Precast Segmental Construction Ensures Faster Infrastructure Construction
The precast segmental construction is the most sought after methodology with its speed as high as 2½ days per span of superstructure construction.
Er. Vinay Gupta - Managing Director, Tandon Consultants
It is well-known that India has been constructing a large number of infrastructure projects. In order to keep pace with the accelerated construction of bridges, flyovers, elevated viaducts, metro structures, etc., employment of several innovative and advanced technologies becomes eminent.
India has taken a giant leap in the field of structural engineering over the past 20-30 years. One of the best solutions found to solve traffic problems is to construct long elevated viaducts and metro structures. Such structures demand accelerated construction to minimise traffic disturbance, as they are located in urban habitats.
Precast concrete being reasonably cheap and versatile material, is found to be the most appropriate choice of the engineers. A recent example is the 10-km long Elevated Viaduct of Bangalore-Hosur BOT Expressway (figs. 1 & 2), wherein the construction speed of the precast segmental superstructure was as high as 1.75 days per carriageway span.
The 26-km long Delhi-Gurgaon Toll Expressway (figs. 3 & 4) was successfully made using 1800 Precast Pretensioned Girders for continuous superstructures.
Similarly, the 4-km long Barapulla Elevated Viaduct (figs. 5 & 6) in Delhi is a unique example of precast segmental construction over a city drain and over the Indian Railway Tracks, yet meeting deadlines of Commonwealth Games 2010. The Badarpur Elevated Corridor is an excellent example of precast segmental superstructure for the sharply curved spans apart from the regular straight ones.
The 26-km long elevated metro LRT-2 (Putra line) has used external prestressing with dry jointed segments (figs. 7 & 8) for speedier construction.
Metro constructions have shown a pathway to high-speed constructions. Starting with the Delhi Metro in the late 90’s, many Indian cities have come up with metro constructions. Precast segmental construction is the most sought after construction methodology as it gives speed of superstructure construction as high as 2½ days per span.
Wherever the site constraints prevailed, other types such as steel superstructures, cast-in-situ voided slab and precast girder-slab arrangements have been employed for elevated structures. A careful planning is required while designing the associated launching girder, which may be over-head or under-slug. Sometimes, the weight of the launching girder has to be suitably taken into account in the design of the permanent superstructure.
Associated risks with precast segmental structure are opening of joints between precast segments, blocking of prestressing ducts by Epoxy Bonding Agent, breakage of prestressing hardware etc. These have to be adequately taken care of in the design, detailing and construction. Provision of Pier Bent (cantilever pier cap), RCC/ PSC portals to negotiate ground level constraints are often utilised. It’s well known that overhead construction is faster and cheaper than underground construction. But, wherever inevitable, under-ground construction has been taken up in many areas. Of late, even 15m diameter road tunnels are under construction, employing TBM technology. It may be noted that TBM (Tunnel Boring Machine) was so far used only for smaller diameters of about 7m in India for underground metros.
