capacity of strengthened reinforced concrete columns

Port Said University

Faculty of Engineering

Civil Engineering Department

Capacity of Strengthened Reinforced

Concrete Columns

Eng. Khaled Mohamed Mahmoud Ahmed

Senior Engineer, Suez Canal Authority

Email: Khaled.mahmoud@suezcanal.gov.eg

Tele: +201003915088

Prof. Dr. Kamal Gad SharobemProfessor of properties and strength of materials

Vice President of Suez Canal University for Community Service

and Environmental Development - Suez Canal University

Prof. Dr. Hassan Mohamed Hassan IbrahimProfessor of concrete structures

Faculty of Engineering

Port Said University

SUPERVISORS OF THESIS

Introduction Column definition : Structural element which transfers the loads

from slabs to foundation.

The reasons of columns strengthening:

1) Error at design or bad detailing.

2) Improper quality control measures (poor construction

materials and workmanship).

3) Unexpected loading, like earthquakes, strong winds,

impact and explosive loading.

4) Functional changes in the service of the structure.

5) Deterioration by time.

The difference between : repair-strengthening-repair and strengthening.

Concrete jacket Steel jacket FRP jacket

The techniques of columns strengthening:

The most common methods for columns' strengthening in Egypt are

concrete or steel jackets.

Structural engineers don't prefer using FRP jackets because of :

1-high cost 2-lacking of trained workers 3-weakness of fire protection.

There is still a need of design equations to refine the design of

strengthening.

So this work focused on the analysis of concrete and steel jackets to :

1- Get simple equations for design.

2- Perform experimental investigation.

3- Compare theoretical analysis by experimental work to validate the

refined equations.

The two types of columns jackets were used at sites :

Concrete jacket The cage

Reinforced concrete jacket:

Advantages:

economy, compatibility with the original concrete

substrate, and the ability to enhance durability and impact

fire protection.

Disadvantages:

the loss of floor space due to enlargement of the column

cross-section, and difficulties in casting and compacting.

Steel jacket:

Advantages:

increase the confinement, capacities, ductility and

stiffness. Steel jackets/cages have also advantageous due

to their light weight and insignificant increase in cross

sectional area of the column compared to concrete

jackets.

Disadvantages:

Compared to concrete jacket: More expensive and

weaker at fire protection.

The main objectives of this research are:

Study analytically and experimentally the efficiency of RC

columns strengthened by concrete and steel jackets.

To study the effect of direct and indirect load applied to jackets

(i.e. to simulate the situation where it is not feasible to connect

the jacket to the roof slabs/beams).

To define the expected failure modes.

To compare the predicted load carrying capacity with that

obtained from the current experimental works and that performed

by others.

To refine the design equations for concrete and steel jackets.

Outlines of Thesis

Chapter 1 Introduction.

Chapter 2 Literature review.

Chapter 3 Strengthening design methods.

Chapter 4 Experimental investigation.

Chapter 5 Theoretical investigation and

parametric study.

Chapter 6 Conclusion and recommendations.

Load transmitting between column and jacket.

Load transmitting between column and concrete jacket

Load transmitting between column and jacket.

Load transmitting between column and steel jacket

- 800 mm height.

- 100x100 mm cross section.

- The main reinforcement is 4Ø6

- seven square stirrups Ø4 mm

spread along the column height

starting at 50 mm from both ends

were used.

Experimental study

Concrete original column (core) :

Materials

Concrete and mortar :

- Concrete strength was 36 MPa .

- Mortar strength was 25.9 MPa .

Steel :

Nominal

diameter

Yield strength

Tensile strength

(MPa) Elongation (%)

4 461.8 652.9 6.25

6 323 410.5 27

External angles 298.7 402.7 28.26

First trial

Experimental Program for Concrete Jacket:

Group Specimen No.Column

dimensions(mm)

reinforcementStirrups

O 1,2,3 100x100 4Ø6 7Ø4

Specimens details of group (O)

Specimens details of group (A)

Specimen No.

Column dimensions

after strengthening

Jacket main

reinforcement

Jacket

stirrupsRemark

A 4,5,6 150x150 4Ø6 7Ø4 Square stirrups

Specimens details of group (B)

Specimen No.

Column dimensions

after strengthening

Jacket main

reinforcement

Jacket

stirrupsRemark

B 7,8,9 150x150 4Ø6 9Ø4 Square stirrups

Specimens details of group (C)

Specimen No.

Column dimensions

after strengthening

Jacket main

reinforcement

Jacket

stirrupsRemark

C 10,11,12 150x150 4Ø6 7Ø4Jacket height shorter

than core by 50mm

Failure mode

Original column failure Jacket failure of group (A)

Failure mode

Jacket failure of group (B) Jacket failure of group (C)

Strengthening

type Gro

Specimen numberMaximum capacity

( KN )

Experimental average

maximum capacity

( KN )

Percentage of

capacity increase

Reference

column (core)O

253.3 NA2 239

Concrete jacket

446.6 76.3%5 424

473.3 86.85%8 481

10 380

382 50.81%11 384

12 382

Experimental results of concrete jackets

Strain measurement

0.000 0.001 0.002 0.003 0.004 0.005 0.006

Strain

original

concrete jacket

Axial load – Vertical strain relationship

Group ( C )

0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016

Horizontal strain

original column

concrete jacket

Axial load – Horizontal strain relationship

Ultimate load ratio of concrete jacket

Original group (A) group (B) group ( c )

Experimental Program for Steel Jacket (Cage)

