MAINTENANCE OF BRIDGES

INTRODUCTION

For safe & uninterrupted traffic flow, it is necessary to have the maintenance of bridges periodically. This maintenance is essentially consists of structural maintenance, rehabilitation & replacement. It provides the guidelines for the engineers to adopt cost effective decisions regarding maintenance work for bridges, canals, tunnels etc & it can be achieved by the optimal use of public funds. To achieve the desired result requires constant alertness & thorough inspection procedures.

WHY WE NEED TO MAINTAIN BRIDGES.

It is necessary for the following factors,

COST.

LIFE TIME.

VERY IMPORTANT STRUCTURE.

PURPOSE. etc

QUALIFICATIONS OF INSPECTION PERSONNEL:

The individual incharge of the bridge should have the

following qualifications & should carry out the following responsibilities.

He should be a qualified Engineer having 10 years experience in bridge constriction & inspection

He shall be responsible for thorough field inspections & recommendations for correction of defects, posting of restricted load or traffic etc.

He must be very familiar with design & construction features of the bridge & he must be capable of analyzing the structural deficiency, taking serious actions to over come it.

CATEGORIES OF BRIDGE INSPECTIONS:

The following are the categories of the bridge inspections.

Routine inspection.

Detailed inspection.

Special inspection.

ACCESS FOR INSPECTION:

The accessibility can be successfully achieved by Snooper-crawlers, which are movable vehicle-mounted crane arms, tailored to the needs of horizontal & vertical accessibility all along the span, substructure & bearings. For very long span, wide bridges with large navigational clearances, like the cable stayed suspension system or steel girder type, permanently installed traveling gantries and lifts are a must for inspection. For piers & pier frame of large height & bearings must be provided from the deck.

EQUIPMENTS NEEDED FOR THE INSPECTION PURPOSES:

Snooper-Crawler & adjustable ladders. Rebound hammer for in situ assessment of the

strength of concrete. Magnetic detector for measuring thickness of

concrete cover, & for locating reinforcement bars. Acoustic crack detector & magnetic crack definer

for detection & location of cracks in steel. Mechanical extensometer & transparent templates

for reading off crack widths from surface contact. Hydraulic devices, pressure transducers or load

cells for measurement of forces or pressures etc. Equipment for measuring vibrations. Electrical resistance meter.

CONDITION RATING SYSTEM:

Structures containing elements which are in good condition without any signs of deterioration & requiring no immediate action.

Structures containing elements which show signs of distress but which, under the operating conditions of the total structure may not having a serious defect & it can be rectified easily.

Structures containing elements which show signs of deterioration & which, due to distress under operating conditions, would affect structural integrity of the elements.

Structures containing elements which show signs of progressive failure due to excessive loading, possibly resulting in use limitations or replacement.

Structures containing elements which have been under designed or poorly constructed or are overloaded & show cracks or deterioration.

SUFFICIENCY RATING SYSTEM:

In the area of performance, a numerical representation can be developed. This system is used in the following areas.

• Load performance.• Safety performance.• Remaining life. CONDITION SURVEY:

Each bridge should be classified, inspected, & evaluated in relation to the various parameters which affect the structural integrity & performance characteristics of the structure. The primary goal is to achieve uniform interactive parameters to inspect bridges.

STRUCTURE IDENTIFICATION:

This is done to establish a uniform code number for each structure, which relates the structure location with respect to highway, district, highway route number & the number of kilometers from an established base point of the highway on which the structure is located.

The type of bridge crossing should be noted & coded as a supplementary identification. Some of the crossings are identified as follows.

• Over water course.• Over highway.• Over railway line.• Over land feature.

For identifying the type of crossing, the structure should be identified by name. Once the preliminary identification of the structure & crossing type has been established, the identification of the type of structure should be made.

• APPROACH TO THE BRIDGES:

Alignment. Grade. Approach slabs. Guard rail. Embankment condition. Relief joints.

• INSPECTION GUIDANCE:

The bridge engineer & inspector should be familiar with terminologies included in bridges & should be well versed in the tools & devices used in bridges. He must have a thorough knowledge of safety precautions & should be properly organize, prepare the necessary documents for the inspection purposes. He shall have good knowledge in judging the construction materials which have used in the construction of bridges such as concrete, steel, timber etc. In addition to it he shall have knowledge in foundations on which the bridge piers will rest.

