Introduction of High Speed Railway Box Girder Formwork

High - speed railway box girder formwork is a set of molds used in the construction of box girders for high - speed railway bridges. Its primary purpose is to shape the fresh concrete into the desired box - like cross - sectional geometry of the girder. The formwork provides the necessary support and confinement for the concrete during the casting process, ensuring that the box girder takes on the correct dimensions, shape, and surface finish.

For example, it helps to form the top flange, bottom flange, and two webs of the box girder precisely. The accurate shaping of these components is crucial as any deviation in the form can affect the structural performance of the box girder and ultimately the safety and smooth running of high - speed trains.

Fixed Formwork: This type of formwork is stationary and is often used for box girders with a relatively simple and repetitive cross - sectional shape. It is usually made of steel or timber and is assembled and bolted together on - site. The fixed formwork provides a stable and reliable mold for the concrete casting process.

Movable Formwork: Movable formwork, also known as slip - form or traveling formwork, is designed to move along the length of the box girder during the casting process. It is particularly useful for long - span box girders. The movable formwork allows for continuous casting, which can improve construction efficiency. It is usually equipped with a system of rails or tracks to ensure smooth movement and accurate positioning.

Pre - fabricated Formwork: Pre - fabricated formwork is manufactured off - site in a factory environment. These formwork units are usually made of high - quality materials and have a high degree of precision. They can be quickly assembled on - site, reducing the construction time and labor cost. The pre - fabricated formwork also offers better quality control as it is produced under factory conditions.

Side Forms: The side forms are responsible for shaping the two side webs of the box girder. They are usually made of steel plates with sufficient thickness to withstand the lateral pressure of the fresh concrete. The side forms are designed to be adjustable to some extent to accommodate different web thicknesses and heights.

Top and Bottom Forms: The top form shapes the top flange of the box girder, and the bottom form shapes the bottom flange. The top form often has openings or inserts to allow for the installation of reinforcing bars and prestressing ducts. The bottom form needs to have a good bearing capacity to support the weight of the concrete and the construction loads.

Bracing and Support Systems: To ensure the stability of the formwork during the casting process, a bracing and support system is essential. This includes vertical and horizontal braces, as well as support legs or frames. The bracing system helps to resist the internal and external forces acting on the formwork, such as the pressure of the concrete and the wind load.

Steel: Steel is a commonly used material for formwork due to its high strength, durability, and reusability. Steel formwork can withstand high concrete pressures and can be precisely fabricated to the required dimensions. It is also resistant to deformation, which is crucial for ensuring the accuracy of the box girder shape.

Timber: Timber formwork is sometimes used, especially for small - scale or less - critical parts of the box girder construction. Timber is easy to work with and can be cut and shaped to fit different geometries. However, it has a lower strength compared to steel and may have a limited number of reuses.

Plastic and Composite Materials: In some cases, plastic or composite materials are used for formwork. These materials can provide a smooth surface finish for the concrete and are lightweight, making them easier to handle and install. They are also resistant to corrosion and chemical attack, which can be beneficial in certain construction environments.

Concrete Pressure Calculation: The formwork must be designed to withstand the lateral pressure exerted by the fresh concrete during casting. The pressure depends on factors such as the height of the concrete pour, the slump of the concrete, and the rate of pour. Accurate calculation of the concrete pressure is essential to ensure the safety and stability of the formwork.

Tolerance and Precision: High - speed railway box girder construction requires a high level of precision. The formwork design must take into account the allowable tolerances for the dimensions and shape of the box girder. Tight tolerances are necessary to ensure the smoothness of the railway track and the proper connection of adjacent box girders.

Erection and Demolition: The formwork should be designed with easy erection and demolition in mind. The design should consider the sequence of construction operations and the use of appropriate lifting and handling equipment. After the concrete has cured to the required strength, the formwork needs to be removed without causing damage to the newly - cast box girder.