Construction Methodology of Major Bridges

Minor Bridge is a reinforced concrete structure used as drainage under road work. In other words, It is a structure of more than 6m to 60m in length between faces of abutments. On the other hand, major bridge, if the length between the faces of abutments is greater than 60m.

The scope of work shall consist of furnishing and providing plain and reinforced concrete placed into a required shape as per the drawing.

MACHINERY

1. Excavator
2. Generator
3. crane
4. Tippers
5. Concrete Batching Plant
6. Transit Mixers
7. Concrete Pump
8. Plate, needle & Shutter vibrators
9. Water tanker
10. Shuttering Plates
11. Bar Bending Machine
12. Bar Cutting Machine
13. Tower Light
14. Survey kit etc.,

MATERIALS

The materials used in the construction of structures over rivers or streams where severe scouring is involved, such as supporting the piers below the water level of a road, railway bridge, barrage, aqueduct, or any monumental structure shall be satisfying the following requirements.

1. Cement conforming to IS: 8112 / IS: 12269 shall be used only after ensuring that the minimum required design strength can be achieved without exceeding the maximum permissible cement content of 450 Km./Cum.

2. Steel shall be corrosive resistance.

3. Super plasticizer shall be conforming to IS: 9103.

4. Water shall be of following permissible limits when tested in accordance with IS: 3025.

5. Coarse aggregate having positive alkali silica reaction shall not be used and it should be conforming to IS: 383 and IS: 2386 (Part I-VIII).

6. Flakiness Index of coarse aggregate shall not be more than 35%.

7. Aggregate Impact value shall bot be more than 45%.

8. Gradation of the coarse aggregate shall be satisfying the following requirements.

9. Fineness modulus of the fine aggregate shall neither be less than 2.0 nor greater than 3.5.

10. Gradation of the fine aggregate shall be satisfying the following requirements.

PROCEDURE

Center-line Fixing

Firstly, The place where the bridge exists shall be located and the centre line shall be marked with the help of a Total station or electronic theodolite.

The located bridge must be either perpendicular or with some skew to the road alignment according to the direction of the flow.

OPEN FOUNDATION

Setting Out

With the use of an electronic theodolite, reference points must be fixed to accurately indicate the X-X axis (typically traffic direction) and the Y-Y axis (normal to the X-X axis).

A temporary benchmark should be set up near the bridge and compared to the permanent benchmark on a regular basis.

The limits of excavation shall be marked on the original ground to the line and length as per drawing.

Original Ground Levels

The original ground level (hereinafter called the “OGL”) shall be taken on the centerline and either side of the centerline of the bridge location to the required width and length as per the drawing. However, the number of rows to be taken is depending on the width of the bridge.

Excavation for Open Foundation

Most importantly,  before starting excavation, proper barricading along with reflective marking with cautionary signboards shall be provided ground excavation area.

Dumpers or tippers will help in loading excavated material by removing it with help of an excavator to the required level, length and width. The excavated portion shall be cleaned off if any slush, and it should be dressed properly.

Controlled blasting/breaking shall be carried out if necessary, to excavate hard rock.

Sand Filling

Fine to coarse sand shall be laid to the required length, width and thickness. However, the sand should be compacted properly with plate vibrators.

Leveling Course

Conformity test for checking bearing capacity shall be carried out before laying PCC.

The shuttering shall be thoroughly cleaned, aligned and marked to the required thickness. The transit mixer will transport concrete mix from batching plant to the site. After that, It shall be laid and compacted properly with plate vibrators.

Raft

Reinforcing steel should be bent to the dimensions and shapes stated in the certified bar bending schedules, and it should be precisely placed in position on levelling coarse with appropriate cover as per the approved design.

The bars crossing one another must be connected together with binding wire at every junction to keep the reinforcement’s skeleton solid and prevent it from sliding during concrete placement.

Greased shutters must be aligned, fastened, and labelled to the appropriate thickness, and concrete of an acceptable mix must be poured properly.

After the concrete has been set, it must be de-shuttered and properly cured for a sufficient period of time.

Abutment Footings

In accordance with the authorised drawing, skin reinforcement must be placed along the length of the bridge with a required cover. Bending must be done correctly to provide for shutter gap spacing. Concrete that satisfies the specifications must be spread out layer by layer and properly compacted using plate vibrators.

