Category: Prestressed Concrete Structures Objective Questions

Prestressed Concrete Structures Multiple Choice Questions on “Loss Due to Friction”.

1. The loss of prestress due to friction generally occurs in case of ____________
a) Post tensioned members
b) Pre tensioned members
c) Chemical members
d) Electrical members
Answer: a
Clarification: Loss of prestress due to friction occurs in the case of post tensioned members, the tendons are housed in the ducts performed in concrete and the ducts are either straight or follow a curved profile depending upon the design requirements.

2. The total loss of prestress due to friction is of ____________
a) 4 types
b) 2 types
c) 8 types
d) 3 types
Answer: b
Clarification: The total loss due to friction is divided into two types:
Loss of prestress due to effect of curvature, Loss of prestress due to wobble effect and frictional losses can be reduced by over tensioning the tendons by an amount equal to the maximum frictional loss and jacking the tendons from both ends of the beam adopted generally, when the tendons are long or when the angles of bearing are large.

3. The loss of stress due to curvature effect depends upon ____________
a) Alignment
b) Midpoint
c) Centerline
d) Exterior point
Answer: a
Clarification: The loss of stress due to the curvature effect, which depends upon the tendon form or alignment which generally follows a curved profile along the length of the beam, curvature coefficient is expressed as μ and wobble coefficient is expressed as k/m.

4. The wobble effect due to loss of stress is also known as ____________
a) Wave effect
b) Ray effect
c) Bubble effect
d) Bulb effect
Answer: a
Clarification: Loss of stress due to wobble effect, which depends upon the local deviations in the alignment of the cable and the wobble effect is also known as wave effect, the friction coefficient values for wave effect k are 0.15 per 100m for normal conditions, 1.5 per 100m for thin walled ducts where heavy vibrations are encountered and in other adverse conditions.

5. The wobble effect is the result of ____________
a) Misalignment
b) Extreme alignment
c) Tensile alignment
d) Anchorage alignment
Answer: a
Clarification: The wobble or wave effect is the result of accidental or unavoidable misalignment since ducts cannot be perfectly located to follow a predetermined profile throughout the length of the beam, the coefficient due to wobble effect may be reduced to zero where the clearance between the duct and cable is sufficiently large to eliminate wave effect so as the sheath is made up of heavy gauge steel tube with water tight joints, where a deformation of duct profile is prevented during the vibration of concrete.

6. The loss of stress due to friction is given as ____________
a) 1-(μα+Kx)
b) 1-(μα+Kx)
c) 1-(μα+Kx)
d) 1-(μα+Kx)
Answer: a
Clarification: The prestressing force at section is given by:
P_x = p˳e^-(μα+kx), Loss of stress (Δf)_f = 1-(μα+Kx),
P_x = Prestressing force at section x, P˳ = initial prestressing force, μ = coefficient of friction between the cable and concrete, k = wobble effect, d = cumulative angle.

7. The value of ‘μ’ in loss of stress equation depends upon ____________
a) Type of curing
b) Type of concrete
c) Type of steel
d) Type of aggregates
Answer: c
Clarification: The values of ‘μ’ (coefficient of curvature effect) depend upon the type of steel and concrete used in construction and are given in Indian standard codes of practice, coefficient of friction can be considerably reduced by variety of lubricants, particularly greases, oil, graphite mixtures, paraffin, the use of paraffin wax gives by far the coefficient of friction especially with high contact pressure.

8. A post tensioned concrete beam 200mm wide and 450mm deep, of span 10m, initial stress of 840n/mm² is available in the un jacked end immediately after the anchoring. Find the angle between tangents to the cable at supports?
a) 0.13
b) 0.12
c) 0.10
d) 0.23
Answer: b
Clarification: b = 200mm, d = 450mm, l = 10m, r = 84m, d = 5m,
Angle between the horizontal tangent drawn to the cable at support sinα = (5/84) = 0.06radians,
Cumulative angle between tangents to the cable at supports = (2×0.06) = 0.12radians.

