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First European Conference on Earthquake Engineering and Seismology (a joint event of the 13th ECEE & 30th General Assembly of the ESC)

Geneva, Switzerland, 3-8 September 2006 Paper Number: 136

SEISMIC PERFORMANCE OF RC BEAM-COLUMNS JOINTS RETROFITTED USING LIGHT RC JACKET – EXPERIMENTAL STUDY

Chris KARAYANNIS1, George SIRKELIS2 and Constantin CHALIORIS3

SUMMARY

The use of a light type of Reinforced Concrete (RC) jacket for damaged RC beam-columns subassemblages due to seismic excitations is proposed and experimentally investigated. The examined jacket has very small thickness (20 mm) and dense reinforcement that consists of small diameter (5.5 mm) horizontal and vertical steel bars and stirrups. This jacketing applies only at the joint region and at a small part (300 to 450 mm) of the columns and beam, close to the joint body. The main advantage of the proposed light jacketing comparing to the commonly used concrete jackets is the fact that it slightly changes the initial size of the elements. Consequently, the whole seismic response and the distribution of shears among the columns remains practically the same. For the needs of this study, test results of 4 external beam-columns joint specimens are presented and examined. Two specimens were constructed and subjected to constantly increasing cyclic (pseudo-seismic) loading. Afterwards, the damaged specimens were retrofitted using the proposed light RC jackets and they were retested with the same cyclic load sequence. Test results indicated that the seismic performance of the retrofitted specimens was ameliorated with respect to the performance of the specimens in the initial loading, since they exhibited higher values of load capacity and hysteretic energy dissipation in the higher deformation loading cycles.

1. INTRODUCTION One of the earliest and most common techniques for the retrofit of Reinforced Concrete (RC) frames and beam-column joints is the use of RC jackets. These jackets are usually constructed with high strength concrete, reinforced with longitudinal and transverse reinforcement and they encase only the existing columns or columns and beams along with the joint regions. Concrete jacketing is a well-known strengthening technique that is frequently used either prior or after the damage of RC members (pre-seismic strengthening or strengthening of damaged elements) [Alcocer and Jirsa, 1993], [Tsonos, 1999], [Lowes and Moehle, 1999], [Dritsos 2005]. It is proved that RC jacketing techniques can provide increased joint strength, shift the failure to the beam, and increase overall lateral strength and energy dissipation [Tsonos, 1999, 2002]. However, jacketing increases the size of the members, reduces the available floor space and increases mass. This way, concrete jacketing techniques alter the dynamic characteristics of the structure that may cause increased demands at unintended locations [Engindeniz, Kahn and Zureick, 2005]. Thus, in most of the cases, an intensive re-analysis of the entire structure is required. Since 1998, the research efforts on upgrading existing RC beam-column joints have focused on the use of Fibre Reinforced Polymer (FRP) composites, mainly in the form of epoxy-bonded fabrics that surround the RC 1 Professor of Democritus University of Thrace, Civil Engineering Department, Reinforced Concrete Laboratory, 67100 Xanthi, Greece Email : karayan@civil.duth.gr 2 Civil Engineer MSc, PhD student of Democritus University of Thrace, Civil Eng. Dept., Reinforced Concrete Lab., 67100 Xanthi, Greece Email: sirkelis@yahoo.gr 3 Lecturer of Democritus University of Thrace, Civil Engineering Department, Reinforced Concrete Laboratory, 67100 Xanthi, Greece Email: chaliori@civil.duth.gr

