prestressed concrete and precast concrete technology.pptx

Precast and Prestressed concrete construction technology

1. BUILDING CONSTRUCITON & MATERIALS PRECAST, PRESTRESSED CONSTRUCTION

2. Process where Concrete elements, cast and cured in a manufacturing plant, then transported to the construction site. Plant casting allows increased efficiency and higher quality control. Durable, permanent steel forms are reused many times, reducing formwork costs compared to sitecast concrete. H igh early strength cement and steam curing allow concrete members to be cast and cured in as little as 24 hours . Controlled casting conditions and high quality forms allow for greater control of surface finishes. INTRODUCTION :

3. Precast Concrete Structural elements are commonly reinforced with tightly stretched pretensioned steel strands , which provide increased structural efficiency . Conventional steel reinforcing is added for resistance to thermal and other secondary stresses. On the construction site, precast concrete elements are lifted into place and assembled into structural assemblies in a process similar to that used for structural steel. Compared to site cast concrete, precast concrete erection is faster and less affected by adverse weather conditions . A vacuum lifting device is used to lift and place precast concrete pranks.

4. P RECAST, P RESTRESSED C ONCRETE S TRUCTURAL E LEMENTS

5. Precast Concrete Slabs Used for floor and roof decks. Deeper elements (toward the right below) span further than those that are shallower (toward the left). Right: Hollow core slabs stacked at the precasting plant . P RECAST, P RESTRESSED C ONCRETE S TRUCTURAL E LEMENTS

6. Precast Concrete Beams and Girders Provide support for slabs . The projecting reinforcing bars will bond with concrete cast on site . Right: Inverted tee beams supported by precast columns. P RECAST, P RESTRESSED C ONCRETE S TRUCTURAL E LEMENTS

7. Precast Concrete Columns and Wall Panels Provide support for beam and slab elements . Since these elements carry mainly axial loads with little bending force , they may be conventionally reinforced without prestressing . M ultistory elements may be prestressed to provide resistance to bending forces during handling and erection (columns at right). P RECAST, P RESTRESSED C ONCRETE S TRUCTURAL E LEMENTS Precast concrete wall panels may be solid (right ), hollow, or sandwiched (with an insulating core). Wall panels can be ribbed, to increase their vertical span capacity while minimizing weight, or formed into other special shapes (below) .

8. Other Precast Concrete Elements Precast concrete stairs ( below) Uniquely shaped structural elements for a sports stadium (right) Etc. P RECAST, P RESTRESSED C ONCRETE S TRUCTURAL E LEMENTS

9. Assembling Concepts for Precast Concrete Buildings Vertical support can be provided by precast columns and beams ( above ), wall panels ( below ), or a combination of all three. The choice of roof and floor slab elements depends mainly on span requirements . Precast slab elements are frequently also used with other vertical loadbearing systems such as sitecast concrete, reinforced masonry, or steel. P RECAST, P RESTRESSED C ONCRETE S TRUCTURAL E LEMENTS

10. P RECAST, P RESTRESSED C ONCRETE S TRUCTURAL E LEMENTS A single story warehouse consisting of double tees supported by insulated sandwich wall panels. Precast concrete structure consisting of solid wall panels and hollow core slabs. A parking garage structure consisting of precast double tees supported by inverted tee beams on haunched columns .

11. M ANUFACTURING OF P RECAST C ONCRETE S TRUCTURAL E LEMENTS

12. Casting Hollow Core Planks Precast elements are manufactured in casting beds , 800 ft or more in length. High-strength steel strands are strung the length of the bed and tensioned . Conventional reinforcing, weld plates, block outs, lifting loops, and other embedded items are added as needed. Concrete is placed. M ANUFACTURING OF P RECAST C ONCRETE S TRUCTURAL E LEMENTS Untensioned prestressing strands can be seen in the left-most casting bed. In the bed second from the right, low-slump concrete for hollow core slabs is being formed over tensioned strands using an extrusion process. A completed hollow core casting is visible at the far right.

13. Prestressing and Reinforcing Steel Many precast elements contain both prestressing strands and conventional reinforcing . M ANUFACTURING OF P RECAST C ONCRETE S TRUCTURAL E LEMENTS Once the concrete has cured to sufficient strength, the castings are cut into sections of desired length. In some cases, transverse bulkheads are inserted to divide the casting bed into sections before concrete is placed . In this case, only the prestressing strands need to be cut to separate the section.

14. Casting Hollow Core Planks Individual sections are lifted from the casting bed and stockpiled to await shipping to the construction site. M ANUFACTURING OF P RECAST C ONCRETE S TRUCTURAL E LEMENTS Precast concrete elements are shipped to the construction site by truck and erected on site by crane . Sample hollow core slab sections of varying depths. At bottom left, note the insulated sandwich floor panel.

