Introduction to Pre Stressed Concrete (2nd Ed.)

Foreword Prestressed concrete is widely used today in many branches of civil engineering and building construction. Elegant and spectacular structures have been created to bridge across rivers and ravines or roof over large areas indeed the history of prestressing is rich with innovation and inspired design by brilliant engineers. Other, more mundane, applications include the manufacture of everyday structural items such as railway sleepers, piles, lintels, beams and flooring units. This publication is intended for those readers requiring an introduction to prestressed concrete, but not wishing to get involved with the complicated mathematical treatment often found in textbooks on the subject. In the following pages prestressed concrete is introduced by words and pictures in a way that should appeal to the student and the practising architect or engineer who needs a basic understanding of the concepts and technique. However, the use of some technical terms is inevitable and some design aspects are also considered. This publication is based on the text of an earlier edition written by A H Allen of the Cement and Concrete Association in 1981. This 2002 edition has been updated and re-written by Tony Threlfall, formerly of the C&CA (now the BCA) and currently an independent training consultant, specialising in the design of concrete structures. 47.022 First published 1981 Reprinted with amendments 1986, 1992 Second edition 2002 ISBN 0 7210 1586 7 Price group E OBritish Cement Association 2002 Published by the British Cement Association Century House, Telford Avenue Crowthorne, Berkshire RG45 6YS Telephone (01344) 762676 Fax (01344) 761214 www.bca.0rg.uk www.concretebookshop.com All advice or information from the British Cement Association is intended for those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted. Readers should note that all BCA publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version. Contents Basic concepts................................... What is prestressing? ......................................... Why prestress concrete? ..................................... Benefits of prestressing ...................................... Mechanics of force system .................................. 2 2 2 2 3 Methods of prestressing ..................... Pre-tensioning .................................................. Post-tensioning ................................................. 4 4 6 Materials ........................................... Concrete .......................................................... Tendons ........................................................... 8 8 9 Equipment........................................ Pre-tensioning ................................................ . . Post-tensioning ............................................... Strand systems ............................................................. 12 12 13 13 15 15 Bar systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications ..................................... Pre-tensioning ................................................ Railway sleepers Piles and pylons ............................................................ ............................................................ 16 16 16 16 17 18 19 Bridge beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flooring and roofing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Handling of units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Post-tensioning ............................................... Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bridges ........................................................................ ........................................................ 19 19 21 22 Containment vessels Appendix.......................................... 23 Brief history and development ............................ 23 Design considerations ...................................... 24 Further reading ................................................ 25 Addresses of companies and organisations ........... 26 Acknowledgments ........................................... 27 An introduction to prestressed concrete 1 I L I cost. steel reinforcing bars are provided in the regions where tension occurs. depending on the magnitude of the prestressing force. f/m/kN Benefits of prestressing 1 Tension and cracking under service loads may be avoided or reduced to a low level. Each book is a discrete element but. Row of books lifted as a single unit Prestressing is best explained by considering a row of books. Downward deflections of beams and slabs under service loads may be avoided or greatly reduced.Basic concepts What is prestressing? fFigure 1. The bicycle wheel (steel rim held in compression by spokes in tension). in this case a row of books. Crack width is a critical design criterion with regard to the appearance. road and rail traffic) is considerably enhanced. if they are stacked closely together and an axial compressive force is applied at each end of the stack. Cracking considerably reduces beam stiffness and increases deflection. In reinforced concrete construction. for instance. Reinforced concrete beam with cracks in tension zone Embed within the concrete reinforcement that is strong in tension reinforced concrete. It provides the unit. steel tendons are stretched and anchored at each end so that compressive forces are applied to the concrete (Figure 3). depending on the method of prestressing. 1 1 Concrete Flexure cracks In the case of concrete. as in the row of books. Fatigue resistance (i. to form beams and similar members. the strength of reinforcement is limited typically to 500 N/mm2. there are two ways of overcoming this deficiency: 1 Reinforcing bars Figure 2. durability and water-tightness of the structure. Since crack widths increase as the reinforcement stress increases.---- I I \ \ \ \ Concrete is a building material that is strong in compression but relatively weak in tension. In prestressed concrete construction. 1 1 500 1000 1500 2000 1 Steel strength. Very high strength steel may be used to form the tendons (Figure 4). due mainly to the capacity to reduce deflection. Relative unit cost @er unit force) of reinforcing and prestressing steels 1 1 2 (An introduction to prestressed concrete . Other forms of prestressed items in everyday use include: 1 The barrel and the cartwheel (timber segments held in compression by iron bands in tension). become a practical reality. the ability to resist the effect of repeated live loading due to. I Anchorage Anchorage I Reinforced concrete members are designed on the assumption that cracks will occur under the design service loads.e. to form cylindrical tanks and silos. Why prestress concrete? I / I / \ 1 / !I /. prestressing is used as in the row of books. The forces are transmitted from the tendons to the concrete either by the bond created between the concrete and the tendons or by embedded anchorages. N/rnmz Figure 4. This is prestressing in its simplest form. Segmental forms of construction. but they pass through the entire wall thickness in cylindrical tanks and silos. In principle. The umbrella (membrane held in tension by ribs in compression). In beams and similar members cracks form only on the tension side of the member. Beam and slab sections may be smaller than in reinforced concrete. and as in the barrel. compensating for the low tensile strength of the concrete (Figure 2). - 1 Avoid the tension altogether by (a) arching the structure or (b) prestressing the concrete. with a strength and stability that it would not othewise possess. it is possible to lift the whole row as a single unit (Figure 1). If the prestressing force is located close to the bottom of the unit.. of length L. Further advantage may be gained if the prestressing force can be either deflected (Figure 9) or draped (Figure 101. it is advantageous to locate the prestressing force towards the bottom of the unit. since 9 is normally small. the system of forces becomes unstable. Consider the row of books. Beam with central point load and deflected tendons Figure 10. If tension is to be avoided entirely in a rectangular section. Cracking at bottom of unit due to apex of force diagram rising above the kern 0. is called the kern of the section. the line of thrust forms a parabolic cuwe with the apex at mid-span. In reality. In this case.5W are produced at each end. If the unit.' - . Similarly.5W L 1 0 5W Figure 9. tension develops at the bottom of the unit within the middle portion (Figure 8). Pa = 0. reactions of 0. rather than Pcos9. is supported at the ends and a vertical load W is applied at the mid-span. This region.. a friction develops between each book that enables the complete row to act as a single unit. which varies with the shape of the cross-section. The load-carrying capacity of the unit and the location of the prestressing force are limited by the compressive stresses in the concrete and also by tension and cracking considerations. Line of thrust 0. In this way. which is permanent.. increasing the load. When a compressive force P is applied at each end of the row. It can be seen that the additional load-carrying capacity of the unit is provided by the vertical reaction of the prestressing force on the concrete. For a given prestressing force. If the vertical load is uniformly distributed. the height of the force diagram.5W x 0. Although the effect of the applied load is to negate the tension within the middle portion of the unit. Prestressed unit with central point load 1111 # .5W x (0. It should be noted that the horizontal component of the prestressing force has been taken as P. in order to mirror the line of thrust.-..5W L 0. a substantial part of the load is due to the self-weight of the unit.