10-SUPER STRUCTURES



SUPER STRUCTURES (BRIDGES)

GENERAL

This part consists of construction of superstructures for bridges and culverts in accordance with the details shown in the Drawings and conforming to the specifications or as directed by the Engineer.

  1. A method statement for construction shall be submitted by the Contractor for the approval of the Engineer well in advance of the commencement of the superstructure work.
  2. The statement shall include:
  1. Sources of materials.
  2. Design, erection and removal of form work.
  3. Production, transportation, laying and curing of concrete.
  4. Launching procedures in case of precast girders/segmental construction.
  5. Personnel employed for execution and supervision.
  6.  Tests and sampling procedures.
  7. Equipment details.
  8.  Quality management system to be adopted as per this Manual.
  9. Safety measures.
  10. Any other relevant information.

MATERIALS

The basic materials shall comply with respective specifications.

REQUIREMENTS/WORKMANSHIP

1.  Cast-in-situ decks

  1. Dimensions, lines and levels shall be set out and checked with respect to permanent reference lines and permanent bench marks so that the completed superstructure is in full and in accordance with the Drawings and as approved by the Engineer.
  2. The formwork, steel reinforcement, structural concrete and pre-stressing for concrete superstructure shall conform to specifications or as directed by the Engineer.

    1.1 Solid slabs

  1. Solid slabs may be constructed with reinforced concrete or prestressed concrete and the cross section of a typical solid slab is shown in figure.
  1. Where adjacent span of slab has already been cast, the expansion joint and filler board shall be placed abutting the already cast span, which shall form the shutter on that side of the new span to be cast.
  2. The reinforcement for the road kerb and railings embedded in the slab shall be tied in position before casting of slab. The entire slab shall be cast in one go. Where the slab is continuous over two spans or more, the entire span of the first slab and the length of the slab in the next adjacent span up to the point of contra-flexure, shall be cast in one go, the same sequence of concreting being repeated for additional spans as required.
  3. Normally no construction joint shall be allowed in deck slabs. In very wide slabs, however, longitudinal construction joints may be permitted with the approval of the Engineer.
  4. Constructions joints, if provided, shall be made in the prescribed manner as per specifications in the codes.
  5. The portions of solid slab near expansion joints shall be cast along with reinforcements and embedment for expansion joints. For this purpose, the portion of solid slab near expansion joints may be cast in a subsequent stage, if permitted by the Engineer or as indicated in the Drawing.
  6. Where wearing coat is required to be provided after the slab has been cast, the surface of the slab shall be finished rough, but true to lines and levels as shown in the Drawing, before the concrete has hardened.
  7. The top of the slab shall be covered with clean moist sand as soon as the surface has hardened. Curing shall be carried out as per specifications.
  8. If bearings are provided for the solid slab, the same shall be placed in position in accordance with the Drawing, before casting of slab.

1.2 Voided slabs

  1. Voided slabs can be either in reinforced concrete or in pre-stressed concrete. Typical cross section of voided slab is shown in figure
  2. Voids can be either circular or rectangular in shape. Void formers may be manufactured from steel sheets, fibre-reinforced cement, expanded polystyrene, HDPE, etc. They are generally corrugated to attain sufficient rigidity in order to prevent distortion or collapse during concreting. They should also be leak proof.
  3. Void formers shall be suitably tied down in order to prevent flotation during concreting. Care shall be taken during placement of concrete to ensure that the concrete flows fully into the space beneath the void formers.

1.3 Box girders

  1. Box girder may be constructed with either reinforced concrete or prestressed concrete. 
  2. Box girders may be simply supported or continuous. Simply supported box girders shall have the minimum construction joints as approved by the Engineer.
  3. In the case of continuous box girders, the sequence of construction and location of construction joints shall be strictly in accordance with the Drawing.
  4. The box section shall be constructed with only one construction joint located in the web below the fillet between the deck slab and the web.
  1. The portions of deck slab near expansion joints shall be cast along with reinforcements and embedment for expansion joints. For this purpose, the portion of deck slab near expansion joints may be cast in a subsequent stage, if permitted by the Engineer.
  2. The surface of the deck slab shall be finished rough but true to lines and levels as shown in the Drawing before the concrete has hardened. Care shall be taken for setting of bearings as indicated in the Drawing.

1.4  RCC T-beam and slab

  1. Provision of construction joint shall conform to the Drawings or as per directions of the Engineer. No construction joint shall be provided between the bottom flange/bulb and the web. If not indicated in the Drawing, construction joint may be provided at the junction of the web and the fillet between the web and the deck slab, with the approval of the Engineer.
  2. The portions of deck slab near expansion joints shall be cast along with reinforcements and embedment for expansion joints. For this purpose, the portion of deck slab near expansion joints may be cast in a subsequent stage, if permitted by the Engineer or as indicated in the Drawing.
  3. The surface of the deck slab shall be finished rough but true to lines and levels as shown in the drawings before the concrete has hardened. Care shall be taken for setting of bearings as indicated in the Drawing.