15. J OINING P RECAST C ONCRETE E LMENTS

16. Column-to-Column Connection Metal bearing plates and embedded anchor bolts are cast into the ends of the columns. After the columns are mechanically joined, the connection is grouted to provide full bearing between elements and protect the metal components from fire and corrosion. J OINING P RECAST C ONCRETE E LEMENTS

17. Beam-to-Column Connection Beams are set on bearing pads on the column corbels. Steel angles are welded to metal plates cast into the beams and columns and the joint is grouted solid . J OINING P RECAST C ONCRETE E LEMENTS

18. Slab-to-Beam Connection Hollow core slabs are set on bearing pads on precast beams. Steel reinforcing bars are in inserted into the slab keyways to span the joint. The joint is grouted solid . The slab may remain untopped as shown, or topped with several inches of cast in place concrete. J OINING P RECAST C ONCRETE E LEMENTS

19. Site cast Concrete Toppings over Precast Slabs Greater floor strength and stiffness Greater fire resistance Greater acoustic isolation Allow easy integration of electrical services into floor system Create a smoother, flatter floor surface . J OINING P RECAST C ONCRETE E LEMENTS

20. Precast Concrete Construction and Seismic Design In areas of high seismic risk, structures must be designed to respond safely to the dynamic forces imparted into the structure. Innovations in joint design are improving the connection systems in precast concrete structures and making them increasingly suitable for use in such areas. J OINING P RECAST C ONCRETE E LEMENTS A large scale test facility for simulating seismic forces on precast concrete structural systems.

21. For large spans self weight of structure becomes important Prestressed concrete results in lesser self weight in comparison with RCC Hence PSC is desirable for all long span applications There are some applications for small products also. Application of Prestressed Concrete

22. Behaviour of Concrete Reinforcement to Concrete

23. Behaviour under Gravity Loads

24. Effect of Prestress

25. Types of Prestressing Pre-tensioning Wires are stressed first and then concrete is poured. Suitable for mass produced small elements Sleepers, slab panels, etc Post-tensioning Concrete is poured and cured. Wires are stressed later. Suitable for long elements. Beams, large slabs, columns, trusses etc.

26. Pretensioning

27. Pretensioning Pretensioned Beams

28. Hollow Core Slab Unit Double Tee Floor Elements

29. Pretensioned Poles Pretensioned Pipes

30. Pretensioned Sleepers High Tensile Wires

31. Strands Sheathing for Beams

32. Sheating for Slabs Sheath and Strand

33. Anchorages Individual Anchorage

34. Prestressing Jack

35. Post-tensioning Systems Each proprietory system has a unique way of anchoring the wires to concrete Freyssinet system Giffor-Udal system VSL system And several others Anchorages

36. Freyssinet Anchorage VSL Anchorage

37. BBR System Sanfield - Alga

38. Gifford Udall Cona System

39. CASE STUDY showing the steps in Post tensioning

40. Post-tensioning Steps - summary Place sheaths and pour concrete. Insert high tensile steel into the sheaths. Pull the wires with jacks and anchor them to the end face of the beams Grout the empty space within the sheath. Beam is now ready to resist design loads Loss of Prestress Slip in anchorage Elastic Shortening of concrete Friction in sheathing Creep of concrete Shrinkage of concrete Relaxation of high tensile steel It is essential to have high strength concrete and high strength steel to keep loss within acceptable percentage

41. Advantages of Prestressed Concrete Self weight of PSC element is lower than self weight of RCC elements and hence longer spans (larger column-free spaces) are feasible. High strength concrete is used, and hence structure is more durable. Compared with steel, PSC has better performancce against corrosion, fire, and fatigue More economical than steel in developing countries Greener than steel Disadvantages of PSC Generally not suitable for small spans Design of PSC elements requires specialist structural engineers. Making alterations to PSC elements is very difficult and requires specialist intervention. High strength concrete requires quality conscious site personnel. In case, the high strength steel gets corroded, repair works are difficult to carry out and failure can be sudden.

42. Applications of Prestressing Long span structures: Bridges Shell roofs Water tanks Flat slab constructions in multi-storied structures Large spacing of columns More space for services Lesser height of floor may permit an additional storey.

43. Bridge Girder Chenab Bridge

44. Orly Hangar PSC Truss at Tuticorin

45. Flat Slab

46. Pretensioned Hollow Core Slabs

47. M/c for Pretensioned Hollow Core Units

48. Pretensioned Hollow Core Slab Span range – 3 to 8m Thickness – span / 30 Advantages of Prestressed Flat Plates & Flat Slabs Reduced thickness of slab results in reduced loads on columns and foundations Less deflection since deflection due to prestress is opposite to the deflection due to gravity loads Faster construction – deshuttering in a week

49. Multi-storied Flat Slab/Plate Building Reduction in Height RCC flat plate – upto about 9m c/c Thickness Prestressed flat plate – upto about 12m c/c Thickness Minimum thickness Limits of Flat Plates

50. COMBINATION OF HOLLOW CORE SLAB & STEEL COLUMNS

51. MODULAR CONSTRUCTION - ASSEMBLY OF PLATES @ 1 FLOOR / DAY

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