# Influence of Concrete Strength Class on the Long-Term Static and Dynamic Elastic Moduli of Concrete
## 4 Conclusions  
The paper presents an experimental program aimed at assessing the long-term material properties of Portland cement concrete. Based on the obtained results, the following conclusions can be drawn:  
There is a clear tendency of the values of the mechanical properties increasing with the age of the concrete and with the concrete strength class. The compressive strength increases by 5.81% to 47.37% compared to the values obtained at the age of 28 days. The increasing trend is not as steep as other results reported in the scientific literature because the specimens were stored in a relatively dry environment. The storage conditions, most importantly the relative air humidity, are known to have a direct influence on the evolution of the material properties in time.  
The values of the dynamic modulus of elasticity are between 18-50% higher than the static modulus of elasticity, with the lower percentage corresponding to the higher concrete strength class.  
The dynamic Poisson's ratio is lower for the higher concrete strength class owing to the higher rigidity of the material. This may have a direct influence on the material damping properties of concrete and the authors believe it warrants further investigation.  
The empirical conversion equations, from the dynamic modulus of elasticity to the static one, provide a good agreement between the analytical values and the experimental ones, with some of the equations being on the conservative side. A similar accuracy was observed when determining the static modulus of elasticity in terms of the compressive strength at the age of 6 years. The available equations from the design code overestimate the experimental results by as much as 8%. This implies that they can be applied with a fair degree of confidence when in situ non-destructive experimental data are available.  
The accuracy and repeatability of the presented data were checked using two statistical parameters that provide valuable information on the spread of data with respect to the median value. For each investigated parameter reported in the paper, the standard deviation and the coefficient of variation showed consistently low values. It can be concluded that the values of the material properties were uniformly distributed over the 15 samples for each mix, the resulting concretes were homogeneous, and the experimental investigations were thoroughly and systematically conducted.  
Figure 7: Samples of failed specimens: (**a**) in compression; (**b**) tensile splitting test.  
**Author Contributions:** Conceptualization, I.-O.T., P.M. and G.T.; methodology, I.-O.T., S.-M.A.-S., P.M. and A.-M.T.; formal analysis, P.M., G.T. and A.-M.T.; investigation, I.-O.T. and S.-M.A.-S.; resources, G.T. and A.-M.T.; writing--original draft preparation, S.-M.A.-S., A.-M.T. and G.T.; writing--review and editing, I.-O.T. and P.M.; funding acquisition, I.-O.T. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The APC was funded by The \"Gheorghe Asachi\" Technical University of Iasi under the Publications grant GI/P12/2021.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data reported in this research is available upon request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest.