Maturity calibration is the laboratory procedure that establishes the strength-maturity relationship for one specific concrete mix, so that recorded temperature data from that mix can be turned into an estimate of in-place strength. It is done once per mix design: cylinders of the mix are cured in the lab, two of them are instrumented to record temperature, the rest are broken for compressive strength at a minimum of five ages, and the resulting strength and maturity pairs are plotted to give a curve. From then on, a sensor embedded in the structure records the temperature history, the maturity method (ASTM C1074) converts that history into a maturity index, and the curve converts the maturity index into an estimated in-place strength. The procedure typically takes 28 days end to end because the last break age is usually 28 days, and it must be repeated whenever the mix changes.
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What is Concrete Maturity Calibration?
Through all of your research on the concrete maturity method you’ve probably read a little bit about the calibration process that is required before you start using maturity sensors on site. The goal of the calibration is to associate the strength of your concrete to a maturity index that is at specific times. A maturity calibration only needs to be completed once for a specific mix and its properties. This maturity-strength relationship is developed in the lab, using cylinder break tests, and then correlated to the strength of the in-place concrete on your jobsite.
While this step may seem a little daunting, we promise you it’s not complicated. In fact, the maturity calibration can be done in 5 easy steps:
How to Calibrate Concrete Maturity: The Step-by-Step Procedure
What calibration requires:
| What you need | Requirement | Notes |
|---|---|---|
| Cylinders | A minimum of 15 per ASTM C1074, 17 in practice to allow for the third break, all from the same batch | 2 instrumented for temperature, 15 available for compressive strength breaks |
| Curing | All cylinders cured together in a moist environment per ASTM C511 | The two temperature cylinders must experience the same conditions as the break cylinders |
| Break ages | A minimum of 5, for example 1, 3, 7, 14 and 28 days | Break 2 cylinders at each age, and a third if the two results differ by more than 10 percent |
| Temperature record | Continuous logging from the 2 instrumented cylinders | Averaged across the two, converted to a maturity index at each break time |
| Maturity function | Nurse-Saul (temperature-time factor) or Arrhenius (equivalent age), per ASTM C1074 | Use the same function later on the jobsite that you used to build the curve |
| Elapsed time | Typically 28 days | Driven by the last break age, not by lab throughput |
| Validation | Additional cylinders on the next pour | Recalibrate if the difference between estimated and broken strength consistently exceeds 10 percent |
- Make a minimum of 15 cylinders per ASTM C1074, 17 in practice to allow for the third break; 2 will be used for temperature monitoring while the other specimens will be used for compressive strength breaks. All cylinders must be cured together in a moist environment according to ASTM C511.
- Select a minimum of 5 break times, for example 1, 3, 7, 14, and 28 days. For each day, obtain the compressive strength of two cylinders, break a third cylinder if the results vary more than 10% from the average. Note the time of the breaks.
- At the time on your breaks, also obtain data from the two cylinders that were used for temperature monitoring and make an average of these values. Input these values into either the Nurse-Saul equation or the Arrhenius method according to ASTM C1074 to obtain a maturity value.
- You will now have a set of at least 5 data points each with a strength associated to a maturity value. Plotting those data points allows you to obtain a curve with a logarithm equation. Here is the formula for calculating the maturity of concrete: Strength=a+ b LOG(maturity)
- Validate your calibration curve by making a couple of additional cylinders on your next pour. Compare the calculated strength obtained from your maturity calculation to the compressive strength obtained in the lab. Up to a 10% difference is acceptable.
Take a look at this video “Real-Time Concrete Strength Monitoring – Maturity Method” for a more thorough explanation on how to perform this calibration process.
Looking for an even easier way? Try our maturity calculator here
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Here’s a Refresher on the Concrete Maturity Method
The maturity method (ASTM C1074) is a non-destructive testing approach that allows you to estimate the early-age and compressive strength of in-place concrete in real-time. The technique is based on the principle that concrete strength is directly related to its hydration temperature history. Though this methodology has been used since the 1970s, it has only recently been gaining popularity on job sites with wireless concrete maturity sensors.
Want an even more thorough explanation of the concrete maturity method? Download our free e-book Concrete Maturity 101: From Theory to Application Here
Frequently Asked Questions About Concrete Maturity & Maturity Calibration
As the maturity method increases in popularity worldwide, more standards are adopting this method. As a result, more questions are being asked about how maturity can be used for different applications and its limitations can be addressed. This section focuses on answering the questions typically asked by engineers, ready-mix producers, contractors, and testing labs regarding the maturity method and its applications.
How many cylinders do you need for a maturity calibration?
A minimum of 15 per ASTM C1074, and 17 in practice to allow for the third break, all from a single batch of the mix being calibrated. Two are instrumented with temperature probes and cured alongside the rest to produce the maturity record, and the remaining fifteen are available for compressive strength breaks at a minimum of five ages, two at each age with a third held in reserve for when the two results differ by more than 10 percent.
