retrofitting of bridges

8/10/2019 Retrofitting of Bridges

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RETROFITTING OF

BRIDGES


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Retrofitting

In repairing deteriorated concrete bridges thepreliminary objective is to restore structure toits original shape.

The need of retrofitting of existing bridges hasa risen preliminary because of age of thebridge, adjacent construction, traffic loadfactor, weather and environmental factor,

maintaince. It is possible to strength many ofexisting bridges against the collapse by usingsimple retrofitting measures.


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TYPICAL DAMAGE OF BRIDGE

STRUCTURES A. Obsolescence

The functional Obsolescence of the structure is frequently encountered due to grade separationor change in pattern of traffic passing over the railway or highway using the bridge, while itmay still have many years of normal life left. The changing of the traffic conditions,particularly on the railways and national highways.

B. Damage

The structure may be damaged due to a serve accident occurring over it, particularly in the

cases of railway bridges. The bridge may be damaged fully or partly due to heavy andunexpected high floods.

C.Weathering

Even a well maintained steel or concrete structure or concrete structure of the bridge candeteriorate over the years from the effects of weathering action of saline atmosphere, fumes,abrasion, etc.

D.Maintenance

The Maintenance cost of the structure generally increases as the bridge nears end of theservice life. With the increase cost of lab our and repair materials, it may be found moreeconomical to replace the structure with modern materials than spending money in oldmaintaining the in old one until theoretical service life of the structure. And also

The age of bridge structure, traffic load factors, weather and environmental factors, adjacentconstruction.


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INSPECTION OF REPAIRS OF

BRIDGES

Inspection is an integral part of maintenance. The structure should beinspected on a regular basis, not after the failure of the bridge. A bridgeinspector must possess the following qualifications.

Must be a licensed structural engineer.

Should have knowledge in structural behavior and design of bridges Should be aware of the changing behavior of materials with age, its fatigue,

chemical properties.

Must be familiar with construction practices and proper execution.

Inspection process

Review available plans and specifications and previous inspection records.

Prepare a set of bridge plans and sketches of framing plan, deck plan, pier,abutment details, joint details, bearings, etc. These are helpful in takingnotes of deteriorated structural and other components.


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Deck slab repairs will be classified as

follows:

(a) Partial-Depth.

Partial-depth repairs shall consist of removing theloose and unsound deck concrete, disposing of the

concrete removed and replacing with new concrete.The removal may be performed by chipping withpower driven hand tools or by hydro-scarificationequipment.

The depth shall be measured from the top of theconcrete deck surface, at least 3/4 in. (20 mm) butnot more than 1/2 the concrete deck thickness.


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(b) Full-Depth. Full-depth repairs shall consist of

removing concrete full-depth ofthe deck, surface.

The removal may be performed with power driven

hand tools, hydraulic impact equipment, or by hydro-

scarification equipment. Full-depth repairs shall be

classified for payment as Full-Depth, Type I and

Full-Depth, Type II according to the Type I Full-depth patches less than or equal to 5 sq.

ft. (0.5 sq m) in area. The minimum dimensions for a

patch shall be 1 ft. x 1 ft. (300 mm x 300 mm).

Type II Full-depth patches greater than 5 sq. ft. (0.5

sq. m) in area.


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Materials-

Materials-

Repairs materials for concrete structures are

1. Cement based mortar or concrete.

These are the most popular because of their common availability andrelative low cost. They may be applied both small & large repairs.

2. Fiber reinforced concrete. These are most commonly used for structural applications, including repair

of concrete bridge structures.

3. Polymer cements concrete.

Pc can be applied for repair to any bridge structural member.

Non-shrink quick-setting mortars with expensive cements and admixturesthat increase strength, bond and workability and reduce curing time.

Epoxy mortars.Used mainly concrete repair, protection of reinforcing steeland improvement in adhesion between fresh and old concrete.


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EQUIPMENTS

(a) Surface Preparation Equipment.Surfacepreparation and concrete removal.

1) Sawing Equipment. Sawing equipment shall be a

concrete saw capable of sawing Concrete to the

specified depth.

2) Blast Cleaning Equipment.

The blast cleaning may be performed by wet

sandblasting, high-pressure water blasting, shotblasting or abrasive blasting. Blast cleaning

equipment shall be capable of removing rust and old

concrete from exposed reinforcement bars, and shall

have oil traps.


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(3) Power-Driven Hand Tools.

Power-driven hand tools will be permitted

including jackhammers lighter than thenominal 45 lb. (20 kg) class.

Chipping hammers heavier than a nominal 15

lb. (6.8 kg) class shall not be used forremoving concrete from below any reinforcing

bar for partial depth repairs, or for removal

within 1 ft (300 mm) of existing beams,girders or other supporting structural members

that are to remain in service or within 1 ft (300

mm) of the boundaries of full-depth repairs.


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4) Hydraulic Impact Equipment.

. Hydraulic impact equipment with a

maximum rated striking energy of 360 ft-lbs

(270 J) may be permitted only in areas of full

depth removal more than 1 ft (300 mm) away

from existing beams, girders or other

supporting structural members that are to

remain in service or more than 1 ft (300 mm)from the boundaries of full-depth repairs.


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(5) Hydro-Scarification Equipment.

The hydro-scarification equipment shallconsist of filtering and pumping unitsoperating with a remote-controlled robotic

device. The equipment may use river, streamor lake water.

. The equipment shall be capable of removingconcrete to the specified depth and removingrust and concrete particles from exposedreinforcing bars.


