Pre Stressed Beams

8/12/2019 Pre Stressed Beams

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1

Pre-stressed Beams

From Reinforced Concrete to

Pre-stressed Applications



2

The Disadvantages of

R/C

Reinforced Concrete does

have shortcomings…



3

Concrete Beams require a large

amount of formwork Timber is expensive

The shuttering is

afterwards mostlythrown away

A lot of material is

wasted for props andsupports



4

R/C Formwork is Time

Consuming

Fabrication takes a long time Craftsmen are required for formwork



5

Concrete requires long curing

The props cannot

be removed for 3

weeks andprevent any

progress



6

Large Amount of Steel

for Reinforcement Mild Steel is used for

Reinforcement bars

The maximum tensilestress mild steel can

take is 3000kg/cm2

This is quite low and

requires relatively

high amounts of steel

in R/C

3 Tons

One

cm2



7

Bar cutting,

bending andtieing is time

consuming

As a result, R/Cbeams are rather

expensive

and therefore not

much used for

roof construction

Stirrups

Reinforcement

Bars



8

Pre stressed beams reduce the

cost of concrete slabsdramatically

You can make a concrete slab for

the cost of an entire roof andhave no trouble with leaks



9

On top of it, you get a

roof top garden free



10

Advantages of Pre-stressed

Concrete

High Tensile steel wire is used instead

of mild steel bars

The cross section area is reduced since

the steel wire can take a much greaterload



11

RC Beam

Pre-stressed

Beam



12

To increase the load capacity, the steelwire is kept under tension of more than2000 kg during casting and curing

This results in a reduction of thenumber of bars and a substantialreduction of concrete material andcost



13

What is more,

The beams can be pre cast on ground

due to their small size which enables

the constructor to lift them on the brickwall to build a slab in a short period of

time.



14

And

there are no shuttering and props

required. This again results in huge

savings



15

What does a concrete slabwith pre-stressed beamslook like ?



16

Pre cast concrete planks

Pre-stressed beams



Plan

In the following section we willshow you how to make

pre cast beams

and how to stress them



Plan

Then we will show you

how to make a slab

and a slab roof.



19

Steel Formwork

Design,Dimensions,Details



20

Formwork Elements

l

These elements of 2.40m lengthare joined serially. As a result, a

number of beams can be cast and

pre-stressed in one stretch.

The wooden blocks

5x5cm are removed after

casting



21

Align the Formwork•Level the floor exactly and alignthe wooden blocks horizontallywith the help of guide lines to

bring them in to one level.

•Align the formwork with guidelines.

The wooden blocks

5x5cm are removed after

casting



22

Dimensions

Steel Formwork for Beams



23

The Metal Sheet Forms –

3m BeamGauge

Side – s 10.5 cmWidth-w 10.0 cm

w

s L= 2.4 m



24

Metal Sheet Form -3.6m Beam

Gauge

Side-s 13.25Width-w 12.7

w

s L= 2.4 m



25


Gauge

Side-s 16 cmWidth-w 15 cm

w

s L= 2.4 m



26


Gauge

Side-s 19 cmWidth-w 18 cm

w

s L= 2.4 m



27

Frame and End Plate

37mm L

angle

The Forms

are

supported

by welded

frames

Each beam is

separated by a

endplate that

determines the

length of the

beam



28

Plates separate

each beamwithin the Formwork

They

determine theLength of each

Beam



29

Apply Formwork Oil

Mix Oil and Grease 1:2



30

Tricks

Formwork oil prevents the formwork

from sticking to the concrete. It eases

dismantling

Formwork oil gives the concrete a finefinish

Never use burnt engine oil. It will blackenthe concrete and spoil the formwork



31

Stressing

Pre-stressing yard



32

Setting up a Pre-stressing yard

Select a flat land in excess of 600m2(25

perches)

Level and compact an area of 10mX30m

Put a 5cm thick lean concrete layer andlevel it perfectly



33

The Yard

Consists of

a number of rows of steel form work

Two concrete abutment blocks on either

end of the formwork to apply tension

onto the wires





35

Build two Abutments with suitable

foundations able to withstand the force

(F) induced by stressed bars

Consider the soil condition of the ground !



