STRENGTHENING OF COLUMN BY RC JACKETING

STRENGTHENING OF COLUMNS BY RC-JACKETING

1. INTRODUCTION

Although hundreds of thousands of successful concrete and

masonry buildings are annually constructed worldwide, there are

large numbers of concrete and masonry structures that

deteriorate, or become unsafe due to changes in loading, changes

in use, or changes in configuration. Also the old structures

designed for gravity loads are not able to withstand seismic

forces and cause wide spread damages. Repair of these structures

is often difficult, expensive, hazardous and disruptive to the

operations of the building. The removal and transportation of

large amount of concrete and masonry materials causes

concentration of weight, dust, excessive noise and requires long

period of time to gain strength before the building can be

reopened for service.

FACTORS INFLUENCING DAMAGE:

Inadequate design strength.

Inadequate reinforcement.

Inadequate compaction of concrete during casting.

Inadequate shear strength due to the change of lateral steelspacing.

Corrosion of steel due to inadequate concrete cover.

DEPT.OF CIVIL ENGG (STUCTURAL ENGG), KLECET, BELAGAUM.


2. NEEDS FOR RE-STRENGTHENING:

The load carried by the column is increased due to either

increasing the number of floors or due to mistakes in the

design.

The compressive strength of the concrete or the percent and

type of reinforcement are not according to the codes’

requirements.

The inclination of the column is more than the allowable.

The settlement in the foundation is more than the allowable.

There are errors in the design or construction.

Structure has to perform well with increasing time.

3. OBJECTIVES Increasing the lateral strength and stiffness of the

building.

Giving unity to the structure.



Eliminating sources of weakness or those that produce

concentration of stresses.

Fig 1(a). Failure at construction joint.

Fig 1(b). Crushing of concrete.



Fig 1(c). Spalling of concrete.

Fig 1(d). Diagonal shear crack.

4. STEPS INVOLVED IN RESTRENTHENING: Investigation of existing columns,

Examination of performances of existing columns,

Selection of re-strengthening method, and

Examination of performances in re-strengthened columns.



The three main weakness of RCC columns that require

attention are:

Loss of reinforcement due to corrosion.

Lack of confinement in concrete especially at the joints.

Deterioration of concrete due to attack of multiple

environmental agencies.

The selected method of repair should achieve following

objectives:

Reinstate the structural integrity of the member by

restoring or increasing its strength and stiffness.

Prevent the ingress of distress promoting agents such as

moisture, chlorides and carbon-di-oxide to improve

durability.

Maintaining the aesthetics/appearance of concrete

structures.

5.PERFORMANCE REQUIREMENTS OF REPAIR SYSTEMS:

Strength, serviceability and durability.

Protection of steel.

Bond with parent surface.

Dimensional stability.

Resistance to environmentally induced damage.

Ease of application.

Appearance.



The present practice of repairs in India is focused towards

delaying the deterioration. In last two decades the attempts on

rehabilitation of damaged RC structures have been mainly

concentrated in two methods external post tensioning, and the

addition of epoxy bonded steel plates to the tension flange. High

strength steel strands are used in external post tensioning to

increase the strength of damaged concrete structures.

6. COLUMN STRENGTHENING TECHNIQUES:

1. Concrete jacketing

2. Steel jacketing

3. Fiber reinforced polymer sheet wrapping.

6.1 REINFORCED CONCRETE JACKET:

Concrete jackets

One of the earliest and the most common solutions for

rehabilitation of concrete frames is to encase the existing

column, along with the joint region, in new concrete with

additional longitudinal and transverse reinforcement.

The Continuity of the added longitudinal bars through the joint

requires opening the slab at the column corners. The addition of

the joint transverse reinforcement makes the process even more



labor-intensive, in which case the beams are also cored, and in-

place bending of the hooks is necessary. The size of the jacket

and the number and diameter of the steel bars used in the

jacketing process depend on the structural analysis that was made

to the column.

