Precast Concrete: A Global Solution
Er. Vinay Gupta, Director & CEO, Tandon Consultants Pvt. Ltd.
The entire world is busy constructing several projects related to infrastructure, real estate, power sector, industrial sector, etc., and it goes without saying that civil construction is the first aspect of project implementation that precedes functional installations. A structure would be disliked by masses, if it does not find enough aesthetic appeal; even the infrastructure projects are making inception of aesthetics and architects are being involved in the design of flyovers. Moreover, while it is important to conserve the environment, it is also equally important to speedily complete the projects, which apart from having financial benefits, reduces disturbance to traffic and the neighbourhood.
Precast concrete is an answer to all the issues discussed. All over the world, people are encouraging the use of precast concrete. Nearly, every second structure in the world employs precasting of small or larger volumes. Though precast concrete is accessible at marginally higher cost, it gets offset by the advantage of speed. Being a factory made like product, it entails a better surface finish, and hence, aesthetics. Other major advantage of precasting is the reduction in pollution at the construction site, which is often witnessed in urban areas; this applies equally well to infrastructure and building projects (factory, institutional, residential, etc). Some of the major examples of such structures are discussed below:
Khalsa Heritage Complex, Punjab
To celebrate Sikhs’ 500 years of history and Khalsa’s 300 years of establishment, the Punjab Government launched the mega project ‘Khalsa Heritage Complex’. The project comprises a 150 m long pedestrian bridge to connect Complex ‘A’ and Complex ‘C’. The special highlights of the project include:
- Inception of large volume of architectural fair faced concrete
- Preparation of mock-ups of all specialized elements, prior to their actual construction
- 26 m span prestressed concrete ramps in Heritage Museum building
- 20 m span RCC roof beams acting as partial catenary in permanent exhibit building
- 35 m span arch bridges incorporating prestressed tie beams
- Precast canopy over the pedestrian bridge
- Specialized mechanical connection between in-fill brick walls and the adjoining beam-column frame structure for sustainability during high seismic forces
The project made use of precast concrete (all architectural fair faced concrete) for elements like window surround (lintel, sill and joints), 157 m long pedestrian bridge canopy, etc. Fig. 1 shows a building model of the Khalsa Heritage Complex project.
The complex is divided into:
- Complex ‘A’- It houses a multi-storied library complex and a 20 m high, 400 seating capacity auditorium (Theatre).
- Complex ‘B’- It incorporates a pedestrian bridge having 4 arch spans of 35 m each and a two-level cafeteria below the bridge.
- Complex ‘C’- It comprises various multi-storied buildings to house high-tech exhibits of various types.
The architectural conception demanded the 4 arch spans (35 m each) to be separated from each other by 6 m. Hence, the longitudinal thrust of one arch could not be balanced by the adjoining arch spans. Therefore, each arch span was made independent, using prestressed tie beam (4 for each arch rib of 7 m width) provided below the ground level.
Further, the pedestrian bridge is provided with a precast canopy, which was precast and erected from top of the bridge itself. Each canopy unit comprises 3 pieces, i.e., columns, slab and a concrete cap; all these elements comprise architectural fair faced concrete using PSC (Fig. 2).
Fig. 1: Khalsa Heritage Complex, Anandpur Sahib, Punjab
Fig. 2: Precast Concrete Canopy over Pedestrian Bridge
Amari Atrium Hotel, Bangkok
The 25 storied Amari Atrium Hotel, Bangkok, built between 1991-93, incorporates prestressed floor slabs for the upper 20 floors; this way, each floor could be completed in an average of 9 days per floor. Fig. 3 shows an overall view of the building.
The structural system comprises banded slab system with frames at 8.2 m spacing. Each frame comprises 12 m centre to centre span between columns flanked by 4 m cantilever on either side. The prestressed band beams are 400 mm deep, which span 12 m from column to column. The prestressed slab, that spans 8.2 m, has a thickness of 200 mm. Further, the prestressing system comprises unbounded tendons using grease coated factory extruded strands, housed in flat prestressing ducts for post tensioning the slab; the prestressing system is shown in Fig. 4 and Fig. 5.
Apart from the speed of construction, prestressing provided a distinct advantage of reduced structural depth, reduced deflections, enhanced durability and reduced consumption of concrete, eventually leading to sustainable construction. This is another method of fast track construction, though not precast.
Fig. 3: Amari Atrium Fig. 4: Prestressed Fig. 5: Flat Prestressing
Hotel, Bangkok Banded Slabs Cables
Samtel Color Ltd., Ghaziabad
The use of prestressing is not only limited to buildings and bridges; it can also be extended to industrial structures. Samtel Color Ltd., Ghaziabad is one such example, wherein precast prestressed trusses (post-tensioned) were used in large scale. This way, the 25,000 sq. m of factory building could be constructed in merely one year.
