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METHODOLOGY FOR CASTING OF CAST IN PLACE GIRDER

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METHODOLOGY FOR CASTING OF CAST IN PLACE GIRDER

1. INTRODUCTION

At intersections or rotaries of the bridge structure, encountering mixed traffic, or curved sections

of bridges subjected to lateral stresses, use of Precast Prestressed (PSC) Girders is substituted with

Cast in Place (CIP) Girders, due to difficulty in prestressing the curved precast concrete member.

Cast-in-situ method of construction of bridges is a flexible method and demands of more unusual

geometrical shapes can be easily coped with the same. As the structural elements are casted

together and intersected to give better finish and appearance. Keeping in view these limitation, the

best option is to go for cast in site concrete at such places.

Moreover the cast in-situ girders can perform other useful structural functions. It can provide

continuity in adjacent span over the supports, have fewer joints and less maintenance cost. In

addition monolithic construction provides lateral stability to the structure. At intersection, lateral

loads act from different direction, thus they are more resistant to lateral and seismic forces.

Construction of cast in place girders comprises of the following activities:

Erection of Falsework

Fixing of Formwork

Fixing of Steel Reinforcement

Placing of Concrete

Finishing and Curing of Concrete

2. ERECTION OF FALSEWORK

Falsework includes the foundations, footings and all structural members used to support a

permanent structure, material, equipment and manpower that may be applied during construction.

Depending upon the type of structure and construction procedure, movable or stationary system of

falsework can be used.

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I. STATIONARY FALSEWORK

The stationary system of falsework shall be used for construction of cast in place girders. This

system can accommodate any deck curvature and varying cross-sectional shapes, including box

section, slabs, and beams. It can be used for any type of concrete and fits well with complicated

configurations. This technique requires full occupancy of the ground and provides stability by

transmitting all loads to the ground and allows easy erection & dismantling. The ground should be

able to sustain loads transmitted and free of any obstacle.

II. COMPACTED GROUND AND EARTH LEVELING

Stationary falsework shall be erected on a leveled surface to prevent the risk of collapse and should

be able to safely support loads that may be applied during execution of work. In order to obtain

leveled ground to withstand all the loads, the surface shall be leveled properly using survey

techniques and the ground be compacted to the appropriate degree of compaction.

III. FOUNDATIONS AND FOOTINGS

Falsework frames and props shall be located on a firm base that will not fail or get washed away.

Foundation and Footing shall be provided under column or vertical components of falsework

frames. Keeping in view the site conditions, lean concrete shall also be provided on natural ground

under the props or scaffolding. Minimum thickness of lean is 100 mm in order to obtain a smooth

working platform to withstand all the loads.

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IV. FALSEWORK ASSEMBLAGE

The erection and dismantling of falsework, shall comply with the standards and shall be tied

systematically and progressively to stabilize the structure and to ensure the health & safety of

workers and the general public. With increasing height of the falsework additional measure shall

be taken with regard to the lateral stability.

Necessary edge protection shall also be provided to avoid any accident. Falsework system

consisting of scaffolding pipe shall be fixed at a distance of 50 X 75 cm with additional diagonal

bracing, and frame system consisting columns, beams and purlins.

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3. FIXING OF FORMWORK

Formwork used to mold concrete into a desired shape, it includes wooden batten/planks, steel

plates and other structural members. Formwork shall be rigid and watertight, constructed and

braced so as to maintain forms in position during all construction activities and to remove easily,

without damaging the formed concrete. In formwork construction process, careful attention shall

be paid on following essential elements:

Construction Details

a) Exposed surface forms.

b) Stay-in-place or lost-deck forming.

Structural Adequacy

Ensuring the adequacy of formwork supporting elements against deflection and settlement.

Alignment and Grading.

Maintaining the horizontal/vertical curvature, lines and levels.

Cast in place box girders can be constructed in stages, beginning with the bottom slab, webs and

finally the top slab. Initially, formwork for bottom slab of the girder shall be fixed and constructed,

afterwards reinforcing and formwork shall be tied for girder webs and casted, lastly the supports

for top slab forming shall be placed in the portion where webs are completed.

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I. BOTTOM SLAB FORMWORK

Once the falsework has been erected wooden battens shall be placed at specified spacing to hold

formwork for bottom slab and cushion for the removal at later stage. Steel plates shall be provided

over the wooden battens to maintain a stable base and leveled surface using wedges for bottom

slab of cast in place girders. Joints in between the plates shall be closed properly.

II. FIXING OF ACCESS DOOR

Access to internal cells is usually necessary through diaphragms or manholes for future

maintenance or inspection. The access doors shall be fixed in the bottom slab which also provides

a convenient way for removal of internal formwork.

III. GIRDER WEBS FORMWORK

After constructing the bottom slab, steel formwork for outer and inner webs (fabricated as per

required size and shape) shall be fixed along the span maintaining the profile and grades.