9. A concrete tank if has a minimum stress in wires 600n/mm² immediately after tensioning and the coefficient of friction is 0.5. Calculate the maximum stress to be applied to the wires at the jack?
a) 900n/mm²
b) 960n/mm²
c) 850n/mm²
d) 800n/mm²
Answer: b
Clarification: P_x = 600n/mm², e = 2.7183, μ = 0.5
P_x = P˳e^-μα,
600 = P˳e-^(0.5×π/2),
P˳ = (600) × (2.7183^0.79) = 1320n/mm², Average stress in wires = (1320+600/2) = 960n/mm².

10. A cylindrical concrete tank, 40m external diameter is to be prestressed circumferentially by means of a high strength steel wire (E_s = 210kn/mm²) jacked at 4 points and 90 degrees apart. Find the expected extension at the jack?
a) 150mm
b) 130mm
c) 144mm
d) 133mm
Answer: c
Clarification: d = 40m, E_s = 210kn/mm², n = 4points, θ = 90˚,
Length of wires = (π×40×1000/4) = 104πmm,
Extension at the jack = (960/210×10³×10⁴π) = 144mm.

Prestressed Concrete Structures Multiple Choice Questions on “Transmission Length”.

1. The transmission length required to attain uniform stress distribution in prestress member is acquired through __________
a) Zero bond stress
b) Middle bond stress
c) Safe bond stress
d) Edge bond stress
Answer: a
Clarification: The transmission length required to attain uniform stress distribution in prestress member with zero bond stress and the length required at the ends of a pretensioned member for the build up to of stress in concrete is of great importance, particularly in short pretensioned units, since it controls the working bending moment and the shear force allowable on the section.

2. The transmission length mainly depends up on how many factors?
a) 8
b) 4
c) 2
d) 12
Answer: b
Clarification: The transmission length mainly depends on 4 factors, diameter of wire, surface characteristics of wire, co-efficient of friction between steel and concrete, elastic properties of steel and concrete.

3. The expression for transmission length based on wedge action is given as?
a) L_t = ϕ/2μ (1+μ_c) (α_c/μ_s-f_pi/E_c) (f_pe/2f_pi-f_pe)
b) L_t = ϕ/2μ (1+μ_c) (α_c/μ_s-f_pi/E_c)
c) L_t = ϕ/2μ (1+μ_c)
d) L_t = ϕ/2μ
Answer: a
Clarification: Hoyer has developed an expression for computing the transmission length, based on wedge action: L_t = ϕ/2μ (1+μ_c) (α_c/μs-f_pi/E_c) (f_pe/2f_pi-f_pe), L_t = transmission length, ϕ = wire diameter, μ = coefficient of friction between steel and concrete, μ_c = Poisson’s ratio for concrete, μ_s = Possion’s ratio of steel, α_c = modular ratio (E_s/E_c), E_c = modulus of elasticity of concrete of concrete, f_pi = initial stress in steel, f_pe = effective stress in steel.

4. The transmission length is predicted considering the equation parameters varying from __________
a) 90 to 150ϕ
b) 100 to 200ϕ
c) 80 to 160ϕ
d) 20 to 60ϕ
Answer: c
Clarification: Under the normal ranges of values of transmission parameters, the transmission length is likely to vary from 80 to 160ϕ, several tests have been carried out by many investigators to determine the transmission length and these methods can be classified into different categories depending upon the principles under which the solutions are obtained.

5. Which one of the empirical formula is used for predicting transmission length?
a) L_t = ((f_cu)^1/2 x 10³/β)^1/2
b) L_t = ((f_cu)^1/2 x 10³/β)
c) L_t = (f_cu)^1/2 x 10³
d) L_t = (f_cu)^1/2
Answer: a
Clarification: The transmission length is predicted generally using the following empirical formula,
L_t = ((f_cu)^1/2 x 103/β)^1/2, L_t = transmission length in mm, f_cu = cube strength of concrete at transfer in n/mm², β = constant depends on the strand and wire, the transmission length changes with time due to the effect of creep and shrinkage of the concrete.

