Pre stressed concrete

for diploma course

1. Pre S tressed Concrete Short-01 and Eassy-02

2. Introduction Concrete is strong in compression and weak in tension. In this reason, reinforcement is provided in concrete members. When ever service load is placed on RC structures, it will undergo deformation.

3. This leads to causing Tensile cracks in the RC members Generally, steel bars are provided to limits the crack width and resist the tensile forces.

4. What is Prestressing? Prestressing is the application of an initial load on a structure, to enable it to counteract the stresses arising from subsequent loads during its service period

5. Brief History

6. Terms Wires: Prestressing wire is a single unit made of steel Strands: Two, three or seven wires are wound to form a prestressing strand Tendon: A group of strands or wires are wound to form a prestresssing tendon. Cable : A group of tendons form a prestressing cable

7. End Block: An end section of a prestressed member that houses one or more anchorage assemblies.

8. Advantages Of Prestressed Concrete Section remains uncracked under service loads Reduction of steel corrosion Increase in durability Full section is utilised Higher moment of inertia(Higher Stiffness) Less deformation(Improved serviceability) Increase in shear capacity It suitable for use in pressure vessels, liquid retaining structures Improved performance under dynamic and fatigue loading.

9. High span to depth ratios Large spans possible with prestressing (45:1) Reduction in self weight More economical section Suitable for precast construction Rapid construction Better quality

10. Limitation of prestressing Skilled technology Use of high strength materials is costly There is additional cost in auxiliary equipments Need for quality control and inspection

11. Types of prestressing Prestressing of concrete can be classified in several ways. Source of prestressing force This classification is based on the method by which the prestressing force is generated. There are four sources of prestressing forces. 1. Mechanical 2. Hydraulic 3. Electrical 4. Chemical

12. 2) Pre-tensioning or Post-tensioning This is based on the sequence of casting the concrete and applying tension to the tendons. Pre-tensioning (M-40): The tension is applied to the tendons before casting of the concrete. The pre-compression is transmitted from steel to concrete through bond. Post-tensioning(M-30): The tensionn is applied to the tendons after hardening of the concrete. The pre-compression is transmitted from steel to concrete by the anchorage device

13. 3)Linear or circular prestressing The classification is based on the shape of the member prestressed . 4)Full, Limited or partial prestressing based on the amount of prestressing force.

14. Necessity of High grade of concrete and steel Higher the grade of concrete higher the bond strength which is very essential in pretensioned concrete. Higher bearing strength which is vital in post-tensioned concrete. Further creep and shrinkage losses are minimum with high-grade concrete. Generally M30 grade concrete is used for post-tension And M40 grade concrete is used for pretensioning .

15. The losses in prestress members due to various reasons are generally in the range of 250 N/mm 2 to 400 N/mm 2 If mild steel or deformed steel is used the residual stress after losses is either zero or negligible. Hence high tensile steel wires are used which varies from 1600 to 2000 N/mm 2 Steel wire of dia 2.5, 3, 4, 5, 7, and 8mm are available.

16. Hoyer’s Long Line method Hoyer’s long line method is often adopted in pre-tensioning. Two bulk heads or abutments independently anchored on the ground several meters apart, says 100m and wire stretched between the bulkheads. Moulds are placed enclosing the wires . Concrete is placed surrounding the wires. With this system, several members can be produced can be produced along one line. And it is economical

17. For tensioning, a hydraulic jack is used. Wires are gripped at the bulkheads , using split-cone wedges. These wedges are made from tapered conical pins . Flat surface of the pin carries serrations to grip the wire

18. The advantage in pre-tensioning system is that there is no expenditure on end anchorages Disadvantages in this system are that the end abutments should be very strong and are provided only in precast factories . This naturally limits the size of the member as large sizes are difficult to transport from factory to the site of construction. Loss is more in pre-tensioned members.

19. Post-Tensioning System A metal tube or a flexible hose following intended profile is placed inside the mould and concrete is laid. Flexible hose is then removed leaving a duct inside the member. Steel cable is inserted in the duct. The cable is anchored at one end of the member and stretched using a hydraulic jack at the other end. After stretching the cable is anchored at the other end also. Therefore post tensioning system consists of end anchorages and jacks.

20. The popular post-tensioning systems are the following: Freyssinet system Magnel Blaton system Gifford-Udall system Lee-McCall system

21. Freyssinet System Freyssinet system was introduced by the French Engineer Freyssinet and it was the first method to be introduced. High strength steel wires of 5mm or 7mm diameter, numbering 8 or 12 or 16 or 24 are grouped into a cable with a helical spring inside. Spring keeps proper spacing for the wire. Cable is inserted in the duct.

22. Anchorage device consists of a concrete cylinder with a concentric conical hole and corrugations on its surface, and a conical plug carrying grooves on its surface . Steel wires are carried along these grooves at the ends. Concrete cylinder is heavily reinforced.

