Set No. 1

Code No: RR310101

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III B.Tech I Semester Supplimentary Examinations, November 2008 CONCRETE TECHNOLOGY AND PRESTRESSED CONCRETE (Civil Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. Write short notes on the following:

(b) Refractory cement (c) Standard consistency of cement (d) Coloured cement.

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(a) High Alumina cement

[4+4+4+4]

2. (a) Explain the purpose of aggregates in concrete.

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(b) How does the source affects the quality of aggregate?

[8+8]

3. (a) What do you understand by “Grade of concrete”? Discuss different grades of concrete as per IS-456-2000.

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(b) Describe in detail the Accerlated curing test for concrete? Bring out its advantages and limitations. [8+8] 4. Design a M25 Concrete Mix using I.S. Method of Mix Proportioning for the following data. (a) Maximum size of Aggregate - 20mm (b) Degree of workabililty - 0.85 (c) Quality Control - Average

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(d) Type of Exposure - Moderate (e) Specific gravity:

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i. Cement - 3.12 ii. Sand - 2.65 iii. Coarse Aggregate - 2.68

(f) Water absorption: i. coarse aggregate -1.0% ii. fine aggregate - 1.8%

(g) Free surface moisture: i. coarse aggregate - NIL ii. fine aggregate - 2.0%

(h) Sand conforms to grading zone II 1 of 2

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Set No. 1

Code No: RR310101 5. With the help of stress diagrams explain the following:(a) Prestressing with a straight concentric tendon. (b) Prestressing with a straight eccentric tendon.

[8+8]

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6. (a) A post-tensioned concrete beam of rectangular cross section 110mm wide and 300mm deep is stressed be a parabolic cable with zero eccentricity at the supports and an eccentricity of 45mm at the centre of the span. The cross sectional area of the cable is 200mm2 . Take Esteel = 206kN/mm2 , EConcrete = 35kN/mm2 . Age of concrete at the time of loading = 28 days. The initial stress in cable is 1200MPa. Find the loss of stress due to creep of concrete.

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(b) A concrete beam is prestressed by a cable carrying an initial prestressing force of 335kN. The cross sectional area of wires is 320mm2 . Calculate the percentage loss of stress in the cable only due to shrinkage of concrete using IS 1343 recommendations assuming the beam to be

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i. Pre-tensioned. ii. Post-tensioned. Take ES = 210kN/mm2 . Age of concrete at the time of transfer = 10 days. [8+8]

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7. A prestressed concrete beam 200 mm wide and 300 mm deep is used an effective span of 5m to support an imposed load of 5kN/m. The density of concrete is 24kN/m3 . At the quarter span section of the beam, find the magnitude of (a) The concentric prestressing force necessary for zero fibre stress at the soffit when the beam is fully loaded and (b) The eccentric prestressing force located 100 mm from the bottom of the beam, which would modify the bottom fibre stress due to loading. [8+8]

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8. A prestressed concrete beam of I-section has triangular bending stress districution 13N/mm2 at top fibre and O at bottom fibre. The total vertical shear in concrete at the section 2400kN. Find the principal tensile stress at the centoridal axis and at the junctions of web and flanges. The dimension of I section are Top flanges: 240 x 40cm (depth) Bottom flange : 120 x 90cm (depth) Web : 60 x 100cm (depth) [16] ⋆⋆⋆⋆⋆

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Set No. 2

Code No: RR310101

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III B.Tech I Semester Supplimentary Examinations, November 2008 CONCRETE TECHNOLOGY AND PRESTRESSED CONCRETE (Civil Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. What are Bouges compounds? Explain in detail how each one of these compounds influence the strength & setting properties of cement. [6+10]

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2. (a) Differentiate between segregation and bleeding. Explain how segregation and bleeding affect the properties of concrete. (b) Explain the test that can be conducted for bleeding of concrete

[8+8]

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3. (a) Why the compressive strength is an important property of concrete. Discuss factors that affect the compressive strength.

(b) Compressive strength that conducted on 6 samples of 150X150X150mm Cubes at 28 days age given the following crushing loads: 110KN,115KN,112KN,108KN,113 KN and 114KN. Compute the cube compressive strength of concrete and comment on its grade. [8+8]

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4. Design a M20 concrete Mix using IS method of Mix design. Use following data. (a) Maximum size of Aggregate - 20mm (Angular) (b) Degree of workability - 0.90 compaction factor (c) Quality Control - good

(d) Type of Exposure - mild (e) Specific gravity

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i. Cement -3.10 ii. Sand -2.60 iii. Coarse aggregate - 2.62

(f) Water absorption:

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i. coarse aggregate-0.60% ii. fine aggregate - 1.50%

(g) Free surface moisture: i. coarse aggregate -NIL ii. fine aggregate -2.5%

(h) Sand conforms to zone III grading. Assume any other Data required suitably.

