This thesis investigates using Ultrasonic Pulse Velocity (UPV) to check for near-surface damage in concrete structures without causing any harm to the material. The research started by reviewing past studies on UPV methods, helping to compare results and evaluate how well different techniques worked. The main goal then became testing three specific UPV methods: Indirect Method 1, Indirect Method 2, and the Pulse-Echo Method. The damages are intended from the opposite surface as which is being tested, and the purpose was to see how well these methods could spot these near-surface damages and whether they were able to measure the thicknesses of these damages. The study also looked at how different water-cement (W/C) ratios impacted the strength of concrete, including the compressive and the flexural strength, and how these changes in the end had influence on the UPV measurements.
The research was done in two parts. First, the three UPV methods were tested on concrete samples with different W/C ratios to see whether the pre-intended damages could be found, and the thickness could be estimated. Then, additional mechanical tests, like the compressive and the flexural strength test, were used to check the overall strength and condition of the concrete.
The results showed that Indirect Methods 1 and 2 worked well for detecting surface-level damage, but they struggled to find and map deeper damage of the concrete. In comparison, the Pulse-Echo Method was more reliable, as it could better detect changes in depth and accurately measure the thickness of near-surface or inner damage. This was partly because the wave direction of the Pulse-Echo Method is perpendicular to the surface of the concrete sample, and therefore a better control over the wave direction is achieved. Although the Pulse-Echo method had some limitations, such as not being able to fully map the shape of the damage due to a lack of measurement points, it outperformed the other indirect methods in detecting and estimating the sizes of the pre-intended damages.
Another important finding was that concrete with a lower W/C ratio had more pores in this specific case, which seemed to be caused by issues with compaction and curing. These problems weakened the mechanical strength of the low W/C ratio concrete. Due to this finding, the study suggests that future studies should perform quality checks before testing starts, so this phenomenon could be prevented in future research. Other recommendations of this study include, upgrading UPV testing (especially for the indirect methods) with better sensors and signal processing, and exploring hybrid non-destructive testing methods. Furthermore, future research should also involve real-world testing to better understand how these methods perform in practical situations.
To sum up, while the Pulse-Echo Method was the best at detecting and measuring the intended damages, there is still room to improve the indirect methods by refining the tools and techniques. This would probably allow them to localize and determine the size of deeper damage in concrete structures.