Maturity is a non-destructive approach to testing concrete that allows you to estimate the early-age compressive strength of in-situ concrete in real-time. Adopting the maturity approach on the jobsite eliminates the need for concrete cube/cylinder break tests, allowing to greatly optimize the construction schedule.
ASTMC1074, the standard practice for maturity, defines the method as: a technique for estimating concrete strength that is based on the assumption that samples of a given concrete mixture attain equal strengths if they attain equal values of the maturity index.
Maturity Index: It represents the progression of concrete curing. The maturity index value considers concrete temperature and curing time. As a result, mix calibration is required to implement this concept in a project. The goal of the calibration is to determine a relationship between maturity and strength for a specific mix.
As the Figure shows, estimation of maturity index requires a plot of temperature versus curing age of the concrete.
The maturity method based on the ASTM C1074 is the most commonly used method to estimate the in-situ strength of concrete. The ASTM C1074 provides two maturity functions: 1) Nurse-Saul function; and 2) Arrhenius function. Based on the Nurse-Saul method, there is a linear relationship between the maturity and the temperature in real time. The underlying assumption is that the strength development in concrete is a linear function of hydration temperature. The Nurse-Saul Equation shows the relationship between maturity and hydration (curing) temperature history, as follows.
M(t)= ∑[(Ta-T0)∆t]
Where: M(t) is the maturity index at age t; Ta is the average temperature during time interval ∆t; T0 is the datum temperature. ASTM C1074 provides a standard procedure to find the datum temperature for a specific mix design. However, most of previous studies suggest a practical estimation of the datum temperature that is between 0 °C and -10 °C. Indeed, this is the temperature at which the hydration of cementitious paste stops; hence the strength development of concrete ceases. In most cases that is taken as 0°C. Ta
is simply
obtained by the maturity monitoring system at a given time. ∆t is the default value given by the
frequency of measurements taken by the maturity meter and is usually defined as 1 hour, 30
min, or less. The maturity Index is defined as the area under the curve for a certain age and calculated using the above equation. The unit is: °C.h
Maturity-Strength Calibration of Concrete Mix: This calibration can be used to determine the in-place strength of the concrete and evidently replace the need for field-cured cubes/cylinders. To perform a maturity calibration, the ASTM C1074 standard must be followed. Here are the five steps in calibration process.
Step - 1: Make a minimum of 17 cubes/cylinders; 2 will be used for temperature monitoring while the other specimens will be used for compressive strength breaks. All cubes/cylinders must be cured together in a moist environment (ASTM C511).
Step - 2: Select a minimum of 5 break times, for example, 1, 3, 7, 14, 28 days. For each day, obtain the compressive strength of two cubes/cylinders, break the third cube/cylinder if the results vary more than 10% from the average. Note the time of the breaks.
Step - 3: At the time of the break, obtain the maturity index value from the two cubes/cylinders that were used for temperature monitoring and make an average of the maturity.
Step - 4: On 28th day, there will be five data points, where strength is associated with the maturity index. Plot the values to fit into the following equation, as shown in the Figure above:
Strength = a + b.log(maturity index)
It should be noted that this is a semi-log plot, with maturity index on x-axis.
Step - 5: Validate the calibration curve by making a couple of additional cubes/cylinders on the next pour, compare the calculated strength obtained from the maturity value to the compressive strength obtained in the lab. Up to a 10% difference is acceptable.
The principal goal of this method is to help estimation of early strength of in-place concrete (up to 14 days). It does not replace the standard laboratory testing of concrete for compressive strength.
Benefits of Maturity:
The benefits of using maturity as contrasted with traditional quality control procedures
are as follows:
• It provides a real-time, in-place indication of the strength of the concrete.
• It is a non-destructive testing method as contrasted to breaking cubes/cylinders in the
laboratory.
• It provides early quality verification of the in-place concrete, often within hours of its
placement.
• It accelerates the construction process.
• It reduces the quantity and cost of sampling and testing by reducing the number of cubes/cylinders that need to be cast and broken to determine strength.
• The maturity method is readily assessable to most materials laboratories because it is
based on traditional cylinder compressive strength tests for its development.
Weaknesses of Maturity:
The maturity method also has its weaknesses.
* Changes in the brand of cement, the
source and type of fly ash, the source of the aggregate or the water to cement ratio can result in a
change in the strength-maturity relationship and require a new calibration curve.
* The method cannot account for humidity conditions during curing, that is, if there is not enough moisture
present for hydration to occur the strength gain will not be realized as predicted by the maturity
curve.
* It is not accurate when there are large temperature swings during the curing process.
* The
method also cannot account for concreting practices that result in inadequate consolidation, poor
placement techniques, inadequate curing, lack of protection during early ages, or fluctuations in
air content.
Reference: "Concrete Maturity - From Theory to Application", e-book, Giatec Scientific Inc., 2018, 1st Edition, Ebook ISBN: 978-1-9994762-0-5
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