# The Use of Non-Destructive Testing (NDT) to Detect Bed Joint Reinforcement in AAC Masonry
## 2 Potential of Non-Destructive Tests for Detecting the Reinforcement  
Currently, non-destructive tests that are conducted to detect reinforcement in structures involve radar, electromagnetic, radiologic (radiographic), ultrasonic and tomography techniques [23, 24, 25, 26]. The radiologic technique was commonly applied in the 1960s and 1970s, since it produced the best results. This technique is based on \(X,\gamma\), or other hard radiation and its recording that follows radiation emitted to the tested facility. During the tests on structures, radiation was recorded using special photographic films sensitive to ionising radiation. Radiographic techniques are now rarely used because of their long test duration, time-consuming elaboration of results and the exposure of operators performing measurements to harmful effects of hard radiation. Accuracy and a wide range (depth) of measurements are the unquestionable advantages of radiology. The ultrasonic technique is an acoustic technique based on the analysis of relations between the velocity of elastic waves with a frequency higher than 20 kHz, which propagate in solid medium, and the properties of that medium. These methods are mainly employed to determine the strength of concrete and masonry, to detect voids and discontinuities in the structure, and to test crack depths. Due to different velocity of an acoustic wave in concrete/masonry and reinforcement, and the effects of wave interference and reflection, the ultrasonic technique can be applied to detect reinforcement. The typical frequency of ultrasonic waves used to detect reinforcement varies from 20 to 100 kHz. Until recently, the method had not been suitable for locating the reinforcement due to its low accuracy. Modern devices for ultrasound tomography increase the measuring accuracy as the elastic wave is induced by a multi-head antenna equipped with many independent ultrasonic transducers. This is why efforts have been made to detect reinforcement using this method [27; 28; 29; 30; 31; 32]. Infrared thermography consists of recording, processing, and visualising the invisible IR emitted by a body. Infrared thermography usually uses the medium IR band with a wavelength of 0.9-14 \(\upmu\)m. A thermal image (thermogram) is the result of tests capturing temperature distribution over the area of a tested body. Non-destructive tests using infrared thermography can be divided into procedures of passive and active tests [26; 33; 34]. Passive thermography analyses heat emitted by material without any additional thermal simulation, whereas active thermography is based on thermal analyses of the material response after its exposure to heat. Thermography can be used to detect reinforcement as concrete and steel have different thermal properties. Hence, this technique gives very promising results [33; 34; 35]. Currently, electromagnetic and radar techniques are the most popular non-destructive techniques for detecting reinforcement. Electromagnetic methods for locating reinforcement in reinforced concrete elements usually include the analysis of changes in the magnetic field near rebars [22; 23; 36]. However, they are suitable only for detecting metallic reinforcement. The GPR (Ground-Penetrating Radar) method is based on emitting electromagnetic waves in ultra-short and short frequencies of radio waves and recording waves reflected from layers characterised by variable dielectric properties. This technique had rarely been used for reinforced concrete structures until the 1980s [24; 25; 37]. At that time, both the measuring technique and the methods of interpreting results were improved. Currently, many measuring systems are available that vary in measurement ranges and levels of result interpretation. Thus, the GPR technique is often used to localise reinforcement [38]. The assumed measurement range of 1.0-3.0 GHz offered by devices available on the market limits the method accuracy. Lower frequencies enable to make tests at greater depths, and higher ones to detect small bodies localised near a tested edge.