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The Myths and Realities
of Prestressing
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The Myths and Reality of
Prestressing
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Rey C. D. Singh
Manager,BBR Philippines Corporation
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Overview
Introduction to prestressing
To compare the myths and realities of
prestressingThe application of prestressing
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What is prestressing ?
Definition of prestressing
Pre-tensioning and Post-tensioning
Bonded and Unbonded Post-tensioningSystem
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Why is there a need for
prestressing ?
Advantages of prestressing:
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Advantages of prestressing
1. Effective use of high strength material
2. Shallow and attractive structure3. Long and economical span
4. Control of deflection
5. Elimination of crack
6. Fire resistant characteristics
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Advantages of prestressing
7. Self testing of structure during
construction8. Constructability of structure
9. Lighter structuresaving on foundation
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Myths of prestressing
1. Prestressing causes delay in construction
works.
2. Prestressing is more costly when compareto conventional reinforced concrete works.
3. Prestressing causes hogging/chamber in
the slabs.4. Prestressing causes cracks and leakage
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Myths of prestressing
5. Not able to create any future opening in
prestressed slab.
6. Prestressing is not able to sustainearthquake design
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Myths and Realities
Myth no. 1 : Prestressing causes delay in
construction works
Reality : Prestressing improves theconstruction cycles of typical concrete slabs
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Myths and Realities
Typical layout of a RC beam & slab scheme
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Myths and Realities
Typical layout of a PT flat plate scheme
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Myths and Realities
Flat Plate
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Myths and Realities
Flat Slab with Drop Panel
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Myths and Realities
Flat Slab with Banded Beam
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Myths and Realities
Myth no.1
Factors which improve productivity
and thus increase the speed of
construction:
1. Utilisation of system formwork
2. Use of wiremesh for bottom reinforcement
3. Ease of M & E installation
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Myths and Realities
I D Task Nam e
1 Post-tensioning works
2 Preparation of Formwork
3 Laying of bottom reinforcement
4 Laying of prestressing tendons
5 Laying of top reinf orcement
6 Inspect ion & casting
7 Curing
8 Stressing
9 Removal of formwork
10
11
12 Conventional Reinforced Concrete
13 Preparation of Formwork
14 Laying of bottom reinforcement
15 Laying of top reinf orcement
16 Inspection & casting
17 Curing
18 Removal of formwork
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1 2 3 4 5 6 7 8 9 10 11 12 13
Time comparison between Prestressed and
Reinforced Concrete slab
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Myths and Realities
Myth no. 2 : Prestressing is more costly
when compare to conventional reinforced
concrete works.Reality : Prestressing is more economical
when compared to conventional reinforced
concrete works
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Myths and Realities
Typical layout of a RC beam & slab scheme
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Myths and Realities
Typical layout of a PT flat plate scheme
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Myths and Realities
Cost comparison of Prestressing &
Reinforced Concrete Works.
DESCRIPTION UNIT QTY RATE AMOUNT
(S $) (S $)
CONCRETE m3/m2 0.200 83 16.60
FORMWORK m2/m2 1.000 21 21.00
REBAR kg/m2 9.0 0.90 8.10
PRESTRESS S $/m2 16.00
TOTAL 61.70
DESCRIPTION UNIT QTY RATE AMOUNT
(S $) (S $)
CONCRETE m3/m2 0.25 83 20.75
FORMWORK m2/m2 1.09 21 22.89
REBAR kg/m2 37.5 0.90 33.75
PRESTRESS S $/m2
TOTAL 77.4
Prestressed Flat Plate Reinforced Concrete Beam & Slab
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DESCRIPTION UNIT QTY RATE AMOUNT
(S $) (S $)
CONCRETE m3/m2 0.20 83.00 16.60
FORMWORK m2/m2 1.00 21.00 21.00
REBAR kg/m2 10.00 0.90 9.00
PRESTRESS S $/m2 16.00
TOTAL 62.60
(A) Residential 7.5 x 3.5 m grid LL = 1.52 kN/m2
(1) Prestressed Flat Plate
DESCRIPTION UNIT QTY RATE AMOUNT
(S $) (S $)
CONCRETE m3/m2 0.201 83.00 16.68
FORMWORK m2/m2 1.40 21.00 29.40
REBAR kg/m2 35.00 0.90 31.50
PRESTRESS S $/m2
TOTAL 77.58
(2) Reinforced Concrete
There is a savings of 20%in cost
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DESCRIPTION UNIT QTY RATE AMOUNT
(S $) (S $)
CONCRETE m3/m2 0.23 83.00 18.68
FORMWORK m2/m2 1.00 21.00 21.00
REBAR kg/m2 12.00 0.90 10.80
PRESTRESS S $/m2 24.00
TOTAL S $/m2 74.48
(B) Office 8 x 8 m grid LL = 3 kN/m2
(1) Prestressed Flat Plate
DESCRIPTION UNIT QTY RATE AMOUNT
(S $) (S $)
CONCRETE m3/m2 0.273 83.00 22.66
