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 ................................... 2

What is prestressing? ......................................... 2

Benefits of prestressing ...................................... 2 Why prestress concrete? ..................................... 2

Mechanics of force system .................................. 3

Methods of prestressing ..................... 4 Pre-tensioning .................................................. 4 Post-tensioning ................................................. 6

Materials ........................................... 8 Concrete .......................................................... 8 Tendons ........................................................... 9

Equipment ........................................ 12 Pre-tensioning ................................................ 12 Post-tensioning ............................................... 13

Strand systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Bar systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Ducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

. .

Applications ..................................... 16 Pre-tensioning ................................................ 16

Railway sleepers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Piles and pylons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Bridge beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Flooring and roofing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Handling of units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Post-tensioning ............................................... 19 Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Containment vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

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 I 1

Basic concepts

L I

f- Figure 1. Row of books lifted as a single unit

Concrete Flexure cracks

/ / / ! I \ \ /.---- I I \ 1 I I \ \

Reinforcing bars

Figure 2. Reinforced concrete beam with cracks in tension zone

I Anchorage

I Anchorage

cost, f/m/kN

500 1000 1500 2000

Steel strength, N/rnmz

Figure 4. Relative unit cost @er unit force) of reinforcing and prestressing steels

What is p rest ressi n g ? Prestressing is best explained by considering a row of books. Each book is a discrete element but, if they are stacked closely together and an axial compressive force is applied at each end of the stack, it is possible to lift the whole row as a single unit (Figure 1).

This is prestressing in its simplest form. It provides the unit, in this case a row of books, with a strength and stability that it would not othewise possess. Other forms of prestressed items in everyday use include:

1 The barrel and the cartwheel (timber segments held in compression by

1 The bicycle wheel (steel rim held in compression by spokes in tension); 1 The umbrella (membrane held in tension by ribs in compression). In the case of concrete, prestressing is used as in the row of books, to form beams and similar members, and as in the barrel, to form cylindrical tanks and silos.

iron bands in tension);

Why prestress concrete? Concrete is a building material that is strong in compression but relatively weak in tension. In principle, there are two ways of overcoming this deficiency:

1 Embed within the concrete reinforcement that is strong in tension -

1 Avoid the tension altogether by (a) arching the structure or (b) prestressing

In reinforced concrete construction, steel reinforcing bars are provided in the regions where tension occurs, compensating for the low tensile strength of the concrete (Figure 2).

reinforced concrete;

the concrete.

Reinforced concrete members are designed on the assumption that cracks will occur under the design service loads. In beams and similar members cracks form only on the tension side of the member, but they pass through the entire wall thickness in cylindrical tanks and silos.

Cracking considerably reduces beam stiffness and increases deflection. Crack width is a critical design criterion with regard to the appearance, durability and water-tightness of the structure. Since crack widths increase as the reinforcement stress increases, the strength of reinforcement is limited typically to 500 N/mm2.

In prestressed concrete construction, steel tendons are stretched and anchored at each end so that compressive forces are applied to the concrete (Figure 3). 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, depending on the method of prestressing.

Benefits of prestressing 1 Tension and cracking under service loads may be avoided or reduced to a

1 Downward deflections of beams and slabs under service loads may be

1 Fatigue resistance (i.e. the ability to resist the effect of repeated live

1 Segmental forms of construction, as in the row of books, become a

1 Very high strength steel may be used to form the tendons (Figure 4) .

1 Beam and slab sections may be smaller than in reinforced concrete, due

low level, depending on the magnitude of the prestressing force.

avoided or greatly reduced.

loading due to, for instance, road and rail traffic) is considerably enhanced.

practical reality.

mainly to the capacity to reduce deflection.

2 (An introduction to prestressed concrete

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, the mechanics of the system must be investigated.

Consider the row of books. When a compressive force P is applied at each end of the row, a friction develops between each book that enables the complete row to act as a single unit. If the unit, of length L, is supported at the ends and a vertical load W is applied at the mid-span, reactions of 0.5W are produced at each end.

Resulting lines of thrust, dependent on both P and W, extend from each support and intersect on the line of action of W. Taking moments about the point of intersection, at height 'a' above the level of the prestressing force, gives Pa = 0.5W x 0.5L and hence W = 4Pa/L (Figure 5).

If the vertical load is uniformly distributed, the line of thrust forms a parabolic cuwe with the apex at mid-span. In this case, Pa = 0.5W x (0.5L-0.25L) and hence W = 8Pa/L (Figure 6).

For a given prestressing force, increasing the load, W, causes a corresponding increase in a, the height of the force diagram. If the apex of the force diagram falls outside the unit, the system of forces becomes unstable. Clearly, it is advantageous to locate the prestressing force towards the bottom of the unit.

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. If tension is to be avoided entirely in a rectangular section, the force diagram should be kept within the middle-third of the section depth. This region, which varies with the shape of the cross-section, is called the kern of the section.

If the prestressing force is located close to the bottom of the unit, tension develops at the top of the unit throughout its length. Although the effect of the applied load is to negate the tension within the middle portion of the unit, tension zones will remain at each end (Figure 7). Similarly, if the magnitude of the applied load causes the apex of the force diagram to rise close to the top of the unit, tension develops at the bottom of the unit within the middle portion (Figure 8).

Further advantage may be gained if the prestressing force can be either deflected (Figure 9) or draped (Figure 101, in order to mirror the line of thrust. In this way, the load-carrying capacity of the unit may be enhanced without developing tension in the concrete at the ends of the unit.

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. If all of the load were to be removed, then tension zones would develop in the top of the unit at mid-span. In reality, a substantial part of the load is due to the self-weight of the unit, which is permanent.

It should be noted that the horizontal component of the prestressing force has been taken as P, rather than Pcos9, since 9 is normally small.

1 1 1 1 " 1 " # - - - ' -

- - - - -..-_ , a

b**-

Line of thrust

Line of 'f " 1 1'

- - - 'thrust - * 4 4

0.5L W

I

I 0.5W

0.5W L

Figure 9. Beam with central point load and deflected tendons

L

0.5W L

Figure 5. Prestressed unit with central point load

1 I f 5

1 Figure Z Cracking at top of unit due to prestressing force being located below the kern

1

1 Figure 8. Cracking at bottom of unit due to apex of force diagram rising above the kern

2PsinWL per unit length

0.5W L 1 0 5W

Figure 10. Beam with uniformly distributed load and draped tendons

An introduction to prestressed concrete I 3

Methods of prestressing

I I I I

I I

I I Stage 1 - Tendons are tensioned and anchored

I

I

I

I

Prestressing tendons may be tensioned before the concrete is placed @re-tensioned) or after the concrete has hardened (post-tensioned). The resulting prestressed concrete members are also frequently described as being either pre-tensioned or post-tensioned.

I I Pre-tensioning

Stage 2 - Concrete is placed

I

I I

I Stage 3 - Tendons are released and force is I transferred to concrete I I

Figure 1 1. Prestressing using pre-tensioned tendons

, Zero stress in tendon after transfer

Transmission length 40 to 60 diameters for strand Maximum stress L b-4- in tendons

Elastic shorleningof member I++ at transfer of prestress

Figure 12. Transmission zone at end of member

Here the tendons are tensioned and anchored between fixed supports before the concrete is placed around the tendons. The concrete is either cast in moulds or formed by an extrusion or slip-form process to provide the required cross-section. When the concrete has achieved sufficient strength, the tendons are slowly released from the support at one end (Figure 11).

The prestressing force is transferred from the tendons to the concrete by the bond existing between the hardened concrete and the tendons. The transfer of force occurs over a short transmission length at each end of the concrete, as the tendons outside the concrete revert to their original untensioned condition (Figure 12). The elastic shortening of the concrete that occurs at this stage causes a corresponding reduction of the tendon force.

Pre-tensioning may be used on site where large numbers of similar precast units are required, but is usually carried out in a factory where permanent stressing beds have been installed. Single units and units cast side-by-side may be produced in rigid steel moulds, against which the tendons are tensioned and anchored until the forces can be transferred to the concrete, but the most effective use of pre-tensioning is in long-line production.

