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EXPERIMENTS ON SEISMIC RETROFIT AND REPAIR OF

REINFORCED CONCRETE SHEAR WALLS

Hamed LAYSSI

PhD Candidate

Department of Civil Engineering and Applied Mechanics, McGill University

817 Sherbrooke St. W., Montréal QC, Canada H3A 2K6

hamed.layssi@mail.mcgill.ca*

Denis MITCHELL

Professor

Department of Civil Engineering and Applied Mechanics, McGill University

817 Sherbrooke St. W., Montréal QC, Canada H3A 2K6

denis.mitchell@mcgill.ca

Abstract

The reversed cyclic loading responses of full-scale shear wall specimens were investigated.

The walls were designed and detailed to simulate non-ductile reinforced concrete construction

of the 1960’s, having lap splices of the longitudinal reinforcement in the potential plastic

hinge region, and having inadequate confinement of the boundary regions. The walls were

tested under reversed cyclic loading with loading applied near the tip of the walls. The

response of the original walls was associated with the brittle failure of the lap splice. The

effectiveness of a retrofit technique and a repair technique were investigated. The retrofit

involved the use of carbon fibre-reinforced polymer (CFRP) wrap for improving the lap splice

behaviour and the shear strength of the walls. The repair of the previously tested specimens

using a steel fibre-reinforced self consolidating concrete (SFRSCC) jacket, and CFRP wrap

was investigated. The retrofit and repair techniques improved the displacement ductility, and

prevented premature failure of the lap splices.

Keywords: Carbon Fibre Reinforced Polymers, Lap splice, Reversed cyclic loading, Repair, Seismic Retrofit, Shear Walls

1. Introduction

Performance-based retrofit and repair of older RC structures can lead to a cost effective

approach where demolishing and reconstruction is not applicable or economical. There are a

large number of existing RC structures designed according to pre 1970’s standards (i.e.,

gravity load design, with no specific seismic provisions), which are vulnerable to seismic

hazards [1].

There has been a tendency among researchers and engineers, over the past two decades, to

provide reliable tools in seismic evaluation of such construction, as well as developing cost

effective practical repair and retrofit solutions to upgrade existing substandard designs [2-4].

General deficiencies of such construction have been studied and reported by several

researchers [5, 6], and include short lap splice lengths of the longitudinal reinforcement in

potential plastic hinge regions, insufficient and poorly detailed transverse reinforcement and

inadequate shear strength required to develop hinging [7].

Jacketing is a common technique in seismic retrofit of existing structures and it can improve

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strength, stiffness and the confinement of existing elements. Steel, concrete (plain concrete,

fibre-reinforced concrete or reinforced concrete) and fibre reinforced polymers (FRP) jackets

can be used.

Recently, the use of FRP has become more popular in the retrofit and repair of certain RC

structural components, especially for beams, and columns [8, 9]. However, little research is

available on the effectiveness of this technique on the improvement of the seismic response of

the shear walls.

This paper presents the results of reversed cyclic loading responses of poorly designed and

detailed shear walls. The effectiveness of a retrofit and a repair method, with different

performance objectives, was investigated. The retrofit technique involved application of

CFRP wrap for improving the behaviour of deficient lap splices and shear strengthening of

the walls. The repair technique, which was carried out on two failed shear walls, combines

adding a steel fibre-reinforced self-consolidating concrete jacket over the failed lap splice

region, and CFRP wrap for improving confinement and shear strengthening of the rest of the

wall.

2. Experimental Program

Wall specimens were designed and detailed to simulate some general deficiencies of older

shear walls including the critical potential plastic hinging region containing lap splices of the

longitudinal reinforcement and single-leg, improperly anchored transverse reinforcement.

2.1 Design of Test Specimens

Originally, a set of four RC shear walls were designed, and constructed in the laboratory. The

cross sectional dimensions and the amount of concentrated and distributed reinforcement

were chosen to comply with the 1963 ACI [10] design provisions.

