Post Tension Bridge Construction Formwork

Post - Tension Bridge Construction Formwork

Overview

Post - tension bridge construction formwork is a crucial component in the building of bridges. It provides the necessary shape and support for the concrete during the casting process, especially for post - tensioned concrete bridge structures. This type of formwork is designed to withstand the pressure exerted by the fresh concrete and to ensure that the concrete is cast to the precise dimensions and shape required for the bridge's design.

Components of the Formwork

Deck Formwork: The deck formwork is used to shape the bridge deck. It usually consists of plywood sheets or steel plates supported by a framework of joists and beams. The framework provides the necessary rigidity to support the weight of the concrete and any live loads during construction. For example, in a box - girder bridge, the deck formwork is carefully designed to create the flat surface of the deck and the correct camber (a slight upward curvature) to allow for drainage and load - distribution.

Side Formwork: Side formwork is used to define the edges of the bridge elements such as girders or the bridge deck. It is typically made of plywood or metal panels and is held in place by ties and braces. These braces are adjusted to ensure that the side formwork is vertical and in the correct position. In a post - tensioned I - girder bridge, the side formwork helps to shape the flanges and the web of the girders precisely.

Internal Formwork: In the case of box - girder bridges or other structures with hollow sections, internal formwork is required. This formwork can be made of inflatable rubber forms, removable plastic or plywood boxes. The internal formwork is used to create the voids within the concrete structure, which can reduce the weight of the bridge while maintaining its strength. For example, an inflatable internal formwork can be inserted into the wet concrete of a box - girder, and once the concrete has set to a certain strength, the form can be deflated and removed.

Design Considerations

Load - Bearing Capacity: The formwork must be designed to bear the weight of the fresh concrete, the self - weight of the formwork itself, and any additional construction loads such as workers and equipment. Engineers calculate the maximum load that the formwork will experience based on the volume and density of the concrete to be poured and the span of the formwork. For post - tension bridges, the formwork also needs to consider the forces that will be exerted during the post - tensioning process.

Dimensional Accuracy: High precision in the formwork's dimensions is essential. The formwork must conform to the exact geometric requirements of the bridge design. Even a small deviation in the formwork's shape can lead to problems in the final structure, such as incorrect alignment of post - tensioning ducts or uneven distribution of loads. Tolerances are carefully specified and monitored during the construction process to ensure that the formwork meets the design standards.

Ease of Assembly and Disassembly: The formwork should be designed for easy and efficient assembly and disassembly. This is important for re - use of the formwork in other parts of the project or for future projects. Modular formwork systems are often used, where the components can be quickly connected and disconnected using bolts, clamps, or other fastening devices. For example, a pre - fabricated steel formwork system can be assembled on - site with minimal effort and time.

Construction Process and the Role of Formwork

Installation: The formwork is first assembled on - site according to the design specifications. The base of the formwork is leveled and anchored to the supporting structure, such as the bridge piers or abutments. The different components of the formwork (deck, side, and internal) are then installed and aligned carefully. Once the formwork is in place, the post - tensioning ducts are usually installed within the formwork. These ducts are the channels through which the post - tensioning cables will be inserted later.

Concrete Pouring: After the formwork and ducts are in place, the concrete is poured into the formwork. The formwork provides the containment for the concrete, ensuring that it takes the desired shape. During the pouring process, the formwork must be monitored to prevent any deformation or displacement. Vibrators are often used to ensure the proper compaction of the concrete within the formwork.

Post - Tensioning: After the concrete has reached a certain strength (usually determined by testing), the post - tensioning process begins. The post - tensioning cables are inserted through the ducts and tensioned using hydraulic jacks. The formwork plays a role in this process by providing a stable reference frame for the tensioning equipment and ensuring that the forces exerted during tensioning are properly distributed through the concrete structure.

Formwork Removal: Once the post - tensioning is completed and the concrete has reached the required design strength, the formwork can be removed. The order of removal is carefully planned to avoid any damage to the newly - formed concrete structure. The formwork components are then cleaned, inspected for damage, and stored for future use if possible.