Smart Options

New materials for bridge construction

Bridge building involves conceptual and computational aspects. The concep­tual aspect focuses on the materials to be used and the combinations to be chosen to build an economical structure that meets safety standards. The computational aspect re­quires considerable application, engineering, judgement and experience. There are various types of bridges that are constructed using a wide range of materials. Bridges can be categorised based on different structures and each structure has different requireme­nts to cover such as span clearage, traffic flow, geometry and characteristics of the place to build. Modern bridges are predo­mi­na­ntly built-in reinforced concrete and desi­gned with a service lifespan of more than a 100 years. Common materials used in construction are structural steel, reinforced concrete, prestressed concrete, or post-tensioned concrete.

Smart materials

Several smart construction materials are emerging and one of the most innovative construction materials considered today for bridge design is self-healing concrete. Smart materials are engineered to respond to cracks, excessive stress,  environmental effects such as te­m­­­perature and pressure, and the presence of oxygen. Smart and new construction materials have also led to the development of standard precast, prestressed concrete beams that are normally designed to act compositely with a cast-in-place reinforced concrete deck slab. Concrete is prone to cracking because of various loads occurring in bridges. However, new concrete mixtures that include limestone-producing bacteria are being developed, which fill the cracks as they form. Smart concrete is still being tested in laboratories to determine how long the bacteria sustains itself. This new innovative material can prevent costly damages that can occur if cracks in concrete are not filled or repaired.

Segmental bridges and launchers for them, as well as shutters for single-span brid­g­es, have become modern. In Hyderabad me­­­­­t­­ro construction, segmental viaduct has bee­n used with some cast in-situ spans in a few ca­ses including eight rail overbridges. The via­duct for the Delhi metro Project comprises simply supported precast pre-tensioned twin U-girder (each U-girder supporting one track only)/post-tensioned segmental box girder with RCC substructure and bored cast in-situ pile/open foundation.

Fly ash

Fly ash can be used as prime material in many cement-based products such as poured concrete, concrete blocks and bricks. Being a generally cohesion-less material, fly ash gets consolidated at a faster rate and primary consolidation is completed quickly. Hence, it has low compressibility and shows negligible post-construction settlement. Fly ash forms a compound similar to portland cement when mixed with lime and water. Thus, fly ash can be suitable as a prime material in blended cement, mosaic tiles and hollow blocks, among other building materials. When used in concrete mixes, fly ash im­proves the strength and segregation of the concrete and makes it easier to pump. As the use of fly ash in cement concrete reduces the requirement of cement and raw materials (such as limestone and coal), it results in cost reduction. Since fly ash can displace cement to a certain extent, it has the potential to eliminate greenhouse gas emissions.

The embankment for the Signature Bridge in Delhi was done with fly ash in waterlogged area. First a geogrid reinforced fly ash approach embankment was constructed for the Okhla flyover bridge in Delhi. Since fly ash is lightweight in nature, it leads to savings in material and re­duces settlement in the underlying soils. Going forward, there is a huge potential for the use of fly ash in bridge construction.

Concretes

Several new types of concretes have been developed. These include self-compacting concrete, high performance concrete, fibre reinforced concrete and ultra high performance concrete. This is a high strength, high stiffness, self-consolidating, ductile material, formulated by combining portland cement, silica fume, quartz flour, fine silica sand, high range water reducer, water, and steel or organic fibre. Each of these materials has its own advantages and can be used judiciously.

The Chenab Bridge in Jammu & Kashmir was designed as a large-span single-arch steel bridge with approach viaducts on either side. The arch is two-ribbed, fabricated from large st­eel trusses. The chords of the trusses are sealed steel boxes, internally stiffened and filled with concrete to assist in controlling wind-induced forces on the bridge. Also, internal pa­in­­ting is not required in the case of concrete filling. The number of bearings has been mini­mi­sed, particularly on the approach viaduct, th­­­ro­ugh the use of continuous construction. This is advantageous, as it reduces the maintenance and inspection efforts, and improves riding qu­a­lity. The viaduct piers are of concrete while the piers near the arch are of steel.

High strength steel

High strength steel is often preferred for constructing bridges. The higher the strength, the smaller the proportional difference between the yield strength and tensile strength. This means that high strength steel is not as ductile as that with normal strength. Nor does fati­gue strength rise in proportion to tensile stren­gth. It is therefore necessary to have a profo­und kn­owledge of the behaviour of these special ste­els before using them. For building pur­po­ses, steel is fabricated in the form of plates of about 6 to 80 mm thickness by means of rolling when red hot. For bearings and some other items, cast steel is used. For compone­nts under constant stress, like ropes or cables, there are special steels, processed in different ways, which allow us to build bold suspension or cable-stay­ed bridges. The high strength of steel offers numerous benefits as it allows small cross-sections of beams or girders and therefore a low dead load of the structure. It has thus been possible to develop lightweight orthotropic plate steel decks for roadways, which have now become common with an as­ph­alt wearing course, 60 to 80 mm thick. The use of high strength steel structure for bridge structures can lead to substantial material savings and has the potential to be more economical and environmentally friendly compared to mild steel.

The Bogibeel Bridge is India’s first fully welded warren truss girder-type steel bridge and has used an incremental launching technique for the erection of the superstructure. The superstructure of the bridge was constr­uc­ted using special copper bearing steel plates in order to increase durability.

Aluminium is also occasionally used for constructing bridges. Aluminium is fabricated by the extrusion process, which allows many varied hollow shapes to be formed. This ma­kes aluminium structures more elegant than those of steel. Aluminium is popular for bridge parapets because it doesn’t need any protective paint. Further, properties such as density and corrosion resistance make aluminium a durable material well suited for brid­ge structures. Large aluminium structures su­ch as a suspension bridge girder or a ship hull are mainly based on flat-rolled plates and extruded profiles. However, a common challen­ge in aluminium welding is reduced mechanical properties of the welded joint compared to the base material. The Arvida Bridge in Sagu­enay, Que­bec, is one of the longest aluminium brid­ges in the world.

The way forward

In order to maintain durability over the lifespan of bridges, there is a need to focus on waterproofing and protection of exposed elements and critical areas such as the bridge deck it­self, to prevent serious damage to the conc­rete and structural reinforcement. Further­more, natural disasters, such as earthquakes and tornados, are one of the most important factors to consider when designing bridges. The damages generated by these seismic and wind loads cause irreversible damages to different structural elements of bridges which require very costly repairs. However, new construction materials are now being used to avoid such issues.

Construction of modern structures for bri­dges is quite complicated. Thus, there is a need for skilled and trained engineers and ex­perts to plan the construction of these structu­res using best methods and equipment. Also, there is a need to have a clear understanding of the quality of construction materials to build sustainable bridges. In addition, periodic moni­toring of bridges is important for safety purposes. Going forward, construction materials will be engineered to be smarter, stronger, self-sustaining and easier on the environment.

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