The four walls (two pairs) were identical in every aspect except for amount of concentrated

flexural reinforcement at both ends. The walls had cross-sectional dimensions of 150 mm by

1200 mm. The total height of the walls was 3400 mm. The walls were tested in horizontal

position, cantilevered out from a heavily reinforced foundation block, anchored to the strong

floor. The reversed cyclic loading was applied at a distance of 3250 mm from the base of the

wall by hydraulic jacks (Figs. 1(a) and 1(b)).

Figure 1. (a) Test setup; (b) the details of the as-built wall specimens

Load Cell

Load Cell

PositiveLoadingJacks

NegativeLoadingJacks

3250 mm

Shear wall

Foundation block

Strong floor

2 - 20M

10M @ 277 mm

10M @ 260 mm

10M @ 250 mm

Single leg stirrup

150 mm

120

0 m

m

46 46

85

2 - 20M

150 mm

4 - 20M

4 - 20M

W1 W2(a) (b)

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For each pair of identical walls, one wall is tested in as-built condition (W1 and W2), while

the companion wall was retrofitted prior to testing (WRT1 and WRT2). The as-built wall

specimens were repaired after being tested, and were retested in a similar approach (WRP1

and WRP2).

2.1.1 As-Built walls W1 and W2

Walls W1 and W2 were tested in their as-built condition. Two walls are only different in the

amount of concentrated flexural reinforcement at the ends. The flexural reinforcement

consists of 2-20M (single bar area of 300 mm2) for W1 and 4-20M (in two layers) for W2,

located at each end of the walls. Uniformly distributed longitudinal reinforcement consists of

3-10M (single bar area of 100 mm2). The lap splice lengths are about 30 times the bar

diameter (600 mm for 20M bars and 350 mm for 10M bars). The transverse reinforcement

consists of single leg 10M bars with a 90° hook and 6db extension (≈ 60 mm) at each end to

simulate the poor details in existing walls, with the hook oriented vertically without

anchorage around the longitudinal bars at the ends of the thin walls.

It is noted that the lap lengths are considerably shorter than that required by the current ACI

Code [11] and the lap splices are located in a critical region of expected plastic hinging. The

lengths and positions of the lap splices are shown in Fig. 2(a).

2.1.2 Retrofitted Walls WRT1 and WRT2

The retrofit technique involves only the use of carbon fibre wrap, in order to study the effect

of retrofitting with minimal intervention. In this method, the main objective was to prevent

premature failure of the lap splice and to achieve some yielding in the concentrated

reinforcement such that a displacement ductility level of about 2.0 could be reached. Walls

WRT1 and WRT2 had the same structural details as their companions W1 and W2, with the

concrete strengths being slightly higher for retrofitted Walls due to the increase in strength

over time.

.

Figure 2. (a) Detail of as-built wall; (b) retrofit details for WRT1 and WRT2

The entire lap splice length and the potential plastic hinging region was wrapped using uni-

directional CFRP over a length of 180051 =wl. mm. Above this region, the walls were

strengthened in shear by applying carbon fibre wrap strips of 100 mm width, spaced

uniformly at 250 mm, centre-to-centre. The preparation of the concrete surfaces and the

application and carbon fibre wrap was done in accordance with the requirements of ACI

Committee 440 [12]. The carbon wrap was provided with an overlap length of 300 mm on

300

mm

ov

erla

p

Rounded, r = 20 mm

t= 1 mm

300

mm

ov

erla

p

WRT1 WRT2

A

100 mm

1800 mm

A

350 mm

10M lap

splice

Reversed

cyclic

loading

150 mm

600 mm

20M lap

splice

250 mm

CFRP

10M single

leg ties @250 mm

(a) (b)

Fibre

direction

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one side of the wall. Fig. 2(b) illustrates the details of the retrofit technique

2.1.3 Repaired Walls WRP1 and WRP2

Walls W1 and W2 were repaired after initial test by removing the damaged concrete over the

lap splice region (600 mm). A steel fibre reinforced self consolidating concrete (SFRSCC)

jacket was added over the lap splice zone, increasing the thickness of the wall from 150 mm

to 350 mm. Supplementary flexural reinforcement, anchored properly into the foundation

block, were added to the boundaries and to the web of this new section, prior to casting.