IV. TOP SLAB FORMWORK

The top slab formwork shall be provided for the portions where internal and outer webs have been

constructed. Forms for top slab includes the wooden or precast planks supported on wooden batten

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with vertical posts, joists and brackets. Purpose-made/permanent deck forms may remain in place

provided they have been accounted for in the design.

Accuracy to line, level and thickness is essential to ensure the correct final grade/slope and

alignment of the girders. The formwork shall be oiled properly so that it can be removed easily

after concreting is being done, it also helps to obtain a smooth Surface of Concrete.

4. REBAR FIXING FOR GIRDER

Reinforcing steel shall be placed in accordance with the drawings and specifications coinciding

with the casting of the cross sectional components i.e., bottom slab, webs, and top slab. Detailing

or fabrication of reinforcing steel shall be carried out, keeping in view the following characteristics

of cast in place bridges to meet structural requirements and also facilitate forming and casting:

• Varying girder spacing.

• Varying deck thickness.

• Large/Varying skews.

• Wide, curved bridges with small curvature radius.

• Future widening.

All reinforcing shall be securely tied and blocked up to prevent any movement during placement

of concrete and by maintaining the required lap splice length and staggering the splices in adjacent

bars. Precast mortar blocks shall be used to assure the required concrete cover on rebar.

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5. PLACING OF CONCRETE

Cast in place box girder can be casted in two or three stages. When casted in two stages, the bottom

slab and webs shall be poured at the same time, followed by the casting of the top slab. In three-

stage casting the bottom slab, webs, and top slab are poured separately, with enough time between

stages to permit sufficient concrete hardening.

I. THREE STAGE CASTING

First stage of CIP box girder construction is the casting of the bottom slab concrete. Placing of

concrete for bottom slab shall commenced from one side (usually the low side of grade and super

elevation) and continues across the width of the slab, along the length of the girder as necessary.

Concrete shall be consolidated using vibratory tools and then struck off to required elevation by

hand, followed by a float finish.

When bottom slab concrete has set and hardened sufficiently, the concrete for girder’s webs shall

be placed and consolidated. Web concrete shall be placed in lifts to control pressure on forms and

consolidated by means of internal poker-type vibrators and/or external vibrators.

Top slab of box girder shall be casted in last. Concrete for top slab shall be placed working across

and longitudinally up hill. Concrete shall be consolidated, struck off to levels with screeds and

floated to a final finish.

The preceding paragraphs describe pouring the superstructure in three phases. Two-phased

construction, in which bottom slab and webs are cast together, followed by the casting of the top

slab, is also commonly used.

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II. CONSTRUCTION JOINT

Construction joint occurred in between new segment’s fresh concrete and the hardened concrete

of a previously produced segment. Longitudinal construction joints shall be located in the webs a

few inches above the top of the bottom slab (three-stage casting) and a few inches above the top

of the webs in the top slab fillets (two-stage and three-stage casting).

Transverse construction joints shall be needed at various locations in a span in order to keep the

total volume of concrete placed within a given work period. Transverse construction joints shall

be provided at distance of Span/4 from the support.

In order to ensure proper structural integrity and function, joints shall be prepared, cleaned and

roughened prior to the next pour.

III. CONSOLIDATION OF CONCRETE

Concrete shall be consolidated by means of high frequency internal and/or external vibrators

within 15 minutes after it is deposited in the forms. Consolidation shall be done in such a way that

do not cause segregation of the aggregate and produce dense, homogeneous concrete without voids

or rock pockets.

In webs or deep sections it shall be pushed into the concrete approximately every 2 ft ensuring the

head of the vibrator entered almost vertically. In thin deck sections vibrator shall also hit the

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concrete approximately every 2 ft and it shall not be dragged horizontally over the top of the

concrete surface, nor be allowed to run continuously.

6. FINISHING AND CURING OF CONCRETE

i. REMOVAL OF FORMWORK

After the concrete has reached specified strength, the side forms shall be removed but bottom

formwork removal shall be delayed until the concrete has attained required strength (up to 28 days)

to avoid any damage.

ii. FINISHING OF CONCRETE SURFACE

Concrete surface that will be visible, such as edge of the girder, soffits of slab, and deck soffits for

box girders often needs finishing. It shall be finished by striking off and screeding to proper

contour and elevation. Concrete surface shall meet specified requirements for longitudinal and

transverse smoothness.

iii. CURING OF CONCRETE

Proper curing of concrete deck surfaces is one of the key elements for strength development and

control cracking. Curing procedures depend on the concrete mix and the environmental conditions

after casting. Concrete. Concrete segments shall be covered with Hazen clothes and shall be kept

continuously wet by applying water for a curing period of at least seven days after the concrete is

placed to maintain a controlled temperature and humidity.