Test: Theory of RCC- 2 - Civil Engineering (CE) MCQ

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.

Longitudinal Reinforcement:

• CI. 26.5.3 of IS 456:2000, specifies that the total area of longitudinal bars in a column section must NOT be less than 0.8% of the gross column area. This limit on minimum reinforcement is imposed because of the following reasons:
  • In order to ensure that a minimum flexural resistance of the column exists due to unexpected eccentricities in the column loading.
  • In compression members, creep under sustained loading is very predominant, especially at low percentages of steel. Thus, the resulting creep stress (due to creep strain) tries to yield the bars. 
• Maximum Reinforcement: The maximum area of cross-section of longitudinal bars must NOT exceed 6% of gross column area. However, in practice, a maximum of 4% is recommended.

Additional Information
Diameter and Number of bar:

• The diameter of longitudinal bars in column NOT be less than 12 mm. These bars must NOT be spaced more than 300 mm apart on the column perimeter.
• For rectangular columns. a minimum of 4 bars is provided.
• For Circular columns, a minimum of 6 bars be provided.

Cover = 40 mm or bar diameter

As per clause No. 26.2.1.1, Design bond stress in limit state method for plain bars in tension for different grades of concrete are:

• As per IS 456 CI. D-1, Restrained slab is the slab whose corners are prevented from getting lifted up and are provided with suitable reinforcement to resist torsion and are called a restrained slab.
• All the four edges of the slab are assumed to be rigidly tied with the beams or walls underneath and the edges may be either continuous (Fixed) or discontinuous.
• Corner restrained in the slab reduces the bending moment and deflection in the middle of the slab just like fixed beam which reduces the mid-span moment and deflection as compared to simply supported beams. 

Important Points
As per IS 456 (B 1.8, B 1.9, B 1.10) Torsion reinforcement provided as:

• B 1.8: Torsion reinforcement is required at the corner where both edges are discontinuous 
  • Reinforcement provided in 4 layers.
  • size of mesh = l_x/5
  • Area of steel in each layer = (3/4) × A_st(+ve)
• ​​B 1.9: Torsion reinforcement is required where at least one edge is discontinuous
  • Provided in 4 layers
  • size of mesh = l_x/5
  • Area of steel in each layer = (3/8) × A_st(+ve)
• _{​B 1.10: Where both edges are continuous no need to provide torsional reinforcement.}

Nominal Shear stress:
IS code recommends the use of nominal shear-stress for RCC structure. The nominal shear stress in beams or slabs of uniform depth is calculated as:
τv = V_u/bd
Where
V_u = Shear force due to design loads,
b = breadth of beam or slab,
d = effective depth
Shear stress for slabs is very low since b is large. Therefore no shear reinforcement is provided in slabs except that the alternate bars are bent up near the supports.
Bond stress (τ_bd):
Bond stress is the shear stress developed along the contact surface between the reinforcing steel and the surrounding concrete which prevents the bar from slipping out of concrete.
It depends upon the grade of concrete and type of steel only.