SUB STRUCTURE

Abutments / Piers

Reinforcing bars must be linked at prescribed intervals, and shutter plates must be positioned according to the drawing’s specifications for size and shape. Up to the top level marks, the concrete must be laid layer by layer with the proper vibration.

For abutments, weep holes must be installed at predetermined intervals. For small bridges, it is best to cast the abutment or pier element as a single piece. Abutment/pier caps must be cast to the line and length.

Wing/Return Walls

Depending on the site’s constraints, work may start on constructing the bridge accompanying safety measures including barricades and warning signs that must be positioned all around the excavation area.

Firstly, Excavation, sand laying, and PCC laying must be done in accordance with the authorised drawing’s line, length, and depth.

Secondly, Shuttering must be carefully cleaned, marked, and aligned in accordance with the design. And, Concrete must be moved by transit mixers from the batching plant to the construction site, laid out in accordance with the drawing, and thoroughly compacted using plate vibrators.

Finally, Curing shall be done for a required period by providing hessian cloth as per clause 1731 of MORT&H specification for foundation and substructure.

Back Filling

Filter media shall be transported by means of end dumpers to the site and should be laid behind the return/wing walls as per specification. For backfilling selected material should be laid behind the abutment walls to the top of the embankment as per specifications and it should be compacted properly till the achievement of the required density.

If there is any presence of unwanted material, it must be removed from the foundation before being backfilled. Moreover, The material for backfilling shall be as per the design drawing.

Pedestals & Bearings

Using the design coordinates, the marking must be done for pedestals. The bent and fixed reinforcement must follow the permitted plan. Shuttering must be set up properly to the required size and shape, and top-level marking must be completed in accordance with the drawing.

The proper vibration must be used when laying the certified concrete mix. Bearings of the appropriate size must be positioned after the concrete has been set and before the beams have been concretized.

A bridge bearing is a part of a bridge that transfers loads from the deck to the substructure while also allowing for controlled movement in response to temperature changes or seismic activity, which lowers the stresses involved. The line dividing the superstructure from the substructure is known as a bearing.

Expansion joint shall be provided surrounding the bearings between beams and pedestals.

Furnishing and fixing of bearings in position in accordance with the details of drawings, to the requirements of MoRT&H specifications of section 2000.

1. All bearings shall be tested in accordance with specifications as per IRS: 82 and checked for overall dimensions.
2. Care shall be taken during installation of the bearings to permit their functioning in accordance with the design scheme.
3. To prevent contamination, dismantling of the bearings at site shall not be done.
4. The load shall be transferred to the bearings only when the bedding material has developed sufficient strength.
5. The props for the form work shall only be removed after lapse of appropriate time. In special cases, this can be ensured by suitable devices like jacks, etc.
6. Temporary clamps and shims shall be removed at an appropriate time, before the bearing is required to permit movement.
7. Permitted installation tolerance of the bearings from plan of sliding shall be maintained.

SUPER STRUCTURE

Girders

A girder is a type of construction support beam. It serves as a structure’s primary horizontal support and holds smaller beams. Girders frequently feature an I-beam cross section made up of two load-bearing flanges spaced apart by a stabilizing web, although they can also take the form of a box, a Z, or the other shape.

Deck Slab

The prescribed formwork must be kept in a level, stable position, and the gaps between the shutter plates must be properly packed. As per the bar bending schedule, reinforcement must be placed and positioned correctly.

Side shuttering must be installed, and they must be properly marked in accordance with the drawing. Before placing concrete, all foreign objects and dust on shutter plates must be removed and a cement slurry must be applied evenly over the surface.

Concrete must be laid in a single, continuous operation using shutter vibration and necessary vibration to properly compact the concrete to the required level.

Additionally, when pouring the concrete, care must be taken to avoid creating any cold joints, and construction joints must adhere to specifications.

Approach Slab

The purpose of the approach slab is to lower the overall settlements of the embankment soil beneath it and the rear wall.

If it is not provided, the soil right behind the rear wall will usually settle, causing a cavity bump, which may result in an additional impact load being applied to the junction when the approaching vehicle rides from the embankment to the top slab. The minimum length of the approach slab shall be 3.5m and 300mm thickness throughout the width of the road.

Mastic Asphalt

Mastic Asphalt is a bituminous design mix made up of bitumen, coarse aggregate, fine aggregate, and filler material. It is utilized as a waterproofing material and as a wearing course in a variety of road and bridge construction projects.

