Bridge decks are constructed through a multi-stage process involving careful preparation, reinforcement, material placement, and crucial integration with the bridge's superstructure to ensure structural integrity and durability. A key aspect, especially for steel superstructure bridges, is creating a composite structure.
Understanding Bridge Deck Construction
The bridge deck is the primary load-bearing surface of a bridge, directly supporting traffic and transferring those loads to the bridge's girders, trusses, or arches. Its construction requires precision to meet stringent safety, durability, and service life requirements.
Key Stages in Bridge Deck Construction
The construction of a bridge deck, most commonly using concrete, follows several distinct phases:
1. Preparing the Substructure and Formwork
Before the deck material can be placed, a robust support system must be in place.
- Falsework and Shoring: Temporary structures, known as falsework or shoring, are erected to support the formwork until the deck material (e.g., concrete) has gained sufficient strength.
- Formwork Installation: This involves setting up the molds, or forms, into which the concrete will be poured. Forms can be made from timber, steel, or specialized plastics. Advanced methods include:
- Slip Forms: Continuously moving forms for faster construction of uniform sections.
- Stay-in-Place Forms: Permanent forms that remain part of the structure, often made of corrugated steel.
2. Reinforcement Installation
For concrete decks, reinforcement is critical for tensile strength and crack control.
- Rebar Placement: Steel reinforcing bars (rebar) are precisely placed within the formwork according to engineering designs. This includes layers of rebar grids, often with two layers for top and bottom reinforcement, and sometimes additional bars for specific load points or edges.
- Spacing and Supports: Rebar is held in place by chairs and ties to ensure correct spacing and concrete cover, which prevents corrosion and ensures proper load transfer.
3. Integrating with the Superstructure: Composite Action
A crucial step for many modern bridges, particularly those with steel girders, is ensuring that the deck works synergistically with the main bridge structure.
- Composite Design: In most applications, the bridge deck is made composite with the steel superstructure through positive attachment to the girders. This composite action means the deck and girders act as a single, unified structural unit, significantly enhancing the bridge's stiffness and strength.
- Shear Connectors: This positive attachment is commonly achieved by using shear connecters to attach the concrete deck slabs to steel girders. These small metal studs are welded to the top flange of the steel girders, and when concrete is poured around them, they create a strong mechanical bond, preventing slippage between the deck and the girders. This allows the concrete deck to resist compressive forces while the steel girders handle tensile forces, leading to a highly efficient structural system.
4. Concrete Placement and Finishing
Once the formwork and reinforcement are ready, the concrete is poured.
- Pouring: Concrete is typically delivered by trucks and placed using pumps or cranes with buckets. Careful planning is essential for large deck pours to ensure continuous placement and uniform strength.
- Vibration: The concrete is vibrated (using internal or external vibrators) to remove air voids, ensure full consolidation around the rebar, and achieve maximum density and strength.
- Finishing: The surface is then screeded to achieve the desired profile, followed by floating and troweling for a smooth finish. Often, a texture (like brooming or grooving) is applied to enhance skid resistance and drainage.
5. Curing and Post-Construction Procedures
Proper curing is vital for concrete to achieve its designed strength and durability.
- Curing Methods: This involves maintaining adequate moisture and temperature for a specified period, typically through methods like:
- Wet Curing: Spraying water, covering with wet burlap, or ponding.
- Curing Compounds: Applying a liquid membrane that seals in moisture.
- Thermal Curing: For cold weather, using blankets or heaters.
- Joint Sealing: Expansion and contraction joints within the deck are sealed with flexible materials to accommodate thermal movements and prevent water intrusion, which can cause damage.
- Barrier and Railing Installation: Once the deck concrete has cured, safety barriers, railings, and drainage systems are installed before the bridge is opened to traffic.
Common Bridge Deck Construction Methods
Beyond cast-in-place concrete, other methods and materials are employed depending on project requirements and bridge type.
| Method | Description | Advantages | Disadvantages |
|---|---|---|---|
| Cast-in-Place Concrete | Concrete is poured directly on-site into formwork. | High strength, versatile for complex geometries. | Weather-dependent, slower construction, more formwork. |
| Precast Concrete Panels | Deck segments are fabricated off-site in a controlled environment and lifted into place. | Faster erection, higher quality control, reduced traffic disruption. | Requires heavy lifting equipment, more complex jointing. |
| Orthotropic Steel Decks | Made of welded steel plates stiffened by ribs, often with a thin asphalt overlay. | Very lightweight, suitable for long spans and movable bridges. | Complex fabrication, high initial cost, fatigue concerns. |
| Timber Decks | Utilizes treated timber planks or glued laminated timber. | Environmentally friendly, aesthetic appeal for certain applications. | Limited spans, requires regular maintenance, fire susceptibility. |
Why Composite Action Matters
The concept of composite action, as achieved with shear connectors, is a cornerstone of modern bridge engineering. It allows the concrete deck and the steel girders to share the load more effectively than if they acted independently. This results in:
- Enhanced Stiffness: The bridge is less prone to deflection under load.
- Increased Strength: The combined section can carry significantly higher loads.
- Optimized Material Use: Leads to more efficient and often lighter bridge designs, reducing material consumption and construction costs.
