Beam Jacketing MS

ORIGINAL ARTICLE

Strengthening of reinforced concrete beams in flexureby partial jacketing

Ibrahim Abd El Malik Shehata ÆLidia da Conceicao Domingues Shehata ÆEuler Wagner Freitas Santos ÆMaria Luisa de Faria Simoes

Received: 24 August 2007 / Accepted: 12 June 2008 / Published online: 22 June 2008

� RILEM 2008

Abstract This work investigates the structural

behaviour of reinforced concrete beams strengthened

in bending by the addition of concrete and steel on

their tension sides using expansion bolts as shear

connectors, technique here denominated partial jac-

keting. The experimental program comprised tests on

eight full-scale reinforced concrete beams, simply

supported, with rectangular cross section

(150 mm 9 400 mm) and 4,500 mm length. Five of

these beams were strengthened in bending by partial

jacketing, while the other three did not receive any

strengthening and served as reference beams. The

flexural reinforcement ratio in the beams varied

between 0.49% and 2.33% and the beams target

concrete strength was 35 MPa. On the basis of the

obtained test results, the studied strengthening tech-

nique proven to be efficient in terms of increasing the

resistance and stiffness of the beams. The used

expansion bolts as shear connectors proven to be

practical and added ease to the application of this

technique.

Keywords Strengthening � Flexural �Beams � Partial jacketing � Reinforced concrete

Notations

Ast Total area of main steel

Asb Area of main steel in the beam

Asr Added area of main steel in the jacket

Asw Area of beam web steel

b Beam breadth

d Effective beam depth

fcm Average concrete compressive strength

fct Tensile strength of concrete

fy Yield strength of reinforcement

L Length, span

Pu Ultimate theoretical load

Pu,exp Ultimate experimental load

s Spacing of web steel

/ Diameter

qst Total geometrical ratio of main reinforcement

qsw Geometrical ratio of web reinforcement

1 Introduction

Strengthening of beam in flexure can be achieved by

composing its section with new structural elements,

either steel or reinforced concrete.

The use of composite beams to resist forces goes

far back in structure history. In old ages, engineers

I. Abd El Malik Shehata (&) �L. da Conceicao Domingues Shehata �E. Wagner Freitas Santos � M. L. de Faria Simoes

Department of Civil Engneering, COPPE—The Federal

University of Rio de Janeiro, P.O. Box 68506,

CEP 21945-970 Rio de Janeiro, RJ, Brazil

e-mail: [email protected]

L. da Conceicao Domingues Shehata

Universidade Federal Fluminense, Niteroi, RJ, Brazil

Materials and Structures (2009) 42:495–504

DOI 10.1617/s11527-008-9397-3

used layered timber beams glued or tied to one

another to form a single strong beam.

In order to get the full benefit of composite beam

section to the flexural strength, the connection

between the assembled parts to compose the beam

section should be able to transmit longitudinal shear

stresses.

Generally, in reinforced concrete beams, the shear

stresses can be transmitted across the connection by

adhesion, by shear-friction at the concrete interface

and by the dowel action of reinforcing bars that cross

the connection.

Equations for evaluation of the shear resistance of

such connections were first suggested by the ACI

code in 1963 and were based on the results of

different research works summarized in the ACI-

ASCE 333 report.

In 1970, the concept of shear-friction was intro-

duced by the ACI and was validated by the results of

direct shear tests on concrete blocks. During the

seventies and eighties, other empirical and analytical

expressions based on direct shear tests were also

proposed.

There are only a few studies on composite beams,

though work started as early as 1964 by Saemann and

Washa [11] followed by Nosseir and Murtha [9],

Loov and Patnaik [8], Araujo [3], Tan et al. [13] and

more recently by Gohnert [6].

Tests on strengthened composite beams by jacket-

ing are also few, and started as early as 1988 by

Alexandre et al. [1] followed by Souza [12], Liew

and Choeng [7], Choeng and Macalevey [4],

Piancastelli [10], and lately by Altun [2]. Only

Piancastelli [11] treated the case of partially jacketed

beams.

The present work aimed to contribute to the

understanding of the behaviour of strengthened

beams with partial jacketing using expansion bolts

as shear connectors.

2 Experimental program

This research program comprised tests on eight

beams divided into three groups. All beams had a

rectangular cross section of 150 mm 9 400 mm and

a total length of 4,500 mm. In these beams, the main

variables were the amount of original main steel and

amount of added steel in the jacket for flexural

strengthening. Both the compressive mounting steel,

composed of two 8 mm bars, and the stirrups,

composed of 8 mm bars spaced at 150 mm, were

kept constant in all beams. The provided stirrups

were such as to prevent shear failure in any of the

beams and so were equal to the required for beam

REF3 with the highest flexural steel ratio (2.33%).

