Introduction MCQ Quiz - Objective Question with Answer for Introduction - Download Free PDF

Explanation:

According to IS 1893:2002 (Criteria for Earthquake Resistant Design of Structures), reinforced concrete (RC) buildings subjected to seismic forces experience lateral loading, which leads to shear stresses in structural elements. These stresses often result in diagonal cracking patterns, particularly in:

• Masonry infill walls: due to the racking motion of frames.

• RC shear walls or columns: when subjected to high shear forces and bending moments during seismic events.

• The diagonal cracks usually originate from the corners of window or door openings and propagate across the wall, indicating in-plane shear failure or frame-infill interaction.

 Additional InformationVertical cracks in beams

• Vertical cracks in beams are typically due to flexural stresses and are normal under loading but are not characteristic of seismic-induced damage.

Horizontal cracks in slabs

• Horizontal cracks in slabs could occur due to shrinkage or temperature effects, not typically as a direct result of seismic action.

Shrinkage cracks in columns

• Shrinkage cracks are caused by drying or poor curing, and while they may exist, they are not characteristic of seismic damage.

Explanation:

• Map cracking, also known as pattern cracking, is a network of fine cracks forming irregular polygons on the concrete surface.

• As per IS 3370-2:2009 (Concrete structures for storage of liquids – Part 2: Reinforced cement concrete structures), this type of cracking is commonly associated with corrosion of reinforcement.

• When reinforcing steel corrodes, the corrosion products (rust) expand, exerting tensile stresses on the surrounding concrete. This leads to microcracking on the surface, often forming map-like patterns.

• These cracks are typically random, interconnected, and shallow, but they can worsen if not addressed, allowing further moisture ingress and accelerating corrosion.

Additional Information

• IS 3370-2:2009 emphasizes durability and crack control in structures exposed to liquids, as such environments are prone to moisture ingress and chemical attack, increasing the risk of reinforcement corrosion.

Preventive measures to avoid map cracking include:

• Proper cover to reinforcement.

• Use of corrosion-resistant steel or coatings.

• Ensuring dense, impermeable concrete.

• Regular maintenance and inspection to detect early signs of deterioration.

Explanation:

According to IS 1904:1986 (Code of Practice for Design and Construction of Foundations in Soils: General Requirements), the primary cause of cracks in foundations is ground movement, which includes:

• Differential settlement due to non-uniform soil bearing capacity.

• Shrink-swell behavior of expansive soils (like black cotton soil).

• Soil erosion, subsurface water movement, or vibration.

• Such movements create uneven stresses on the foundation, leading to cracks in both the foundation and the superstructure (especially in masonry and plaster).

Additional Information IS 1904:1986

This IS code provides guidance for:

• Types of foundations based on soil conditions.

• Requirements for design loads, bearing capacity, and settlement limits.

• Precautions to prevent damage due to soil behavior.

It emphasizes the importance of:

• Soil investigation.

• Drainage control.

• Designing for differential settlements in clayey or collapsible soils.

Explanation:

• According to IS 456:2000, proper curing is essential to control plastic and drying shrinkage cracks.

• Clause 13.5 of IS 456 states that curing shall be done for at least 7 days in normal conditions (and at least 10 days in dry or hot weather) using methods like water ponding, wet coverings, curing compounds, or moist curing.

• Curing maintains adequate moisture and temperature in the concrete, which allows proper hydration of cement and prevents early-age shrinkage cracking due to rapid drying.

 Additional Information

Purpose of Curing

• Maintains moisture content and temperature in concrete to ensure proper hydration of cement.

• Prevents early-age cracking due to drying shrinkage and thermal stress.

• Ensures development of desired strength and durability.

Minimum Duration of Curing (Clause 13.5)

• Ordinary Portland Cement (OPC): Minimum 7 days.

• Blended cements (PPC, PSC): Minimum 10 days.

• In hot and dry weather: Curing should be continued for at least 10 days for OPC and 14 days for blended cements.

Explanation:

As per IS 3370-2:2009 (Concrete Structures for Storage of Liquids – Part 2: Reinforced Concrete Structures), strict crack width control is essential for water-retaining structures.

• Clause 6.2.1.1 of IS 3370-2:2009 specifies that the maximum crack width should not exceed 0.1 mm for concrete surfaces in contact with liquid, especially when impermeability is critical.

• The aim is to prevent leakage, minimize corrosion risk, and enhance durability.

• The code emphasizes that water-tightness is as important as structural safety in such structures.

Additional Information IS 3370-2:2009:

• Crack width limits are based on exposure conditions and service requirements.

• For critical structures like water tanks, limiting crack width is a serviceability criterion, not just an aesthetic or structural one.

• Crack control is achieved through:

  • Proper detailing of reinforcement

  • Control of shrinkage and thermal movements

  • Adequate cover and concrete quality

As per IS 875 Part 2, clause 3.1, Imposed Floor load for residential building are:

As per IS 456: 2000, ANNEX B

This value of the modular ratio partially takes into account the long-term effects of creep.

\({\sigma _{cbc}}\) for M20 is 7 MPa and the modular ratio comes out to be 13.

