detailed study and execution work in post tension slabs

8/22/2019 DETAILED STUDY AND EXECUTION WORK IN POST TENSION SLABS

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Project Report

On

DETAILED STUDY AND EXECUTION WORK INPOST TENSION SLABS

A project report submitted in the partial fulfilment ofRequirement for the award of the degree of

Bachelor of Technology

In

Civil Engineering

(2011-12)

By

DIVYA KAMATH (08241A0113)

KASIREDDY VANDANA REDDY (08241A0155)

Under the esteemed guidance of

Mr. K. Suresh Reddy

Managing Director

Crux Prestressing Systems Pvt.Ltd.

(External Guide)

Mr. B.H. Mahesh ChandrakanthAssistant professor

Department of Civil Engineering

Gokaraju Rangaraju Institute of Engineering and Technology

(Affiliated to JNTU),

Bachupally, Nizampet Road,Hyderabad-90.


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GOKARAJU RANGARAJU

INSTITUTE OF ENGINEERING AND TECHNOLOGY

HYDERABAD

CERTIFICATE

This is to certify that the dissertation entitled DETAILED STUDY AND EXECUTION WORK

IN POST TENSIONED SLABS is a bonafide project work done under the guidance ofMr. K.

SURESH REDDY (MANAGING DIRECTOR, CRUX PRESTRESSING SYSTEMS

PVT.LTD.) and Mr. B.H.MAHESH CHANDRAKANTH (ASSISTANT PROFESSOR IN

THE DEPARTMENT OF CIVIL ENGINEERING, GRIET.)

GRIET, Hyderabad.

Project by


K.VANDANA REDDY (08241A0155)

Prof. Dr. G. Venkata Ramana Dr. J.N. Murthy B.H. Mahesh Chandrakanth

HOD, Civil Engineering Principal, GRIET Internal guide, Civil Engineering


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STUDENT DECLARATION

We hereby declare that the project entitled "Detailed Study and Execution work in

Post Tensioned Slabs" is the work done by us during the academic year 2011-2012

with Crux Prestressing Systems Pvt. Ltd. The site is situated at Kothaguda Village

nearby hi-Tech city. The building proposed is on a site with an area of 70,500 sq ft.

and is a 2 basement + G + 5. This was taken up by Crux Prestressing Systems Pvt.

Ltd. for Ektha builders. The name of the building is Ektha Pearl and it is a

Commercial Complex. This project report is submitted in partial fulfillment of the

requirements for the award of degree of Bachelor of Technology (B.Tech) in

CIVIL ENGINEERING from Gokaraju Rangaraju Institute of Engineering and

Technology, affiliated to JNTU, Hyderabad.


K.VANDANA REDDY (08241A0155)


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ACKNOWLEDGEMENT

Salutations to our beloved and highly esteemed institute "Gokaraju

Rangaraju Institution of Engineering and Technology" for having well qualified

staff and labs furnished with necessary equipment and computers.

First of all we would like to express our deep sense of gratitude towards Our

Principal, Dr. Jandhyala.N.Murthy and Head of the Department, Civil Engineering

Dr. G. Venkata Ramana, for giving us the opportunity to do an industry oriented

project work. We would also like to thank Mr. K. Suresh reddy, Managing

Director, CRUX Prestressing Pvt. Ltd. who has given us the opportunity to work in

his company.

We are very grateful to Mr. T. Anil Kumar, Senior Consultant Engineer at

CRUX Prestressing Pvt.ltd. who has guided and explained every detail concernedwith the execution work at the project site.

Finally, we would like to thank our project guide, Mr.B.H.Mahesh

Chandrakanth, Asst.Professor, Department of Civil Engineering at Gokaraju

Rangaraju Institute Of Engineerinng And Technology for always being available

when we required his guidance as well as for motivating us throughout the project

work.

Special thanks all our friends for their help and constructive criticism duringour project period for always being available and guiding us throughout the

project.


K. VANDANA REDDY (08241A0155)


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AbstractPost-Tensioning is a method of reinforcing concrete, masonry and other structural

elements.Post-Tensioning is a method of pre-stressing. Pre-stressed concrete or masonry has

internal stresses (forces) induced into it during the construction phase for the purpose of

counteracting the anticipated external loads that it will encounter during its lifecycle.

Post-Tensioned reinforcing consists of very high strength steel strands or bars. Typically, strands

are used in horizontal applications like foundations, slabs, beams, and bridges; and bars are used

in vertical applications like walls and columns. A typical steel strand used for post-tensioning

has a tensile strength of 1860 N/mm2. In comparison, a typical non-pre-stressed piece of

reinforcing bar (rebar) normally has a tensile strength of about 600 N/mm2.

In this project, we have seen and studied thoroughly some works on the site related to post

tensioned slabs. The various sizes of tendons available, which are the materials imparting

prestress to the structure were studied thoroughly. We have also understood and performed

building column line staking on site. We have visited the site and taken part in the execution

work under the structural engineer in-charge. The unbondedtendons are typically prefabricated

at a plant and delivered tothe construction site, ready to install. The tendons are laidout in the

forms in accordance with installation drawings that indicate how they are to be spaced, what

their profile (height above the form) should be and where they are to be stressed. After the

concrete is placed( from the RMC trucks) and has reached its required strength, usually about

75% of its final strength, then the prestressing process begins. The concrete grade that was used

was M35 and hence, after 7 days when it achieved the strength of 25 N/mm2, prestressing was

achieved through Prestressing powerpack using a mono strand stressing jack. The principle is

that when the tendons are stretched, want to return to their original length but are prevented from

doing so by the anchorages. The fact that the tendons are kept in a permanently stressed

(elongated) state causes a compressive force to act on the concrete. The compression that results

from the post tensioning counteracts the tensile forces created by the prestress applied. This

significantly increases the load-carrying capacity of the concrete.


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Since post-tensioned concrete is cast in place at the job site, there is almost no limit to the shapes

that can be formed. Curved facades, arches and complicated slab edge layouts are often a

trademark of post-tensioned concrete structures. Post tensioning is only advancing more and

more with increasing innovation of creative and environment friendly materials. An important

aspect to be kept in mind is the sustainability. Post tensioned concrete has a large number of

advantages when compared to conventional RCC construction, one of the most important being

the reduction in self weight and overall reduction in the requirement of materials. It is of course

mandatory to use high quality materials which should be able to withstand the prestress.

However, one thing is for certain. Post tensioned concrete is going to serve the building

community for many more years to come given the numerous advantages it bears.


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NOTATIONS

fcr = flexural tensile strength of the concrete.

fck = characteristic compressive strength of cubes in N/mm

2

fc = compressive stress

c = compressive strain

o = strain corresponding to fck

cu = ultimate compressive strain

Ec = elastic modulus

cr,ult = ultimate creep strain

= creep coefficient

l = elastic strain

sh = ultimate shrinkage strain

s = distance between points of inflection

a = drape of tendon measured at centre of profile between points of inflection.

Pav = average prestressing force in tendon

Zt = the top section modulus

Zb = the bottom section modulus

M = the total out of balance moment

ft = Top fibre stress

fb = Bottom fibre stress

E = eccentricity of tendons, taken as positive below the neutral axis


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Ma = applied moment due to dead and live loads

Ms = moment from prestress secondary effects

Vc = shear resistance

b = width of the section

d = effective depth of tension reinforcement or tendons

bv = breadth of member for T I L beams the breadth of the rib

As = area of shear reinforcement

Aps = area of prestressing tendons

Mo = moment necessary to produce zero stress in the concrete at the extreme tension fibre

P = the total prestress force over the panel width after all losses

Ac = concrete section area across the full panel width

Zt = section modulus for the top fibre over the width of the side of the critical perimeter

P = the total prestress force for all tendons passing through the side of the critical perimeter

e = eccentricity of the prestress force


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S.No Contents Page No.

