Maturity Calibration

To be able to estimate the in-place concrete strength using the maturity method, you need to determine the relationship between time, temperature, and strength for each concrete mix design you intend to use.

In short, you need to make some specimens from a test batch of each mix design, monitor the maturity in some of the samples, and break the specimens at different test ages. This will give you what is known as the strength-maturity relationship. This correlation is used to create a maturity curve, from which the concrete strength can be estimated at different values of maturity.

The strength-maturity relationship must be established in the laboratory and the calibration can afterward be used for determining the in-place strength. The ASTM C1074 (Standard Practice for Estimating Concrete Strength by the Maturity Method) explains the entire procedure in detail and is a highly recommended read. Below we have made a more visual guide based on the standard practice. 

How To Make a Maturity Calibration

Step 1: Prepare and Cure Specimens

  • Prepare at least 15 specimens, where you embed temperature sensors in at least two of the specimens*. The sensors must be placed within ±15 mm of the center.
  • Connect the temperature sensors to maturity instruments or temperature-recording devices as soon as possible and start recording the temperature history.
    • The interval between the temperature measurements must be 30 minutes or less for the first 48 hours – and 1 hour or less thereafter.
  • Once the specimens have been prepared, cure them according to standard procedures (e.g. ASTM C511) and cure all the specimens under the same conditions, e.g. in a water bath or moist room.
    • It is important to have the same curing conditions for all the specimens in order to obtain a correct correlation between the strength and maturity.
 
* The consensus is that using thermocouples as temperature sensors should have no significant effect on the compressive strength. However, if you are using larger temperature sensors, we recommend that you prepare two additional specimens only for measuring the temperature history. 

Step 2: Perform Break Tests

  • Perform compression tests at minimum five different test ages, e.g. after 1, 3, 7, 14, and 28 days. The specimens with the temperature sensors embedded should be tested last to remain for maturity monitoring.
    • For each test age, break two specimens and compute the average strength. This improves the reliability of the results.
      • If the difference between the two tests is higher than 10% from their average, break the third specimen and use the average of the three.
      • If you obtain a low test result due to an obviously defective specimen, discard the low test result and use the average of the other two specimens.
    • For each test age, record the average maturity index from the two specimens with the temperature sensors embedded.
      • If you use maturity instruments, note down the average maturity index.
      • If you use temperature recorders, calculate the maturity index using one of the maturity functions.

Step 3: Create Maturity Curve

  • You should now have at least 5 data points, where each point is the correlation between the compressive strength and the maturity index
    • Plot the average strength as a function of the average maturity index
  • The resulting curve is the strength-maturity relationship – and can be used to estimate the strength of the concrete mix cured under other temperature conditions.

Step 4: Validate Test Results

  • To validate the calibration and maturity curve, make some specimens during the next pour/batch and compare the strength predicted using your maturity calibration with the compressive strength results obtained from laboratory break tests.
    • If the difference is more than 10%, you should perform a re-calibration and develop a new strength-maturity relationship for the concrete mix.

FAQ - Frequently Asked Questions About Calibration

Yes. A calibration is only valid for a specific concrete mix design as every mix design will have different properties and curing. Some concrete mixes might be designed to reach fast early strengths, while others might have a much slower strength development.

In addition to this, it is important to manage the mixing variances. Minor variances between the mix you made your concrete calibration with in the laboratory and the one you use for your construction might only have a small impact. However, if you have high variances in your concrete mix, then the strength predictions might be wrong or misleading.

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