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members (FRP jacketing). This strengthening technique eliminates some of the previous mentioned important limitations that concrete jacketing induces [Engindeniz, Kahn and Zureick, 2005]. FRP jacketing has been also used as a rehabilitation method for damaged beam-columns RC joints that were first repaired using high strength concrete [Ghobarah and Said, 2001] or using epoxy resin injections [Karayannis and Sirkelis, 2002]. Although the fact that the use of FRP jackets enhanced the joint seismic performance in most of the examined cases, the FRP failure is dominated by premature debonding of the composite material from the concrete surface and substantial reductions of the potential capability in shear strength were reported. Thus, it is proved that the use of FRP jackets in beam-column joints with common constructional limitations (spandrel beams, existence of slab and transverse beams) needs reliable mechanical anchorage methods that would lead to effective joint confinement and full development of fibre strength [Engindeniz, Kahn and Zureick, 2005]. Although there is a plethora of known and well-established strengthening techniques of RC beam-column joints (reinforced or prestressed or steel fibre reinforced concrete jackets, steel jackets, external steel elements, FRP jackets), the only efficient and established repair technique is the use of epoxy resin injections [Karayannis, Sideris and Economou, 1995], [Karayannis, Sirkelis and Chalioris, 2003]. This repairing method has been extensively applied for damaged concrete structures after earthquakes and could be combined with partial or total removal and replacement of concrete in cases of heavily damaged areas with crushed concrete [Karayannis, Chalioris and Sideris, 1998]. In this study, the use of a light RC jacket for the rehabilitation of damaged RC exterior beam-columns subassemblages due to cyclic deformations is experimentally investigated. The examined jacket has very small thickness and dense steel reinforcement that consists of small diameter bars and stirrups. Further, this jacketing is not applied along the entire height of the existing columns, but it only encases the joint region and a critical part of the conjuncted columns and beam, close to the joint. The advantages of the proposed light jacket in respect to the commonly used concrete jacketing are focused on the fact that the dimensions of the retrofitted elements slightly change regarding to their initial size. Thus, the available floor space and the mass practically are not modified and thus, the dynamic characteristics of the structure remain approximately the same. This way, the proposed technique should not be considered as a strengthening technique, since the character of the proposed method is more focused on the repair and rehabilitation of damaged RC joints than on the strengthening.

2. EXPERIMENTAL PROGRAM The experimental study of this work includes the test results of 4 external beam-columns joint specimens. Two specimens were constructed and they were tested under constantly increasing cyclic loading (original specimens J0 and J1). Afterwards, the damaged specimens were rehabilitated using the proposed light RC jackets (retrofitted specimens J0R and J1R) and retested with the same loading sequence. 2.1 Original specimens The geometry of the specimens was the same; total columns length and cross-section dimensions 1800 mm and 300×200 mm (Ac = 0.06 m2), respectively, whereas beam length and cross-section dimensions were 1100 mm and 200/300 mm, respectively. Reinforcement arrangements of the columns and beam of these specimens were also the same; columns reinforcement comprised 4 longitudinal bars of 10 mm diameter (4∅10) and stirrups of 8 mm diameter at a uniform spacing of 150 mm (∅8/150 mm), whereas beam reinforcement comprised longitudinal bars 6∅10 top, 6∅10 bottom and stirrups ∅8/150 mm. The joint area of the specimen J0 had no shear reinforcement whereas specimen J1 had one stirrup ∅8. Geometry and reinforcement arrangement details of the specimens J0 and J1 are shown in Figure 1. The concrete mean cylinder compressive strength was fcm = 31.6 MPa (age of 28 days) whereas steel yield strength was 580 MPa (deformed steel bars and stirrups). The thickness of the concrete cover of stirrups was approximately 15 mm. The anchorage arrangement of the beams’ bars in the joint body was anchorage with bend and has been designed according to EC2 standards [Eurocode 2, 2002].

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1800

200

200

30030

0

1325

100

286

166

80

80 186

286

∅10

∅8/150

∅8/150

750

750

1100150

150

6∅10

4∅10

6∅10

∅8

in specimen J1

Figure 1: Geometry and reinforcement arrangements of specimens J0 and J1 2.2 Retrofitted specimens After the initial loading, both damaged specimens were rehabilitated using light RC jackets. The retrofitted specimens (J0R and J1R) were retested with the same cyclic loading sequence. The light jackets encased (a) part of the columns, equal to 300 mm close to the joint for each column, (b) the joint area and (c) part of the beam, equal to 450 mm close to the joint (Figure 2). The thickness of the light jacket was only 20 mm. The jackets were made of a premixed, non-shrink, flowable, rapid, high-strength cement-based mortar (SikaGrout 212) that was mixed with water in a proportion 1:0.14. This grout had density 2250 kgr/m3 and maximum aggregates size equal to 4 mm. Supplementary compression tests showed that the mean cube compressive strength at the age of 5 days was 43.4 MPa. The reinforcement of each jacket consisted of: - 4 vertical straight longitudinal bars (placed at the back side of the columns along the joint area), - 2 vertical straight longitudinal bars (one of each was placed at the long side of the columns along the joint area

