Introduction to design of prestressed concrete structures - Part 1
- 1. 1 Presentation on Prestressed Concrete by Prof. N. Rajagopalan (Retd. Professor & Dean, I.I.T. Chennai) Chief Technical Advisor – Bridges & Structures M/s. L&T-RAMBØLL Consulting Engineers Limited, Chennai
- 3. 3 Prestressing means application of a pre- determined force on a system, for resisting the internal stresses that may be developed in the system, due to external loads.
- 4. 4 If the system is expected to have tensile stresses due to external loads, pre- compression can be given to the system to a desired level even before the application of the external loads. Then the tension developed due to the external loads, will get largely nullified by the initially applied compression. This pre-compression is called prestressing. Generally prestressing is understood as application of pre-compression and this understanding will be followed hereafter unless otherwise stated.
- 6. 6 Prestressing force could be applied internally or it could also be applied by external systems. External application of prestressing force calls for reaction frame and hence not preferred. Normally, prestressing (pre-determined force) is achieved using steel elements with a fixed profile along the length of the element, inside the element or outside the element, concentric or eccentric to the axis of the element.
- 7. 7 Wires (cables) are pretensioned by stretching. Pre-compression on concrete is achieved by: (a) anchoring the wires (cables) to the concrete through external anchorages. (b) bonding the wires to concrete over a definite length. If there are external anchorages, there will be an anchor plate to distribute the concentrated force to the bigger area.
- 9. 9
- 10. 10 The type of steel used for such prestressing will be different from normal steel (mild steel). Such steel is called high tensile steel. Mild steel can be stressed normally upto 140 N/mm2 with a strain of 0.00067 (Es=2.1 x 105 Mpa).
- 11. 11 After the force is transferred to concrete there will be a compression (strain) in concrete due to shrinkage and creep of concrete, which are due to natural phenomena and independent of pre- compression. This will be to an amount of 0.0006. Hence the tensile strain in steel will get reduced by 0.0006.
- 12. 12 The left over tensile strain in steel will be 0.00007. This is only 10% of the applied strain. This would lead to only 10% of the applied stress on steel namely 14 N/mm2 . The corresponding stress on concrete with a vast area will be negligible.
- 13. 13 The creep and shrinkage strain in concrete (say 0.0006) is nearly constant. To have a good residual strain/stress, the applied stress/strain in steel should be high. Hence the steel used should be capable of being stressed to around 1000 N/mm2 . This could be achieved by using high tensile steel which will have a ultimate strength of 1600 N/mm2 .
- 14. 14 Residual prestress in prestressed elements
- 15. 15 Advantages of Prestressed Concrete When tension occurs in concrete due to external loading, it will be nullified by the initial compression and there will not be any residual tension in concrete and hence there will not be any cracking. This would prevent atmospheric ingression inside the concrete element and hence save steel from corrosion and concrete from chemical attack.
- 16. 16 The concrete in the tension zone (uncracked) will also be useful in resisting external forces. The concrete properties of full section will be useful. This would lead to slender structures. The stiffness of the structure is increased (because of un-cracked concrete) and hence the structure has a reduced deflection, leading to better serviceability.
- 17. 17 Pre-compression introduced in concrete increases shear capacity. This leads to heavier load carrying capacity of the element.
- 18. 18 Shear resisting capacity in prestressed concrete beams
- 19. 19 Classification of types of prestressing Prestressing of concrete can be achieved in number of ways and hence can be classified depending on the way in which pre- compression is transferred on concrete. Accordingly, the classification is identified in the following four groups:
- 20. 20 External or internal prestressing Linear or circular prestressing Pretensioning or post tensioning Partial prestressing, limited prestressing or full prestressing.
- 22. 22 Force is transferred by bond. The length this is required to transfer is called bond length. The force is picked up by concrete after a length called transmission length. Upto this transmission length, the force will not be felt fully. The force is felt in the full cross-section only after the transmission length. Transmission length is little more than bond length.
- 23. 23 The disadvantages in this system are: high tensile steel cannot be given any profile under normal conditions. the prestressing cannot be transferred immediately on to cocnrete and one has to wait till the concrete acquires sufficient bond strength to transfer the precompression generated by the prestressing to concrete.
