Between 7 days and 28 days, the estimated ratio of strength of cement concrete is:
Correct Answer: D. 2/3
📊 Ratio of 7-Day to 28-Day Strength
For OPC concrete, the 7-day compressive strength is approximately 2/3 (two-thirds) of the 28-day compressive strength. This relationship is used in practice to predict 28-day results early from 7-day cube tests.
Rule of thumb:
7-day strength ≈ (2/3) × 28-day strength
So 28-day strength ≈ (3/2) × 7-day strength
7-day strength ≈ (2/3) × 28-day strength
So 28-day strength ≈ (3/2) × 7-day strength
7-day : 28-day strength ratio ≈ 2/3 — option D.
Key takeaway
If your 7-day cube is 20 N/mm², the predicted 28-day strength is about 30 N/mm². The 2/3 rule is the exam answer for OPC.