Precast concrete girder beams are often used to form the main structural elements of the bridge decks. It can range from 12m to 45m . When it is too long, the girder beams are heavy, then transportation, lifting to place and launching will become problems. This 40m beam weighs about 70 tonnes and is about 2m high.
How to construct the beam then ?
(1) preparation of steel bed and steel form-work.
Make sure the bed is level. If the middle section sinks down, then the shape will be sagging which is not acceptable in term of appearance and design. Sometimes, piling is required if the soil beneath is poor. (Never provide piling on the two ends only, as the middle will definitely sink after a while and this will require adjustment and much works.) Then set up one side of the form-work to guide the fabrication of steel reinforcements.
2. Assemble and fabricate steel reinforcements which have been cut and bent to shapes in the workshop, to fit into the steel form-work. Make sure that the minimum cover of 30-40mm is observed to ensure durability and no scaly rust to the steel.
3. Install the pre-stressing ducts onto to the reinforcement cage. The positions of the ducts have to be checked and firmly tied to the reinforcements so that they will not move during concreting. The ducts have to be at centre to ensure no lateral bowing during pre-stressing. Check to ensure there is no puncture of the ducts and the joints are done properly so that no cement mix will leak into the ducts (If the cement leaks inside, pre-stressing cannot be done as intended, the beams may have to be abandoned!)
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