Construction Methodology for Pre-Cast Reinforced Concrete Girder Bridge
SCOPE This methodology relates to the construction of foundations and superstructures for a pre-cast reinforced concrete girder type bridge. The scope of works comprises the construction of two abutments, five piers, two lanes deck slab, approach slabs and grouted riprap slope protection.
GENERAL APPROACH At the initial stage, significant number of manpower, field personnel and sufficient materials and equipment will be deployed at site. Detour roads will be constructed before starting the bridge construction. Detour signs and warning materials and equipment will be provided for the safety purpose of the public motorists, pedestrians as well as construction equipment and workers. Within the right of way limit, the area will be cleared of obstruction and other unnecessary objects. Material designated to be removed shall be disposed of at places designated and or approved by the authority. Facilities which are designated to remain will be protected from damage. The main construction of bridge will follow after benchmarks and centerlines had been established and approved by the consultant’s representative. The consultant‘s representative will be notified with sufficient time in advance before commencing any excavation of structure. Foundation footing will be excavated using suitable excavation equipment and trimmed to the required outlines and elevation. Backfill placed around piers and abutments will be deposited on both sides at approximately same elevation to prevent wedging against the structure. The materials shall be placed in layer and compacted by suitable compaction machine to the required density. After structural excavation and blinding concrete had been undertaken and approved, placing of reinforcing steel in accordance with the required number, spacing and location will be started immediately. Concrete covering will be maintained as per type of structure according to specification. The reinforcing
steel shall be free from dirt, oil and detrimental scales. It shall be accurately placed and secured during placing of concrete. Falseworks shall be assembled and erected on site by competent skilled workers. False work columns shall be supported by wood or metal bases. It shall not be supported on any part of the structure. Forms shall be mortar–tight and rigid to prevent distortion due to concrete pressure and vibration. They will be checked before, during , and after placing of concrete. Concreting of structure will commence after all necessary parameters have been checked and approved by the consultant’s representative. Sequence and method of pouring concrete will be formulated and implemented according to site condition. The forms shall be clean before concrete is placed. During placing of concrete , all necessary care in handling will be taken to avoid segregation of aggregates and displacement of reinforcing bars. External surface of concrete will be worked thoroughly to avoid air pockets and honeycombs. Concrete shall be deposited in horizontal layer in fast and continuous manner. Forms will remain in placed at specified period of time. All top surfaces such as top of retaining wall, curb, abutment, etc., will be treated by tamping and floating with a wooden float. Bridge deck on wearing surface will be floated, straight- edged and belted until finished surface is in conformity with the cross section shown on plans. Concrete will be cured for a period of time required. The method, materials, and equipment for curing is per approval of the consultant’s representative. Additional burlap, straw or other suitable blanketing and housing materials will be provided in case temperature of air drop or rise at critical temperature. Risers will be checked for correct elevation, orientation and location prior to placing of elastomeric bearing pad. Bearing pads will be maintained in correct position during placing of girders. Construction Joints shall be installed in accordance with the manufacturer’s recommendation with suitable materials and tools recommended by them. The position of all bolts cast into concrete and holes drilled in plates shall be accurately determined from templates. Threads will be kept clean free from dusts and relative movement will be prevented during placing and hardening of concrete or mortar under joints. Slope protection will be constructed following the required lines, grades, thickness and cross section. Sub-grade or base will be compacted to the required density after excavation and trimming of bed and footing trench. Placing of stones and concrete will be started after all excavations and footing trenches had been approved by the consultant’s representative. Stones will be placed in a
manner that spaces between them are minimal. Larger stones will be placed in the footing trench and on the outside surface of the slope. For grouted riprap, all spaces between stones will be filled with concrete mortar and rodded into place until the voids are completely filled. Front face of riprap will be spaded and clean before concrete hardened. Curing will follow immediately after completion of grouting process.
REFERENCE DRAWING •
General Plan and Specification
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Bridge Alignment
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Plan and Profile
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Cross Section and Elevation
CONSTRUCTION ACTIVITIES WILL INCLUDE: •
Site establishment including offices and services.
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Ground preparation for the casting yard of pre-cast concrete girder.
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Ground preparation for the excavation of footings foundation.
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Construction of temporary fences and access gates.
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Construction of temporary environmental protection works, including storm water management and truck wheel and concrete mixer washing facilities.
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Construction of temporary structures required for launching of pre-cast girder
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Relocation or construction of protection works of existing services likely to be affected by the launching operations.
