Bridge construction has evolved significantly over the years, and one of the most versatile methods currently in use is the cast-in-situ technique. This method provides flexibility in constructing bridges of unusual geometrical shapes and is especially useful when transporting pre-fabricated parts is challenging due to size or accessibility. In this article, we will explore the various cast-in-situ methods of bridge construction, their advantages and disadvantages, and the ideal situations where they should be applied.
Overview of Cast-in-Situ Bridge Construction Methods
The cast-in-situ method involves the construction of a bridge directly on-site, where the bridge superstructure and foundation are cast in place rather than being pre-fabricated and transported to the site. This method is particularly advantageous for projects in remote or inaccessible locations and when the bridge design requires unusual geometries. There are three primary techniques that fall under the cast-in-situ method, each suited to different project needs:
- Incremental Launching Method
- Balanced Cantilever Method
- Cast-in-Situ Post Tensioned Method
Let’s dive into each of these methods in more detail.
Incremental Launching Method
The Incremental Launching Method is a highly mechanized approach used in the construction of multi-span post-tensioned bridges. This method is time-efficient and cost-effective, making it a preferred option for many modern bridge projects.
Process:
- First, the bridge piers are constructed in alignment, creating the necessary support structure.
- The superstructure of the bridge is divided into sections, and one section is cast at a time.
- A casting bed is set up behind one of the abutments, where the first section is cast in place.
- Once the first section hardens, it is pushed along the bridge axis using a jack system or friction launching system.
- The process is repeated with each section being cast and pushed forward.
Advantages:
- The use of a launching nose—a steel truss attached to the front of the bridge deck—helps reduce bending moments as the structure moves forward.
- The bridge alignment must be straight or feature constant curvature for best results.
- The construction takes place on-site, reducing transportation costs and risks, as there is no need to move large prefabricated pieces.
- High-quality finishes are possible because the work is done on a stable, well-prepared casting platform.
Balanced Cantilever Method
The Balanced Cantilever Method is an advanced bridge construction technique that does not require any temporary support structures or falsework, making it highly effective for building bridges of great height and irregular span lengths.
Process:
- The construction starts by building the piers that will serve as the anchor points for the cantilevered sections.
- The cantilever parts of the bridge are constructed symmetrically from the fixed piers, with sections being cast in place and extended outward on both sides.
- As the cantilever sections grow, they balance each other, ensuring the structure remains stable without the need for temporary scaffolding or falsework.
Advantages:
- This method is ideal for bridges with varying span lengths and depths, as it allows flexibility in how the structure is built.
- No temporary support structures are required, which can save time and costs.
- It is suitable for taller bridges or those built over challenging terrain, such as rivers or valleys.
Cast-in-Situ Post Tensioned Method
The Cast-in-Situ Post Tensioned Method is one of the most demanding techniques, but it offers exceptional durability and flexibility, especially for bridges with complex curves or long spans. This method combines concrete, reinforcement, and post-tensioned steel tendons to achieve greater load-carrying capacity.
Process:
- Tendons, which are steel strands or cables, are arranged within the reinforcement framework before concrete is poured.
- After the concrete hardens, hydraulic jacks are used to pull the tendons, applying a predetermined tensile force to the structure.
- The tendons are then locked in place, and the post-tensioning process increases the bridge’s load-bearing capacity and enhances its durability.
Advantages:
- This method eliminates the need for intermediate supports, making it ideal for long-span bridges.
- It also improves the durability of the bridge and allows for better performance under heavy loads.
- It is particularly well-suited for complex geometries, including bridges with curves and varying cross-sections.
Advantages of Cast-in-Situ Methods
There are several compelling reasons why the cast-in-situ construction method is preferred for certain bridge projects:
- No Need for Heavy Transport or Lifting Equipment: Since the bridge superstructure is cast directly on-site, there is no need for transporting heavy prefabricated parts, reducing the cost and complexity of transportation.
- Safety from Transport and Lifting Damages: The superstructure is built in place, eliminating the risk of damage that often occurs during the transportation and lifting of prefabricated components.
- Ideal for Inaccessible Areas: Cast-in-situ methods are perfect for constructing bridges in remote or difficult-to-reach areas where it would be impractical or impossible to transport large prefabricated pieces.
- Cost-Effective: These methods can be more affordable for building solid, ribbed, or voided reinforced concrete slab bridges.
Disadvantages of Cast-in-Situ Methods
While there are many advantages, there are also some drawbacks to the cast-in-situ approach:
- Time-Consuming: Cast-in-situ methods typically take longer to complete compared to precast construction methods, as they require on-site casting, curing, and assembly.
- Vulnerability to Weather Conditions: When constructing bridges over water bodies or in areas with unpredictable weather, the unset concrete can be damaged by floods or adverse weather conditions.
- Labor-Intensive: This method requires a large number of skilled workers and careful supervision throughout the construction process to ensure the quality and safety of the structure.
Conclusion
The cast-in-situ method of bridge construction is a flexible and cost-effective approach, especially when constructing bridges in challenging locations or with unusual geometrical designs. Whether utilizing the Incremental Launching Method, the Balanced Cantilever Method, or the Cast-in-Situ Post Tensioned Method, this technique offers significant benefits, including reduced transportation costs, improved durability, and the ability to construct complex structures on-site. However, it does come with challenges such as longer construction times and dependency on favorable weather conditions. Overall, cast-in-situ methods are an excellent choice for bridges that require precision, adaptability, and robustness in their design and construction.