myths & realities of prestressing.ppt

8/14/2019 Myths & Realities of Prestressing.ppt

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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


(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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(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


TOTAL 62.60

(A) Residential 7.5 x 3.5 m grid LL = 1.52 kN/m2

(1) Prestressed Flat Plate


(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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(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


TOTAL S $/m2 74.48

(B) Office 8 x 8 m grid LL = 3 kN/m2

(1) Prestressed Flat Plate


(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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(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


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



tiles.

Public Housing at Choa Chu Kang, Singapore


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Myths and Realities



tiles.

Public Housing at Sembawang, Singapore


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Myths and Realities



tiles.

Public Housing at Woodlands, Singapore


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Myths and Realities



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


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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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Lifting in progress


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Fitting the segment in place


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Construction of the pier

San Juan Bridge, Laguna


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Cantilevering construction of the bridge span


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Another view of the San Juan Bridge, Laguna


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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

myths & realities of prestressing.ppt

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