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Q. What is pre stressed concrete?Ans: Pre stressed 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 pre stress is commonly introduced by tensioning the steel

reinforcement.

Q. Discuss general concepts of prestressed concrete.First Concept

• Prestressing to transform concrete into an elastic material

• Tensile stress due to external load counteracted by the compressie stress

due to prestress

• !ue to prestress force F, a uniform compressie stress,f" F#A

• If $ is the external moment at a section due to load,the stress at any pointacross the section due to external load f "F#A % $y#I

Q. What are the types & classification of pre stress concrete?

Describe in brief.Ans: Pre stressed concrete structures can be classified upon their features of 

design and construction:

&. 'xternally or Internally pre stressed: (enerally pre stressed

concrete structures are internally pre stressed with high tensile

steel. It may be mentioned that it is sometimes possible to pre

stress a concrete structure by ad)usting its external reactions. *ut

shrin+age creep in concrete may completely offset that artificial

strain unless they can be read)usted from time to time.

-. inear or Circular pre stressing: Circular pre stressing is a term

applied to pre stressed circular structures such as round tan+s, pipes

where the pre stressing tendons are wound around circles. The term

linear pre stressing is often employed to include all other structuressuch as beam slabs. The tendons are not necessarily straight/

they can be either be bent or cured.

0. Pre tensioning Post tensioning: The pre tensioning in which the

tendons are tensioned before the concrete is placed. The pre stress

is transferred to the concrete after it has set. The post tensioning in

which the tendon is tensioned after the concrete has hardened. Thus

the pre stressing is always performed against the hardened concrete

and the tendons are anchored against it immediately after transfer.

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This method can be applied to members either pre cast or cast1in1

 place.

2.  'nd anchored or 3on1anchored tendons: 4hen pre tensioning the

tendons are anchored at their end by means of mechanical deicesto transmit the pre stress to the concrete. 5uch a member is termed

as end anchorages. A post1tensioned member may hae both end

anchorage and grouting. In pre tensioning the tendons generally

hae their pre stress transmitted to the concrete by their bond

action near the ends. 4ires of small si6e and larger diameter 

strands, which possess better bond properties, are suitable.

7.  *onded and 3on1bonded tendons: *onded tendons denote those

 bonded throughout their length to the surrounding concrete. 3on1

end anchored tendons are bonded ones but end anchored tendons

may be either bonded or unbonded to the concrete. Post1tensioned

tendons are bonded by grouting to protect from corrosion.

8. Pre cast, cast1in1situ, composite construction: Pre casting permits

 better control in mass production and is often economical. Cast in

 place concretes re9uires more form and false wor+ but saer the

cost of transportation and erection and it is a better for aggregatesand heay members. 5ometimes it is economical to pre cast part of 

a member erect wbgb; it and the cast the remaining portion in

 place. This procedure is called composite construction.

<. Partial or fully pre stressing: 4hen a member is designed so that

under wor+ing load there are no tensile stresses in it, the concrete is

said to be fully pre stressed. If some tensile stress will be produced

under wor+ing load then it is termed partially pre stressed.

Q. What are the advantages of pre stressed concrete over R.C.C?Pre stressed concrete offers great technical adantages oer =.C.C .The

adantages of pre stressed concrete comparing with =.C.C are gien below1

&. In the case of fully pre stressed members, which are free from tensile

stresses under wor+ing loads the cross section is more efficiently utili6ed

comparing with =.C.C section, which is crac+ed under wor+ing loads.

-. Pre stressed concrete member possesses improed resistance to forces,

due to the effect of compressie pre stress, which reduces the principaltensile stress. The use of cured cables, particularly in long span

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members, helps to reduce the shear forces deelops at the support

sections.

0. A pre stressed concrete flexural member is stiffer under wor+ing loads

than a reinforced concrete member at the same depth.

2. The use of high strength concrete and steel in pre stress members resultsin lighter slender member that it is impossible with reinforced concrete.

7. The ability to resist repeated wor+ing loads has been proed to be as good

in pre stressed as in reinforced concrete.In the long span range, pre

stressed concrete is generally more economical than reinforced concrete

steel. The saings in steel are een higher, 8> to ?>@. For the oerall

economy in using pre stressed concrete, decrease in dead weight reduces

the design loads the cost of foundations

8. Pre stressed concrete has considerable resilience due to its capacity for 

completely recoering from substantial effects of oerloading withoutundergoing any serious damage.

Q. Why high strength concrete is required for pre stress?&. Ans: 5tronger concrete is usually re9uired for pre stressed than for 

reinforced wor+1

-. In order to minimi6e their cost, commercial anchorages for pre stressing

steel are always designed on the basis of high strength concrete. ence

wea+er concrete either will re9uire special anchorages or may fail under 

the applications of pre stress. 5uch failures may ta+e place in bearing of in bond between steel or concrete or in tension near the anchorages.

0. Concrete or high compressie strength offers high resistance in tension

shear, as well as in bond and bearing and is desirable for pre stressed

concrete structures where arious portions are under higher stresses than

ordinary reinforced concrete.

2. igh strength concrete is less liable to the shrin+age crac+s whose

sometimes occur in low strength concrete before the application of pre

stress.

5. igher strength concrete has a higher modulus of elasticity and smaller creep strain, resoling in smaller loss of pre stress in the steel.

 

Q. What is partially pre stressing? What are the

advantages and disadvantages? What are the conditions?Ans: It is a common practice in Ciil 'ngineering constructions. Partially pre

stressed members are those which are designed to allow significant tensile

stresses to occur at serice loads and such tensile regions are usually

additionally reinforced with non pre stressed reinforcement. 5ome fully pre

stressed structures hae deeloped too much upward deflection or camber 

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which is not desirable. Partial pre stressing is for eliminating or controlling

crac+ width at serice loads by setting allowable tensile stresses, which are

slightly less than crac+ing stresses for the concrete.

Advantages

&. *etter control of camber.-. 5aing in the amount of pre stressing steel.

0. 5aing in the wor+ of tensioning and end anchorages.

2. Possible greater resilience in the structure.

7. 'conomical utili6ation of mild steel.

Disadvantages

  &. 'arlier appearance of crac+s.

-. (reater deflection under oerloads.

0. igher principal tensile stress under wor+ing loads.

2. 5light decrease in ultimate flexural strength for the same amount ofsteel.

Conditions

  1Tensile stresses are permitted in the concrete under wor+ing load.

13on1pre stressed reinforcements are employed in the member.

Q. What happens hen pre stressing ill be done by using

ordinary steel?Ans: 4hen mild steel is pre stressed against concrete, low pre stress produced

in the steel was soon last as a result of shrin+age creep of concrete. At last,

only a small portion of the pre stress could be retained and method cannot be

completely economical with conentional reinforcement of concrete. *esides,

mild steel yield point is ery low in comparison of high strength steel. ence

ordinary steel should not be used for pre stressing.

Q. !s it possible to design a pre stress concrete bea" ith no shear

carried by the concrete section? #ustify your anser.

Ans: If tendons are inclined its ertical components can resist the full shear atany section then it will say no shear. *y following the balanced load approach

for pre stressed concrete, it is possible to design a beam with no shear in a

concrete under a gien condition of loading.

 Now taking moment about the lowest point we have,

-P sin∝  *L/2 + P cos∝  *Y 0 + Wp *L/2 0

⇒  P cos∝  *Y 0 P sin∝  *L/2 - Wp*L2 /!

⇒  P Y 0 Wp*L2 /" - Wp*L2 /! # P sin∝   Wp *L/2 $ cos∝ 

negligible %

⇒  P Y 0 WpL2 /!

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⇒ Wp !PY 0 /L2

&o, when WWp, then no shea' in beam(

Q. Co"pare the pre stressed concrete ith reinforced concrete in

econo"y$ serviceability and safety?Ans:

%re stressed concrete Reinforced concrete

.'cono"y

a; 5maller 9uantities of materials both

steel concrete are re9uired to carry

the same loads.

 b; $ore labor is re9uired.

(. )erviceability

a; It is more suitable for structure of 

long spans those carrying heay

loads.

 b; Pre stressed structure do not crac+ 

under wor+ing load.

c; !eflection is less than =.C.C.

*.)afety

a; Pre stressed concrete members do

re9uire more care in design,

construction erection than those of 

ordinary concrete.

 b; The resistance to corrosion is better 

than that of reinforced concrete.

a; arger 9uantities of materials both

steel concrete are re9uired to carry

the same loads.

 b; ess labor is re9uired.

a; It is less suitable for structures of 

long spans these carrying heay

loads.

 b;It may crac+ under wor+ing load.

c;!eflection is more than pre stressed

concrete.

a; =einforced concrete members do

re9uire less care in design, construction

erection that those of pre stressed

concrete.

 b; The resistance to corrosion is not

 better than that of pre stressed concrete.