In the article the impact of varying humidity conditions on selected strength properties of hardened steel fibre-reinforced concrete (such as Young’s modulus and the compressive strength of concrete) were analysed. The analysis shows that the value of the Young’s modulus of concrete cured under 100% humidity conditions is the highest; 35% higher compared to concrete hardening in the laboratory hall and about 18% higher than modulus measured in the concrete samples maturing in the chamber. A much higher compressive strength of approx. 25% was observed in samples hardening under 100% humidity conditions than in the other two environments. It was also observed that the non-uniform distribution of samples in the chamber and opening and closing the chamber during sample maturation had an impact on the dispersion of the results and due to the coefficient of variation, results can be even worse than in free-maturing samples.