1.5  PSC/RCC/Steel girder and composite RCC slab

  1. PSC/RCC girder may be precast or cast in-situ. Girders may be post tensioned or pre-tensioned. Steel girders may also be adopted instead of concrete girders. Where precast construction is required to be adopted, selection of casting yard and details of methodology and equipment for shifting and launching of girders shall be included in the method statement.

In case of cast in-situ construction, the sequence of construction including side shifting of girders, if required, and placing on bearings shall be in accordance with the Drawing.

  1. The PSC/RCC girder constituting the top flange, web and bottom flange shall be concreted in a single operation without any construction joint.
  2. The portions of deck slab near expansion joints shall be cast along with reinforcements and embedment for expansion joints. For this purpose, the portion of deck slab near expansion joints may be cast in a subsequent stage, if permitted by the Engineer or as indicated in the Drawing.
  3. The surface of the deck slab shall be finished rough but true to lines and levels as shown in the Drawings before the concrete has hardened.
  4. Care shall be taken for correct alignment and setting of bearings as indicated in the Drawing.

1.6 PSC/RCC Bow string girder

  1. Bow string girders are cast in-situ type superstructure as shown in a below figure.
  2. Bottom string (tie) beam may be post tensioned or RCC and top arch rib is RCC.
  3. High strength steel hangers may also be adopted instead of traditionally used concrete hangers.

1.7 Tolerances

Table: Permissible tolerance for cast in-situ superstructures

Sl.

No.

Item

Permissible limits

1

Variations in thickness of top and bottom slab for box girders, top and  bottom flange for T-girders and slabs.

-5 mm/+10 mm.

2

Variations in web thickness.

-5 mm/+10 mm.

3

Variations in overall depth or width.

±5 mm.

4

Variation in length overall and  length between bearings.

±10 mm or ±0.1% of

the span length,

whichever is less.

5

Surface unevenness in deck slab   measured with 3 m straight edge.

5 mm.

2.  Pre-cast girders

  1. All forms shall be of steel and of sufficient thickness, with adequate external bracing, stiffened and adequately anchored to withstand the forces due to placement and vibration of concrete.
  2. Compaction of concrete may be achieved through needle vibrators or form (shutter) vibrators along with needle vibrators.
  3. For casting of precast components, any of the two commonly used techniques of precasting viz. Long Line method or Short Bench method may be used. After the first segment of each unit is cast, succeeding segments shall be match cast against the previous ones and shall be given a unique identification mark so that it is placed at the intended locations in the superstructure.
  4. A bond breaking material such as flax, soap, talc, wax or any other approved material shall be used between previously cast segment and newly cast segments, as well as the end headers, where ever required.
  5. Segments shall not be moved from the casting yard until stipulated strength requirements have been met. They shall be supported in a manner that will minimise warping.
  6. Under all circumstances, the concrete shall have attained a minimum compressive strength of 20 MPa at the time of removal of forms. At the time of lifting and assembly of precast segments, the concrete shall have attained sufficient strength to withstand the handling stresses.
  7. Curing of segments may be achieved through water curing or steam curing followed by water curing. Approved curing compound may also be used.
  8. In case of spliced girder system, match casting is not necessary because the gap between the girder segments is filled with concrete or epoxy material at the location of splices.
  9. The faces which are required to receive the cast-in-situ stitch concrete shall be adequately roughened and prepared as construction joint before pouring the stitch concrete. In case of epoxy jointed spliced girder system (with no gap between the girder segments), match casting shall be resorted to and all provisions of epoxy jointed segmental structure shall apply.
  10. A full scale mock-up of the lifting and holding equipment (including assembly truss, cantilevering formwork etc.) shall be performed to demonstrate their adequacy and efficacy prior to start of erection/assembly of the segments.

2.1 Tolerances

The permissible tolerances for precast girders shall be as given in table.

Sl.

No.

Item

Permissible limits

 

1

Variations in thickness of top and   bottom slab for box girders, top and bottom flange for T- girders and slabs.

 

±5 mm.

2

Variations in web thickness.

-5 mm/+10 mm.

3

Variations in overall depth or width.

±5 mm.

4

Variation in overall length and length

between bearings.

±10 mm or ±0.1% of the span

length, whichever is less.

5

Surface unevenness in deck slab

measured with 3 m straight edge.

5 mm.

 3.  LOAD TEST ON SUPERSTRUCTURE

  1. The materials and work shall be tested in accordance with the relevant sections of this Manual and shall meet the prescribed standards of acceptance.
  2. When a newly constructed bridge is to be accepted, where the design is of an unusual type or if the quality of construction is suspected to be not as specified, then a full-fledged testing of the bridge has to be resorted to, as an assurance test, by the application of loads atleast equivalent to the design loads.
  3. Such load testing shall also to be employed if it is specified in Contract conditions.
  4. The load testing of bridges shall be carried out as per IRC: SP: 51.

RELATED CODES

1.IRC: 112:-Code of practice for concrete road bridges.

2.IRC: SP:51:-Guidelines for load testing of bridges.

3.MoRTH:-Specifications for road and bridge works (5th revision).

*****