How long does a maturity calibration take?
Around 28 days in most cases, because the calibration curve is only as complete as its last break age and 28 days is the usual final age. The lab work itself is a few hours spread across those weeks. This elapsed time is the reason calibration has to be planned into the schedule well before the first pour that depends on it, and it is the main practical argument for self-calibrating sensors on fast-moving projects.
Is the maturity calibration still valid if there is additional water or air added to the mix?
A calibration is specific to one mixture design with specific properties. Any additional changes in materials or mixture properties would affect the strength (or rate of hydration), which would make the calibration invalid. For any change in the mixture, a new calibration should be created.
Unfortunately, it is unrealistic to assume that the exact same mixture used for calibration is used in an actual construction jobsite. It is normal that some additional water or variation in air content occurs in the mixture. For this reason, it is recommended to use concrete that is delivered to the jobsite for the calibration curve which minimizes some of these potential sources of error. Another approach would be to build the calibration curve with a mixture that still meets the specifications but with slightly higher w/cm and air content. This will, subsequently, incorporate some safety margins to the calibration.
Is the calibration still valid if retarders or accelerators are added to the mix?
The goal of the retarder is to delay the setting time. This will affect the rate of strength development by shifting the hydration to a later time. On the other hand, the goal of accelerators is to expedite the setting time and strength gain at an early age. There exists multiple types of chemical admixtures, some with or without water, reducing specifications. Retarders and accelerators can affect the compressive and flexural strength properties of the concrete specifically at an early age. For this reason, it is important that the maturity calibration be done with the accelerator or retarder incorporated into the mix. If the dosage of the chemical admixture varies from the one used during maturity calibration, a validation of this measurement is necessary and recalibration could be required. Depending on the admixtures, it’s possible that a different dosage of the admixture has only minimal effect on maturity calibration.
How long is the calibration valid for?
Technically, the calibration is valid until an element from the mix is changed. In general, validation should be done periodically, or before/during critical operations. During validation, if the differences consistently exceed 10%, a new maturity calibration needs to be developed. This is particularly essential for the general contractors who are doing the mix calibration as they do not have full control over the mixes. However, ready-mix producers who are performing maturity calibrations have better knowledge and control of their mixes.
What should be expected when comparing the results from field-cured specimens and in-place strength calculated by maturity?
One should expect a difference between the results of the cylinders and the in-place concrete. In general, the maturity method would produce higher strength results because of the larger size of the structural elements compared to concrete cylinders, which makes the process of heat dissipation slower. The in-place concrete is able to retain more heat for a longer period of time compared to the cylinders, even if the cylinders were placed under the same curing conditions. In general, the maturity method will allow you to perform critical operations faster than the break test.
It is also possible to observe the opposite effect, where the cylinders reach the required strength while the in-place elements do not. Those cases occur more frequently in cold regions or during seasons when the cylinders might have been cured in different conditions than the structural element. An interruption to the heating system or exposure to colder temperatures would make the structural element cure slower compared to the cylinders.
Who is responsible for maturity calibration, the contractor or the ready-mix producer?
Either can do it, and the answer usually depends on who controls the mix. Ready-mix producers have better knowledge of and control over batching, which makes their calibrations more stable over time. General contractors who calibrate themselves are working from mixes they do not control, so they should validate more often and treat any change in materials as a trigger for recalibration.
Self-Calibrating Sensors: A New Frontier in Concrete Strength Monitoring
Traditional concrete maturity methods rely on establishing a calibration curve by testing cylinders at various ages to correlate maturity with compressive strength. While this process is standardized and effective, it introduces challenges such as the need for consistent mix properties, controlled curing, and access to lab facilities.
A newer alternative is the self-calibrating sensor, which removes the need to develop a mix-specific calibration curve before the pour. Rather than relying on a relationship built in the lab from that mix, these sensors apply a built-in model to the temperature history they record in the element itself, and produce a strength estimate from it. The strength value is still an estimate derived from temperature, not a direct measurement, but it does not depend on a curve that a mix change can invalidate.
Where self-calibrating sensors help:
- No lab calibration step. No cylinders to cast, cure and break before the mix can be used on site, which removes 28 days from the front of the process.
- Tolerant of mix variation. Batch to batch variation in water content or air content does not invalidate a curve, because there is no mix-specific curve to invalidate.
- Practical on multi-mix projects. Projects running several mixes would otherwise need a separate calibration for each one.
- Fewer specialist dependencies. Less reliance on lab scheduling and third-party testing turnaround to get started.
This approach is particularly valuable for projects involving multiple mix designs or where time and resource constraints limit the feasibility of traditional calibration.
Giatec’s SmartRock® Pro is an example of this approach, offering self-calibrating, mix-independent in-place strength estimation on the jobsite. Read more about self-calibrating concrete strength monitoring here!
Editor’s Note: This post was originally published in November 2017 and has been updated for accuracy and comprehensiveness.
