Test 38 - Prestressed Concrete Concepts, Materials and Systems

Question 1 ప్రశ్న 1

Which statement correctly identifies the current principal Indian Standard framework for the general structural use of prestressed concrete?

A IS 1343:2012, Second Revision, read with its applicable amendment, remains the operative prestressed-concrete code while its proposed revision has not yet replaced it B IS 456 alone has already completely withdrawn and replaced IS 1343 C IS 800:2007 is the principal design code for prestressed concrete D IS 1343 applies only to structural steel members

Question 2 ప్రశ్న 2

What is the fundamental structural principle of prestressing concrete?

A Introduce a deliberately applied compressive stress so that tensile stresses produced by service loads are reduced or counteracted B Increase the self-weight of concrete so that external loads become negligible C Eliminate compressive stress from concrete before service loading D Replace concrete entirely by high-tensile steel

Question 3 ప్రశ్న 3

Which is a major structural advantage commonly associated with prestressed concrete compared with ordinary reinforced concrete?

A Improved control of cracking and deflection, allowing efficient use of relatively slender members and longer spans B Prestressed members require no concrete quality control C Prestressing completely eliminates every type of structural failure D Prestressed concrete can be made effectively without any steel

Question 4 ప్రశ్న 4

Which of the following is a genuine limitation or additional requirement associated with prestressed-concrete construction?

A It requires careful control of materials, stressing operations, anchorages or transfer, and losses of prestress B It requires no specialized equipment under any circumstances C Prestress remains exactly equal to its initial value throughout the service life D Concrete strength at stressing or transfer is irrelevant

Question 5 ప్రశ్న 5

According to IS 1343:2012, what are the minimum concrete grades generally applicable to pre-tensioned and post-tensioned prestressed-concrete members respectively?

A M30 and M20 B M30 and M40 C M40 and M30 D M50 and M40

Question 6 ప్రశ్న 6

Why is relatively high-strength concrete particularly desirable in prestressed-concrete members?

A It can sustain high compressive stresses at transfer and service, generally provides better bond and stiffness, and helps reduce time-dependent deformation B Its only purpose is to increase the colour density of the member C High-strength concrete is required because prestressed concrete carries no compression D It allows tendons to be omitted after stressing

Question 7 ప్రశ్న 7

Why is high-tensile-strength steel used for prestressing rather than ordinary mild-steel reinforcement stressed to a relatively low level?

A A high initial tensile stress and elastic strain can be introduced so that useful prestress remains after shrinkage, creep, relaxation and other losses B High-tensile steel has zero elastic strain C Ordinary mild steel cannot carry any tensile stress D Prestressing steel is selected only because its density is lower than that of concrete

Question 8 ప్రశ్న 8

Which group correctly consists of recognized forms of prestressing steel covered by relevant Indian Standards?

A High-tensile wires, indented prestressing wire, high-tensile prestressing bars and stress-relieved prestressing strands B Only ordinary structural-steel I-sections C Only Fe250 mild-steel plates D Timber strips, aluminium sheets and cast-iron bars

Question 9 ప్రశ్న 9

Which sequence correctly describes pre-tensioning?

A Stress the tendons against external abutments or a stressing bed, cast concrete around them, allow the concrete to gain adequate strength, and then release the tendons B Cast the concrete first, grout every duct, and only then insert unstressed tendons C Stress the concrete directly before placing any steel D Anchor unstressed tendons only after the member has entered service

Question 10 ప్రశ్న 10

How is the prestressing force transferred from the steel to the concrete in an ordinary pre-tensioned member after the tendons are released?

A Primarily by bond between the prestressing steel and the surrounding hardened concrete over a transfer length B Only through permanent mechanical anchor blocks at both ends C Only through friction beneath a steel base plate D By applying an external bending moment throughout the life of the member

Question 11 ప్రశ్న 11

Which sequence correctly describes post-tensioning?

A Cast and harden the concrete member first, then stress the tendons against the member and secure the prestress by permanent anchorages B Stress the tendon on a long external bed before concrete is cast and remove all anchorages permanently before transfer C Cast concrete only after the permanent structure has been fully loaded D Apply prestress solely by increasing the dead load

Question 12 ప్రశ్న 12

What makes an internal post-tensioned tendon a bonded tendon after stressing?

A The duct surrounding the stressed tendon is properly filled with grout so that force can be transferred continuously between tendon and concrete along its length B The tendon remains permanently free to slide relative to the concrete at every section C The end anchorages are removed after stressing D The tendon is located outside the concrete cross-section

Question 13 ప్రశ్న 13

Which statement correctly describes an unbonded post-tensioned tendon?

A It is intentionally prevented from developing continuous bond with the surrounding concrete, so the prestressing force is transmitted primarily through its end anchorages B It has no anchorage at either end C It must be cast directly into concrete without any sheathing or protection D It is always a pre-tensioned tendon

Question 14 ప్రశ్న 14

Which statement correctly distinguishes internal and external prestressing?