Long-line production In this case, a number of similar units are produced in line at the same time. Tendons, generally 7-wire strands, are tensioned between anchor plates placed at opposite ends of a long stressing bed. 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, in most cases, the abutments are sufficiently massive to be independently stable. In very long stressing beds, intermediate abutments with preformed pockets to receive temporary steel joists may be provided, so that a shorter stressing bed can be created should the need arise.

Positioning the tendons At one end the anchor plate bears directly against the supporting joists. At the other end, the jacking end, temporary steel struts are introduced between the anchor plate and the joists. The anchor plates are large thick steel plates with holes through which the tendons can be passed and anchored. For items such

as bridge beams, where the concrete is placed within moulds, stop-end plates are provided at the ends of

Temporary struts

Steel ioists

each unit. These plates contain standard patterns of holes to accommodate the particular layout

Fixed :: I: :: : : J a c k i n g of tendons required for each unit. A typical arrangement for long-line production is shown in Figure 13.

1, ,I

::::; end abutment I:.::;

Figure 13. Typical arrangement for long-line production

Stressing the tendons Tendons are drawn down the full length of the stressing bed, threaded through the holes in the stop-ends and anchor plates and anchored-off at the fixed abutment. At the other end, when all the tendons have been positioned, each one is tensioned. A stressing jack is attached to the tendon, the slack taken up and the load applied. The extension of the tendon is checked against a calculated value, the tendon is anchored-off and the jack

4 1 An introduction to prestressed concrete

is released. The operation is then repeated on the remaining tendons. The stressing sequence is not important in pre-tensioning but, as with all prestressing, the stressing operation needs to be carefully controlled

Fixing additional reinforcement If reinforcement is required within the transmission zones and for shear purposes, the amount necessary for each unit will have been grouped near the stop-ends when the tendons were being drawn down the stressing bed. Once the tendons have been stressed, the reinforcement can be spaced out and fixed in its final position. The moulds can then be assembled in preparation for concreting. Care needs to be taken to ensure that the tendons are kept free of any material, such as mould oil, that would impair the bond with the concrete.

I

I I

I Compacting and curing the concrete Vibrators are used to achieve full compaction of the concrete. Internal vibrators, if badly handled, can result in small pockets of water adjacent to the tendons that will reduce the effective bond. External vibrators are generally more effective provided there are enough of them and the moulds are sufficiently rigid. As with all concrete, proper curing is essential. In order to obtain a high concrete strength at an early age, the hardening process is often accelerated by raising the temperature of the concrete; this enables a more rapid turn-round in production.

Releasing the tendons When the concrete has attained sufficient strength, typically after 8 to 16 hours, the temporary struts at the jacking end of the bed are replaced by large jacks that can be slowly released. As the tendons attempt to return to their original length, the concrete in each unit is put into compression, provided the units are free to slide back along the bed. The tendons can now be cut. using an abrasive disc cutter, in the gaps between the units. The end plates are removed and the projecting lengths of tendon cut back to the end faces of the units.

In some prestressing arrangements, 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.

Extrusion and slip-forming processes For many proprietary items, where additional reinforcement is not required, moulds are not needed and the concrete can be simultaneously placed and compacted. Small beams, solid slabs and lintels can be slip-formed, while hollowcore slabs (see later) can be either slip-formed (Figure 141 or extruded. After releasing the tendons the concrete strip is cut into lengths, using a circular saw, to form separate units. At this stage a small pull-in of the tendons occurs at the ends of each unit.

Altering the prestress along the length of the unit In the arrangements considered so far, the tendons have all been straight and bonded to the concrete for their entire length. Although most pre-tensioned units are made in this way, the arrangement does not provide the most efficient use of the prestressing force in members of constant cross-section. The location of the prestressing force is limited by the conditions that can be permitted at the ends of the member, as shown earlier in Figure Z In large units, where self-weight is significant, 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. Typically, the tendons are arranged in several layers with multiple tendons in each layer, 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.

Figure 14. Long-line production of prestressed hollowcore flooring. After 8- 16 hours the tendons will be released and the concrete cut into lengths of 3 in to 20 in.

An introduction to prestressed concrete I 5

- -

e--- 1 R :: I; t n\ t n\ t ji j j

1%

,I :: :: .. . :: +

., . . . :: ..... .....

Figure 15. vpical arrangement for deflecting pre-tensioned tendons

Figure 16. Production of SY bridge beams incorporating deflected tendons

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

r g e 1 - Concrete cast with tendons in duct , 3

Stage 2 - Tendons tensioned after concrete has hardened

Stage 3 - Tendons secured at anchorages - Figure 18. Prestressing using post-tensioned internal tendons

Deflecting typically involves holding down the tendons at two symmetrically placed positions within each unit, and holding up the tendons within the gaps between units and at the ends of the line, as shown in Figure 15.

The tendons are usually tensioned before being deflected. They may be held in the upper part of the section during tensioning, and then pushed down from above at the hold-down points. Alternatively, 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, at the hold-down points, a steel tube with projecting lugs to deflect the tendons is bolted to the base of the bed. After the concrete has gained sufficient strength, the bolts are removed before releasing the tendons. A typical set-up for a bridge beam, before the side-moulds are put in place, is shown in Figure 16.

Debonding is a more straightforward procedure, in which specified lengths of plastic tubing are placed around several tendons in different layers, so that no bond can develop between the tendons and the concrete. In this way, the transmission lengths for the encased tendons begin at the end of the tubing and, by varying the lengths of tubing, both the magnitude and the eccentricity of the prestressing force may be adjusted in steps, as shown in Figure 17.

Post- ten si o n i n g Here the tendons, which may be located inside or outside the concrete, are tensioned after the concrete has hardened. Internal tendons, which are contained within ducts or sheaths to prevent them from bonding to the concrete, can be arranged to follow the curvature of the structure and provide the most appropriate profile to suit the applied loading. External tendons can be arranged as a series of straight lengths with changes of direction.at specific positions. In cylindrical structures, external tendons may be placed directly against the surface of the concrete.

In all cases, after the concrete has attained sufficient strength, the tendons are tensioned using jacks that bear upon the exposed face of cast-in anchorages at the ends of the tendon. When the required force has been attained, the tendons are made secure at the anchorages (Figure 18).

Post-tensioning may be used in the factory production of single special-purpose precast units, but is usually associated with construction on site where the concrete is formed entirely in situ or by assembling a series of precast segments. Structures may be fully formed or assembled before any prestressing is carried out. but there is great benefit in being able to assemble and prestress the structure in stages, as will be shown later.

Protection of tendons An important consideration is the long-term protection of the tendons against corrosion. Two different methods are used producing either bonded or un-bonded forms of construction. The particular form has no significant effect upon the behaviour of the member under normal loading conditions, but does affect the nature of the cracking that would occur in the event of over-loading, and the ultimate strength. 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.

Ducts to contain groups of tendons can be formed within the concrete, either by using removable void formers or by casting in steel or plastic sleeves. Tendons may be contained within the sleeves before the concrete is placed or may be threaded through the ducts after the concrete has hardened. After tensioning, the ducts are injected with a cement grout to provide a bonded construction (Figure 19).

Alternatively, tendons may be individually encased in plastic tubing, having been first coated with protective grease (Figure 20). These tendons are then fixed in position before the concrete is placed and, after tensioning, remain

6 IAn introduction to prestressed concrete

un-bonded to the concrete. The system, which results in Grout tube Vent -. "-Grout tu be very low tendon friction during stressing, IS particularly useful in thin sections such as floor slabs and the walls of cylindrical structures.

Stressing anchorage Dead-end anbhorage All duct-formers, sleeves and internal sheathed tendons must be fixed securely so that they are not displaced during concreting. It is important that the concrete is properly cured but, unlike pre-tensioning, accelerated curing is unnecessary as the age of the concrete at transfer of prestress is typically between 3 days and 28 days, according to the level of prestress to be applied.

Figure 19. Typical arrangement for internal post-tensioned tendons in bonded construction

Tensioning Tendons may be tensioned one at a time or in groups, depending on the system and the type of jack being used. When there are several tendons or groups of tendons at different positions within the cross-section of a member, the tensioning sequence specified by the designer must be followed in order to avoid over-stressing the concrete at any stage. The elastic shortening of the concrete that occurs during tensioning has no effect on the force in the tendons being stressed. However, with each stressing operation, there is a progressive reduction of force in any tendons that have been previously anchored.

