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This article published in News Letter of Southern Railway
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Maintenance of Pre-stressed concrete girder bridges
G.Radhakrishnan,
SE/Br./E/TPJ @ GOC.Introduction From pyramids to modern pavements, quality concrete is the choice for a structurally sound, everlasting product. Modern development of prestressed concrete is credited to F.Freyssinet of France, who in 1928 started using high strength steel wires for Prestressing. Since 1960, the use of prestressed concrete bridges has become a standard in United States and now a days in our Railway. Advantages
1. Inherently safe Prestressed concrete has many things going for it: no
fatigue-prone details; built-in redundancy; no special design; overloads and excess capacity; better deck durability because of reduced deflections; better substructure stability from increased dead-loads.
2. Excellent riding characteristics The public will not only be safe, but also feel secure and
comfortable on a bridge that can hold traffic vibrations to a minimum.
3. Efficient material usage Prestressed concrete is engineered high technology at its
best.It is carefully designed using a variety of materials, each selected only for the benefits it brings to get the job done.
o High strength steel prestressing strands to provide load bearing capabilities as needed.
o Concrete to give permanence and strength and good riding characteristics.
o Reinforcing bars to further improve the long-term quality of the concrete.
4. Low life cycle costs
The overall economy of a structure is measured in terms of its life cycle costs. This includes the initial cost of the structure plus the maintenance cost. Prestressed concrete bridges do well in both these aspects.
5. Durability. Studies have shown that precast concrete products can
provide a service life in excess of 100 years. For severe service conditions, additional design options are available which can extend the life of the precast concrete product. This is extremely important when calculating life-cycle costs for a project.
6. Minimal maintenance Of course no painting is needed. Some engineers believe
that this alone adds about 10 to 20 percent of the initial costs of a steel bridge over the period of its useful life.
7. UV Sensitivity. Unlike some other materials, precast concrete does not
degrade from exposure to sunlight.8. Strength.
The strength of precast concrete gradually increases over time. Other materials can deteriorate, experience creep and stress relaxation, lose strength and/or deflect over time.
Most of these bridges are relatively new and their numbers are growing rapidly now; therefore, assessing the condition of concrete bridges and planning a repair and rehabilitation strategy is a complex task. However, failure of the high-strength prestressing steel can compromise the integrity of the prestressed concrete bridge (corrosion-related deterioration compromising the structural integrity of a conventional concrete structure is highly unlikely). This makes close attention to construction details and subsequent monitoring and inspection of the prestressed concrete bridges critical.
Some Larger Prestressed Concrete Girder Bridges in the world
Name Location Country Year Span in
‘m’Remarks
Stolmasundet Austevoll Norway 1998 301
Raftsundet Lofoten Norway 1998 298
Humen Pearl River China 1998 279
Varodd Kristiansand Norway 1994 260
Gateway Brisbane Australia 1986 260 Double pier supports
Skye Skye Island Britain 1995 250
Effective maintenance
If the intention is to provide a long-term benefit for new and existing bridge structures, then we need to address the issue of effective bridge maintenance. Long-term bridge deterioration can only be effectively resolved through advanced technologies that have the superior ability to enhance and preserve both existing and newly erected structures.
SCHEDULE OF INSPECTION By SSE/SE/Bridges
PSC girders irrespective of spans and their bearingsInitial inspection
Within one year of installation {(As per IRBM-1998, Para No. 107(1)a}
Initial inspection being carried out one year after installation {(As per the same IRBM-1998, Para No. 1102(1)c}(Hence the safer side of within one year after installation may be adopted)Routine inspection
Once in five years Recording of camber
Once in a yearBy SSE/SE/P.Way
Once a year during the prescribed months prior to the monsoon season, the Inspector shall inspect every bridge including road under/over bridges in his section. The inspection shall cover the following :
Foundations and substructures,Protective worksSuperstructures of all RCC, PSC slab and masonry bridges,General condition of superstructure of all other types of bridges and their bearings,Obstruction of waterways.The track and approaches of all the bridges,Run off frames if any and foot path on bridges.
By ADEN Once a year after the monsoon every bridge including road over /
under bridges. The inspection should commence soon after the cessation of the monsoon and completed by a date to be specified by the Chief Engineer.