1. The air compressor will clean the surface of any foreign substances and dust.
2. Tack coat will be applied as necessary.
3. Bitumen will be heated to the requisite temperature, and the authorised design mix of stone dust and lime powder will be blended with bitumen.
4. The prepared mix will be put down to the required thickness on the surface.

Drainage Spouts

Rainwater is diverted down from a platform into a pipe that serves as the bridge drainage spout. Additionally, it is made of mild steel and galvanized components. It safeguards it and stops corrosion or rusting from happening as a result of water passing through the pipes.

1. The drainage assembly will be constructed according to the drawing’s specifications, welded to ensure water tightness, and then hot-dip galvanized.
2. Before casting the Deck slab, the reinforcement will be trimmed to the necessary size and matched reinforcement will be added at the assembly’s corners.
3. The appropriate cutouts for the deck slab will be made before assembling the galvanized assembly in precise location, lines, and levels as shown on the drawings.
4. The drainage spouts shall be spaced no more than 10 meters apart.
5. The drainage spouts will be linked to appropriately placed pipe lines to discharge surface run-off to ground-level drains.
6. Around the drainage assembly, sealant will be applied in a thin layer that covers at least 50 mm of the surface.

Strip Seal Expansion Joint

Strip Seal Expansion Joint is a mechanical device designed to seal an extended gap formed between two adjacent road slab sections by providing continuous support for vehicles passing the gap while letting the road slab sections to move as needed in response to changing temperatures.

1. The recess’s surface will be thoroughly cleaned, and all dust and dirt will be properly removed.
2. Exposed reinforcement will be adjusted appropriately to allow the joint to be lowered into the recess without being obstructed.
3. The recess formwork will be connected and positioned according to the measurements in the drawing.
4. The joint will be leveled and aligned before the anchor loops on one side of the junction are welded to the exposed reinforcing bars of the structure, and the same will be done on the other side.
5. High-quality certified concrete will be placed into the recess and properly compacted.
6. A crow bar will be used to insert a neoprene strip seal between the edge beams once it has been cut to the desired length.
7. The surface of the carriageway will be completed flush with the tops of the steel sections.

Crash Barrier

concrete crash barriers prevent the vehicles from diverting into the opposing lane and off the road.

1. While fastening the reinforcement for the task, it should be tied in position.
2. Smooth, consistent, and lubricated shutter plates will be installed with appropriate cover, and gaps between shutter plates will be packed.
3. Shutters should not bulge during vibrating.
4. Transit mixers will transport the concrete mix from the batching facility.
5. The vibration will continue until there is no longer any honeycomb.
6. The approved concrete mix will be placed in layers with sufficient compaction.
7. De-shuttering will be performed after the required strength is reached.

Bridge Furniture

Vertical posts, Hand rails and other sorts of furniture are generally required and should be in accordance with clause 2703 of MoRT&H specifications.

1. Firm ground / flat form shall be prepared for casting the handrails.
2. Smooth surfaces shuttering sheets shall be prepared to the required size and shape fixing bolts and nuts.
3. Handrails shall be caste by pouring approved grade of concrete into prepares mould and these should be kept damp for a period of atleast 14 days.
4. Precast handrails shall be places in position in site and then reinforcement & shuttering plates shall be arranges for handrail posts to the alignment as per the drawing.
5. Concrete of approved mix shall be laid for handrail posts by proper vibration till to fill all voids for smooth surface.

Uses

1. Allowing passage over the obstruction, which is generally something difficult or impossible to cross.
2. They are cost-effective since they are simple and affordable.
3. To maintain network traffic control.
4. Usually designed to ease movement through various terrains.
5. Allow people to cross the river.
6. It is expected to have a lifespan of more than 100 years
7. Delivering a more secure and faster mode of transportation.
8. Resistant to many sorts of loads and impacts

In conclusion, A bridge allows traffic to pass on it. Furthermore, construction must be carried out in a safe manner, with workers wearing personal protective equipment (PPE) and the minimum possible interface to traffic flow by using appropriate safety devices on site.

Above all, this is the general procedure for the construction of bridges like Minor Bridges, Major Bridges, Flyover, Viaducts etc. The procedure varies depending on design of structure.

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Also Read: Construction Methodology of Box Culvert