CONSTRUCTION SEQUENCE AND METHODS
Upon setting and approval of required elevation, centerlines and limits, the construction of each structure will follow. A. Substructure 1. Foundation Works •
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Excavation using backhoes in supervision of a competent supervisor to monitor the depth and width of excavation to prevent any overshoot in dimensions and to ensure that the footing bed is undisturbed prior to placing of concrete. Trimming to desired width and depth manually using light equipment and tools. Removal of loose materials Disposal of excavated materials to designated and approved area. Placing of lean concrete using approved concrete chutes or by concrete pump. Using lightweight crane and bucket is also an option. Concreting of footing s using concrete pump in fast and steady manner to ensure consistency of concrete poured. Curing using burlap and water or approved curing compound. Application of bituminous paint to footing for waterproofing Backfilling and compaction by layer using portable compaction machine. A field density test will be conducted in every layer and must be subsequently approved.
2. Columns / Pedestal •
Fabrication of column forms corresponding the true diameter of columns based on plan dimension. Column forms will be checked for roundness, and dimensions ( based on column heights) . Dried concrete will be removed from the inside and form oil will be applied prior to installation.
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Installation of reinforcing steel bars based on required spacing, numbers and positions. Erection / aligning of column forms using lightweight crane for lifting. Columns wilLbe checked for plumbness on all sides and fixed securely to avoid any movement during placing of concrete. Concreting by concrete pump. Curing and Removal of forms. Surface finishing
3. Cap Beams / Pier Head / Wingwall / Backwall / Corbel/Risers • • • • • • •
Ground base will be compacted and leveled prior to placing of wooden or metal bases for scaffoldings. Erection of false works by experienced scaffolders ( checking consistently for tower plumbness and rigidity.) Installation of reinforcing steel bars Installation of forms and chamfers Concreting using concrete pumps Curing and removal of forms Surface finishing
B. Superstructure 1. Launching of Pre-Cast Girder • •
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Additional structure / embankment will be needed to ensure flat surface and ground stability for the positioning of cranes. The launching operation will be supplemented by two 90 tons crane or a 150 ton cranes if only one crane will be utilized. An auxiliary crane and suitable transport equipment will also be utilized for transporting of precast girder from casting yard to launching area. Erection / placing of girder to final position will be done by skilled personnel under a qualified supervisor. Concreting of diaphragms.
2. Deck Slab • •
Installation of pouring guide/rails to provide the required crown and grade. Placing of concrete in consistent and steady manner to ensure smooth workmanship.
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The screed will be operated from a position transverse to the bridge roadway centerline. The surface will be floated and straight edged until the finished surface is in conformity with cross section of shown on plans. After the water sheen has disappear visually, the brooming operation shall be drawn transversely across the surface with adjacent stroke overlapping slightly. Approved working platform or bridges not touching the surface will be provided for brooming operation. Curbs, sidewalks, and parapet will be rubbed surface finish.Concrete will be cured for a period of time required to obtain the full specified strength.
C. Slope Protection Excavation and trimming of riprap bed and footing trench • • •
Placing of stones Grouting Surface finishing
SAFETY GUIDELINES The contractor will ensure that activities on site is carried out in safe and healthy manner as much as possible to avoid or minimize the risks of injury to its employees and any other persons who could possibly be affected. •
All site personnel will wear suitable Personal Protective Equipment (PPE) while on site. Minimum Requirements ; 1. Hard Hat 2. High Visibility Vest 3. Safety Shoes Additional Safety Gears : 1. 2. 3. 4. 5.
Harnesses and Lanyards Gloves Safety Goggles Hearing Protection Gas and Dust Masks
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All waste materials, hazardous materials and rubbish will be disposed of immediately in accordance with environmental requirements of the surrounding municipality.
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Fire precautions will be implemented strictly. Open fire will not be permitted on site and an adequate supply of water for fire fighting purposes and suitable fire extinguishers will be provided and maintained at all times. Escape route sign will be posted on every possible exits.
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All visitors and/or delivery drivers will be required to report on site office upon arrival. Delivery drivers must remain inside or immediately near their vehicles.
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Only trained and authorized persons are allowed to operate plant and machineries. All plant/machinery will be switch off when not in use. Electrical safety guidelines and warnings will be posted near electrical equipment and machineries.
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No Smoking areas will be implemented and strict compliance will be enforced. Alcohols and other prohibited substances will be immediately reported to law enforcing authority.
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Entering confined spaces will not be permitted unless a permit to enter has been approved. Only authorized persons with suitable PPE’s may enter. Proper ventilating equipment and /or suitable blower will be installed before allowing any persons to enter.
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Excavations will be immediately backfilled after completion of works . Effective barriers and edge protection will be installed around excavation pit to keep unauthorized persons away from the perimeter. Proper shoring and supports will be installed around deep excavations.
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Safety nets will be provided for overhead construction and guard rails will be installed on all scaffold platforms. Scaffolding works will be prohibited during strong winds.
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Construction equipment especially cranes and associated rigging equipment must be inspected regularly to identify any existing potentially unsafe conditions. Location and voltage of existing overhead and underground power lines and utilities must be identified to determine
precautions. Supporting surface must be level as much as possible and firm.