1. Introduction

1.1 History

1.2 Types of prestress

1.3 Important terminology

1.4 Applications

1.5 Materials used in Prestressing

1.6 Post Tensioning Systems

1.7 Devices

1.8 Structural Behaviour

1.9 Structural form

1.10 Losses in prestress

1.11 Construction of prestressed

Concrete Structures

1.12 Maintenance and rehabilitation

of Prestressed Structures

2 Site Inspection

2.1 Initial Assessment

2.2 Soil Testing

2.3 Acquiring of raw materials

2.4 Modes of Transport


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3 Preliminary Works at Site

3.1 Material procuring

3.2 Site cleaning

3.3 Building column line staking

4 The Design Process

4.1 Introduction

4.2 Basic analysis

4.3 Structural Layout

4.4 Loading

4.5 Equivalent Frame Analysis

4.6 Tendon Profile and Balance load

4.7 Flexural section design

5 Execution

5.1 Materials

5.2 Machinery used

5.3 Anchorage markings

5.4 Laying of tendons

5.5 Concrete pouring

5.6 Prestressing

5.7 Grouting

6 CONCLUSIONS

7 BIBLIOGRAPHY


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DETAILED STUDY AND

EXECUTION WORK IN

POST TENSIONED SLABS


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1.INTRODUCTIONThe present report is based on the project work on Detailed study and Execution work in post

Tensioned Slabs. The site was a commercial complex at Kothaguda Village nearby Hi-Tech

city. The building proposed is on a site with an area of 70,500 sq ft. and is a 2 basement + G + 5

building. This was taken up by the pioneer company Crux Prestressing Systems Pvt. Ltd. for

Ektha builders. The name of the building is Ektha Pearl.

Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and

distribution are introduced so that the stresses resulting from external loads are counteracted to a

desired degree. In Reinforced Concrete members, the prestress is commonly introduced by

tensioning the steel reinforcement. The earliest examples of wooden barrel construction by force

fitting of metal bands and shrink fitting of metal tyres on wooden wheels indicate that the art of

prestressing has been practiced from ancient times. The tensile strength of plain concrete is only

a fraction of its compressive strength and the problem of it being deficient in tensile strength

appears to have been the driving factor in the development of the composite material known as

Reinforced Concrete.

The development of early cracks in reinforced concrete due to incompatibility in the strains of

steel and concrete was perhaps the starting point in the development of a new material like

prestressed concrete. The application of permanent compressive stress to a material like

concrete, which is strong in compression but weak in tension, increases the apparent tensile

strength of that material, because the subsequent application of tensile stress must first nullify the

compressive prestress.

1.1 HistoryIn 1904, Freyssinet attempted to introduce permanently acting forces in concrete to resist the

elastic forces developed under loads and this idea was later developed under the name ofprestressing. In 1886, Jackson of San Fransisco applies for a patent for construction of

artificial stone and concrete pavements, in which prestress was introduced by tensioning the

reinforcing rods set in sleeves. Dohring of Germany manufactured slabs and small beams using

embedded tensioned wires in concrete to avoid cracks. However, the idea of prestressing to


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counteract the stresses due to loads was first put forward by Mandl in 1896. Later on, the

importance of losses in prestressing due to Shrinkage of concrete was recognized.

The use of unbounded tendons was first demonstrated by Dischinger in 1928, in the construction

of a major bridge of the deep girder type, in which prestressing wires were placed inside the

girder without any bond. Losses of prestress were compensated by the subsequent re-tensioning

of the wires. Other advancements like development of vibration techniques for the production of

high strength concrete and the invention of the double-acting jack for stressing high tensile steel

wires are considered to be the most significant contributions.

1.2Types of prestressing1) Pre tensioning:

A method of prestressing concrete in which the tendons are tensioned before the

concrete is placed. In this method, the prestress is imparted to concrete by bond between

steel and concrete. The concrete is cast around already tensioned tendons. This method

produces a good bond between the tendon and the concrete, which both protects the

tendon from corrosion and allows for direct transfer of tension. The cured concrete

adheres and bonds to the bars and when the tension is released, it is transferred to the

concrete as compression by static friction. However, it requires stout anchoring points

between which the tendon is to be stretched and the tendons are usually in a straight line.

Thus, most pretensioned concrete elements are prefabricated in a factory and must be

transported to the construction site, which limits their size. Pre-tensioned elements are

mostly balcony elements, lintels, floor slabs, beams or foundation piles. An innovative

bridge construction method using prestressing is the stressed ribbon bridge design

2) Post tensioning:It is a method of prestressing concrete by tensioning the tendons against hardened

concrete. In this method, the prestress is imparted to concrete by bearing. Post tensioned

concrete may be either bonded or un-bonded.

Bonded post tensioned concrete:

The term used for a method of applying compression after pouring concrete and

the curing process. The concrete is cast around plastic, steel or aluminum curved duct to


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follow the area where otherwise tension would occur in the concrete element. A set of

tendons are fished through the duct to follow the area where otherwise tension would

occur in the concrete element, and then concrete is poured. Once the concrete is

hardened, the tendons are tensioned by hydraulic jacks that react (push) against the

concrete member itself. When the tendons have stretched sufficiently, according to the

designed specifications, they are wedged in position and maintain tension after the jack is

removed, transferring pressure to the concrete. The duct is then grouted to protect the

tendons from corrosion. The method is commonly used to create monolithic slabs for

house construction in locations where expansive soils (such as adobe clay) create

problems for the typical perimeter foundation. All stresses from seasonal expansion and

contraction of the underlying soil are taken into the entire tensioned slab, which supports

the building without supports the building without significant flexure. Post tensioning is

also used in the construction of various bridges, both after concrete is cured, support by

false work and by the assembly of prefabricated sections.

Advantages

Large reduction in traditional reinforcement requirements such as tendons cannotdistress in accidents.

Tendons can be easily woven allowing a more efficient design approach.

Higher ultimate strength due to bond generated between the strand and concrete. No long term issues in maintaining the integrity of the anchor end.

Un Bonded post tensioned concrete:

It differs from bonded post tensioning by providing each individual cable permanent

freedom of movement relative to the concrete. To achieve this, each individual tendon is coated

with grease (generally lithium base) and covered by a plastic sheathing formed in an extrusion

process. The transfer of tension to the concrete is achieved by the steel cable acting against steel

anchors embedded in the perimeter of the slab. The main disadvantage over bonded post

tensioning is the fact that a cable can distress itself and burst out of the slab if damaged (such as

during repair on the slab).


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The advantages are:

The ability to individually adjust cables based on poor field conditions (For ex: shifting agroup of four cables around an opening by placing two to either side.)

The procedure of post stress grouting is eliminated. The ability to distress the tendons before attempting repair work.

1.3Important Terminology1) Tendon: A stretched element used in a concrete member of structure to impart prestress

to the concrete. Generally, high tensile steel wires, bars, cables or strands are used as

tendons.

2) Anchorage: A device generally used to enable the tendon to impart and maintain prestressin the concrete. The commonly used anchorages are the Freyssinet (widely used in India),

Magnel Blaton, Gifford-Udall.

3) Partial prestressing: The degree of prestress applied to concrete in which tensile stressesto a limited degree are permitted in concrete under working loads. In this case, in addition

to tensioned steel, a considerable proportion of untensioned reinforcement is generally

used to limit the width of cracks developed under service loads.

4) Transfer: This stage corresponding to the transfer of prestress to concrete. For posttensioned members, it takes place after the completion of the tensioning process.

5) Supplementary or untensioned reinforcement: It is the Reinforcement in prestressedmembers not tensioned with respect to the surrounding concrete before the application of

loads. These are generally used in partially stressed members.

6) Transmission length: It is the length of bond anchorage of the prestressing wire from theend of a pre-tensioned member to the point of full steel stress.

7) Cracking load: The load on the structural element corresponding to the first visible crack.8) Creep in concrete: Progressive increase in the elastic deformation of concrete under

sustained stress component.

9) Shrinkage of concrete: Contraction of concrete on drying.10)Relaxation in steel: Decrease of stress in steel at constant strain.11)Proof stress: The tensile stress in steel which produces a residual strain of 0.2 per cent of

the original gauge length on unloading.


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12) Creep Co-efficient: The ratio of the total creep strain to elastic strain in concrete.13)Cap cable: A short curved tendon arranged at the interior supports of a continuous beam.

The anchors are the compression zone; cable is the curved portion is in the tensile zone.