-_--. the force diagram should be kept within the middle-third of the section depth. W. If the apex of the force diagram falls outside the unit.25L) and hence W = 8Pa/L (Figure 6). dependent on both P and W. tension zones will remain at each end (Figure 7). extend from each support and intersect on the line of action of W. tension develops at the top of the unit throughout its length. If all of the load were to be removed. Resulting lines of thrust. the load-carrying capacity of the unit may be enhanced without developing tension in the concrete at the ends of the unit.5L and hence W = 4Pa/L (Figure 5). at height 'a' above the level of the prestressing force. causes a corresponding increase in a.5W 44 Line of 'thrust L If 1 5 Figure Z Cracking at top of unit due to prestressing force being located below the kern 1 1 1 Figure 8.5W L Figure 5. if the magnitude of the applied load causes the apex of the force diagram to rise close to the top of the unit. Beam with uniformly distributed load and draped tendons An introduction to prestressed concrete 3 I . gives Pa = 0. the mechanics of the system must be investigated. Taking moments about the point of intersection. Clearly.Mechanics of force system In order to understand how the magnitude and position of the prestressing force affects the strength and stability of a beam. then tension zones would develop in the top of the unit at mid-span..5L-0.5L W I 2PsinWL per unit length 0.a " 1 " 'f " 1 -* 1 ' b**I 0.. intermediate abutments with preformed pockets to receive temporary steel joists may be provided. The resulting prestressed concrete members are also frequently described as being either pre-tensioned or post-tensioned. For items such as bridge beams. are tensioned between anchor plates placed at opposite ends of a long stressing bed. generally 7-wire strands. . Pre-tensioning may be used on site where large numbers of similar precast units are required. but the most effective use of pre-tensioning is in long-line production. stop-end Temporary struts plates are provided at the ends of each unit. 1. I I I I I I I + I+ Elastic shorleningof member at transfer of prestress Figure 12. The concrete is either cast in moulds or formed by an extrusion or slip-form process to provide the required cross-section.Tendons are tensioned and anchored I I Prestressing tendons may be tensioned before the concrete is placed @re-tensioned) or after the concrete has hardened (post-tensioned).Concrete is placed I 1 . so that a shorter stressing bed can be created should the need arise. Prestressing using pre-tensioned tendons . At the other end. the jacking end. At the other end.I Steel ioists Fixed abutment::. a number of similar units are produced in line at the same time. The extension of the tendon is checked against a calculated value.transferred toreleased and force is concrete I I Here the tendons are tensioned and anchored between fixed supports before the concrete is placed around the tendons. the tendons are slowly released from the support at one end (Figure 11). as the tendons outside the concrete revert to their original untensioned condition (Figure 12). The transfer of force occurs over a short transmission length at each end of the concrete. Tendons. I L I I Zero stress in tendon after transfer Transmission length 40 to 60 diameters for strand -4 b Maximum stress in tendons Long-line production In this case. When the concrete has achieved sufficient strength.:I . Transmission zone at end of member Positioning the tendons At one end the anchor plate bears directly against the supporting joists. where the concrete is placed within moulds.:::: end typical arrangement for long-line production is shown in Figure 13. A stressing jack is attached to the tendon. These plates contain standard patterns of holes to accommodate the particular layout :: : : J a c k i n g of tendons required for each unit. threaded through the holes in the stop-ends and anchor plates and anchored-off at the fixed abutment. the slack taken up and the load applied. The prestressing force is transferred from the tendons to the concrete by the bond existing between the hardened concrete and the tendons. :: I: Figure 13. The anchor plates bear against steel joists embedded in concrete abutments. The base to the casting surface may sometimes act as a strut between the abutments but. temporary steel struts are introduced between the anchor plate and the joists. each one is tensioned. Pre-tensioning I Tendons are IStage 3 .Methods of prestressing IStage I Stage 2 . The elastic shortening of the concrete that occurs at this stage causes a corresponding reduction of the tendon force. Typical arrangement for long-line production Stressing the tendons Tendons are drawn down the full length of the stressing bed. the abutments are sufficiently massive to be independently stable. but is usually carried out in a factory where permanent stressing beds have been installed. the tendon is anchored-off and the jack 4 1 An introduction to prestressed concrete . The anchor plates are large thick steel plates with holes through which the tendons can be passed and anchored. I I Figure 1 1. A . against which the tendons are tensioned and anchored until the forces can be transferred to the concrete. in most cases. In very long stressing beds. Single units and units cast side-by-side may be produced in rigid steel moulds. when all the tendons have been positioned. typically after 8 to 16 hours. large jacks between the anchor plate and the joists form an integral part of the system and are used to both tension and release all the tendons simultaneously. the reinforcement can be spaced out and fixed in its final position. the stressing operation needs to be carefully controlled Fixing additional reinforcement If reinforcement is required within the transmission zones and for shear purposes. The end plates are removed and the projecting lengths of tendon cut back to the end faces of the units. Although most pre-tensioned units are made in this way. After 8. the tendons are arranged in several layers with multiple tendons in each layer. the arrangement does not provide the most efficient use of the prestressing force in members of constant cross-section. the hardening process is often accelerated by raising the temperature of the concrete. and the eccentricity and magnitude of the prestressing force are progressively reduced towards the ends of the unit by deflecting and/or debonding some of the tendons. Once the tendons have been stressed. The tendons can now be cut. An introduction to prestressed concrete 5 I . using an abrasive disc cutter. Altering the prestress along the length of the unit In the arrangements considered so far. the amount necessary for each unit will have been grouped near the stop-ends when the tendons were being drawn down the stressing bed. moulds are not needed and the concrete can be simultaneously placed and compacted. the tendons have all been straight and bonded to the concrete for their entire length. Care needs to be taken to ensure that the tendons are kept free of any material. In order to obtain a high concrete strength at an early age. that would impair the bond with the concrete. where additional reinforcement is not required. Long-line production of prestressed hollowcore flooring. After releasing the tendons the concrete strip is cut into lengths. The operation is then repeated on the remaining tendons. in the gaps between the units. In some prestressing arrangements. as shown earlier in Figure Z In large units. The location of the prestressing force is limited by the conditions that can be permitted at the ends of the member. as with all prestressing. Internal vibrators. while hollowcore slabs (see later) can be either slip-formed (Figure 141 or extruded. to form separate units. using a circular saw. this enables a more rapid turn-round in production.16 hours the tendons will be released and the concrete cut into lengths of 3 in to 20 in. Releasing the tendons When the concrete has attained sufficient strength. I I I I Compacting and curing the concrete Vibrators are used to achieve full compaction of the concrete.is released. At this stage a small pull-in of the tendons occurs at the ends of each unit. solid slabs and lintels can be slip-formed. proper curing is essential. a smaller force can be used if the eccentricity of the force can be increased within the central portion of the span without exceeding the critical value at the ends. Figure 14. can result in small pockets of water adjacent to the tendons that will reduce the effective bond. where self-weight is significant. As the tendons attempt to return to their original length. The moulds can then be assembled in preparation for concreting. provided the units are free to slide back along the bed. As with all concrete. such as mould oil. the temporary struts at the jacking end of the bed are replaced by large jacks that can be slowly released. Small beams. Typically. the concrete in each unit is put into compression. Extrusion and slip-forming processes For many proprietary items. if badly handled. External vibrators are generally more effective provided there are enough of them and the moulds are sufficiently rigid. The stressing sequence is not important in pre-tensioning but. . which are contained within ducts or sheaths to prevent them from bonding to the concrete. a steel tube with projecting lugs to deflect the tendons is bolted to the base of the bed. Figure 15. after the concrete has attained sufficient strength. by varying the lengths of tubing.. . In all cases.. Post-tensioning may be used in the factory production of single special-purpose precast units.Concrete cast with tendons in duct Protection of tendons An important consideration is the long-term protection of the tendons against corrosion. the tendons are made secure at the anchorages (Figure 18). Alternatively..:: . Plastic tubing Tendon layers 1 2 3 4 Number of tendon layers effective of fubing IS placed around all tendons in each layer) Figure 1Z vpical arrangement for debonding pre-tensioned tendons . the bolts are removed before releasing the tendons. before the side-moulds are put in place.Tendons secured at anchorages Stage 2 . but there is great benefit in being able to assemble and prestress the structure in stages. external tendons may be placed directly against the surface of the concrete. and the ultimate strength.. After the concrete has gained sufficient strength. Production of SY bridge beams incorporating deflected tendons Post. The particular form has no significant effect upon the behaviour of the member under normal loading conditions.. is shown in Figure 16. Tendons may be contained within the sleeves before the concrete is placed or may be threaded through the ducts after the concrete has hardened. Ducts to contain groups of tendons can be formed within the concrete. can be arranged to follow the curvature of the structure and provide the most appropriate profile to suit the applied loading. are tensioned after the concrete has hardened. and then pushed down from above at the hold-down points. as shown in Figure 17.. Alternatively. but does affect the nature of the cracking that would occur in the event of over-loading. as will be shown later. after tensioning. and holding up the tendons within the gaps between units and at the ends of the line. . which may be located inside or outside the concrete.. the tendons are tensioned using jacks that bear upon the exposed face of cast-in anchorages at the ends of the tendon. Two different methods are used producing either bonded or un-bonded forms of construction. vpical arrangement for deflecting pre-tensioned tendons The tendons are usually tensioned before being deflected. A typical set-up for a bridge beam. In this way.I I. but is usually associated with construction on site where the concrete is formed entirely in situ or by assembling a series of precast segments. They may be held in the upper part of the section during tensioning. they may be held in the bottom part of the section during tensioning and then hoisted up from above at the hold-up points In this case.