Calcady™
Home / Trade / Construction / Concrete Maturity Index Calculator (Nurse-Saul)

Concrete Maturity Index Calculator (Nurse-Saul)

Calculate the Nurse-Saul temperature-time factor (Maturity Index) of curing concrete. Predict in-place concrete strength accurately without breaking field cylinders.

Concrete Maturity Index Calculator (Nurse-Saul)

Calculate the ASTM C1074 temperature-time factor (TTF) to predict in-place concrete strength development during curing — without breaking field cylinders.

Typical: 50–90°F

Standard: 14°F (−10°C)

= 3.0 days  |  Max 8,760 hrs (1 yr)

M = (T − T₀) × Δt  =  (65 − 14) × 72  =  51.0 × 72 = 3,672 °F·hrs
Maturity Index (M)
3,672
°F·hrs
Equivalent Age at 68°F
68.0
hours  |  2.83 days
Typical Strength Milestone Reference (°C·hrs, ASTM C1074)
Form Stripping
500–800
~50% f'c
28-Day Equiv.
~14,000
~100% f'c
Design Strength
16,000+
108%+ f'c

Practical Example

A bridge deck pour is monitored during winter. Sensors embedded in the slab report an average temperature of 20°C (68°F) over a 3-day (72-hour) cure. Using a standard datum of −10°C: M = (20 − (−10)) × 72 = 30 × 72 = 2,160 °C·hrs. The project specification requires 2,000 °C·hrs before form stripping — so the slab clears this milestone and forms can safely be removed.

Email LinkText/SMSWhatsApp

What is ASTM C1074: Concrete Maturity Method?

The Maturity Method estimates in-place concrete strength without field cylinder breaks. It is based on the principle that concrete strength is a function of both time and temperature. The Nurse-Saul equation quantifies this relationship as a single number: the Temperature-Time Factor (TTF), measured in degree-hours.

Mathematical Foundation

Nurse-Saul Temperature-Time Factor (TTF)
M=∑(T−T0)×ΔtM = \sum (T - T_0) \times \Delta t
MM= Maturity Index (Temperature-Time Factor) in degree-hours. Higher values correlate directly to greater concrete strength development.
TT= Average concrete temperature during the time interval. Measured by embedded sensors or thermocouples at mid-depth of the concrete member.
T0T_0= Datum temperature below which hydration stops. ASTM C1074 default: -10 degrees C (14 degrees F) for most Portland cement mixes.
Δt\Delta t= Time interval (hours). Each hour contributes (T - T0) degree-hours to the cumulative maturity index.

Laws & Principles

  • ASTM C1074 Standard: The governing standard for the Maturity Method. Requires establishing a strength-maturity relationship curve for each mix design before the method can be used for form-stripping decisions.
  • Datum Temperature Rule: At or below the datum temperature (T0 = -10 degrees C by default), chemical hydration stops. Curing time below the datum contributes zero to the maturity index.
  • Cold Weather Concrete: This is why cold-weather placements use heated enclosures or insulated blankets — to keep T above T0. A slab at -5 degrees C adds zero maturity per hour.
  • Form-Stripping Threshold: Most project specifications require a minimum maturity index (e.g., 2,000 degree-C hours) before forms can be stripped. This corresponds to roughly 50-60% of the 28-day design strength.
  • Sensor Placement: ASTM C1074 requires sensors at the critical location — typically the cross-section center where heat dissipates slowest.

Step-by-Step Example Walkthrough

" A contractor places a bridge deck in late autumn. Embedded sensors report 20 degrees C for the first 48 hours, then 12 degrees C for the next 24 hours. Datum is -10 degrees C. Spec requires 2,500 degree-C hours before post-tensioning. "

  1. 1. Phase 1 (0-48 hrs at 20 degrees C): M1 = (20 - (-10)) x 48 = 30 x 48 = 1,440 degree-C hours.
  2. 2. Phase 2 (48-72 hrs at 12 degrees C): M2 = (12 - (-10)) x 24 = 22 x 24 = 528 degree-C hours.
  3. 3. Total at 72 hours: M = 1,440 + 528 = 1,968 degree-C hours.
  4. 4. Not yet at 2,500 threshold. Extend cure 24 more hours at 12 degrees C: 1,968 + 528 = 2,496. Still short.
  5. 5. One more hour: 2,496 + 22 = 2,518 degree-C hours. Spec met at 97 hours.
Final Result: Post-tensioning can proceed after 97 hours. If temperature had been constant 20 degrees C, the 2,500 threshold would be reached in only 83 hours.

Quick Answer: What is the concrete maturity index?

The maturity index is a cumulative measure of curing progress calculated as M = ∑(T - T0) × Δt in degree-hours. It predicts in-place concrete strength without breaking test cylinders. A slab cured at 20°C for 48 hours with a datum of -10°C accumulates 1,440 °C·hrs. Most specifications require 2,000-3,000 °C·hrs before form stripping or post-tensioning.

Nurse-Saul Equation

M = ∑ (T - T0) × Δt

Each hour of curing contributes (T - T0) degree-hours. At 20°C with a -10°C datum, each hour adds 30 °C·hrs. At 5°C, each hour adds only 15 °C·hrs — it takes twice as long to reach the same maturity. Below the datum temperature, hydration stops and zero maturity accumulates.

