undefined | unit 1 prestressed concrete analysis

SDD3

UNIT 1Question bank Q1) What is the basic principle of Pre-stressed Concrete?A1) Strength of concrete in weak in tension, hence it is ignored in reinforced concrete design. In RCC design assume compressive force resist in concrete and tensile force resist in steel i.e. reinforced we know stress strain diagram R.C. beam.

In this diagram concrete above neutral axis is useful in resisting applied load while the concrete below neutral axis is not useful in resisting any load. The concrete in tension side only acting as a cover to reinforcement only. This disadvantage of RCC, made the concrete lover to think of improving the use of concrete and it resulted into development of prestressed concrete.Prestressed concrete is that can create in which the compressive stresses are induced in the concrete section before the member is loaded by external loads.IS code-1343-2012; concrete in which permanent internal stresses are deliberately introduced usually by tensioned steel, to counteract to the desired degree the stresses caused in the member in service. Q2) What is the need of prestressing a concrete member?A2) Concrete which is strong in compression but weak in tension; however concrete is very weak in tension and it is very difficult to have concrete of tensile strength greater than 2.5

In RCC use concrete and reinforcement bonded to perfect bond. Due to loading cracks get developed in concrete as the steel yields.In prestressing concrete member reduces these deflects and use high strength concrete and high tensile steel i.e. tendon. Q3) Difference between pre-stressed concrete P.S.C. and reinforced cement concrete (R.C.C.)?A3)

R.C.C.	Pre-stressed
30% to 40% of total cross-sectional area is utilized	Hundred percent cross-sectional area is utilized
Section is an economical beyond spans of 10m to 12m (load heavy)	Economical up to spans of 25 to 30 m (load small)
Self weight of section is higher	Self weight of section is lesser
Cracking and corrosion problems are more	No cracking and corrosion problems
Reinforcement is straight	Reinforcing tendons can be bent or have parabolic shapes
Tension is developed below neutral axis	The whole cross section is in compression
Stresses are developed only after external load is allowed	Stresses are developed before counting due to pre compressive load
Generally large in size and require additional share reinforcement to resist shear forces	In P.C. section are smaller in size and due to pre stressed tendon a considerable amount of shear reinforcement is avoided
It does not require any auxiliary units much for construction	Auxiliary units are required live anchoring devices, equipments for prestressing

Q4) Difference between pre-tensioning and Post-tensioning.A4)

SR.No.	Pre-Tensioning	Post-Tensioning
1.	Tendons are tensional before costing the concrete.	Tendons are tensioned after the concrete has fully hardened
2.	One end of tensioned is secured an abutment and another is pulled by using jack then concrete is poured. After hardening the concrete the ends of tendon are released prestress is transmitted to the concrete by action of bond between the reinforcement and surrounding concrete.	The concrete member is casted leaving duct for placing the tendons. One end of the tendon is anchored to the member . After the tendons reaches the desired stress by pulling with it Jack through the duct provided. The other end is also properly anchored to the concrete and the duct is completely filled with cement slurry by grouting. Prestress is his transmitted to concrete.
3.	Loss of prestress is more.	Loss of prestress is less.
4.	The cost of pre tensioning system is less.	The cost of post tensioning system is more.
5.	Pre tensioning is mostly used for small member.	Post tensioning is used for span member.
6.	Grade of concrete shall not be less than 45	Grade of concrete shall not be less than 35
7.	For ex:- Hoyer system	For ex:-Preyssinet system.

Q5) Why high strength concrete used in prestressed concrete?A5) (1) High strength concrete is having low shrinkage and creep value, high impermeability, high resistance to abrosion and more ductility.(2) High strength concrete permits hire bond between steel and concrete because stresses transformed from tendons to concrete.(3) It has high modulus of elasticity leading to smaller elastic strain when tendons are cut. This will reduce relaxation loss in steel.(4) Use of high strength concrete is justified in terms of cost also, as in prestressed concrete the entire cross section is in compression and hence full concrete area is utilized in resisting compressive force.(5) To sustain the high stresses at anchorage regions, due to large prestressing force to be applied by tendon.(6) To have higher stiffness for reduced deflection.(7) In factories, with high degree of control it is easier to produce high strength concrete.(8) For post tensioned prestress concrete, the grade of concrete shall not be less than