Measurements during tensioning It is important to verify the extension of the tendon by recording the movement of the jack, as well as recording the jacking load. The extension is checked against a calculated value, allowance having been made for the effects of friction between the tendon and the surface of the duct during the tensioning process. An irregularity in the observed extension during tensioning may indicate that a duct has developed a blockage, in which case remedial action will need to be taken. Any significant difference between the observed and calculated extensions may mean that the effects of friction are not as assumed, and the jacking load may need to be adjusted.

External tendons

Permanent corrosion preventing grease

I Plastic sheath I Strand

Figure 20. Sheathed-strand used in un-bonded construction

In major bridge construction, external tendons contained within sheaths can be installed in the voids of hollow box sections. The tendon profile takes the form of a series of straight lengths with deviating saddles provided at intermediate diaphragms (Figure 21). Un-bonded tendons are used in structures where the design requires the tendons to be examined at intervals and, if necessary, removed and replaced.

In cylindrical tanks and silos, external tendons may be used to form a series of separate rings or a continuous band encircling the outside of the structure. Protection against corrosion and accidental damage is provided by a spray application of a cement-based mortar.

Stressing anchorage Tendon in sheathing Stressing anchorage

Figure 21. Typical arrangement for external post-tensioned tendons in bridge construction

Anchorage zones There are several different post-tensioning systems, as will be shown later. Each one involves the use of anchorages that remain part of the finished structure. Anchorages apply large concentrated compressive forces to the concrete with an effect like that of driving a wedge into a block of wood. The resulting bursting forces in the concrete are resisted by concentrations of reinforcement in the regions containing the anchorages. It is very important that the concrete in these regions is thoroughly compacted, despite the congestion caused by the anchorages, ducts and reinforcement. In order to make concreting easier, the end-block containing the anchorages is sometimes produced as a precast item in advance of the main structure.

An introduction to prestressed concrete I 7

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, particularly with regard to the losses of prestress that occur at various stages.

Some losses occur immediately, due to friction during the tensioning of the tendons and elastic shortening of the member during the transfer of prestress to the concrete. Other losses occur over an extended period of time, due to steel relaxation and creep and shrinkage of the concrete. The sources and magnitude of the losses, which differ for pre-tensioning and post-tensioning, will be examined in more detail later.

Concrete I

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). 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. It is sufficient here to mention particular factors that are important in relation to prestressing; namely the strength of the concrete and the deformations that occur before, during and after the transfer of prestress.

Strength Concrete needs to be workable when fresh, and strong when it has hardened. The stiffening and hardening is due to a chemical reaction between the cement and water in the mix. The workability (consistence) of fresh concrete is enhanced by good aggregate grading and by using admixtures. The strength of hardened concrete, which increases with age, is enhanced by good compaction and by reducing the watedcement ratio. The condition at transfer, when the prestress is applied, is normally critical, and requires concrete with a high early strength, e.g. one using CEM I42,5 or higher strength class cement, with thorough vibration of the fresh concrete.

In order to check the concrete strength at transfer, samples are taken from the fresh concrete. Test specimens, usually in the form of cubes, but cylinders may also be used, are made in steel moulds. The cubes, which are subjected to the same curing conditions as the concrete units, are placed in a testing machine and the crushing strength is recorded. Alternatively, the in-situ concrete strength can be measured by pull-out tests or by using a calibrated rebound (Schmidt) hammer.

With pre-tensioning, 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, since a rapid turn-round is vital to the success of the production process. 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. In practice a rapid- hardening cement such as CEM 152.5 is used.

Concrete at normal temperatures could take several days to develop the required strength for transfer, but the process can be accelerated so that the strength is reached in 8 to 16 hours. 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.

The required concrete strengths depend on the type of unit and the level of prestress applied. . For flooring units, the cube strengths are typically 28 - 40 N/mm’ at

transfer and 50 - 60 N/mm’ at 28 days. . For standard bridge beams, the values are typically 40 N/mmz at transfer and 60 N/mm2 at 28 days.

i I

8 I An introduction to prestressed concrete

With post-tensioning, the age at transfer is less critical and accelerated curing is normally not necessary. . For floors in buildings, where the level of prestress is low and transfer

takes place at 3 to 7 days, cube strengths are typically 25 N/mm2 at transfer and 40 N/mmz at 28 days.

1 For other forms of construction, where transfer usually occurs at a later stage, the cube strengths at 28 days are typically 40 N/mm’ for cylindrical structures and 50 N/mm2 for bridges. I

I Loss of prestress

i When stress is applied to concrete, it undergoes dimensional changes: an immediate elastic deformation followed by a time-related creep deformation

in moisture content. 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.

Concrete shrinks over time by an amount that varies with the initial water content of the mix, the thickness of the section and the relative humidity of the environment. The shrinkage develops rapidly at first and continues at a reducing rate for many years. The resulting loss of prestress that occurs in the tendons depends on the age of the concrete at transfer, and is greater with pre-tensioning than with post-tensioning.

The loss of prestress due to the elastic deformation of the concrete that occurs at transfer is greatest in pre-tensioning, since the tendons are already anchored by bond, and all the stress is applied to the concrete at the same time. In post-tensioning, there is no loss if all the tendons are stressed at the same time, since the elastic deformation takes place before the tendons are anchored. When the tendons are stressed sequentially, a progressive loss occurs in any tendons that are already anchored. The total loss is then intermediate between nil and half the value that occurs in pre-tensioning.

Concrete under applied stress also undergoes an inelastic creep deformation. Like shrinkage, creep develops rapidly at first and continues at a decreasing rate for many years. The creep value depends upon the thickness of the section, the relative humidity of the environment and the maturity of the concrete at transfer of prestress. As a result, the loss of prestress that occurs in the tendons is greater with pre-tensioning than with post-tensioning.

I (Figure 22). These changes are in addition to the shrinkage caused by changes

Tendons Prestressing tendons are usually formed from high tensile steel wires or alloy steel bars. The wires can be used singly or twisted together to form strand (usually of seven wires). Several tendons may be arranged in a group with a common anchorage to form a cable (Figure 23).

Wire Cold-drawn wire is produced in coil form from hot-rolled rod which is heat treated to make it suitable for cold drawing. The wire surface is initially smooth but may be indented by a subsequent mechanical process. In the as-drawn condition, the wire has a natural curvature approximately equivalent to the capstan of the drawing machine. 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. The stress-relieving treatment pre-straightens the wire, so that it will pay out straight from the coil, and enhances its elastic and relaxation characteristics. 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. Wire is used in factory-produced items such as lintels and small flooring units.

Strain 1 _ _ - - _ _ -

- - - - - - Elastic deformation

Figure 22. Typical curve of elastic and creep deformation for concrete

Figure 23. Types of tendon (from the top): wire, seven-wire strand, drawn strand, cable of seven strands, Dpidag bar and Macalloy bar

An introduction to prestressed con- 9

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. The diameters of the outer wires have to be slightly less than that of the king wire to allow for their helical form. 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. In BS 5896, there are three types of seven- wire strand: standard, super and drawn (Figure 24).

Figure 24. Cross-sections of (a) standard and super strand, and (b) drawn strand

Standard and super strands are visibly similar, but the outer wires of drawn strand are clearly flattened as a result of the strand having been drawn through a die. All strands are given a final stress-relieving treatment in the same way as wire before being wound into coils. Strand is widely used in factory-produced items such as railway sleepers, bridge beams, sign gantries, flooring and terracing, and for post-tensioning in all forms of in-situ and segmental construction.

Greased and plastic-coated strand is produced for designs involving the use of un-bonded or external tendons. The process involves the application of a protective fluid, which penetrates to the centre wire of the strand, followed by coating with a special corrosion-resistant grease and a continuous hot extrusion of a high density polypropylene or polyethylene sheath. The strand is wound onto a wooden reel for extra protection during transit and in use.

Bar There are two types of bar in common use:

1 Macalloy bars are produced from hot-rolled carbon-chrome steel bars that are then cold-worked by stretching to obtain the specified properties. The bars are available in lengths up to 17.8 m for diameters between 25 mm and 50 mm. Stainless steel bars are available in lengths up to 6 m for diameters between 20 mm and 40 mm. Both types of bar are provided with cold-rolled threads at each end, or over the full length if needed, and can be joined together by threaded couplers to obtain longer tendon lengths.

diameters between 20 mm and 40 mm, with a coarse thread extending over the full length of the bar. 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.

