Question

Design strength for M25 concrete in direct compression, bending compression and flexural tension are, respectively:

• A. 25 MPa, 11.15 MPa and 2.57 MPa
• B. 10 MPa, 11.15 MPa and 3.5 MPa
• C. 25 MPa, 11.15 MPa and 3.0 MPa
• D. 10 MPa, 12.5 MPa and 3.5 MPa

Hint:

Always remember: Concrete is strong in compression and weak in tension. The direct compression limit ($0.4f_{ck}$) is stricter than the bending compression limit ($0.446f_{ck}$) because there is no neutral axis to provide relief.

Answer 1

The Correct Option is B

Concept: The design strength of concrete varies depending on the type of stress and the code-specified factors of safety applied to the characteristic strength ($f_{ck} = 25 \text{ MPa}$ for M25).

Step 1: Direct Compression.
For columns or members in direct compression, the design strength is generally taken as $0.4 f_{ck}$: \[ 0.4 \times 25 = 10 \text{ MPa} \]

Step 2: Bending Compression (Flexure).
According to IS 456, for bending, we first reduce the strength to $0.67 f_{ck}$ to account for the difference between cube and actual structure, then apply a factor of safety of 1.5: \[ \text{Design Strength} = \frac{0.67 f_{ck}}{1.5} = 0.446 f_{ck} \] \[ 0.446 \times 25 = 11.15 \text{ MPa} \]

Step 3: Flexural Tension (Modulus of Rupture).
The characteristic flexural strength is given by: \[ f_{cr} = 0.7\sqrt{f_{ck}} \] \[ f_{cr} = 0.7\sqrt{25} = 0.7 \times 5 = 3.5 \text{ MPa} \]