- - :: . t + ::. . Un-bonded forms of prestressed construction are provided with some ordinary reinforcement to ensure that the post-cracking behaviour and the ultimate strength are satisfactory. either by using removable void formers or by casting in steel or plastic sleeves. the ducts are injected with a cement grout to provide a bonded construction (Figure 19)..tensioning Here the tendons. tendons may be individually encased in plastic tubing. :: n\ 1 t n\ R 1 % t ji jj Deflecting typically involves holding down the tendons at two symmetrically placed positions within each unit. both the magnitude and the eccentricity of the prestressing force may be adjusted in steps. After tensioning.e . Internal tendons. Debonding is a more straightforward procedure. as shown in Figure 15. having been first coated with protective grease (Figure 20).at specific positions.. Structures may be fully formed or assembled before any prestressing is carried out. the transmission lengths for the encased tendons begin at the end of the tubing and. When the required force has been attained. Figure 16. . In cylindrical structures. in which specified lengths of plastic tubing are placed around several tendons in different layers.:: . These tendons are then fixed in position before the concrete is placed and. remain Stage 3 . at the hold-down points. 3 r g e1 .Tendons tensioned after concrete has hardened Figure 18. so that no bond can develop between the tendons and the concrete.. Prestressing using post-tensioned internal tendons 6 IAn introduction to prestressed concrete . External tendons can be arranged as a series of straight lengths with changes of direction. It is very important that the concrete in these regions is thoroughly compacted. which results in very low tendon friction during stressing. I Plastic sheath I Strand Figure 20. Figure 21. allowance having been made for the effects of friction between the tendon and the surface of the duct during the tensioning process. Anchorages apply large concentrated compressive forces to the concrete with an effect like that of driving a wedge into a block of wood. if necessary. accelerated curing is unnecessary as the age of the concrete at transfer of prestress is typically between 3 days and 28 days. the end-block containing the anchorages is sometimes produced as a precast item in advance of the main structure. external tendons may be used to form a series of separate rings or a continuous band encircling the outside of the structure. the tensioning sequence specified by the designer must be followed in order to avoid over-stressing the concrete at any stage. The tendon profile takes the form of a series of straight lengths with deviating saddles provided at intermediate diaphragms (Figure 21). according to the level of prestress to be applied. The elastic shortening of the concrete that occurs during tensioning has no effect on the force in the tendons being stressed. unlike pre-tensioning. Un-bonded tendons are used in structures where the design requires the tendons to be examined at intervals and. Each one involves the use of anchorages that remain part of the finished structure. The resulting bursting forces in the concrete are resisted by concentrations of reinforcement in the regions containing the anchorages. However. Typical arrangement for external post-tensioned tendons in bridge construction Anchorage zones There are several different post-tensioning systems.un-bonded to the concrete. ducts and reinforcement. It is important that the concrete is properly cured but. Sheathed-strand used in un-bonded construction External tendons In major bridge construction. there is a progressive reduction of force in any tendons that have been previously anchored. IS particularly useful in thin sections such as floor slabs and the walls of cylindrical structures. When there are several tendons or groups of tendons at different positions within the cross-section of a member. in which case remedial action will need to be taken. and the jacking load may need to be adjusted. The system. Stressing anchorage Dead-end anbhorage Figure 19. sleeves and internal sheathed tendons must be fixed securely so that they are not displaced during concreting. depending on the system and the type of jack being used. external tendons contained within sheaths can be installed in the voids of hollow box sections. Protection against corrosion and accidental damage is provided by a spray application of a cement-based mortar. An irregularity in the observed extension during tensioning may indicate that a duct has developed a blockage. as well as recording the jacking load. An introduction to prestressed concrete I7 . The extension is checked against a calculated value. Permanent corrosion preventing grease Measurements during tensioning It is important to verify the extension of the tendon by recording the movement of the jack. Any significant difference between the observed and calculated extensions may mean that the effects of friction are not as assumed. "-Grout tu be All duct-formers. Grouttube Vent . removed and replaced. as will be shown later. In order to make concreting easier. despite the congestion caused by the anchorages. Stressing anchorage Tendon in sheathing Stressing anchorage In cylindrical tanks and silos. Typical arrangement for internal post-tensioned tendons in bonded construction Tensioning Tendons may be tensioned one at a time or in groups. with each stressing operation. samples are taken from the fresh concrete. during and after the transfer of prestress.Materials It is important to consider the physical properties of both the concrete and the tendons in order to understand the effect that each has on the other.g. In order to check the concrete strength at transfer. This can be achieved either by introducing saturated steam into an enclosure containing the units or by circulating hot water in pipes embedded in the stressing bed. I Strength Concrete needs to be workable when fresh. particularly with regard to the losses of prestress that occur at various stages. will be examined in more detail later. With pre-tensioning. is enhanced by good compaction and by reducing the watedcement ratio. usually in the form of cubes. one using CEM I42.40 N/mm’ at transfer and 50 . since a rapid turn-round is vital to the success of the production process. Concrete at normal temperatures could take several days to develop the required strength for transfer. The required concrete strengths depend on the type of unit and the level of prestress applied. which are subjected to the same curing conditions as the concrete units. but cylinders may also be used. The sources and magnitude of the losses.5 is used. the cube strengths are typically 28 . The strength of hardened concrete. e. The stiffening and hardening is due to a chemical reaction between the cement and water in the mix. the in-situ concrete strength can be measured by pull-out tests or by using a calibrated rebound (Schmidt) hammer.5or higher strength class cement. It is sufficient here to mention particular factors that are important in relation to prestressing. it is important to be able to achieve a high concrete strength at early age so that the prestressed units can be lifted from the bed as soon as possible. Grades or strength classes of concrete are selected in accordance with the recommendations of codes of practice and standards such as BS 5328 and BS EN 206-1/8500. namely the strength of the concrete and the deformations that occur before.60 N/mm’ at 28 days. due to friction during the tensioning of the tendons and elastic shortening of the member during the transfer of prestress to the concrete. are placed in a testing machine and the crushing strength is recorded. are made in steel moulds. is normally critical. The specification of cement and combination types should be based on durability considerations and the producer will select from these the one that is most cost-effective for their process. when the prestress is applied. The condition at transfer. For standard bridge beams. The cubes. The workability (consistence) of fresh concrete is enhanced by good aggregate grading and by using admixtures. In practice a rapidhardening cement such as CEM 152. the values are typically 40 N/mmz at transfer and 60 N/mm2 at 28 days. Concrete The selection of suitable materials and the specification of concrete with regard to durability and resistance to chemical attack are adequately dealt with elsewhere (see Appendix for further reading). Other losses occur over an extended period of time. Alternatively. due to steel relaxation and creep and shrinkage of the concrete. . but the process can be accelerated so that the strength is reached in 8 to 16 hours. and strong when it has hardened. which increases with age. i I 8 An introduction to prestressed concrete I . Some losses occur immediately. For flooring units. and requires concrete with a high early strength. with thorough vibration of the fresh concrete. Test specimens. which differ for pre-tensioning and post-tensioning. . The resulting loss of prestress that occurs in the tendons depends on the age of the concrete at transfer. the thickness of the section and the relative humidity of the environment. the wire has a natural curvature approximately equivalent to the capstan of the drawing machine. These changes are in addition to the shrinkage caused by changes in moisture content. Wire Cold-drawn wire is produced in coil form from hot-rolled rod which is heat treated to make it suitable for cold drawing. and is greater with pre-tensioning than with post-tensioning. the relative humidity of the environment and the maturity of the concrete at transfer of prestress. The shrinkage develops rapidly at first and continues at a reducing rate for many years. Concrete shrinks over time by an amount that varies with the initial water content of the mix.Elastic deformation Figure 22. where the level of prestress is low and transfer takes place at 3 to 7 days. The stress-relieving treatment pre-straightens the wire. Figure 2 . When the tendons are stressed sequentially. and all the stress is applied to the concrete at the same time. i I Strain 1 _ _ . Any shortening of the concrete that occurs after the tendons have been tensioned and anchored causes a loss of prestress that must be allowed for in the design of the member. Wire is used in factory-produced items such as lintels and small flooring units._ _ . The creep value depends upon the thickness of the section. 3 seven-wire strand. As a result.. Dpidag bar and Macalloy bar An introduction to prestressed c n o- 9 . Several tendons may be arranged in a group with a common anchorage to form a cable (Figure 23). Typical curve of elastic and creep deformation for concrete Tendons Prestressing tendons are usually formed from high tensile steel wires or alloy steel bars. the age at transfer is less critical and accelerated curing is normally not necessary. where transfer usually occurs at a later stage. For other forms of construction.. In post-tensioning. since the tendons are already anchored by bond. drawn strand. . it undergoes dimensional changes: an immediate elastic deformation followed by a time-related creep deformation (Figure 22).. and enhances its elastic and relaxation characteristics.With post-tensioning. A final stress-relieving heat treatment to improve some of the mechanical properties of the wire is carried out before it is wound into large diameter coils. cube strengths are typically 25 N/mm2 at transfer and 40 N/mmz at 28 days. The total loss is then intermediate between nil and half the value that occurs in pre-tensioning.. there is no loss if all the tendons are stressed at the same time. Types of tendon (from the top): wire. creep develops rapidly at first and continues at a decreasing rate for many years. so that it will pay out straight from the coil. the loss of prestress that occurs in the tendons is greater with pre-tensioning than with post-tensioning. The wire surface is initially smooth but may be indented by a subsequent mechanical process. the cube strengths at 28 days are typically 40 N/mm’ for cylindrical structures and 50 N/mm2 for bridges. Like shrinkage.. Concrete under applied stress also undergoes an inelastic creep deformation. Wire to be used for pre-tensioning is supplied in a de-greased condition and is often indented to ensure that the maximum bond is obtained between steel and concrete. The wires can be used singly or twisted together to form strand (usually of seven wires). 