modern technique of retrofitting 1

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MODERN TECHNIQUE OF RETROFITTING Introduction The choice of retrofitting basically depends upon the structural scheme and the employed building material in the parent construction along with the feasible and economical technology. Moreover an understanding in failure mode, structural behavior with the weak and strong aspect of design as derived from the earthquake damage surveys also the section of the retrofitting schemes. Numerous technique used to retrofit seismically deficient or damaged buildings, may be broadly classified into three categories on the basis of their effect on structural performance namely 1)improving the existing masonry strength and deformability , not related to any specific objective which is similar to the repairing process of masonry structures. 2) improving the in plane strength of the wall or any weak zone of the section akin to local/member retrofitting.3) improving the structural integrity of the whole structure in terms of in plane and out of plane strength Some of the technique which mainly used are 1) Member retrofitting: Shotcrete,Fibre reinforced plastic(FRP),RC and steel frames 2)Structural retrofitting/Structural level Retrofitting method: Jacketing, Pre-stressing, splint and bandage technique, Exterior supplemental elements, Inserting new walls, Confinement with RC elements, Confinement with steel elements, Adding reinforce to masonry walls, Connection between walls and floors, Strengthening of parapets. STRUCTURAL FIRST AID AFTER DISASTER EARTHQUAKE EFFECTS TO STRUCTURES Earthquakes are natural hazards under which disasters are mainly caused by the damage to or collapse of buildings and other manmade structures.

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Page 1: Modern Technique of Retrofitting 1

MODERN TECHNIQUE OF RETROFITTINGIntroduction

The choice of retrofitting basically depends upon the structural scheme and the employed building material in the parent construction along with the feasible and economical technology. Moreover an understanding in failure mode, structural behavior with the weak and strong aspect of design as derived from the earthquake damage surveys also the section of the retrofitting schemes. Numerous technique used to retrofit seismically deficient or damaged buildings, may be broadly classified into three categories on the basis of their effect on structural performance namely 1)improving the existing masonry strength and deformability , not related to any specific objective which is similar to the repairing process of masonry structures. 2) improving the in plane strength of the wall or any weak zone of the section akin to local/member retrofitting.3) improving the structural integrity of the whole structure in terms of in plane and out of plane strength

Some of the technique which mainly used are

1) Member retrofitting: Shotcrete,Fibre reinforced plastic(FRP),RC and steel frames

2)Structural retrofitting/Structural level Retrofitting method: Jacketing, Pre-stressing, splint and bandage technique, Exterior supplemental elements, Inserting new walls, Confinement with RC elements, Confinement with steel elements, Adding reinforce to masonry walls, Connection between walls and floors, Strengthening of parapets.

STRUCTURAL FIRST AID AFTER DISASTER

EARTHQUAKE EFFECTS TO STRUCTURES

Earthquakes are natural hazards under which disasters are mainly caused by the damage to or collapse of buildings and other manmade structures.

Until today, there is not much that can be done to prevent the occurrence of earthquakes. However, the earthquake-induced damage can be reduced with planning and constructing earthquake resistant buildings or evaluating and strengthening existing buildings before an earthquake.

Earthquake damages depend on many parameters, such as:1. Earthquake shaking characteristics:Intensity, duration, and frequency content of ground motion2. Soil characteristics:Topography, geologic, and soil condition3. Building characteristics:Building’s stiffness, strength, ductility, and integrity

Beside the parameters mentioned above, earthquake casualties depend on important sociologic factors, such as:

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1. Density of population2. Time of day of the earthquake occurrence3. Community preparedness

2.2. Basic causes of earthquake - induced damage1. Ground shakingGround shaking is the main cause of damage by earthquakes.2. Ground failureTypes of ground failure:-Landslide,-Settlement, and Liquefaction3. TsunamiTsunami is a sea current that is triggered by an undersea event such as explosion of oceanic volcano (Krakatau), undersea landslide, rock slides into the ocean, tectonic fault, and nuclear explosion. The conditions for a tsunami to occur:

under-sea earthquake-reverse or normal fault shallow earthquake magnitude, m >6.5 The tsunami destruction is bigger if the beach is slightly sloped.