Bridges, whose condition warrants special attention should be inspected more frequently.
SEQUENCE OF INSPECTION1. Foundation and flooring
Visual examination for cracks, unequal settlement & sinking Observe exposed element Take scour reading in rainy season during maximum floods
by using
Echo sounder Under water inspection in case of perennial rivers– As per
instructions by RDSO Inspect quality of material probably in dry season and
telltale marks for defects
Permanent kink in alignment – tilt of pier Inscription plaques on bridges showing nature and depth of
foundations.
The information to be recorded should be:o Nature of foundation e.g. wells, piles and open,o Nature of strata passed through and reached,o Depth from rail level to the bottom of foundations and
top of foundations. In case of shallow foundations - flooring, upstream side
curtain wall and scour on downstream side drop wall to be checked.
Better to dump wire crates filled with boulders in case of scour
2. Sub structure The inspection report of sub-structure may reveal the following
Establish procedures for early detection of defects and deterioration.
Assess deficiencies of substructures below the water line. Verticality of abutment and pier Uneven settlement Inadequate/abnormal clearance between ballast wall and
end of girder leaning, bulging of abutments Abnormal level difference Ensure timely repair of scour damage. Structural, weight capacity, and traffic flow considerations
should increase or decrease the frequency of bridge maintenance.
Availability and functioning of weep holes 3. Training and protective works including vent way
Pitching: In approach banks, check the condition of pitching. Toe wall is an important component of the pitching
provides a proper foundation to the toe wall. Guide Bunds: Provided in alluvial rivers to train the river stream.
Repair the disturbed pitching stones before every
monsoon. Aprons : Check whether the apron of design length and thickness is available. Recoup with more stones if
necessary. Vent way : Check the silted bed, obstructions by vegetation, Debris, Released concrete blocks, Boulders etc.,
4. Bed blocks/trestles Soundness of Bed block/trestles Look for improper seating of bearing-gap Cracking & crushing of masonry No cavity/ gap between Bearing and bed block
5. Bearings and expansion arrangements Bearings normally used
Elastomeric bearings (Elastomeric bearings permitting rotation and Translation)
Elastomeric bearings are very stiff in resisting volume change but are very flexible when subjected to shear or pure uni-axial tension. They are
generally reinforced with steel plates in alternate layers to reduce bulging. When used with a steel or concrete girder these permit moderate longitudinal movements and small rotations at the ends.
PTFE bearings
The coefficient of friction between steel and PTFE is quite low.
The mating surface which forms the upper component of the bearing is stainless steel with good surface finish.
The PTFE can be unfilled or filled with glass fibre or other reinforcing material. Its bonding property is very poor.
Hence it is preferable to locate the PTFE by confinement and fitting of half the PTFE thickness in recess in a metallic matrix.
These are used either to provide rotation by sliding over cylindrical or spherical surfaces or to provide horizontal sliding movement over flat surface or a combination of both.
Where there are large displacements accompanied with relatively small loading, as in case of centrifugal loads, wind loads or seismic loads, PTFE sliding bearings are utilised.
Role of bearings Transmission of vertical & horizontal forces from superstructure to
substructure To permit the longitudinal movement due to temperature
variations. Translation Rotation
Care on bearings
Check both ends of girder are in same level. Uniform seating check with feeler gauge The longitudinal alignment - straight and central w.r.t. girder The girders - symmetrically placed Elastomeric bearings require periodic cleaning. They may require
replacement in Gap, if any, in between girder & elastomeric pad and top of bed
block/Trestle & elastomeric pad (Ensure 100% of Bearing area is subjected to bear and transmit the super structure load)
service depending on the condition and usage.PTFE BEARINGS
PTFE bearings also require periodic cleaning of the bearing surface.
The interface should be protected from dust. Lubricating the mating surface by silicon grease reduces the
coefficient of friction and is desirable.
The following properties of the bearings have to be checked
Sufficient ability to allow movement, taking into account the temperature of the superstructure
Correct positioning of the bearings themselves and of parts of bearing relative to each other
Uncontrolled movement of bearing Fracture, cracks and deformations of parts of the bearings Cracks in the bedding or in adjacent parts of sub and super
structure Condition of the anchorage Condition of sliding or rolling surfaces Condition of anti-corrosive protection, against dust and
other scaling
6. Super structure General Observe for
Condition of concarete in general
Cracks if any at near mid span near supports Anchorage zones near junction of main girder and diaphragms. In diaphragms.