Before this technique is carried out, we need to reduce or even

eliminate temporarily the loads applied to the column; this is

done by the following steps:

Putting mechanical jacks between floors.

Putting additional props between floors.

Moreover, in some cases, where corrosion in the reinforcement

steel bars was found, the following steps should be carried

out:

Remove the concrete cover.

Clean the steel bars using a wire brush or sand compressor.

Coat the steel bars with an epoxy material that would

prevent corrosion.

6.1.1 PREPARATION OF COLUMN WHICH IS TO BE STRENGTHENED

Mechanical cleaning of reinforcement and thorough cleaning

of damaged concrete surface after removal of weak concrete.



In the joints between old and new concrete, epoxy based

bonding material to be applied on old concrete surfaces.

For RCC members, where overall integrity found good, cracks

to be filled by putting epoxy grouting under pressure. For

spalled concrete polymer modified mortar/ concrete with

epoxy bond cote to be provided.

The exposed concrete surface and the reinforcement bars were

covered with epoxy mortar made of epoxy and cement (acting as

filler material). This provides an impervious membrane against

further ingress of corrosive agents.



Fig: Preparation of column which is to be

strengthened

6.1.2 PROCESS OF JACKETING:

The jacketing process could start by the following steps:

Adding steel connectors into the existing column in order to

fasten the new stirrups of the jacket in both the vertical

and horizontal directions at spaces not more than 50cm.Those

connectors are added into the column by making holes 3-4mm

larger than the diameter of the used steel connectors and

10-15cm depth.

Filling the holes with an appropriate epoxy material then

inserting the connectors into the holes.

Adding vertical steel connectors to fasten the vertical

steel bars of the jacket following the same procedure in

step 1 and 2.

Installing the new vertical steel bars and stirrups of the

jacket according to the designed dimensions and diameters.



Fig: Installation of dowels to fastening of reinforcement



Fig: Installation of new reinforcement



6.1.3. COATING THE SURFACE WITH EPOXY

Coating the existing column with an appropriate epoxy material

that would guarantee the bond between the old and new concrete,

and also it prevents the reinforcement from corrosion attack.

6.1.4. POURING NEW CONCRETE

Pouring the concrete of the jacket before the epoxy material

dries. The concrete used should be of low shrinkage and consists

of small aggregates, sand, cement and additional materials to

prevent shrinkage.



F

ig: Pouring new concrete



Fig: Process of strengthening of column by RC jacketing.



7. CONCLUSIONS

This method is successful in creating

strong column

Requires highly skilled professionals

Concrete jacketing of columns and encasing the joint region

in a reinforced fillet is an effective but the most

labor-intensive strengthening method due to difficulties

in placing additional joint transverse reinforcement

Increase in column size intern reduce floor space and

disruption to building occupancy;

Walls adjacent to columns must be demolished before

jacketing and then reconstructed.

Cost of construction is high



8. REFERENCES:

1. Shri. Pravin B. Waghmare “Materials And Jacketing Technique For

Retrofitting Of Structures” IJAERS/Vol. I/ Issue I/October-December, 2011”

2. Eduardo N, Fernando A, B. Branco, and Vítor D.

Silva .“Reinforced Concrete Jacketing—Interface Influence on Monotonic

Loading Response, ACI Structural Journal/March-April 2005”

3. A.R. Shantha Kumar, IIT Madras “retrofitting of buildings, Master

Builder Vol. 9, Sept. 2007.”



4. E S Ju lio, F Branco and V D Silva.“Structural rehabilitation of

columns with reinforced concrete jacketing.”

5. Amlan K. Sengupta, Dept of Civil Engg. IIT Madras “Seismic

Analysis and retrofit of existing multi-storeyed buildings in India-An over

view, 13th World Conference on Earthquake Engineering Aug,2004”


STRENGTHENING OF COLUMN BY RC JACKETING

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