. Dywidag threadbars are produced to a German Standard specification in

Relaxation When tensioned, a steel tendon undergoes a relaxation of stress over time that is proportional to the initial load applied. Relaxation, like creep in concrete, develops rapidly at first and continues at a decreasing rate for many years. Standards are set in terms of maximum values, after a period of 1000 hours, for the percentage reduction of load for initial loads of 600/0, 70% and 80% of the breaking load. The values are based on test conditions of constant strain and a temperature of 20°C. Much higher values can occur at elevated temperatures. In practice, the 1000-hour value is multiplied by a factor to allow for the long-term relaxation.

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). In practice, wire and strand are generally produced to the class 2 requirements.

10 I An introduction to prestressed concrete

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 0.1% proof load, which is defined as the load that produces a permanent elongation equal to O.lO/o of the gauge length. For wire and strand, the load at lO/o elongation may be used as an alternative to the proof load (Figure 25).

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, a selection of which is shown in Table 1.

Table 1. Dimensions and properties of wires, 7-wire strands and bars

Wires to BS 5896

Standard strands to BS 5896

Super strands to BS 5896

Drawn strands to BS 5896

Bars to

(Macallo)i BS 4486

m

4 7

9.3 12.5 15.2

8.0 12.9 15.7

12.7 15.2 18.0

25 32 40 50

12.6 38.5

52 93 139

38 100 150

112 165 223

49 1

1257 1963

a04

1770 1670

1770 1770 1670

1 a60 1 a60 1770

1860

1700

1030 1030 1030 1030

1 a20

@i&

m 22.3 64.3

92 164 232

70

265

209 300

1 a6

380

a28 506

1295 2022

18.5

78

53.4

139 197

59

225 158

178 255 323

410 670 1050 1639

m 19.0 54.7

a1 144 204

61 163 233

1 a4 264 334

Care of tendons Prestressing steel is very different to ordinary reinforcement and particular care must be taken to protect tendons against corrosion, and to avoid damage by mechanical means or heating while handling. Tendons should be stored clear of the ground and protected from the weather at all stages. 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. Tendons should not be left unprotected inside ducts for prolonged periods of time. All tendons are eventually highly stressed, and failure to observe simple precautions in storage and handling has led to unfortunate consequences.

Elongation, %

Figure 25. ljpical load-elongation culve for strand showing A: 0.1% proof load, B: load at 7% elongation, C: breaking load

I

I An introduction to prestressed concrete 1 1 1

Equipment Pre-tensioning

figure 26. Open grip for pre-tensioning

Figure 27. Spring-loaded anchor for pre-tensioning

With pre-tensioning, the wires or strands are held by temporary grips during and after tensioning. The method of tensioning may vary but in all cases the grip consists of a barrel and wedge (Figure 26).

The wedge is generally in two or three pieces with a collar and wire clip, or ‘0’ ring, to keep the pieces in the correct position. It is important that the wedge is fixed so that the wire or strand is in a central position, with each segment driven the same distance into the barrel. The wedges, which have grooves on the surface in contact with the tendon, will be used many times, and they need to be carefully cleaned and examined before each use to ensure that they are not worn or damaged.

Stressing is carried out either by extending the tendons one at a time, or by multi-stressing, where all the tendons are extended at the same time. In both cases, the process starts at the non-jacking end, where grips are forced onto the unstressed tendons close to the anchor plate. 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).

With conventional stressing, when individual tendons are being stressed, a grip is then placed onto the unstressed tendon close to the anchor plate at the jacking end. The jack is attached to the tendon and stressing begins, with the tendon being pulled through the grip. When the required load and tendon extension have been reached, the wedge is forced onto the tendon and the jack is released. As the tendon attempts to pull back, the wedge is drawn further into the barrel and the tendon is firmly gripped.

Multi-stressing uses the same wedge anchor approach. When the full length of the stressing bed is not being used, double-ended anchors are introduced between the abutment and the last concrete unit. These anchors enable two lengths of wire or strand to be joined together and so avoid wastage (Figure 28).

For tendons that are stressed individually, a relatively small power-operated jack is used to enable stressing to be carried out quickly and efficiently. A popular jack for this purpose is shown in Figure 29. Once the controls have been set to pre-determined values, the stressing and anchoring operations are carried out automatically.

Figure 28. Double-ended anchor for pre-tensioning

Figure 29. CCL Stressomatic jacks and pump I

12 I An introduction to prestressed concrete

Post -t e n s i o n i n g A large number of systems have been developed and used throughout the world over the past 60 years. The main aspects of the systems in current use and available in the UK will be briefly described but, for full details of the full product range, reference should be made to the particular company concerned (see Appendix). Each company has been at the forefront of development, to meet the changing needs of design and construction, for many years. They each provide a range of anchorages, ducts, jacks and pumps, as well as servicing, replacement parts and calibration service accessories. Three companies use strand systems and two companies use bar systems.

Strand systems Suppliers include CCL Stressing Systems, Freyssinet and VSL Systems (UK). Each system has its own range of anchorages, couplers and equipment for installing, stressing and grouting the tendons - a selection is shown in Figures 30 to 34.

Multi-strand systems are available that can accommodate up to 55 strands, all contained within a circular duct and tensioned by a large jack in a single stressing operation. A selection of frequently used strand groupings with the corresponding ultimate load capacities and nominal internal duct sizes is shown in Table 2. The information is indicative only and many other tendon groupings can also be provided.

Table 2. Ultimate load capacities of multi-strand cables

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

11495 -

900

2100

3600

5700

8100

11100

16500

35

50

65

80

100

120

140 -

40

60

80

100

120

140

160

Figure 30. CCL multi-strand stressing anchorage and jack

Grout inlet Guide

---__ J

Secondaw tendon /A

Figure 3 1. Freyssinet multi-strand stressing anchorage (above) and coupler (below)

The tendons are generally installed by using a machine that pushes each strand into the duct from one end. At the stressing (or live) end, each strand is gripped by the jaws of a wedge that is forced into a tapered hole in the anchorage block. 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, the

terminated before the end of the member and the strands are fanned out, with each one anchored against a steel plate or provided with a bulbous shaped end that resists pull-out. Couplers and intermediate anchorages are also available to meet specific design and construction requirements.

tendons can be anchored directly into the concrete. In this case, the duct is Grout tube

Figure 32. VSL multi-strand dead-end anchorages

An introduction to prestressed concrete I 13

I-) Placing of anchor head and wedges

Lr YJ Positioning of jack

,I 7 - 7 Stressing

Figure 34. Multi-strand jacking

I1 I I

i i Figure 35. Freyssinet anchorages for bonded slab tendons. Left. live (jacking) end, right. dead end

Figure 36. Bonded slab construction, with tendons shown emerging from sheaths.

Fiaure 37. Un-bonded slab construction. Note

Figure 33. freyssinet strand-pushing and jacking in operation

The stressing jacks, which need to be hung from a lifting frame, are compact, easy to operate and highly manoeuvrable. The body can be rotated with respect to the lifting points, enabling easy access to the hydraulic connections. The inner workings of the jack can also be rotated, for easy alignment with the tendons. The jacks can be operated in a vertical or horizontal position and provide automatic de-wedging and hydraulic locking-off of jaws, to ensure correct seating of the wedges and minimise losses at transfer.

Bonded slab systems incorporate groups of three, four or five strands, of 13 mm or 15 mm nominal diameter, contained in one layer within a flat duct. The flat duct, which may be of metal or plastic, enables all the strands to be positioned close to the surface so as obtain the maximum eccentricity within the slab. The strands are individually tensioned by means of a hand-held stressing jack. After stressing, the duct is filled with a cement grout.

Un-bonded slab systems incorporate 15 mm nominal diameter strands, each of which has been coated with a corrosion-preventing grease and enclosed in an extruded plastic sheath. The strands, which are installed either singly or in bundles of two, three oifour, are individually tensioned using a mono-strand stressing jack.

