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.
• High 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. PRECAST,
PRESTRESSED
CONCRETE
STRUCTURAL
ELEMENTS

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.
PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS

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.
PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS

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.
• Multistory elements may be prestressed
to provide resistance to bending forces
during handling and erection (columns at
right).
PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS
• 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.
PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS

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.
PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS

10. PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS
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. MANUFACTURING OF
PRECAST CONCRETE
STRUCTURAL
ELEMENTS

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.
MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS
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.
MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS
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.
MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS
• 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. JOINING PRECAST
CONCRETE ELMENTS

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.
JOINING PRECAST CONCRETE ELEMENTS

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.
JOINING PRECAST CONCRETE ELEMENTS

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 p
concrete.
JOINING PRECAST CONCRETE ELEMENTS

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.
JOINING PRECAST CONCRETE ELEMENTS

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.
JOINING PRECAST CONCRETE ELEMENTS
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

48. 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

49. 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.

50. 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.

51. Bridge Girder
Chenab Bridge

52. Orly Hangar
PSC Truss at Tuticorin

53. Flat Slab

54. Pretensioned Hollow Core Slabs

55. M/c for Pretensioned Hollow Core
Units

56. 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

59. 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

60. COMBINATION
OF HOLLOW
CORE SLAB &
STEEL COLUMNS

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

62. T
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