The on-site calibration finds the relationship between degree of hydration and uniaxial compressive strength. This is necessary because as well as the degree of hydration, several other factors affect strength including the type, quantity and grading of aggregates, the water/cement ratio, and the level of compaction achieved by the placement method.
For shotcrete, this means that spraying at full-scale is necessary, either on an easily accessible part of the tunnel or test panels. Then, temperature history is measured by a thermal imaging camera to calculate degree of hydration and compressive strength is measured using in situ methods (such as needle penetrometer and stud driving tests) and coring. An example of the calibration relationship is shown below:
The calibration relationship is always linear, and this has been found to be true for all types of concrete. A 'critical degree of hydration' (also known as the 'percolation threshold') is needed before strength begins to develop, and this is because crystals need to grow on the surface of cement grains to the point where they are no longer floating around in a solution but beginning to interlock and create an initial skeleton.