*3.2. Proposed Method and SonReb Method Compared to Destructive Method*

For a better validation of the proposed method, the obtained results are compared to both the SonReb method and the DT. For the SonReb method, six structural elements (columns) were investigated on-site by using ultrasonic pulse velocity and Rebound Hammer Schmidt. Each element was tested in three sections, with five UPV and nine RHS measurements/section. In Table 3, the resulting mean values of the UPV, RHS, and compressive strength from each method are presented.


**Table 3.** The results obtained from each presented method.

A graphical representation of the accuracy is presented in Figure 9, namely a comparative analysis between the proposed method vs. SonReb method, both of them evaluated with respect to the reference, namely, the destructive method.

**Figure 9.** Graphical representation of the accuracy evaluation of each method, proposed and SonReb method, with respect to the reference method (destructive testing).

For the six elements investigated through both NDT methods, the precision rate in the case of the proposed method reaches up to 96%, while in the case of the SonReb method the precision reaches up to 82%.

#### **4. Conclusions**

The aim of this paper is to present the results obtained by combining the on-site measurements of UPV and theoretical interpretation using a set of equations developed by different researchers linking the values of ultrasonic pulse velocity to the dynamic modulus of elasticity, static modulus of elasticity, and finally concrete compressive strength.

Estimating concrete compressive strength through this method delivered results with high accuracy, which, in this case, ranged between 84 and 100%. It can be noticed that the high level of accuracy remains the same regardless of the range value of compressive strength, which in this study is between 15.8 and 38.9 MPa. Additionally, the coefficient of variation (CoV) shows reduced values, ranging along compact intervals, both for the dry-air density evaluation (from 0.4 to 0.8%, for the theoretical, Salman approach) and also for the compressive strength evaluation (from 7.7 to 10%, for proposed method). Further investigations will also consider the methodology and statistical approach proposed by Breysse et al. [62].

As this method relies only on UPV measurements, the on-site surface preparation and testing process must be performed with a high level of precision; otherwise, the results will have a higher level of uncertainty.

In contrast to the SonReb method, the proposed one has the advantage, so far, that there is no requirement of information about the classical concrete mix design such as cement type and dosage, granulometry, and nature of aggregates. This information is often difficult to obtain, especially in the case of buildings where the concrete mix was produced on-site with no known recipe. In the SonReb method, not knowing these parameters correct can lead to errors up to ±30%. In this case, although the concrete mix design was known, hence all the coefficients were correctly assumed, the accuracy of the SonReb method had a lower value than the proposed method on each analyzed concrete column when compared to DT.

The current results are clearly encouraging, offering new research perspective for method optimization and further confirmation to prove its viability, especially in terms of concrete mix design diversity, which has experienced an exponential growth in the last decades. The single and multiple additions in concrete compositions, waste, or by-products generated by the industry, may induce concrete hardened state changes which lead to complex investigation in terms of overall behavior, NDT included. A preliminary approach on this area represents the on-going study of the current research.

**Author Contributions:** Conceptualization, B.B. and A.G.; methodology, B.B. and V.V.; validation, B.B. and V.V.; formal analysis, C.B. and D.-D.B.-N.; investigation A.G.; resources, B.B. and S.D.; data curation, D.-D.B.-N. and S.D.; writing—original draft, B.B. and C.B.; writing—review and editing, V.V. and D.-D.B.-N.; visualization, B.B. and V.V.; supervision, C.B., A.G. and S.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** This paper is supported by: Programme: Research for sustainable and ecological integrated solutions for space development and safety of the built environment, with advanced potential for open innovation—"ECOSMARTCONS", Programme code: PN 19 33 04 02: "Sustainable solutions for ensuring the population health and safety within the concept of open innovation and environmental preservation", financed by the Romanian Government. Project "Entrepreneurial competences and excellence research in doctoral and postdoctoral programs—ANTREDOC", project co-funded by the European Social Fund financing agreement no. 56437/24.07.2019. The authors would like to thank Michael Grantham of Sandberg LLP, University of Leeds and Queen's University Belfast, Institute of Concrete Technology and Editorial Board of the Journal "Case Studies in Construction Materials", UK for his support and contribution in technical editing, language editing and proofreading.

**Conflicts of Interest:** The authors declare no conflict of interest.
