*6.3. Ultrasonic Pulse Velocity*

Ultrasonic pulse velocity (UPV) testing is used to verify the integrity and quality of structural concrete alongside voiding, honeycombing, cracking, and other defects. According to BS 1881-203 [83], the ultrasonic pulse velocity, which is greater or equal to 4.5 km/s, is considered as excellent concrete quality, while less than 2.0 km/s is classified as very weak concrete quality. Based on the study conducted by Tanaka et al. (2020) [4], the pulse velocity is 3.5 km/s to 4.4 km/s, which is slower as compared to the control sample due to the large amount of air voids in the artificial aggregate in the concrete, which affects the quality of the concrete. In study of Rehman et al. (2020) [68], it was found that geopolymer-based lightweight concrete has a higher ultrasonic pulse velocity, which is 2936 m/s to 3016 m/s as compared to cement-based lightweight concrete, which is 2601 m/s to 2835 m/s. This can be explained by the higher strength and more complex microstructure of the geopolymer-based concrete. Further, the occurrence of large amounts of micro-cracks in the concrete can cause the ultrasonic pulse velocity to be lowered, which is 3.42–4.51 km/s when the replacement of coarse and fine aggregates is increased with artificial lightweight aggregate (Satpathy et al., 2019) [63]. For geopoly-

mer concrete specimens containing artificial lightweight aggregate, the ultrasonic pulse velocity value vary from 4.15 km/s to 4.35 km/s, which can be considered good quality concrete (Abbas et al., 2018) [46]. Furthermore, the UPV of the concrete mixes decreased from 4.29 km/s to 3.58 km/s as the lightweight expanded clay aggregate (LECA) and expanded perlite aggregate (EPA) replacement percentage increased, and this could be due to the existence of voids in LECA and EPA lengthening the travel path of the ultrasonic pulse and resulting in a lower UPV value [84].

From Table 6, it can be concluded that the lightweight aggregate concrete will provide good ultrasonic pulse velocity to prevent the defects of the concrete. It is also possible to deduce that the majority of the ultrasonic pulse velocity of concrete is determined by the use of commercialized lightweight aggregate, such as LECA, which is manufactured using the sintering method. As a result, the ultrasonic pulse velocity for concrete with cold-bonded lightweight aggregate and autoclaved lightweight aggregate is limited, and more research is needed.

**Table 6.** Previous studies on ultrasonic pulse velocity of lightweight aggregate concrete.