1.4ApplicationsThe use of prestressed concrete has revolutionized the entire building industry in the erstwhile

U.S.S.R., U.S.A, U.K, Japan and the Continent. Prestressed concrete building components

comprising hollow cored and ribbed slabs are widely used in the erstwhile Russia. Single and

double tee units and channel sections are popular in the U.S.A. for the construction of floors in

buildings. Prestressed concrete is ideally suited for long-span bridge construction. A typical

twin-box girder bridge under construction, using the segmentally cast cantilever method

Fig2.GangaBridge,Patna.Lengthof

5575metresandconsistsofspansof

121.65mlongprestressedconcrete

Griders

Fig1.CN,Toronto,Canada Worlds

tallestPrestressedbuildingwhichis

553mtall

The present trend is to adopt prestressed concrete for long span cable-stayed bridge of 365 m

Tower main span, constructed at Tampa Bay, Florida, U.S.A. The longest precast prestressed

concrete cable-stayed box girder, the chaco-Corrientes bridge was constructed in Argentina,

South America. Typical use of prestressed concrete simple-span box girders for the Bay area


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rapid-transit system is displayed in bridge in San Fransisco, California. Prestressed concrete has

found extensive applications in the construction of long-span folded plate roofs, aircraft hangers,

nuclear containment vessels, pavements, rail road sleepers, poles, piles, television towers and

masts.

Notable examples of prestressed concrete structures in India

The Lubha Bridge, the nations longest single-span 172 m long prestressed concrete box-girder type continuous bridge built across a 30m deep gorge of the Lubha river in Assam.

Ball tank, Trombay, Maharashtra, consisting of a prestressed concrete, tank of 4 millionlitre capacity for the department of atomic energy.

Ganga bridge at Patna, the longest prestressed concrete bridge in the world has a lengthof 5575 m consisting of continuous spans of 121.65 m long prestressed concrete girdersof variable depth.

1.5 Materials used in prestressingMaterials used in prestressing are

1) High strength concrete2) High tensile steel

Constituents of concrete:-

Concrete is a composite material composed of gravels or crushed stones (coarse aggregate), sand

(fine aggregate) and hydrated cement (binder).

Properties of Hardened Concrete:

The concrete in prestressed applications has to be of good quality. It requires the following

attributes:-

1) High strength with low water-to-cement ratio

2) Durability with low permeability, minimum cement content and proper mixing, compaction

and curing.

3) Minimum shrinkage and creep by limiting the cement content.


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Strength of Concrete:-

For prestressed concrete applications, high strength concrete is required for the following

reasons:-

1) To sustain the high stresses at anchorage regions.

2) To have higher resistance in compression, tension, shear and bond.

3) To have higher stiffness for reduced deflection.

4) To have reduced shrinkage cracks.

Compressive strength:

The minimum grades of concrete for prestressed applications are as follows:-

35 MPa for post-tensioned members

40 MPa for pre-tensioned members.

The maximum grade of concrete is 60 MPa.

Tensile strength :-

The tensile strength of concrete can be expressed as follows:-

1) Flexural tensile strength: It is measured by testing beams under 2 point loading (also called 4

point loading including the reactions).

2) Splitting tensile strength: It is measured by testing cylinders under diametrical compression.

3) Direct tensile strength: It is measured by testing rectangular specimens under direct tension.

fcr= 0.7(fck)

where,

fcr= flexural tensile strength of the concrete.

fck= characteristic compressive strength of cubes in N/mm2


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1.5.1High Performance Concrete

With the advancement of concrete technology, high performance concrete is getting popular in

prestressed applications. The attributes of high performance concrete are as follows:-

1) High strength

2) Minimum shrinkage and creep

3) High durability

4) Easy to cast

5) Cost effective.

Traditionally high performance concrete implied high strength concrete with high cement

content and low water-to-cement ratio.

But higher cement content leads to autogenous and plastic shrinkage cracking and thermal

cracking.

At present durability is also given importance along with strength.

Some special types of high performance concrete are as follows:-

1) High strength concrete

2) High workability concrete

3) Self-compacting concrete

4) Reactive powder concrete

5) High volume fly ash concrete

6) Fibre reinforced concrete

In a post-tensioned member, the concrete next to the anchorage blocks (referred to as end block)

is subjected to high stress concentration. The type of concrete at the end blocks may be different


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from that at the rest of the member. Fibre reinforced concrete is used to check the cracking due

to the bursting forces.

Allowable Compressive Stresses under Direct Compression:-

For direct compression, except in the parts immediately behind the anchorage, the maximum

strain is equal to 0.8 times the maximum compressive stress under flexure.

Allowable Tensile Stresses under Flexure:-

The prestressed members are classified into three different types based on amount of

prestressing:

Type -1 : No tensile stress

Type - 2 : 3 N/mm2

to 4.5 N/mm2

Type - 3 : hypothetical values

Stress-strain Curves for Concrete:-

Curve under uniaxial compression

The stress versus strain behavior of concrete under uniaxial compression is initially linear (stress

is proportional to strain) and elastic (strain is recovered at unloading). With the generation of

micro cracks, the behavior becomes nonlinear and inelastic. After the specimen reaches the peak

stress, the resisting stress decreases with the increase in strain

Fig3.ConcreteCubeunder

Compressionandstressstraincurve

underflexure


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The equation for the design curve under compression due to flexure is as follows:-

Forco

fc = fck(2(c/o) (c/o)2Forc


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Fig4.StressStraincurveunderCompression

Curve under uniaxial tension:

The stress versus strain behavior of concrete under uniaxial tension is linear elastic initially.

Close to cracking nonlinear behavior is observed.

Fig5.ConcreteundertensionandStressstraincurveforconcreteundertension

In calculation of deflections of flexural members at service loads, the nonlinearity is neglected

and a linear elastic behavior fc = Ecc is assumed. In the analysis of ultimate strength, the

tensile strength of concrete is usually neglected.

Creep of Concrete :

Creep of concrete is defined as the increase in deformation with time under constant load. Due to

the creep of concrete, the prestress in the tendon is reduced with time.


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Hence, the study of creep is important in prestressed concrete to calculate the loss in prestress.

The creep occurs due to two causes:-

1. Rearrangement of hydrated cement paste (especially the layered products)

2. Expulsion of water from voids under load

The creep strain depends on several factors. It increases with the increase in the following

variables:-

1) Cement content (cement paste to aggregate ratio)

2) Water-to-cement ratio

3) Air entrainment

4) Ambient temperature.

The creep strain decreases with the increase in the following variables :-

1) Age of concrete at the time of loading.

2) Relative humidity

3) Volume to surface area ratio.

The creep strain also depends on the type of aggregate.

cr,ult = l

where,

cr,ult = ultimate creep strain ; = creep coefficient

l= elastic strain


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Fig 6. Variation of strain with time for

concrete under compression

Shrinkage of Concrete:

Shrinkage of concrete is defined as the contraction due to loss of moisture. The study of

shrinkage is also important in prestressed concrete to calculate the loss in prestress.

The shrinkage occurs due to two causes

1. Loss of water from voids

2. Reduction of volume during carbonation

The following figure shows the variation of shrinkage strain with time. Here, t0 is the time at

commencement of drying. The shrinkage strain increases at a decreasing rate with time. The

ultimate shrinkage strain (sh) is estimated to calculate the loss in prestress.


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Fig 7. Variation of shrinkage

strain with time

Like creep, shrinkage also depends on several factors. The shrinkage strain increases with the

increase in the following variables:

1) Ambient temperature

2) Temperature gradient in the members

3) Water-to-cement ratio

4) Cement content.

The shrinkage strain decreases with the increase in the following variables:- (spacing

corrections)

1) Age of concrete at commencement of drying

2) Relative humidity

3) Volume to surface area ratio.

The shrinkage strain also depends on the type of aggregate.

IS:1343 - 1980 gives guidelines to estimate the shrinkage strain in Section 5.2.4. It is a simplified

estimate of the ultimate shrinkage strain (sh).


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For pre-tension

sh = 0.0003

For post-tension

sh = (0.0002/log10(t+2))

Where,

t is the age at transfer in days.

Note that for post-tensioning, t is the age at transfer (in days) which approximates the curing

time.

1.5.2 Prestressing Steel

The development of prestressed concrete was influenced by the invention of high strength

steel. It is an alloy of iron, carbon, manganese and optional materials. The following material

describes the types and properties of prestressing steel.

Wires

A prestressing wire is a single unit made of steel. The nominal diameters of the wires are 2.5,

3.0, 4.0, 5.0, 7.0 and 8.0 mm. The different types of wires are as follows:-

1) Plain wire:No indentations on the surface.

