The Bridge decks are one of the basic components of the bridge that forms its surface. The bridge deck may either be cast-in-situ, precast, steel, wood, concrete, or other pavement systems which are supported on concrete or steel girders or beams (composite bridges). The bridge decks are most often defines the characteristics of the bridge. In a suspension bridge, the deck is suspended from the main structural element.
In the case of a Tied arch or cable-stayed bridge, the deck is the main structural element that carries the load and transfers it via cables. Also, the deck has an important role in establishing the aesthetic appearance of the bridge. So while designing a bridge, it is very important to give special attention to decks to maintain good serviceability, safety, and appearance, etc. The deck system varies according to the bridge structure, types of construction methods. Here we discuss various types of bridge decks;
Types of Bridge Decks
Reinforced Concrete Decks
Concrete Decks are most common in bridge construction and this is especially because of the ease of preparation and placement to various shapes and sizes. Concrete decks have good durability, are very resistant to wear, and also provide good traction control. Among these design features the durability of the concrete decks is also very dependent on the drainage system provide, reinforcement cover, bridge skew, and thickness of the slabs.
The main cause of concrete slab deterioration is due to reinforcement corrosion. The corrosion is typically due to the chloride attacks (penetration of chloride molecules through the cracks on the decks). So as said earlier the provision of adequate reinforcement cover is very important in deck slabs. There are different types of concrete decks are used in bridges and some of them are;
Solid Slab Bridge Decks
Solid rectangular (fig: 1) is not very efficient structural form in bridges as because of the moment of area is small relative to its area and weight. This type of decks mostly adopted in relatively small span (upto 20m) and the cost of construction is comparatively low and reinforcement is simple.
Voided Slab Deck
Voided rectangular slab sections are used when the span range of bridge in between 20-30m. It is common in some countries to use situ concrete with polystyrene voids as shown in figure. This can be constructed using ordinary reinforced concrete or can be post tensioned.
The presence of voids may increases the cost of construction and this is because of the voids adds complexity to the reinforcement particularly the reinforcement in transverse bending section. But it reduces the self-weight and area of the concrete to be prestressed with affecting the second moment of area. The diameter of the void most often given 60% less the depth, also the void must accounted for considering the design to resist transverse bending.
T Slab Sections
T sections are mostly adopted in span range of 20-40m as an alternative to void slabs in bridges. The beams can be done on either precast or on site construction. The top portion of the T section called flange which supports the slab work with beam to reduce the compression and deflection can be controlled to a great extent.
Also it has greater moment of resistance compared to rectangular sections and cast monolithically with slab. The main challenges in T section bridges are the action of shear force. Cracks can develop in web and flange due to the changes in cross section.
Box Slab or girder section
Box sections are used when the span more than 40m and box sections are more economical in constructions of decks in this span range. T section has higher area moment of inertia per unit weight compared to T sections and voided slab. The box girders can also be constructed as precast section or composite with precast, pre-tensioned U section and an in situ concrete slab.
The box girders have many advantages like good resistance to torsion and this because of the deck is in curved plane. Also the girder can be constructed in wider because of the presence of two webs and strong flanges. The interior of the box girders can be used for service such as gas lines, pipe lines, cables etc.
Steel Orthotropic Bridge Decks
Steel orthotropic decks are now extensively used in Europe, South America and Asia. The main advantages of these decks are the weight of the structure can be considerably reduced (due to the decks are designed integral with girders as a common flange), have a long life and easy to install.
Another feature of steel orthotropic Bridge Decks is that in short span bridge it can be installed as a single unit. A major drawback in steel bridges is the effect of fatigue. A detailed design is necessary to overcome fatigue issues in steel bridges.
Composite Bridge Decks
Composite Bridge Decks can generally refer to the interaction of structural members of concrete and steel. This is used in bridge construction is mainly because of the ability to adapt the geometry and design constraints and also the reliability of steel members. The shear transmission in composite decks is done through shear connectors which are embedded in concrete and the other end is welded on the steel beam.
The concrete is good in compression and steel is good in tension and compression. Hence the deflection can be controlled to a large extent. Also, the steel members can be recycled in the future during the time of demolition. The steel as the main member is easier to erect when compared to the concrete girder.
Fibre Reinforced Polymer Decks
Fibre Reinforced Polymers are mostly used in the aerospace industry and now it’s also used in bridge construction. FRP composite materials are made up of fibre aligned within resin materials and this makes the panels very strong, customizable, and very light. Carbon and glass fibres are most often adopted for bridge decking. It is installed as full-depth deck panels after molding it into cellular panels.
Fibre Reinforced Polymer Decks are used as an alternative to cast in place concrete Bridge Decks. These panels are designed to interlock each other with male and female shear keys. To connect it over the steel frames, pockets are formed over the beam and then welded by shear connectors.
FRP materials can lose strength when exposed to high heat and result in a vehicle accident. Asphalt, concrete or polymer materials are given in wearing surfaces as overlays. FRC is very useful in movable bridges, decks or slabs subjected to the highly corrosive environment