6. The transmission length prevailing at the time of transfer does not remain __________
a) Zero
b) Constant
c) Without reaction
d) With reaction
Answer: b
Clarification: The transmission length prevailing at the time of transfer does not remain constant but increases at a decreasing rate with time due to the effect of creep and shrinkage of concrete, using radiographic strain measure techniques Evans and Robinson have conclusively shown that the influence of passage of time is to increase the transmission length and to move it bodily away from the end of the member.

7. The surface of drawn round wires is roughened by __________
a) Pickling
b) Bending
c) Bonding
d) Tensioning
Answer: a
Clarification: The surface of drawn round wires is roughened by pickling to produce fine irregularities which interlock with the concrete this is the method of obtaining shear bond, twisting of round wires into strands.

8. The Shear bonding is obtained by cold rolling of __________
a) Square indentations
b) Rectangle indentations
c) Elliptical indentations
d) Circular indentations
Answer: c
Clarification: Cold rolling of elliptical or rhombic shallow indentations on the surface of wires and hot rolling of oblique transverse ribs on wires which are subsequently heat treated methods of obtaining shear bond.

9. According to R_os, the transmission length of smooth round wires of 1.5-5mm diameter varies approximately from __________
a) 1500-600
b) 100-300
c) 200-100
d) 1100-800
Answer: b
Clarification: According to R_os, the transmission length of smooth, round wires of 1.5-5mm diameter varies approximately from 100-300 diameter respectively, after allowing for the time dependent in elastic effects like creep with crimping or indentations, these values could be considerably reduced.

10. Estimate the transmission length at the ends of a pretensioned beam prestressed by 7mm diameter wires. Assume the cube strength of concrete at transfer as 42n/mm² (adopt empirical method)?
a) 550mm
b) 610mm
c) 420mm
d) 210mm
Answer: b
Clarification: Lt = ((f_cu)1/2 x 10³/β)^1/2 = Transmission length,
7mm diameter smooth wires, β = 0.0174, f_cu = 42n/mm², L_t = ((42)^1/2 x 10³/ 0.0174) = 610mm = 87ϕ.

Prestressed Concrete Structures Multiple Choice Questions on “Design for Shear and Torsion”.

1. Which type of cables are advantages in reducing the effective shear?
a) Straight
b) Curved
c) Trapezoidal
d) Longitudinal
Answer: b
Clarification: Curved cables are advantageous in reducing the effective shear and together with the horizontal compressive prestress, reduce the magnitude of the principal tension and the effect of shear is to induce tensile stresses on diagonal planes and prestressing is beneficial since it reduces the magnitude of the principal tensile stress in concrete.

2. The various codes recommend empirical relations to estimate ____________
a) Ultimate shear resistance
b) Ultimate torsional resistance
c) Ultimate bending resistance
d) Ultimate load
Answer: a
Clarification: The various codes recommend empirical relations to estimate the ultimate shear resistance of the section by considering the flexure shear and web shear cracking modes and the design shear resistance should exceeded the ultimate shear due to the transverse loads and if not, suitable transverse reinforcements are designed to resist the balance shear force.

3. Which type of shear reinforcement should be provided for members with thin webs?
a) Maximum shear reinforcement
b) Minimum shear reinforcement
c) Nominal shear reinforcement
d) Tensile reinforcement
Answer: c
Clarification: In members with thin webs such as I and T sections nominal shear reinforcements have to be provided to prevent cracking due to variations in temperature the provisions for design of shear reinforcements prescribed in British, American and Indian standard have been dealt with in the case off structural concrete members subjected to torsion, shear stresses develop depending upon the type of cross section and magnitude of torque and the shear stresses in association with the flexural stresses may give rise to principal tensile stress the value of which when it exceeds tensile strength of the concrete results in the development of cracks on the surface of the member.