2) Indented wire: There are circular or elliptical indentations on the surface.

1.5.3 Strands

A few wires are spun together in a helical form to form a prestressing strand. The different types

of strands are as follows:-

1) Two-wire strand: Two wires are spun together to form the strand.

2) Three-wire strand: Three wires are spun together to form the strand.


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3) Seven-wire strand:In this type of strand, six wires are spun around a central wire. The central

wire is larger than the other wires.

1.5.4 Tendons

A group of strands or wires are placed together to form a prestressing tendon. The tendons are

used in post-tensioned members. The following figure shows the cross section of a typical

tendon. The strands are placed in a duct which may be filled with grout after the post-tensioning

operation is completed.

Cables

A group of tendons form a prestressing cable. The cables are used in bridges.

Bars

A tendon can be made up of a single steel bar. The diameter of a bar is much largerthan that of a

wire. Bars are available in the following sizes: 10, 12, 16, 20, 22, 25, 28 and 32 mm.

Fig. 8 Forms of reinforcing and

prestressing steel

Types of Prestressing Steel:

The steel is treated to achieve the desired properties. The following are the treatmentprocesses:

Cold working (cold drawing):

The cold working is done by rolling the bars through a series of dyes. It re-aligns the crystals

and increases the strength.


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Stress relieving:-

The stress relieving is done by heating the strand to about 350 C and cooling slowly. This

reduces the plastic deformation of the steel after the onset of yielding.

Strain tempering for low relaxation:-

This process is done by heating the strand to about 350 C while it is under tension. This also

improves the stress-strain behavior of the steel by reducing the plastic deformation after the onset

of yielding. In addition, the relaxation is reduced.

Behavioural Properties of Prestressing Steel:

The steel in prestressed applications has to be of good quality. It requires the following attributes

1) High strength

2) Adequate ductility

3) Bending ability, which is required at the harping points and near the anchorage

4) High bond, required for pre-tensioned members

5) Low relaxation to reduce losses

6) Minimum corrosion.

Strength of Prestressing Steel :

The tensile strength of prestressing steel is given in terms of the characteristic tensile

strength (fpk). The characteristic strength is defined as the ultimate tensile strength of the coupon

specimens. The ultimate strength of a plain hard drawn steel wire varies with its diameter. The

tensile strength decreases with increase in the diameter of wires.

For high tensile steel bars (IS: 2090), the minimum tensile strength is 980 N/mm2. The proof

stress should not be less than 80% of the specified tensile strength.


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Stiffness of Prestressing Steel :

The stiffness of prestressing steel is given by the initial modulus of elasticity. The modulus

of elasticity depends on the form of prestressing steel (wires or strands or bars).

IS:1343 - 1980 provides the following guidelines which can be used in absence of test

Type of steel Modulus of elasticity

Cold drawn wires 210 KN/mm2

High tensile steel bars 200 KN/mm2

Strands 195 KN/mm2

Allowable Stress in Prestressing Steel :

As per Clause 18.5.1, IS: 1343 1980 the maximum tensile stress during prestressing (fpi) shall

not exceed 80% of the characteristic strength.

Fpi = 0.8fpk

There is no upper limit for the stress at transfer (after short term losses) or for the effective

prestress (after long term losses).

Stress-Strain Curves for Prestressing Steel:

The stress versus strain behaviour of prestressing steel under uniaxial tension is initially

linear (stress is proportional to strain) and elastic (strain is recovered at unloading).

Beyond about 70% of the ultimate strength the behaviour becomes nonlinear and inelastic. There

is no defined yield point. The yield point is defined in terms of the proof stress or a specified

yield strain. IS:1343- 1980 recommends the yield point at 0.2% proof stress. This stress

corresponds to an inelastic strain of 0.002.

1.5.5Relaxation of steel

Relaxation of steel is defined as the decrease in stress with time under constant strain. Due

to the relaxation of steel, the prestress in the tendon is reduced with time.


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Hence, the study of relaxation is important in prestressed concrete to calculate the loss in

prestress. The relaxation depends on the type of steel, initial prestress and the temperature.

The following figure shows the effect of relaxation due to different types of loading conditions:-

Fig 9. Effect of relaxation due to different

types of loading

Fatigue :

Under repeated dynamic loads the strength of a member may reduce with the number of cycles

of applied load. The reduction in strength is referred to as fatigue. In prestressed applications, thefatigue is negligible in members that do not crack under service loads. If a member cracks,

fatigue may be a concern due to high stress in the steel at the location of cracks.

Durability :

Prestressing steel is susceptible to stress corrosion and hydrogen embrittlement in aggressive

environments. Hence, prestressing steel needs to be adequately protected. For bonded tendons,

the alkaline environment of the grout provides adequate protection.

For unbonded tendons, corrosion protection is provided by one or more of the following

methods:-

1) Epoxy coating

2) Mastic wrap (grease impregnated tape)


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3) Galvanized bars

4) Encasing in tubes.

1.6 Post tensioning systems

The prestressing systems and devices are described for the two types of prestressing :-

pre-tensioning

post-tensioning

Stages of Post-tensioning :

In post-tensioning systems, the ducts for the tendons (or strands) are placed along with the

reinforcement before the casting of concrete. The tendons are placed in the ducts after the

casting of concrete. The duct prevents contact between concrete and the tendons during the

tensioning operation.

Unlike pre-tensioning, the tendons are pulled with the reaction acting against the hardened

concrete. If the ducts are filled with grout, then it is known as bonded post-tensioning. The

grout is a neat cement paste or a sand-cement mortar containing suitable admixture. In unbonded

post-tensioning, as the name suggests, the ducts are never grouted and the tendon is held in

tension solely by the end anchorages. Most of the commercially patented prestressing systems

are based on the following principles of anchoring the tendons:

1. Wedge action producing a frictional grip on the wires.2. Direct bearing from rivet to bolt heads formed at the end of the wires.3. Looping the wires around the concrete

The various stages of the post-tensioning operation are summarised as follows :-

1) Casting of concrete.

2) Placement of the tendons.

3) Placement of the anchorage block and jack.


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4) Applying tension to the tendons.

5) Seating of the wedges.

6) Cutting of the tendons.

The tensioning of tendons and pre-compression of concrete occur simultaneously. A system of

self-equilibrating forces develops after the stretching of the tendons

Fig10.Stagesinposttensioning

Advantages of Post-tensioning :

The relative advantages of post-tensioning as compared to pre-tensioning are as follows:

Post-tensioning is suitable for heavy cast-in-place members.

The waiting period in the casting bed is less.

The transfer of prestress is independent of transmission length.

Disadvantage of Post-tensioning:

The relative disadvantage of post-tensioning as compared to pre-tensioning is the requirement of

anchorage device and grouting equipment.


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1.7 Post Tensioning Devices:

1) Casting bed (1line)

2) Mould/Shuttering (aluminium shuttering, resist impcts, highly corrosive)

3) Ducts

4) Anchoring devices

5) Jacks

6) Couplers (optional)

7) Grouting equipment (optional)

Fig 11 . Casting bed, mould and duct

Anchoring Devices:

In post-tensioned members the anchoring devices transfer the prestress to the concrete.

The devices are based on the following principles of anchoring the tendons:-

1) Wedge action

2) Direct bearing

3) Looping the wires


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Wedge action:

The anchoring device based on wedge action consists of an anchorage block and wedges. The

strands are held by frictional grip of the wedges in the anchorage block.

Direct bearing:

The rivet or bolt heads or button heads formed at the end of the wires directly bear against a

block.

Looping the wires:

The Baur-Leonhardt system, Leoba system and also the Dwidag single-bar anchorage system,

work on this principle where the wires are looped around the concrete. The wires are looped to

make a bulb.

Jacks:

Fig12.Jackingandanchoringwithedges

Couplers:

The couplers are used to connect strands or bars. They are located at the junction of the

members, for example at or near columns in post-tensioned slabs, on piers in posttensioned

bridge decks.


