The maturity concept uses the time/temperature history of concrete to develop a maturity-strength curve that is specific to a mix. The curve thus generated helps estimate the in-place early-age compressive (and flexural) strength of concrete during construction.

Several methods are followed to conduct the maturity test on concrete. Each method proposes several equations to calculate the maturity of concrete. This article explains the two major methods used to calculate the maturity of concrete - **The temperature-time factor method** and **the equivalent age method** as per ASTM standards.

## Maturity Test on Concrete as per ASTM C 1074

The ASTM C 1074 provides standard procedures to estimate the concrete strength using the maturity concept. The code is designated as "Standard Practice for Estimating Concrete Strength by the Maturity Method."

The procedure starts by developing a temperature-time-strength relationship of a concrete mixture, which is also called as a calibration chart for maturity test on concrete. The test and reading are collected by conducting the laboratory test.

At the construction site, the measured value of temperature and age gives the benefit of the maturity index. The maturity index value is then used to estimate the concrete's strength by a standard graph showing the maturity-strength relationship.

All the maturity methods follow the maturity concept i.e. the concrete samples of a given mix possess the same strength if they have the same maturity index. For example, if a concrete sample cured at a temperature of 10°*C* for 7 days and a sample cured at 27°*C* for 3 days have the same maturity index, then they gain similar strengths.

## Temperature-Time Factor Method to Calculate Concrete Maturity

The temperature-time factor method is also called a Nurse-Saul Maturity function method, which was developed in the 1950s by Nurse, Macintosh, and Saul. This is the first maturity function method developed and mentioned in ASTM standards C 1074 in 1987. The Nurse-Saul function is the most commonly used maturity method in North America.

In this method, the maturity index is measured in terms of temperature-time factor (TTF) as the product of temperature and time in °*C* hours or °*C* days. Here, the method follows the concept that the maturity value is dependent on the temperature linearly, which can be represented as the area under the curve, as shown in figure-2.

The area under the curve = Average Recorded Temperature - The datum temperature (T).

The datum temperature is defined as the temperature below which there is no hydration process, or there is no strength developing through hydration. Hence, the Nurse-Saul Equation can be given by the formula:

The temperature 'Ta' is obtained by a maturity monitoring system at a given time. The time intervals are the frequency of measurements taken by the maturity meter that can be 1 hour or 30 minutes or a lesser value. The datum temperature 'Td' can be determined by the procedure given by ASTM C 1074.

The accuracy of the Nurse-Saul function reduces as there is a wide range of curing temperatures.

## Equivalent Age Method to Calculate Concrete Maturity

In 1977, Freiesleban-Hansen and Pedersen developed an equation to determine the maturity of concrete, which is an exclusive function of temperature alone. It was developed to mitigate the inaccuracy faced by the temperature-time factor method due to varying curing temperatures, especially for the temperature outside the range of 0 to 40°*C*. This method is called as Equivalent Age Method that was formulated based on the Arrhenius equation.

In the Equivalent Age Method, the maturity index is expressed in terms of equivalent age for a reference temperature. It is given by the following relation:

As described above, the maturity meter helps to determine the value of Ta and time interval. The specified temperature 'Ts' is mainly taken to an equivalent age at 20 or 23 ^{o}C. The value of activation energy 'Q' is determined by the procedure as per ASTM C 1074. It also suggests a 'Q' value between 40000 to 45000 J/mol as a reasonable assumption for concrete that uses Type-1 cement as per ASTM standards.

The Equivalent age method is standardized in both ASTM C 1074 and European Standards. The use of the Arrhenius equation by following proper assumption provides accurate results compared to the Nurse-Saul function, but the Nurse-Saul function is gaining popularity due to its simplicity.

## Maturity Method Procedure

- Determine the strength-maturity relationship graph by employing concrete specimens made by the same concrete mix as proposed by the project. The samples are checked for temperature using temperature probes embedded in one or two specimens. Standard brake tests at different ages determine the compressive strength. This data is recorded, and the maturity function is defined (Nurse-saul or Arrhenius function).
- Now consider the in-place concrete structure and measure the temperature history by embedding sensors at critical locations (Based on exposure conditions and structural requirements).
- Use the temperature and age values to determine the maturity index.
- Compare this maturity index with the already prepared strength-maturity relationship and estimate the in-place strength of the field concrete.

## Frequently Asked Questions

How do you calculate the maturity of concrete?

The temperature-time factor is the most common method to determine the maturity of concrete. The method measures concrete maturity by a parameter called the maturity index. This is given by the formula:

What are the methods to test concrete maturity?

The two primary methods used to calculate the maturity of concrete - The temperature-time factor method and the equivalent age method as per ASTM standards.

What are the steps involved in determining the maturity of concrete?

The general steps involves in determining the maturity of concrete at the site are:

1. Determine the strength-maturity relationship graph by employing concrete specimens made by the same concrete mix as that proposed by the project. The samples are checked for temperature using temperature probes embedded in one or two specimens. Standard brake tests at different ages determine compressive strength. This data is recorded, and the maturity function is defined (Nurse-saul or Arrhenius function).

2. Now consider the in-place concrete structure and measure the temperature history by embedding sensors at critical locations ( Based on exposure conditions and structural requirements)

3. Use the temperature and age values to determine the maturity index.

4. Compare this maturity index with the already prepared strength-maturity relationship and estimate the in-place strength of the field concrete.

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