of each hysteresis loop up to the current displacement. Fig. 10 shows the cumulative energy dissipation curves for each specimen and Table 6 lists the cumulative energy dissipation for each specimen at yield load, peak load and ultimate displacement. The jacketed columns exhibited an increase in cumulative energy dissipation in comparison to the control specimens ranging between 31% and 227% at the yield displacement, from 1% to 383% at the peak

C-1 C-2 C-3 C-4 C-5 C-6 C-7

Cumulative energy dissipation E/ (kN mm) Yield load

Peak load

Ultimate displacement

3236 4262 4659 5789 1570 3830 5135

2184 26,300 29,953 21,985 8450 21,741 40,785

37,989 53,889 76,928 86,945 15,089 54,112 97,950

Fig. 10. Cumulative energy dissipation-displacement curves.

542

Engineering Structures 160 (2018) 535–545

M. Deng et al.

displacement, and from 41% to 549% at the ultimate displacement. The ECC jacket performed better at improving the energy dissipation capacity of the original columns than the ferrocement jacket, and the BECC jacket was the most effective strengthening scheme for improving the energy dissipation capacity of the original columns. The B-ECC jacketed columns exhibited much more stable hysteresis response and higher plastic deformation capacity after the peak load when compared with the control specimens. As a result, the energy dissipated by specimens C-4, C-6 and C-7 at the ultimate displacement were 128%, 258% and 549% higher, respectively, than the corresponding control specimens. In addition, the cumulative energy dissipated by specimen C-7 at the ultimate displacement was 81% higher than specimen C-6. This is because flexural failure occurred in specimen C-7 due to a relative high transverse mesh ratio in its B-ECC jacket. The increased axial load had a negative influence on the energy dissipation capacity of the short columns. In specimen C-5, a 60% decrease in cumulative energy dissipation was measured at the ultimate displacement compared with specimen C-1, because premature failure occurred in specimen C-5. However, the decrease in cumulative energy dissipation in specimen C-6 was 37.8% compared to specimen C-4, which indicates that the ECC jacketing effectively enhanced the damage tolerance of the specimens.

cosθ =

5.1. Measured shear deformation

(Δ −Δ )−(δ 3−δ 4) γ= 1 2 2acosθ Δ3−Δ 4 d

(7)

where VC and Vj are the shear contribution from the original RC columns and the newly added RC jacket respectively. The value of VC and Vj are evaluated using provisions in existing design codes [i.e., GB 500112010, GB 50367-2013]:

VC =

A 1.75 f bh 0 + fyv sv h 0 + 0.07N λ+1t s

Vj = α c

Asvj 1.75 f Aj + α s fyvj h 0j λ + 1 tj sj

(9)

In Eq. (8), represents the shear contribution of concrete, the tensile strength of concrete is given by ft = 0.395fcu0.55 in accordance with GB50010-2010. The contribution from hoops is calculated by A fyv ssv h 0 based on the truss model. The contribution from axial load to the shear strength is 0.07 N (N ≤ 0.3fc A). In Eq. (9), α c (=0.7) and α s (=0.9) are the effective strength coefficient of the newly added

S2

S1

ǻ4 a

(8)

1.75 f bh 0 λ+1 t

(4)

į4

a

(6)

Vmc = VC + Vj

(3)

a2 + d 2 − (a−Δ 4)2 + d 2

ǻ3

+ d2

According to the Chinese Standard GB 50376-2013 [41], the shear strength of the RC jacketed structure member can be calculated by Eq. (7) based on additive approach:

(2)

į3

d a2

5.2. Shear strength

As shown in Fig. 5, the shear deformation of the plastic hinge region of the columns was measured by two cross dial gauges, and LVDT-3 and LVDT-4 were used to measure the flexural deformation of the plastic hinge region. Because the lowest point of the cross dial gauges and LVDT-3/4 were fixed on the column rather than the foundation beam, the measured shear deformation does not include sliding movement. Thus, the deformation of the measured region includes flexural deformation and shear deformation, and Fig. 11 shows the flexural deformation and shear deformation diagram of the measured region. The data measured by the cross dial gauges can not reflect the actual shear deformation, because the flexural deformation calculated by Eq. (3) can change the reading data of the cross dial gauges, thus the shear strain of the plastic hinge region was calculated by Eq. (2), and the calculation results are listed in Table 7.

