Experimental Response and FRC Strengtheningof Existing RC Members

Brian Stratman – Business Development Leader - MAPEIGiulio Morandini, Alberto Balsamo, Ciro Del Vecchio, Marco Di

Ludovico, Andrea Prota

University of Naples “Federico II”Dept. of Structures for Engineering and Architecture

Milano, Italy

Recent seismic events outlined the main structural weaknesses of existing RC buildings:

- Brittle failure of structural members due to lack ofproper seismic details

INTRODUCTION

- Joint panel shear failure due to the lack of stirrupsin the joint panel

- Buckling of longitudinal reinforcement due to thehigh spacing of stirrups (200-300 mm)

OBJECTIVES

- Identify the main structural weaknesses of the case study building viaexperimental tests

- Sampling of the specimens from real building damaged by the L’ Aquila 2009earthquake and demolished

- Design and testing of a proper seismic retrofit solution (HPC material) for shearstrengthening of beam-column joints

- Design and testing of a proper retrofit solution (HPC material) to improve theconfinement of columns

CASE STUDY BUILDING

Case Study 5 storey RC building built in 1963

severely damaged by the L’Aquila earthquake (2009)

Perimetral frame with corner joints

BUILDING DEMOLITION

The building was demolished due to:

- Economic inconvenience of structural retrofitting (after a detailed seimic assessment)

- Poor-quality concrete (fc < 8 MPa in some portion of the building)

Perimetral frame with corner joints

SPECIMEN SAMPLING

Two beamcolumn joints

were extractedat the first floor

JOINT 1

JOINT 2

350

500 6φ16

1350

Stir

rups

φ 6

/200

SPECIMEN SAMPLING

Column 2Column 1

Two Columnswere extractedat the second

floor of the same frame

(b)

S12

S7 + S17

S8

S9 S10

S11

S13

S14

S15

S1 – TRAVE PIANO TERRA

S2 – PILASTRO PRIMO PIANO

S3 – TRAVE PRIMO PIANO

S4 – PILASTRO PIANO SECONDO

S5 – PILASTRO PIANO TERRA

S6 – PILASTRO PIANO TERRA

COL 1 – COLUMN SECOND FLOOR

S16 – TRAVE PRIMO PIANO

S18 – PILASTRO PRIMO PIANO

S19 – PILASTRO PIANO TERRA S20 – PILASTRO PIANO TERRA

NON-DESTRUCTIVE TESTS

DESTRUCTIVE TESTS

EXTRACTED COLUMN

Investigated frame

fc =12 MPa

MATERIAL PROPERTIESDestructive and non-destructive material characterization tests were conducted on the entirebuilding and on the extracted portions

Concrete

Reinforcing steel diameter fy εsy Es εsh fu εsu [mm] [MPa] [mm/mm] [MPa] [mm/mm] [MPa] [mm/mm]

Longit. reinforc.

16 400.4 0.0020 203579 0.020 586.6 0.128 16 395.4 0.0022 178424 0.021 587.6 0.137 16 374.1 0.0019 196000 0.024 548.3 0.135 Mean (16) 390.0 0.0020 192667 0.022 574.2 0.133 12 360.4 0.0019 192217 0.024 504.4 0.236 12 357.5 0.0020 181415 0.022 501.9 0.148 12 362.5 0.0024 149463 0.014 506.3 0.010 14 364.2 0.0021 171254 0.019 526.2 0.213 Mean (12/14) 361.1 0.0021 173587 0.020 509.7 0.152

Stirrups

6 447.0 0.0025 181691 0.016 574.0 0.070 6 433.2 0.0027 160459 0.013 548.0 0.060 6 392.0 0.0018 213043 0.011 550.0 0.150 6 400.0 0.0026 153846 0.010 549.0 0.110 Mean (6) 418.1 0.0024 177260 0.013 555.3 0.098

smooth internal reinforcements

2080

1450

500

400

500

6φ12

Fd

constructionjoint

1600

400

400

FRP

wra

ppin

g

400400

FRP wrapping

+

400

500

1450

400

230

400

400

FRP

wra

ppin

g400

40

190

St. φ

6/20

0St

. φ6/

200

St. φ6/200

Setup steel pipeφ=130mm s=2mm

High strength mortar s=40mm

2φ14

2φ12

2φ14+2φ12

30

30

A

A

B

B

Stirrups with 90°bend

1450

500

400

500

4φ12350

400

FRP

wra

ppin

g400

1450

400

230

400

400

FRP

wra

ppin

g400

40

190

St. φ

6/20

0St

. φ6/

200

St. φ6/200

Setup steel pipeφ=130mm s=2mm

High strength mortar s=40mm

3φ14

30

30

C

C

Transverse beam (C-C)

2φ16

2φ14+2φ16

400

500

30

2φ14+1φ12

Top column (E-E)

EE

FFBottom column (F-F)

2φ16

2φ14+2φ16

400

30

2φ14+1φ12

550

250

250 200

HPFRCC jacketingjoint T_SJ

Lack of stirrups in the joint panel

- Smooth internal reinforcements

- Lack of transverse reinforcements (φ6 /200mm)

- No stirrups in the joint panel

- Bad quality construction joint

STRUCTURAL DETAILS

AC

TUA

TOR

Hc

= 3.