A An internal tendon lies within the concrete member cross-section, whereas an external tendon lies outside the concrete cross-section and transfers force through anchorages and, where provided, deviators B Internal prestressing uses steel, whereas external prestressing cannot use steel C An external tendon must lie outside the entire building or bridge D There is no structural distinction between tendon location inside and outside the concrete section

Question 15 ప్రశ్న 15

Which statement correctly distinguishes concentric and eccentric prestressing of a beam section?

A Concentric prestress acts through the centroid and ideally produces uniform direct compression, while eccentric prestress produces both direct compression and a bending effect Pe B Concentric prestress always produces a bending moment Pe but no direct stress C Eccentricity of the tendon has no effect on concrete stress distribution D Eccentric prestressing can produce only tensile stress in concrete

Question 16 ప్రశ్న 16

A rectangular prestressed-concrete member 300 mm wide and 500 mm deep is subjected to a concentric prestressing force of 1200 kN. Ignoring losses, what uniform compressive stress does the prestress produce?

A 4 N/mm^2 B 6 N/mm^2 C 8 N/mm^2 D 12 N/mm^2

Question 17 ప్రశ్న 17

A 300 mm × 600 mm rectangular beam carries an effective prestressing force of 900 kN acting 100 mm below the centroid. Considering prestress only, what are the stresses at the top and bottom fibres respectively, taking compression as positive?

A 5 MPa and 5 MPa B 0 MPa and 10 MPa C 10 MPa and 0 MPa D 5 MPa tension and 15 MPa compression

Question 18 ప్రశ్న 18

Why is eccentric prestressing commonly used in flexural members rather than placing every tendon concentrically?

A The moment Pe created by eccentricity can be chosen to oppose part of the bending effect produced by gravity and service loads B Eccentric prestressing eliminates the direct compressive stress P/A C Eccentric prestressing guarantees zero shear force at every section D A tendon below the centroid can never influence the bottom-fibre stress

Question 19 ప్రశ్న 19

Which statement correctly identifies the Hoyer system of prestressing?

A It is a pre-tensioning system commonly associated with long-line production in which tendons are stressed between external abutments before concrete is cast B It is a post-tensioning system based on threaded high-tensile bars and nuts C It is a bonded post-tensioning system using sandwich plates and flat wedges D It is an external prestressing system that does not use a stressing bed

Question 20 ప్రశ్న 20

Which feature is characteristically associated with the Freyssinet post-tensioning system?

A A group of high-tensile wires forming a cable is commonly stressed together and anchored by a conical wedge or plug arrangement B Only one ordinary reinforcing bar is used and no anchorage is required C The tendons are always tensioned before concrete is cast on a long-line bed D Prestress is developed solely by expansive cement without mechanical tensioning

Question 21 ప్రశ్న 21

Which anchorage arrangement is characteristically associated with the Magnel-Blaton post-tensioning system?

A Flat steel wedges used with sandwich plates to anchor pairs of wires B A long pre-tensioning bed with no permanent anchorage C Only threaded bars anchored by nuts D A single unstressed reinforcing bar cast concentrically

Question 22 ప్రశ్న 22

Which statement correctly describes the Gifford-Udall post-tensioning system?

A Prestressing wires are commonly stressed and anchored individually using gripping or split-cone type anchorage units bearing against an end plate B All prestressing is produced chemically without steel tendons C It is exclusively a pre-tensioning long-line process D It uses only threaded bars with ordinary hexagonal nuts

Question 23 ప్రశ్న 23

Which feature is most characteristically associated with the Lee-McCall prestressing system?

A High-tensile steel bars with threaded ends are anchored using high-strength nuts bearing against end plates B Pairs of wires are anchored in sandwich plates by flat wedges C A multi-wire cable is always anchored by a central conical concrete plug D The tendons are released from a long-line casting bed and transfer force only by bond

Question 24 ప్రశ్న 24

Which comparison between pre-tensioning and post-tensioning is structurally correct?

A Pre-tensioning normally needs an external stressing bed and transfers prestress to concrete mainly by bond after release, whereas post-tensioning stresses hardened concrete members and initially transfers tendon force through permanent anchorages B Pre-tensioning always uses permanent end anchorages while post-tensioning can never use anchorages C Post-tensioning must always be carried out before the concrete is cast D There is no difference in the stage at which the tendon is stressed

Question 25 ప్రశ్న 25

Which statement best explains the relationship among the classifications pre-tensioned/post-tensioned, bonded/unbonded, internal/external and concentric/eccentric?

A They describe different characteristics of a prestressing arrangement, so a tendon may be classified simultaneously by stressing stage, bond condition, location and eccentricity B They are four different names for exactly the same classification C Every post-tensioned tendon must be internal, bonded and concentric D An external tendon cannot be post-tensioned

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