Non-Destructive Evaluation (NDE) Methods Used for Condition Assessment of Concrete Structures

Non-destructive Evaluation (NDE) Methods Used for Condition Assessment of Concrete Structures
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Non-destructive Evaluation (NDE) Methods Used for Condition Assessment of Concrete Structures
Introduction
Today, there are thousands of publicly controlled concrete structures in China, many of which have existed for several decades. Equally, the number of concrete structures, including bridges and buildings, is increasing each year significantly due to rapid real estate development and urbanization. The movement from rural to urban areas has resulted in a sharp increase in urban residents. Urbanization and the need for bigger residential space have increased the need to construct new structures, such as bridges and buildings (Duan et al., 2021). Likewise, concrete structures have been replacing structures made up of clay bricks, wood, and other primitive materials all over China. Testing the quality of concrete at different stages of the lifecycles of a structure is crucial in ensuring the safety of the people using such structures. Thus, to properly rehabilitate, maintain, and repair public infrastructures, engineers must use effective structure assessment methods (Saremi et al., 2023). Better inspection methods are required for deteriorating as well as newly constructed infrastructures. There are several new and emerging techniques for evaluating the quality of concrete in the civil infrastructure. Some of these methods include ultrasonic wave, acoustic emission, microwave, and rebound methods, among other techniques. All of these methods are non-destructive, meaning that a building or any other structure being tested for the quality of its concrete remains intact, and its integrity is maintained (Saremi et al., 2023). The conventional methods of assessing the quality of concrete in public structures focus mainly on testing specimens cast concurrently for flexural, tensile strength, and compressive. The traditional methods have several limitations, making it difficult for engineers to predict results immediately after the test. Specimen concrete, in some instances, differs from the actual structure and the strength properties of specimen concrete are usually affected by shape and size, which can result in inaccurate conclusions (Saremi et al., 2023). Thus, various non-destructive evaluation (NDE) techniques have been devised to overcome these limitations. NDE techniques are premised on the fact that it is possible to relate some chemical and physical properties of concrete to the durability and strength of structures. Such techniques have existed for the last three decades, and engineers in different countries have used them to evaluate the infrastructure condition (Saremi et al., 2023). Today, NDE methods have become more advanced as non-destructive testing technology has transitioned from a hammer to impact impulse response and echo to the application of waves such as microwave, ultrasonic, and acoustic emission. The current paper evaluates different NDE methods, such as ultrasonic, acoustic emission, and microwave techniques, among other methods.
NDE Overview
NDE is a technology that uses techniques to evaluate materials, objects, and systems without impairing their usefulness. Engineers measure or inspect structures without little or no damage to them (Saremi et al., 2023). Thus, these methods are today considered powerful tools for assessing existing concrete structures’ durability and strength. They are drawing increased attention in terms of effectiveness and reliability. Their usefulness in testing in situ is becoming one of their distinguishing features and compared to the traditional technique of random sampling of concrete for analysis of the material, and most engineers prefer NDE. There are different NDE techniques: rebound hammer tests, ultrasonic tests, acoustic emission tests, leak testing, liquid penetrant and magnetic particle (Saremi et al., 2023). Five significant factors, including required penetration depth, required lateral and vertical resolution, the difference in physical properties between the structure and its surroundings, and past information about the techniques used in constructing the structure under test, influence the choice of the NDE technique.
NDE techniques have various functions, including ranking concrete structures based on their present condition, detecting the state of RC structures and comparing various properties according to threshold values. Civil engineers must have deep knowledge and training on various NDE techniques available for testing the integrity and quality of concrete in different structures. With such knowledge, they will be able to select the best technique from the available methods, depending on the condition of the structure under test (Saremi et al., 2023). It is important to note that using different NDE techniques to evaluate a given parameter increases confidence and validates the result. Coalescing results of different NDE techniques while evaluating the quality of infrastructure is desirable for better results.
NDE Techniques
Rebound Tests: Rebound Hammer Test
The rebound test is an NDE category that entails testing the compressive strength of concrete in a given structure. The method provides a rapid and convenient result, and engineers can use the method for fast and reliable results regarding the compressive strength of concrete. Engineers under this category use the rebound hammer test, also known as the Schmidt hammer (Brencich et al., 2020). The hammer comprises a spring-controlled metal that moves on a plunger inside the tubular house. Pressing the plunger against the concrete surface result in the spring-controlled metal containing constant energy hitting the concrete surface and rebounding back. The degree of the rebound, which is the measure of the hardness of the concrete surface, is recorded on the calibrated scale. The measured value is typically designated a rebound index or rebound number. The rebound hammer test technique is premised on the hardness and strength of a concrete surface against which a mass strikes influences the rebound of elastic masses (Gehlot et al., 2016). A concrete surface with low stiffness and strength absorbs more energy and yields a lower rebound value. Therefore, one can relate the hardness or stiffness of a concrete surface to the rebound reading and obtain the compressive strength of the concrete under test (Gehlot et al., 2016). Usually, the body of the rebound hammer has a graph from which an individual can directly read the compressive strength of concrete under test.