4. The pre and post tensioned members with bonded tendons bond stress between ____________
a) Steel and concrete
b) Steel and water
c) Steel and aggregates
d) Steel and plastic
Answer: a
Clarification: Pre tensioned or post tensioned members with bonded tendons develop bond stresses between steel and concrete when the sections are subjected to transverse shear forces due to the rate of change of moment along length of the beam and in the case of type 1 and 2 members, which are uncracked at service loads, the flexural bond stresses developed are computed by considering the complete section.

5. In case of pre tensioned member, the computations of transmission length are influenced by?
a) Bond
b) Flexure
c) Torsion
d) Tension
Answer: a
Clarification: In the case of pre tensioned members, the computations of the transmission length at the ends is of practical significance since the support positions are influenced by bond and transmission length.

6. When prestress is transferred to concrete by means of external anchorages which pressure is developed?
a) Bearing
b) Twisting
c) Torsion
d) Bent
Answer: a
Clarification: In the case of post tensioned members where prestress is transferred to concrete by means of external anchorages, the bearing pressures develop behind the anchorages have to be investigated and suitably controlled to prevent crushing failure of the end block zone.

7. The bearing pressure on the concrete is given as ____________
a) 0.4f_ci
b) 0.8f_ci
c) 0.12f_ci
d) 0.2f_ci
Answer: b
Clarification: According to the Indian standard code IS: 1343-1980, the permissible unit bearing pressure on the concrete after allowing for all losses is limited to: 0.48f_ci (A_br/A_pan) or 0.8 f_ci, f_ci = cube strength of concrete at transfer, A_br= bearing area, A_pan = punching area.

8. The effective punching area A_pun is generally the contact area of ____________
a) Tendon device
b) Anchorage device
c) Stress device
d) Strain device
Answer: b
Clarification: The effective punching area is generally the contact area of the anchoring device and the bearing area is taken as the maximum area of that portion of the member which is geometrically similar and concentric to the effective punching area.

9. The end block of a beam has a rectangular section 100mm wide by 200mm deep, the force of cable is 200kn. Find the actual bearing pressure?
a) 30n/mm²
b) 40n/mm²
c) 20n/mm²
d) 10n/mm²
Answer: c
Clarification: force of cable = 200kn, b = 100mm, d = 200mm
Bearing area A_br = (100×100) = 10000mm²,
Actual bearing pressure = (200×10³) / 10000 = 20n/mm².

10. The end block of a prestressed concrete beam has a rectangular section; a cable carrying a force of 200kn is to be anchored against the end block at the centre if the cube strength of concrete at transfer is 30n/mm². Design the maximum permissible bearing pressure?(A_br = 10000mm²)
a) 24n/mm²
b) 48n/mm²
c) 54n/mm²
d) 12n/mm2²
Answer: a
Clarification: Assuming an anchor plate of size 50mm by 50mm, punching area A_pun = (50×50) = 2500mm², A_br = 10000mm², maximum permissible bearing pressure = 0.48fci(A_br/A_pun) or 0.8f_ci whichever is smaller, f_b = 0.48x30x(10000/2500)1/2 = 28.8n/mm² or (0.8×30) = 24n/mm2².

Prestressed Concrete Structures Multiple Choice Questions on “Prestressing of Tanks”.

1. Prestressed concrete tanks have been widely used for the storage of __________
a) Gas
b) Air
c) Fluids
d) Water
Answer: c
Clarification: Prestressed concrete tanks have been widely used for storage of fluids, such as water, oil, gas, sewage, granular materials like cement, process liquids and chemicals, slurries and more recently cryogens water storage tanks of large capacity are invariably made of prestressed concrete recent applications include special forms of prestressed concrete tanks, which are triaxially prestressed and serve as containment vessels and biological shields for nuclear reactors.

2. Prestressed concrete although it is water tight, it is not __________
a) Gas tight
b) Liquid tight
c) Vapour tight
d) Material tight
Answer: a
Clarification: Tanks have been built for storing liquid oxygen at 230 degrees with capacities up to one million liters and prestressed concrete, although water tight, it is not gas tight were vapours under pressure are to be stored and in such cases, a thin membrane linear of steel provides rigidity and increases the steel tensile capacity of the pretressed concrete.

