Practical Aspects of PSC

1. Practical Aspects of PSC IRSE Phase II Course

2. What is Pre Stressing ? • It is intentional application of a predetermined force on a system for resisting the internal stresses due to external loads. P P

3. Thus PSC……. …is the special reinforced concrete which makes use of the intrinsic properties of steel and concrete i.e. using the properties they are good at CONCRETE in compression STEEL in tension

4. PSC is called Active concrete • Because steel is tensioned to compress the concrete so that there is no or hardly any tension in concrete under the service loads • This system needs high strength concrete (brittle) and high tensile steel (ductile) • Makes effective use of modern high strength materials

5. In-spite of the good designs… • There will be problems in the construction due to • Improper understanding or lack of understanding of the • basic principles • Right method of the application of the principles • Practical aspects of execution (because everything can’t be reduced to writing) • In fact the failures enlighten us to highlight the inconsistencies between assumptions on the paper and the understanding in the field

6. Practical problems and remedies • Specifications of works are based on the theory and also to a large extent on observations from minor to major deformations, observed in already executed works. • There are many factors which are too difficult to be precisely laid down. In such cases the decisions are based on discretion or intuition of the Engineer-in-Charge or the field executive

7. Practical problems and remedies Broadly there are following four causes of failures. • Defective design. • Faulty methods or wrong sequence of construction. • Natural causes, such as, unanticipated floods, scouring and settlement of foundations, etc. • Sub-standard specifications. • Majority of the cases of failures are found to be on account of (2) and (3) above.

8. PRE-STRESSING STEEL • Uncoated Stress Relieved Strand As Per IS :6006 • Uncoated Stress Relieved Low Relaxation Strands to IS : 14268 • Hard Drawn Plain Steel Wires ( Cold Drawn Stress Relieved Wires) to IS :1785(part-I) 1983 • High Tensile Steel Bars to IS: 2090

9. PROPERTIES • THE TWO WIRE AND THREE WIRE STRANDS ARE DESIGNATED BY NUMBER OF ELEMENTAL WIRES AND DIA. OF ELEMENTAL WIRES. • Nomenclature A-B • AREPRESENTS NO. OF WIRES IN THE STRAND • BREPRESENTS DIA. OF INDIVIDUAL WIRE IN THE STRAND TWO WIRE STRAND THREE WIRE STRAND

10. DIA. OF CENTRAL WIRE IS 1.5% MORE THAN THE SURRONDING WIRE PROPERTIES • SEVEN WIRE STRAND • Outer wires enclose inner wire in a helix with a uniform pitch of 12 to 16 times nominal diameter • Nomenclature - A-B • AREPRESENTS NO. OF WIRES IN THE STRAND • BREPRESENTS NOMINAL DIA. OF STRANDS

11. PROPERTIES • THE STRAND SHALL BE EITHER CLASS I OR CLASS II DEPENDING UPON THE BREKING STRENGTH OF STRAND.THE BREAKING STRENGTH OF CLASS II STRAND IS MORE. • THE TOTAL ELONGATION UNDER LOAD SHALL NOT BE LESS THAN 3.5%.

13. Detailing of Reinforcement and Cables

14. Cable Layout • Cable layout means • Deciding about the location of cable at various section • Vertical profile • Horizontal profile • The locations between which the cable will be in straight and on curve • Working out the ordinates at every meter and at every change of curvature from curved to straight and vice versa in vertical as well horizontal plane

16. Importance of Cable Layout • Proper moment resisting couple so as to • Carry the dead and live load moments • Not to induce tension in the concrete under dead load as well as live load • Local Imperfections • Cause increase in the losses due to friction on account of the wobble effect

17. Loss due to Friction (Wobble) The permissible tolerance in the location of the pre-stressing tendons (sheathing duct) shall be ± 5 mm

18. Cable Profile and Ordinate details for half span

19. END BLOCK DEATAILS

22. How Proper Positioning of Cable is ensured • Cable tends to sag due to its self weight if not supported properly on reinforcement chairs and supports • Cable tends to float and move upwards due to buoyancy effect when concrete is poured (and is in liquid form), if not tied down properly • So cable has to be secured against downward as well as upward movement unlike reinforcement

23. What else is important • The angle of the cable at the end • To provide the proper force • Not to induce unintended forces causing tensions in the direction not catered to for in design • This can be ensured and checked only at the time of fabrication of shuttering for end block

26. What else is important? • Sequencing of the stressing operations in Post-tensioned construction is important and that given in the drawing should be followed. • If not given in the drawing this should be asked for from the designer.

27. Why Reinforcement is required in PSC • In the end block • To take the local transverse tension around the tendon behind the anchorage • To cater for the tension developed between two or more anchorages, which tends to split the member

31. Why Reinforcement is required in PSC • In the web for carrying shear • Shear is carried in PSC by • the vertical component of tendon • the concrete section • vertical reinforcement in the form of stirrups

32. Why Reinforcement is required in PSC • When the concrete section is sufficient to take the shear, theoretically no web reinforcement is required • This is seldom the case and shear reinforcement in the form of vertical stirrups is provided

35. Vertical Shear links

36. Why Reinforcement is required in PSC • At the junction of the web and the flange • As shear connectors for transferring the forces for enabling the member to carrying the moment • These are required between the bottom flange and the web • As well as between the top flange and the web

38. Shear Connectors

39. Why Reinforcement is required in PSC • Over the bearing area • This is required to distribute the stresses due to distribute the reaction to the larger section of the concrete

41. Material test data 4. Strand /wire coil no. = Tested UTS value = 5. Design Area of Cable (Ad ) = mm2 Measured Area of Cable (Am ) = mm2 6. Design Value of E (Ed) = kg/cm2 Measured Value of E (Em) = kg/cm2

42. Modified elongation : 8. Jack Area (Aj ) = cm2 9. Design jack efficiency (nd) = 10. Measured jack efficiency ( nf ) = (as per certificate) 11. Pre-stressing design force (Pd ) = t x 103 kg 12. Modified pressure = Pd /Aj x nd /nf kg/cm2

43. STRESSING OF CABLES

44. LOSS DURING ANCHORAGE • This loss • occurs when Pre-stressing force is transferred from tensioning equipment to anchorage. • It is particularly important in short members • It should be cross checked at site & compared with the values adopted by designer • ( it depends on type of anchorage and pre stressing system)

45. FUTURE CABLES • For easy installation at later date • Made in box girder to cater for increased pre-stress force • Provision of 15% (minimum) of design pre-stressing force.

46. EFFECT OF PRESTRESS

47. Other Important Issues • Proper Storage of the HTS – HTS coils should be stored in a closed go-down to protect it from the harmful effects of atmosphere and protect it from corrosion • Use of water soluble oil coating – Insist on the factory application of the water soluble oil coating on the HTS to prevent corrosion

48. Other Important Issues • HTS should be handled with great care like a baby so that it does not get a cut or even a minor nick. The handling should be done on raised supports avoiding dragging on ground. • Cable should be grouted after stressing without delay – and in no case it be allowed to remain un-grouted after 7 days of stressing.

49. Difference between pressure and elongation The difference between the elongation and the pressure should not be more than 5%

50. Other Important Issues • Grouting of the ducts – Non shrink grout or non shrink admixture to be used (but take care to use admixtures that do not cause corrosion like Aluminum salts • For longer Girders, it is preferable to provide Air Vents to release trapped air and ensure complete filling of the ducts with grout.