Group Specimen No.Column

dimensions(mm)

reinforcementStirrups

O 1,2,3 100x100 4Ø6 7Ø4

Specimens details of group (O)

Specimens details of group (D)

Specimen No. CoreCorner angles

size (mm)Strips plates(mm) No. of strips

D 13,14,15As columns

at group O30x30x3

4 plates

100x30x36

Specimens details of group (D”)

D” 19,20,21As columns at

group O30x30x3 4 plates 100x30x3 6

Jacket height

shorter than

core by 10mm

E 16,17,18As columns

at group O30x30x3 4 plates 100x30x3 8

Specimens details of group (E)

Failure mode

Jacket failure of group (D) Jacket failure of group (E)

Strengthening

type Gro

up Specimen

number

Maximum

capacity

( KN )

Experimental average

maximum capacity

( KN )

Percentage of capacity

increase

Original column

(core)O

253.3 NA

11 384

12 382

Steel jacket

(cage)

13 235

264.7 4.5%14 295

15 264

D" D" 340 340 34.22%

E 16 453 465.7 83.85%

Experimental results of steel jackets

Strain measurement

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010

Strain

original

steel jacket

Axial load – Vertical strain relationship

Group ( D” )

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004

Horizontal strain

original column

steel jacket

Axial load – Horizontal strain relationship

Original group (D) group (D") group ( E )

Ultimate load ratio of steel jacket

Refined Equations

Concrete jacket

By first principle method:

Pu = (4*w1*L + 4*w2*L*n)*tan

Steel cage

Adam Equation:

Pu = 0.85 * Ac* fc+ As * fys

+ K * fl* Ac+ NL

w1 confinement pressure at strip 1

w2 confinement pressure at strip 2

L original column width

n number of strips

Tan (ᶱ) friction factor, its range between

1.1 and 1.5

Ac cross section area of concrete

As cross section area of longitudinal

reinforcement

Fc compressive strength of concrete

Fys yield strength of reinforcement steel

F1 confinement pressureN

L axial load supported by the cage

Comparison of experimental and theoretical results using concrete jackets :

Strengthening

type Gro

Experimental

average maximum

capacity

( KN ) Per

ease Analytical

capacity (KN)

Pexperimental

PtheoreticalRemarks

Original

column (core)O 253.3 NA NA NA NA

Concrete

jacket

A 446.6 76.3% 373.7 1.2 20%

additional

capacityB 473.3 86.9% 392 1.2

C 382 50.8% 368.8 1.03F.O.S

needed

Comparison of experimental and theoretical results using steel jackets :

Strengthening

type Gro

Experimental

average maximum

capacity

( KN )

Analytical

capacity

Remarks

Original

column (core)O 253.3 NA NA NA

Steel jacket

(cage)

D 264.7 386.6 4.5% 0.68Buckling

failure

D" 340 346 22.4% 0.98 F.O.S needed

E 465.7 464.8 24.4% 1 F.O.S needed

luanalytica

erimentalu

Analytical parameter study1- Concrete jacket :

100.00

120.00

140.0030*3

Concrete jacket dimension,cm

c20*20 c25*25 c30*30

c40*40 c50*50

Ø6 Ø8 Ø10 Ø12

Diameter of stirrups at upper region of column

6Ø6 8Ø6 9Ø6 10Ø6

stirrups of jacket upper region

20*20 25*25

30*30 40*40

1.1 1.2 1.3 1.4 1.5

tan (angle of fricition)

Stirrups of jacket near loaded zone Diameter of stirrups near loaded zone

2- Steel jacket :

angles of the cage, mm

1 3 5 8 10

Thickness of cage batten plates, mm

100 200 300 410 500

Distance between batten plates of cage, mm

0.1 0.2 0.3 0.4 0.5

Friction coefficient

Analytical parameter study

Conclusions & Recommendations

1- The length of concrete jacket should by equal to the original column's length if possible to

achieve the best benefit of the concrete jacket. and the capacity of original column will be

increased up to 87% of the original column capacity by confinement.

1- The length of cage's angles should be shorter than original column's length to achieve the

benefit of cage. If the length of cage angles is equal to column length, the applied load will

transmit to the angles as a vertical load and this lead to angle yielding by buckling and

cause failure before benefiting of cage confinement.

2- The stirrups near loaded area should be intensified to a distance of (1-1.5) the jacket

width. The efficiency of jacket will be increased by decreasing the distance between

stirrups for the tested specimens.

2- The recommended design equation for steel cage is Adam’s equation:

Pu = 0.85 * Ac* fc+ As * fys + K * fl* Ac+ NL

3- The increase of stirrups (number/diameter) is more important than (increase of longitudinal

bars or increase of jacket cross-section).

4- The recommended design equation for concrete jacket is : Pu = (4*w1*L + 4*w2*L*n)*tan ᶱ

For concrete jackets:

For steel cages:

Recommendations and Future Work

As a result of this work, several items should be considered in future

research :

1- Study the design of concrete and steel jackets for rectangular and

circular columns as well as L and T – section columns.

3- Study strengthened columns which subjected to eccentric loads.

5- Performing experimental study to validate the present parameter

study in this thesis.

6- Study the efficiency of strengthening of loaded columns by

percentage from ultimate capacity using jackets without removing its

loads during strengthening process.

2- Study experimentally the strengthening using full scale or nearly full

scale specimens.

4- Study jacketing from 2 sides or 3 sides of strengthened columns.

Eng. Khaled Mahmoud

Best regards

Khaled.mahmoud@suezcanal.gov.eg

Khaled.mahmoud@eng.psu.edu.eg

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