Additional transverse reinforcement was provided in accordance to modern seismic

provisions. A combination of steel threaded rods and surface roughening was used for a better

shear transfer between old and new concrete. Details of the repair technique are shown in Fig.

3.

Figure 3. (a) Details of the repaired walls; (b) cross section of repaired walls WRP1 and WRP2

2.2 Material Properties

The target strength of the concrete used in construction of walls was 30 MPa, a typical

concrete strength for construction in the 1960’s. The steel reinforcement consisted of Grade

400 (minimum specified yield strength of 400 MPa) deformed reinforcing bars.

The carbon fibre wrap and epoxy composite laminate has a design thickness of 1 mm and a

corresponding design strength of 834 MPa in the fibre direction and a breaking strain of

0.85%, according to the supplier’s specifications. The carbon fibre wrap has no strength in the

direction perpendicular to the strips.

The SFRSCC consists of a dry mix SCC concrete (typical 28 days compressive strength of 40

MPa), mixed with 25 mm straight steel fibres. Fibre content was 0.5% of concrete volume.

The actual material properties of the concrete and the reinforcing steel are summarized in

Tables 1, and 2.

2.3 Loading and Instrumentation

Each reversed cycle of loading consisted of positive (upwards) and negative loading

(downwards). The walls were cycled to selected load levels up to general yielding. After

general yielding the walls were cycled to multiples of the yield deflection (1.5∆y, 2.0∆y, …).

Three reversed cycles were carried at each load or deflection level. The reversed cyclic

loading histories of the specimens are illustrated in Fig. 4.

4 - 15M

10M @ 80 mmStirrups overlayed

at the Centre

Threaded Rod

10M @ 80 mmStirrups overlayed

at the Centre

2 - 15M

4 - 20M

2 - 20M

350 mm 350 mm

120

0 m

m

WRP1 WRP2

75 115

FRSCC

jacket

100 mm

1200 mm

250 mm

CFRP

600 mmB B

350 mm

150 mm

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Table 1. Average concrete material properties.

Wall cf ′ cε ′

rf spf

(MPa) (mm/mm) (MPa) (MPa)

W1 31.2 0.0023 3.79 3.34

W2 30.4 0.0021 4.74 3.50

WRT1 32.4 0.0021 4.06 3.37

WRT2 32.8 0.0021 4.73 4.05

Table 2. Reinforcing Steel Material Properties

Specimen Diameter Area yf yε

uf

(mm) (mm2) (MPa) (mm/mm) (MPa)

10M 11.3 100 470 0.0024 727

15M 16.0 200 426 0.0021 728

20M 19.5 300 460 0.0023 637

0 6 12 18 24 30 36 42 48-320

-240

-160

-80

0

80

160

240

320

Load (

kN

)

Cycle

W1

Load control

0 6 12 18 24 30 36 42 48-320

-240

-160

-80

0

80

160

240

320

Load (

kN

)

Cycle

W2

Load control

0 6 12 18 24 30 36 42 48-320

-240

-160

-80

0

80

160

240

320

Load (

kN

)

Cycle

WRT1

Displacement controlLoad control

Dy

2.0Dy

0 6 12 18 24 30 36 42 48-320

-240

-160

-80

0

80

160

240

320

Load

(kN

)

Cycle

WRT2

Load control Displacement control

Dy2.0D

y

0 6 12 18 24 30 36 42 48-320

-240

-160

-80

0

80

160

240

320

Loa

d (

kN

)

3.5Dy

Dy

Cycle

WRP1

Load control Displacement control

0 6 12 18 24 30 36 42 48-320

-240

-160

-80

0

80

160

240

320

Lo

ad (

kN

)

2.5DyDy

Load control

Cycle

WRP2

Displacement control

Figure 4. Reversed cyclic loading histories

Walls W1 and W2 were tested entirely in load control until failure because flexural yielding

did not occur. Other specimens which experienced flexural yielding, were tested in both load

and deflection control. Testing was stopped at the positive and negative peaks of each cycle to

take photographs and to determine crack widths and the cracking pattern. During loading,

data was collected from the load cells, linear voltage differential transformers (LVDTs), and

strain gages on the reinforcing bars. A potentiometer was used to measure the tip deflection of

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the specimen at the loading point. The LVDTs enabled average strains to be determined in key

locations with localized steel strains obtained from strain gages.

