METHODOLOGY FOR BRIDGES

 

METHODOLOGY FOR BRIDGES

General :

Concrete bridge can be grouped according to their type into ordinarily reinforced and PSC bridges, ordinary reinforced bridges cannot be used in long span bridges, pre-stressed bridge are utilize to over come the above mentioned limitations of ordinary reinforced concrete bridges.

 

With reference to the Construction Methodology Concrete bridge can be classified into six techniques:-

1.      Cast-in-situ

2.      Cantilever Carriageway

3.      Flying shuttering

4.      Launching girder

5.      Pre-cast bal cantilever

6.      Incremental Launching.

 

We are presently using Cast-in-situ, before start of construction work care must be taken to ensure that the following documents are available:

 

(i)     Sanction letter and technical note, if any enclosed with the letter.

(ii)   Copy of set of plans and approved detailed working drawings.

(iii) Standard specification, guidelines, codes of practices etc according to which the work must be executed as per contract.

(iv) Survey, investigation and sub-soil test reports

 

Based on the study of the document, drawings and visits to site decide the following:

 

(i)     Location of camp site which should include space for batching plant, storage sheds, casting yards, sheds for equipment and machinery, labour camp etc. Care should be taken to see that the land selected for the above purposes is on high ground well above the highest flood level recorded in the area with good drainage facilities, convenient approaches from existing roads etc.

(ii)   Time required for site clearance and functioning of camp site.

(iii) Evaluate the existing plants and equipments to ascertain their output and plan for the procurement of additional units, if required, for the timely completion of the project.

(iv) Evaluate the number of skilled and unskilled labour and the period required for bringing them to site.

 

 

1. ALIGNMENT

After having cleared the site and finished the camp work, the next step is to fix the alignment of the bridge on ground. This should be done by following the layout plan with reference to the permanent benchmarks or pillars which may have been fixed at site during the location survey. The bridge alignment can then be marked on the ground so that the distances and radii of curvature as laid on the ground tally with those shown on the plan.

 

Once the alignment of the centre line of the bridge has been transferred to the ground, this line should be extended on both ends to about 50-60 meters outside from either end of the bridge and pucca pillars should be fixed at such extended points and the centerline precisely marked on top of these pillars. In between the reference pillars and the end abutment, another reference pillar about 20-30 meters away from the abutment should also be fixed.  

 

The construction of various components of bridge works and protective works shall conform to Ministry of Transport Specification for Road and Bridge Works and relevant IRC Bridge Codes/ Standards. However, some of the salient points are mentioned below:-

 

2. Materials

All materials to be used in the work shall be tested to ascertain that they conform to the standards laid-out in the prescribed specifications. The basic standards to be referred are as follows:

 

1.  Cement.                      IS 12269

2.   Coarse Aggregates – IS:383 and Table no 1000-1 of MORT&H

3.  Sand –                         IS: 383 for cement concrete work.

4.  Steel

All reinforcing bars shall be high yield strength deformed bars ( Grade designation S415) conforming to IS 1786 ( expect for mesh reinforcement which shall be MS bar grade designation S 240 conforming to IS : 432 part I ).

 

5.  Water: Normally potable water free from deleterious materials may be considered satisfactory for mixing and curing concrete. Water to be used in concreting and curing shall conform:

 

           PH value             Min.6

           Alkalinity            Max. 10ml  HCL to neutralize 200 ml water.

           Acidity                Max 2 ml NaOH to neutralize 200ml water

           Organic               200 mg /ltr

           Inorganic             300 mg/ltr

           Sulphate              500 mg/ltr

           Chloride              500  mg/ltr

           Suspended matter 200mg/ltr

          

 6. Admixtures: Admixtures shall confirm to the requirement of IS 9103, in addition    to the conditions mentioned in cl 1012 of MORT&H shall be satisfied.

Admixtures are used to modify the following properties

a)      Workability of fresh Concrete

b)      Pumpability

c)      High Strenth through reduction of Water content.

 

Minimum quantity of cement to be used shall not be less that 210 Kg per cubic meter in plain concrete and not less than 300 Kg per cubic metre in reinforced concrete structural members. The minimum quantity of cement for prestressed concrete members shall neither be less than 360 Kg per cubic metre of concrete nor shall it be more than 540 Kg per cubic metre of concrete. This shall conform to MORT&H  specification NO 1700 & Table  nos 1700-2 ,1700-3 ,1700-4,   for Road and Bridge Works.

 

3. Requirements for Design Mixes

These shall conform to MORT&H Specification NO 1700 & TABLE 1700-1  for Road and Bridge Works.