40 m

L = 1.65 m

TEST FRAME STEEL HINGE

PRESTRESSEDBAR

LOAD CELL

+

STRONG FLOOR

Fd

Stiff steel frames

TEST SETUP

Constant axial loadon the column

Cyclic displacementat the beam end

TEST RESULTS – AS-BUILT JOINT

Poor seismic performancedue to the joint panel shearfailure (in the positive loaddirection)

Beam yielding (in thenegative load direction)

-50

-40

-30

-20

-10

0

10

20

30

40

50

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

Vc[kN]

Drift [%]

T_CJ

Beam yieldingVc= 19kN

Joint panel shear failure

Vc= 35 kN

TEST RESULTS – AS-BUILT JOINT

TEST RESULTS – AS-BUILT JOINT

TEST RESULTS – AS-BUILT JOINT

Close-up on the joint panel

Concrete cover removal (40 mm by using a

jackhammer to have a rough surface)

Stiff and hydraulic sealed formwork

Anti-corrosioncementitious mortar

HPC STRENGTHENING

Cast of the HPC strengthening

View of the strengthened specimen

HPC STRENGTHENING

Beam hinge in both the

directions

TEST RESULTS- HPC JOINTHigh effectiveness of theHPC strengthening whichavoided the joint panelshear failure promoting amore ductile failure mode

TEST RESULTS- HPC JOINT

TEST RESULTS- HPC JOINT

Close-up on the joint panel

COMPARISON

-50

-40

-30

-20

-10

0

10

20

30

40

50

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

Vc[kN]

Drift [%]

T_HPCT_CJTh. Beam yielding (My-)Th. Beam yielding (My+)

Joint shear failure

+38% Strength

+54% Strength

+85% Dissipated energy

More details can be found in:

Del Vecchio C., Di Ludovico M., Balsamo A., Prota A. (2018)Seismic Retrofit of Real Beam-Column Joints Using Fiber-Reinforced Cement Composites. ASCE Journal of StructuralEngineering. Vol. 144(5), https://doi.org/10.1061/(ASCE)ST.1943-541X.0001999

https://doi.org/10.1061/(ASCE)ST.1943-541X.0001999

Experimental performances of sampled columns

COLUMNS

0

500

1000

1500

2000

2500

3000

0 2 4 6 8 10 12 14

N [kN]

d [mm]

TEST RESULTS-AS-BUILT COLUMN

Concrete crushing and barbuckling limited the axialload capacityMega testing machine (30.000 kN,

3000 tons axial capacity)

Ing. Ciro Del Vecchio

PRE-DAMAGE ON COLUMN 2

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

0 2 4 6 8 10 12 14

N [kN]

d [mm]

Column 2 was pre-damageduntil the spalling of concretecover and then repaired andstrengthened

Ing. Ciro Del Vecchio

HPC STRENGT. COLUMN 2Concrete cover removal (by using a jackhammer to have a rough surface)

Stiff and hydraulic sealed formwork Cast of the HPC

strengthening

Experimental test demonstrated the high effectiveness of the HPC strengtheningincreasing the axial load capacity and delaying bar buckling

TEST RESULTS - HPC COLUMN

Cut of the HPC jacketbefore the test(it is working only inconfinement)

0

500

1000

1500

2000

2500

3000

3500

4000

0 5 10 15 20 25 30 35 40

N [kN]

d [mm]

Col_1Col_2Col_2-FRC

The confinement by using the HPCstrengthening increased the axial loadcapacity about the 37%

Strengthincrease (+37%)

COMPARISON

The experimental results can be summarized as follows:

• The premature shear failure of poorly detailed beam-column joints maysignificantly limit the seismic performance of existing buildings

FINAL REMARKS

• An innovative strengthening solution consisting of a thin FRC jacketing hasbeen proposed and tested

• The experimental test outlined the high effectiveness of the proposedstrengthening solution which changed the brittle failure mode in a ductile one(+38 to 54% strength increase, +85% energy dissipation)

• The same strengthening technique has been used also for columnconfinement with satisfactory results (+37% strength increase)

Thank you

Giulio Morandini, Alberto Balsamo, Ciro Del Vecchio, Marco Di Ludovico, Andrea Prota

University of Naples “Federico II”, ItalyDept. of Structures for Engineering and Architecture

Milano, Italy

MorandiniG@mapei.it

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