Because the walls were tested in horizontal position, the effects of the self weight of the wall

and the loading devices were accounted for in determining the actual loads applied to the

wall.

3. Response of the Walls

3.1 As-built walls W1 and W2

The shear versus tip deflection response of Walls W1 and W2 are described in Fig. 5(a). The

non-ductile response of both walls was due to brittle side splitting failure of the lap splices of

the 20M bars prior to yielding, and led to a significant drop in capacity. The predicted

nominal flexural resistance of walls W1 and W2, neglecting any strain hardening, were 386

kNm and 673 kNm, respectively. The wall W1 reached a shear of 95.2 kN, corresponding to

an applied moment of 309 kNm (80% of the predicted flexural capacity). The maximum shear

reached for W2 was 140.5 kN, corresponding to an applied moment of 455 kNm (68% of the

predicted nominal flexural capacity).

3.2 Retrofitted Walls WRT1 and WRT2

The general yield deflection, ∆y, was determined using the secant stiffness of the response as

proposed by Park (1988) [13]. The maximum deflection at which the wall could endure

without the capacity dropping below 80% of the maximum load was considered as the

ultimate deflection, ∆u.

Fig. 5(b) shows the shear versus tip deflection responses of specimens WRT1 and WRT2. The

response of the walls indicates that the premature brittle failure of the lap splice was delayed

and the walls achieved a displacement ductility level of 2.0. Retrofitted walls, WRT1 and

WRT2, experienced 21% and 54% increase in the flexural strength compared to the

companion specimens W1 and W2.

Strains on the main longitudinal bars and the dowel bars indicated yielding at the critical

section as well as spreading of yield along the bars and yield penetration into the foundation.

Above the potential plastic hinging region, the carbon fibre strips controlled the diagonal

shear cracks.

3.3 Repaired Walls WRP1 and WRP2

The shear versus tip deflection response of WRP1 and WRP2 are presented in Fig. 5(c). Wall

WRP1 achieved a displacement ductility of about 3.5, while WRP2 achieved a ductility of

2.5. The maximum shear load experienced for WRP1 and WRP2 was 157 kN, and 270 kN,

respectively.

For these specimens, the critical section has been shifted from the base of the wall to a

location at the end of the SFRSCC jacket (600 mm from the base of the wall). The hysteresis

loops indicate that a significant amount of energy was dissipated through formation of the

plastic hinge for specimens WRP1 and WRP2. The repair prevented the failure of the lap

splice, and both specimens had a large reserve of strength after general yielding. The

resistance of the wall gradually degraded due to crushing of concrete just above the jacket.

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-30 -15 0 15 30

-150

-100

-50

0

50

100

150

Tip Deflection (mm)

Ap

plie

d s

he

ar

(kN

)

Maximum Load

W1

-45 -30 -15 0 15 30 45

-250

-200

-150

-100

-50

0

50

100

150

200

250

W2

Maximum Load

Tip Deflection (mm)

Applie

d s

hear

(kN

)

-30 -15 0 15 30

-150

-100

-50

0

50

100

150

WRT1

Maximum LoadGeneral yield

Tip Deflection (mm)

Ap

plie

d s

he

ar

(kN

)

-45 -30 -15 0 15 30 45

-250

-200

-150

-100

-50

0

50

100

150

200

250

WRT2

General yieldMaximum Load

Tip Deflection (mm)

App

lied

sh

ea

r (k

N)

-60 -45 -30 -15 0 15 30 45 60

-200

-150

-100

-50

0

50

100

150

200

WRP1

General yieldMaximum Load

Tip Deflection (mm)

Ap

plie

d s

he

ar

(kN

)

-75 -60 -45 -30 -15 0 15 30 45 60 75

-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

300

WRP2

General yieldMaximum Load

Tip Deflection (mm)

Ap

plie

d s

he

ar

(kN

)

Figure 5. Reversed cyclic response of a) W1 and W2, b) WRT1 and WRT2, c) WRP1 and WRP2

4. Conclusions

The reversed cyclic responses of existing deficient shear walls were studied. The as-built walls had inadequate lap splices in the flexural reinforcement at the base of the wall and inadequately anchored transverse reinforcement offering no confinement at the ends of the walls. These walls experienced sudden failure of the lap splice prior to general yielding.