1 For floors in buildings. a progressive loss occurs in any tendons that are already anchored. The loss of prestress due to the elastic deformation of the concrete that occurs at transfer is greatest in pre-tensioning. I I Loss of prestress When stress is applied to concrete. cable of seven strands. since the elastic deformation takes place before the tendons are anchored. In the as-drawn condition. Standards are set in terms of maximum values. flooring and terracing. the 1000-hour value is multiplied by a factor to allow for the long-term relaxation. Figure 24.Strand Strand is made from cold-drawn wires: a seven-wire strand consisting of a straight core wire (the king wire) around which are spun six helical wires in one layer. with a coarse thread extending over the full length of the bar. Macalloy bars are produced from hot-rolled carbon-chrome steel bars that are then cold-worked by stretching to obtain the specified properties. and can be joined together by threaded couplers to obtain longer tendon lengths. there are three types of sevenwire strand: standard. The values are based on test conditions of constant strain and a temperature of 20°C. and for post-tensioning in all forms of in-situ and segmental construction. after a period of 1000 hours. and (b) drawn strand Standard and super strands are visibly similar. sign gantries. Greased and plastic-coated strand is produced for designs involving the use of un-bonded or external tendons. Dywidag threadbars are produced to a German Standard specification in diameters between 20 mm and 40 mm. Bar There are two types of bar in common use: 1 . bridge beams. Relaxation values for both wire and strand depend on the way in which the final stress-relieving treatment is carried out and are defined in BS 5896 as class 1 (normal) or class 2 (low). or over the full length if needed. 70% and 80% of the breaking load. followed by coating with a special corrosion-resistant grease and a continuous hot extrusion of a high density polypropylene or polyethylene sheath. The diameters of the outer wires have to be slightly less than that of the king wire to allow for their helical form. Relaxation When tensioned. like creep in concrete. The bars may be cut to finished length at the factory or on site and couplers can be used to connect or extend bars as required. Strand can be supplied with the outer wires having either a left-hand or a right-hand twist and the stressing jacks need to be adjusted accordingly. develops rapidly at first and continues at a decreasing rate for many years. a steel tendon undergoes a relaxation of stress over time that is proportional to the initial load applied. All strands are given a final stress-relieving treatment in the same way as wire before being wound into coils. which penetrates to the centre wire of the strand. super and drawn (Figure 24). In practice. Strand is widely used in factory-produced items such as railway sleepers. Stainless steel bars are available in lengths up to 6 m for diameters between 20 mm and 40 mm. Cross-sections of (a) standard and super strand. but the outer wires of drawn strand are clearly flattened as a result of the strand having been drawn through a die. The bars are available in lengths up to 17. wire and strand are generally produced to the class 2 requirements. The strand is wound onto a wooden reel for extra protection during transit and in use. In practice. In BS 5896. Much higher values can occur at elevated temperatures.8 m for diameters between 25 mm and 50 mm. The process involves the application of a protective fluid. for the percentage reduction of load for initial loads of 600/0. Both types of bar are provided with cold-rolled threads at each end. Relaxation. 10 An introduction to prestressed concrete I . Table 1. Tendons should be stored clear of the ground and protected from the weather at all stages. 7-wire strands and bars Elongation. B: load at 7% elongation.1% proof load. 57 12. ljpical load-elongation culve for strand showing A: 0.7 Figure 25.4 78 139 17 9 59 158 225 178 255 323 410 670 1050 1639 BS 5896 Super strands to BS 5896 Drawn strands to a1 144 204 6 1 163 233 1 a4 264 334 12.2 8.3 12. I I An introduction to prestressed concrete 1 1 1 .lO/o of the gauge length. Care must be taken to prevent the tendons coming into contact with splashes of material from oxy-acetylene torch or arc-welding operations being carried out in the vicinity. which is defined as the load that produces a permanent elongation equal to O.0 54.7 1. and to avoid damage by mechanical means or heating while handling.6 38.5 15. C breaking load : 4 7 9. Dimensions and properties of wires.3 92 164 232 70 1 a6 18.Strength of tendons The strength of a prestressing tendon is specified in terms of characteristic load values for the breaking (or failure) load and the 0. The British (BS 5896 and BS 4486) and European (pr EN 10138) standards include a range of sizes and strengths for each type of tendon. the load at lO/o elongation may be used as an alternative to the proof load (Figure 25).5 53. Tendons should not be left unprotected inside ducts for prolonged periods of time.0 12.1% proof load.9 1.3 64. 80 2 5 32 40 50 265 209 300 380 506 a28 1295 2022 BS 5896 Bars to BS 4486 (Macallo)i 1860 1 a20 1700 1030 1030 1030 1030 Care of tendons Prestressing steel is very different to ordinary reinforcement and particular care must be taken to protect tendons against corrosion. 52 1. For wire and strand. % m Wires to BS 5896 Standard strands to @ i & m m 19. and failure to observe simple precautions in storage and handling has led to unfortunate consequences. a selection of which is shown in Table 1.5 52 93 139 38 100 150 112 165 223 491 a04 1257 1963 17 70 1670 17 70 17 70 1670 1 a60 1 a60 17 70 22. All tendons are eventually highly stressed. the wires or strands are held by temporary grips during and after tensioning. the wedge is forced onto the tendon and the jack is released. where grips are forced onto the unstressed tendons close to the anchor plate. which have grooves on the surface in contact with the tendon. Multi-stressing uses the same wedge anchor approach. For tendons that are stressed individually. when individual tendons are being stressed. CCL Stressomatic jacks and pump I 12 An introduction to prestressed concrete I . Open grip for pre-tensioning Figure 27. Once the controls have been set to pre-determined values. When the required load and tendon extension have been reached. will be used many times. The wedges. to keep the pieces in the correct position. or ‘0’ ring. A popular jack for this purpose is shown in Figure 29. Spring-loaded anchors are often used to apply a consistent force and retain the anchor in position when the tendons are being handled (Figure 27). The method of tensioning may vary but in all cases the grip consists of a barrel and wedge (Figure 26). with each segment driven the same distance into the barrel. a relatively small power-operated jack is used to enable stressing to be carried out quickly and efficiently. The jack is attached to the tendon and stressing begins. These anchors enable two lengths of wire or strand to be joined together and so avoid wastage (Figure 28). Double-ended anchor for pre-tensioning figure 26. It is important that the wedge is fixed so that the wire or strand is in a central position. the wedge is drawn further into the barrel and the tendon is firmly gripped. with the tendon being pulled through the grip. When the full length of the stressing bed is not being used. With conventional stressing. double-ended anchors are introduced between the abutment and the last concrete unit. Spring-loaded anchor for pre-tensioning Figure 29. Figure 28. In both cases. and they need to be carefully cleaned and examined before each use to ensure that they are not worn or damaged. the stressing and anchoring operations are carried out automatically. Stressing is carried out either by extending the tendons one at a time. The wedge is generally in two or three pieces with a collar and wire clip.Equipment Pre-tensioning With pre-tensioning. the process starts at the non-jacking end. where all the tendons are extended at the same time. a grip is then placed onto the unstressed tendon close to the anchor plate at the jacking end. or by multi-stressing. As the tendon attempts to pull back. CCL multi-strand stressing anchorage and jack Strand systems Suppliers include CCL Stressing Systems. for full details of the full product range. with each one anchored against a steel plate or provided with a bulbous shaped end that resists pull-out. They each provide a range of anchorages. Three companies use strand systems and two companies use bar systems. to meet the changing needs of design and construction. Couplers and intermediate anchorages are also available to meet specific design and construction requirements. all contained within a circular duct and tensioned by a large jack in a single stressing operation. Grout tube Figure 32.Post-t ensioning A large number of systems have been developed and used throughout the world over the past 60 years. the tendons can be anchored directly into the concrete. In this case. Freyssinet multi-strand stressing anchorage (above) and coupler (below) - 11495 - 140 The tendons are generally installed by using a machine that pushes each strand into the duct from one end. ducts. A selection of frequently used strand groupings with the corresponding ultimate load capacities and nominal internal duct sizes is shown in Table 2. jacks and pumps. The information is indicative only and many other tendon groupings can also be provided. each strand is gripped by the jaws of a wedge that is forced into a tapered hole in the anchorage block. couplers and equipment for installing. Figure 30.a selection is shown in Figures 30 to 34. replacement parts and calibration service accessories. At the stressing (or live) end. Table 2. VSL multi-strand dead-end anchorages An introduction to prestressed concrete 13 I . the duct is terminated before the end of the member and the strands are fanned out. Multi-strand systems are available that can accommodate up to 55 strands. Ultimate load capacities of multi-strand cables Grout inlet Guide J ---__ 3 7 12 19 27 37 55 492 1148 1968' 3116 4428 6068 9020 696 1624 2784 4408 6264 8584 12760 558 1302 2232 3534 5022 6882 10230 795 1855 3180 5035 7155 9805 14575 627 1463 2508 397 1 5643 7733 900 2100 3600 5700 8100 11100 16500 35 50 65 80 100 120 40 60 80 100 120 140 160 Secondaw tendon /A Figure 3 1. Each system has its own range of anchorages. The main aspects of the systems in current use and available in the UK will be briefly described but. Each