Rebound Hammer Test Procedure
The first step during the rebound hammer test is calibrating the hammer, which is tested against an anvil, a steel device with a Brinell hardness value of 5000N/mm2. Following the calibration, which is usually done to test the hammer’s accuracy, the hammer is positioned at a right angle on the concrete surface of the structure under study and readings are recorded (Gehlot et al., 2016). The hummer is held horizontally at a right angle for vertical concrete surfaces, and readings are recorded. It is crucial to note that holding the hammer at an intermediate angle can result in different rebound numbers for the same concrete surface. The most accurate way of obtaining the relationship between the rebound number and compressive strength of the concrete surface is by testing the concrete cubes utilizing the compression testing machine and rebound hammer simultaneously. The procedure entails first recording the rebound number of the concrete cube followed by testing for compressive test on the compression machine. When the energy’s impact of the rebound hammer is approximately 2.2 Nm, the fixed load needed is of the order of 7N/mm2 (Gehlot et al., 2016). This indicates that the load should be increased when calibrating the hammer of high energy impact, and when calibrating a hammer of low energy impact, the load should be decreased. Similarly, it is also important to note that the test specimen should be larger enough to reduce the side effects on the test results of the actual structure. Cube specimens of approximately 150mm are recommended to calibrate rebound hammers of lesser energy impact. For hammers of more significant energy impact, cube specimens of approximately 300mm are recommended. It is advisable to store the cube specimens at room temperature for 24 hours after removing them from the curing pond before subjecting them to the rebound hammer (Brencich et al., 2020). Following establishing the correlation between rebound number and compressive strength, the strength of concrete can be evaluated. Generally, the rebound number is directly proportional to the strength of the structure. Still, it is essential to note that the rebound number can be impacted by various parameters such as aggregate type, cement type, moisture content and surface condition of the concrete, age and curing of the concrete and carbonation of concrete surface, among other factors. However, the rebound number reflects the compressive strength of a given surface up to a limited depth (Brencich et al., 2020). Likewise, the rebound number does not indicate the concrete’s internal flaws, cracks, or heterogeneity. Thus, estimating concrete strength using the rebound hammer technique does not produce a highly accurate result, and the prediction may be about ± 25 % inaccurate. Obtaining a correlation between compressive strength and rebound number by testing core samples acquired from the structure under test or the standard specimen made from the same concrete materials and mix proportion increases the accuracy of the results.
Ultrasonic Testing
Ultrasonic testing is another NDE technique that engineers can use to perform various tests on concrete to determine its integrity and strength. The test utilizes high-frequency sounds that human ears cannot process. The ultrasonic sounds fall between 16 kHz and 20 kHz depending on an individual’s hearing state and health (Ensminger & Bond, 2012). Ensminger, D., & Bond, L. (2012).. The ultrasonic technique of testing the quality of concern is based on the principle that sound can travel through solid objects, and depending on the intactness of a material, the quality of sound may differ among different materials. Thus, the sound waves travel through the concrete under investigation using this technique. They detect alterations or flaws in the structure, allowing engineers to characterize the concrete and make crucial decisions regarding repair and maintenance (Xu & Wei, 2019). Most equipment used during ultrasonic testing comprises a standard display device, a transducer, and a pulse system. These elements can be altered, or other devices can be added depending surface examined and the user. The pulse generator produces short impulses directed into the transducer on the equipment, which generates high-speed waves (high frequency), generating ultrasonic energy. The produced energy on the equipment searches for cracks, breaks and other flaws within the concrete. When the interacting waves encounter a discontinuity in the concrete, they change direction slightly (are reflected). The transducer transforms the reflected waves into an electrical signal which is recorded on the graph and can be read directly (Xu & Wei, 2019). Ultrasonic waves occur in various amplitudes and patterns depending on the equipment used and the test performed. Longitudinal designated as P-Waves (pressure waves) waves usually occur in equipment that performs tests in deeper or longer material using high frequencies and short waves. These waves bounce around the material and create excited zones and wavelengths. Such zones are regarded as compression zones and are produced when a wave moves through a body. P-waves typically oscillate in the propagation direction. Transverse waves, also known as S-Waves (shear waves), occur within the material being ...