Prestress Concrete MCQ Quiz - Objective Question with Answer for Prestress Concrete - Download Free PDF

Explanation:

• Elastic Shortening of Concrete occurs when the pre-stressing force is transferred to the concrete, causing the concrete to shorten elastically.

• In pre-tensioning, tendons are tensioned before casting concrete, so when concrete is cast and the force transfers, elastic shortening happens immediately.

• In post-tensioning, tendons are tensioned after concrete has hardened, so the elastic shortening loss in concrete is negligible or does not occur at the same stage.

• Other losses like shrinkage, creep, and friction can occur in both systems (though friction loss specifically applies only to post-tensioning).

 Additional Information

• Prestressed concrete involves introducing internal stresses in concrete using high-strength steel tendons to counteract tensile stresses during use.
• It enhances the load-carrying capacity and reduces cracking by keeping concrete mainly under compression.
• There are two main methods: pre-tensioning, where tendons are stressed before concrete casting, and post-tensioning, where tendons are stressed after the concrete hardens.
• This technique is commonly used in bridges, beams, and slabs that require longer spans and higher strength.
• Compared to ordinary reinforced concrete, prestressed concrete results in lighter, more durable structures with better control over deflections and cracks. 

Explanation:

• In pre-stressed concrete, the concrete is pre-compressed by applying a compressive force using tendons or cables.

• However, concrete is weak in tension, so reinforcement (steel tendons or wires) is provided to resist tensile stresses that occur when the structure is loaded.

• The reinforcement carries tensile forces and prevents cracking in the concrete.

• The initial compressive stress is imparted by the pre-stressing process itself (not the reinforcement alone).

• Bond stress develops between concrete and steel to transfer forces, but the primary purpose of reinforcement is to resist tension.

Additional Information

• Pre-stressed concrete combines concrete's strength in compression with steel's strength in tension.

• Reinforcement ensures durability and serviceability by controlling cracks.

• It allows longer spans and thinner sections compared to conventional reinforced concrete.

Explanation:

As per IS 456: 2000,

• A standard hook has an anchorage value equivalent to a straight length of 16ϕ.
• The anchorage value of the standard U-type hook shall be 16 times the diameter of the bar. 
• The anchorage value of the standard bend shall be considered as 4 times the diameter of the bar for each 45o bend subject to a maximum value of 16 times the diameter of the bar.

Calculation:

Given:  

Diameter = ϕ = 25 mm

Anchorage value for 45° = 25 × 4 = 100 mm

Anchorage value for 90° = 25 × 8 = 200 mm

Concept:

As per IS 1343:2012, Clause 19.6.2.1 (a), the permissible unit bearing stress on concrete in the end zone of post-tensioned members (areas immediately behind external anchorages), after accounting for losses due to relaxation of steel, elastic shortening, and seating of anchorages, shall NOT exceed:

\( 0.48f_{ci} \sqrt{\frac{A_{br}}{A_{pun}}} \) or \( 0.8f_{ck} \), whichever is smaller.

Where:
• \( f_{ci} \) = Cube strength at transfer
• \( f_{ck} \) = Characteristic compressive strength of concrete
• \( A_{br} \) = Bearing area
• \( A_{pun} \) = Punching area

Explanation:

Characteristics of High Strength Concrete

High Strength Concrete (Grade M70 and above) possesses certain notable properties which distinguish it from normal strength concrete. These properties include:

• High durability: This type of concrete has enhanced durability due to its dense microstructure and lower water-cement ratio.

• Reduced permeability: The dense microstructure also results in lower permeability, making the concrete less susceptible to water ingress and chemical attacks.

• Increased shrinkage: Despite its advantages, high strength concrete can experience increased shrinkage due to its lower water content and higher cementitious material content.

• Lower cement content: This is not a characteristic of high strength concrete. In fact, higher grade concrete generally requires more cementitious materials to achieve the desired strength and durability.

Analyzing the Given Options

1. "High durability" (Characteristic)

   • High strength concrete is known for its enhanced durability due to its dense microstructure and low porosity.

2. "Reduced permeability" (Characteristic)

   • The dense microstructure of high strength concrete results in lower permeability, reducing the risk of water and chemical ingress.

3. "Increased shrinkage" (Characteristic)

   • High strength concrete can experience increased shrinkage due to its lower water content and higher cementitious material content.

4. "Lower cement content" (NOT a characteristic)

   • High strength concrete typically requires more cementitious materials to achieve the desired strength, making this statement incorrect.

Explanation

Minimum grade of concrete to be used in the design of prestressed concrete structure as per IS 1343 is as below:

1. For Post tensioning minimum grade of concrete used is M-30.

2. For Pre-tensioning minimum grade of concrete used is M-40.

Cover to be used in the design of prestressed concrete structure as per IS 1343 is as below:

1. For Posttensioning minimum cover to be used is 30 mm.

2. For Pre-tensioning minimum cover to be used is 20 mm.

The correct answer is longs spans.Key Points

• Post tensioning is a construction technique that involves the use of high-strength steel strands or cables to reinforce concrete structures.
• The suitability of post tensioning depends on various factors such as span length, design requirements, and construction constraints.
• Post tensioning allows for longer spans to be achieved without the need for intermediate supports.
• This is because the high-strength steel cables can provide the necessary tensile strength to counteract the weight of the structure and any applied loads.
• Longer spans can result in more open and flexible interior spaces, which can be beneficial for certain building types such as sports arenas, exhibition halls, and airports.
• Post tensioning can also help reduce the overall weight of the structure, which can lead to cost savings in materials and construction.
• The use of post tensioning can improve the durability and resilience of concrete structures, as the cables can help prevent cracking and deformation due to temperature changes, shrinkage, and other factors.

