Paper Title
RELATIONSHIP BETWEEN ULTRASONIC PULSE VELOCITY AND SCHMIDT REBOUND NUMBER IN HIGH TEMPERATURE-DAMAGED CONCRETE

Abstract
Throughout service life, structures can be exposed to fire and high temperatures. It is crucial for structural health to know and check the compressive strengths of concrete in different structural elements like columns and beams, especially after such events. With updated regulations, determining concrete strength has become a priority for inspection and evaluation. Scmidt rebound hammer and ultrasonic pulse velocity tests are non-destructive, rapid, economical, and practical methods for assessing concrete. This study aims to investigate the relationship between ultrasonic pulse velocity and rebound hammer numbers in concrete samples damaged by high temperatures. Concrete samples produced with two different types of cement (CEM I and CEM II) were exposed to different temperatures (0°C, 200°C, 400°C, 600°C, 800°C), and their ultrasonic pulse velocities and Schmidt rebound number were measured. Additionally, compressive strength tests were conducted on 10 cm concrete cube samples. Concrete samples were exposed to temperatures ranging from 200 to 800°C, the weight of the samples decreased by approximately 5% to 13%. Except for 200°C, the compressive strength tends to decrease with increasing temperature.Samples exposed to 800°C were crumbled and fragmented, making it impossible to obtain reliable compressive strength results due to the loss of integrity. There is a low coefficient of determination (R2 = 0.578) between the average rebound number and ultrasonic pulse velocity of concretes produced with CEM I type cement. There is a moderate coefficient of determination (R2 = 0.717) between the average rebound number and ultrasonic pulse velocity of concretes produced with CEM II type cement. Therefore, it has been concluded that measurements taken with the Schmidt hammer can be misleading in predicting the compressive strength of concrete that has been damaged due to high temperatures or fire. Keywords - Ultrasonic Pulse Velocity, Schmidt Rebound, High Temperature-Damaged, Cement Type, Statical Evaluation.