4. FireIt is difficult to extinguish the fire because of the loss of water supply and the infrastructure is broken. The main cause of damage is ground shaking (no. 1). Others (ground failure, tsunami and fire) are secondary disaster. Therefore the main priority is making earthquake resistant structures.

RETROFITTING (REPAIR, RESTORE, STRENGTHENING)Problem classification1. before a damaging earthquake:The required strengthening to be determined by a survey and analysis.2. Just after a damaging earthquake:Temporary supports & emergency repairs are to be carried out. Those actions are needed so that the buildings can be re-functioning and not collapsed due to aftershocks.3. after a damaging earthquake when things start settling down:Distinction to be made in the type of action required:

Repair Restoration Strengthening

1. RepairThe main purpose is to bring back the architectural shape of the building so that all services start working & the functioning of building is resumed quickly. The actions will include the following:

Patching up of defects such as cracks, and fall of plaster Repairing doors, windows, replacement of glass panes Checking and repairing electric wiring. checking and repairing gas pipes, water pipes & plumbing services

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Rebuilding non-structural walls, boundary walls, etc. Re-plastering of walls as required Rearranging disturbed roofing tiles

2. Restorationthe main purpose is to carry out structural repairs to load bearing components to restore its original strength. The actions will include the following:

Injecting epoxy like material, which is strong in tension, into the Cracks in walls, columns, beams, etc.

Addition of reinforcing mesh on both faces of the cracked wall, holding it to the wall through spikes or bolts & then covering it suitably.

Removal of portions of cracked masonry walls & piers and rebuilding them in richer mortar. Use of non-shrinking mortar will be preferable.

Remove the damage column or beam, fix the reinforcing, add reinforcing if needed, and re-concreting.

3. Strengthening the main purpose is to make buildings stronger than before. The actions will include the

following: demolish the weakness sources or sources that can make stress concentration in several

parts: columns distribution are not symmetric walls distribution are not symmetric different stiffness from one to another floors excessive openings Make the building as a unity by tying together all components of building Avoid brittle failure by re-arranging, adding reinforcing bars, and make the details in

accordance with ductility requirement. Increasing the lateral strength by adding walls, columns, etc.

STEPS OF RETROFITTING:1. Determine as accurate as possible how the building behave when shaken by an

earthquake check the building check building material quality list all components of the building that are damaged2. Perform a dynamic analysis for the building to get an idea of the causes of damage and

determine the load paths when shaken by the earthquake.3. Determine the causes of damage of components; caused by shear, compression, tension,

flexure, anchoring, etc.4. As soon as the type of damage can be identified, repair and restoration of the components

can be done separately in order that the original strength of the components can be restored.

5. If results of analysis indicate that the building with restored components can withstand the maximum expected earthquake for that area based on the latest code, then there is no need to strengthen.

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6. However, if the building with restored components was not designed or designed for a lower than the maximum expected earthquake specified by the latest code, then the building needs to be strengthened

for strengthening, the restored building must be re analyzed to identify which components must be strengthened

for engineered buildings with severe damage and if the building needs to be strengthened, 3d non-linear analysis performance based design should be done

if cost for strengthening the building to its original function is not feasible, one option that can be chosen is to change the building function with less stringent requirement. Therefore, the cost will be reduced.

7 After the strengthening works is completed, the building must be reanalyzed to ensure that the strengthened building is earthquake resistant.

TECHNIQUES OF REPAIR & STRENGTHENING Repair of cracks in walls

Example of cracks at corners of opening and diagonal cracks in walls

For small cracks (cracks width is less than 5 mm):i. the old plaster around the crack is removed _ 50cm, and then injected with cement grout or epoxy.ii. After the crack is sealed, apply new plaster made of mortar

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For large cracks (crack wider than 5mm): The old plaster around the crack is removed minimum 50cm, and then injected with

cement grout or epoxy. After the crack is sealed, a thin bed of mortar, 1cm thick, and 2cm width must be made.