The first group (A) was composed of three beams

with original amount of main steel equal to 285 mm2

(qsb = 0.49%) that were strengthened in flexure by

adding three different areas of external steel,

300 mm2, 600 mm2 and 800 mm2. The second group

(B) consisted of two beams with 600 mm2

(qsb = 1.08%) of original main steel and likewise

the first group was strengthened in flexure by adding

steel areas of 300 mm2 and 600 mm2. Three reference

beams formed the third group (C), with main steel

areas equal to 285 mm2 (qsb = 0.49%), 600 mm2

(qsb = 1.08%) and 1,230 mm2 (qsb = 2.33%). Two

reference beams had amounts of steel equal to the

original steel of the first and second groups while the

third one had an amount corresponding, approxi-

mately, to the balanced one. All used steel had

nominal yield strength of 500 MPa and was of the

round ribbed type. Details of the reinforcement of all

beams are given in Table 1 and are shown in Fig. 1.

The concrete used for the fabrication of the

beams was made of coarse aggregate with

maximum size of 19 mm (crushed stone type-

gneiss), river sand and rapid hardening cement. The

mix proportion, by mass, was of 1:2.71:3.58

Table 1 Characteristics of tested beams

Beam fcm

(MPa)

d(mm)

Asb

(mm2)

Asr

(mm2)

qst

(%)

qsw

(%)

Asw/s(mm2/mm)

Group A

V1-A 41.6 382 285 300 1.02 0.45 0.67

V2-A 38.6 402 285 600 1.47 0.45 0.67

V3-A 39.2 409 285 800 1.77 0.45 0.67

Group B

V1-B 36.4 360 600 300 1.67 0.45 0.67

V2-B 41.4 377 600 600 2.12 0.45 0.67

Group C

REF1 36.2 386 285 – 0.49 0.45 0.67

REF2 41.4 369 600 – 1.08 0.45 0.67

REF3 40.8 351 1230 – 2.33 0.45 0.67

496 Materials and Structures (2009) 42:495–504

(cement: sand: coarse aggregate), with water/cement

ratio = 0.6 and cement content equal to 300 kg/m3.

The average compressive strength of the concrete

for all beams is given in Table 1. The concrete

used for the jackets had the same mix proportion

and constituents as that of the beams except for

the maximum aggregate size which was 10 mm,

and had an average compressive strength of

32 MPa and an average indirect tensile strength of

2.5 MPa.

3 Beams strengthening

After the two initial loading cycles (explained in

Sect. 4) applied to pre-crack the beams, two lines, one

on each side of the beams, of expansion bolts holes

spaced at 150 mm (inner stirrups spacing) were drilled

(see Fig. 2). Each hole was distant 50 mm from the

bottom face of the beams and had a depth of 65 mm.

The position of each expansion bolt hole was chosen

to be as close as possible to an original beam stirrup

r = 90

4470N2 - 5140mm

N1 - 4450mm

N3 - 4470mm

r = 90

N4 - Ø8 @150mm

A

A

7575

300

N1-2Ø8 mm

N2-2Ø16mm

N3-Ø16mm

385

370

120

80

80

Ø8 @150mm

Section A-A for beams V1-B, V2-B & VREF2

150

N1-2Ø8 mm

N2- 3Ø16mm

N3-2Ø20mm

385

370

120

80

Section A-A for beam VREF3

150

Ø8 @150mm

N1-2Ø8 mm

N2-2Ø10mm

N3-Ø12.5mm

400

380

120

80

80

Ø8 @150mm

150

Section A-A for beams V1-A, V2-A, V3-A and REF1

Fig. 1 Details of beam

original reinforcement

(dimensions in mm)

Materials and Structures (2009) 42:495–504 497

and just above the beam main steel, in order to provide

good anchorage condition for the expansion bolts even

in the cracked stages of the beams.

As the bolts spacing was fixed to 150 mm (stirrups

spacing), the definition of the bolts diameter depended

upon the maximum shearing force that would occur

between the beam and the jacket. The value of this

force corresponded to the one of beam V3-A, which

had the maximum jacket steel (800 mm2). For a

nominal jacket steel yield strength of 500 MPa, this

force is equal to 400 kN and the force per bolt,

considering 24 bolts in half length of the jacket, is

found to be 16.67 kN (giving an average shear

strength = 0.79 MPa). If the contribution of concrete

to the shear resistance of the interface between the

beam and the jacket is ignored, considering the bolt

nominal shear strength (half its yield strength—Tresca

criteria of failure) equal to 250 MPa as provided by the

manufacturer, the required bolt area is 67 mm2, which

lead to a commercial bolt diameter of 10 mm.

