A concrete sample is cured at 15°C for 28 days. If the datum temperature is taken as −11°C, what is the maturity (°C·days) of the concrete sample?
Correct Answer: D. 728 °C·days
📚 Detailed Explanation: Concrete Maturity Calculation
Why D (728 °C·days) is correct: The maturity method quantifies the combined effect of time and temperature on the strength development of concrete. The Nurse-Saul maturity formula uses a datum (reference) temperature (typically −10°C or −11°C), below which hydration effectively stops. Maturity M = ∑[Δt × (T − T0)], where Δt = time interval, T = curing temperature, T0 = datum temperature.
Step-by-Step Calculation
Nurse-Saul Maturity Formula:
M = Σ [Δt × (T – T0)]
M = Σ [Δt × (T – T0)]
Given:
Curing duration (Δt) = 28 days
Curing temperature (T) = 15°C
Datum temperature (T0) = -11°C
Calculation:
M = 28 × (15 – (-11))
M = 28 × (15 + 11)
M = 28 × 26
M = 728 °C·days
Maturity Method: Concept and Application
| Aspect | Detail |
|---|---|
| Purpose | Predicts in-situ concrete strength without destructive cube testing; accounts for variable temperature curing conditions |
| Datum temperature (T0) | −10°C or −11°C (Nurse-Saul); the temperature below which no strength gain is assumed to occur |
| Maturity units | °C·days (or °C·hours if Δt in hours) |
| Maturity-strength relationship | Pre-established curve (maturity vs. strength) calibrated for the specific concrete mix |
| Use case | Cold weather concreting; formwork removal decisions; precast production scheduling; in-situ assessment |
Why Other Options Are Wrong
| Option | Calculation Attempted | Error |
|---|---|---|
| A. 112 | Perhaps 28 × (15 − 11) = 28 × 4 = 112 | Treated datum as +11°C instead of −11°C; sign error |
| B. 308 | Perhaps 28 × (15 − 4) or similar miscalculation | Incorrect temperature difference |
| C. 402 | Perhaps used 15 × 26 + 12 or other error | Incorrect combination of values |
| D. 728 | 28 × (15 − (−11)) = 28 × 26 = 728 | Correct |
- Maturity M = ∑[Δt × (T − T0)] = 28 × (15 − (−11)) = 28 × 26 = 728 °C·days.
- Key: datum temperature is −11°C; subtracting a negative number adds to the temperature difference.
- The maturity method (ASTM C1074; ACI 228) allows early formwork removal and in-situ strength estimation without coring.