## 1 Introduction  
Building fires occur more frequently than other fires, causing considerable property damage and human casualties [1]. This has prompted extensive research on reducing fire damage in buildings. Several major factors are responsible for the degraded mechanical properties of concrete buildings exposed to fire. Accelerated carbonation by the dehydration of calcium hydroxide (CaOH\({}_{2}\)) in concrete and microcracks caused by the different thermal expansion properties of aggregate and paste significantly degrade concrete durability. In addition, long-term exposure to high temperatures causes porosity and significant cracking because of the chemical decomposition of the concrete matrix, resulting in significant deterioration in the structural performance [2; 3; 4]. Therefore, this warrants the accurate measurement of the mechanical properties of concrete after exposure to fire, which can enable the accurate assessment of repair or demolition, thereby saving cost and time, as well as reducing additional human casualties and property damage caused by secondary collapse [5; 6; 7]. The most accurate method to characterize the mechanical properties of a building degraded after fire is the direct destructive test of the degraded concrete members. Core extraction for the destructive test of the degraded members is required for building fire safety diagnosis, which presents the following problems: (1) core extraction and destructive testing is expensive and (2) continuous research is necessary to address problems with equipment, manpower, and time required. Thus, building safety diagnosis and assessment through non-destructive methods are preferred today. Among the several non-destructive methods, ultrasonic pulse velocity method is proposed as a method to estimate mechanical properties in this study. The ultrasonic pulse velocity is a function of the time and distance that the ultrasonic pulse transmitted from the transmitting transducer measures the time when it arrives at the receiving transducer and represents a value since it shows different tendencies depending on the internal structure and defects of the concrete, it has a high possibility in predicting defects according to the internal state of the concrete. The equipment of this method is portable, while safety diagnosis does not require expert skills and can be performed at a relatively low cost and in short time, compensating for the drawbacks of destructive testing. Several studies on predicting the mechanical properties of concrete through ultrasonic pulse velocity analysis after high temperature exposure have been reported [8, 9, 10, 11]. However, these have focused on compressive strength prediction, with limited information on the prediction of the modulus of elasticity, i.e., stiffness, which is an equally important for accurate safety diagnosis of fire-damaged concrete. The residual modulus of elasticity of concrete after high temperature exposure varies depending on the aggregate type [12] (Figure 1). Since the modulus of elasticity is significantly affected by the stiffness of the coarse aggregate, it is necessary to consider the aggregate type to accurately predict the residual modulus of elasticity of concrete after high temperature exposure [13, 14, 15]. The ultrasonic pulse velocity is significantly affected by the materials mixed in concrete, including the aggregate [16]. Therefore, the ultrasonic pulse velocity method that considers the influence of the aggregate type is expected to be suitable for accurately measuring the residual modulus of elasticity after high temperature exposure. In this study, the mechanical properties of concrete mixed with normal aggregate (normal concrete (NC)) and lightweight aggregate (lightweight concrete (LC)) were evaluated at various water-to-binder (W/B) ratios (0.41, 0.33, and 0.28). The amount of cement and aggregate mixed varies according to the W/B ratio. It is necessary to consider W/B because the ultrasonic velocity shows a different tendency depending on the material mixed in the concrete.