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[16]

Set No. 2

Code No: RR310101

5. A pretensioned concrete beam of 300 mm x 600 mm cross section is stressed by 20 - 8 mm high tensile steel wires, located at 200 mm below the centre line of the section. If the characteristic strength of concrete is 45N/mm2 and the characteristic strength of prestressing steel is 1400N/mm2 , determine the moment of resistance of the section. [16]

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6. A concrete beam of 10m span, 120mm wide and 310mm deep is presstressed by three cables. The cross sectional area of each cable is 220mm2 and the initial stress is 1075N/mm2 cable 1 is parabolic with an eccentricity of 45mm above the centroid at the supports and 55mm below at the centre of span. Cable 2 is also parabolic with zero eccentricity of 55mm below the centroid at the centre of span. If the cables are tensioned from one end only estimate the percentage loss of stress in each cable due to friction. Take coefficient of friction = 0.40 and friction coefficient for wave effect = 0.0015 per m. [16] 7. Design a prestressed concrete beam to the following requirements: (a) Span : 15 m

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(b) Superimposed load : 34 KN/m

(c) Cube strength of concrete at 28 days : 35N/mm2

(d) Safe stress in concrete at transfer of prestress fs : 0.5f ck

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(e) Safe stress in concrete due to final prestress fc : 0.4 fck (f) Total loss of prestress : 20%

(g) Allowable tensile stress in concrete : 0.129

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fck

(h) Ultimate stress in steel: 1500N/mm2 (i) Safe stress in steel : 60 % ultimate stress

[16]

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8. Explain how the behaviour of P.S.C. beam in shear differs from that of an R.C. beam. Illustrate your answer with neat sketches. [16]

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Set No. 3

Code No: RR310101

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III B.Tech I Semester Supplimentary Examinations, November 2008 CONCRETE TECHNOLOGY AND PRESTRESSED CONCRETE (Civil Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. Explain with details, in which situations each of the following cements are used

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(a) Air Entraining cement (b) Sulphate resisting cement (c) Low heat cement (d) Oil well cement.

[4+4+4+4]

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2. ”Slump test is the most commonly used method of measuring workability of fresh concrete is field”. Explain in detail the slump test with the help of a neat sketch. Discuss its merits and limitations. [8+8] 3. (a) State and explain Abram’s w/c ratio law. Discuss its limitations.

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(b) Draw the normal curves depicting the variation of compressive strength of concrete with w/c ratio and cement /water ratio. [8+8] 4. (a) Explain the relationship between Mean Design Strength and Specified Minimum strength. (b) It is required to design a M25 concrete mix. Calculate the mean target strength. Use I.S. Code method of approach. Explain the basic principle involved. [8+8] 5. With the help of stress diagrams explain the following:-

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(a) Prestressing with a straight concentric tendon.

(b) Prestressing with a straight eccentric tendon.

[8+8]

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6. A concrete beam of 10m span, 120mm wide and 310mm deep is presstressed by three cables. The cross sectional area of each cable is 220mm2 and the initial stress is 1075N/mm2 cable 1 is parabolic with an eccentricity of 45mm above the centroid at the supports and 55mm below at the centre of span. Cable 2 is also parabolic with zero eccentricity of 55mm below the centroid at the centre of span. If the cables are tensioned from one end only estimate the percentage loss of stress in each cable due to friction. Take coefficient of friction = 0.40 and friction coefficient for wave effect = 0.0015 per m. [16]

7. (a) Write in detail the approximate method of design of prestressed concrete Beams? (b) Derive the expression for eccentricity to be used in design? 1 of 2

[8+8]

Set No. 3

Code No: RR310101

8. A P.S.C beam ( 150 x 300mm) is prestressed by a parabolic tendon having an eccentricity of 100mm at mid span and zero at ends. It carries a live load of 2 k/m over a span of 8m. Determine the effective force in the tendon to balance the dead and live loads on the beam. Estimate the principal stresses at the supports. Take density of material = 24KN/m3 . [16]

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Set No. 4

Code No: RR310101

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III B.Tech I Semester Supplimentary Examinations, November 2008 CONCRETE TECHNOLOGY AND PRESTRESSED CONCRETE (Civil Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) With the help of neat sketches explain how the initial and final setting time test are conducted in the laboratory using Vicot apparatus.

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(b) What are I.S.codal specifications on setting times of ordinary Portland cement. [8+8] 2. (a) Distinguish between Aggregate crushing value and Aggregate abrasion value.

(b) Describe Devals Attrition test to determine the abrasion value of coarse aggregate. [8+8]

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3. (a) What is shrinkage of concrete?

(b) Explain various factors affecting shrinkage of concrete.

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(c) Describe the laboratory test for measurement of Drying shrinkage of Concrete. [4+4+8] 4. Write short notes on:

(a) Weight batching and volume batching. (b) Absolute volume method of proportioning mixes. (c) Workability viz-a-viz Durability.

[6+4+6]

5. Give a brief account of the properties of “High Tension Steel” used in PSC. [16]

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6. (a) A prestressed concrete beam 125mm wide and 325mm deep is prestressed by a straight wire carrying an initial force of 200kN at an eccentricity of 50mm. The modulus elasticity of steel and concrete are 210kN/mm2 respectively. Estimate the percentage loss of steel due to elastic deformation of concrete if the area of steel wires is 206mm2 . Derive the expression involved.

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(b) A concrete beam is prestressed by a cable carrying an initial prestressing force of 310kN. The cross sectional area of wires is 320mm2 . Calculate the percentage loss of stress in the cable only due to shrinkage of concrete using IS1343 recommendations assuming the beam to be i. Pretensioned. ii. Post tensioned.

[8+8]

7. (a) Write in detail the approximate method of design of prestressed concrete Beams? (b) Derive the expression for eccentricity to be used in design? 1 of 2

[8+8]

Set No. 4

Code No: RR310101

8. A P.S.C beam ( 150 x 300mm) is prestressed by a parabolic tendon having an eccentricity of 100mm at mid span and zero at ends. It carries a live load of 2 k/m over a span of 8m. Determine the effective force in the tendon to balance the dead and live loads on the beam. Estimate the principal stresses at the supports. Take density of material = 24KN/m3 . [16]

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