Vinay Gupta, Director & CEO, Tandon Consultants, New Delhi
Prestressing works as an active force in the structure, opposite to applied tensions. There are several types of prestressing such as Internal Prestressing, External Prestressing, Bonded Prestressing, Unbonded Prestressing, etc. Pre-tensioning and Post-tensioning are some of the broad forms of prestressing. Protection of prestressing cables / strands is more importantly needed as steel under higher stresses is more susceptible to corrosion. Prestressing results in sleeker, more crack free, and durable structures that deflect less and entails a higher speed of construction. Building industry is also using prestressing for its distinct advantages.
The concept of prestressing started in the 1940s. Since the start, there have been various doubts about prestressing as a reinforcement. In 1970, UK had banned external unbonded prestressing after detecting some problems with it. The ban was later lifted once proven external unbonded prestressing alright. In 1992, UK banned prestressing altogether, seeing the problems of corrosion, etc. This ban was lifted four years later after firming up the grouting specifications.
However, bridge and any large span structure is practically unimaginable without prestressing. Solutions need to be found to solve the problems associated with prestressing from time to time. A prestressed structure entails lesser cracking and deflection apart from enabling sleeker structure. The word ‘tendon’ is used to denote any prestressing material, strand or cable, whereas, a cable is a combination of several strands. Each strands comprises 7 wires stranded together.
Basics of Prestressing
Prestressing works as active reinforcement in the structural member. It is placed at the locations of tensions so that it can counteract the tensile stresses, partly or completely. Over the years several types & forms of prestressing have been invented. Namely, Pre-tensioning and Post-tensioning. These can be Internal Bonded, Internal Unbonded, External Unbonded Prestressing, etc. In case of precast segmental construction, Internal Bonded or Unbonded prestressing may be used for Epoxy Jointed segments. However, in case of Dry Jointed segments, the prestressing has to be of External Unbonded type for the reasons of durability. Pre-tensioning has to be of Internal Bonded type only due to its very nature. Fig 1 depicts various types of conditions of prestressing. Similarly, Figs 2 & 3 depict pictorial comparison of Post-tensioning and Pre-tensioning.
Types of Prestressing
One of the most conventional types of prestressing is Internal Bonded type of Post-tensioning. Wherein, a duct (Metallic or High Density Polythene (HDPE)), see Fig 4, is left in concrete during casting. This leaves a hollow space, in order to insert cables which are then stressed. After stressing, the cables they are grouted with cement to create a bond between cables and concrete. Grease or Wax grout may also be used, that will make it Unbonded Internal prestressing system. In this case, strains in prestressing are not the same as those in the surrounding concrete and the design has to take care of this fact, suitably. Another type is Pre-tensioning wherein individual strands are stresses before casting of concrete element. This becomes internal bonded individual strand prestressing. In this type, the strands have to run straight, as the concrete is not available to hold them in any profile other than straight during their stressing. In case prestressing tendon is required to be continued for stagewise construction, Prestressing Coupler as indicated in Fig 5 may be used.
Internal Bonded and Internal Unbonded Post-tensioning
This method of prestressing has been most commonly used in India due to its simplicity. Cement grouted system is cheaper compared to grease or wax grouted system. Latter may be preferred in some cases as it makes the cables replaceable. Some structures incorporate galvanised or epoxy coated strands for enhanced durability. Fig 6 indicates an example of preparation of Ground Anchors of a vehicular underpass project wherein apart from Epoxy coating of strands, individual HDPE tubes with grease infill have been used, see Fig 6. This is further elaborated in Fig 7. Internal Post-tensioning entails, generally, larger concrete sections, as some of the web thickness is occupied by the cable ducts, requiring relevant considerations in the shear design. A 25 storied Amari Atrium Hotel in Bangkok incorporates internal unbonded prestressing for its slabs, wherein factory extruded grease coated strands have been used, see Figs 8A, 8B, 8C & 8D.
Fig 9 depicts anchorage components of Post-tensioning. The duct and anchorage cone remain inside concrete. The bearing plate which is outside concrete incorporates conical holes in order to house conical wedges (usually a 3 to piece assembly with a circlip). These wedges have serrations on their internal surfaces. Each strand comprises 7 wires stranded together. Central wire is about 1.5% larger in diameter. Combination of strands forms a cable. A hydraulic jack is used to stress each cable. Once, the jack is released and the strands tend to move inward, serrations of the wedges start biting the strands and hold them against moving inward. This way the prestressing force is imparted to the concrete structure. Subsequent to stressing, grout is sent under pressure, in order to protect the cables against weathering and to create a bond with concrete in case of cement grout.