FORMWORK m2/m2 1.50 21.00 31.50
REBAR kg/m2 47.00 0.90 42.30
PRESTRESS S $/m2
TOTAL S $/m2 96.46
(2) Reinforced Concrete
There is a savings of 23%in cost
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DESCRIPTION UNIT QTY RATE AMOUNT
(S $) (S $)
CONCRETE m3/m2 0.29 83.00 23.90
FORMWORK m2/m2 1.08 21.00 22.68
REBAR kg/m2 15.00 0.90 13.50
PRESTRESS S $/m2 30.25
TOTAL S $/m2 90.33
(D) Office 12 x 8 m grid LL = 3 kN/m2
(4) Prestressed Banded Beam & Slab
Even at longer span, there is savings ( S$90.06 against S$96.46)
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Myths and Realities
Myth no. 3 : Prestressing causes
hogging/chamber in the slabs works.
Reality : Prestressing has been applied inthe construction of hotels, offices, high end
condominium, public housing & shopping
complexes with marble tiles, ceremic tiles
finishing.
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Myths and Realities
Photograph showing the high end
condominium and the public housing with
tiles.
High end residential and commercial
development in downtown Singapore
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Myths and Realities
Photograph showing the high end
condominium and the public housing with
tiles.
Public Housing at Choa Chu Kang, Singapore
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Myths and Realities
Photograph showing the high end
condominium and the public housing with
tiles.
Public Housing at Sembawang, Singapore
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Myths and Realities
Photograph showing the high end
condominium and the public housing with
tiles.
Public Housing at Woodlands, Singapore
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Myths and Realities
Photograph showing the high end
condominium and the public housing with
tiles.
Commercial and hotel development at Ermita Manila
(Robinson Twin Tower)
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Myths and Realities
Myth no. 4 : Prestressing causes cracks and
leakage in the concrete.
Reality : Prestressing induces thecompression in the concrete thereby
reducing the tensile stress that causes cracks
in the concrete.
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Myths and Realities
The layout of the proposed prestressed watertank
construction in Singapore
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Myths and Realities
Myth no. 5 : Not able to create future
openings in prestressed slab.
Reality : Openings can be created in theprestressed slab as per reinforced concrete
slab
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Myths and Realities
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Myths and Realities
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Myths and Realities
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Myths and Realities
Cost and complexity comparison between
RC and PS slab openings
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Myths and Realities
Creating opening in prestressed slab for escalator pit
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Myths and Realities
Creating opening in the prestressed slab for fountain feature
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Myths and Realities
Locking back of tendons after cutting the strands
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Myths and Realities
Creating a large opening and its structural support system
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Myths and Realities
Therefore, openings can be made in both RC andPS structures.
Creating opening in PS structure will cost slightly
more than in RC structure.Creating opening in PS structure will requireslightly more engineering consideration andanalysis.
Savings in the initial construction of Prestressedbuildings far outweigh the slightly higher cost ofalteration work.
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Myths and Realities
Myth no. 6 : Prestressing is not able to
withstand earthquake loading.
Reality : Prestressing has used in manyareas where earthquake has occurs
frequently, for example, in Los Angeles,
San Francisco in the US.
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Some application of prestressing
In civil engineering works
Commercial building,residential building
Others
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Segment being off-loaded from the low-loader
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Some application of prestressing
Lifting in progress
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Some application of prestressing
Fitting the segment in place
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Some application of prestressing
Construction of the pier
San Juan Bridge, Laguna
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Some application of prestressing
Cantilevering construction of the bridge span
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Some application of prestressing
Another view of the San Juan Bridge, Laguna
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Some application of prestressing
Closing the last segment
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Conclusion
Prestressing is comparable to RC structure,
if not better, in terms of:
Cost savings
Time savings
Ease of constructionFlexibility