A thin bed of mortar serves as the support for the wire mesh. Place wire mesh on both sides of the wall and tie those wire mesh to each other. The wall is re-plastered with mortar

crack min. 50 cm

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Note:If the wall is confined with beams and columns, the wire mesh to strengthen the cracked wall can be placed directly on the wall. Wire mesh on both sides can be nailed with concrete nails (see figure IV). However, the best is as described below.Unconfined masonry walls when strengthened using wire mesh: The wire mesh must be placed on a thin bed of mortar and wire mesh on both sides must be tied (see figure iii). In this case, both sides of wire mesh with the plasters and the brick wall act as “sandwich” construction.

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Retrofitting of damaged beams and columns cracks in concrete: 1. hair-line cracks in concrete (width less than 0.2 mm) or no visible cracks indicate insignificant damage.2. Generally, cracks up to 2 mm wide in concrete components are not considered significant (and may signify low damage)3.Cracks up to 5 mm wide in concrete components may indicate moderate damage4. Cracks greater than 5 mm indicate heavy damage (with significant loss of strength)5. Buckling of reinforcement in concrete components signifies heavy damage, irrespective of crack width in concrete

Generally, the techniques to strengthen concrete column / beam are as follows: 1. To add reinforcement and stirrups outside the concrete column / beam and subsequently cover by concrete. 2. To bandage the concrete column / beam with welded wire fabric and subsequently plaster with mortar.3. To cover the concrete column with rectangular steel profile or pipe and subsequently the gap between concrete and steel is grouted. 4. To bandage with steel plate welded to 4 steel angle profiles placed in each corner of the column and subsequently the gap between concrete and steel is grouted. To increase the column shear strength without increasing its flexural capacity, a gap must be provided at the top and bottom of the column. Therefore, the additional reinforcement is not anchored to the beam.

For columns with moderate cracks, remove the damage part, clean, and pour new concrete. Columns must be supported during retrofitting works

For heavily damaged columns, namely columns where its strength is substantially reduced (based on observation as well as analysis). The damaged part is removed and if

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the analysis shows that additional reinforcement must be placed, one of the above explained method can be applied. Before removing the damaged part of the column, beams and slabs around the column must be supported.

Retrofitting of top & bottom of columns that are damaged and placing additional stirrups of beams and columns

Example of upper & lower parts of columns that are damaged

Implementation of retrofitting: 1. Support the beams around the column to be retrofitted with timber posts 2x8/10 cm at suitable intervals. Timber size to be adjusted based on the load to be supported. Steel support can also be utilized.

2. Strip the concrete cover of columns / beams to know the number of reinforcement & stirrups. (figure II)

3. If analysis shows that the number of column & beam reinforcement & stirrups are not sufficient, additional reinforcement must be placed. (Figure III (a).)

4. If the detail of the column-beam joint reinforcing is not in accordance with the seismic resistant requirements, additional anchoring from column to beam must be placed. in this case, part of the beam concrete must be removed for anchoring. (figure III.b.)

(I) (II)

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(IIIa) (IIIb) if additional beam stirrups is necessary:Concrete must be removed ± 2m.Place stirrups with suitable size & spacing inaccordance with the analysis result.

If the column / beam reinforcement is not adequate, there are 2 choices that can be implementeda. Remove the entire column / beam concrete & install the additional reinforcementb. Additional reinforcement is installed outside the column / beam and concreted

(jacketing).

If no additional reinforcement for the column is necessary, but the detailing of the joints must be rectified, the beam concrete must be removed ±2m and install the necessary anchoring. for additional column stirrups, strip the column concrete cover min. 1m length and subsequently place the stirrups with suitable size & spacing in accordance with the analysis result. (Figure III.b.)

For placing new concrete, the formwork must be shaped like a cone(figure iv.(a). & iv.(b).).The upper part of cone formwork must exceed theBoundary of old & new concrete.

Pour concrete with the desired strength up to the top of the cone.

If there is not enough space, concreting can be done by drilling a hole in the slab. Figure iv.(b).