The 25 m span trusses are spaced at 10 m and are supported over elastomeric bearings over the 10 m high columns. The elastomeric bearing also acts as seismic isolator to reduce seismic demand of the structure, leading to economy in columns and foundations, as illustrated in Fig. 6 and Fig. 7. The roof slab in this case, comprises precast RCC inverted channel units topped with cast-in-situ RCC. The constructed structure, apart from being fast has also proved to be weather tight for the air-conditioned electronics factory. Thus, the high speed of construction (25,000 sq. m took 11 months in 1989) enabled the entrepreneur to have an edge over the other manufactures.
Elevated Viaduct Over Barapulla Nallah
An innovative idea of providing an elevated viaduct over a city drain has been successfully implemented, just before commencement of the Commonwealth Games in Delhi. The main attribute of this project included the construction of 4 km + 4 km = 8 km viaduct in mere 20 months for Commonwealth Games 2010, wherein the end date was fixed. The project comprises of the construction of an elevated road over Barapulla Nallah from Ring Road near Sarai Kale Khan to Jawahar Lal Nehru Stadium. The piers for the entire project are in the Nallah bed. The alignment crosses Ring Road, Railway Tracks, Jangpura Road to Nizamuddin Railway Station, Monumental Bridge over Nallah, Mathura Road, and Lala Lajpat Rai Marg (Fig. 8). Further, the project lies in close vicinity of the historical monument, Tomb of Khan-i-Khana. 9.0 m carriageways are provided for each traffic direction with structures provided for both the carriageways. A major portion of project comprises of two type of construction:
- Stilted portion of standard spans constructed span-by-span with precast segments employing launching girder.
- Stilted portion of special modules constructed on major crossings by precast segmental structure employing cantilever construction technique using segment lifter.
Fig. 6: Samtel Color Ltd. (Ghaziabad) Precast Post-Tensioned Roof Trusses
Fig. 7: Precast Trusses Resting over Elastomeric Bearings
Additionally, standard span modules are provided with deck continuity and regular continuous length of 3 m x 34 m = 102 m, supported over two rows of elastomeric bearings at each pier location, making all piers and pier caps aesthetically identical. The lateral forces have been transferred through elastomeric bearings in vertical plane, attached to concrete upstand over the pier cap. Fig. 9 shows a general view of the standard spans, and Fig. 10 demonstrates standard span construction.
Fig. 8: Alignment of Elevated
Viaduct of Barapulla Nallah
Fig 9: View of Viaduct
Fig 10: Construction of
Standard Span Modules
Besides standard span modules, there are special span modules at the road and railway crossings; these are 3 to 5 span units with the largest central span of 84 m. The special span modules have been constructed as precast segmental balanced cantilever structures, employing specially designed segment lifter; Fig. 11, Fig.12 and Fig. 13 depict this type of construction. In the case of the railways span, a block time of only 2 hours per day, from 12:00 midnight to 2:00 am was given, when construction activity over the tracks could take place and no train would run during this period. Hence, the structural planning incorporated only dead end of cables over the railway side and live end (prestressing end) on the remote side. While the preparatory works and segment erection of remote side were done during the remaining 22 hours, the segment erection (including its prestressing) on the railway side was done during the specified 2 hours; this way, speedy construction by over-coming all the hurdles leads to sustainable construction.
Fig. 11: Use of Segment Lifter
Fig. 12: Balanced Cantilever Construction using Segment Lifter
Fig. 13: Cantilever Construction over Railways
The project alignment passes through the Tomb of Abdul Rahim Khan-i-Khana at Mathura Road. The mandate given by the ASI Department was not to obstruct the view of the tomb. For this reason, the level of elevated viaduct was raised to obtain a clear height of 12 m (Fig. 14 and Fig. 15).
Viaduct Of Bangalore Hosur Elevated Expressway
In line with promise to the nation, NHAI has completed the mega project of 10 km long elevated viaduct from Silk board Junction to Electronic City in Bangalore. Twin carriageway (2 x 2 lanes) is carried over 16.3 m wide twin cell box girder. Typical modules are 8 span continuous superstructure with module length of 262 m between expansion joints. About 3,000 precast segments were constructed in 3 different precast yards, specially designed to carry out short bench method of precasting. Erection of segments was carried out using 3 nos. of specially designed overhead launching girders. Refer Fig. 16 and Fig. 17 for some of the operations of launching girder. The launching girder was designed to be operated using wireless remote control for speedy operations, see Fig. 18. This way, even handling of segments including lowering and raising of segments for epoxy application could be made fast and convenient, see Fig. 19. Thus, a peak speed of 3.5 days per span of the twin carriageway superstructure could be achieved for the important BOT project.
