Over the past many years, there have been several advancements in the areas of bridge design, engineering and construction in India, with the adoption of the latest technology and smart construction materials. In addition, monitoring has become mexpansion joints have supplanted strip seal expansion joints on many bridges.
Bridges can be categorised based on their structures, and each structure has its own set of requirements, such as span clearance, traffic flow, geometry and site characteristics. Bridge construction has both conceptual and computational aspects. The conceptual aspect emphasises on the materials to be used and the cominations to be selected in order to construct a safe and cost-effective structure. The computational aspect requires considerable effort, engineering, judgement and experience.
Evolving methods
Before a bridge is constructed, an appropriate construction method must be selected. A few of the principal factors affecting the selection of a bridge construction method are the regularity of the span lengths, the horizontal and vertical profiles of the bridge decks, and soil strata.
- Cast-in-situ: This is a flexible construction method that can easily suit the needs of bridges with unusual geometrical shapes. Cast-in-situ methods are adopted when it is difficult to transport prefabricated parts due to their size, location, etc. Concrete, wires, bars and steel strands are utilised for this method of construction. The Barak bridge in Silchar has been constructed using this method.
- Balanced cantilever: This is the most advanced of all construction techniques, as it requires no temporary support structures. Bridges at greater heights can also be constructed using this method without the use of falsework. When a bridge is long and has irregular span lengths, the cast-in-place technique for the balanced cantilever method of construction is preferred. This method was used for the 560 metre-long metro bridge constructed over the Yamuna river by the Delhi Metro.
- Span by span: In the span-by-span method, an entire span is assembled, post-tensioned and erected so that it is self-supporting before the next span is erected. Span-by-span erection is the most common, simplest and the most cost-effective construction method for precast segmental bridges. This method of construction is suitable for spans up to 60 metres. The Krishna bridge at Deodurg was built using this method.
- Full span: The full span method is one of the fastest, and is primarily used in the construction of high speed rails, bridges and viaducts. It involves casting the whole bridge span in the casting yard and transporting it with a multi-axle tyre trolley to the bridge site. After this, the span is lifted from the transporter and handled using the launching gantry. After completion of the span, the launching gantry is auto-launched and is ready for the next span erection. In November 2021, the first full-span pre-stressed concrete box girder was erected for the Mumbai-Ahmedabad high speed rail project.
- Precast segmental construction: This technique has gained popularity as it accelerates bridge construction, provides better quality control and lowers the life cycle cost. Moreover, elements with difficult shapes and congested reinforcement can be cast at ground level. There are various types of segmental bridges, such as precast segmental superstructure, simply supported or continuous, internally prestressed or externally prestressed, epoxy-jointed or dry-jointed, balanced cantilever constructed using cast-in-situ or precast segments, and spliced girder superstructure constructed using concrete stitch or epoxy joints. A key example is the Bandra-Worli Sea Link project in Mumbai.
- Incremental launching method: This method is mostly used for bridge decks longer than 250 metre. Using this, the bridge deck is constructed in sections by pushing the structure outward from the abutments towards the pier. It is ideal for the rapid construction of bridges with a constant radius of curvature. The Bogibeel bridge in Assam has been constructed using this method.
Emerging material choices
Modern bridges are predominantly built with reinforced concrete. Other common construction materials include structural steel, prestressed or post-tensioned concrete, and post-tensioned concrete.
Several smart construction materials are emerging today. Smart materials are engineered to respond to cracks, excessive stress, environmental effects such as temperature and pressure, and the presence of oxygen. One of the most innovative material is self-healing concrete. Concrete is prone to cracking because of the various loads that a bridge has to bear. However, new concrete mixtures that include limestone-producing bacteria are being developed, which can fill the cracks as they form.
Fly ash can be used as the prime material in many cement-based products such as poured concrete, concrete blocks and bricks. Being a generally cohesion-less material, fly ash is consolidated at a faster rate, and primary consolidation is completed quickly. Hence, it has low compressibility and shows negligible post-construction settlement.
The use of glass fibre-reinforced polymer in bridge construction is gaining popularity. It is an alternative to steel reinforcement bars, which are susceptible to rust and decay and can cause structural damage to bridges. Additionally, it is cost effective, easier to fabricate and durable.
Using high performance concrete with reduced permeability is the topmost mitigation strategy with respect to corrosion management. Epoxy-coated reinforced steel has been the most popular choice among transportation agencies to deal with the problem of premature bridge corrosion. Stainless steel is also a popular option for reinforcement in extremely harsh environments. Additionally, high-strength steel is often preferred for constructing bridges. Aluminium is also occasionally used.
Another emerging trend is post-concrete protection of the bridge structure, reducing the permeability of concrete and the need for special finishing materials. Grounded granulated blast furnace slag and ultrafine fly ash are used while designing the concrete mix. These strengthen the structure while reducing the carbon footprint in concrete production.
Going digital
As bridge designs become more complicated, powerful, all-in-one computer-aided software will be needed. The software must be able to process complex modelling challenges and process complicated structural analysis including finite element analysis, time history analysis, etc. Building information modelling (BIM): BIM is a set of technologies and processes used to create a three-dimensional virtual model of a project. This model contains all project information. The temporary cables and related anchoring towers for the Chenab bridge were designed using BIM. It was also used for the first metro bridge over the Yamuna river.
- Bridge asset management system: Information technology plays a key role in any bridge asset management system. The Ministry of Road Transport and Highways has prepared the Indian Bridge Management System to collect information about all bridges in the country. The purpose is to identify distressed bridges that need immediate attention, and to sensitise the concerned implementation agency regarding corrective measures such as repair, rehabilitation, reconstruction/new construction, etc.
In sum
To extend the longevity of bridges, waterproofing and protection of exposed elements and critical areas, such as the bridge deck itself, must be prioritised in order to prevent severe damage to the concrete and the need for structural reinforcement. Geospatial technology for remotely monitoring the condition of bridges is also an area that needs focus. Innovations in design and construction techniques, out-of-the-box thinking, and the adaptation of global best practices to Indian conditions (for both equipment and construction methods) can unlock great potential.
