The pattern of increase in the strength of concrete with time is:
Correct Answer: B. Non-Linear
📚 Detailed Explanation: Concrete Strength Gain is Non-Linear
Why B (Non-Linear) is correct: The strength of concrete does not increase at a constant rate with time. Instead, it follows a non-linear pattern: very rapid gain in the first 3–28 days, progressively slowing rate from 28 days to 90 days, and extremely slow gain beyond 1 year. The curve is steep initially and flattens over time — characteristic of a non-linear, diminishing-return relationship.
Strength-Time Relationship Pattern
| Time Period | Rate of Strength Gain | Curve Behaviour |
|---|---|---|
| 0–7 days | Very rapid (0 → 65% of f28) | Steep rising curve |
| 7–28 days | Rapid (65% → 100% of f28) | Still rising but rate decreasing |
| 28–90 days | Moderate (100% → 120% of f28) | Noticeably flatter |
| 90 days – 1 year | Slow (120% → 135% of f28) | Nearly flat |
| > 1 year | Very slow (→ asymptotic limit) | Approaching horizontal asymptote |
Why Not the Other Options?
| Option | Why Incorrect/Incomplete |
|---|---|
| A. Linear | A linear relationship would mean strength increases at a constant rate per day forever — this is not what happens; rate clearly decreases over time |
| B. Non-Linear | Correct: rapid early gain, slowing rate, flattening curve — the complete descriptor of the entire strength-time relationship |
| C. Asymptotic | Partially correct for long-term behaviour only; the curve does approach an asymptote after years, but asymptotic alone does not describe the early rapid gain phase |
| D. All of the above | Cannot be all: A (linear) is clearly wrong, so “all” is wrong |
Approximate OPC Concrete Strength Gain:
f(t) = f28 * [t / (a + b*t)] (Plowman/CEB-FIP type expression)
f(t) = f28 * [t / (a + b*t)] (Plowman/CEB-FIP type expression)
At t=3d: f3 ≈ 0.40 * f28
At t=7d: f7 ≈ 0.65 * f28
At t=28d: f28 = 1.00 * f28 (reference)
At t=90d: f90 ≈ 1.20 * f28
At t=365d: f365 ≈ 1.35 * f28
- Concrete strength increase is non-linear: rapid early gain, progressively decreasing rate, asymptotic at long ages.
- IS 456 uses 28-day strength as the design reference (fck).
- PPC/fly ash concrete has slower early gain but similar or higher long-term strength than OPC concrete.