4. Equipment

(a) Batching Plant (Make SCHWING STETTER) (Capacity 30Cums/hr)

 

(b) Concrete Transportation

(a)    Concrete dumpers

(b)   Power hosts

(c)    Transit truck mixer

(d)   Concrete pump

 

(C) Compaction of concrete

(a)    Internal vibrators

(b)   Form vibrators

                      

Mixing of Concrete

Concrete shall be mixed in Batch Mixing plant (Schwing Stetter 30cu.m/hr).Concrete mixer or Hand mixing is not allowed.

 

Transportation Laying and Compaction of concrete: 

Concrete shall be transported and placed as near as practicable to its final position, so that no contamination, segregation or loss of its constituent materials takes place. Concrete shall not be freely dropped into place from a height exceeding 1.5 meters.

Internal vibrators shall normally be used for compaction of concrete. The diameter and size of the vibrators shall be chosen, depending on the thickness of the member, maximum size of aggregates, spacing of reinforcement etc. The vibrator shall be inserted and withdrawn out of the concrete slowly avoiding over-vibration and segregation. The withdrawal of the needle shall be completed by the time the cream of mortar appears on the surface. External vibrators may be used for slender prestressed members in addition to internal vibrators. For large horizontal areas not exceeding 200mm in thickness, screed vibrators may be advantageously used. The entire vibration activities must be completed before the concrete starts setting.

 

Curing: All concreting shall be cured for a period of not less than 14 days from the date of placing. Curing usually consists of maintaining the formwork in place and curing the concrete with materials such as straw, hessian, sand, polythene sheet or curing compound or with an absorbent material which is kept damp. Special methods of curing shall be restored to where the structural members are of considerable depth or bulk or have an unusually high proportion of cement content. Alternate wetting and drying shall be avoided especially in the form of cold water applied to warm concrete surface.

Construction joints: The location of construction joints shall be determined before the concreting starts. Any dirt or unsound concrete shall be removed before the concreting is restarted. Dry old concrete surface against which the fresh concrete is to be placed shall be kept moist for at least 24 hours to minimize the differences in shrinkage between the new and the old concrete. However at the time of adding new concrete, the surfaces of the old concrete shall be dried partially to enable the cement paste of the newly placed concrete to bound properly with the old concrete.

 

OPEN FOUNDATION CONSTRUCTION

 

1. Sources of Material: -

The Coarse Aggregates will be procured from our own Crusher Plant. Fine Aggregates will be procured in the form of river sand, which is readily available in and around the project corridor. Cement and Steel will be procured from approved manufacturer and the manufacturer’s test certificate will be obtained. Materials shall conform to Section 1000 of Ministry’s Specifications. Concreting shall be done as per Section 2300 of Ministry’s Specifications

 

2. Earth Work

    Excavation for Foundations shall be carried out in accordance with Section 300 of MORT&H.

 

3. Design, Erection and Removal of Form work: -

All formwork and reinforcement contained in it shall be cleaned and made free from standing water, dust, snow or ice immediately before placing of concrete. No formwork is necessary for the lean concrete layer but it may be done if directed by the Engineer. Side formwork shall be used for foundation concrete work. All materials for formwork will conform to IRC: 87.

 

Formwork shall be removed not earlier than 24 hours after placing of concrete. The design and erection of formwork shall be as per IRC-87 and conforming to the specifications.

 

4. Dewatering and Protection: -

 

Open foundations shall be laid dry. Where water is met with in excavation due to stream flow, seepage, springs, rain or other reasons, we will be resort to providing drainage channels, and the foundations trenches dry when so required and to protect green concrete/masonry against damage by erosion or sudden rising of water level. The methods to be adopted in this regard and other details thereof shall be left our choice but subject to approval of the Engineer.

 

5. Preparation of Foundations: -

 

The bottom of the foundations shall be leveled both longitudinally and transversely or as directed by the Engineer. Before footing is laid, the surface shall be slightly watered and rammed. In the event of excavation having been made deeper than that shown on the drawings or as otherwise ordered by the Engineer, the extra depth shall be made up with concrete or masonry of the foundations.

 

 

 

6. Production, Transportation, laying and Curing of Concrete: -

 

The concrete design mix will be furnished by us for your approval before the inclusion of the same in the actual work. The concrete will be produced by Batch Mixing Plant and will be transported to the site of placement with the help of Transit Mixers. The concrete will be laid in place and compacted to the form with the help of needle vibrator.

The curing of concrete exposed surface will be done by applying wet Hessian/Jute cloth and sprinkling of water on the vertical faces. The horizontal faces will be cured by making water pond.