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Fig 13. coupler for strands

1.8 Structural Behavior

1) Effects of prestress:The primary effects of prestress are a pre compression of the floor and an upward

load within the spam which balances pat of the downward dead and live loads. In a

Reinforced concrete floor, tensile cracking of the concrete is a necessary accompaniment to

the generation of economic stress levels in the reinforcement. In post tensioned flows with

the pre compression and the upward load in the span act to reduce the tensile stresses in the

concrete. However, the level of prestress is not usually enough to prevent all tensile

cracking under full design live loading at serviceability Limit State. Under reduced live load

much of the cracking will not be visible.

The act of prestressing causes the floor to bend, shorten, deflect and rotate. If any of

these effects are restrained, secondary effects of prestress are set up. As stated above,

2N/mm2.The secondary effects due to the restraint to shortening are usually neglected.

However, unless the floor can be considered to be statically determinate, the displacements

of the floor set up secondary moments which cannot be neglected.

2) One-way and two-way spanning floors:There are several different types post tensioned floor. An important distinction between

types of floors is whether they are one-way or two-way spanning structure.


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3. Flexure in flat slabs (two-way spanning):In the context of this report, flat slabs are those floors which can carry loads in two

different directions to discrete column supports. One misconception held by some engineers is to

consider a reduced load when analyzing the slab in one direction using the equivalent frame

method. A flat slab supported on columns, rather than perimeter beams can fail as a one-way

mechanism just as a single way slab and it should be reinforced to resist the moment from the

full load in each orthogonal direction.

Tests and applications have demonstrated that post tensioned flat slabs behave as a flat plate

almost regardless of tendon arrangement. The effects of tendons are critical to the behavior as

they exert loads in the span as well as provide reinforcement. The tendons exert equivalent

vertical load on the slabs known as equivalent loads and these loads may be considered like any

other dead or live loads. Since the tendon effect is opposite to the effect of gravity loads, the net

load causing bending is reduced. An additional effect of the tendons is the axial pre compression

which counteracts flexural tensile stresses. Therefore, at service dead load, the net downward

load causing bending in the slab is normally very low and the floor is essentially under uniform

axial compression.

Examination of the distribution of the moment in reveal that hogging moments across

panel are sharply peaked in the immediate vicinity of the column and the moment at the column

face is several times the moment midway between the columns. In contrast the sagging moments

across the slab in mid span regions are almost uniformly distributed across the panel width. It is

helpful to the understanding of post tensioned flat slabs to forget the arbitrary column strip,

middle strip and moment percentage tablets which have been long familiar to the designer of

reinforced concrete floors. Instead, the mechanics of the action of the tendons will be examined

first.

The load balancing approach is an even more powerful tool for examining the behavior of two-way spanning systems than it is for one way spanning members. By the balancing load approach,

attention is focused on the loads exerted on the floor by the tendons, perpendicular to the plane

of the floor. As for one-way floors, this typically means a uniform load exerted upward along the

major portion of the central length of a tendon span and statically equivalent downward load

exerted over the short length of the reverse curvature. In order to apply an essentially uniform


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upward load over the entire floor panel these tendons should be uniformly distributed and the

downward loads from the tendons should react against another structural element. The additional

element could be a beam or a wall in the case of one way floors or columns in a two-way system.

Fig14.BendingMomentSurfacesfor

differentArrangementoftendons


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Fig15.Appliedloadbendingmomentsinasolidflatslab

The load balancing approach is an even more powerful tool for examining the behavior of two-

way spanning systems than it is for one way spanning members. By the balancing load approach,

attention is focused on the loads exerted on the floor by the tendons, perpendicular to the plane

of the floor. As for one-way floors, this typically means a uniform load exerted upward along the

major portion of the central length of a tendon span and statically equivalent downward load

exerted over the short length of the reverse curvature. In order to apply an essentially uniform

upward load over the entire floor panel these tendons should be uniformly distributed and the

downward loads from the tendons should react against another structural element. The additional

element could be a beam or a wall in the case of one way floors or columns in a two-way system.

However, a look at a plan view if a flat slab reveals that columns provide an upward reaction for

only a very small area. Thus, to maintain statistical rationality, we must provide reinforcement

perpendicular to the above tendons, a second set of tendons to provide an upward load to resist

the downward load from the first set. Remembering that the downward load of the uniformly

distributed tendons occur over a relatively narrow width under the reverse curvatures and that the

only available exterior reaction, the column is also relatively narrow, it becomes obvious that the

second set of tendons should be in narrow strips or bands passing over the columns.

There are two ways of accompanying this two part tendon system to obtain the nearly uniform

upward load we desire for ease of analysis. In the first method, tendons are spaced uniformly in


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Fig16.Loadbalancingforprestressedtendonsforregularcolumnlayouts

each of the two directions and react against banded tendons along the column grid lines in each

direction. This results in some of the tendons in each direction being banded over the columns

and some uniformly distributed between these bands. This method works well where the

columns are arranged on a rectangular grid.

The balanced load provided by the tendons in each direction is equal to the dead load. It gives

the most uniform distribution of moments and provides a practical layout of tendons. This

arrangement gives 70% of the tendons in the banded zone and the remaining 30% between the

bands. It should be noted that, since the width of the banded zone is 0.4 times the width of bay,

this arrangement is identical to providing 50% of the tendons evenly distributed over the full

width of the bay in addition to 50% concentrated in the band. The detailed distribution is not

critical, provided that sufficient tendons pass through the column zone to give adequate

protection against punching shear and progressive collapse.

Flexural cracking is initiated at column faces and can occur at load levels in the

serviceability range. While these and early radial cracks remain small, they are unlikely to affect

the performance of the slab. Compression due to prestress delays the formation of cracks, but it


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is less efficient in controlling cracking than un-tensioned reinforcement placed in the top of

floors, immediately adjacent to and above the column.

Flat Slab Criteria:

For a prestressed floor to be considered as a flat slab the following criteria apply:

9 Pre compression should be applied in two orthogonal directions. Such a floor withno or moderate, crack formation performs as a homogenous elastic plate with its

inherent two-way behavior.

9 The pre compression at the edges of the slab is concentrated behind the anchoragesand spreads into the floor with increasing distance from the edge. Floors with banded

post-tensioning and floors with wide shallow beams also qualify for two-way action

at regions away from the free edges where pre compression is attained in both

directions.

Aspect ratio of any panel should not be greater than 2. This applies to solid flat

slabs, supported on orthogonal rows of columns. The ratio of the stiffness of the slab in two

orthogonal directions should not be disproportionate.

1.9 Structural form

1) Column Layout:In general, the ideal situation is, of course to think prestressing from the initial concept of

the building and to choose suitably longer spans. However, current experience in many

countries indicates a minimum span is of approximately 7m. In choosing column layouts

and spans for a prestressed floor, several possibilities may be considered to optimize the

design, which include:

a) Reduce the length of the end spans or if the architectural considerations permit, insert thecolumns from the building perimeter to provide small cantilevers. Consequently end span

bending moments will be reduced and a more equable bending moment configuration is

obtained.

b) Reduce, if necessary the stiffness of the columns to minimize the prestress lost inovercoming the restraint offered to floor shortening.


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c) Where span lengths vary, adjust the tendon profiles and the no. of tendons to provide theuplift required for each span. Generally this will be a similar percentage of the dead load

for each span.

Once the column layout has been determined, the next consideration is the type of floor to be

used. This again is determined by number of factors such as lengths, magnitude of loading,

architectural form and use of the building, special requirements such as services, location of

building, and the cost of materials available .

2) Floor thickness and types:The slab thickness must meet two primary functional requirements-structural

strength and deflection. Vibration should also be considered where there are only a few

panels. There selection of thickness or type (ex : plate without drops, plate with drops,

coffered or waffle, ribbed or even beam and slab) is also influenced by concrete strength and

loading. There are likely to be several alternative solutions to the same problem and a

preliminary costing exercise may be necessary in order to choose the most economical. The

info given in fig 14, 15 & 17 will assist the designer to make a preliminary choice of floor

section. Fig 15 is appropriate for all types of prestressed floors. Fig 15 & 17 are only

appropriate for flat slabs but fig 15 is not appropriate for coffered slabs which do not have a

solid section over the columns

Fig17.Preliminaryselectionoffloorthicknessformulti

s anfloors


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Flat slabs tend to exceed punching shear limits around columns and often need shear

reinforcement at these locations. The graphs in fig 18 provide a preliminary assessment as to

whether shear reinforcement is needed or not. As the shear capacity of a slab is dependent on the

dimensions of the supporting columns or column heads, each graph is based on different column

dimensions.