The live-end anchorage comprises a metal casting, wedges and plastic fittings to facilitate the fixing of the anchorage to the formwork and to form a stressing access pocket. After stressing, the strand is cropped, and the strand end and wedges are sealed with a grease-filled plastic cap. The dead-end anchorage usually incorporates a factory-applied compression fitting to the strand, which is secured against the metal casting to prevent displacement during concreting.

A selection of anchorages and equipment for both slab systems is shown in Figures 35 to 38.

- _ _ ~ - - -

Figure 38, Freyssinet anchorages for un-bonded tendon slabs Left. live (jacking) end, - the draped profile of the tendons. right: dead end

14 introduction to prestressed concrete

Bar systems Suppliers include McCalls Special Products and Dywidag Systems International.

Although developed initially for use in prestressed concrete, high tensile alloy steel bars are also used for many other structural applications, including ground and rock anchorages. The prestressing force is transferred to the concrete by means of a threaded nut bearing directly against a steel distribution plate. As a result, there is little or no movement of the bar or loss of prestress at transfer. This is particularly important when short tendons are required.

Macalloy bars are normally supplied as smooth bars with rolled-on threads at each end, but fully threaded bars are also available. The bars are generally supplied straight, but they may be tensioned in a curved profile if the radius of curvature is not less than 200 bar diameters.

Dywidag bars are continuously threaded and are anchored by means of a domed shaped nut, which locates into a cone shaped recess in a surface-mounted distribution plate. The domed nut, which may also be used with a recessed bell anchorage, can tolerate small deviations in the direction of the bar.

The prestressing force may be increased in stages to suit design or construction requirements, and to compensate for any losses of prestress prior to grouting. In designs where the prestress is required only temporarily during construction, bars may be de-tensioned for removal. The bars are separately tensioned, but groups of up to four bars may be contained within a single duct. Hydraulic jacks are provided with gauges calibrated against a certified load cell to register the force applied to the bar. In addition, load cells are available to give an independent check on the accuracy of the pump gauge. Typical details of each bar system are shown in Figures 39 and 40.

Ducts Ducts which are normally circular or flat, 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. Ducts for internal tendons are made from corrugated steel or plastic sheathing. For bonded forms of construction, grout and air vents have to be provided at intervals along the length of the duct; these are usually formed by drilling through the sheathing and attaching a plastic vent tube.

Internal ducts can also be created by means of re-useable pneumatic rubber tube void formers. The tubes are inflated and fixed in position before the concrete is poured around them. After the concrete has hardened, the tubes are deflated and withdrawn to leave circular voids.

Ducts for external tendons are normally made from high-density polyethylene. The ducts are either connected to, or continued through, steel deviation pipes that are embedded in concrete at positions such as transverse diaphrams, where the direction of the duct needs to be changed.

In bridges and other structures in very severe environments, where a fully encapsulated system is required, plastic sheaths complete with caps to enclose both live and dead-end anchorages are provided. An electrically insulated system may be provided in structures for railways or light-rail transit systems, where stray currents may affect durability.

External tendons in major bridges are normally left un-bonded so that they can be checked after a number of years and, if necessary, removed and replaced. In this case, if bare strands are used, the duct is injected with petroleum wax. Otherwise, pre-greased and plastic coated strands may be used, and the duct left unfilled. Some examples of ducts and duct assemblies are shown in Figures 41 and 42.

Figure 39. Macalloy bar anchorage

Figure 40. Dywidag anchorages

Figure 4 7. Examples of corrugated steel and plastic ducts

Insulation plate HDPE liner

I I plastic plug Plastic sleeve

I

. Plastic or Intermediate plate Heat shrink sleeve or tape

plastic coated cast iron grout cap

Figure 42. Electrically insulated tendons

An introduction to prestressed concrete I 15

Applications

Figure 43. Some of the 35 million prestressed concrete railway sleepers used so far in the UK

figure 44. High-strength spun concrete pylon prestressed with carbon- fibre reinforced plastic

Figure 45. Bridge deck using inverted tee beams (showing alternative positions for sewices)

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.

Pre-tension i ng Pre-tensioning is best suited to the factory production of large numbers of similar precast units by the long-line method. Standard items of excellent quality and durability, ranging from small lintels to large bridge beams, can be produced efficiently and economically. Pre-tensioning is also used for specially designed elements, such as large grandstand roof beams, that would be impossible to precast on site within the space and time available. In order to minimise site storage requirements and construction time, 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.

Railway sleepers During the war from 1939 to1945, it became essential to find an alternative to timber for railway sleepers and the manufacture of prestressed concrete sleepers was developed. Factories set up specially to produce these units are highly mechanised with a low labour requirement. Production lines can output up to 400 sleepers at a time on stressing beds that are about 135 m long. A large number of wires was required in the original sleepers but a small number of 9 mm diameter strands, made from indented wires to minimise the transmission length, is used now. Prestressed concrete is ideally suited to meet the need for resilience and high fatigue resistance with excellent durability in a hostile environment. Concrete railway sleepers in use are shown in Figure 43.

Piles and pylons Where the ground conditions are suitable for the use of precast concrete driven piles, prestressed concrete offers several advantages over reinforced concrete. Prestressed piles can be made longer than is practical with reinforced piles, due to their greater ability to resist bending stresses during handling. The tensile stresses that occur in the concrete, due to the pile rebounding elastically from the driving hammer, are reduced by prestressing and the risk of cracking is minimised. Long-line production methods may be used to form solid sections in conventional moulds or hollow sections by extrusion.

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. The procedure consists of assembling the tendons and a reinforcing cage in steel moulds, tensioning the tendons, and spinning the mould on revolving wheels as the concrete is placed. The centrifugal force compacts the concrete and forces out the excess water.

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. Spun concrete pylons have been used in some countries, 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. Recent development work in Switzerland, using high-strength concrete and carbon-fibre reinforced plastic (CFRP) instead of steel, has made it possible to reduce the weight considerably. 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. The mast, which has an outer diameter varying from 850 mm at the base to 530 mm at the top, is prestressed longitudinally by 5 mm diameter CFRP rods and reinforced circumferentially by a CFRP spiral tape (Figure 44).

16 1 An introduction to prestressed concrete

In-situ slab - Bridge beams Precast prestressed concrete beams are widely used in bridge construction. As precast construction takes less time on site, these beams are especially useful when bridging over roads, railways and waterways, 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. A set of inverted tee beams, of various depths, was introduced for the span range 7 - 15 m. The beams, which have holes at regular centres through the bottom of the web, are placed on the supports side by side at 508 mm centres. Ordinary reinforcing bars are threaded through the holes, and in-situ concrete is added between and over the tops of the beams to form a composite solid slab, as shown in Figure 45.

During the 1960s. sets of box beams and I-section beams, of various depths, were introduced for the span range 15 - 30 m. The box beams, which were placed on the supports side by side at 1 m centres, were provided with transverse diaphragms at 2.4 m centres. The narrow gaps between the beams were filled with in-situ concrete and prestressing bars, threaded through ducts in the diaphragms, were post-tensioned. The I-section beams, which were formed with holes at 3 m centres through the web, were spaced out at 1.5 m centres. An in-situ reinforced concrete slab and transverse diaphragms were then formed to create a composite tee section. The box and the I-section beams were not widely used and alternative designs were sought.

Eventually, a further set of larger inverted tee beams was introduced to cover the span range 15 - 30 m. As the beams are usually placed side by side at 1 m centres, they have come to be called M 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, with a transverse diaphragm formed at each end of the span. It was also possible to create a pseudo-box section, by forming an in-situ reinforced concrete bottom slab as well as a top slab, but this option was soon discarded as being uneconomical in practice. During the same period, a set of U-section beams was also introduced. These were spaced at centres between 1.5 m and 2 m, with permanent formwork placed between each leg to support an in-situ top slab.

In the 199Os, the Prestressed Concrete Association introduced new sets of inverted tee beams, which are easier to manufacture and have extended the span range to meet the requirements of motorway widening schemes. The beams, which have a 750 mm wide bottom flange, are of three forms as follows:

TY for spans up to 17 m (simply supported) = Y for spans up to 32 m (simply supported) = SY for spans up to 40 m (simply supported).

The TY beams may be either placed side-by- side, with in-situ concrete added to form a composite slab, or spaced out at 1 m centres to support an in-situ concrete top slab. The Y and SY beams are spaced at centres between 1 m and 2 m according to design requirements, to support an in-situ top slab. Typical details of M, U and Y beam sections are shown in Figures 46 and 47.