 

7. Person employed for execution and supervision: -

 

Bridge Engineer will be employed at site of work for control of various activities and will be assisted by site Engineer and supervisor who will be posted at each  location.

 

8. Tests and sampling procedure: -

 

The materials will be test as per Sec 900 of MORT&H and the procedure of relevant IS and IRC Codes will be followed.

 

6. Equipment Details: -

 

Following equipments will be used for the production, placing, and compaction of concrete:

 

(a) Batch Mixing Plant (Make Schwing Stetter)(Capacity 30Cums/hr).

(b) Transit Mixers (TATA)

(c) Concrete Pump (Make Schwing Stetter)(Capacity 46Cums/hr)

(d) Needle Vibrators – As per the requirements

 

7. Your staff shall be apprised before the commencement of each item of work and request for checking of every step of work through documented Requisition of Inspection of Work.

   

Placing Of Reinforcement

Reinforcement steel shall conform to the dimensions and shapes given in the approved Bar Bending Schedules.

Reinforcement bars shall be placed accurately in position as shown on the drawings. The bars crossing one another shall be tied together at every intersection with binding wire .the diameter of binding wire shall not be less than 1 mm.

Necessary stays, concrete / polymer cover blocks, metal chairs, metal hangers, supporting wires etc or other subsidiary reinforcement shall be provided to fix the reinforcement firmly in its correct position.

 

   Placing and fixing of reinforcement shall be inspected and approved by the Engineer before concrete is deposited.

 

 

 

 Sub-structure Construction of Minor Bridges and Box Culverts

 

EXCAVATION FOR STRUCTURES

 

1. Scope

 

Excavation for structures shall consist of the removal of the material for the construction of foundations for bridges, culverts, retaining walls, head walls, cutoff walls, pipe culverts and other similar structures, in accordance with the requirement of MOST and additional (as per contract) specifications and the lines and dimensions shown on the drawings or as indicated by the Engineer. The work shall include draining and pumping. The removal of all logs, stumps, grubs and other deleterious matter and obstructions, necessary for placing the foundations; trimming of excavations; backfilling and clearing up the site and disposal of all surplus material.

 

1. Sources of Materials: -

 

The Coarse Aggregates will be procured from our own Crusher Plant. Fine Aggregates will be procured in the form of river sand, which is readily available in and around the project corridor. Cement and Steel will be procured from approved manufacturer and the manufacturer’s test certificates will be obtained. Materials shall conform to Section 1000 of Ministry’s Specifications. Concreting shall be done as per Section 2300 of Ministry’s Specifications.

 

 

2. Design Erection and Removal of Formwork: -

 

Side form work shall be used fro foundation concrete work. All materials for formwork will conform IRC:87. Formwork shall be removed not earlier than 24 hours after placing of concrete. The design and erection of formwork will be as per IRC-87 and conforming to the specifications. We will furnish the design of formwork for your approval before the erection of the same. It shall be ensured that the forms are removed without disturbing the concrete.

 

3. Production, Transportation, Laying and Curing of Concrete: -

 

The concrete design mix will be furnished by us for your approval before the inclusion of the same in the actual work. The concrete will be produced by Batch Mixing Plant and will be transported to the site of placement with the help of Transit Mixers. The concrete will be laid in place and compacted to the form with the help of needle vibrator.

 

The curing of concrete exposed surface will be done by applying wet Hessian/jute cloth and sprinkling of water on the vertical faces. The horizontal faces will be cured by making water pond.

 

 

 

4. Person employed for execution and supervision: -

 

Bridge Engineers will be employed at site of work for control of various activities and will be assisted by supervisors who will be posted at each location.

 

 

5. Tests and sampling procedures: -

 

The materials will be test as per Sec 900 of MORT&H and the procedure of relevant IS and IRC Codes will be followed.

 

Equipment details: -

 

Following equipments will be used for the productions, placing, and compaction of concrete.

 

(a) Batch Mixing Plant (Make Schwing Stetter)(Capacity 30Cums/hr).

(b) Transit Mixers (TATA)

(c) Concrete Pump (Make Schwing Stetter)(Capacity 46Cums/hr)

(d) Needle Vibrators – As per the requirements

 

 

 

7. I C staff will be apprised before the commencement of each item of work and request for checking of every step of work through documented Requisition of Work

 

 

 

3.5 Backfilling: -

 

Backfilling shall be done with approved material after concrete or masonry is fully set and carried out in such a way as not to cause undue thrust on any part of the structure

 

 

 Super structure Construction of Minor Bridges

 

1. Sources of Material: -

 

The Coarse Aggregate s will be procured from our own Crusher Plant. Fine aggregates will be procured in the form of river sand, which is readily available in and around the project corridor. Cement and Steel will be procured from approved manufacturer and the manufacturer test certificates will be obtained. Materials shall conform to Section 1000 of Ministry’s Specifications. Concreting shall be done as per Section 2300 of Ministry’s Specifications.

