PSC • April 2016

External Prestressing of ConcreteMembers

Praveen.S.K Vishwanath.H Vinuth.M

Abstract

The normal interaction between steel and concrete is lost because the tendonsare unbonded with the concrete cross section. External pre-stressing has beenproven cost effective and technically attractive worldwide, as it leads to substan-tial cost savings and a large decrease in construction time. If the purpose of theproject is to improve the load carrying capacity of existing bridges, then tendonsare usually, placed outside the bridge girders, tensioned and anchored at theirends. The external tendons are usually made of steel, but also fibre reinforcedpolymeric (FRP) materials can be used to replace steel which would prove moreeconomical.

I. INTRODUCTION

Pre-stressed concrete is reinforcedconcrete in which the steel re-inforcement has been tensioned

against the concrete, which counteractsthe downward deflection caused dueto its dead load and the imposed liveload.

II. SYSTEM OF PRESTRESSING

A prestressed system induces a knownamount of stresses in a concrete mem-ber before the live load is applied.These stresses are induced by highstrength steel strands by mechanicalmeans. These cables are generallyplaced inside a cylindrical duct madeout of either metallic or HDPE mate-rial. The Anchorages, one of the maincomponents of the prestressing activ-ity, are used to anchor the cable af-

ter inducing the load. The whole as-sembly of the Anchorage and the H.T.Cable is named as "TENDON". ThePrestress system counterbalances thestresses, produced by additional load-ing on the members like live/imposedload, thereby improving the range ofstresses to which a structural membercan safely be subjected.

III. METHODS OF PRESTRESSINGIN CONCRETE

Prestressing System can be classifiedby two basic methods as:

I. External Prestressing

When the prestressing is achieved byelements located outside the concrete,it is called external prestressing. Thetendons can lie outside the member orinside the hollow space of a box girder.

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This technique is adopted in bridgesand strengthening of buildings.

II. Internal Prestressing

When the prestressing is achieved byelements located inside the concretemember (commonly, by embedded ten-dons), it is called internal prestressing.Most of the applications of prestressingare internal prestressing.

IV. EXTERNAL PRESTRESSING

External prestressing refers to a post-tensioning method in which tendonsare placed on the outside of a struc-tural member.The first use with exter-nal steel tendons was in the 1950s, butafter that it lay dormant for some time.Now external prestressing techniqueswith steel tendons have been widelyused with success to improve existingstructures in the United States, Japanand Switzerland. However there can bea problem with corrosion in the steelthat forces the use of steel protectionon the external tendons, for exampleby plastic sheeting. This problem canbe resolved by the use of FRP materi-als. Therefore research in the area hasbeen conducted since the early 1970s.In the beginning, glass FRP was usedbut at the moment aramid and carbonare mainly used due to higher modu-lus of elasticity. FRP tendons lack theductility under extreme loading exhib-ited by steel, which means that a FRPprestressed beam may simultaneouslyprovide greater ultimate load capacityand lower energy absorption than asimilar steel prestressed beam.

V. NEED FOR EXTERNALPRESTRESSING

• Because of the absence of bond be-tween the tendon and structure,external prestressing allows the re-moval and replacement of one ortwo tendon at a time so that thebridge could be retrofitted in theevent of deterioration and their ca-pacity could be increased easily.This is essential for bridges in ur-ban areas where traffic disruptionis undesirable.

• It usually allows easy access to an-chorages and provides the ease ofinspection.

• It allows the adjustment and con-trol of tendon forces easily.

• It permits the designer more free-dom in selecting the shape of crosssection of bridges.

• Members could be made thinnerso that there is a reduction of deadload.

• Concreting of new structures is im-proved as there are no or few ten-dons and bars in the section

• Friction losses are significantly re-duced because external tendonsare linked to the structure onlyat the deviation and anchoragezones.

VI. DISADVANTAGES OFEXTERNAL PRESTRESSING

• External tendons are more easilyaccessible than internal ones, be-

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cause of that they are more vulner-able to corrosion.

• Anchorage zones are vital addi-tions to the cross section. Theseelements must be designed to sup-port large longitudinal and trans-verse forces.

• In the case of internal tendonswith grouting, the long-term fail-ure of anchor heads has limitedconsequences because prestress-ing may be transferred to the struc-ture by bond. In the case of exter-nal tendons, the behaviour of an-chor heads is much more critical.

• At ultimate limit states, failurewith little warning due to insuf-

ficient ductility is a major concernfor externally prestressed struc-tures.

VII. FUTURE DEVELOPMENT OFEXTERNAL PRESTRESSING

The use of Fibre Reinforced Polymers(FRP) as prestressing material has a lotof advantages over conventional pre-stressing materials such as the corro-sion of prestressing steel is one of themajor reason for adopting FRP as pre-stressing material. However the useof FRP materials can be limited dueto some drawbacks such as relaxationlosses. These need to be further inves-tigated.

VIII. CONCLUSION

External Prestressing has been widely used in construction of new bridges aswell as strengthening of existing bridges. Use external prestressing has resultedin construction of several innovative bridges with large eccentricities and lightweight concrete.

While this technology is widely used in construction of bridges, there is needfor further research especially in the use of alternate prestressing material.

References

1) Prof. Yogesh Ravindra Suryawanshi, Ms. Manisha Bhise "Study ofParameters Which Affect the Strength of Concrete by External Prestressing"2) HÃekan Nordin "Strengthening structures with externally prestressedtendons"3) Thiru Aravinthan "Overveiw of External Post-Tensioning in Bridges"

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