• DETERIORATION IN CONCRETE: Freezing & Thawing: Porous concrete absorbs water & creates high expansive

pressures which results in cracking, scaling & spalling of concrete. Salt Action: The use of salt contribute to weathering through

recrystallization & it also increases the chemical attack on concrete & steel. Unsound Aggregates: These are structurally weak & readily cleavable. Leaching: Water seeping through cracks & voids in the hardened

concrete leaches or dissolves the Ca(OH)2. It results in efflorescence encrustation

at the surface of the cracks with evidence of CaCO3. Chemical Attack: A number of chemicals attack concrete.

Wear or Abrasion:

Traffic abrasion & impact cause wearing of bridge decks & kerbs, parapets, piers

gets damaged by the scraping action of vehicles. Foundation Movements: These movements can cause serious cracking in

structures. Shrinkage & Flexure forces: Shrinkage & flexure forces set up tensile stresses

causing serious cracks. Rusted reinforcing steel: Corrosion of steel causes increase in its volume &

exherts radial expansive pressure on concrete, leading to cracking & for

further rusting.

Cracks in the column and beam.

DETERIORATION IN STEEL:

Air & Moisture:

This will cause corrosion of steel, especially in a marine environment.

Industrial Fumes: This fumes mainly cause deterioration of steel. De-icing Agents: Under damp conditions, it will attack steel causing

corrosion. Sea-water & Mud: Unprotected steel piles if immersed in sea water &

embedded in mud results in serious deterioration.

Thermal strains or Overloads:

Where movement is restrained, the steel may yield, buckles.

Fire:

Extreme heat will cause serious deformation of steel members & loss of strength.

Collisions: The vehicles may strike the steel beams,

columns causing damage to the bridge. Animal Wastes:

This will results in rusting of steel items. Galvonic action:

FOUNDATION MOVEMENTS: These movements are caused by the

movement of the supporting strata. In this case the soil deformations are caused by volume changes & settlement or subsidence in the soil or by a shear failure. The movements can be classified into four types. They are

Lateral Movements: Earth retaining structures such as abutments,

retaining wall,piers will undergo this type of movement.

Vertical Movements: Any type of structure which was not founded on a

solid rock may undergo this type of movement.

Pile Settlements:

Rotational Movements:

CHECKED DURING INSPECTION:

Checking the alignment: Any abrupt change in the alignment of the bridge indicates a movement of the pier or bearings. Sight along railings: A sudden dip in the rail line is indicating the result of displacement of a pier or abutment. Run profile levels: This inspection will not only help us to establish the existence of any settlement, but will also identity any differential settlements across the roadway. Check piers, pile bents, abutment faces: This inspection is employed to check the plumbness. An out of plumb pier in either direction usually signifies foundation movement & it also indicates a superstructure displacement. Observing expansion joints: This should be checked for the signs of opening or rotating. It will indicate the movement of subsurface soils which results in abnormally small or large openings, differential elevation etc. Observe slabs, walls & members: The cracks, buckling, & other deteriorations should be noted. Checking back walls & Beam ends: Back walls should be checked for cracking developed by the abutment rotation, sliding or pavement thrust. This is a sign of horizontal movement of the abutment. Unbalanced post-construction embankment or fill: Embankments should be checked for balance & positioning. Unbalanced embankments may cause a

variety of soil movements which may impair the structural integrity

• UNDER WATER INVESTIGATIONS:

This investigation has to be performed by the personnel experienced in these types of inspections. However, the bridge inspector is responsible for the bridge inspection & for the evaluation of the underwater portions of the bridge. Relatively new structures have collapsed due to the corrosion of the steel piles below the concrete protection. PSC piles will not fail below water level, while timber piles are known to be vulnerable. This type of investigations is very important in the case of constructions to be done in the sea. During inspection the following points may be checked.

Pile bents. 1.Steel piles. 2.Timber piles 3.Concrete piles. Dolphins & fenders. Pier & Abutment Conditions. Scour.

Approaches:

Approach pavement is to be checked for the unevenness, settlement or roughness. Existence of defects may cause vehicles coming onto the bridge to induce undesirable impact stresses in the structure. Cracking in slab indicates a void under the slab from fill settlement. The joints between approach pavement & the abutment backwall is to be examined to determine if there is adequate clearance. The condition of the shoulders, slopes, drainage & approach guard rail should also be checked.

• INVESTIGATIONS OF STRUCTURAL COMPONENTS:

Note down any structural cracks more than 0.3mm in width & any signs of deterioration & distress. In a structure this will range from usual surface cracks to serious structural cracks & manifestation of distress zones.

If the observed manifestations of distress in the structure are so serious, then it will require the detailed structural investigation in order to enable to decide between repair & part or complete demolition & replacement. A detailed explanation supporting reasons & the details of the likely causes that obviously lead to such distress is given.