company has been at the forefront of development. stressing and grouting the tendons . Freyssinet and VSL Systems (UK). reference should be made to the particular company concerned (see Appendix). The same type of anchorage can also be used at the non-stressing (or dead) end but when the tendons are installed before the member is concreted. for many years. as well as servicing. The live-end anchorage comprises a metal casting. which need to be hung from a lifting frame. for easy alignment with the tendons. which are installed either singly or in bundles of two. three oifour. contained in one layer within a flat duct. The inner workings of the jack can also be rotated. Multi-strand jacking I1 I ii Figure 35. right. Bonded slab systems incorporate groups of three. which may be of metal or plastic. The flat duct. _ _ ~ -. Figure 36. freyssinet strand-pushing and jacking in operation 3 The stressing jacks. the strand is cropped. Freyssinet anchorages for un-bonded tendon slabs Left. Freyssinet anchorages for bonded slab tendons.). A selection of anchorages and equipment for both slab systems is shown in Figures 35 to 38. are compact. and the strand end and wedges are sealed with a grease-filled plastic cap. Bonded slab construction. which is secured against the metal casting to prevent displacement during concreting. dead end Un-bonded slab systems incorporate 15 mm nominal diameter strands. four or five strands. The jacks can be operated in a vertical or horizontal position and provide automatic de-wedging and hydraulic locking-off of jaws. Left. easy to operate and highly manoeuvrable. enables all the strands to be positioned close to the surface so as obtain the maximum eccentricity within the slab. are individuallytensioned using a mono-strand stressing jack. with tendons shown emerging from sheaths. The body can be rotated with respect to the lifting points. right: dead end 14 introduction to prestressed concrete . I Figure 34. The strands. live (jacking) end. each of which has been coated with a corrosion-preventing grease and enclosed in an extruded plastic sheath. Note 7 the draped profile of the tendons.- - Fiaure 3 . live (jacking) end. The strands are individually tensioned by means of a hand-held stressing jack. The dead-end anchorage usually incorporates a factory-applied compression fitting to the strand. After stressing. 7 7 Stressing Figure 3 . enabling easy access to the hydraulic connections. wedges and plastic fittings to facilitate the fixing of the anchorage to the formwork and to form a stressing access pocket. to ensure correct seating of the wedges and minimise losses at transfer. of 13 mm or 15 mm nominal diameter. After stressing. the duct is filled with a cement grout. - Figure 38.I Placing of anchor head and wedges Lr Y Positioning of jack J I .Un-bonded slab construction. Otherwise. As a result. Ducts for internal tendons are made from corrugated steel or plastic sheathing. Although developed initially for use in prestressed concrete. if necessary. The ducts are either connected to. can tolerate small deviations in the direction of the bar. An electrically insulated system may be provided in structures for railways or light-rail transit systems. Electrically insulated tendons An introduction to prestressed concrete 15 I . In this case. Macalloy bars are normally supplied as smooth bars with rolled-on threads at each end. but they may be tensioned in a curved profile if the radius of curvature is not less than 200 bar diameters. there is little or no movement of the bar or loss of prestress at transfer. but groups of up to four bars may be contained within a single duct. these are usually formed by drilling through the sheathing and attaching a plastic vent tube. bars may be de-tensioned for removal. where a fully encapsulated system is required. Ducts for external tendons are normally made from high-density polyethylene. and to compensate for any losses of prestress prior to grouting. and the duct left unfilled. Macalloy bar anchorage Figure 40. Insulation plate plastic plug Figure 4 7. have to be flexible enough to follow the required profile yet strong enough to keep their shape during threading of the tendons and/or concreting. including ground and rock anchorages. In addition. The prestressing force is transferred to the concrete by means of a threaded nut bearing directly against a steel distribution plate. Internal ducts can also be created by means of re-useable pneumatic rubber tube void formers. Figure 39. which may also be used with a recessed bell anchorage. load cells are available to give an independent check on the accuracy of the pump gauge. Hydraulic jacks are provided with gauges calibrated against a certified load cell to register the force applied to the bar. the tubes are deflated and withdrawn to leave circular voids. high tensile alloy steel bars are also used for many other structural applications. The bars are generally supplied straight. This is particularly important when short tendons are required. which locates into a cone shaped recess in a surface-mounted distribution plate. grout and air vents have to be provided at intervals along the length of the duct. In bridges and other structures in very severe environments. if bare strands are used. the duct is injected with petroleum wax. The bars are separately tensioned. External tendons in major bridges are normally left un-bonded so that they can be checked after a number of years and. plastic sheaths complete with caps to enclose both live and dead-end anchorages are provided. pre-greased and plastic coated strands may be used. removed and replaced. After the concrete has hardened. Some examples of ducts and duct assemblies are shown in Figures 41 and 42. The domed nut. Examples of corrugated steel and plastic ducts I HDPE liner I Plastic sleeve I Plastic or plastic coated cast iron grout cap Intermediate plate . where the direction of the duct needs to be changed.Bar systems Suppliers include McCalls Special Products and Dywidag Systems International. or continued through. For bonded forms of construction. Typical details of each bar system are shown in Figures 39 and 40. The prestressing force may be increased in stages to suit design or construction requirements. In designs where the prestress is required only temporarily during construction. The tubes are inflated and fixed in position before the concrete is poured around them. Dywidag anchorages Ducts Ducts which are normally circular or flat. Heat shrink sleeve or tape Figure 42. steel deviation pipes that are embedded in concrete at positions such as transverse diaphrams. Dywidag bars are continuously threaded and are anchored by means of a domed shaped nut. but fully threaded bars are also available. where stray currents may affect durability. and spinning the mould on revolving wheels as the concrete is placed. Long-line production methods may be used to form solid sections in conventional moulds or hollow sections by extrusion. High-strength spun concrete pylon prestressed with carbon-fibre reinforced plastic Piles and pylons Where the ground conditions are suitable for the use of precast concrete driven piles. it became essential to find an alternative to timber for railway sleepers and the manufacture of prestressed concrete sleepers was developed. are reduced by prestressing and the risk of cracking is minimised. The wall thickness of a 27 m high mast for an electric power line has been reduced from 100 mm for a traditional spun reinforced concrete pylon to 40 mm for the new design. A large number of wires was required in the original sleepers but a small number of 9 mm diameter strands. Concrete railway sleepers in use are shown in Figure 43. prestressed concrete offers several advantages over reinforced concrete. Standard items of excellent quality and durability. figure 44. In order to minimise site storage requirements and construction time. due to the pile rebounding elastically from the driving hammer. Hollow section spun piles may also be produced in individual moulds designed to resist the forces in the tendons during the casting and curing of the concrete. has made it possible to reduce the weight considerably. but the need to protect the steel reinforcement against corrosion with at least 30 mm of concrete cover means that the pylons are very heavy. Figure 43. using high-strength concrete and carbon-fibre reinforced plastic (CFRP) instead of steel. Figure 45. that would be impossible to precast on site within the space and time available. Pylons supporting electric cables are required to sustain comparatively light loads but may be subject to considerable bending and torsion effects due to wind and in the event of the breakage of one or more cables. is used now. made from indented wires to minimise the transmission length. Prestressed piles can be made longer than is practical with reinforced piles. such as large grandstand roof beams. both standard and special units can be produced and delivered to site to suit the construction programme Some examples of the most commonly produced items are discussed below. Bridge deck using inverted tee beams (showing alternative positions for sewices) 16 An introduction to prestressed concrete 1 . Spun concrete pylons have been used in some countries. which has an outer diameter varying from 850 mm at the base to 530 mm at the top. The centrifugal force compacts the concrete and forces out the excess water. Production lines can output up to 400 sleepers at a time on stressing beds that are about 135 m long. Factories set up specially to produce these units are highly mechanised with a low labour requirement. The mast. due to their greater ability to resist bending stresses during handling. Pre-tensioning is also used for specially designed elements. can be produced efficiently and economically. The tensile stresses that occur in the concrete. Pre-tension ing Pre-tensioning is best suited to the factory production of large numbers of similar precast units by the long-line method. Prestressed concrete is ideally suited to meet the need for resilience and high fatigue resistance with excellent durability in a hostile environment. is prestressed longitudinally by 5 mm diameter CFRP rods and reinforced circumferentially by a CFRP spiral tape (Figure 44). Some of the 35 million prestressed concrete railway sleepers used so far in the UK Railway sleepers During the war from 1939 to1945. tensioning the tendons.Applications The choice between pre-tensioning and post-tensioning is determined essentially by economic considerations and practical aspects such as the nature of the structure and the method of construction on site. ranging from small lintels to large bridge beams. Recent development work in Switzerland. The procedure consists of assembling the tendons and a reinforcing cage in steel moulds. Over the years. and continuity between spans is a requirement of the current UK highway standards. which have holes at regular centres through the bottom of the web. a further set of larger inverted tee beams was introduced to cover the span range 15 . were post-tensioned. were spaced out at 1. causing serious deterioration of the bearing plinths and supporting structures. . The box beams. The I-section beams. which were formed with holes