# Evaluation of Condition of Concrete Structures Using Ultrasonic Pulse Velocity Method
## 6 Conclusions  
The purpose of this study was to estimate the compressive strength of concrete using the ultrasonic pulse velocity method, one of the nondestructive testing methods, for compressive strength estimation according to the age of the concrete. To achieve this purpose, a total of 123 concrete specimens were fabricated by setting nine variables based on the ages of 16, 20, 24, 48, 72, 120, 168, 360, and 672 h for the designed strengths of 24, 30, and 40 MPa. An experiment was performed to estimate the compressive strength of concrete according to its age. From the experiment, the following conclusions were obtained:  
For the designed strengths of 24, 30, and 40 MPa, the ultrasonic pulse velocity in each specimen sharply increased from 16 to 72 h of age. It slowly increased to 120 h and then maintained a very slow increasing tendency up to 672 h. As a result of measuring the compressive strength, the average compressive strength was 2.07% of the designed strength at 16 h of age, 2.25% at 20 h, 3.21% at 24 h, 13.55% at 48 h, 31.42% at 72 h, 64.71% at 120 h, 80.89% at 168 h, 87.98% at 360 h, and 91.09% at 672 h. From the analysis of the correlation between the ultrasonic pulse velocity and the compressive strength, a concrete compressive strength estimation equation was proposed as Equation (6). The proposed estimation equation confirmed that it is possible to estimate the compressive strength of concrete according to its age using nondestructive testing methods. In particular, the proposed estimation equation is expected to be helpful in preventing problems caused by formwork removal by providing construction sites with valuable information at early ages of the concrete.  
[PERSON], [PERSON], [PERSON], and J.K. conceived and performed the experiments and analyzed the data and wrote the paper. [PERSON] and S.K. supervised this project as a research director. All authors have read and agreed to the published version of the manuscript.  
National Research Foundation of Korea (NRF) funded by the Ministry of Education. Award Number: NRF-2017R1A2B2009743 (Recipient: [PERSON], Ph.D).  
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1A2B2009743).  
The authors declare no conflict of interest.  
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