After 24 hours, the formwork can be removed and the protruding concrete part (due to the cone shape) can be chipped. (Figure v(a)&v(b) )

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IV(a) IV(b) example of formwork used for beam concreting

V (a) V (b)

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IV(b) V(b)

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SHOTCRETE / GUNITEShortcrete is defined as pneumatically applied concrete or mortar place directly to a surface. The shotcrete shall be placed by either the dry mix or wet mix processThe dry mix process shall consist of

Thoroughly mixing the dry materials. Feeding of these materials into mechanical feeder or gun Carrying the material by compressed air through a hose to a special nozzle Introducing water at nozzle point and intimately mixing it with other ingredients at

nozzle Jetting the mixture from the nozzle at high velocity on the surface to receive the shotcrete

Wet mix shotcrete The wet mix process shall consist of

Thoroughly mixing all the ingredients with the exception of the accelerating admixture, if used

Feeding the mixture into the delivery equipment Delivering the mixture by positive displacement or compressed air at the nozzle Jetting the mixture from the nozzle at high velocity on to the surface to receive the

shotcrete

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Dry mix shotcrete The Force of impact compacts the material. In good quality work, density of around 2100 kg/m3 is achieved. For effective gunniting, the nozzle should be kept 60cm to 150cm from the work preferable normal to the surface. Dry-mix shotcrete suffers high dust generation and rebound loss varying from about 15% up to 50% . Wet mix shotcrete must contain enough water to permit pumping through the delivery line. Wet mix shot crete as a result may experience significantly more cracking problems due to the excess water and drying shrinkage. However ,advances in the development of the high range water reducing admixtures, pumping aids, and concrete pumping equipment have greatly reduces the problem, and wet-mix shotcrete is now being used more frequently in repair construction

.Shotcrete is a versatile construction material that can be readily placed and successfully used for a variety of concrete repair applications. Because of the considerable advantage of the shotcrete process and its ability to provide finished work of the highest quality, Shotcrete has been used to repair cannalsand spillway lining and walls, the faces of dams, tunnel linings, highway bridges and tunnels, deteriorating natural rock walls and earthen slopes, and to thicken and strengthen existing concrete structures

More important with shotcrete than with other standard concrete repair method, if highly qualified ,well trained and competent workmen cannot be employed, it is advisable to consider using some other repair procedure. The quality of Shotcrete closely depends upon the skill and experience of one person, the nozzelman.

JACKETINGJacketing is the process of fastening a durable material over concrete and filling the gap

with the grout that provides needed performance characteristics. The materials used are metals, rubber, plastics, Ferro cement and concrete. Reinforced concrete jacketing increase the member size significantly. This has the advantage of increasing the member stiffness and is useful where deformations are to be controlled. If columns in a building are found to be slender, RC jacketing

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provides a better solution for avoiding buckling problems. Design for strengthening/repair work is based on composite action between the old and the new work. Strain compatibility calculations may have to be carried out carefully giving due accounts to factors such as creep. As the new jacket is to behave compositely with the parent member, the new jacket can take can take additional loads only with the increase in the stress & strain in the old one. The problem arises if the

Old concrete has reached limiting strain and is not likely to sustain any more significant strain

Old concrete is weak and porous and started deteriorating due to weathering action and corrosion of reinforcement.

The question then arises as to whether the composite action should be abandoned and the new jacket (plate of RC) design to carry the entire load. It is perhaps best to design the strengthening in this manner, but detailing must be right to ensure transfer of load to the new jacket, if the old concrete fails. It is however, necessary to ensure perfect bond also between the old and new concrete by providing shear keys and effective bond coat with the use of epoxy or polymer modified cement slurry giving strength not less than that of new concrete.

Plate bonding and RC jacketing are the common method of strengthening RCC structures. The cost difference between the two methods is not significant. A choice has to be made between the two methods based on the actual needs and the suitability of each method with respect to the architectural and other details of buildings.

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REFERENCE: Rehabilitation of concrete structures by Dr. B.Vidvelli Disaster mitigation and Rehabilitation of structures by Dr. C.Anthony Jeyasehar &

Dr. S.Thirugnanasambandam Handbook of CPWD Earthquake Resistance Design of structures by Pankaj Agarwal &

Manish Shrikhande