Following the holes drilling, the beams bottom

surfaces and two bottom side bands of 80 mm in

width were prepared by removing the concrete cover

and leaving out a rough surface on a central extension

of 3,840 mm of the beam length. Expansion bolts of

10 mm in diameter and 110 mm in total length (see

Fig. 3) were then installed in the previously drilled

holes, leaving out, approximately, half of their

lengths exposed without the outer sleeves. The outer

sleeves were removed after bolts fixation, in order to

assure good anchorage between the bolts used as

shear connectors and the jacket concrete.

150

150

100100

470

70

Strengthening Jacket

Original Beam

SECTION A-A

A

A

80

4500

3840

80Strengthening Jacket

Expansion Bolts10mm x 110 mm

25 expansion bolts @150mm each side

A

A

65 6570 12

0

SECTION A-A

50mm

40mm

Stirrups

BDETAIL B

Fig. 2 Strengthening

details (dimensions in mm)


The strengthening steel cages shown in Fig. 4

were then tied to the exposed expansion bolts as seen

in Fig. 5. Following the fixation of trapezoidal

formwork to the bottom of the beam, the jacket

concrete was cast. After the complete curing of the

jacket concrete for about 14 days, the strengthened

beams were replaced in the test rig for the application

of the last loading cycle.

4 Test procedure and results

The beams were simply supported at a span of

4,000 mm (from centre to centre) in the test rig seen

in Fig. 6. They were loaded at their centre by means

of a servo controlled hydraulic jack (load/displace-

ment) with a 500 kN capacity. All the beams had

instrumentation to measure the longitudinal steel

deformations (both beam and jacket steel), the

Fig. 3 Expansion bolts used as shear connectors

Strengthening of beams V1-A and V1-B

2Ø8

N1 - 4Ø8 - 3810mm

N2 - 2Ø8 - 3810mm

N3-Ø5 @150mm (see detail)

Strengthening of beams V2-A and V2-B

N1 - 4Ø8 - 3810mm

N4 - 2Ø16 - 3810mm


Strengthening of beams V3-A

N1 - 4Ø8 - 3810mm

N4 - 3Ø16 - 3810mm


4Ø8

2Ø16

4Ø8

3Ø16

4Ø8

Expansion Bolts

A

A

B

B

C

C

Section A-A

Section B-B

Section C-C34

438

644

6

344

386

442

344

386

442

Ø10 @150mm

120

7070

120

180

80

N3 - Ø5.0 - 715mm

Stirrups detail - N3

Fig. 4 Details of

strengthening reinforcement

in the jackets (dimensions

in mm)


concrete deformations, the beams deflections at

sections near mid-span and the slipping between

the jacket and beam along the interface.

The beams were loaded in steps of 10 or 20 kN,

according to the beam estimated loading capacity,

that lasted for about 10 min each, during which the

measurements were taken together with crack map-

ping and crack width measurement.

Generally, all the beams had three loading cycles.

In the first, the beams initially uncracked and still

without strengthening were loaded until the strains in

their flexural steel reached a value close to 2%, and

then unloaded. The second cycle was a repetition of

the first one but with the beams in the pre-cracked

state. The last load cycle, which had lead to the

beams failure, was applied to the reference beams

soon after the second cycle, while to the beams of

group A and B after strengthening.

Table 2 gives the experimental ultimate load for

the tested beams and the obtained modes of failure.

Figures 7 and 8 show the aspect of the typical

flexural and shear modes of failure occurred in the

tested beams. It is worth noting that, in beam V2-A,

the shearing of the jacket occurred long after the

yielding of the flexural reinforcement, while, in

beam V3-A, both occurred practically at the same

time.

The obtained results for the beams deflections, the

beams main steel strains, the jackets main steel

strains and the maximum relative displacement

between the beams and the jackets are shown in

Figs. 9–14. Relative displacement between the beam

and jacket occurred only in beams V2-A and V3-A

Fig. 5 Fixation of the

strengthening steel cages

Reaction floor

Beam

Jack

250 00020002 250

Hinge Roller

(dimensions in mm)

Fig. 6 Test rigTable 2 Ultimate loads and modes of failure

Beam qst (%) d (mm) Pu,exp (kN) Failure mode

V1-A 1.02 382 150 Flexural

V2-A 1.47 402 205 Flexural/Shear

V3-A 1.77 409 229 Flexural/Shear

V1-B 1.67 360 186 Flexural

V2-B 2.12 377 235 Flexural

REF1 0.49 386 72 Flexural




and, hence, only two curves are provided at the

locations where maximum displacements occurred,

which were at the jacket ends (Figs. 13 and 14).