- 24. 24 The prestressing can be done only where there is a prestressing bed or a self straining frame namely stress bench. the bond properties between concrete and steel should be good. the prestressing force is transferred gradually from the end and there is a transmission distance before the prestress is felt on the full cross-section of the structural elements.
- 25. 25 Post-tensioning system In case post tensioning, structural elements are cast first with holes left for prestressing elements to run through. These holes are provided with metal sheathing or HDPV ducts placed in proper position and they are called cable ducts. The prestressing cable (steel) is threaded through these holes and tensioned. The tensioned steel are anchored at the ends by special type of anchoring units.
- 26. 26 The vacant portion of the ducts are then grouted. The anchorages transfer the force at the end. The concrete and the system should be able to take it. The end portion called end block (anchor zone) has to be separately designed.
- 27. 27
- 28. 28 Pre-tensioning is preferred when: the structural element to be prestressed is small and can easily be transported. more numbers of similar prestressed structural elements are to be produced, such as prestressed concrete sleepers. labour at site is costly. number of external anchorages and the corresponding costs are high.
- 29. 29 Post tensioning is preferred when: the structural elements to be prestressed is very heavy. the product is unique and not repeated many times. labour at site is not very costly number of external anchorages and the corresponding costs are not high.
- 30. 30 Partial prestressing, limited prestressing or full prestressing This type of classification was introduced in CEB/FIP recommendations in 1970 depending on the level of prestress introduced in the structural element to nullify the stress that may develop during applications of external load. IS-1343 calls partial prestressed structures as Type III structure.
- 31. 31 Type 1 refers to a fully prestressed concrete structure and Type 2 structure with limited prestress where tensile stresses do not exceed the cracking stresses of concrete. Type 3 structures are those which allow tensile stresses to occur leading to limited crackwidths and that too under extraordinary leading conditions which may occur rarely on the structure.
- 32. 32 The cracks may get closed as soon as that extraordinary loading moves out from the structure. The permissible tensile stresses for such conditions are related to permissible crack width.
- 33. 33 CEB/FIP has classified them as Class I (fully prestressed), Class II (limited prestressed) and Class III (partially prestressed) and Class IV (Reinforced concrete structures). In Class III type, the level upto which precompression is done can be achieved: (a) by prestressing the high tensile steel upto desired level (much below the permitted level of stress for prestress for such steel) and allowing the same steel to act of reinforcement for taking care of extra
- 34. 34 extra tensile force that may occur due to occasional large loads or (b) by prestressing the high tensile steel near to permitted level of stressing and providing untensioned steel to take care of the extra tensile force that may occur due to occasionally occurring large loads. The first is called partially prestressed structures and the second is called prestressed reinforced concrete structures.
- 35. 35 Spectrum of concrete structures
- 36. 36 Tendons and Cables The prestressing steel in a prestressed concrete element is normally referred to as a tendon. A tendon may consist of one or more individual wires, one or more number of rods, number of wires 2,3 or 7 twisted together to form an individual strand and number of strands grouped together.
- 37. 37 The number of tendons grouped is called a cable. The tendons grouped together in a cable will be anchored with one external anchor. The external anchorage can be pulled with all the tendons together by a single jack and this is called bulk pulling. Individual tendons in the cable can also be pulled and anchored. The tendon is accommodated in a cable duct. The duct has a sheathing of a minimum thickness governed by relevant codes. The duct is also to be tested for containing the cables and grouting materials.
- 38. 38 Concepts of prestressing Stress concept Force concept Load balancing concept
- 40. 40 Force Concept
- 42. 42 Bruggeling has defined a full spectrum of concrete structures as: a) plain concrete with nominal steel to take care of shrinkage cracks b) reinforced concrete c) partially prestressed (prestressed reinforced) concrete. d) concrete with limited prestressing e) fully prestressed concrete
- 43. 43 Hence the studies are only concrete structures with various external inputs in the form of material or forces. Hence Prestressed Concrete is not a New Materials