at 3 m centres through the web. During the same period. The Y and SY beams are spaced at centres between 1 m and 2 m according to design requirements. sets of box beams and I-section beams. with in-situ concrete added to form a composite slab. In the 199Os. many bridges incorporating standard bridge beams have been constructed as a series of isolated spans separated by movement joints._ .. of various depths. but this option was soon discarded as being uneconomical in practice. During the 1960s. with permanent formwork placed between each leg to support an in-situ top slab. of various depths. Y beams used in continuous span construction An introduction to prestressed concrete 17 I . An in-situ reinforced concrete slab and transverse diaphragms were then formed to create a composite tee section. Several forms of span-to-span and deck-to-abutment connection have been used. U and Y beam sections are shown in Figures 46 and 47.side. Continuous decks generally ensure more durable structures. . the relative merits of which are outside the scope of this publication.30 m. where interruptions to traffic must be minimised The standardisation of beams in the UK began during a period of major road construction in the 1950s. UM beam formwork 160 m m r Permanent formwork U beam 1500 10 2000 mm centres Figure 46. these beams are especially useful when bridging over roads. and in-situ concrete is added between and over the tops of the beams to form a composite solid slab. railways and waterways. Ordinary reinforcing bars are threaded through the holes. The narrow gaps between the beams were filled with in-situ concrete and prestressing bars. . The beams. It was also possible to create a pseudo-box section. or spaced out at 1 m centres to support an in-situ concrete top slab.4 m centres. was introduced for the span range 7 .. These were spaced at centres between 1. were introduced for the span range 15 . by forming an in-situ reinforced concrete bottom slab as well as a top slab.5 m centres. As precast construction takes less time on site. Typical details of M. they have come to be called M beams. as shown in Figure 45. Too often. All of the standard bridge beams have proven long-term durability and are eminently suitable for continuous and integral bridge construction (Figure 48). with a transverse diaphragm formed at each end of the span. %.30 m. which were placed on the supports side by side at 1 m centres. Dimensions of 7 ! Y and SY beam l sections The TY beams may be either placed side-by.15 m. The box and the I-section beams were not widely used and alternative designs were sought. to support an in-situ top slab.5 m and 2 m. Eventually. which have a 750 mm wide bottom flange. are placed on the supports side by side at 508 mm centres. which are easier to manufacture and have extended the span range to meet the requirements of motorway widening schemes. threaded through ducts in the diaphragms. Cross-sections through bridge decks using M beams and U beams 240 mrn - 750 rnrn 750 rnm = Y for spans up to 32 m (simply supported) = SY for spans up to 40 m (simply supported). a set of U-section beams was also introduced. were provided with transverse diaphragms at 2. . the Prestressed Concrete Association introduced new sets of inverted tee beams. As the beams are usually placed side by side at 1 m centres. A set of inverted tee beams. Permanent formwork is placed between the tops of adjacent beams and an in-situ reinforced concrete slab is added to create a composite tee section. are of three forms as follows: TY for spans up to 17 m (simply supported) 750 rnrn 160mm1' In-situ slab . Figure 47. The beams. _I Figure 48. the joints have allowed salty water to leak through to the piers and abutments.Bridge beams Precast prestressed concrete beams are widely used in bridge construction. Bridges up to 60 m long also have to be constructed as integral bridges without movement joints between the deck and abutments. This may be full continuity of the whole deck structure or partial continuity of the top slab alone. Flooring and roofing Precast prestressed concrete units are widely used to form floors and roofs in buildings. The units may be used with a 75 mm topping for spans up to about 5 m (unpropped) and 7 m (propped). Hollowcore slabs being lowered into position t 595 mm 2390 mrn 1200 rnni 595 rnm 1 1 1 Hollowcore slabs are precast prestressed concrete units with continuous voids that reduce self-weight and provide an efficient structural section (Figure 50). Beam and block construction using 225 mrn deep prestressed concrete beams Figure 50. using slip-form and extrusion techniques or by casting in moulds. For a 2-hour fire period. figure 53. U 140 rnm Figure 51. For applications where particularly long spans are required. For roofs. units with wider ribs are used and a 100 rnm thick topping is required. the 400 mm deep unit can be used for spans up to about 15 m and the 800 mm deep unit for spans up to about 27 m. the following three forms of construction are available. to 225 mm for spans up to about 8 m. with welded shear connections between adjacent units. For general office loading. which may be propped or un-propped according to design requirements. Other depths of topping may be used depending on design requirements. a range of double tee units is available. Standard double tee unit showing section profile I Figure 52. a mesh-reinforced concrete topping of 50 mm minimum thickness is needed to assist in load distribution and provide a minimum fire resistance of 1 hour. Standard units are produced with a flange width of 2390 mm and ribs at 1200 mm centres. 600 and 800 mm (Figure 51). and the precast units may be used either on their own or in combination with in-situ concrete. For floors. Ridged units of varying depth with a level soffit but with drainage falls of 1 in 20 in the top flange are produced for roofing members. Beam and block construction. The units are placed side by side. Special ridged units of uniform depth but with minimum drainage falls of 1 in 50 are produced for parking decks. a 75 mm thick topping is needed and. is ideal for ease of handling in developments with limited access. for a 4-hour fire period. The depths range from 110 mm for spans up to 5 m. The supporting structure may be of concrete. The beam depths range from 150 mm for spans up to 6 m. in overall depths of 400. Tapered reductions can be made in the flange width for applications such as curved ramp decking. Solid composite slabs are formed of precast prestressed solid units acting as fully participating formwork. the 400 mm deep unit can be used for spans up to about 11 m and the 800 mm deep unit for spans up to about 22 m. Double tee walling units in office and maintenance depot for British Gas 18 An introduction to prestressed concrete I . The precast units are usually manufactured in depths of 75 mm and 100 mm. Figure 49. a lightweight insulating screed and mastic asphalt or similar waterproofing is placed on top of the units. and widths of either 600 mm or 1200 mm. to 450 mm for spans up to about 18 m. which combines precast prestressed concrete beams and infill blocks (Figure 493. acting in conjunction with an in-situ concrete topping. A double tee unit being craned into position for use as a floor A range of single tee units with a maximum flange width of 3000 mm and overall depths up to 1200 mm is also available for extra large spans. In this case. masonry or steelwork. For most applications. The unit width is usually 1200 mm but other widths are available. The long span units are ideal for multi-story car parks and for open-plan offices where flexibility of use is required. Examples of the use of double tees units are shown in Figures 52 and 53. Double tee units may be used also for applications such as structural walling and multi-span footbridges. The units are manufactured on stressing beds between 50 m and 200 m long. The main forms of construction. but a more general use of post-tensioning in the floors of multi-storey buildings has developed only within the last 25 years. ducts and corrosion protection to the tendons is an important factor in determining the economic viability of post-tensioning. The benefits of reduced building height and rapid construction are particularly attractive to commercial developers. (d) ribbed slab An introduction to prestressed concrete 19 I . I I No higher than chest height It I I I Correct Incorrect Figure 54. (b) waffle slab. The dimensions of the cross-section and the position of the prestressing force may be varied along the member to maximise load capacity and minimise deflection. simply supported at the ends. This manual of industry best practice is to be used for the training of erectors. b W Figure 55. as shown in Figure 55.Handling of units In most cases. competent personnel. control and safe working methods are covered in the Code of Practice for the safe erection of precast concrete flooring and associated components. foremen and supervisors. and this is how they should be handled and stacked. Othetwise. Stacking of precast prestressed concrete units Post-te nsioning Post-tensioning is. or to the magnitude of the prestressing force that may be provided. are flat slab (solid or waffle) or band beam and slab (solid or ribbed). The Precast Flooring Federation has produced a publication to help improve safety and prevent accidents on sites. Most units will be designed to act as single span members. slings or other lifting devices should be positioned near the ends of the units. The bearers should provide support over the full width of the unit. in principle. it is the responsibility of the manufacturer of factory-produced units to deliver them safely to site. However. the cost of providing anchorages. The subsequent handling and temporary stacking of the units is very important. All aspects of management. training. so that there is no tendency for the unit to twist. post-tensioning starts to become cost-effective and the economic span range of concrete floors is considerably extended. and arranged to be vertically above each other (Figure 54). There are no limits to the size or shape of the structure. Unless lifting hooks have been cast in. whenever possible. so that storey heights are less than with most other forms of construction. (c) band beam and slab. who are responsible for ensuring that all handling operations are carried out by skilled. in both reinforced and prestressed concrete. For spans greater than 6 m. Floor thickness is kept to a minimum. a Buildings Post-tensioning has been used for many years to prestress large-span beams supporting heavy loads. the units should be stacked where the ground is level on timber bearers placed at or near the lifting positions. be off-loaded from the delivery vehicle and placed directly in their final position in the structure. Concrete floors: (a) solid flat slab. Units should not be allowed to rotate during lifting and should. more versatile and more efficient than pretensioning. A typical example is shown in Figures 57 and 58. The tensile zones can be prestressed with longitudinal tendons located in the shells or in edge beams. this was generally provided by a mass structure and. Single and multi-span barrel vault roofs are formed of one or more cylindrical arch