Maturity Accumulation by Temperature

Concrete Temp (°C) °C·hrs per hour Hours to 2,500 °C·hrs Approx. Days
35°C (hot summer)4556 hrs2.3 days
25°C (warm)3571 hrs3.0 days
20°C (standard)3083 hrs3.5 days
10°C (cool autumn)20125 hrs5.2 days
5°C (cold weather)15167 hrs7.0 days
0°C (near freezing)10250 hrs10.4 days

Datum T0 = -10°C per ASTM C1074. Hot weather curing reaches maturity 4x faster than near-freezing conditions. These values assume constant temperature — real projects have temperature swings that require hourly accumulation.

Real-World Scenarios

Summer Bridge Deck Pour

A bridge deck placed in July with average concrete temp of 30°C. Each hour adds 40 °C·hrs. The spec requires 3,000 °C·hrs before removing falsework. Time to threshold: 3,000 / 40 = 75 hours (3.1 days). Hot weather actually helps maturity accumulation — but watch for thermal cracking if the concrete exceeds 70°C internally due to heat of hydration.

Winter Foundation Pour

A foundation wall poured in December with insulated blankets maintaining 8°C average. Each hour adds 18 °C·hrs. Time to reach 2,500 °C·hrs: 2,500 / 18 = 139 hours (5.8 days). Without blankets (2°C ambient), time extends to 208 hours (8.7 days). The $500 cost of blankets saves 3 days of schedule — often worth $5,000+ in project delay costs.

Pro Tips

Do This

  • ✓Establish a strength-maturity curve for each mix design. ASTM C1074 requires lab testing to correlate maturity values with compressive strengths for your specific concrete mix. Without this calibration, the maturity number is meaningless — different mixes reach different strengths at the same maturity index.
  • ✓Place sensors at the coldest point of the pour. In winter, the coldest spot is at the edges exposed to ambient air. In massive pours, the hottest spot is the center. ASTM C1074 recommends placing sensors at the critical location that controls the decision (form stripping, tensioning).
  • ✓Log temperatures at least every hour during critical phases. Temperature swings (especially overnight cooling) affect maturity accumulation. Averaging a full 24-hour period masks critical dips below the datum temperature that contribute zero maturity.

Avoid This

  • ✗Don't use air temperature as a proxy for concrete temperature. Fresh concrete generates heat from hydration, so internal concrete temperature can be 10-20°C higher than ambient air. Using air temperature under-estimates maturity in hot weather and over-estimates it in cold weather.
  • ✗Don't apply one mix design's maturity curve to a different mix. A 4,000 PSI mix and a 6,000 PSI mix reach different strengths at the same maturity value. Each mix design needs its own calibration per ASTM C1074. Using the wrong curve leads to stripping forms before the concrete has enough strength.
  • ✗Don't let concrete freeze before reaching 500 PSI. Concrete that freezes before reaching 500 PSI (approximately 500-700 °C·hrs) suffers permanent strength loss of 50% or more. ACI 306 cold-weather concrete protection standards require maintaining concrete above 10°C for a minimum period.

Frequently Asked Questions

What is the datum temperature and why is it -10°C?

The datum temperature T0 is the threshold below which cement hydration effectively stops. ASTM C1074 uses -10°C (14°F) as the default because laboratory tests show that Portland cement continues slow hydration down to approximately -10°C due to dissolved minerals lowering the freezing point of pore water. Below this temperature, hydration rate is negligible and no strength is gained. Some supplementary cemite materials (fly ash, slag) may have slightly different datum values.

Can the maturity method replace cylinder break testing?

Partially. The maturity method can replace field-cured cylinder breaks for operational decisions like form stripping, post-tensioning, and opening to traffic — saving 2-7 days of waiting for lab results. However, it does not replace standard 28-day laboratory cylinder tests for official acceptance testing. Most DOT and building specifications allow maturity-based decisions for construction operations while still requiring traditional cylinder tests for final strength verification and project acceptance records.

How does hot weather affect the maturity calculation?

Hot weather accelerates maturity accumulation because each hour adds more degree-hours (e.g., 45 °C·hrs at 35°C vs 30 at 20°C). However, the Nurse-Saul equation has a known limitation: it assumes linear behavior at all temperatures. In reality, very high temperatures (>40°C) can cause rapid early strength gain but lower ultimate strength (the crossover effect). The Arrhenius-based equivalent age method handles this better for extreme temperatures, but for typical construction temperatures (5-35°C), the Nurse-Saul equation is accurate and widely accepted.

What maturity index corresponds to form-stripping strength?

It depends on the mix design and the calibration curve. As a rough guideline for typical 4,000 PSI concrete: 1,500-2,000 °C·hrs corresponds to about 50% of 28-day strength (2,000 PSI), which is a common form-stripping threshold for vertical elements. 2,500-3,500 °C·hrs corresponds to 65-75% of design strength, typically required for post-tensioning or shoring removal on elevated slabs. These values are approximate — always use your project-specific maturity-strength calibration curve.

Related Calculators