The target mean strength of M20 grade concrete is 27 N/mm². What is the value of the standard deviation?

A. 2.01

B. 3.5

C. 4.24

D. 5

Correct Answer: C. 4.24

📊 Finding Standard Deviation from Target Mean Strength (M20)

Use the target mean strength formula from IS 10262:

f_m = f_ck + 1.65σ

Step 1 — Known values. f_m = 27 N/mm², f_ck = 20 N/mm² (for M20).

Step 2 — Substitute.27 = 20 + 1.65σ

Step 3 — Solve for σ.σ = (27 − 20) / 1.65 = 7 / 1.65 = 4.24 N/mm²

Standard deviation σ = 4.24 N/mm² — option C.

Rearrange f_m = f_ck + 1.65σ to get σ = (f_m − f_ck) / 1.65. The factor 1.65 comes from the 5% tail of the normal distribution.