5 Analysis of the results

Considering the rectangular stress block as defined by

the CEB-FIP MC-90 [5] and the yielding of all

longitudinal steel, the resistance flexure moment of

Fig. 7 Typical flexural failure for the strengthened beams

(V1-B)

Fig. 8 Typical shear failure between the beam and jacket for

the strengthened beams (V3-A)

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80

Deflection (mm)

Lo

ad (

kN)

V1-A V2-A V3-A REF1 REF3

Fig. 9 Load-deflection curves for the beams of the first group

together with reference beams REF1 and REF3

0

50

100

150

200

250

0 20 40 60 80 100

Deflection (mm)

Lo

ad (

kN)

V1-B V2-B REF2 RFF3

Fig. 10 Load-deflection curves for the beams of the second

group together with reference beams REF2 and REF3

0

50

100

150

200

250

0 5 10 15 20

Steel strain (E-3)

Lo

ad (k

N)

V1-A JV1-A V2-A

JV2-A V3-A JV3-A

REF1

Fig. 11 Load-main steel strain at mid span curves for the

beams of the first group together with reference beam REF1

(letter J refers to the Jacket)

0

50

100

150

200

250

0 10 20 30 40 50

Steel strain (E-3)

Lo

ad (k

N)

V1-B JV1-B V2-B JV2-B REF2

Fig. 12 Load-main steel strain at mid span curves for the

beams of the second group together with reference beam REF2

(letter J refers to the Jacket)


the strengthened beam section can be calculated as

(see Fig. 15):

Mu ¼ As fyðd � 0:4xÞ þ Asr1 fyðd1 � 0:4xÞ þ Asr2 fy

ðd2 � 0:4xÞ þ A0

s fyð0:4x� d0 Þ ð1Þ

with the neutral axis positioned at

x ¼ As fy þ Asr1 fy þ Asr2 fy � A0s fy

0:85fcð Þ � 0:8bð Þ ð2Þ

The ultimate load (Pu) and average ultimate shear

stress (su) at the interface between the original beam

and the jacket, considering the load arrangement of

the beam (simply supported with concentrated load at

mid span), can be calculated from the resistance

moment and from the force in the jacket steel at yield

stress, so as (see Fig. 16)

Pu ¼4 �Mu

Lð3Þ

su ¼P

Asr � fy

Aið4Þ

where Ai is the interface area between the beam and

the jacket, L is the beam nominal span.

For the tested beams the interface area (Ai) is (see

Fig. 16)

Ai ¼ 80þ 120þ 80ð Þ � 1920 ¼ 537600 mm2

Table 3 gives the calculated and experimental

values for the ultimate load together with the ultimate

shear stress at yield of the jacket steel. In this table it

is also quoted the ratios between the experimental

and theoretical values of the ultimate load together

with the ratio between the experimental ultimate load

for the strengthened beams and the ultimate load of

the reference beam for the corresponding group.

From the comparisons made in that table (Pu,exp / Pu

and Pu,exp / PREF*), it is evident that the composite

strengthened section of the tested beams acted

monolithically until yielding of the main reinforce-

ment and the increase in the beams strength varied

0

50

100

150

200

250

0

Relative displacement (mm)

Lo

ad (k

N)

V2-A

0.2 0.4 0.6 0.8 1 1.2

Fig. 13 Load-maximum relative displacement curves between

the beam V2-A and the jacket

0

50

100

150

200

250

0 5 10 15 20

Relative displacement (mm)

Lo

ad (

kN)

V3-A

Fig. 14 Load-maximum relative displacement curves between

the beam V3-A and the jacket

Tr1

CcCs

As'

As

ε'sε

ε

c

0.8x x

0.85fcb

h

Asr,1

d1dd2

Asr,2sr2

ε sε sr1

T

Tr2

Fig. 15 Section analysis


between 43% and 210% according to the added

amount of steel.

Only beams V2-A and V3-A had relative dis-

placement between the beams and jackets after yield

loads, 150 kN and 175 kN, respectively, as seen in

Figs. 13 and 14. In beam V2-A, the relative dis-

placement stayed below 1 mm up to the ultimate

load, while in beam V3-A it reached 17 mm at the

end of the jackets, where maximum displacement

occurred in both beams. From the values of Pu,exp / Pu

of those beams quoted in Table 3, it can be concluded

that the shear strength was very close to the

theoretical flexural strength of both beams and the

shear strength slightly affected the flexure strength by

dropping the value of Pu,exp / Pu from 1.14, average

value obtained for the beams failed in flexure, to 1.06

and 1.00 for V2-A and V3-A, respectively.