The retrofit method consisted of applying CFRP wrap that was designed as a minimal intervention technique, aimed to prevent the premature failure of the lap splice and provide some yielding. The retrofitted walls were able to develop their nominal flexural capacities,

(a)

(b)

(c)

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and achieved a ductility of 2.0.

The repair technique consisted of a SFRSCC jacket over the lap splice region, which increased the nominal flexural capacity of the wall at its base. The walls developed significant yielding in the flexural bars and achieved higher displacement ductilities and flexural moment capacities.

This research provides a simple, cost-effective means of retrofitting and repairing deficient RC walls

5. Acknowledgements

The authors gratefully acknowledge the financial support provided by the Canadian Seismic

Research Network (CSRN), funded by the Natural Sciences and Engineering Research

Council of Canada (NSERC).

6. References

[1] GHOBARAH, A. “Seismic assessment of existing RC structures”, Journal of Progress in Structural Engineering and Materials, Vol. 2, No. 1, Jan/March 2000, pp. 60-71.

[2] PRIESTLEY, M. J. N., SEIBLE, F. “Design of seismic retrofit measures for concrete and masonry structures”, Journal of Construction and Building Materials, Vol. 9, No. 6, Month 1995, pp. 365-377.

[3] FIORATO, A.E., Oesterle, R.G., and Corley, W.G. “Behavior of Earthquake Resistant Structural Walls Before and After Repair”, ACI Structural Journal, Vol. 80, No. 5, September 1983, pp. 403-413.

[4] VECCHIO, F.J., Haro de la penta, O.A., Bucci, F., and Palermo, D., “Behavior of Repaired Cyclically Loaded Shearwalls”, ACI Structural Journal, Vol. 99, No. 3, May 2002, pp. 327-334.

[5] Applied Technology Council (ATC) “Seismic evaluation and retrofit of concrete buildings” (ATC-40 Report), Redwood City, CA, November 1996, 612 p.

[6] HARRIES, K.A., RICLES, J.R., PESSIKI, S., and SAUSE, R. “Seismic retrofit of lap-splices in non-ductile square columns using carbon fiber-reinforced jackets” ACI Structural Journal., Vol. 103, No. 6, pp. 874-884.

[7] PATERSON, J., and MITCHELL, D. “Seismic retrofit of shear walls with headed bars and carbon fiber wrap” ASCE Journal of Structural Engineering, Vol. 129, No. 5, May 2003, pp. 606-614.

[8] ELGAWADY, M., ENDESHAW, M., McLean, D., and SACK, R. “Retrofitting of rectangular columns with deficient lap splices” ASCE Journal of Composites for Constrcution, Vol. 14, No. 1, January 2010, pp. 22-35.

[9] Colalillo, M. A., Sheikh, S.A. “Seismic retrofit of shear-critical reinforced concrete beams using CFRP”, Journal of Construction and Building Materials, Available online April 2011, In press

[10] American Concrete Institute (ACI) Committee 318 “Building code requirements for reinforced concrete” ACI 318-63, Detroit, MI., 1963

[11] American Concrete Institute (ACI) Committee 318 “Building code requirements for reinforced concrete” ACI 318-2011, Farmington Hills, MI., 2011

[12] American Concrete Institute (ACI) Committee 440 “Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures”, ACI 440-02, Farmington Hills, MI, 2002, 45 p.

[13] Park, R. “Ductility evaluation from laboratory and analytical testing.” Proc. 9th World Conf. Earthquake Eng. Tokyo-Kyoto, Japan, VIII, 1988, pp. 605–616.