36

Concrete

Anchoring

Formwork,

Wires

Jacks



37

The Hydraulic Unit

To apply tension on the steel wires, a

hydraulic unit is required

It consists of 2 hydraulic jacks mounted on

a base plate, a pressure plate with wedgingcones to lock the wires



38

Base

Plate

5cm Opening

Tension is applied on the wires

with two 10T jacks

welding



39

A Pressure Plate is added



40

A Pressure Plate is added



41

And mounted on a Abutment

Cast Beam

Abutment

1m

50

cm

Jack

Steel

wire



42

The Wires are Passed Through



43

Two Wedges are inserted and the

Wires Passed trough them as well

Wires

Wedging Cones



44

And tighten the wedge to fix the

Wires



45



46

Pump up the Jacks equally and apply

tension on the wires



47

Pump up the Jacks equally and apply

tension on the wires



48

Mark the Wire to determine the

Tension

Pre-stressing means putting the wire

under a tension of at least 2000 kg

In the process the wire extends andgets longer

This can be observed by marking and

measuring the extension which is calledelongation





50

The elongation is

of the entire length of the wire

0.353 %



51

The Wire Length

The wire length is determined by your yard:the entire distance from anchoring block tothe wedge of the hydraulic unit.

length



52

Elongation Table

Length of Wire Elongation

10m 3.5cm

20m 7.0cm

30m 10.6cm

50m 17.0cm



53

Concreting



54

Concrete Mixture

Cement : Sand : Coarse aggregate

Mixture Grade Beam length1:1½:3 40 < 4m

1:1:2 50 4m< <5m





56

Without Interruption

Complete Casting within the shortest

time possible Vibrate while concreting using Form

Vibrators



57

Vibrate until air

bubbles popping

stops Vibrate evenly

along the

formwork

About Vibration……..



58

Over vibration

Over vibration is dangerous to the concrete

Aggregates settle to sediment at the bottom

and only cement and sand would collect attop

As a result aggregate would not bond and

remain weak and break

See that the aggregates remain visible on the

top!



59

Under Vibration

If the concrete is not sufficiently vibratedair bubbles remain between the

aggregates and become later visible! And honeycombs

These honeycombs are specially unacceptable for fair face concrete

Honeycombing is also the cause forwater leakage



60

Leveling

Use the upper edge of the form work as

the guide to level the concrete

Use a large heavy trowel and floated onthe surface exactly down to formwork

edges until the beams are absolute

straight and even



61

Make yourself a wooden Float

This wooden plank with the handle is ideallysuited for floating, plastering and leveling

(60X15X2cm plank)

The thickness of the handle should be 4cm



62

Removal

The formwork can be removed after onefull day

Do not remove the hydraulic unit.The beams need to be cured for 4-5days under tension

Remove the blocks and drop theformwork on the floor

Set the formwork up for the next cast



63

Curing

Keep the beams covered with bags toprevent them from heating up. Heat isharmful to the bonding process of the

cement. Water the beams every day. Cement

cannot harden without water.



64

Making a Floor Slab

Cast a slab with pre-cast

beams



65

Set The Pre-stressed Beams



66

Add the pre-cast concrete planks



67

Pre-cast Slab Planks !

Pre-cast the slab planks to be ready

when they are needed for the floor slab

Reinforce them with 10X10cm mesh Cast them on the ground in a matrix of

60X60cm

The thickness should be about 5 ..7cm

C i C Pl k f



68

Casting Concrete Planks for a

Floor Slab



69

Curing

Remove the planks carefully on the next

day and keep them under cover over a

waterbed The humidity is sufficient to cure the

cement

W t b d ith Pl ti Sh t



70

Waterbed with Plastic Sheet

Cover Makes a Curing Chamber

Reapers



71

Method

L l th ll ith 10



72

Level the walls with a 10 cm

Screed Concrete



73

Lift the Beams onto the Walls



74

Place them in a 60cm Pitch

Fill U th S ith



75

Fill Up the Spaces with

Brickwork

Pl th t t



76

Place the pre-cast concrete

Planks evenly

Add a brick frame as form ork



77

Add a brick frame as formwork

for the rendering



78

Add a wire mesh on spacers

Do the rendering section by



79

Do the rendering section by

section



80

Roof Slab

The pre-cast slab is a very

good roof



81

Concrete does not leak

Usually people think that flat roofs leak.

This is not necessarily true.

For one, concrete does not leak if there isno honeycombing.

and if the concrete is not exposed to the

sun.

I i h



82

It is the exposure

to sun that makesthe concrete slab

crack,

not the water !



83

The best heat insulation

Compacted clay is the perfect insulation

against heat and rain. This property hasbeen used for roof insulation by the

ancient and even now in Pakistan and

India.