3. The metal linear concept in prestressed tanks has proved to be success in case of __________
a) Air tanks
b) Water tanks
c) Fluid tanks
d) Vapour tanks
Answer: b
Clarification: The metal linear concept has proved so successful that it is being increasingly used in America, even for large water tanks and in the case of sanitary structures like sludge digestion tanks, spherical shapes are preferred and for practical reasons, the tank is made up of a top and bottom conical shell connected by a circular cylindrical intermediate portion.

4. In the case of large tanks, the base slabs is subdivided by __________
a) Water
b) Joints
c) Scale
d) Lines
Answer: b
Clarification: In the case of large tanks, the bars slab is subdivided by joints which are sealed by water stops and the floor slabs are cast in panels and according to the British standard the maximum length of side of such panels should not exceed 7.5m for reinforcement slabs and 6m for nominal slabs and they may be formed out of 50 to 80mm thick gunite reinforced with 0.5 percent of steel distributed in each of the principal directions.

5. The nominal reinforcement provided for floor slabs stipulated by Indian standard code is not less than?
a) 0.5
b) 0.7
c) 0.15
d) 0.8
Answer: c
Clarification: The Indian standard code stipulates the floor slabs of tanks resting on the ground should be provided with a nominal reinforcement of not less than 0.15 percent and the floor slabs should be cast in panels of area not more than 4.5m² with contraction or expansion joints and these slabs are to be cast over a layer of concrete not less than 75mm thick with a sliding layer of bitumen paper provided to prevent the bond between the screed and the floor slab.

6. In the fixed base joint the junction is between the tank wall and __________
a) Slab
b) Footing
c) Beams
d) Columns
Answer: b
Clarification: In fixed base joint the junction is between the tank wall and footing is the most vulnerable location as far as leakage is concerned and hence in the case of tanks storing penetrating liquids, it is necessary to form the wall and footing in monolithic construction and this type of connection is generally well suited for shallow tanks with diameters up to 30m, where the fixing moment developed at the wall base does not result in excessively high stresses and congestion of reinforcement.

7. When a sliding joint is made what is interposed at the junction of wall and base?
a) Rubber
b) Timber
c) Plastic
d) Soil
Answer: a
Clarification: A sliding joint is made by interposing rubber or neoprene pads at the junction of the wall and the base and the preload engineering company has developed this type of sliding base in which a vertical water stop is inserted between two rubber strips and in the present state of art, single neoprene pads have also used and the main function of these pads is to allow for free horizontal movement of the wall relative to the base by shear deformation of the rubber joint, which does not exceed a critical value of 30 degrees.

8. The most common method of wire wrapping for circular tanks consists of __________
a) VBR machine
b) Slump cone
c) Cassagrande apparatus
d) Traction machine
Answer: d
Clarification: The most common method of wire wrapping circular tanks consists of a traction machine, and it is suspended from a trolley which runs along the top of the tank walls and the high tensile wire is drawn through a die while it is wound on the tank to achieve the designed tension in the wire and as a precaution the wires are anchored by clips, the wall at regular intervals to ensure that in the event of wire fracture, the winding does not get detached.

9. Calculate minimum wall thickness given a cylindrical prestressed water tank of internal diameter 30m over a depth of 7.5m and the permissible compressive stress at transfer is 13n/mm² and the maximum compressive stress under working pressure is 1n/mm² and the loss ratio is 0.75?
a) 43.8
b) 82.3
c) 64.5
d) 90.4
Answer: b
Clarification: D = 30m, H = 7.5m, N_d = 720n/mm, ɳ = 0.75, f_ct = 13n/mm², pressure is 1n/mm²
T = N_d/ɳf_ct-f_min.w = 720/ (0.75X 13) – (1) = 82.3mm.