Additional Information

• End spans refer to the sections of a structure that are adjacent to a support, such as a column or a wall.
  • Post tensioning can be used for end spans, but it may not be as necessary as for long spans.
• Break spans are sections of a structure that are interrupted by an expansion joint or a construction joint.
  • Post tensioning can be used for break spans, but it may require additional design considerations and construction techniques.
• Edge spans are sections of a structure that are located at the perimeter, such as a balcony or a cantilevered slab.
  • Post tensioning can be used for edge spans, but it may require additional reinforcement to account for wind loads and other lateral forces.

Explanation:

As per IS 1343 Clauses 6.2.4:

Shrinkage

• The total shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions.
• For a given humidity and temperature, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing.
• The total shrinkage strain is composed of two components, (i) the autogenous shrinkage strain and  (ii) the drying shrinkage strain

Autogenous shrinkage strain:

• Develops during hardening of concrete
• The major part develops in the early days after casting.
• It is a linear function of concrete strength

Drying shrinkage strain:

• The drying shrinkage strain develops slowly as it a function of migration of the water through the hardened concrete.

Explanation:

Prestressing is the process by which a concrete element is compressed, generally by steel wires or strands.

The purpose of reinforcement in prestressed concrete is to impart initial compressive stress in concrete

Precast elements may be prestressed during the construction process (pre-tensioning) or structures may be stressed once completed (post-tensioning).

Prestressing compensates for the tensile stresses introduced when the element is loaded. Hence the concrete generally remains in compression.

Prestressing serves two main purposes:

1. To improve the resistance of the member to the dead and live loads.

2. To modify the behavior of the member or structure in such a way so as to make it more suitable for its intended purpose.

The primary purpose of prestressing steel is to apply a force to a concrete, either by bond or by means of special anchoring devices.

Explanation:

As per CI. 19.5.1 of IS 1343 At the time of initial tensioning, the maximum tensile stress, f_pi immediately behind the anchorages shall not exceed 76 percent of the ultimate tensile strength, f_pu of the wire or bar or strand.Important Points 

Grade of concrete:

• M 40 for pre-tensioned members
• M 30 for post-tensioned members

Minimum grade of concrete to be used in the design of prestressed concrete structure as per IS 1343 is as below:

1. For Post tensioning minimum grade of concrete used is M-30.

2. For Pre-tensioning minimum grade of concrete used is M-40.

Hence it can be seen that grade of concrete used for prestressed member lies in the range of M30 to M60

Important Points

Cover to be used in the design of prestressed concrete structure as per IS 1343 is as below:

1. For Posttensioning minimum cover to be used is 30 mm.

2. For Pre-tensioning minimum cover to be used is 20 mm.

A minimum grade of concrete to be used in the design of the prestressed concrete structure as per IS 1343 is as below:

1. For Post-tensioning minimum grade of concrete used is M-30.

2. For Pre-tensioning minimum grade of concrete used is M-40.

Cover to be used in the design of the prestressed concrete structure as per IS 1343 is as below:

1. For Posttensioning minimum cover to be used is 30 mm.

Concept:

Prestress concrete is the concrete in which internal stresses are produced due to compression or tension applied before applying external load and these stresses are counter balanced by the applied load to the desired degree.

Calculation:

Given,

Prestressing force = 2500 KN

Effective span length = 10 m

Total external load = 40 kN/m

We know that,

 \(M = P.e = {WL^2 \over 8}\)

\(e = {WL^2 \over 8P} = {40\times10^2 \over 8\times2500} = 0.2 m = 200 mm\)

Concept:

Shrinkage loss

Time-dependent strain measured in an unloaded and unrestrained specimen at a constant temperature.

Loss of pre-stress (Δf_p) due to shrinkage = E_P × ϵsh

Where E_p is the modulus of pre-stressing steel and

ϵsh is shrinkage strain

The approximate value of shrinkage strain for design shall be assumed as follows (IS 1383):

For pre-tensioning = 0.0003

Calculation:

Given, 

initial Prestressing force, P_o = 300 kN, Area = 300 mm²

Assume, E_p of steel = 2 × 10⁵ N/mm²

Initial stress, f = P_o/A = 300000/300 = 1000 N/mm²

Loss of pre-stress due to shrinkage, (Δfp) = EP × ϵsh

(Δfp) = 2 × 10⁵ × 0.0003 = 60 N/mm²

Loss of stress (%) = \(\frac{{{\Delta f_p}}}{f} \times 100 = \frac{{60}}{{1000}} \times 100 = 6\%\)

So, the percentage of loss of stress due to shrinkage in pre-tensioned members is 6 %. Hence the most appropriate option is 1 i.e 6.3 %.

Concept:

Loss due to friction and anchorage slip occurs only in the post-tensioned beam.

• In prestressed concrete, the most important parameter is the prestressing force.
• If the prestressing force reduces with time, prestresses also get reduced. 
• Various reductions of the prestressing force are termed as the losses in prestressing.
• Total loss of prestress consists of those losses which are instantaneous at the transfer stage as well as those which are time-dependent. 

Important Points

Losses of prestress: