USE OF NONDESTRUCTIVE ULTRASONIC TECHNIQUES FOR MATERIAL ASSESSMENT AND IN-SERVICE MONITORING OF CONCRETE STRUCTURES

摘要

Cracks often form in concrete structures as a result of external loading or environmental factors. Cracks allow ingress of deleterious chemicals, such as deicing salts, leading to corrosion of steel reinforcement. As the steel corrodes it expands, thereby further accelerating the rate of corrosion. This leads to reduced service life of the structure. There is a need for in-service monitoring of the concrete infrastructure and assessment of existing damage in structures is essential to target the critical damaged portions and initiate repair. Recent research at the NSF Center for Advanced Cement Based Materials has led to the development of nondestructive test methods for evaluating the in-service performance of materials. A nondestructive test method based on measuring the attenuation of propagating surface waves has been developed for assessing the imparted damage owing to surface breaking cracks. The test configuration and the experimental findings are presented in this paper. In addition, improved performance of concrete structures requires better material processing techniques and early detection of defects. Poor service performance is often a result of improper material processing. Defects are often introduced during casting and measuring the 28-day compressive strength is often too late to remedy the situation. Early development of damage in concrete structures is also related to insufficient strength development prior to loading the structure. An NDE technique that monitors the setting and hardening process of portland cement concrete is presented. The technique is based on measuring the ultrasonic wave reflection factor (WRF) between the hardening concrete and a steel mold. This method can be used to assess the in situ properties of concrete at very early ages so that corrective measures can be taken.