Plain and Reinforced Concrete – Code of Practice
2000 Edition

The minimum concrete grades and nominal mix ratios prescribed by IS 456 are as follows:

Minimum Grades (Clause 6.1.2 and Table 2)

• The minimum recommended grade for plain and reinforced concrete is generally M20.
• Grades range from M10, M15, M20, M25, up to M80 for high-strength applications.
• Characteristic compressive strength (fck) is measured on 150 mm cube specimens at 28 days.

Nominal Mix Ratios (Clause 9.3 and Table 9)

• The fine aggregate ratio varies with grading and maximum aggregate size (typically Zone II fine aggregate).
• Water-cement ratio limits are imposed to ensure required durability and strength.

Additional Notes:

• Design mixes will vary according to specific mix design requirements.
• Use well-graded coarse aggregates.
• Adjust fine to coarse aggregate proportions based on grading and maximum aggregate sizes (10, 20, 40 mm).

Summary:

• M20 is the minimum structural concrete grade generally used.
• A nominal mix example for M20 is Cement : Fine Aggregate : Coarse Aggregate = 1 : approximately 1.4 to 2.7, with water limited to a maximum of 30 litres per 50 kg cement.
• Adhere strictly to water-cement ratio limits and exposure conditions as mandated by IS 456.

Anchorage and detailing of reinforcement in slabs and beams as per IS 456 require:

• Slab Reinforcement Anchorage (Clause 31.7.4):

  • Positive reinforcement bars perpendicular to discontinuous edges must extend at least 150 mm beyond the internal face of supports such as spandrel beams, walls, or columns.
  • When fabric reinforcement with fully welded transverse wires is used, anchorage length can be reduced to the greater of half the support width or 50 mm.
  • Negative reinforcement must develop full design stress at the internal support face.
  • For cantilever slabs or those without spandrel support, anchorage should be provided within the slab itself.

• Beam Reinforcement Anchorage (Clauses 26.2.2 and 26.3):

  • At least 50% of the main reinforcement should extend through the support and be securely anchored.
  • For continuous slabs treated as simply supported for design, provide reinforcement above supports equal to at least 25% of the mid-span reinforcement, extending a minimum of one-tenth of the clear span into adjacent spans.
  • Maintain adequate concrete cover (minimum 10 mm with permanent cover blocks, otherwise as per Clause 26.4) for durability and fire protection.

• General Detailing Guidelines (Clause 21.3):

  • Ensure the reinforcement layout includes sufficient ties, bonds, and anchorage to maintain structural integrity and fire resistance.

Anchorage Length Summary:

This detailing ensures effective load transfer, crack control, and durability.

IS 456 outlines safety factors and load combination procedures as follows:

• Design Loads (Clause 19.9):

  • For the Working Stress Method, characteristic loads are used directly.
  • For Limit State Design, characteristic loads are multiplied by partial safety factors.

• Partial Safety Factors (γ) (Table 18, Clause 36.4.1): | Load Combination | Limit State of Collapse (γ) | Limit State of Serviceability (γ) | |------------------|-----------------------------|-----------------------------------| | | DL | IL | WL | DL | IL | WL | | DL + IL | 1.5| — | 1.0 | 1.0| 1.0| — | | DL + WL | 1.5 or 0.9| — | 1.5 | 1.0| — | 1.0 | | DL + IL + WL | — |1.2 | — | 1.0|0.8 | 0.8 |

• Legend:

  • DL = Dead Load
  • IL = Imposed Load
  • WL = Wind Load

• For members subjected to combined axial load and bending moments, apply these partial factors prior to limit state checks (Clause 39.5).

Summary:

• Dead loads typically use a factor of 1.5.
• Imposed and wind loads have factors usually between 1.0 and 1.2 for collapse limit states, and lower for serviceability.
• Loads should be combined as per Clause 19.7, applying factors from Table 18 for structural design verification.

IS 456 emphasizes concrete durability and exposure conditions primarily in Clause 8, focusing on minimizing permeability and categorizing exposure severity.

Durability Essentials (Clause 8.1.1):

• Durability depends on concrete's resistance to ingress of water, oxygen, chlorides, sulfates, and other deleterious agents.
• Achieved through:
  • Adequate cement content
  • Low water-cement ratio
  • Thorough compaction
  • Proper curing
• Influencing factors include environmental severity, reinforcement cover, material quality, mix design, workmanship, and member geometry.

Exposure Classifications (Clause 8.2.2.1, Table 3):

Concrete Mix Requirements (Clause 8.2.4.1, Partial Table 5):

Additional Measures:

• Control chloride content as per Table 7.
• Use low-alkali cement and supplementary cementitious materials like fly ash and slag.
• Ensure adequate cover and curing practices.
• Apply impermeable membranes where necessary.

This approach ensures concrete longevity by addressing environmental exposure and material quality.

Per IS 456 (Clause B-2.2 and Table 22), the allowable stresses for steel reinforcement are categorized by steel type:

Additional considerations:

• Partial safety factor for steel (γm) is 1.15.
• Minimum strain at failure must conform to code requirements.
• Stress-strain behavior is detailed in Fig. 23 of the code.

These values ensure reinforcement operates safely within elastic and yield limits, accounting for bar diameter and steel grade.