segments. The tendons are flexible and can be easily fixed to different profiles. with welded plate connectors. The versatility of post-tensioning is demonstrated in structures that comprise an assembly of precast concrete elements. Canada 20 An introduction to prestressed concrete I . and with no subsequent grouting. Manufacture of precast units for Unicorn Hotel and car park Figure 58. Un-bonded tendons displaced to accommodate changes in slab shape Figure 57. compared with 50 mm to the duct for the bonded system. exhibitions and sport. Typical examples include the famous Sydney Opera House (Figure 59) and the Calgary Saddledome (Figure 60). but the un-bonded system has particular advantages. Sydney Opera House. Many exciting shell and hanging roof forms have been used to cover the large open spaces required in facilities for concerts. The beam action produces compressive stresses along the crown of the arch and tensile stresses along the edges. Paul’s Cathedral in London. require a containment ring at the bottom edge. In modern concrete domes. These were used to form a series of welded connections in the vertical direction. formed of spherical arch segments. Calgaty Saddledome. the whole system is well suited to rapid construction methods. Australia Figure 60. Horizontally. post-tensioned tendons are used. 3 m high by 3 m wide. using a small hand-held jack. by a ring chain. in the case of St. Figure 56. The beam supports 16 m span pre-tensioned flooring units. In the direction of curvature the action is that of an arch. Both bonded and un-bonded forms of construction are used. Shell roofs are another example of the versatility of post-tensioning. In traditional masonry domes. where the doublediamond shaped units were used to provide the external structural frame of the building. . Stressing of the tendons is simple.I . The units. whilst longitudinally the action is that of a beam. External frame of Unicorn Hotel and car park at Bristol Figure 59. They can also be displaced locally around holes and to accommodate changes in slab shape (Figure 56). with each segment supported at the four corners. tendons were fed through the pre-formed ducts in each unit and post-tensioned to form a continuous edge beam. the cover being typically 30 mm. are provided with steel bearing plates at the top and bottom of each diamond. Dome roofs.. The tendons can be located closer to the surface of the concrete. joined together by means of internal tendons. there is a need to reduce the self-weight of the slab and so longitudinal voids are created by using polystyrene formers. the weight of concrete to be supported at any one time is reduced. it is attached to its predecessor on sliding bearings and launched into the span by jacks. These segments. extending the cantilevers to mid-span. in order to minimise the cost of construction. is to build them in short lengths at one end. cast in-situ) Figure 64. prestressed concrete single or multi-cell box girders are generally used (Figure 61). Subsequently. Slab bridges can be cast in-situ in either reinforced or prestressed concrete. Precast box segments An introduction to prestressed concrete 1 21 . Span-by-span construction -?. figure 61.. For longer spans. in lengths ranging from 5 m to 30 m. This length is then prestressed and the falsework is moved forward. Figure 62. Reinforced concrete voided slabs are usually more economical than the prestressed alternative for spans up to about 25 m. which are then stressed together. Kylesku Bridge! Scotland Ipost-tensioned in-situ box section) . During erection. Another technique that is used for bridges of constant cross-section that are either straight or curved to a single radius. by dividing each span into a series of transverse segments. As the spans increase. Another span length is then formed and stressed back to the previous cantilever (Figure 62). They need to be held firmly in position to prevent flotation during concrete placing and compaction. These are usually of circular section to enable the concrete to flow more easily under them. which can be formed insitu (Figure 63) or precast (Figure 64). As the spans increase. each segment is match-cast against the previous one. One span plus a cantilever of about one quarter of the next span is cast first on supporting falsework. Further segments are then added.L I I Launching nose Temporary t Figure 65. Skye Bridge. When each segment is complete. they are jointed with epoxy resin before being stressed together. are normally erected on either side of each pier to form balanced cantilevers. The span-by-span method of construction is often adopted for multi-cell viaducts with individual spans up to 60 m. where an in-situ closure is formed to make the spans continuous. with prestressed concrete voided slabs being used up to about 50 m span. The segments are cast in-situ. When the segments are precast. and incrementally launch the structure outwards from the abutment (Figure 65). Scotland (balanced cantilever.Bridges For short spans. and work can advance simultaneously on several fronts. the leading segments are supported from gantries erected on the piers or completed parts of the deck. the simplest form of bridge deck is a concrete slab. Incremental launching i" Figure 63. Tendons are stressed and anchored at these positions. automatically tensions the wire. A similar pattern may be adopted for external tendons. Lincolnshire Figure 67. which allows the wall to move radially inwards as the prestress is applied. The joint may be designed to remain in a free-to-slide condition or can be secured against further movement after prestressing has been completed. The post-tensioned tendons are used to create ring compression in the structure to counteract the ring tension caused by the internal pressure of the contained liquid. After all the rings have been stressed. with external tendons. Cylindrical tanks are generally provided with a sliding joint at the bottom of the wall.Wrre-winding for a water tower at Appleby. conical and spherical forms are used for tanks. Leicestershire One of the oldest applications of circumferential prestressing is by wire winding. Small groups of tendons may be contained within narrow ducts that are subsequently filled with cement grout or unbonded tendons may be used. Containers of cylindrical. On completion. each tendon normally extending between alternate piers. with several layers of wire within each band (Figure 67). requires the provision of several piers or pilasters at regular intervals around the structure where the tendons are brought to the surface (Figure 66). silos and pressure vessels. and to obtain a more uniform stress distribution. Wrapping may be continuous from top to bottom of the structure or in a series of bands. 22 [An introduction to prestressed concrete . a sprayed concrete protection is applied. having been first coated with protective grease. Bonded and un-bonded forms of construction are used with both internal and external tendons. The process. are also used. In this method. a sprayed concrete protection is provided. These are individually encased in plastic tubing. Circumferential prestressing. using un-bonded tendons and special anchorage connectors. in which the wire is drawn through a die or a system of braked pulleys. The tendons in alternate rings are anchored at different piers. Most structures are cast in-situ but precast forms.Containment vessels The major benefit of prestressing in containment vessels is that tension can be avoided in the concrete under service conditions. a continuous length of wire is wrapped around the outside of a tank or silo by a winding machine that travels around the structure.Stressing internal tendons for a sugar silo at Newark. gas or other material. using internal tendons within the wall thickness. to reduce the congestion of anchorages that would otherwise occur. !' Figure 66. bridges and flyovers. with factory-made components assembled on site. setting up the Prestressed Concrete Development Group in 1948 and the international organisation. Hoyer patented a long-line system of prestressing for factory production. Eugene Freyssinet (Figure 68) is widely regarded as the ‘father’ of prestressed concrete. Freyssinet was to devote all his time to refining the techniques and materials. Freyssinet and Seailles patented the principle and. multi-storey buildings.Appendix Brief history and development Examples of prestressing are not new . The UK road-building programme of the late 1950s to the early 1970s saw its extensive use for elevated motorways. a brilliant young engineer had developed an interest in the technique. Some of the most significant developments were in prestressing equipment. as well as for marine structures such as the massive offshore oilrigs of the 1970s. In Belgium Professor Magnel developed a two-wire system. lintels.but its application as an engineering principle began just over 100 years ago. early attempts to prestress concrete by means of pre-tensioned metal reinforcing bars met with little success due to the lack of suitable materials. and shells. He was a practical engineer.classical arches relied upon prestress for stability and wooden barrels have long gained their strength from tensioned hoops . commercial expansion and the resulting need for rapid construction saw the introduction of prefabricated building systems. Meanwhile. In 1945. In 1928. Eugene Freyssinet (1879 -1962). water towers and reservoirs were typical. Freyssinet duly developed his theory of prestressing and his first treatise on the subject was aptly entitled A revolution in The art of building. In Germany. This period saw the first use of prestressed concrete for nuclear pressure vessels. One of the first exponents was French engineer Considere. from then on. Significant examples include the Hammersmith Flyover in West London and the Spaghetti Junction interchange near Birmingham. the ‘father’ f prestressing o An introduction to prestressed concrete 23 I . Prestressing became part of that mass-production. Population booms. who used vertically tensioned iron bars to prestress granite for the walls of the harbour at Finistere. Many famous engineering practices were established at this time and the use of prestressed concrete expanded rapidly. which held twelve wires simultaneously. joists and floor systems. The Cement & Concrete Association in Britain was to lead the way. Tests carried out in the early 1900s led him to believe that prestressed concrete would be a practical proposition if high strength steel and high strength concrete were available. and was also a prolific builder of innovative and outstanding structures. The 1930s saw a tremendous boom in prestressed concrete work and a rapid development in applications and techniques. Freyssinet perfected his prestressing jack and conical anchorage. He was greatly helped by Glanville’s investigations at the Building Research Establishment into the effects of creep and shrinkage of concrete. The shortage of steel and timber in the postwar years gave an additional boost to developments in concrete structures. while in Germany. bridges. and pre-tensioned units continue to be used for items such as railway sleepers. with continental European engineers leading the world in this new method of