In this article we will explain why it is important to validate your maturity calibration, when to do it and how to do it.
Why is validating calibration important?
The maturity calibration is only valid for a specific concrete mix design as each mix design will have a different relationship between maturity and strength.
For example, some concrete mixes are designed to develop strength fast, while others have a much slower strength development curve. This is important because it means that if you try to apply a maturity calibration from a fast curing concrete mix to predict the strength development of a slow curing mix, then the strength estimations will be wrong. The strength will be perceived as much higher than the actual strength of the concrete.
The opposite will happen if you use a maturity calibration from a slow curing concrete mix to predict the strength development of a fast curing mix. In this case, the estimations will also be wrong, as these will be much lower than the actual concrete strength.
Another important factor is variances in the concrete mix. Minor variances will probably not have a big impact on the reliability of your maturity calibration. However, if you have high variances, e.g. significant changes in your water-cement ratio, then this will directly affect the concrete strength. The calculations using the maturity method will be wrong, since the concrete’s strength development may be faster or slower than normally (and according to the calibration).
As big changes or high variances can affect the accuracy when using the maturity method, then it is important to pay attention to these factors and perform validation on a regular basis and when considered necessary. This will guarantee that the strength estimation reflects the actual strength of your concrete mix.
If you change your mix design (w/c ratio, cement type, aggregate type, admixtures, etc.) then you must perform a recalibration to ensure that your strength estimation is precise.
When to calibration?
It is recommended to validate maturity calibrations often. There are standard steps one can take to better embed this practice.
Let’s validate a calibration
Make samples
Cast three samples using the same procedure as used to develop the current maturity calibration. Remember to use moulds of the same size and dimensions.
Embed temperature sensors
Place a temperature sensor in one of the three samples and connect it to a data logger or transmitter. Cure all the samples under the same conditions.
Test the samples
Perform break tests of the samples as close as possible to the maturity value which represent the target strength. Calculate the average strength from the test results.
When using thermocouples, the consensus is that you can break test all three samples. When using digital sensors, test the two samples that are not wired.
Validate the strength estimation
Compare the compressive strength result from the break test with the strength estimated using the maturity method. If the difference is more than 10%, you should perform a recalibration.
Accuracy testing
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