as vertical shear reinforcement of the joint), - 6 horizontal Π -formed longitudinal bars (placed at the vertical sides of the beam and around the joint area as

transverse shear reinforcement of the joint), - 8 L-formed longitudinal bars (4 of them placed at the upper and 4 at the lower side of the beam; all of them

were bended to the columns), - 7 closed stirrups (placed at the beam) and - 10 closed stirrups (5 of them placed at the upper column and 5 at the lower column).

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The diameter of all the above reinforcement was 5.5 mm (∅5.5 plain mild steel bars and stirrups) with steel yield strength equal to 390 MPa. In Figure 2 the detailed arrangement of the jackets’ reinforcement is displayed. It is emphasized that the new dense reinforcement have not been welded or joined with the existing reinforcement, whereas no drilling took place and dowels have not been installed.

Figure 2: Reinforcement and geometry of the light jackets (for the retrofitted specimens J0R and J1R)

450 mm

900 mm

thickness= 20 mm

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2.3 Test setup Test rig and instrumentation details are shown in Figure 3. Supports that allow rotation were used to simulate the inflection points assumed to occur at a point of the columns in a laterally-loaded frame structure. Column axial load equal to Nc = 0.05×Ac×fcm was applied during the tests in all the specimens. All specimens were subjected to full cycle deformations imposed near the free end of the beam by a pinned-end actuator. The moment arm for the applied load was 1.0 m. The load sequence of the specimens was performed the way it is shown in Figure 4. The imposed load was measured by a load cell with accuracy equal to 0.025 kN and the displacements of the beam at its end were measured by linear variable differential transducer (LVDT) with accuracy equal to 0.01 mm. LVDTs were also placed at each end of the column part of the specimens, as shown in Figure 3, in order to check the supports during the test.

1.50

1.10

+

-

actuator

load cell actuator

load cell

LVDT

Load frame

LVDT

LVDT

Nc

P1.00

1.80

Figure 3: Test rig and instrumentation

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-20

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20

40

60

Def

orm

atio

n (m

m)

1 2

3 4

5 67 8

Figure 4: Cyclic loading history

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3. TEST RESULTS AND COMPARISONS 3.1 Hysteretic responses To assess the effectiveness of the applied light jacketing technique the hysteretic responses of the original beam-columns subassemblages (J0 and J1) are examined and compared with the hysteretic responses of the retrofitted specimens (J0R and J1R). The hysteretic responses of the loading cycles for the specimens J0 and J0R are presented in Figure 5, whereas for the specimens J1 and J1R in Figure 6. In these figures the solid line represents the response of the original specimens, whereas the dashed line the response of the corresponding retrofitted ones. Further, the hysteretic energy dissipation in terms of the area of the response loading cycle of each tested specimen is presented in the Table 1.

Table 1: Values of energy dissipation per loading cycle

Energy dissipation (kN·mm)

Cycle Deformation (mm) J0 J0R increase* J1 J1R increase*

1 + 6 mm 177.9 161.8 -9.1% 189.1 176.3 -6.8%

2 – 6 mm 86.8 126.1 45.3% 110.4 148.7 34.7%

3 + 20 mm 1036.8 1149.2 10.8% 1029.3 1220.2 18.5%

4 – 20 mm 629 815.3 29.6% 709 917.2 29.4%

5 + 40 mm 2511.2 3133.7 24.8% 2970.5 3059.7 3.0%

6 – 40 mm 1563.1 1741.7 11.4% 1499.1 1838.9 22.7%

7 + 60 mm 2816.8 2965.7 5.3% 3324.3 3614.8 8.7%

8 – 60 mm 1452.6 2004.6 38.07% 1918.7 2346.7 22.3% * Increase of the energy dissipation of the repaired specimens (J0R & J1R) compared to the initial ones (J0 & J1)