construction. In 1939. FIP (Federation lnternationale de la Precontrainte) in 1952. in France. Applications such as long-span roof structures for aircraft hangers and industrial sheds. . Figure 68. It was used unseen in piles and other foundations. the end of the world war marked the beginning of a major programme of rebuilding throughout Europe. who relied on hard work and experience rather than complex mathematics. . Prestressed concrete floors are capable of long spans with relatively shallow depths. As a result.There have been no fundamental discoveries in prestressing since the 1950s. No tension is permitted circumferentially in a cylindrical tank or across the joints in segmental construction. The dynamic response of the floor is normally considered only for very shallow slabs and beams. In bridges. as recommended in a relevant code of practice. will not necessarily be the same. but a continuing improvement in its application. when placed on the supports. in cases where synchronised crowd loads can occur or the proposed use is particularly sensitive to vibration. they may be more sensitive to vibration than heavier reinforced concrete floors. The member retains its uncracked stiffness and the eccentric prestress creates a precamber. Load-deflection diagrams for reinforced and prestressed members 24 An introduction to prestressed concrete ~~~ ~~~ I . the limits are varied according to the combination of loads under consideration. so that little or no downward deflection occurs under service loads (Figure 70). Figure 70. Figure 69. but steel tendons remain the norm for the foreseeable future. deflection is often critical in determining the thickness of a reinforced concrete slab. In the case of a prestressed concrete slab. glass fibre. Deflection Reinforced concrete cracks in tension zones and the stiffness of the member is reduced. and allowance must be made for this when considering the thickness of finishing screed and the overall dimensions of floor zones and storey heights. Camber due to prestress Deflection Cracking The ability to avoid cracking under service loads has potential benefits with regard to aspects such as durability and water-tightness. for example. . Tendons made from composite materials (aramid. a limited tensile stress or crack width is permitted. carbon fibre) have been investigated. For many other structures. There are some important differences in the serviceability behaviour of reinforced and prestressed concrete. The Ravenspurn North Sea oil platform being towed into position off Scarborough Design considerations Concrete structures are designed to satisfy certain requirements with regard to serviceability limit states (cracking. There may be little scope for further development but prestressed concrete will continue to be used in novel and exciting ways in major structures [Figure 69). The basic components of concrete and steel have been further developed to give higher strength and more consistent quality. deflection. and some have been used in bridges. The upward camber exhibited by a precast prestressed flooring unit will depend on the time since its manufacture. As a result. it is possible to prevent cracking and also offset deflection. strength). as well as the length of the unit and the magnitude of the prestressing force. and anchorages and corrosion protection systems with enhanced reliability and durability have been introduced. Stressing jacks have been made lighter and easier to handle. The camber of adjacent units. vibration) and ultimate limit states (stability. depending upon the system. A different approach is necessary in cases such as bridges. The compression due to the prestressing force is then utilised to offset the effects of the transient loads. the algebraic sum of the support reactions due to prestress will be zero). The tendons are arranged in a series of parabolic curves that trace the bending moment diagram determined for the permanent load. The losses depend on many factors. These support reactions cause a secondary (or parasitic) set of moments and shears that need to be considered in the design of the rnember. Subsequent losses are generally less than for pre-tensioning and about 15% of the transfer force would be typical. A prestressing force is chosen so that the effect of the permanent load is balanced by the vertical reactions of the tendons on the slab. the application of the prestress will induce a set of support reactions that are in mutual equilibrium (i. a cylindrical tank. including the inherent properties of the materials and the method of prestressing. In this case vertical bending and radial shear also occur. are normally free to shorten but are not free to deflect at the support positions. When the tank is full. the hydrostatic pressure due to the contained liquid varies linearly from zero at the top to a maximum at the bottom. the long-term effects of creep have to be taken into account. For pre-tensioning. the length and curvature of the tendon and the characteristics of the duct or sheath. Designers begin by assuming values for the expected losses at each stage. as a proportion of the initial prestressing force. and the wall can be prestressed circumferentially with ring compression that varies in the same way.Losses of prestress In determining the amount of prestress necessary to control deflection. the losses can be calculated more precisely from information given in codes of practice and by systems suppliers: the design is then modified if necessary. the losses are due mainly to the elastic shortening of the member at transfer. where there are more load combinations to consider and considerably higher stresses are applied to the concrete. Members that are continuous over two or more spans. at the time of prestressing. If the wall is free to slide at the bottom. which need to shorten and deflect under the action of the prestress. The wall will move inwards in response to the prestress and. This is generally the case for pre-tensioned members at transfer. If the bottom of the wall is not free to slide. the resulting ring tension will also vary linearly from top to bottom. although the conditions may change subsequently if the members are used in continuous forms of construction. Typical losses for a bridge beam. for example. A concept of load balancing is usually employed in the design of post-tensioned slabs. For post-tensioning. the wall will continue to move out and in as the tank is filled and emptied. subsequently.e. no ring forces are possible at the bottom and the force distribution is affected throughout the lower portion of the wall height. An introduction to prestressed concrete 25 I . allowance has to be made for the losses of prestress that occur at various stages. Consider. now. Movement and restraint An important consideration in determining the effect of a prestressing force is the extent to which the structure is free to move. losses due to elastic shortening are less than 5% but friction losses during tensioning can vary enormously. In most cases. the more general case of beams and slabs. and the combined effects of steel relaxation. In this case. A single simply supported span is normally free to shorten and deflect without restraint. Once the prestressing details have been determined. or to avoid tension or limit crack widths. Consider. would be about 10% at transfer and a further 20% after transfer. concrete creep and shrinkage after transfer of stress. Cardiff CF24 2WR Tel: 02920 256100 www. British Cement Association. Crowthorne. 72 pp.performance. Crowthorne.cclstressing. Tebbutts Road. Hurst. Pengam Works. Millennium Way. Westland Road. Specification for hot rolled and hot rolled and processed high tensile alloy steel bars for the prestressing of concrete. Unit 4.com 26 An introduction to prestressed concrete I . CALcrete: computer-aided learning for concrete materials. John Wiley & Sons. P. Post-tensionedconcrete floors: design handbook. Orchard House. Ref. Box 56. Code of practice for the safe erection of precast concrete flooring and associated components. Prestressed concrete design: 2nd edition. B C. 2000. M K. F K & Evans. Carr Hill. Gerwick.co.freyssinet. Shropshire TF3 3DE Tel: 01952 201901 www. 2002. Prestressing steel (in 5 parts]. BS 5896: 1980.O.bridon.carringtonwire. Cambridgeshire PE19 1AW Tel: 01480 404401 www. South Yorkshire DN4 8DG Tel: 01302 382217 www. 1987.037. Technical Report 47. 1998.vsl-intl. Concretepractice: 3 d edition.358. 1997.u k VSL Systems (UK) Ltd. Concrete Society. R H. Stafford Park 1. Specification for high tensile steel wire and strand for the prestressing of concrete. CD-ROM. 288 pp. Leeds LS11 5AL Tel: 01132 701221 www. 70 pp. RCC/British Cement Association. Concrete . 591 pp. 174 pp.. . Chapman & Hall. 97. E & F N Spon.com Freyssinet Ltd. BS EN 206-1 : 2000. Durable bonded post-tensioned concrete bridges.com Carrington Wire Cardiff. Ref. Not yet published. 508 pp. BS 4486: 1980. Park 2000. Crowthorne. BS 5328-1 : 1997. 1996. Reinforced andprestressed concrete: 3r6edition. 2001. Telford.com Suppliers of strand post-tensioning systems CCL Stressing Systems Ltd. The Federation.. Precast Flooring Federation. The Concrete Society. 1994. Concrete Society.Part 1: Specification. 88 pp. 7 Hollinswood Court. production and conformity pr EN 10138. 48. Suite 5. design and construction. Further reading British Cement Association. The Concrete Society. St Neots. Doncaster. Kong. Technical Report 43. Reinforced Concrete Council. Addresses of companies and organisations Suppliers of prestressing wire and strand Bridon Wire Construction Products. Concrete .Part 1: Guide to specifiing concrete. Construction of prestressed concrete structures: 2"d edition. 47. 64 Dywidag Systems International .macalloy. 28. Leicester LE1 1FB Tel: 01162 536161 www.Figures 19. Hawke Street.pff.Figure 60 . Sheffield S9 2LN Tel: 01 142 426704 www. 41. The UK certification authority. Kent TN13 1XR Tel: 01732 450000 www.Figures 43. 5600 Lenzburg. 34. 58 Tarmac Topfloor .Figure 44 Tarmac Precast . wire and strand and the supply and installation of post-tensioning systems UK CARES. 60 Charles Street.ukcares. Century House. 21.Figure 16 CCL Stressing Systems Ltd . 46. 60 Charles Street.Org.Figures 14. 38. 21 Pembroke Road.uk 1 Arup . Box 71.Figures 62.Figure 69 Jan Bobrowski & Partners CV Buchan . 33. 49. Southam.com . 50 I VSL Systems (UK) Ltd . 42.britishprecast. 63. 29. 27. Leicester LE1 1FB Tel: 01 162 5361 61 www.0rg. Sevenoaks. Berks RG45 6YS. 57.uk I Post-tensioning Association C/o Balvac Whitley Moran Ltd. 35.uk Prestressed Concrete Association.O. 20.dywidag-systems. Warwickshire CV47 OFG Tel: 01926 813980 www. 37 . 52. Pembroke House. Tel: 01344 725727 www. 53. Alfreton.com Dywidag Systems International Ltd. covering the production of high tensile steel bars. 68 Lancashire County Council . Birchwood Way. Somercotes.Figures 26. Switzerland .Figure 40 Freyssinet Ltd . Crowthorne. 36. Telford Avenue. 30 Concrete Bridge Development Group . I I I . 32. P.Figure 39 SACAC AG.Figure 48 Platts .cbdg.Figures 31.0rg.com ' Industry associations Concrete Bridge Development Group. Northfield Road.I Suppliers of bars and bar post-tensioning systems McCalls Special Products Ltd. Derbyshire DE55 4QQ Tel: 01773 542600 Precast Flooring Federation.