A recently presented numerical tendency model has been applied on an assumed civil engineering structure. With the model parameters for heat and strength development calculations in early age period can be calculated. This paper shows the possibility to evaluate e.g. form removal times and estimations of need for protection against early freezing for concrete mixes containing fly ash in different amounts, with different water-to-cement ratios and at different outer conditions. The tendency model has shown to be a useful tool for production planning for concrete containing fly ash. According to the performed calculations, any replacement of cement with fly ash will significantly influence the young concrete properties. The effect on delayed strength growth increases with the increased amount of fly ash and will also increase for lower temperatures. In addition, the effect from fly ash increases at higher water-to-cement ratios