The following procedure should be followed while obtaining a flat slab section:-

Step 1: Knowing the span and imposed loading requirements, Fig 15 can be used to choose a

suitable span/ depth ratio

Step 2: Check the shear capacity of the section using fig 14. Obtain the imposed load capacity for

the chosen section. If this exceeds the imposed load, then shear reinforcement is not needed. If

not, reinforcement will be needed.

Step 3: Check the shear capacity at the face of the column using Fig 17. If the imposed load

capacity is exceeded, increase the slab depth and check again

3). Effect of restraint to floor shortening:

Post tensioned floors must be allowed to shorten to enable the prestress to be applied to the floor.

Shortening occurs because of :-

(a)Elastic Shortening due to the prestress force(b)Creep shortening due to the prestress force.(c)Shrinkage of concrete

Elastic shortening occurs during stressing of the tendons, but the creep and shrinkage are long

term effects.

The floor will be supported on columns or a combination of columns and core walls. These

supports offer a restraint to the shortening . There are no firm rules which may be used to

determine when such restraint is significant. A simple method of ascertaining the restraint

offered by the supports is to calculate the elastic creep and shrinkage strains expected in the slab

and then to calculate the forces required to deflect the supports.

Typical strains for a 300mm internal floor with a prestress of 2 N/mm2


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Elastic Strain 100 x 10-6

Creep strain 250 x 10-6

Shrinkage strain 300 x 10-6

Total long term strain 650 x 10-6

The force required to deflect each column may be assumed to be calculated as follows:

i = x Ii

Hi = 12 Ec Iii(Hcol)

3

For the purpose of calculating Hi, the value of Ec for the column may be reduced by creep in the

column and in some cases cracking. A reduction of at least 50% from the short term elastic

properties is normally justifiable.

The total tension in the floor due to the restraint to shortening is the sum of all the column forces

to one side of the stationary point the tension is H1 + H2 + H3. This tension acts as a reduction in

the pre compression of the floor by prestress.

1.10 Losses in prestress:

In prestressed concrete applications, the most important variable is the prestressing force. In

the early days, it was observed that the prestressing force does not stay constant, but reduces with

time.Even during prestressing of the tendons and the transfer of prestress to the concrete member,

there is a drop of the prestressing force from the recorded value in the jack gauge. The various

reductions of the prestressing force are termed as the losses in prestress.

The losses are broadly classified into two groups:-

1) Immediate2) Time-dependent.


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The immediate losses occur during prestressing of the tendons and the transfer of prestress to the

concrete member.

The time-dependent losses occur during the service life of the prestressed member. The losses

due to elastic shortening of the member, friction at the tendon-concrete interface and slip of the

anchorage are the immediate losses. The losses due to the shrinkage and creep of the concrete

and relaxation of the steel are the time-dependent losses.

1.11 Construction of prestressed concrete structures

Rapid development in construction techniques of prestressed concrete structures over the last

decades has resulted in several novel methods of construction.

Prestressed concrete being ideally suited for larger spans, its application is much less for sub

structures than for super structures.

Construction of any structure forms only a small part of the whole gamut of construction

management. Construction management functions comprise the following central activities:-

1. Tendering and winning the contract of given work2. Contract negotiations3. Developing liaison with clients4. Mobilizing financial resources for the work5. Maintaining proper accounts6. Work planning7. Work supervision8. Project progress control and monitoring9. Maintenance of good labour relations10. Engineering and completion of work.

Before starting the construction of any prestressed concrete structure, it is essential to consider

minimum requirements for material and workmanship which will result in a structure that will

perform satisfactorily in various limit states.


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The most important consideration of prestressed concrete structure is the design, production, and

control of high strength concrete with desirable properties.

Most of the long span bridges are built using prestressed concrete and those built by the

cantilever method demonstrate the latest refinements of this construction technique.

This method eliminates the use of expensive form work and scaffolding especially for bridges in

deep valleys and rivers with large depth of water.

1.12 Maintenance and rehabilitation of prestressed concrete structures

The fundamental objective of maintenance management of prestressed concrete structures in

such a way that it will function satisfactorily at various limit states immediately after

construction and also over a period covering the life span of the structure.

Good maintenance practice requires periodical surveillance, identification of local damage,

deterioration and loss of durability of the structure due to environmental and other load effects.

In prestressed concrete structures the primary problem encountered is the damage caused to the

anchorages and unbounded tendons due to rusting under the exposure to humid weather

conditions.

The overall objective of the maintenance of prestressed concrete structures is to identify the need

for structural integrity, periodical surveillance, repairs, rehabilitation, and replacement,

depending upon the local conditions.

Inspection of structures:

All types of remedial and preventive maintenance or minor repair work, including

replacement of components should be planned at periodical intervals without causing

inconvenience to the users of the structure.

1. Routine Inspection: under this category, general inspections are carried outquickly and frequently by highway maintenance engineers. This type inspection is

required to identify the obvious deficiencies which could lead to accidents or major

future repairs or maintenance problems.


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2. Detailed Inspection: This type of inspection can be further divided as follows :-General Inspection

Major Inspection

Special Inspection

General inspection is normally made annually it should cover all the structural elements and it is

mainly a visual inspection supplemented by standard instrumental aids, invariably followed by a

written report.

Major inspection is generally more intensive involving detailed examination of all

structural elements even requiring setting up of special access facilities where required. It is

generally done in a span of 2 to 3 years.

3. Special Inspection: These are resorted to under extraordinary situations such asearthquakes, high intensity or abnormal loadings, floods etc. These inspections should be

exhaustive including structural testing (using instruments like ultrasonic pulse velocity

apparatus to detect micro cracks and excessive deflections using dial gauges)and

computations using structural analysis.

The timing of this type of inspection should be such that the most critical evaluation of the

performance of the structure is obtained.

Inspection instrumentation:

Prestressed concrete structures showing visible signs of distress in the form of surface

cracks, spanning of concrete should be subjected to special inspection. Modern testing equipment

which could be of use to the specialized inspection team is listed below:

1. Rebound hammer for in situ evaluation of compressive strength of grade ofconcrete.

2. Ultra sonic pulse velocity apparatus for the detection of cracks in the concrete.3. Snooper crawler and adjustable ladders.4. Magnetic detector for measuring the thickness of concrete cover and for locating

reinforcement bars.


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5. Mechanical extensometer, transparent templates and microscope for reading ofcrack widths on the surface of the concrete.

6. Hydraulic jacks, pressure transducers or load cells for the measurement of forcesetc.

7. Electronic strain gauges for measurement of strains in concrete and steel.8. Vibration measuring equipment.9. Electrical resistance meter.

The instruments listed above are very useful to evaluate the strength of in situ concrete and the

distress caused due to the development of micro cracks in the concrete.


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2. SITE INSPECTION :A site investigation report is the basis for all the subsequent decisions regarding cleanup of a

contaminated site.This report describes the findings of the desk study and the field work and

discuss their implications with respect to the proposed development of the site. An assessment is

made in terms of likelihood of the presence of contamination that may affect the feasibility of the

site for the intended use.

2.1Initial assessment:Site characterization:

The site is part of an open car park but lies within a mixed software offices and retail

commercial area.

Site location:

The site is located at kothegudam village, near Hi tech city in Hyderabad. It is

boundedby commercial complexes to the south east and its fronting the main road.

Desk study:

The desk study is carried out in accordance with the technical report-43 by concrete

society. The report provides the base for an opinion on the condition of site.

Hydrological information:

The site is classified as being under laid by a major aquifer of high permeability associated with

soils of high leaching potential.

Other environmental information:

The environment check has revealed that there is no registered landfill or waste

management facilities within the 1km vicinity of the site. There are no recorded pollution

incidents to the controlled waters or other known site processes which could potentially impact

the development.


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2.2 Soil testing:

The field work was carried out under the supervision of a geo tech - environmental engineer and

comprised of the following elements:

1. Cable percussive bore holes to 30m depth2. Window sampler bore holes3. Dynamic probes up to a maximum depth of 10m.

Standard penetration tests were carried out in the cable percussive bore holes at regular intervals

to the full depth of boring. Distributed samples were recovered from the various bore holes for

subsequent testing in the laboratory and/or for contamination testing.