δ3 =

(5)

where a = 275 mm, d is horizontal distance between the two LVDTs (d = 250/300 mm), Δ1 and Δ2 were the measurements from the cross dial gauges, Δ3 and Δ 4 were the data measured from the LVDTs, and δ3 and δ4 were the reading changes of the cross dial gauges caused by flexural deformation. At the peak load, the jacketed columns except specimen C-7 all exhibited an increase in shear strain, and the shear strain of specimen C2 is just 3.1% higher than specimen C-1. This is because many flexuralshear cracks formed in specimen C-2 and specimen C-7, thus the development of shear deformation is to be delayed. At the ultimate displacement, the shear strain of the ferrocementjacketed column is just 13.8% higher than the control specimen because of the premature crushing of mortar in the column foot, while the increase of the shear strain in the ECC-jacketed columns is at least 40.5%. In particular, the shear strain of specimen C-7 is 82.7% higher than specimen C-5. This indicates that the strain-hardening behavior and the multiple crack mechanism of ECC materials effectively improved the shear deformation capacity of the ECC-jacketed columns. Besides, the influence of the axial load on the shear strain of the columns is relatively small.

5. Shear behavior

φ=

(a + Δ3)2 + d 2 − a2 + d 2

δ4 =

Ȗ ș

b

b

(a) flexural

(b) shear 543

Fig. 11. Deformation diagram of the measured region.

Engineering Structures 160 (2018) 535–545

M. Deng et al.

ranged between 2.33% and 3.14%. (4) Under high axial load ratios, the B-ECC jacketed columns still exhibited high seismic performance, with high plastic deformation, high energy dissipation and slow stiffness degradation after the peak load, which means that the B-ECC jacketing technique is feasible for pre-earthquake strengthening of high load level RC short columns. (5) The shear deformation and shear strength of the original RC columns are improved significantly due to the high tensile property of the jacketing materials (ECC), and the calculation method for RC jacketed columns can be used to predict the shear strength of the ECC jacketed columns (failed in shear). (6) Based on the test results, the effectiveness of the manually plastering ECC jacket has been certified. The advantages of this strengthening scheme are its ease-of-construction and time saving application, making its application an inviting prospect in structure strengthening engineering.

Table 7 Shear strain of the plastic hinge region. Shear strain γ /(×10−3 mm/mm)

Specimens

C-1 C-2 C-3 C-4 C-5 C-6 C-7

Peak load

Ultimate displacement

6.4 6.6 7.9 7.4 7.1 7.5 6.0

10.9 12.4 15.9 15.9 11.6 16.3 21.2

Table 8 Shear strength of the columns. Specimen

Vm

Vmc

Vm/Vmc

C-1 C-2 C-3 C-4 C-5 C-6 C-7

212.34 kN 317.30 kN 309.11 kN 318.55 kN 229.32 kN 319.43 kN 316.30 kN

210.07 kN 313.67 kN 270.17 kN 316.88 kN 210.07 kN 316.88 kN 377.41 kN

1.01 1.01 1.14 1.01 1.09 1.01 0.84

Acknowledgements The research presented herein was funded by the National Natural Science Foundation of China PR(No.51578445) and the Project of Department of Construction of Shanxi Province (NO. 2015-K142), which is gratefully acknowledged.

concrete and reinforcements respectively. The tensile strength of ECC jackets is ftj = 5 MPa based on the test result, and the tensile strength of the mortar is calculated by ftj = 0.395fcu0.55 (see Table 8). For specimen C-5, the predicted value is smaller than the test value, because the maximum contribution from the axial load to the shear strength of the RC columns is 0.07 × 0.3fc A (0.3fc A = 494 kN). The prediction for specimen C-3 shows higher deviations, which indicates that the effective strength coefficient for the ECC jacket without mesh bars is conservative. In addition, the calculated value of specimen C-7 is 19% higher than the test value, this is because specimen C-7 failed in flexural before the shear strength reached. In all, the formulas according to the current Chinese codes can be adopted to predict the shear strength of the ECC jacketed columns.