23. Wires are pulled by Freyssinet double acting jacks which can pull through suitable grooves all the wires in the cable at a time. One end of the wires is anchored and the other end is pulled till the wires are stretched to the required length. An inner piston in the jack then pushes the plug into the cylinder to grip the wires.

24.   Magnel Blaton system In Freyssinet system several wires are stretched at a time. In Magnel Blaton system, two wires are stretched at a time. This method was introduced by a famous engineer, Prof. Magnel of Belgium. In this system, the anchorage device consists of sandwich plate having grooves to hold the wires and wedges which are also grooved. Each plate carries eight wires.

25. Between the two ends the spacing of the wires is maintained by spacers. Wires of 5mm or 7mm are adopted. Cables consists of wires in multiples of 8 wires. Cables with as much as 64 wires are also used under special conditions. A specially devised jack pulls two wires at a time and anchors them. The wires with the sandwich plate using tapered wedge

26. Gifford Udall System This system originated in Great Britain, is widely used in India. This is a single wire system. Each wire is stressed independently using a double acting jack. Any number of wires can be grouped together to form a cable in this system. There are two types of anchorage device in this system. a) Tube anchorages b) Plate anchorages

27. Tube anchorage consists of a bearing plate, anchor wedges and anchor grips. Anchor plate may be square or circular and have 8 or 12 tapered holes to accommodate the individual prestressing wires. These wires are locked into the tapered holes by means of anchor wedges.

28. In addition, grout entry hole is also provided in the bearing plate for grouting. Anchor wedges are split cone wedges carrying serrations on its flat surface. There is a tube unit which is a fabricated steel component incorporating a thrust plate, a steel tube with a surrounding helix. This unit is attached to the end shutters and form an efficient cast-in component of the anchorage

29. Lee McCall System This method is used to prestress steel bars. The diameter of the bar is between 12 and 28mm. bars provided with threads at the ends are inserted in the performed ducts. After stretching the bars to the required length, they are tightened using nuts against bearing plates provided at the end sections of the member

30. Losses In Prestress

31. In pre-stressed concrete applications, most important variable is the pre-stressing force. In the earlier days, it was observed that the pre-stressing force does not stay constant. • Even during pre-stressing of the tendons and the transfer of pre-stress to the concrete members , there is a drop of the pre-stressing force from the recorded value in the jack gauge.

32. The various reductions of the pre-stressing force are termed as the losses in pre-stress. Early attempts to produce prestressed concrete was not successful due to loss of prestress transferred to concrete after few years.

33. Prestress loss is nothing but the reduction of initial applied prestress to an effective value. In other words, loss in prestress is the difference between initial prestress and the effective prestress that remains in a member. Loss of prestress is a great concern since it affects the strength of membe r and also significantly affects the member’s serviceability including Stresses in Concrete, Cracking, Camber and Deflection.

34. Immediate Losses

35. Elastic Shortening 1. Pre-tensioned Members: When the tendons are cut and the prestressing force is transferred to the member, concrete undergoes immediate shortening due to prestress. 2. Tendon also shortens by same amount, which leads to the loss of prestress.

36. Elastic Shortening 1. Post-tensioned Members: If there is only one tendon, there is no loss because the applied prestress is recorded after the elastic shortening of the member. 2. For more than one tendon, if the tendons are stretched sequentially, there is loss in a tendon during subsequent stretching of the other tendons.

37. F p = Change in Prestress/Losses in prestress m= modular ratio E s / E c f c = Prestress in concrete

38. Frictional Loss In Post-tensioned members, tendons are housed in ducts or sheaths. If the profile of cable is linear, the loss will be due to straightening or stretching of the cables called Wobble Effect. If the profile is curved, there will be loss in stress due to friction between tendon and the duct or between the tendons themselves.

39. Time dependent Losses

40. Shrinkage The shrinkage of concrete is defined as the contraction due to loss of moisture . Due to the shrinkage of concrete, the prestress in the tendon is reduced with time .

41. Creep of Concrete Time-dependent, increase of deformation under sustained load . Due to creep, the prestress in tendons decreases with time.

42. The ratio of the ultimate creep strain to the elastic strain is defined as the ultimate creep coefficient or simply creep coefficient , θ. ε cr,ult = θε el

43. Relaxation Relaxation is the reduction in stress with time at constant strain. – decrease in the stress is due to some of the initial elastic strain is transformed in to inelastic strain under constant strain . – stress decreases according to the remaining elastic strain.

44. This losses is generally 2% to 8% of the initial stress. Initial Stress Relaxation stress 0.5fy 0 0.6fy 35 0.7fy 70 0.8fy 90

45. Type of Losses Percentage loss of stress Pretensioning Post Tensioning Elastic Shorting of concrete 3 1 Creep of concrete 6 5 Shrinkage of concrete 7 6 Creep in steel 2 3

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