The evaluation of the material strength of concrete over time is a significant concern in many civil engineering construction projects. Several reference documents, including BAEL 91, provide formulas to estimate the compressive strength of concrete at J days based on the strength measurements obtained at 7 days. However, noticeable differences exist between the estimated values and the actual values obtained during construction when applying these formulas. The objective of this study is to assess a reliable age coefficient for estimating the concrete strength at J days based on the strength measurements at 7 days, 28 days, and 90 days, depending on the locally used materials (Togo) for concrete production. The methodological approach adopted involves collecting rock samples (amphibole and biotite gneiss) from 33 sites in Southern Togo, sampling them, and conducting various identification tests on the obtained gravel samples. The chosen sand is from the Mono River, and a commonly used cement from the local market is used. The theoretical concrete mix design is performed using the Dreux Gorisse method. The results obtained from the hardened concrete allowed the examination of different ratios according to the concrete's age. It is found that the average RC28/RC7 and RC90/RC7 ratios obtained for B20 concrete are 1,35 and 1,56, respectively, and for B25 concrete, they are 1,43 and 1,67. Considering a risk level of P=10%, these ratios become 1,42 and 1,59 for B20 concrete and 1,37 and 1,75 for B25 concrete. These ratios differ from those provided by BAEL 91, which range from 80% to 93% for B20 and 86% to 99% for B25.