10. Calculate circumferential prestress of a cylindrical prestressed concrete water tank given that the thickness is 12mm, loss ratio is 0.75, the maximum stress under working pressure is 1n/mm²(N_d value is 720)?
a) 9.4n/mm²
b) 5.6n/mm²
c) 11.2n/mm²
d) 15.2n/mm²
Answer: a
Clarification: N_d = 720, f_min.w = 1, ɳ = 0.75, t = 120mm
F_c = N_d/ ɳ_t+ f_min.w/ ɳ = 720/0.75 x 120+1/0.75 = 9.4n/mm².

11. Calculate vertical prestressing force if wires of 5mm diameter with an initial stress of 1000n/mm² are available for circumferential winding and Freyssinet cables made up of 12 wires of 8mm diameter stressed to 1200n/mm² are to be used for vertical prestressing?
a) 15
b) 12
c) 8
d) 4
Answer: b
Clarification: 5mm diameter wires stress is 1000n/mm², 12 wires of 8mm diameter are stressed to 1200n/mm², f_c = (12x1000x200)/(1000) = 2400kn.

12. Calculate circumferential prestress if loss ratio 0.75, thickness is 120mm, working pressure is 1n/mm² and Nd is given as 840n/mm?
a) 10.75n/mm²
b) 14.8n/mm²
c) 12.6n/mm²
d) 10.65n/mm²
Answer: a
Clarification: Given N_d = 840, f_min.w = 1, ɳ = 0.75, t = 120mm,
F_c = N_d/ ɳ_t+ f_min.w/ ɳ = 840/0.75×120+ 1/0.75 = 10.75n/mm².

13. Calculate the spacing of 5mm wires having a loss ratio of 0.075, compressive stress is 10.75n/mm², 5mm diameter wires stress is 1000n/mm², 12 wires of 8mm diameter are stressed to 1200n/mm²(N_d = 840n/mm²)?
a) 15.4mm
b) 11.6mm
c) 12.4mm
d) 18.5mm
Answer: b
Clarification: ɳ = 0.075, t = 120mm, internal diameter is 30×10³, N_d = 840
S = 2×840/0.075x1000x20/10.75x30x103x120 = 11.6mm.

14. Calculate the maximum vertical moment due to prestress if given self weight moment is 16.5kn/m, thickness is 0.115m and loss ratio is 0.0075?
a) 15.4
b) 21.5
c) 25.4
d) 2.6
Answer: c
Clarification: M_w = 16.5kn/m, t = 0.115, ɳ = 0.075
M_t = M_w x W_t / ɳ 16500(0.11/0.075) = 25.4×10⁶nmm/m.

15. Find vertical prestressing force if characteristic strength is 8.2, wires are stressed at 1000n/mm², diameter is 150mm?
a) 1500kn
b) 1230kn
c) 4567kn
d) 8967kn
Answer: b
Clarification: f_c = 8.2, stress = 1000, diameter = 150mm
F = (8.2 x 1000 x 150)/1000 = 1230kn.

Prestressed Concrete Structures Multiple Choice Questions on “Structural Form Bridges”.

1. One of the structural forms used for bridges are ____________
a) Solid slabs
b) Dense slabs
c) Rigid slabs
d) Open slabs
Answer: a
Clarification: For medium and long spans prestressed concrete is ideally suited for the super structure of bridges and the structural forms generally used for bridges are listed as follows solid slabs(10-15m), voided or hollow slabs(15-25m), rigid frame bridges(15-30m), Tee beams and slabs(20-40m), two cell box girders(30-70m), multi cell box girders(40-80), balanced cantilever type bridges(20-30m), continuous girders of variable depth(30-40), cable stayed bridges(100-500m).

2. Prestressed concrete has more or less replaced ____________
a) Reinforced concrete
b) Prestressed concrete
c) Aluminium concrete
d) Voids concrete
Answer: a
Clarification: Prestressed concrete has more or less replaced reinforced concrete as the most suitable material for bridge construction due to its inherent advantages of high strength coupled with durability, energy absorption under dynamic loads, ability to resist repetitive loads, freedom from cracks, easy mould ability to desired shape, economy and ease of maintenance.