A stand pipe was installed in one of the cable percussive boreholes in order to determine the

depth of the ground water table. It is noticed that ground water level is standing at approximately

9.74m below the ground level, one day after installation.

Bearing capacity is the capacity of soil to support the loads applied to the ground. The bearing

capacity of soil is the maximum average contact pressure between the foundation and the soil

which should not produce shear failure in the soil.

Ultimate bearing capacity is the theoretical maximum pressure which can be supported without

failure; allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety.

There are three modes of failure that limit the bearing capacity: general shear failure, local shear

failure, and punching shear failure. The safe bearing capacity of soil is 140 N/mm2.

The consistency limits of soil are as follows:

Soil type Liquid limit Plastic limit

Sand 20 0

Slits 27 20

Murrum 100 45


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2.3 Acquiring of raw materials :

Concrete

Typically concrete is of M35 grade manufactured by Aparna cement industry. The ready

mix design arrives at the site and begins to prepare the concerned RMC in situ. If any concrete is

being transported, then admixtures are added in order to delay the setting time.

Prestressing steel (strands & bars)

Strand for post-tensioning is made of high tensile strength steel wire. A strand is comprised

of 7 individual wires, with six wires helically wound to a long pitch around a center wire. All

strand should be Grade 1860 MPa (270 ksi) low relaxation, seven-wire strand conforming to the

requirements of ASTMA 416 Standard Specification for Steel Strand, Uncoated Seven Wire

Strand for Prestressed Concrete. ASTMA 416 provides minimum requirements for mechanical

properties (yield, breaking strength, elongation) and maximum allowable dimensional tolerances.

Strand from different sources may meet ASTMA 416 but is not necessarily identical

in all respects.

Strand is mostly available in two nominal sizes, 13mm (0.5in) and 15mm (0.6in)

diameter, with nominal cross sectional areas of 99mm2

and 140mm2

(0.153 and 0.217 square

inches),respectively. The majority of post-tensioning hardware and stressing equipment is based

on these sizes. Strand size tolerances may result in strands being manufactured consistently

smaller than or larger than nominal values. Recognizing this, industry (Acceptance Standards

for Post-Tensioning Systems, Post-Tensioning Institute, 1998refers to the Minimum Ultimate

Tensile Strength (MUTS) which is the minimum specified breaking force for a strand. Strand

size tolerance may also affect strand-wedge action leading to possible wedge slip if the wedges

and strands are at opposite ends of the size tolerance range.

Strand conforming to ASTM A 416 is relatively resistant to stress corrosion and

hydrogen embrittlement, due to the cold drawing process. However, since susceptibility to

corrosion increases with increasing tensile strength, caution is necessary if strand is exposed to

corrosive conditions such as marine environments and solutions containing chloride or sulfate,

phosphate, nitrate ions or similar. Consequently, ASTMA 416 requires proper protection of


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strand throughout manufacture, shipping and handling. Protection during the project, before and

after installation, should be specified in project specifications, details, drawings and documents.

In recent years, various innovations have been developed in order to provide additional corrosion

protection. Some of these measures include:

Plastic coated strand for un bonded tendons has been widely used in buildings, but not

generally in bridges in the United States. However, greased and sheathed mono-strands are now

available for cable-stays or external tendon applications for new structures and the repair of old

ones.

Epoxy coated strand meeting the same requirements as ASTMA 416 is available and should

also conform to ASTMA 882 Standard Specification for Epoxy-Coated Seven Wire Strand.

Epoxy coated strand is available as an outer coating only, or as a coating that also fully fills the

interstices between wires. The latter is preferred for post-tensioning or cable stay applications.

Special wedges are required that bite through the thickness of the coating and engage the strand;

power seating of the wedges is usually required.

Strand made from fiber material (such as carbon or aramid fibers) has limited application as

post-tensioning to date. These composite materials offer advantages for enhanced corrosion

resistance, but lack the benefit of a high modulus of elasticity that is routinely provided by steel

and which is crucial to good load-deflection behavior of a prestressed structure without excessive

cracking under service loads.

Few manufacturers supply galvanized strand. Heating during galvanizing reduces the tensile

strength to about 1660MPa (240 ksi). This strand is not used in bridges.

Tendons in prestressed concrete structures do not experience stress cycling significant enough to

induce fatigue problems. Fatigue is a concern only in certain applications such as cable stays in

cable-stayed bridges where traffic loads significantly affect stresses.

Bars:

Bars should be of Grade 1035 MPa (150 ksi), high strength, thread bar meeting the

requirements of ASTM A 722, Standard Specification for Uncoated High-Strength Steel Bar for

Prestressing

Concrete, Type II bar. Coarse thread bars are used for most permanent and temporary

applications. Fine thread bars are available if necessary for special applications. It is good

practice to limit the stress level and number of re-uses for temporary applications, according to


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recommendations of the Manufacturer. In the absence of such information, it is suggested that

for new bars, the stress should not exceed 50% MUTS and the number of re-uses be less than ten

for applications such as temporary stressing or lifting. Post-tensioning bars are available in

various sizes from 16mm (5/8in) to over 50mm (2in) diameter. However, for convenience in

handling, installation, and removal and re-use in normal applications for post-tensioned bridges,

32mm (1-1/4in) or 35mm (1-3/8in) diameter bars are typically used.

Bars are not as easily damaged by corrosion as strands because of their lower strength,

large diameter and smaller ratio of exposed surface to cross section area. Hot rolled bars also

acquire a natural surface oxidation from the rolling process that enhances their protection.

Nevertheless, bars need to be protected during extended periods of exposure especially in

aggressive environments. Hot-dip galvanizing and epoxy coating are available for corrosion

protection if necessary.

Grout:

Grouting can be defined as the filling of duct with a material that provides an anti-corrosive

alkaline environment to the prestressing steel and also a strong bond between the tendon and the

surrounding grout. The major part of grout comprises of water and cement, with a w/c ratio of

about 0.5 together with some water-reducing admixtures, expansion agent and pozzolonas.

Cement and other Pozzolans for Grout:

The primary constituent of grout is ordinary Portland cement (Type I or II). Other

cementitious material may be added to enhance certain qualities of the final product. For

example, fly ash improves corrosion resistance in aggressive environments. The addition of dry

silica fume (micro-silica) also improves resistance to chloride penetration because the particles

help fill the interstices between hydrated cementitious grains thus reducing the permeability.

The water-cementitious material ratio should be limited to a maximum of 0.45 to

avoidvexcessive water retention and bleed and to optimize the hydration process. Any temptation

to add water to improve fluidity on-site must be resisted at all times. Fluidity may be enhanced

by adding a high range water-reducer, HRWR, (Type F or G)


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Pre-bagged Grouts

Grouts made of cementitious materials, water and admixtures batched on site do not

always have uniform properties. This arises from variations in materials, day to day mixing

differences,crew changes, weather conditions and so forth. Grouts made of only cement and

water often exhibit segregation and voids due to excessive bleed water. In an endeavor to

eliminate problems related to grout variations and voids, several State DOTs have obtained

greater quality control by requiring pre-bagged grouts. In a pre-bagged grout, all the

constituent (cementitious) materials have been thoroughly mixed and blended at the factory in

the dry condition. This ensures proper blending and requires only that a measured amount of

water be added for mixing on site.

A manufacturer of a pre-bagged grout may already have had the material pre-qualified by a

State DOT or other agency. In this case, it is appropriate to accept it on the basis of a written

certification; providing that the manufacturer has on-going quality control tests that can be

confirmed by submitting test reports to the Engineer. The certification should show the mixed

grout will meet the pre-qualified standard. On site, daily grout production must be monitored by

various field tests in order to maintain quality control and performance.

Thixotropic vs. Non-Thixotropic Grout:

A thixotropic grout is one that begins to gel and stiffen in a relatively short time while

at rest after mixing, yet when mechanically agitated, returns to a fluid state with much lower

viscosity. Most grouts made with cementitious materials, admixtures and water are non-

thixotropic. Thixotropy may be exhibited by some, but not necessarily all, pre-bagged grouts.

A critical feature of a grout is that it should remain pump-able for the anticipated time to fully

inject the tendon. This may be significant for long tendons or where a group of several tendons is

to be injected in one continuous operation. Some thixotropic grouts can have very low viscosity

after agitation, becoming easy to pump.

