Laboratory testing of small samples of the shotcrete mix ingredients is required to determine the thermodynamic parameters that govern hydration. Hydration is the chemical reactions between water and cement to create crystal hydrates, which is what gives concrete its strength.
Since concrete hydration is exothermic, this makes it very sensitive to temperature. If the concrete temperature goes up by 10°C, the rate of hydration doubles. This is shown in the following graph where samples of cement paste have been cured at different constant temperatures:
The laboratory testing will determine parameters that model the temperature sensitivity of the shotcrete and also how the reaction rate varies with maturity. 'Maturity' is defined by 'degree of hydration', which is literally a measure of how many reactions between cement and water have occurred as a proportion of the total possible reactions. It therefore varies between 0 and 1 (though it never gets to 1 as there is always unhydrated cement even in very old concrete.
Using this model allows us to calculate the degree of hydration of the shotcrete for any temperature histry. To calculate strength, we need to find the relationship between degree of hydration and uniaxial compressive strength. This is done in the next step, on-site calibration.