 

 

2. Designs, Erection and Removal of Formwork: -

 

The design and drawings of complete formwork will be produced for the approval of the Engineer, before the erection is taken up. Not withstanding any approval of review of drawing and design by you, we shall be entirely responsible for the adequacy and safety for formwork and staging,

 

The designing of the formwork shall conform to the provisions of IRC: 87.

 

(a) Soffit of Slabs (with props left under)      :           3 days

(b) Props (left under slabs)                              :           14 days

 

3. Production, Transportation, Laying and Curing of Concrete: -

 

The concrete design mix (if required) will be furnished by us for your approval before inclusion of the same in the actual work. The concrete will be produced by Batch Mixing Plant and will be transported to the site of placement with the help of Transit Mixers. The concrete will be laid in place and compacted to the form with the help of needle vibrator.

 

The curing of concrete exposed surface will be done by applying wet Hessian/Jute cloth and sprinkling of water on the vertical faces. The horizontal surfaces of slab will be cured by making water pond.

 

4. Personnel employed for execution and supervision: -

 

Bridge Engineer will be employed at the site of work for execution of superstructure work. Pour cards will be maintained at site for concrete pouring record and supervision by our staff.

 

5. Tests and sampling procedures: -

 

The materials will be test as per Sec 900 of MORT&H and the procedure of relevant IS and IRC Codes will be followed.

 

6. Equipment details: -

 

Following equipments will be used for the productions, placing, and compaction of concrete.

 

(a) Batch Mixing Plant (Make Schwing Stetter)(Capacity 30Cums/hr).

(b) Transit Mixers (TATA)

(c) Concrete Pump (Make Schwing Stetter)(Capacity 46Cums/hr)

(d) Needle Vibrators – As per the requirements

 

 

7. Our staff will be apprised before the commencement of each item of work and request for checking of every step of work through documented Requisition of Inspection of Works, will be forwarded.

 

 

Bearings: Installation procedure for bearings shall conform to the provisions contained under section 2000 of the “Specifications for Road and Bridge Works”. and IRC : 83-part II -1987 for Electrometric bearings However some of the important points are as follows:-

 

(i)     Prepare wooden casing with inner dimensions as per the size of pedestal. Place the reinforcement and wooden template along with foundation bolts inside the wooden casing.

(ii)   Pour concrete inside wooden casing as per specifications and allow it to set for a day.

(iii) Next day take levels of all pedestals.

(iv) Put thick concrete paste, if required to make height of all pedestals equal and allow it to set.

(v)   Pour liquid slurry over bearing area, place the bearing and tighten foundation nuts. This removes unevenness in pedestal-top and gives uniform support for bearing.

(vi) Cross check levels for the bearing top surface after placing all the bearings.

(vii)Fix anchor bolts to the top plates of bearings.

(viii) Place the reinforcement for superstructure onto the bearings and cover bearing with cotton-waste from sides to protect from ingress of dust.

(ix)     Pour concrete for superstructure and allow it to cure.

(x)       Remove temporary fixtures provided for bearings. The bearings are then ready for use.

(xi)             All bearings of the lot shall be visually inspected for any defects in surface finish, shape or any other discernable superficial defects.

(xii)           Each elastomeric bearing shall be clearly labeled or marked, The bearing shall be warped in a cover. They shall be packed in timber crates with suitable arrangement to prevent their movement and to protect corners and edges.

(xiii)         Care shall be taken to avoid mechanical damage, contamination with oil, grease, dirt, undue exposure to sunlight and weather to the bearings during transport storage and handling prior to and during installation.

(xiv)         Bearings must be placed between the two horizontal surfaces (maximum tolerance 0.2 per cent perpendicular

 

 

Expansion Joints:

 

Studies approved drawings and ensure that adequate anchorage arrangements are available for anchoring of expansion joints to the body of the main bridge deck. The expansion joint shall be placed in position in accordance with the approved drawings and specifications Section 2100 of “Specifications for Road and Bridge Works” maintaining  specified cross slope of the deck, expansion gap, placement of seal etc.

