# Evaluation of Condition of Concrete Structures Using Ultrasonic Pulse Velocity Method
## 3 Ultrasonic Pulse Velocity Method  
Structural safety diagnosis examines the status of the members in an existing structure for safety, durability, and usability using destructive and nondestructive testing as well as visual observation, measurement, and analysis. Nondestructive testing methods are used for the safety diagnosis of buildings. The definition of nondestructive testing methods includes a wide range. First, nondestructive testing refers to a diagnosis that does not deform concrete and other specimens by the test itself. Second, the test refers to methods that do not degrade the function of the structures. For example, core testing is also involved in the range of nondestructive testing. Third, the test refers to a diagnosis that causes less damage to structures than core testing. For the construction management of structures and the judgment of durability of existing structures, it is necessary to be aware of the strength and condition of the structures. For structures under construction, typically, the compressive strengths of concrete specimens fabricated on site are checked. However, it is not possible to estimate accurate strength using the concrete of a structure and the concrete of a specimen; differences exist between on-site conditions (such as placing, compaction, and curing) and laboratory conditions. Therefore, measuring the compressive strength of the concrete of a structure in a nondestructive manner to secure accurate data is important. It is difficult to apply nondestructive evaluation to concrete because large uncertainties arise from differences in skill levels in the processes of concrete mixing, pouring, and curing; thus, related studies are being conducted constantly. Currently, the American Society of Testing Materials (ASTM) and International Standard Organization (ISO) are performing standardization work based on the research results for nondestructive testing methods. Nondestructive evaluation methods can be applied to various areas including concrete. Moreover, owing to advances in technology, new nondestructive evaluation methods have been developed. The ultrasonic pulse velocity method estimates the compressive strength of concrete by measuring the ultrasonic pulse velocity from the pulse passing time between the transmitter and receiver at certain distances in a concrete structure, as described in ASTM C597-16 and KS F 2731. The ultrasonic pulse velocity can be used for evaluating concrete quality factors such as the elastic modulus, crack depth, and internal defects, and is applied by obtaining a correlation between the ultrasonic pulse velocity and the compressive strength. As ultrasound is affected by many factors owing to the non-uniform and nonspecific concrete density, estimating the accurate compressive strength of concrete using ultrasound alone is difficult in many cases. However, if the major conditions are similar, estimate the strength to a certain degree is possible because of the correlation between ultrasound and strength. ASTM C597-16 deals with a method of measuring the propagation velocity of the ultrasonic pulse in the longitudinal direction of concrete; it specifically indicates that stress waves are not applied to other types of radio waves. It also establishes appropriate safety and healthcare implementation standards and discusses the evaluation of the applicability of the management limit before use. In the ultrasonic pulse velocity method, a short and strong electrical signal is transmitted to the transducer to make it vibrate according to the resonance frequency. The vibration of the transducer is transferred to the concrete by the contact medium and detected by the receiving transducer on the opposite side. As the time between the generation and arrival of the wave is recorded by the electrical equipment, the wave velocity can be obtained if the distance traveled by the wave is known. Assuming the behavior of concrete to be elastic, the propagation velocity of the wave can be expressed as Equation (1).