Cracks requiring epoxy grouting along with details of location size etc
Spalled concrete with details of location, extend etc. Condition of concrete in soffit of deck slab. Inside portion of box girders-cracks, condition of concrete etc. Rust streaks, cracks etc. –location with details. Condition of concrete around prestressed cables with reference to
cable profile. Condition of concrete near end blocks and condition of
anchorages. Actual camber compared with original camber. –Bottom surface of
girder at ends and 1/4th, mid span & 3/4th span to be recorded and compared with previously recorded camber. Recording of camber is the only way to know the health of the girder and the recording should be more sensitive
Progressive loss of camber is an important indication of deterioration in the condition of bridge
Condition of drainage holes in the deck slab. Provision of wearing coat on top of slab and its conditions. Condition of concrete in deck slab. Special repairs required, if any.
Corrosion of Prestressing Steel and Corrosion ProtectionThe basic types of corrosion in prestressing steel can be categorized as follows:
Uniform corrosionIn uniform corrosion, the surface of steel is uniformly affected. This condition generally occurs when the steel is left unprotected and exposed to the environment.
Pitting corrosionPitting corrosion results from dissimilar materials and environments, and results in stress concentration in prestressing steel that can lead to cracking of the prestressing wire due to the high levels of tension typically present in such wires. Cracking in these cases generally originates in the base of the corrosion pit.
Stress corrosionCorrosion can be highly detrimental since prestressing wires are under significant mechanical stress.The basic protective system for the prestressing strand in prestressed concrete is the concrete itself.
Consideration should be given to encapsulation of anchorages in corrosion resistant materials,
Use of corrosion inhibitors in surrounding concrete, and use of very low permeability, high performance concrete mixtures.
In structures where these precautions have not been taken, corrosion may occur and significant concrete distress, in the form of cracking and spalling, may develop.
An inquisitive mind, backed by accurate record keeping, is required to provide the necessary documentation for any prioritized repairs."
The construction drawings must show in which areas or at which points hydraulic jacks can be set, what are the maximum lifting forces and upto which level the bridge may safely be lifted.
7. Track structure
ON BRIDGE No eccentricity of track is permitted w.r.t centre of girder Track in good line and level 100% track fittings Proper guard rail & its fixing arrangement Proper spacing of sleepers Full Ballast profile up to top of sleeper with 300mm clear
cushion
APPROACHES Ballast profile as per standard to a length of 100m Effective drainage and clear cushion of 300mm 100% track fittings Ballast retaining wall for one rail length Alignment should be straight Level should be same as on bridge Sleeper spacing of 600mm for one rail length No rail joint within 3m from abutment SEJs beyond 10m from the abutments Standard guard rail arrangement Well anchored track Service life of rails on bridges and approaches as per ACS
No.95 No OBS flaws permitted on bridges and approaches All AT welds to be protected with Joggled / Bent fish plates Approach slabs to be provided for new bridges and during
rehabilitation
BRIDGE INSPECTION AND REPAIR VEHICLE.
Plasser & Theurer recently introduced its BCR 100 Bridge Inspection and Repair Vehicle. The machine, which looks like a cherry picker, can be used for inspection work as well as for repair work.
The BCR 100 has a moving, elevating work platform that can hold up to three people with tools and has a working height of approximately 87 feet over top of rail, a maximum working depth down to 84 feet below top of rail and a horizontal range below a bridge of up to 51 feet.
The platform has double-secured program control monitors, regulates the work sequence and prevents operating errors by personnel. It can also be turned in the horizontal plane through a full 180-degrees and horizontal leveling is performed automatically.
In Southern Railway, the population of PSC girders is going on increasing and in TPJ division itself, at present 250 spans are available. Such Bridge inspection and repair vehicle will be more
effective in inspection and maintenance activities and hence procurement of such machine is justifiable when compared to the asset value of PSC girder bridges and also identification of defects at earlier stage & rectification of same in time resulting premature renewal. Moreover, the bridge section is not having any infrastructure for handling the PSC girders even for replacing the damaged elastomeric bearing.
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