3. The floors and girders of through type structures should be placed in ____________
a) Two continuous
b) One continuous
c) Span continuous
d) Limit continuous
Answer: b
Clarification: The floor and girders of through type structures should be placed in one continuous unless otherwise specified in which case a special sear anchorage should be provided to ensure monolithic action between the girders and floor and the case of tee beam slab floors, it is preferable to deposit concrete first up to the top of the girder ribs followed by concreting of the slab in one continuous operation and if the slab concrete is delayed, suitable shear keys should be formed by roughening the top of the girder before depositing the centre in the slab.

4. Prestressed concrete members should be compacted by which type of vibration?
a) Mechanical vibration
b) Structural vibration
c) Internal vibration
d) External vibration
Answer: a
Clarification: Prestressed concrete members should be compacted by mechanical vibrations expect when certain types of extrusion machines are used which consolidate the concrete by tamping and depending on the type of structural members, internal, external or surface type vibrators are used and normally vibrators having frequencies in the range of at least 3200-3600 cycles per minute are used.

5. The vibrations should be used only for ____________
a) Densation
b) Compaction
c) Rotation
d) Spinning
Answer: b
Clarification: Vibration should be well distributed so that the concrete reaches a state of plastic mass with uniform density and vibrators should be used for compaction only and not for moving concrete horizontally along the forms.

6. When vibrators are used for horizontal and vertical operation, the spacing should ____________
a) Overlap
b) Attached
c) Hardened
d) Made wet
Answer: a
Clarification: When vibrators are used for horizontal and vertical operation, the spacing of points of vibration should be such that their zones of influence overlap and when concrete is deposited in layers such as that in column, the vibrator should be inserted vertically.

7. When concrete is deposited in layers it should be allowed to ____________
a) Sink
b) Collapse
c) Cut
d) Restrain
Answer: a
Clarification: When concrete is deposited in layer it is allowed to sink due to its own weight to the bottom of the layer and then slowly withdrawn and during vibration of succeeding layers, the vibrator should preferably penetrate the surface of the preceding layer by at least 150mm and good vibration should result in a surface without honey combing, aggregate or mortar pockets or excessive air bubbles.

8. The self compacting concrete (SCC) is generally referred as ____________
a) Spatial concrete
b) Pores concrete
c) Rheodynamic concrete
d) Flexible concrete
Answer: c
Clarification: Rheodynamic concrete generally referred to as self compacting concrete (SCC) is able to flow under its own weight and completely fill the formwork, even in the presence of dense reinforcements without the need for any vibration whilst maintaining homogeneity and resulting in concrete of high early strength and durability.

9. Degussa-MBT construction chemicals (INDIA) have developed revolutionary type of admixtures based on?
a) Micro science
b) Nano science
c) poly science
d) Animal science
Answer: b
Clarification: Degussa-MBT construction chemicals (INDIA) have developed revolutionary type of admixtures using nano polymers which can be used to bring together functional groups aimed at targeted performances in concrete and based on nano science.

10. A system of polymers with longer side and shorter side chains is called ____________
a) Zero energy system
b) Elastic energy system
c) Bonded energy system
d) Force energy system
Answer: a
Clarification: A system of polymers with longer side chains and shorter main chains to facilitate high early strengths in concrete without steam curing and with specific applications in precast reinforced and prestressed concrete units manufacturing industry.

Prestressed Concrete Structures Multiple Choice Questions on “Pretensioning Systems”.

1. The tendons in the pretensioning system are tensioned between ___________
a) Rigid anchorages
b) Hydraulic jacks
c) Concrete beds
d) Variable beams
Answer: a
Clarification: In the prestressing system, the tendons are first tensioned between rigid anchorage blocks cast on the ground or a column or a unit mould type pretensioning bed, prior to the casting of concrete in the moulds.

