*3.3. Ultrasonic Pulse Velocity (UPV)*

In contrast to destructive testing, the research group focused on determining the mechanical properties of biochar-based concrete using non-destructive testing. Figure 8 shows the ultrasonic pulse velocity (UPV) testing results for control and biochar concrete. According to ASTM C 597 [29], the test technique has been validated and standardised. It indicates that the values of UPV were around 7.54 and 8.04 km/s, with a mean of 7.79 km/s, according to the given experimental data. However, compared to control concrete, the biochar-concrete samples had higher UPVs. The development in UPV results within biochar-concrete can be attributed to the pozzolanic activity due to the incorporation of biochar. Similar conclusions have been observed by other researchers [34,35]. It is worth mentioning that the UPV values for all the biochar-concrete samples were over 7.79 km/s, indicating that the quality of concrete is exceptional [36]. According to the literature and standard, it can be concluded that the biochar-concrete with UPV value > 7.79 km/s indicates that it is free from internal flaws, large voids, cracks, and segregation leading to reduced structural integrity and good concrete quality in terms of density, uniformity, homogeneity [37,38].

**Figure 8.** Ultrasonic pulse velocity for various concrete specimens.
