The strength-maturity relationship identifies the relationship between compressive strength and maturity index.

The compressive strength is obtained through compression tests of specimens and the maturity index is obtained by recording the temperature history of the specimens up till the time of the test.

The strength-maturity relationship must be determined for each unique concrete mix that is used for construction.

### Procedure

*The following procedure is according to the ASTM C1074 standard practice.*

- Prepare at least 15 specimens
- Mixture proportions and constituents of the concrete shall be similar to those of the concrete to be used in construction
- The preparation should be according to the usual procedures for making and curing test specimens in the laboratory
- The concrete can be taken from a fresh batch just before pouring

- If two batches are needed to prepare the required number of specimens, cast an equal number of specimens from each batch, and test one specimen from each batch at the each test age

- Mixture proportions and constituents of the concrete shall be similar to those of the concrete to be used in construction
- Embed temperature sensors within ± 15 mm of the centers of at least two specimens and immediately connect them to maturity instruments or temperature recording devices
- Maturity instruments are devices that automatically compute the maturity
- Maturix can be used as a maturity instrument to determine the maturity index

- If temperature recorders are used, use a time interval of 30 minutes or less

- Maturity instruments are devices that automatically compute the maturity
- Cure the specimens in a water bath or in a moist curing room
- All the specimens must be cured in accordance with standard procedures (to ensure appropriate cement hydration) and under the same conditions

- Perform compression tests at five different ages and record the results
- Test two specimens at each age and compute the average strength
- In-situ concrete: Unless specified otherwise, perform the compression tests at ages of 1, 3, 7, 14, and 28 days
- Precast concrete: Specifying exact test ages for specimens within precast production is difficult, since every concrete mix cures differently and the area of interest is in the early-age before and around the target strength (required strength before demolding). A practice could be to follow the above procedure with a lower amount of specimens, e.g. to test two specimens after 8 hours and two specimens after 20 hours. The two last specimens should be the ones embedded with temperature sensors. These exploratory tests will give more knowledge about what test ages to choose.

- If the range of the two specimens exceeds 10% of their average strength, test another specimen and compute average of the three tests
- If a low test result is due to an obviously defective specimen, discard the low test result

- The instrumented specimens should be crashed as the last

- Test two specimens at each age and compute the average strength
- Record the average maturity index from the instrumented specimens at each test age
- If maturity instruments are used, record the average of the displayed values
- If temperature recorders are used, calculate the maturity index manually

- Plot the average compressive strength as a function of the average value of the maturity index and draw a best-fit curve through the data, which will show the strength-maturity relationship
- When using Maturix, the average compressive strength and maturity index from each test age can be entered into the software – and the maturity curve will be generated

The aim of the procedure is to have sufficient data points from at least five test ages in order to create a valid maturity curve.

The strength-maturity relationship and the resulting maturity curve is used as a reference for estimating the strength of the concrete mix cured under other temperature conditions.