1 6 0 m m 1 ' . . %. . . ._ .. .

formwork

UM beam

160 m m r

Permanent formwork

U beam

1500 10 2000 mm centres

Figure 46. Cross-sections through bridge decks using M beams and U beams

240 mrn -

750 rnrn 750 rnrn 750 rnm

Figure 47. Dimensions of 7l! Y and SY beam sections

Over the years, many bridges incorporating standard bridge beams have been constructed as a series of isolated spans separated by movement joints. Too often, the joints have allowed salty water to leak through to the piers and abutments, causing serious deterioration of the bearing plinths and supporting structures. Continuous decks generally ensure more durable structures, and continuity between spans is a requirement of the current UK highway standards. This may be full continuity of the whole deck structure or partial continuity of the top slab alone. Bridges up to 60 m long also have to be constructed as integral bridges without movement joints between the deck and abutments. Several forms of span-to-span and deck-to-abutment connection have been used, the relative merits of which are outside the scope of this publication. All of the standard bridge beams have proven long-term durability and are eminently suitable for continuous and integral bridge construction (Figure 48).

_I

Figure 48. Y beams used in continuous span construction

An introduction to prestressed concrete I 17

Figure 49. Beam and block construction using 225 mrn deep prestressed concrete beams

Figure 50. Hollowcore slabs being lowered into position

2390 mrn

595 mm 1200 rnni 595 rnm 1 1

t 1

U 140 rnm

Figure 51. Standard double tee unit showing section profile

I

Figure 52. A double tee unit being craned into position for use as a floor

Flooring and roofing Precast prestressed concrete units are widely used to form floors and roofs in buildings. The supporting structure may be of concrete, masonry or steelwork, and the precast units may be used either on their own or in combination with in-situ concrete. The units are manufactured on stressing beds between 50 m and 200 m long, using slip-form and extrusion techniques or by casting in moulds. For most applications, the following three forms of construction are available.

Solid composite slabs are formed of precast prestressed solid units acting as fully participating formwork, which may be propped or un-propped according to design requirements, acting in conjunction with an in-situ concrete topping. The precast units are usually manufactured in depths of 75 mm and 100 mm, and widths of either 600 mm or 1200 mm. The units may be used with a 75 mm topping for spans up to about 5 m (unpropped) and 7 m (propped). Other depths of topping may be used depending on design requirements.

Beam and block construction, which combines precast prestressed concrete beams and infill blocks (Figure 493, is ideal for ease of handling in developments with limited access. The beam depths range from 150 mm for spans up to 6 m, to 225 mm for spans up to about 8 m.

Hollowcore slabs are precast prestressed concrete units with continuous voids that reduce self-weight and provide an efficient structural section (Figure 50). The unit width is usually 1200 mm but other widths are available. The depths range from 110 mm for spans up to 5 m, to 450 mm for spans up to about 18 m. The long span units are ideal for multi-story car parks and for open-plan offices where flexibility of use is required.

For applications where particularly long spans are required, a range of double tee units is available.

Standard units are produced with a flange width of 2390 mm and ribs at 1200 mm centres, in overall depths of 400, 600 and 800 mm (Figure 51). The units are placed side by side, with welded shear connections between adjacent units.

For roofs, a lightweight insulating screed and mastic asphalt or similar waterproofing is placed on top of the units. In this case, 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.

For floors, 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. For general office loading, 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. For a 2-hour fire period, a 75 mm thick topping is needed and, for a 4-hour fire period, units with wider ribs are used and a 100 rnm thick topping is required.

Special ridged units of uniform depth but with minimum drainage falls of 1 in 50 are produced for parking decks. 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.

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. Tapered reductions can be made in the flange width for applications such as curved ramp decking.

Double tee units may be used also for applications such as structural walling and multi-span footbridges. Examples of the use of double tees units are shown in Figures 52 and 53.

figure 53. Double tee walling units in office and maintenance depot for British Gas

18 I An introduction to prestressed concrete

Handling of units In most cases, it is the responsibility of the manufacturer of factory-produced units to deliver them safely to site. The subsequent handling and temporary stacking of the units is very important. Most units will be designed to act as single span members, simply supported at the ends, and this is how they should be handled and stacked. Unless lifting hooks have been cast in, slings or other lifting devices should be positioned near the ends of the units. Units should not be allowed to rotate during lifting and should, whenever possible, be off-loaded from the delivery vehicle and placed directly in their final position in the structure. Othetwise, the units should be stacked where the ground is level on timber bearers placed at or near the lifting positions, and arranged to be vertically above each other (Figure 54). The bearers should provide support over the full width of the unit, so that there is no tendency for the unit to twist.

The Precast Flooring Federation has produced a publication to help improve safety and prevent accidents on sites. All aspects of management, training, control and safe working methods are covered in the Code of Practice for the safe erection of precast concrete flooring and associated components. This manual of industry best practice is to be used for the training of erectors, foremen and supervisors, who are responsible for ensuring that all handling operations are carried out by skilled, competent personnel.

Post-te n s i on i n g Post-tensioning is, in principle, more versatile and more efficient than pre- tensioning. There are no limits to the size or shape of the structure, or to the magnitude of the prestressing force that may be provided. 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. However, the cost of providing anchorages, ducts and corrosion protection to the tendons is an important factor in determining the economic viability of post-tensioning.

Buildings Post-tensioning has been used for many years to prestress large-span beams supporting heavy loads, but a more general use of post-tensioning in the floors of multi-storey buildings has developed only within the last 25 years. The main forms of construction, in both reinforced and prestressed concrete, are flat slab (solid or waffle) or band beam and slab (solid or ribbed), as shown in Figure 55.

For spans greater than 6 m, post-tensioning starts to become cost-effective and the economic span range of concrete floors is considerably extended. Floor thickness is kept to a minimum, so that storey heights are less than with most other forms of construction. The benefits of reduced building height and rapid construction are particularly attractive to commercial developers.

I No higher than chest height

I It

I I

I Correct Incorrect

Figure 54. Stacking of precast prestressed concrete units

a

b W

Figure 55. Concrete floors: (a) solid flat slab, (b) waffle slab, (c) band beam and slab, (d) ribbed slab

An introduction to prestressed concrete I 19

I . , .

Both bonded and un-bonded forms of construction are used, but the un-bonded system has particular advantages. The tendons can be located closer to the surface of the concrete; the cover being typically 30 mm, compared with 50 mm to the duct for the bonded system. The tendons are flexible and can be easily fixed to different profiles. They can also be displaced locally around holes and to accommodate changes in slab shape (Figure 56). Stressing of the tendons is simple, using a small hand-held jack, and with no subsequent grouting, the whole system is well suited to rapid construction methods.

The versatility of post-tensioning is demonstrated in structures that comprise an assembly of precast concrete elements, joined together by means of internal tendons. A typical example is shown in Figures 57 and 58, where the double- diamond shaped units were used to provide the external structural frame of the building. The units, 3 m high by 3 m wide, are provided with steel bearing plates at the top and bottom of each diamond. These were used to form a series of welded connections in the vertical direction. Horizontally, tendons were fed through the pre-formed ducts in each unit and post-tensioned to form a continuous edge beam. The beam supports 16 m span pre-tensioned flooring units, with welded plate connectors.

Shell roofs are another example of the versatility of post-tensioning. Single and multi-span barrel vault roofs are formed of one or more cylindrical arch segments, with each segment supported at the four corners. In the direction of curvature the action is that of an arch, whilst longitudinally the action is that of a beam. The beam action produces compressive stresses along the crown of the arch and tensile stresses along the edges. The tensile zones can be prestressed with longitudinal tendons located in the shells or in edge beams. Dome roofs, formed of spherical arch segments, require a containment ring at the bottom edge. In traditional masonry domes, this was generally provided by a mass structure and, in the case of St. Paul’s Cathedral in London, by a ring chain. In modern concrete domes, post-tensioned tendons are used.

Many exciting shell and hanging roof forms have been used to cover the large open spaces required in facilities for concerts, exhibitions and sport. Typical examples include the famous Sydney Opera House (Figure 59) and the Calgary Saddledome (Figure 60).

