Ever thought about estimating concrete strength by a calculation? There are several ways to do strength investigation in a non-destructive way. In fact, the concrete maturity method is one of the best methods to estimate real strength.

The best gain from the maturity method is the time. You can cut out the waiting time to check the 28 days strength of laboratory specimens if you estimate the real-time strength using the maturity method at the site. The concept was established in 1950 and got accepted in ASTM and various standards during the time.

The concrete maturity method is simply defined as the relationship of strength to the temperature & age of concrete. Some of the accepted standards of this method are ASTM C1074, ASTM C918, ACI 228 – 1R, AASHTO T325.

### Different approaches of concrete maturity method

There are 3 common ways to estimate the maturity of the concrete.

**Temperature – Time factor method (TTF)**:

This is also known as the Nurse – Saul maturity function. The first method developed using the concept and still using in the ASTM C1074 due to its simple calculation.

TTF concrete maturity method assumes a linear relationship between maturity and the strength of concrete. You can either select the datum temperature or calculate for accurate results. Typical datum temperature is -10C because that considered as the freezing point of concrete where no longer water will involve in cement hydration. However, from 0°C to -10°C segment, the hydration is slower. It is conservative to omit that part and consider the datum temperature at 0°C.

The following equation calculates the maturity in the time factor method.

The maturity can graphically present as the area of temperature vs. time graph

So it’s clear that the high curing temperature results in high maturity as well as strength gain in the concrete.

Use the maturity to calculate the strength of the concrete using the following equation

These A, B values are constant for the mix properties. Materials and their proportions will define these constants. Then maintaining the exact mix proportions at the site throughout the time is essential. Use 15 specimens for compressive strength test and 2 samples for temperature measurements and calculate A & B. It’s always good to calculate the A, B values than using values corresponding to target 28-day strength.

You can use Plowman’s coefficients for A & B values are in the table below for a rough estimation of strengths.

**Equivalent Age method**:

An exponential relationship between temperature and maturity is assuming in this method. It estimates the age of curing at a specific temperature, which gives the same maturity at different temperatures. This allows us to convert the actual age and temperature into an equivalent curing age and predict the strength.

**Weighted maturity method**:

Relationship similar to Nurse – Saul method is using in this method too. Summation of hardening time and few cement factors required to calculate the maturity. However, the weighted maturity method is not accepted in American standards.

### Calibration

The most crucial part of the maturity method is calibration. According to the concrete maturity concept, identical mixes having similar maturity should give you equal strengths. This allows you to do the calibration in a laboratory.

The recommendation in the standard to use 17 number of specimens for the calibration. You can use 5 sets of ( 5 x 3 = 15) samples for the strength testing while using the remaining 2 for measuring temperatures. Testing specimens in 1, 3, 7, 14, and 28 days is the typical way of distributing. However, some suggest 1, 2, 3, 7, and 14 days’ time because the maturity method is mostly used in early strength evaluations.

Soon after the calibration, calculate A & B values and use any time where the same mix is using. Simple spreadsheet calculation will quickly solve the most relevant A, B coefficients.

### Typical applications of the concrete maturity method

The concrete maturity method is commonly using in building constructions. It’s essential to estimate the supports and form removal time in highrise buildings. Maturity of slabs, beams, columns, and footings are separately calculated to assess the strengths and curing time.

Pre-cast constructions are another situation where concrete maturity is using. The earliest possible from removal can save time and labor. The same method can use to estimate the curing, transportation time from the pre-cast yard to the site.

Post-tensioning constructions widely use concrete maturity method in estimating tensioning time. It is mandatory to reach the required strength before post-tensioning.

The maturity technique is using in estimating thermal stresses and evaluating the capacity in mass concretes. Concrete dams and foundations typically have large concrete pours that need to investigate the possibility of thermal cracks.

Pavements and bridges commonly use the maturity method to calculate traffic pass on time. It’s essential to reach strength before allowing the traffic load.

Shotcrete is a widely using ground stabilization method in the industry. It’s useful to estimate the strength of a newly placed shotcrete before you return to the site. Maturity is one of the ways available to assess the early strength in shotcrete.

### Advantages and limitations

The number one advantage of the concrete maturity method is the cost advantage. Rather than waiting for the laboratory results, this method allows us to estimate the strength quickly at any time. Removing the forms and moving into the next layer of construction can speed up according to the results. These will cost 1000-2000 USD for the calibration, and 1-2 USD/m^{2} and the saving will be several 10 to 15 thousand dollars by cutting of delays and curing times.

Another advantage is that this method allows us to estimate the strength at any desired place. It’s just a matter of placing some thermocouples in the concrete, and you are capable of estimating strength on the point.

Other than above, less laboratory involvement and quick estimation of strength are significant advantages in the concrete maturity method. With the record of temperature history, just a simple calculation will estimate the strength.

Even with many plus points, the maturity method has its limitations. Understanding them before using the technique is a must.

- The method is a strength approximation technique, not the exact strength.
- Mostly accurate in early strength calculations
- Strength estimation is not as straightforward as a cube or cylindrical specimen testing. Need to arrange the thermocouples and collect readings throughout the time
- Your datum temperature can affect the maturity
- Any change in the mix can make the calibration invalid. So keeping the exact mix proportion is essential throughout the time.