References [1] Tsonos AG. Lateral load response of strengthened reinforced concrete beam-tocolumn joints. ACI Struct J 1999;96(1):46–56. [2] Pampanin S, Calvi GM, Moratti M. Seismic behavior of R.C. beam-column joints designed for gravity only. University of Canterbury Civil Engineering; 2002. [3] Faleschini F, Hofer L, Zanini MA, et al. Experimental behavior of beam-column joints made with EAF concrete under cyclic loading. Eng Struct 2017;139:81–95. [4] Decanini LD, Sortis AD, Goretti A, et al. Performance of reinforced concrete buildings during the 2002 Molise, Italy, earthquake. J Earthq Spectra 2015;20(6):S301–14. [5] Doǧangün A. Performance of reinforced concrete buildings during the May 1, 2003 Bingöl Earthquake in Turkey. J Eng Struct 2004;26(6):841–56. [6] Ye LP, Yue QR, Zhao SH, Li QW. Shear strength of reinforced concrete columns strengthened with carbon fiber reinforced plastic sheet. J Struct Eng 2002;128(12):1527–34. [7] Galal K, Arafa A, Ghobarah A. Retrofit of RC square short columns. Eng Struct 2005;27(5):801–13. [8] Colomb F, Tobbi H, Ferrier E, Hamelin P. Seismic retrofit of reinforced concrete short columns by CFRP materials. Compos Struct 2008;82(4):475–87. [9] Promis G, Ferrier E, Hamelin P. Effect of external FRP retrofitting on reinforced concrete short columns for seismic strengthening. Compos Struct 2009;88(3):367–79. [10] Dai JG, Lam L, Ueda T. Seismic retrofit of square RC columns with polyethylene terephthalate (PET) fibre reinforced polymer composites. Constr Build Mater 2012;27(1):206–17. [11] Nagaprasad P, Sahoo DR, Rai DC. Seismic strengthening of RC columns using external steel cage. Earthq Eng Struct D 2010;38(14):1563–86. [12] Garzón-Roca J, Ruiz-Pinilla J, Adam JM, Calderón PA. An experimental study on steel-caged RC columns subjected to axial force and bending moment”. Eng Struct 2011;33(2):580–90. [13] Vandoros KG, Dritsos SE. Concrete jacket construction detail effectiveness when strengthening RC columns. Constr Build Mater 2008;22(3):264–76. [14] Takiguchi K. Shear strengthening of reinforced concrete columns using ferrocement jacket. ACI Struct J 2001;98(5):696–704. [15] Takiguchi K. An investigation into the behavior and strength of reinforced concrete columns strengthened with ferrocement jackets. Cem Concr Compos 2003;25(2):233–42. [16] Kazemi MT, Morshed R. Seismic shear strengthening of RC columns with ferrocement jacket. Cem Concr Compos 2005;27(7):834–42. [17] Mourad SM, Shannag MJ. Repair and strengthening of reinforced concrete square columns using ferrocement jackets. Cem Concr Compos 2012;34(2):288–94. [18] Li VC, Wang S, Wu C. Tensile strain-hardening behavior of PVA-ECC. ACI Mater J 2001;98(6):483–92. [19] Li VC. On engineered cementitious composites (ECC)—a review of the material and its applications. J Adv Concr Technol 2003;1(3):215–30. [20] Kanda T, Li VC. Practical design criteria for saturated pseudo strain hardening behavior in ECC. J Adv Concr Technol 2006;4(1):59–72. [21] Shimizu K, Kanakubo T, Kanda T, Nagai S. Shear behavior of steel reinforced PVAECC beams. World Conference on Earthquake Engineering; 2004. [22] Yuan F, Pan JL, Dong LT, Leung CKY. Mechanical behaviors of steel reinforced ECC or ECC/concrete composite beams under reversed cyclic loading. J Mater Civ Eng