Figure 56. Un-bonded tendons displaced to accommodate changes in slab shape

Figure 57. Manufacture of precast units for Unicorn Hotel and car park

Figure 58. External frame of Unicorn Hotel and car park at Bristol

Figure 59. Sydney Opera House, Australia Figure 60. Calgaty Saddledome, Canada

20 I An introduction to prestressed concrete

Bridges For short spans, the simplest form of bridge deck is a concrete slab. Slab bridges can be cast in-situ in either reinforced or prestressed concrete. As the spans increase, there is a need to reduce the self-weight of the slab and so longitudinal voids are created by using polystyrene formers. These are usually of circular section to enable the concrete to flow more easily under them. They need to be held firmly in position to prevent flotation during concrete placing and compaction. Reinforced concrete voided slabs are usually more economical than the prestressed alternative for spans up to about 25 m, with prestressed concrete voided slabs being used up to about 50 m span.

For longer spans, prestressed concrete single or multi-cell box girders are generally used (Figure 61). The span-by-span method of construction is often adopted for multi-cell viaducts with individual spans up to 60 m. One span plus a cantilever of about one quarter of the next span is cast first on supporting falsework. This length is then prestressed and the falsework is moved forward. Another span length is then formed and stressed back to the previous cantilever (Figure 62).

As the spans increase, in order to minimise the cost of construction, the weight of concrete to be supported at any one time is reduced, by dividing each span into a series of transverse segments. These segments, which can be formed in- situ (Figure 63) or precast (Figure 64), are normally erected on either side of each pier to form balanced cantilevers, which are then stressed together. Further segments are then added, extending the cantilevers to mid-span, where an in-situ closure is formed to make the spans continuous. During erection, the leading segments are supported from gantries erected on the piers or completed parts of the deck, and work can advance simultaneously on several fronts. When the segments are precast, each segment is match-cast against the previous one. Subsequently, they are jointed with epoxy resin before being stressed together.

Another technique that is used for bridges of constant cross-section that are either straight or curved to a single radius, is to build them in short lengths at one end, and incrementally launch the structure outwards from the abutment (Figure 65). The segments are cast in-situ, in lengths ranging from 5 m to 30 m. When each segment is complete, it is attached to its predecessor on sliding bearings and launched into the span by jacks.

i" Launching nose Temporary

t

figure 61. Kylesku Bridge! Scotland Ipost-tensioned in-situ box section)

-?.L .. I I

Figure 62. Span-by-span construction

Figure 63. Skye Bridge, Scotland (balanced cantilever; cast in-situ)

Figure 65. Incremental launching Figure 64. Precast box segments

An introduction to prestressed concrete 1 21

! ' Figure 66. Stressing internal tendons for a sugar silo at Newark, Lincolnshire

Figure 67. Wrre-winding for a water tower at Appleby. Leicestershire

Containment vessels The major benefit of prestressing in containment vessels is that tension can be avoided in the concrete under service conditions. Containers of cylindrical, conical and spherical forms are used for tanks, silos and pressure vessels. 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, gas or other material. Bonded and un-bonded forms of construction are used with both internal and external tendons. Most structures are cast in-situ but precast forms, with external tendons, are also used. Cylindrical tanks are generally provided with a sliding joint at the bottom of the wall, 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.

Circumferential prestressing, using internal tendons within the wall thickness, requires the provision of several piers or pilasters at regular intervals around the structure where the tendons are brought to the surface (Figure 66). Tendons are stressed and anchored at these positions, each tendon normally extending between alternate piers. The tendons in alternate rings are anchored at different piers, to reduce the congestion of anchorages that would otherwise occur, and to obtain a more uniform stress distribution.

Small groups of tendons may be contained within narrow ducts that are subsequently filled with cement grout or unbonded tendons may be used. These are individually encased in plastic tubing, having been first coated with protective grease.

A similar pattern may be adopted for external tendons, using un-bonded tendons and special anchorage connectors. After all the rings have been stressed, a sprayed concrete protection is applied.

One of the oldest applications of circumferential prestressing is by wire winding. In this method, a continuous length of wire is wrapped around the outside of a tank or silo by a winding machine that travels around the structure. The process, in which the wire is drawn through a die or a system of braked pulleys, automatically tensions the wire. Wrapping may be continuous from top to bottom of the structure or in a series of bands, with several layers of wire within each band (Figure 67). On completion, a sprayed concrete protection is provided.

22 [An introduction to prestressed concrete

Appendix Brief history and development Examples of prestressing are not new - classical arches relied upon prestress for stability and wooden barrels have long gained their strength from tensioned hoops - but its application as an engineering principle began just over 100 years ago. One of the first exponents was French engineer Considere, who used vertically tensioned iron bars to prestress granite for the walls of the harbour at Finistere. In Germany, early attempts to prestress concrete by means of pre-tensioned metal reinforcing bars met with little success due to the lack of suitable materials. Meanwhile, in France, a brilliant young engineer had developed an interest in the technique.

Eugene Freyssinet (Figure 68) is widely regarded as the ‘father’ of prestressed concrete. 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. He was greatly helped by Glanville’s investigations at the Building Research Establishment into the effects of creep and shrinkage of concrete. 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 1928, Freyssinet and Seailles patented the principle and, from then on, Freyssinet was to devote all his time to refining the techniques and materials. He was a practical engineer, who relied on hard work and experience rather than complex mathematics, 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, with continental European engineers leading the world in this new method of construction. Some of the most significant developments were in prestressing equipment. In 1939, Freyssinet perfected his prestressing jack and conical anchorage, which held twelve wires simultaneously. In Belgium Professor Magnel developed a two-wire system, while in Germany, Hoyer patented a long-line system of prestressing for factory production.

In 1945. the end of the world war marked the beginning of a major programme of rebuilding throughout Europe. The shortage of steel and timber in the post- war years gave an additional boost to developments in concrete structures. The Cement & Concrete Association in Britain was to lead the way, setting up the Prestressed Concrete Development Group in 1948 and the international organisation, FIP (Federation lnternationale de la Precontrainte) in 1952.

Many famous engineering practices were established at this time and the use of prestressed concrete expanded rapidly. Applications such as long-span roof structures for aircraft hangers and industrial sheds, and shells, multi-storey buildings, bridges, water towers and reservoirs were typical. This period saw the first use of prestressed concrete for nuclear pressure vessels. It was used unseen in piles and other foundations, as well as for marine structures such as the massive offshore oilrigs of the 1970s. The UK road-building programme of the late 1950s to the early 1970s saw its extensive use for elevated motorways, bridges and flyovers. Significant examples include the Hammersmith Flyover in West London and the Spaghetti Junction interchange near Birmingham.

Population booms, commercial expansion and the resulting need for rapid construction saw the introduction of prefabricated building systems, with factory-made components assembled on site. Prestressing became part of that mass-production, and pre-tensioned units continue to be used for items such as railway sleepers, lintels, joists and floor systems.

Figure 68. Eugene Freyssinet (1879 -1962), the ‘father’ o f prestressing

An introduction to prestressed concrete I 23 ,

Figure 69. The Ravenspurn North Sea oil platform being towed into position off Scarborough

. . Camber due to prestress Deflection

Figure 70. Load-deflection diagrams for reinforced and prestressed members

There have been no fundamental discoveries in prestressing since the 1950s. but a continuing improvement in its application. The basic components of concrete and steel have been further developed to give higher strength and more consistent quality. Tendons made from composite materials (aramid, glass fibre, carbon fibre) have been investigated, and some have been used in bridges, but steel tendons remain the norm for the foreseeable future. Stressing jacks have been made lighter and easier to handle, and anchorages and corrosion protection systems with enhanced reliability and durability have been introduced. 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).

Design considerat ions Concrete structures are designed to satisfy certain requirements with regard to serviceability limit states (cracking, deflection, vibration) and ultimate limit states (stability, strength). There are some important differences in the serviceability behaviour of reinforced and prestressed concrete.

Deflection Reinforced concrete cracks in tension zones and the stiffness of the member is reduced. As a result, deflection is often critical in determining the thickness of a reinforced concrete slab. In the case of a prestressed concrete slab, it is possible to prevent cracking and also offset deflection. The member retains its uncracked stiffness and the eccentric prestress creates a precamber. so that little or no downward deflection occurs under service loads (Figure 70).