2. When the concrete attains sufficient strength, which elements are released?
a) Jacks
b) Casting bed
c) Tendons
d) Beams
Answer: a
Clarification: High early strength concrete is often used in factory to facilitate early stripping the reuse of moulds and when the concrete attains sufficient strength, the jacking pressure is released and the edge of tendon at its either side is formed to an abutment and its other edge is to be pulled with the application of jack.

3. Which is one of the systems used for pretensioning?
a) Magnel-Balton system
b) Freyssinet system
c) Gifford-Udall system
d) Hoyer’s long line method
Answer: d
Clarification: Hoyer’s long line method is the system used in pretensioning and the other systems like Freyssinet, Gifford-Udall, and Magnel-Balton are post tensioning systems and large numbers of beams are produced in an individual alignment.

4. Hoyer’s system of pre tensioning is generally adopted for ___________
a) Small scale members
b) Large scale members
c) Middle span members
d) End members
Answer: b
Clarification: Hoyer’s system is generally recommended when the production of pretensioned members is required on a large scale is the principle which are used to precast the beams the post tensioning system was considered to pretensioning system when used for large spans, due to this reason the pre tensioning system was replaced by post tensioning system.

5. The transfer of prestress of concrete is achieved by ___________
a) Plates
b) Rings
c) Steel bars
d) Jacks
Answer: d
Clarification: The transfer of prestress to concrete is usually achieved by large hydraulic or screw jacks by which all the wires are simultaneously released after the concrete attains the required compressive strength generally strands of up to 18mm diameter and high tensile wires of up to 7mm diameter anchor themselves satisfactorily with the help of surface bond and the interlocking of the surrounding matrix in the micro indentations on the wires.

6. The bond of prestressing wires in Hoyer’s system can be formed by ___________
a) Helical crimping
b) Tangential crimping
c) Circular crimping
d) Diode crimping
Answer: a
Clarification: Bond of prestressing wires may be considerably improved by forming surface indentations and by helical crimping of wires in Hoyer’s system, strands have considerable better bond characteristics than plain wires of equal cross sectional area supplementary anchoring devices are required when single wires of larger diameter are used in the pretensioned units.

7. The Hoyer’s method of prestressing is done by ___________
a) Pulling out of wires
b) Pushing wires
c) Heating of wires
d) Stressing of wires
Answer: a
Clarification: Hoyer’s system of prestressing involves pulling out of wires between two bulkheads which are separated at large distances in order to produce a larger number of beams in an individual alignment, the concrete is to be poured by providing appropriate shuttering between the beams and then the wires are united after hardening of concrete and cutoff.

8. Hoyer’s system of pretensioning can be done for beams.
a) 1
b) More than 2
c) Less than 2
d) 2
Answer: b
Clarification: United beams more than 2 are hardened in Hoyer’s method of pretensiong in case of large distances between each beam, the most commonly used devices are the Weinberg clip developed in France and the Dorland clip developed in the united states these clips are clamped on to the tensioned wires close on the end diaphragms of the units before concreting operations.

9. The Hoyer’s system of prestressing proves to be economical for ___________
a) Pre tensioning system
b) Post tensioning system
c) Beam casting
d) Bed casting
Answer: b
Clarification: The post tensioning system was considered as economical when compared to pre tensioning system, when used for larger spans due to this reason pre tensioning system was replaced by post tensioning system, for mass production of pretensioned elements, the long line process developed by Hoyer is generally used in the factory and in this method the tendons are stretched several hundred meters apart so that a number of similar units may be cast along the same group of tensioned wires.

10. In Hoyer’s system the projection of plugs left in concrete exceeds beyond ___________
a) Middle of member
b) End of member
c) First of member
d) Transfer part of member
Answer: b
Clarification: One of the disadvantages of Hoyer’s system is the projection of plugs which are left in concrete exceeds beyond the end of the member, application of heavy jacks which are uneconomical, projection of plugs which are left in concrete exceeds beyond the ends of the member, additional reinforcement is required to prevent failure of shear.