# Assessment of Strength Development at Hardened Stage on High-Strength Concrete Using NDT
## 4 Conclusions  
In this study, we performed regression on the experimental concrete strength values obtained from rebound and ultrasonic methods. The results of this study are summarized as follows:  
1. Overall, the core specimens exhibited higher compressive strengths than the cylinder mold specimens. As the W/C decreased, the maturity of concrete increased, thereby resulting in an increase in the maximum temperature history. Owing to the difference in maturity, the strengths of the core specimens were 5-10% higher than those of the cylinder molds, caused by differences in the initial temperature history.  
Figure 12: Regression equation of compressive strength of concrete by ultrasonic pulse velocity.  
2. The ultrasonic pulse velocity and rebound value increased until 7-14 days of age, but converged in a manner similar to that of the rebound value after 14 days. This appears to be due to the surface conditions and soundness of the internal structure.
3. The relationship between the rebound value and ultrasonic pulse velocity shows a parabolic correlation, but it is a low correlation of R\({}^{2}\) = 0.68, and the difference is found to be larger with increasing compressive strength of concrete.
4. When the existing regression equations were compared and examined, we found that their values were within the \(\pm\)20% error range for strengths \(<\)40 MPa, but their reliability significantly decreased for higher strengths over 40 MPa. Therefore, this study proposed a regression equation that can converge 40 MPa or more of experimental data, and has a reliability of \(\pm\)20% error range in the range of 24-65 MPa compared with a previous study.  
This study proposed a regression equation for the compressive strength of concrete based on experimental rebound values and ultrasonic pulse velocities in the concrete strength range of 24 to 60 MPa. It is expected that the proposed regression equation can estimate strengths close to the measured strengths within the scope of high-strength concrete.  
There are some limitations to this study: because we used concrete mixes based only on cement, factors such as the use of various admixtures, aggregate types, and curing conditions were not considered. Thus, it is necessary to acquire additional relevant data in future studies.  
[PERSON], [PERSON], and [PERSON] equally contributed to the research conceptualization and all investigations performed under its purview. Resource procurement was handled by [PERSON] and [PERSON]. The original draft of the manuscript was prepared by [PERSON] and [PERSON], whereas the final draft was reviewed and edited by [PERSON], [PERSON], and [PERSON] All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2018R1A5A1025137).  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.