84

The best heat insulation

If the concrete slab is insulated by a

layer of 30cm clay the slab does notcrack and never leaks.





86

A cool house

The clay layer

absorbs heat and

water

This ensures very

good cooling of the

rooms

Instead of cement

plaster clay or

cement tiles can be

laid in cement as

well

A balustrade and

foliage makes it arooftop garden

After all, you get a roof



87

y g

top garden free



88

A pre-cast roof slab

does not cost more !

Compare the cost of a roof

construction with a pre-castslab



89

Comparison of Rates /sq.m

Roof

construc-

tion with

tiles cover

Roof

constructi

on with

asbestos

sheet

cover

Asbestos

Ceiling

Pre-

stressed

beams

with floor

slab

Rs1280

Rs1024

Rs500

Rs1382

Dimensions of Pre-stressed



90

Dimensions of Pre-stressed

Beams

There are standard lengths available for

house construction !



91

Dimensions

Length- l

Height-h

Bottom width – 5.04 cm

Top width-w



92

The cross sectional area of beams

depend on their length

The table shows dimensions and loads

permitted

Dimensions in Relation to Length



93

g

& Load

Length

-l-

Height

-h-

Top

Width

-w-

UDL concentrated

> 3m 10 cm 10 cm 153kg/m2

200 kg

>3.65m 13 cm 13 cm Do Do

>4.25m 15.5cm 15.5cm Do Do

>5.25m 18 cm 18 cm Do Do

Load

Introduction to Load



94

Introduction to Load

Parameters

In this section we show you

to differentiate betweenload parameters



95

Dead Load

The dead load is the weight of the slab

including beams and any other layer like

rendering, tiles etc





97

Concentrated Load

This term denotes the maximum load

allowed on a spot of a slab.



98

The load parameters of slabs

The rated live

load is usually= 153 kg/m2

The rated

concentratedload is usually

= 215 kg



99

Live Load on a Slab=

Area of the slab x rated live load

1.0 X 0.6 X 15391.8 kg



100

Calculation of Dead Load

In the next exercises we learn how tocalculate the dead load of a beam, this

is the weight which we must know for

transport and lifting

Calculate the Dead Load of



101

Calculate the Dead Load of

Beams:

The Weight is

= Volume x weight of concrete

Th C S i l A



102

The Cross Sectional Area

A = c x ½ (a + b) c

b

a

Th V l f B



103

The Volume of a Beam

A

l

V = A x

Th W i ht f B



104

The Weight of a Beam

W = Volume x

2300kg



105

The Weight of Pre-stressed

concrete is 2300kg/m3



106

Weight of beam =

Sectional Area x length xwt of concrete

0.0075 X 1 X 2300

17.25 kg

Calculate the Weight of a



107

C cu e e We g o

Concrete Slab

Length x Width x Thickness x Wt of Mass concrete

1

w

t

x w x t x 2200

Th W i ht f M C t



108

The Weight of Mass Concrete

2200 kg/m3



109

Weight of Slab =Length X Width X Thickness

X Wt of Mass Concrete

1.0 X 0.6 X 0.08 X 2200

105kg



P t d B i P h



111

Pre-stressed Beams in a Porch

Brick

border

Plaster Clay

Prestressed

Beam

ColumnHouse

Slab



112

House roof slabPrestressed

beams

Over lapping

Main

wall

Column

Provide



113

Insert the

anchor through

the slots.

slots for the

anchorwhile

casting by

inserting

rigid foam

T32



114

Add a

formwork in

the size of

the column.

And cast



115

And cast

the entirecorner on

top of the

column



116

An Alternative

Commercially available

Pre-cast beams with soffitmasonry blocks

Pre stressed Beam



117

Pre-stressed Beam

3.5 cm

3.96m - 4.87m

4.5cm

110cm

3.5cm

Soffit Masonry Block



118

Soffit Masonry Block

52.7cm

10.0cm

20.3cm



119

Construction Method

Place the Beams on the Wall



120

(single beam for housing)

57.2cm

Insert soffit masonry blocks



121

Insert soffit masonry blocks

Double beam (for shop & Office)



122

Double beam (for shop & Office)

57.2cm11cm11cm

Lay a 10 x10 mesh on spacer



123

blocks

Lay a 10 x10 mesh on spacer



124

blocks

Add the Formwork



125

Add the Formwork

Lay a 5 cm concrete Layer



126





(Grade 20)

Documents

Pre Stressed Beams