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(3) Reinforcement Treatment.

Care shall be exercised during concrete

removal to protect the reinforcement bars and

structural steel from damage.

Any damage to the reinforcement bars or

structural steel to remain in place shall be

repaired or replaced.


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(c) Placement & Finishing of

Concrete Repair

Grout Placement:

After the repair areas have been cleaned and

immediately prior to concrete placement, the

grout shall be applied to a dampened surface.

A thin layer of grout shall be thoroughly

scrubbed into the deck surface.


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Caste concrete

In deck slabs, where the spells extend below thetop reinforcement, the area is built up withconventional concrete. On vertical and overheadsurfaces, forms are normally necessary if concrete is

to be cast. Forms are first attached to the soundconcrete surrounding the shall either by expansionbolts or any other suitable anchoring devices. Cast inconcrete is then deposited from the top by gravity or

by pressure grouting. Normally, superplasticisers are

admixed for the production of free flowing concretein hot weather. It is also used as a water reducingagent leading to high early strength


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Pneumatically applied concrete mortar

For overhead repairs, there is no better method than

sprayed concrete (shotcrete).this methods is also very

usual for vertical repairs. Shot Crete needs no

bonding compound. A latex modified shot Crete isused in inaccessible areas because it needs no forms

and has high bond strength.

The repair surfaces should not be wetted just before

applying the shotcrete, but shot Crete should notapplied where there is standing water.


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TYPICAL SURFACE REPAIR


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Curing and Protection

. Concrete patches shall be cured by the

Wetted Burlap or Wetted Cotton Mat Method


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(e) Opening to Traffic.

. No traffic will be permitted on a patch untilafter the specified cure period, and theconcrete has obtained a minimum compressive

strength of 4000 psi (27.6 MPa) or flexuralstrength of 675 psi (4.65 MPa). Constructionequipment will be permitted on a patch duringthe cure period if the concrete has obtained the

minimum required strength. In this instance,the strength specimens shall be cured with thepatch.


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DECK SLAB CRACK REPAIR Cracks in concrete are caused by many factors, e.g.

shrinkage, excessive thermal stresses, improper

placement or compaction.

Uneven foundation settlements, overstressing,

inadequate reinforcement, improper detailing andcorrosion of reinforcement.

For crack repair, epoxy grouts are generally used

because of their excellent bonding qualities.

A modified Iignosulphonate based admixture should

be added to cement grout for improved workability

without an increase in water content\.


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Concrete deck slab maintenance and

Repair

Concrete deck slabs are constantly pounded by

traffic, and are directly exposed to weather,

consequently they deteriorate faster than other parts

of the bridge. Spell over 30 to 40% of the total deck area implies

that the entire deck slab may need replacement.

. An inspector uses a hammer to find out concrete

delaminating and hollowness of concrete deck.

Half cell potentiometer is used to determine the

conditions of reinforcing steel.


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Crack repair

The cracks required to be repaired can be

classified into two main groups as follows:

a) Cracks causing only a leakage of the

structural element without its weakening.

b) Cracks causing a weakening of the

structural element.


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Materials and techniques Crack repair in the concrete structures is performed

with use of injection technique.

Injection technique consisting of sealing or bondingthe narrower or shallower cracks and are required tofilled using pressure injection.

Low pressure injection is usually applied when

concrete is rather weak and cracks are large widths. Mean pressure injection is usually applied when the

crack width are at least 0.5mm or more so as to fillthe cracks in prestressed concrete members or

reinforced concrete members with highlyconcentrated reinforcing bars.

High pressure injection is usually applied to fillnarrow cracks with relatively strong concrete.


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Typical crack repair procedure

A. The surface along the cracks which is requiredshould be clean.

After cleaning, injection nipples are installed at

regular intervals along the cracks and remainingcrack surface is temporarily scaled.

C.There is many types of injection nipples, mostlymetal or plastic ones.

Flanged injection nipples (metal or plastic) which can befixed temporarily to the concrete surface with an adhesive.

Deep injection nipples (mostly metal) ones which areinstalled a small holes drilled along the cracks.


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D.location of flanged or deep injection nipples is a veryimportant factor in the injection operation. In these cases offlanged injection nipples, their spacing a long crack depends

mainly on its width. The hole should be normally be drilled at an angle of 45

degree with respect to the crack direction direction, the holesshould intersect the crack approximately at half of its depth.

The hole should be normally be drilled at an angle of 45

degree with respect to the crack direction direction, the holesshould intersect the crack approximately at half of its depth.

E. in case of vertical cracks, injection starts from the bottomhole and continues until Injection material appear in next one.A similar procedure is used for injection of horizontal cracks.

F. after the injection material is partially consolidate, the holesand temporary surface seal are removed, the crack surface iseither be alone or cosmetically finished e.g. by cement routs orother surface protection materials


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SOME GENERAL PROBLEMS

OF BRIDGE RETROFITTING

Economic Problems

The additional costs (Construction of temporary

bridge) resulting from disturbances to normal

traffic are an important component of the totalretrofitting costs.


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CONSULUSION

It is possible to strength many of existingbridges against the collapse by using simpleretrofitting measures.

In country like Indian the construction of newbridge is uneconomical hence retrofitting isdone so as to attain its original strength.

If bridge is collapse there is heavy or hugeloss of life and property hence the retrofittingof bridge is became necessary.

REFERENCE


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REFERENCE

Indian concrete journal February 2008, repairof concrete bridges page 26-28.

Construction of bridges text book.

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