Fig. 14: Tomb of Abdul Rahim Khan-i-Khana (1556- 1627): View from Mathura Road
Fig. 15: View of Khan-i-Khana Tomb from Mathura Road with Elevated Road at a Height of 12 m
Delhi-Gurugram Toll Expressway
A large-scale use involving 1,800 precast pre-tensioned I-girders was made for the Delhi-Gurugram toll expressway about 10 years ago. The system has a distinct advantage of lighter weight, lesser concrete and sleeker structure, making it fast, efficient and cost-effective.
Fig. 16: Launching Operation Fig. 17: Launching Truss
Supported on Front Pier
Fig. 18: Cordless Remote for Fig. 19: Segments Suspended
Launching Girder from Launching Girder
Precast pre-tensioned girders integrated with cast-in-situ deck slabs and diaphragms were used on a large scale for the superstructure of the 2 x 4 lanes of the access-controlled expressway incorporating several grade separators aggregating 11 km of stilted structure. The requirement of precasting 1,800 girders was met through a casting yard comprising 14 beds and a process of steam-curing to obtain a time cycle of up to 4 girders a day. Further, specially fabricated low-bedded trailers were deployed to transport the 30 m long girders (weight 60 T) to the site and a pair of Goliath cranes operating in tandem was then used for their erection. A pair of 150 T tyre-mounted mobile cranes was also engaged at designated locations for the purpose. Special highlight of the design included the ‘integral’ sharply curved (radius 50 m) prestressed cast-in-situ box girder continuous decks for the ‘horse-shoe loop’ connections towards the international and domestic terminals of the IGI airport, Delhi.
Evolution: Several structural schemes were considered for the BOT project. A detailed comparison of the following types was made in the beginning of the project, i.e., the year 2002:
- Cast in-situ/precast spine beam superstructure
- Precast segmental epoxy jointed box girder superstructure
- Precast pre-tensioned/post-tensioned girder superstructure
The attributes to the study were construction speed, economics and aesthetics. Further, the option of precast pre-tensioned girder superstructure with cast-in-situ deck slab and diaphragm was found to have the best combination of speed and economics. In order to further economise and improve riding quality, typical modules of 3 span continuous structures 25 m + 30 m + 25 m were provided. Continuous supports were provided with single diaphragm and single row of bearings (pot bearings), as shown in Fig. 20.
Fig. 20: Delhi Gurugram Expressway
Precasting: Long line method of casting of pre-tensioned girders was adopted. A combination of well-arranged precasting yard and steam curing made it possible to achieve a precasting speed of 105 girders per month. For this purpose, 14 precasting beds, 6 30 m girders and 8 25m girders were provided (Fig. 21). Also, large stroke hydraulic jacks with screw lock system, each of 500 T capacity were employed, 2 for each line of girders stressing. After 12 hrs of steam curing, the girders attained about 80% strength, when the prestressing could be imparted. Subsequently, they were moved to water curing and stacking beds. Thus, a construction cycle of 1 girder in 3 days per precasting bed could be achieved, which is about double the speed of equivalent completely water cured precasting. Before transporting the girders, the end faces were properly roughened by 100% hacking, to develop effective bond with the cast-in-situ diaphragm.
Fig. 21: Precasting of Pretensioned Girders
Transportation and Erection: Specially fabricated low-bedded trailers were employed to transport 60 T 30 m long girders. The trailers contained dual power steering for better turning (Fig. 22). It was also ensured that the total height after placing of girder over the trailer was less than 5.5 m, so that the same could pass below the flyovers on the way. All the transportation was done at night, to reduce traffic disturbance.
The erection of these girders was facilitated in two ways. In case of shorter stretches, two type mounted cranes worked in tandem; and in case of longer stretches, parallel tracks were laid to move two Goliath cranes to move with the girder in tandem.
BBBE Expressway, Bangkok:
A very interesting project comprising 57 km long and 6-lane wide elevated expressway, Bangna Bangpli Bangpakong Expressway was constructed between 1995 and 1999 in Thailand. The precast segmental box girder structure was erected using a specialised launching girder supported over horizontal member of the H-shaped piers (Fig. 23 and Fig. 24).
Longitudinally, there is dry jointed precast segmental structure with external unbonded prestressing, making the structure light and faster to construct and transverse prestressing in the deck, which is conventional internal bonded type, helping to reduce self-weight. Further, the presence of RCC struts has made the segments much lighter as compared to multi-cell box girder of 27 m width. Thus, the project exhibits excellent combination of speed, quality and aesthetics.
Engineers have found ways and means to negotiate the ground features and make a structure that is economical, aesthetically pleasing and fast to construct. It is important to have an aesthetically pleasing structure, for example, the Khalsa Heritage Complex, Punjab, along with high speed of construction, for example, the Bangalore-Hosur Elevated Expressway. Moreover, the use of precasting and prestressing in buildings leads to a fast, economical and sleek structure; these structures are durable and weather resistant. The solutions of using segment lifter in the case of Barapulla Nallah Viaduct, etc. are some of the examples. Overall, a carefully planned structure is bound to produce a fast, aesthetic and efficient structure
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