Figures 9 and 10 show the comparison between the

load-deflection curves for both strengthened (groups A

and B) and reference beams (group C). From these

figures it can be seen that the strengthened beams have

gained both rigidity and strength as the amount of steel

added to them in the jackets increased. The load-

deflection curves of the strengthened beams lie above

that of the reference beam of each group (REF1 or

REF2) and show comparable or even better behaviour

than the one of the reference beam with highest steel

ratio (REF3). The higher strength and/or rigidity of the

strengthened beams V3-A (Ast = 1,085 mm2) and

V2-B (Ast = 1,200 mm2) in comparison to the refer-

ence beam REF3 (Ast = 1,230 mm2) can be attributed

to the differences in the effective depth of those beams:

409mm, 377mm and 351 mm, respectively.

As for the steel strain, the comparison of Figs. 11

and 12 show that the jacket steel strain followed closely

the original beam steel strain at all load levels till the

yield of both steels. After this stage, there are slight

differences between the strains of both steels, except

for beams V2-A and V3-A, which showed higher

differences due to the exertion of beam-to-jacket shear.

As for the concrete contribution to the shear

strength of the connection between the beam and

jacket, it is evident from the result of beam V3-A that

such contribution does not exist in the ultimate limit

Section

Shear stress

Tr = ΣAsr.f y

ττ

Ls = 1920mm

80

120

PFig. 16 Shear stress

transfer at the interface

Table 3 Comparisons between estimated and experimental failure loads

Beam qst d (mm) Pu (kN) Pu,exp (kN) Pu,exp / Pu Pu,exp / PREF* su (MPa)

V1-A 1.02 382 130 150 1.15 2.08 0.28

V2-A 1.47 402 193 205 1.06 2.84 0.56

V3-A 1.77 409 229 229 1.00 3.10 0.74

V1-B 1.67 360 156 186 1.19 1.43 0.28

V2-B 2.12 377 212 235 1.11 1.80 0.56

REF1 0.49 386 64 72 1.13 – –

REF2 1.08 369 112 130 1.16 – –

REF3 2.33 351 197 219 1.11 – –

Pu = theoretical ultimate load based on the rectangular stress block for concrete and nominal yield strength for steel = 500 MPa

Pu,exp = experimental ultimate load

* For Group A PREF = Pu,exp for beam REF1 = 72 kN and for Group B PREF = Pu,exp for beam REF2 = 130 kN


state. This beam had the shear strength of its

expansion bolts designed to be equal to the maximum

force in the jacket (400 kN) and, at ultimate load, the

jacket sheared off the beam right after yielding of the

jacket steel, which means that concrete did not

provide any contribution to the shear resistance of the

connection. The lack of concrete contribution can be

explained by the modified Mohr–Coulomb criteria of

failure for concrete, which predicts zero shear

strength combined with a tensile normal stress equal

to fct (tensile concrete strength), state of stress for

concrete at the connection.

For all other beams, where the shear strength of the

bolts were higher than the maximum force in the jacket,

shearing of the connection did not occur or was a

secondary mode of failure as happened in beam V2-A.

6 Conclusions

This work presents a simple and efficient technique to

strengthen beams in flexure using traditional materi-

als and construction procedures. The introduction of

expansion bolts as shear connectors added quickness

and ease to the application of the strengthening.

The test results have proven that this technique is

efficient once the connection is properly designed. As

a general rule, for the design of the connection it is

recommended that:

• No count is made for concrete contribution to the

shear strength of the connection.

• The amount of the expansion shear bolts is

calculated so as their shear strength be more than

or equal to the maximum force in the jacket.

• The insertion of the expansion bolts in either the

beam or in the jacket should be greater than five

times the bolt diameter and not lesser than 50 mm

(based on manufacturer recommendations and on

test carried out in another research program that

will be the subject of another paper), in order to

get proper anchorage.

• Holes of the expansion bolts should be as close as

possible to the original stirrups and original main

steel of the beams.

• Exposed part of the expansion bolts should be left

without the extension (outer) sleeves and should

be as close as possible to a jacket stirrups and

jacket main steel.

• Although no count for concrete contribution to

the shear strength of the connection is made, it is

recommended that a proper surface roughness of

the beam surface is made, in order to get good

adhesion between the beam and the jacket con-

cretes for durability purposes.

Acknowledgments The authors would like to thank

HOLCIM and the Brazilian government financing agencies

CNPq and CAPES for supporting this project.

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