The upward camber exhibited by a precast prestressed flooring unit will depend on the time since its manufacture, as well as the length of the unit and the magnitude of the prestressing force. The camber of adjacent units, when placed on the supports, will not necessarily be the same, and allowance must be made for this when considering the thickness of finishing screed and the overall dimensions of floor zones and storey heights.

Prestressed concrete floors are capable of long spans with relatively shallow depths. As a result, they may be more sensitive to vibration than heavier reinforced concrete floors. The dynamic response of the floor is normally considered only for very shallow slabs and beams, in cases where synchronised crowd loads can occur or the proposed use is particularly sensitive to vibration.

Cracking The ability to avoid cracking under service loads has potential benefits with regard to aspects such as durability and water-tightness. No tension is permitted circumferentially in a cylindrical tank or across the joints in segmental construction. For many other structures, a limited tensile stress or crack width is permitted, as recommended in a relevant code of practice. In bridges, for example, the limits are varied according to the combination of loads under consideration.

24 I An introduction to prestressed concrete

~~~ - ~~~

Losses of prestress In determining the amount of prestress necessary to control deflection, or to avoid tension or limit crack widths, allowance has to be made for the losses of prestress that occur at various stages. The losses depend on many factors, including the inherent properties of the materials and the method of prestressing.

For pre-tensioning, the losses are due mainly to the elastic shortening of the member at transfer, and the combined effects of steel relaxation, concrete creep and shrinkage after transfer of stress. Typical losses for a bridge beam, as a proportion of the initial prestressing force, would be about 10% at transfer and a further 20% after transfer.

For post-tensioning, losses due to elastic shortening are less than 5% but friction losses during tensioning can vary enormously, depending upon the system, the length and curvature of the tendon and the characteristics of the duct or sheath. Subsequent losses are generally less than for pre-tensioning and about 15% of the transfer force would be typical.

Designers begin by assuming values for the expected losses at each stage. Once the prestressing details have been determined, 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.

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. Consider, for example, a cylindrical tank. 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. If the wall is free to slide at the bottom, the resulting ring tension will also vary linearly from top to bottom, and the wall can be prestressed circumferentially with ring compression that varies in the same way. The wall will move inwards in response to the prestress and, subsequently, the wall will continue to move out and in as the tank is filled and emptied. If the bottom of the wall is not free to slide, no ring forces are possible at the bottom and the force distribution is affected throughout the lower portion of the wall height. In this case vertical bending and radial shear also occur.

Consider, now, the more general case of beams and slabs, which need to shorten and deflect under the action of the prestress. A single simply supported span is normally free to shorten and deflect without restraint. This is generally the case for pre-tensioned members at transfer, although the conditions may change subsequently if the members are used in continuous forms of construction. In this case, the long-term effects of creep have to be taken into account.

Members that are continuous over two or more spans, at the time of prestressing, are normally free to shorten but are not free to deflect at the support positions. In most cases, the application of the prestress will induce a set of support reactions that are in mutual equilibrium (i.e. the algebraic sum of the support reactions due to prestress will be zero). These support reactions cause a secondary (or parasitic) set of moments and shears that need to be considered in the design of the rnem ber.

A concept of load balancing is usually employed in the design of post-tensioned slabs. The tendons are arranged in a series of parabolic curves that trace the bending moment diagram determined for the permanent load. 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 compression due to the prestressing force is then utilised to offset the effects of the transient loads. A different approach is necessary in cases such as bridges, where there are more load combinations to consider and considerably higher stresses are applied to the concrete.

An introduction to prestressed concrete I 25

. . .

Further reading British Cement Association. Concrete practice: 3 d edition, 2002, British Cement Association, Crowthorne. Ref. 48.037. 70 pp.

Concrete Society. Post-tensioned concrete floors: design handbook, Technical Report 43. 1994, The Concrete Society, Crowthorne. 174 pp.

Concrete Society. Durable bonded post-tensioned concrete bridges, Technical Report 47. 1996. The Concrete Society, Crowthorne. 72 pp.

Gerwick, B C. Construction of prestressed concrete structures: 2"d edition, 1997, John Wiley & Sons. 591 pp.

Hurst, M K. Prestressed concrete design: 2nd edition, 1998, E & F N Spon. 288 pp.

Kong, F K & Evans, R H. Reinforced andprestressed concrete: 3r6 edition, 1987. Chapman & Hall. 508 pp.

Precast Flooring Federation. Code of practice for the safe erection of precast concrete flooring and associated components. 2001, The Federation, 88 pp.

Reinforced Concrete Council. CALcrete: computer-aided learning for concrete materials, design and construction. 2000, RCC/British Cement Association, CD-ROM. Ref. 97.358.

BS 4486: 1980, Specification for hot rolled and hot rolled and processed high tensile alloy steel bars for the prestressing of concrete.

BS 5328-1 : 1997, Concrete - Part 1 : Guide to specifiing concrete.

BS 5896: 1980, Specification for high tensile steel wire and strand for the prestressing of concrete.

BS EN 206-1 : 2000, Concrete - Part 1 : Specification, performance, production and conformity

pr EN 10138, Prestressing steel (in 5 parts]. Not yet published.

Addresses of companies and organisations Suppliers of prestressing wire and strand Bridon Wire Construction Products, Carr Hill, Doncaster, South Yorkshire DN4 8DG Tel: 01302 382217 www.bridon.com

Carrington Wire Cardiff, P.O. Box 56, Pengam Works, Cardiff CF24 2WR Tel: 02920 256100 www.carringtonwire.com

Suppliers of strand post-tensioning systems CCL Stressing Systems Ltd, Unit 4, Park 2000, Millennium Way, Westland Road, Leeds LS11 5AL Tel: 01132 701221 www.cclstressing.com

Freyssinet Ltd, 7 Hollinswood Court, Stafford Park 1, Telford, Shropshire TF3 3DE Tel: 01952 201901 www.freyssinet.co.u k

VSL Systems (UK) Ltd, Suite 5, Orchard House, Tebbutts Road, St Neots, Cambridgeshire PE19 1AW Tel: 01480 404401 www.vsl-intl.com

26 I An introduction to prestressed concrete

I Suppliers of bars and bar post-tensioning systems McCalls Special Products Ltd, P.O. Box 71. Hawke Street, Sheffield S9 2LN Tel: 01 142 426704 www.macalloy.com

Dywidag Systems International Ltd, Northfield Road, Southam, Warwickshire CV47 OFG

, Tel: 01926 813980 I www.dywidag-systems.com I

Arup - Figure 69

Jan Bobrowski & Partners - Figure 60

CV Buchan - Figure 16 CCL Stressing Systems Ltd - Figures 26, 27. 28, 29, 30

Concrete Bridge Development Group - Figures 62, 63, 64

Dywidag Systems International - Figure 40

Freyssinet Ltd - Figures 31, 33, 35, 36. 38, 41, 42, 68 Lancashire County Council - Figure 48

Platts - Figure 39 SACAC AG, 5600 Lenzburg, Switzerland - Figure 44 Tarmac Precast - Figures 43, 46, 47, 52, 53, 57, 58 Tarmac Topfloor - Figures 14, 49, 50 VSL Systems (UK) Ltd - Figures 19, 20, 21, 32, 34, 37 I

The UK certification authority, covering the production of high tensile steel bars, wire and strand and the supply and installation of post-tensioning systems UK CARES, Pembroke House, 21 Pembroke Road, Sevenoaks, Kent TN13 1XR Tel: 01732 450000 ' www.ukcares.com

I ,

Industry associations Concrete Bridge Development Group, Century House, Telford Avenue, Crowthorne, Berks RG45 6YS. Tel: 01344 725727 www.cbdg.0rg.uk

C/o Balvac Whitley Moran Ltd, Birchwood Way, Somercotes, Alfreton, Derbyshire DE55 4QQ Tel: 01773 542600

1 Precast Flooring Federation,

I Post-tensioning Association

60 Charles Street, Leicester LE1 1 FB Tel: 01162 536161 www.pff,Org.uk

Prestressed Concrete Association, 60 Charles Street, Leicester LE1 1 FB Tel: 01 162 5361 61 www.britishprecast.0rg.u k