The comparison of the seismic performance between the original and the retrofitted specimens (Figures 5 & 6 and Table 1) indicated that the joint subassemblages with the proposed light RC jackets exhibited, in general, enhanced behaviour in respect to the original ones. In particular, the load capacity values of specimens J0R and J1R were increased till 91% in most of the loading cycles and in both loading directions compared with specimens J0 and J1, respectively. This improvement was mainly observed in the higher deformation loading cycles (± 60 mm). Nevertheless, it is noted that in a few sporadic loading cycles the observed ultimate loads of the retrofitted specimens were below the maximum load capabilities of the original ones. However, this fact does not alter the general improvement of the entire seismic response due to the proposed jacketing. Besides, retrofitted specimens demonstrated higher energy dissipation values than the original ones after the first loading cycle (Table 1). The average values of the increase of the energy dissipation were 19.5% and 16.6% for the specimens J0R and J1R, respectively. 3.2 Failure modes Specimen J0: The joint body of this specimen, as expected, exhibited X-shaped diagonal cracks during the test and the specimen exhibited typical brittle shear failure, since no shear reinforcement in the joint area has been provided. One main diagonal crack has been extensively widened in the 7th and 8th loading cycle. Vertical cracks have also been appeared at the junction of the beam with the joint. The deformations of the bend anchorage of the beam’s bars (anchorage failure) in combination with the absence of stirrups in the joint area have contributed to significant damage the concrete cover at the back of the joint area. Specimen J0R: The crack pattern of this retrofitted specimen was quite different than the one of the original specimen. The applied light RC jacket at the joint area mainly inhibited the beam’s bars anchorage failure and the damage of the concrete cover at the back of the joint area. Numerous shear X-shaped diagonal cracks have been developed at the jacket of joint area during the loading procedure. However, these cracks were rather thin and the joint body has not been seriously damaged due to the shear reinforcement of the jacket. In the subsequent loading cycles partially spalling of jacket’s mortar cover has been observed whereas the main damage was localized at the junction of the beam with the joint.

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-60 50 -40 -30 -20 -10 0 10 20 30 40 50 60Deformation (mm)

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0

10

20

30

40

50

60

70

Load

(kN

)

Specimens without stirrups in the joint body

Original specimen (J0)

Repaired specimen with light jacket (J0R)

Figure 5: Comparison of the hysteretic responses of specimens J0 and J0R

-60 50 -40 -30 -20 1 0 10 20 30 40 50 60Deformation (mm)

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Load

(kN

)

Specimens with one stirrup in the joint body

Original specimen (J1)

Repaired specimen with light jacket (J1R)

Figure 6: Comparison of the hysteretic responses of specimens J1 and J1R

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Specimen J1: The failure mode and the cracks propagation of this specimen were close to these of specimen J0, without shear reinforcement in joint area. This proves that the use of only one closed stirrup as joint shear reinforcement, although it improves the seismic performance, it is inadequate to alter the failure mode and shift the damage to the beam. Specimen J1 exhibited shear failure and X-shaped diagonal cracks have been developed in the joint body from the early stages of the loading procedure. One main diagonal crack has been extensively widened at the last loading cycles according with concrete crushing at the back concrete cover of the joint area due to beam’s bars anchorage failure. Cracks at the junction of the beam and the joint body have also been formed. Specimen J1R: This retrofitted specimen exhibited better cracking distribution, ameliorated seismic performance and different failure mode than the original one. In general, the response of specimen J1R was very alike to the response of the other retrofitted specimen (J0R). No failure of the concrete cover at the back of the joint has been observed. Thin shear X-shaped diagonal cracks have been distributed all over the jacket’s joint area. In the high deformation loading cycles partially spalling of jacket’s mortar cover has been observed.