The modular ratio is given by

\(m = \frac{{280}}{{3{\sigma _{cbc}}}}\)

For M20 concrete

σcbc = 7 N/mm2

\(\therefore m = \frac{{280}}{{3 \times 7}} = 13.33\)

Note: It is expected from students to know value of \({\sigma _{cbc}}\) which is nearly 1/3rd of characteristics compressive strength. Please don't report questions for no data or wrong question.

Additional InformationThe permissible stresses under bending and direct compression as per IS 456 for different grades of concrete are given below in the tabulated form.

Explanation:

As per codal provisions of IS 3370:

• Minimum grade of concrete for the R.C.C water tank is M30.
• Maximum cement content is 400 kg/m³ to take care of shrinkage effect.
• Minimum cement content is 320 kg/m³.
• Minimum grade of concrete for P.C.C water tank is M20.
• Maximum w/c ratio is 0.45.
• Minimum nominal cover is 45 mm.
• Maximum allowed crack width is 0.2 mm in the LSM design.
• To reduce cracking due to temperature, shrinkage, and moisture loss at an early stage of concrete, curing should be done for at least 14 days.
• Permeability of concrete must be least so use leaser value of w/c ratio.
• No porous aggregate should be used.
• Part of structure retaining liquid and enclosing space above liquid should be taken under server exposure condition.
• All the structures to be designed shall be designed for both empty and full condition.
• Cracking of concrete can be controlled to some extent by maintaining a slope filling rate of 1 m in 24 hours at the first time of filling.

Permissible stress of the material is as follows:

a) Mild Steel - 115 N/mm² and HYSD bar - 130 N/mm²

b) Concrete

• If the thickness is more than 200 mm, the reinforcement is provided in 2 layers, one on each face
• Minimum steel is 0.64% and 0.4% of the surface zone for mild steel and HYSD respectively
• The above percentage values can be reduced to 0.35% and 0.24% for tanks with no dimension more than 15 m.

Explanation:

Limit state of serviceability of prestressed concrete should satisfy cracking, deflection, and maximum compression also.

The crack width & deflection should not exceed the permissible limit and the maximum compressive force also should not exceed the strength of concrete.

Note: See article 19.2 & 19.3 in IS code 1343:1980

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.

Confusion Points
As per IS 1343:2012, The limit state of serviceability deals with the deflection, cracking, and maximum compression. While As per 456:2000, The limit state of serviceability deals with deflection and cracking. Since the question is asking for prestressed concrete, we will go with IS 1343: 2012. 

Concept:

Creep is the plastic permanent deformation of a structure under constant load for a very long period of time.

It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.

It occurs due to dead load only.

Factors affecting creep

1. Type of loading

2. Magnitude of load

3. Time

4. Temperature

If temperature is more, creep is more and if the temperature reaches to half of melting point temperature the creep is intolerable. Such a temperature is defined as homologous temperature. 

Explanation:

According to IS 456 (Clause 26.4.1),

Nominal concrete cover (clear cover) can be defined as the distance from the outer surface of the concrete members to the outer surface of steel (either main reinforcement or stirrups) reinforcements, including links.

Minimum concrete grade and nominal cover requirement based on exposure condition are:

​

• IS 456 specifies tolerance levels ranging from 0 mm to + 10 mm i.e. no reduction in clear cover permitted but can be increased up to 10 mm.
• Nominal cover can be decreased by 5 mm for main reinforcing bars of less than 12 mm diameter.
• Nominal cover can be decreased by 5 mm if the concrete grade is M35 or higher.

 Confusion Points This question asked minimum clear cover for the water tank as per IS-456 therefore we will choose 30mm as the minimum clear cover for moderate exposure conditions, whereas as per IS-3370 minimum clear cover for the water tank is 45mm. Reading questions properly and answering what is asked is very important.

Clear cover(nominal cover) is the distance between the outer external surface of the concrete and the nearest exposed reinforcement bar. It is the measurement from the concrete surface to the outermost surface of the reinforcement.It is a function of exposure conditions to reinforced concrete.

As per IS:456 - 2000, Exposure conditions are of the following types:

As per IS 456: 2000, CL 21.0, following should be remembered for fire resistance in respect of beam and floors.

Explanation:

As per IS 456: 2000,

Explanation:

Methods of designing an RCC structure:

There are 3 methods of design an RCC structure:

1. Working stress method:

• This was the traditional method of design.
• The method basically assumes that the structure behaves in a linear elastic manner.
• The actual stresses developed due to the actual working load.
• Stresses developed are kept within permissible stress value.
• The factor of safety is applied to stress values only.
• The design usually results in relatively large sections of the structural members.

2. Ultimate load method:

• This method is sometimes also referred to as the Load factor method.
• In this method, the stress condition at the site of the impending collapse of the structure is analyzed, and the nonlinear stress-strain curves of concrete and steel are made use of.
• The concept of modular ratio and its associated problems are avoided entirely in this method.
• This method is more economical than the working stress method.

3. Limit state method:

• A condition of the structure once achieved, the structure cannot perform its intended function satisfactorily in the future in terms of safety or serviceability.
• Condition of a structure, once achieved the structure cannot be used satisfactorily in future is called limit state condition.
• Limit state can be achieved due to Limit state of collapse and Limit state of serviceability.
• This method is based on the Ultimate Strength of the structure, satisfactory performance at the working stage, and satisfying the serviceability & durability requirements.