However for pre tensioned concrete is shall not be less than

(9) High strength concrete will have modulus of elasticity of concrete

[ IS-1343-2012,clause no 6.2.3.1 ,pp-5](10) Hence elastic and cracks trains are very low resulting in smaller loss of prestressing force.(11) Just of high strength concrete reduces the size of concrete member thereby resulting in of concrete in reduction of self weight of the member. Q6) Why high strength steel used in prestressed concrete?A6)1) A large prestressing force can be developed on smaller steel area, there force, high tensile strength wires, strands and tendons used in prestressing members.2) When concrete shortens due to compressive stress, creep and shrinkage, a large amount of prestress is lost. If high tensile bars are used for prestressing, permitting a large initial prestress, the loss of prestress will be small percentage of total initial prestress.3) For post tensioned structures, anchorage regions have concentrated and very high stresses.4) High tensile steel give high bond strength to avoid slippage. Q7) Explain the term a) wires b) strands c) tendon d) cable and e) bars?A7)a) Wires:- prestressing wire is a single unit made of steel.b) Strands:-2, 3 or 7 wires are wounds to form prestressing strands.c) Tendon:-a group of strands or wires are wounds to form a prestressing tendon.d)Cable:-a group of tendons forms a prestressing cable.e)Bars:-tendon can be made up of a single steel bar the diameter of bar is much larger than that of a wire. Q8) Explain the term a) Bonded tendon b) Unbonded tendonA8)Bonded tendon: -when there is adequate bond between the prestressing tendon and concrete, it is called as bonded tendon. Pre-tensioned and grouted post tensioned tendons are bonded tendons.Unbonded tendon: -when there is no bond between the prestressing tendon and concrete, it is called as bonded tendon. When grout is not applied after post-tensioning the tendon is an unbounded tendon. Q9) State the advantages of prestressed concrete?A9)1) This provides a type of construction which is always free from cracks under full working load. Due to this reason, such type of construction is suitable where corrosion is a great danger.2) Sufficient horizontal compression due to prestress reduces principal tensile stresses and shear resistance is developed without heavy reinforcement of large webs.3) Less steel is used in the members, it is easier to place good quality of concrete with high compressive strength than in the case of reinforced concrete.4) This type of construction saves material as well as dead loads and hence considerable savings in supports and foundations is also achieved.5) Smaller sections are used in this type of construction. For this reason very large spans can be built in prestressed concrete then in reinforcement concrete.6) For precast prefabricated construction the advantages of a reduction in self weight of the unit are obvious. Also prestressing counteracts handling stress. Hence we can use this method for precast work.7) The cost of shuttering and centring in large structures is reduced when prestressed precast elements are assembled.8) The amount of steel required is very low.9) Deflections of structures are reduced.10) The time of construction is much less if precast prestressed members are used. Q10) State the Disadvantages of prestressed concrete?A10)1) It requires high tensile steel and high strength concrete of grades more than

And above. Hence it will become more expensive for small jobs.2) Prestressing is a tedious job. It requires expertise supervision.3) Cables are required to be anchored by mechanical means. However, full prestressing force cannot be obtained due to losses in prestress to the extent of 25%.4) Special prestressing techniques are required for both pre tensioned and post tensioned prestressed concrete members.5) It does not increase ultimate strength of concrete.6) It requires special equipment such as Jacks, anchorages, high tensile cables, etc. Q11) Explain the Limitations of prestressing?A11) Although prestressing is advantageous, some aspects need to be careful address.1) Prestressing need steel technology hence it is not common as reinforced concrete structures.2) Reuse of high strength concrete is costly.3) There is additional cost in auxiliary equipment.4) There is need of quality control and inspection.5) Prestressed concrete technology is not effective for small structure. Q12) What is Systems of prestressing and explain any one?A12) The various systems of fabricating a post tensioned member are described below. In all the systems the cables and the hydraulic jacks employed for stretching the cables are specially designed to perform the job of prestressing1.Freyssient system 2. Magnet-Blaton system3. Lee- mccall system FREYSSINET SYSTEM:-

The freyssinet cable consists of a number of parallel wires.

The wires of each cable form a ring in cross- section and are place in a position of helical spring .

The cable is put into a flexible metal tube or sheathing before playing it in the formwork of the member to be concreted.

Proper stretching grip, the cable projects about 60 to 75cm beyond the ends of the metallic tube.

The entire assembly of the cable is placed in the prepared formwork of the member and tied in position. The anchorage fittings used in the system consists of two components i.e. i.)the female cone ii.)the male cone figure

The female cones are threated over the cables at each end and fixed to the inside face of the end shuttering of the member.

As concrete is poured in the formwork, the female cone is embedded in the concrete.

Due to the corrugations on outer surface, they develop good bond with the concrete. The member is called. When the concrete has set and attend required strength, the tendons are tensioned by using free Freyssinet Jacking Unit.

The jacking unit is kept at the tensioning end of the member and the cable wires are passed through the grooves and gripped by the wedges on the main piston.

The pump of the jacking unit is operated to move the pistons and the wires are stretched.

After stretching the wires of the desired limit, the male cone is put inside the female cone by operating the inner piston.

This keeps the tensioned cable firmly in position. The jacking unit is removed for used in stretching the outer cables.

Cement grout is then forced through the narrow pipe sleeve in the male cone to fill the annular space between the metallic tube and the cable completely.

Finally the project in wire cut off and then anchorage is grouted to protect the steel from corrosion due to atmospheric actions.

Q13) what are the Assumptions In The Design Of Prestressed Concrete Members?A13)i) A transverse plane section of the member will remain a plan after bending also.ii) With the limits of the deformations taxing place, Hooke’s law is applicable to concrete and steel components.iii) The stress in the reinforcement does not change along the length of the reinforcement breast changes take place for the concrete component only. Variation of stress in the reinforcement due to changes in the external loading is ignorable.iv) In case of bonded tendons, the bond between concrete and tendons is perfect and they act monolithically.v) In prestressing stress in steel due to flexure caused by external loads is negligible compared to applied stress for prestressing.vi) The sectional properties are taken as that of gross sections of concrete, sends the cross sectional area of steel is very small as compared to that of concrete. Q14) what are the Loss in prestress?A14)It has been observed that the prestressing force induced in a member does not remain constant but it decreases with the passage of time on account by various reasons.The total loss may be as high as 15 to 20% of the initial press tracing force.The designer must take into account all such losses. The important causes of loss in prestressing force are the following.1) Due to elastic shortening of concrete2) Due to creep in concrete3) Due to Shrinkage of concrete4) Frictional loss5) Due to creep in steel6) Due to slip at anchorages.

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