 

 

Concrete Crash Barrier

The location of crash barrier shall be strictly adhered to as shown on the drawing. Concrete crash barriers shall present smooth, uniform appearance in their final position, conforming to the horizontal and vertical lines shown on the plans and shall be free of lumps, sags or other irregularities. The top and exposed faces of the barriers shall conform to the specified tolerances, as defined in clause 809.4, when tested with 3 m straight edge  laid on the surface.

All materials shall conform to section 1000-Materials for structures as applicable, and relevant Clauses in Section1600 shall govern the steel reinforcement.

 

Formwork, Centering etc:

The designs and erection of formwork and centering shall conform to IRC:87 ‘Guidelines for the design and erection of False work for Road Bridges” and Section 1500 of  ‘ Specifications for Road and Bridge Works’ and got approved by the Engineer-in-Charge before start of work.

 

Design Standards and Specifications:

Bridges

           1. Design Criteria

           The design is in accordance with the following codes.

 a)  Foundation (open)

             1)    IRC: 6-1966

             2)    IRC : 21-1987

             3)    IRC :  78-1983

b)  Substructure

             1)    IRC : 6-1966

             2)    IRC : 21-1987

             3)    IRC :  78-1983

c)  Superstructure

             1)   IRC  : 21-1987

d)  Bearings

             1) IRC : 83-part II -1987 for Electrometric bearings

e)  Expansion Joints  -  In accordance with MORTH letter No. RW/NH-34059/1/96-                     S&R  dated  30.11.2002.

f)  Protection Works

            1)  IRC : 89-1997

 

The following loads have been considered in design :

       1)  One lane of IRC class 70R or two lanes of IRC class A or one lane of IRC

            Class AA tracked vehicle on carriageway, whichever governs.

       2)  Seismic Analysis -  The proposed bridges fall in seismic zone IV as per the                                                                                                                                        

            Classifications given in IRC : 6-1966 and are designed for seismic coefficient

            Of 0.05 , importance factor of 1.5 and Value of B = 1.2.

       3)  Effect of Wind   -  The proposed Structures have been designed for wind effects as per clause 212 of IRC : 6-1966

       4)  Safe Bearing Capacity of soils -  As per site condition at foundation level.

       5)  Wearing Coat --- Asphaltic concrete wearing coat of 50mm thick provided in

             Two layers of 25mm thick each overlaying 6mm thick mastic asphalt.

g)    Reinforcement (As stated above)

 h)   Water                (As stated above)

         

i) Bearings

           Elastometric bearings shall conform to IRC : 83-1987(Part II)

 

Workmanship/Detailing

           1. Min.clear cover to all reinforcement including stirrup’s shall be 50mm unless shown otherwise in the drawings.

           2. For ensuring proper cover of concrete to reinforcement specially made polymer cover blocks shall only be used.

           3. Construction Joints

           a) Construction Joints shall be provided only at location directed/approved by Engineer in charge shown on the drawing. Concreting operation shall be carried out continuously up to the construction Joints.

           b)  The concrete surface at the joint shall be brushed with a stiff brush after casting while the concrete is still fresh and it has only slightly hardened.

          c)  Before new concrete is poured, the surface of old concrete shall be prepared as under.

          i)  For hardened concrete, the surface shall be thoroughly cleaned to remove debris and laitance and made rough so that ¼ of the size of aggregate is exposed but without dislodging the aggregate or structurally damaging the concrete.

         ii)  For partially hardened concrete, the surface shall be treated by wire brush followed by an air jet. The old surface shall be soaked with water , without leaving puddles, immediately before staring concreting to prevent absorption of water from new concrete.

d) Welding to reinforcement bars shall not be permitted.

e) Laps in reinforcement;

 

a.                   Min .lap length of reinforcement shall be kept as.

                                                            i.            83d for superstructure

                                                          ii.            55d for substructure and foundation (where d is the diameter of reinforcement bar.)

 

b.                  Not more than 50% of reinforcement shall be lapped at any one location.

f)       Bending of reinforcement bars shall be as per IS: 2502.

g)      Supporting chairs shall be provided at suitable intervals as per IS: 2502.

h)      Concrete shall be produced in a mechanical mixer of capacity of not less than 200 liters having integral weighed batching facility and automatic water measuring and dispensing device. Proper compaction of concrete shall be ensured by use of form and / or needle vibrators. Use of full width screed vibrators for compaction of concrete in deck slab shall ensured. Shuttering plates shall suitably be stiffened to enable the compaction by form vibrators if used sharp edges of concrete shall be chamfered. The location of jacks for lifting up the superstructure to replace bearings etc is indicated in drawing. This shall be distinctly etched on one cross girders and pier/abutment caps.   

 

 

 

        

 

 

 

 

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