Pre-tensioning
As the name suggests, tensioning of tendons is carried out before casting of the concrete using two abutments, one on either side (dead end & stressing end) and stressing carried out using high capacity hydraulic jacks in a manner indicated in Figs 10 to 13. Delhi – Gurgaon Expressway is one of the biggest examples where 1800 Pre-tensioned girders have been used, see Fig 14. Similarly, large scale use of Pre-tensioned U-girders has been in 30 km long Noida – Greater Noida metro for speedy construction, see Fig 15. As far as the economics alone is concerned, pre-tensioning is found to be cheaper when the number of girders are more than 200. Hence, in the stations of Bangalore Metro Pre-tensioned I-girders have been made for Concourse and Platform, see Fig 16. The bridges of Kochi Port Connectivity project incorporate 9m long Pre-tensioned girder segments joined through Post-tensioning (Spliced Girder System), see Fig 17.
Prestressing for Precast Segmental Bridges
Precast segmental bridges have their own applicability & limitations of types of prestressing systems, see Fig 18. Epoxy jointed Precast Segmental structures can incorporate Internal Bonded, Internal Unbonded as well as External Unbonded prestressing systems. There can, even, be a combination of two or more types of prestressing in a structure / span. Why internal prestressing works appropriately with epoxy jointed precast segmental structure is because the epoxy seals the minute gaps between the segments and prevents corrosion of the internal prestressing. Figs 19 to 21 depict the example of a 10km long elevated expressway incorporating internal bonded prestressing for epoxy jointed precast segments. Fig 22 depicts application of Epoxy and Fig 23 depicts new types of Duct Couplers to effectively prevent ingress of the Epoxy. In the case of Dry Jointed Precast Segmental structure this facility of sealing the minute gaps is absent, requiring to use external unbonded prestressing with continuous duct, not requiring protection at segment joints. Figs 24 to 26 depict the use of External Unbonded Prestressing in combination with Dry Jointed Precast Segmental superstructure of metro structure in Malaysia. Similarly, Fig 27 indicates the use of External Unbonded Prestressing in combination with Dry Jointed Precast segments of 550m long DND Flyway at Delhi – Noida border of river Yamuna.
Other Applications of Prestressing
There are a large number of variety uses of prestressing. Samtel Color Ltd factory incorporates precast post-tensioned concrete roof trusses for a large scale application, see Fig 28. Now-a-days, use of prestressing is in vogue in buildings, flat slabs, banded slab system, pretensioned hollow slab, etc.
Design Aspects
Prestressing steel undergoes several types of losses after its stressing. These have been enumerated in Fig 29. Handmade Fig 30 depicts how the losses at seating (ie. Just after releasing the prestressing jack) take place. There is a slippage of strands at the wedges. Friction always act against the applied movement. Hence, the diagram. Elastic Shortening of concrete is also a part of the seating losses. Relaxation of strands is more in the beginning and reduces later. As the concrete shrinks with time, shrinkage losses take place. Under the stress the concrete creeps causing creep loss of prestressing.
One of the important aspects of prestressing is the anchorage zone (both, live end as well as dead end), wherein, concentrated stresses apply and the small concrete are acts as column subject to stress concentration. This leads to provision of heavy reinforcement. It needs to be detailed adequately and concreted properly to ensure no honeycombing, see Fig 31.
In case of continuous superstructures, areas other than the one stressed get affected by the stressing. For example, compressive stress may be imparted at mid-span bottom of the first span, it may give rise to compressive / tensile stresses at the next support as well as the next mid-span, etc. This effect is called Hyperstatic effect, see Fig 32 and it needs to be adequately accounted for in the design.
Conclusion
The author is Director & CEO, Tandon Consultants, New Delhi. A Civil Engineer from BITS, Pilani (1983) he specializes in bridges, flyovers, underground & elevated metro structures, precast & long span buildings, 275m tall chimneys, etc. He is all India President of IIBE and ICI and Vice President - ACF and IAStructE.