Admixtures:

Like concrete, admixtures may be used to improve workability and reduce the water

required, reduce bleed, improve pumping properties or entrain air. Care must be exercised to use

the correct quantities in the proper way according to manufacturers instructions and to remain


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within the mix properties established by qualifying laboratory tests. Calcium nitrite may help to

improve corrosion resistance in some situations by bonding to the steel to form a passive layer

and prevent attack by chloride ions. High range water reducer (HRWR) improves short term

fluidity. However, a grout with HRWR may lose fluidity later when being injected through hoses

and ducts. Unlike a concrete mix, it is not possible to re-dose a grout especially when it is in the,

pump, hoses and ducts. Also, HRWR tends to cause bleed in grouts. On-site grout mixing with

HRWR is not recommended. Other admixtures include:

Shrinkage compensating agents

Anti-bleed admixtures

Pumping aids

Air-entraining agents

The addition of these should be strictly in accordance with manufacturers recommendations.

Furthermore, the mix should be qualified by appropriate laboratory testing. On site, daily grout

production must be monitored by various field tests in order to maintain quality control and

performance.

Ducts: The hollow materials made out of HDPE (High Density Poly Ethylene) or Aluminium

that holds the tendons within and is responsible for protecting the tendons, which impart

prestress to the slab are called Ducts.

Ducts for Tendons

Corrugated Steel :

Ducts are spirally wound to the necessary diameter from strip steel with a minimum

wall thickness of 0.45mm (26-gauge) for ducts less than 66mm (2-5/8 in) diameter or 0.6mm

(24-gauge) for ducts of greater diameter. The strip steel should be galvanized to ASTM A653

with a coating weight of G90. Ducts should be manufactured with welded or interlocking seams

with sufficient rigidity to maintain the correct profile between supports during concrete

placement. Ducts should also be able to flex without crimping or flattening. Joints between

sections of duct and between ducts and anchor components should be made with positive,

metallic connections that provide a smooth interior alignment with no slips or abrupt angle

changes.

Corrugated plastic duct to be completely embedded in concrete should be


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constructed from either polyethylene or polypropylene. The minimum acceptable radius of

curvature should be established by the duct supplier according to standard test methods.

Polyethylene duct should be fabricated from resins meeting or exceeding the requirements of

ASTM D3350 with a cell classification of 344434C. Polypropylene duct should be fabricated

from resins meeting or exceeding the requirements of ASTM D4101 with a cell classification

range of PP0340B44544 to PP0340B65884. The duct should have a minimum material

thickness of 2.0 mm + 0.25 mm (0.079 in + 0.010 in). Ducts should have a white coating on the

outside or should be of white material with ultraviolet stabilizers added.

Smooth, High Density Polyethylene Pipe (HDPE) for External Tendons:

HDPE smooth pipe is available in different diameters, wall thickness, physical and chemical

properties. There is significant variability in commonly available materials. It is very important

that it has satisfactory properties for handling, storage, installation and durability for the

application.

Duct Material:

High Density Polyethylene or Polypropylene Plastic duct is available in a flat oval section for

transverse tendons in deck slabs or similar application Ribs against degradation from ultraviolet

light. The wall thickness, diameter and physical strength(Hydrostatic Design Basis) should be

sufficient to initially withstand grouting pressures. In thelong term it should not deteriorate or

split. The requirements should be in accordance with AASHTO LRFD Bridge Construction

Specifications.

Plastic Fittings and Connections for Internal Tendons:

All duct splices, joints and connections to anchorages should be made with couplings and

connectors that produce a smooth interior duct alignment with no slips or kinks. Special duct

connectors may be used in match-cast joints between precast segments and similar situations

ifnecessary to create a continuous, air and water-tight seal. Duct tape should not be used to

join or repair ducts or make connections. All fittings and connections between lengths of

plastic duct and between ducts and steelcomponents (e.g. anchors or steel pipe) should be

made of materials compatible withcorrugated plastic ducts. Plastic materials should contain

antioxidant stabilizers and have anenvironmental stress cracking of not less than 192 hours


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as determined by ASTMD 1693Standard Test Method for Environmental Stress-Cracking

of Ethylene Plastics, Condition C.

External Tendon Duct Connections:

Connections between sections of plastic pipe should be made using heat welding

techniques or with mechanical couplers on the manufacturers recommendations or as approved

by the Engineer. Connections should have a minimum pressure rating of 1 MPa (145 psi) and

produce a smooth interior alignment with no slips or kinks.

Connections between external HDPE pipe and steel pipe embedded in the concrete

should be made using circular sleeve (boot) made of Ethylene Propylene Deine Monomer

(EPDM) having a minimum (working) pressure rating of 1 MPa (145 psi). EPDM should have

100% quality retention as defined by ASTM D1171 Standard Test Method for Rubber

Deterioration Surface Ozone Cracking Outdoors or Chamber (Triangular Specimens) Ozone

Chamber Exposure .

Method B: The minimum wall thickness should be 10mm (3/8 inch) reinforced with a minimum

of four ply polyester reinforcement. Sleeves should be secured with 10mm (3/8 in) wide power

seated, 316 stainless steel band clamps, using one on each end of the sleeve (boot) to seal against

leaking grout. The power seating force should be between 356 and 534 N (80 and 120lbf).

Alternatively, connections may be made using mechanical couplers with plastic components

made of approved plastic resins meeting the same requirements as for external plastic pipes and

metal components of grade 316 stainless-steel. Mechanical connections should meet the same

pressure rating requirements (above) and have seals to prevent grout leaks.

Steel and plastic pipe may be connected directly when the outside diameters do not vary

by more than + 2mm (0.08in). A reducer or spacer should be used when outside this tolerance.

When installed correctly, a single band clamp around each end of the sleeve should be sufficient.

Double banding may be necessary to fix an apparent leak of air, water or grout.


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Shrink Sleeves:

In some cases, external tendon connections may be enhanced by the use of shrink sleeve

wrap overlaying the connection and portions of adjacent plastic and steel pipes. This may be

used in tendons.

Tendons are transported to the site in bundles or packed loose in special transport frames.

Ducts are made of bright or galvanized steel or plastic. The ducts have a corrugated surface to

guarantee the adherence between the cable and surrounding concrete.

Ducts thickness varies for 0.3mm to 0.6mm. Technical features of duct:

No of strands 4 7 9 12 15

Internal dia

(mm)

45 62 72 80 85

Grout (l/m)

requirement

1.2 2.3 2.8 3.6 3.8

Cement(kg/m) 1.9 3.6 4.5 5.8 6.1

Main features of cables using 12.7mm diameter strands.

No. of strands 4 7 9 12

Nominal c/s area

of steel Ap(mm2)

600 1050 1350 1800

Nominal mass of

steel(kg/m)

467 820 1055 1758

Characteristic

tensile

strength(fpi) MPa

1.860 1.860 1.860 1.860

Characteristic

ultimate resisting

force of tendon

(fpk)KN

1.116 1.953 2.511 3.348


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2.5 Mode of transport

Prestressing Steel

All prestressing steel should be protected against physical damage and corrosion at all

times from manufacture to final installation and grouting. It should be packed in containers for

shipping handling and storage. A rust-preventing corrosion inhibitor should be placed in the

package or be incorporated in the carrier type packaging material. Corrosion inhibitor should

have no deleterious effect on the steel or grout or on the bond strength of steel to grout. Inhibitor

carrier type packaging should conform to Federal Specification MIL-P-3420. Damaged

packagingshould be replaced or restored to its original condition.

Shipping containers should be clearly marked with a statement that it contains high-

strength prestressing steel, the type of care needed for handling, the type and amount of

corrosion inhibitor used and the date it was placed, and any other safety precautions and

instructions. Strand should be clearly identified that it is low-relaxation (stabilized) strand per the

requirements of ASTMA 416 and the corresponding LOT number for which quality control test

samples have been taken. Strands not so designated should be rejected. Reels of strand should be

examined by the Contractor and inspected by the CEI when first received on site and periodically

while in storage. During use, any reel that is found to contain broken wires or corrosion should

be carefully examined. Lengths of strand containing broken wires or corrosion should be

removed and discarded. Prestressing steel should also be protected during installation in the

detailed study and execution work in post tension slabs

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