A study on the construction process (Precast concrete, In-situ cast concrete, Shoring, Underpinning)

1. A Study on the
Construction Process
Precast Concrete
In-situ cast Concrete
Shoring and Underpinning

2. Introduction
Precast concrete
In-situ cast concrete
Shoring
Underpinning

3. Precast Concrete

4. What is Precast Concrete?
•
Cast into specific shapes at another location
•
Developed in the late 1960s
•
First use: stair treads, coping, lintels, and window sills
•
Known for its high quality architectural products

5. Characteristics
•
Great accuracy and high quality
•
Additional features: reinforcing, moles, anchor bolts, etc
•
Characteristics controlled by different materials
•
Reinforcement: Conventional reinforcing or prestressed
steel

6. Manufacture
• Casting beds used
• High strength steel strands
pretensioned
• Transverese bulkhead
seperators inserted
• Additional reinforcement added
• Concrete poured and vibrated
• Cured with live steam or radiant
heat

7. Slabs
•
The most standardised precast elements
•
Includes solid flat slab, hollow core slab, double tee
slab, and single tee slab

8. Solid flat slab

9. Hollow core slabs

10. Double tee slabs

11. Single tee slabs

12. Beams
• Made in different sizes
• Rectangular beams, L shaped
beams, inverted T beams and
AASHTO beams

13. Wall panels
• Prestressed or conventional
reinforcement
• Used as load bearing walls
• Includes solid wall panels,
hollow core wall panels and tee
wall panels

14. Solid wall panels

15. Hollow core wall
panels

16. Tee wall panels

17. How are precast buildings put together?
Double tee slabs supported on frame of columns and
girders

18. Hollow core slabs supported on load bearing walls

19. Double tee slabs supported on load bearing walls,
columns and inverted tee beams

20. In-situ cast concrete

21. What is in-situ cast concrete?
•
Concrete that is cast into forms on the building site
•
Any shape that can be formed can be cast
•
Certain types of concrete elements cannot be precast,
and can only be cast in-situ

22. Selecting an in-situ cast concrete framing system
•
Are the bays of the building square or nearly square?
•
How long are the spans?
•
How heavy are the loads?

23. In-situ Concrete mix
•
Appropriate concrete properties designed
•
Appropriate mix design developed
•
Mixing, transporting and handling coordinated with
placing and finishing

24. In-situ concrete placement
•
Should not be placed more rapidly than it can be spread
•
Deposit as near as possible to the final position
•
The concrete is placed in forms and consolidated

25. Curing
•
Satisfactory moisture content and temperature required
•
Concrete in formwork is protected from drying out
•
Curing compounds and treatments prevent loss of
moisture
•
Large surface areas problematic during curing

26. Advantages
•
Easy transportation of wet concrete
•
Flexible when it comes to geometric shapes
•
Relatively easy to do late changes to structure
•
Structure becomes monolithic

27. Disadvantages
•
Produced in an unprotected environment
•
Additional time required for drying out process
•
Requires more temporary work
•
Complex process with many inputs and flows

28. Shoring

29. What is shoring?
•
Construction of a temporary structure to support an
unsafe structure

30. When do we need shoring?
•
When walls bulge out
•
When walls crack
•
When an adjacent structure needs to be pulled down
•
When openings are to be newly made or enlarged in a
wall
•
During construction

31. Shoring during
construction

32. Raking shores
•
Rest upon the ground and
support the wall by being inclined
against it

33. Using raking shores
• Size of the raker decided on
anticipated thrust from the wall
• Center of the raker and wall
should not meet at floor level
• Shoring may be spaced at 3 4.5m
• Wedges should not be used on
sole plates
• For tall buildings, rider raker can
be used

34. Dead Shores

35. Flying Shores

36. Scaffold type shoring
• Scaffolding was first designed to
support loads imposed by
workers
• Often used as support for
formwork
• Provides rapid utilisation for
formwork support

37. Underpinning

38. What is
underpinning?
• Strengthening and stabilising
the foundation of an existing
building
• Installation of temporary or
permanent support
• Provides additional depth or
increase bearing capacity

39. When do we need
underpinning?
• Construction of a new building
adjacent to the old one
• Settlement of an existing
building
• Change in how a structure is
used
• Need to add a basement below
an existing building

40. Temporary support with maintenance jacking
•
Light structures may be supported with timber or concrete mats
•
If settlement occurs, mechanical or hydraulic jacks keep the
structure level

41. Bracket pile
underpinning
• Steel bracket piles driven
adjacent to the structure
• Load is transferred from the
structure into the pile through a
steel bracket
• Backfilled with lean sand
cement mix

42. Minipiles
• Minipiles inserted through the
existing foundation
• Generally do not require
temporary support of the
building

43. New foundation wall
and footing constructed
underneath the existing
foundation
•
•
•
The area is excavated
The new footing is constructed
The excavated area is backfilled

44. Support during
underpinning
•
•
Digging trenches at intervals
This leaves most of the
foundation supported by the soil

45. Support during
underpinning
•
•
•
Using needle beams
Beams are threaded through
holes cut in the wall
Entire wall can be exposed at
once