6. Conclusions Based on the experimental results and analysis above, the following conclusions can be drawn: (1) The control specimens and the ferrocement jacketed column failed in brittle mode because of the brittleness of the concrete and mortar. In contrast, the ECC jacketed columns failed in much more ductile modes with good integrity of the column cross section due to the strain-hardening behavior of ECC, and no bond failure was observed on the interface between the manually plastered jackets and the core RC columns at the ultimate displacements. (2) The hysteresis performance of the ECC jacketed columns was significantly improved compared with the control specimens, which was characterized by plumper and more hysteresis loops with greatly improved plastic deformation capacity. As a result, the energy dissipation capacity was improved tremendously. This directly reflects the effectiveness of the ECC jacketing in enhancing the seismic performance and the collapse resistance of the original RC short columns. (3) The peak load (shear strength) of the ferrocement jacketed column was almost equal to the ECC jacketed columns. The ferrocement jacketed column (C-2) exhibited almost no improvement in plastic deformation, but the ECC jacketed columns showed obvious increase in the ductility factor (μ) and the ultimate drift ratio (θu) due to the high ductility of ECC. For the ECC jacketed columns, the ductility factor reached 3.88–6.05 and the ultimate drift ratio 544

Engineering Structures 160 (2018) 535–545

M. Deng et al.

Chinese]. [32] Hung CC, Chen YS. Innovative ECC jacketing for retrofitting shear-deficient RC members. Constr Build Mater 2016;111:408–18. [33] GB 50010–2010. Code for design of concrete structures. Beijing, China: Chinese standard press; 2010 [in Chinese]. [34] GB 50010–2010. Code for seismic design of buildings. Beijing, China: Chinese standard press; 2010 [in Chinese]. [35] JGJ 101–2015. Specification of Testing Methods for Earthquake Resistance Building. Beijing, China: Chinese Building Industry Press; 2015 [in Chinese]. [36] Parra-Montesinos GJ. High-performance fiber-reinforced cement composites: an alternative for seismic design of structures. ACI Struct J 2005;102(5):668–75. [37] Fischer G, Li VC. Influence of matrix ductility on tension-stiffening behavior of steel reinforced engineered cementitious composites (ECC). ACI Struct J 2002;99(1):104–11. [38] Xu L, Pan JL, Chen JH. Mechanical behaviors of steel reinforced ECC and ECC/RC composite columns under reversed cyclic loading. J Mater Civ Eng 2017;04017097. [39] Yuan F, Pan JL, Dong LT, Leung CKY. Mechanical behaviors of steel reinforced ECC or ECC/concrete composite beams under reversed cyclic loading. J Mater Civ Eng 2014;26(8):04014047. http://dx.doi.org/10.1061/(ASCE) MT.1943-5533. 0000935, 04014047. [40] Mahin SA, Bertero V. An evaluation of inelastic seismic design spectra. J Struct Eng 1981;107(9):1775–95. [41] GB 50367–2013. Code for design of strengthening concrete structure. Beijing, China: Chinese standard press; 2010 [in Chinese].

2013;26(8):04014047. [23] Fischer G, Li VC. Effect of matrix ductility on deformation behavior of steel-reinforced ECC flexural members under reversed cyclic loading conditions. ACI Struct J 2002;99(6):781–90. [24] Wu C, Pan Z, Su RKL, Leung CKY, Meng S. Seismic behavior of steel reinforced ECC columns under constant axial loading and reversed cyclic lateral loading. Mater Struct 2017;50(1):78. [25] Xu L, Pan JL, Chen JH. Mechanical behavior of ECC and ECC/RC composite columns under reversed cyclic loading. J Mater Civ Eng 2017;29(9):04017097. [26] Qudah S, Maalej M. Application of engineered cementitious composites (ECC) in interior beam–column connections for enhanced seismic resistance. Eng Struct 2014;69(9):235–45. [27] Said SH, Razak HA. Structural behavior of RC engineered cementitious composite (ECC) exterior beam–column joints under reversed cyclic loading. Constr Build Mater 2016;107:226–34. [28] Li VC. Engineered cementitious composites-tailored composites through micromechanical modeling. In: Banthia N, Mufti A, editors. Fiber reinforced concrete: present and the future, Canadian Society for Civil Engineering, Montreal; 1998. [29] Zhou J, Pan J, Leung CKY. Mechanical behavior of fiber-reinforced engineered cementitious composites in uniaxial compression. J Mater Civ Eng 2014;17(1):04014111. [30] Li VC, Mishra DK. Structural applications of engineered cementitious composites. Indian Concr J 1996;70(10):561–74. [31] Deng MK, Gao XJ, Liang XW. Experimental investigation on aseismic behavior of brick wall strengthened with ECC splint. Eng Mech 2013;36(6):168–74. [in

545