4. CONCLUDING REMARKS

The effectiveness of a RC jacket with small thickness and dense steel reinforcement (with small diameter) for the rehabilitation of RC exterior beam-columns joints damaged under cyclic imposed deformation was experimentally investigated. Although the preliminary and exploratory character and the present experimental study, based on the tests results reported herein the following concluding remarks can be deduced: - The proposed light RC jacketing seems to be an easy-to-apply, reliable and effective method for the repair and

the rehabilitation of damaged RC joints since the entire response of the retrofitted specimens was ameliorated in respect to the response of the original specimens in the initial loading.

- The retrofitted specimens compared with the original ones exhibited higher values of load capacity in most of

the loading cycles and increased hysteretic energy dissipation practically in the entire loading sequence. This improvement was mainly observed in the higher deformation loading cycles.

- The applied light RC jacketing should not be considered as a strengthening technique since it did not shift the

failure to the beam and did not dramatically increase the specimens’ load capacity, as the commonly used jacketing usually does. However, the overall seismic performance, the joint strength and the hysteretic energy dissipation have been enhanced. Further, retrofitted specimens demonstrated different failure modes than the original ones. Thin shear diagonal cracks have been distributed all over the jacket’s joint area whereas the concrete cover at the back of the joint did not crush.

- The advantages of the proposed light RC jacket in respect to the commonly used jacketing are focused on the

fact that the dimensions of the retrofitted elements slightly change regarding to their initial size. Thus, the available floor space and the mass practically are not modified and the dynamic characteristics of the structure remain approximately the same.

5. REFERENCES

Alcocer, S.A., Jirsa, J.O. (1993), Strength of Reinforced Concrete Frame Connections Rehabilitated by Jacketing, ACI Structural Journal, 90, n° 3, 249-261.

Dritsos, S.E. (2005), Seismic Retrofit of Buildings a Greek Perspective, Bulletin of New Zealand Society of Earthquake Engineering, 38, n° 2.

Engindeniz, M., Kahn, L.F., Zureick, A-H. (2005), Repair and Strengthening of Reinforced Concrete Beam-Column Joints: State of the Art, ACI Structural Journal, 102, n° 2, 1-14.

Eurocode 2 – Part 1 (2002), Design of Concrete Structures – Part 1: General Rules and Rules for Buildings, CEN, prEN 1992-1-1, Brussels.

Ghobarah, A., Said, A. (2001), Seismic Rehabilitation of Beam-Column Joints using FRP Laminates, Journal of Earthquake Engineering, 5, n° 1, 113-129.

Karayannis, C.G., Sideris, K.K., Economou, C.M. (1995), Response of Repaired RC Exterior Joints under Cyclic Loading, Proceedings of the 5th SECED Conference on European Seismic Design Practice - Research and Application, 285-292.

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Karayannis, C.G., Chalioris, C.E., Sideris, K.K. (1998), Effectiveness of RC Beam-Column Connection Repair using Epoxy Resin Injections, Journal of Earthquake Engineering, 2, n° 2, 217-240.

Karayannis, C.G., Sirkelis, G.M. (2002), Effectiveness of RC Beam-Column Connections Strengthening Using Carbon-FRP Jackets, Proceedings of the 12th European Conference on Earthquake Engineering, Paper 549.

Karayannis, C.G., Sirkelis, G.M., Chalioris, C.E. (2003), Repair of Reinforced Concrete T-beam – Column Joints using Epoxy Resin Injections, Proceedings of the 1st International Conference on Concrete Repair, Vol. 2, 793-800.

Lowes, L.N., Moehle, J.P. (1999), Evaluation and Retrofit of Beam-Column T-Joints in Older Reinforced Concrete Bridge Structures, ACI Structural Journal, 96, n° 4, 519-533.

Tsonos, A.G. (1999), Lateral Load Response of Strengthened Reinforced Concrete Beam-to-Column Joints, ACI Structural Journal, 96, n° 1, 46-56.

Tsonos, A.G. (2002), Seismic Repair of Exterior R/C Beam-to-Column Joints using Two-sided and Three-sided Jackets, Journal of Structural Engineering and Mechanics, 13, n° 1, 17-34.