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It is important for nations to preserve historical monuments and conduct necessary repair and strengthening practices for their survival for the generations to come. Prior to the application of any repairment/strengthening method, a damaged structure must be thoroughly analyzed and the reasons behind the damage must be studied. Such information can be obtained without damaging the structure or altering its original conditions.
Concrete hammer test and ultrasound test methods are now considered technologically backward techniques as the new advanced test methods are widespread in structural investigations. Some of the advanced non-destructive test methods are flat-jack, ground-penetrating radar, endoscopy, infrared thermal imaging, and tomography methods.
Flat-jack test is a very powerful non-destructive test used for identifying the mechanical properties of historic structures. Mechanical properties such as compressive strength, elasticity modulus, and Poisson's ratio can be measured with the flat-jack method.
It is possible to detect the existence of metallic materials in the bearing elements using radar images. The use of IR cameras for tomographic images allows for data on the cracks and humidity content of the structural elements. The endoscopy method, on the other hand, does not offer detailed information when compared to other methods.
This article describes the non-destructive test methods used in the assessment of the historical structures. Through the discussed non-destructive test methods, one can obtain data on the material decay, invisible cracks formed in the structure, humidity problems, and thermal properties along with values such as elasticity modulus, Poisson's ratio, and compressive strength.
Thus, it is possible to conduct repairment or maintenance practices on historic structures, which represent the historical identity of that region, preserving their natural conditions and without causing any further damage.
1. Infrared Thermal Imaging Method
Infrared thermal imaging is commonly used for efforts being made towards fuel and energy preservation. Today, this method is used extensively in the field of military, industrial usage, medicine, meteorology, architecture, and engineering. Moreover, many researchers have preferred this method for their studies, primarily focusing on the detection of materials and structural issues in historic buildings.
This method is especially used to detect heat loss, humid sections, gas escape, thermal bridge, and the status of thermal insulation. In addition, infrared thermal imaging is also used for failures in historic structures, rainwater and wastewater drainage system issues, and structural cracks.
This non-destructive test method, which uses thermal cameras, provides a thermal map of the material used in accordance with the color scales at threshold value defined with the detection of the radiation emitted by the material. Thus, the area to be examined is defined. This method allows for the detection of thermal issues, humid sections, gas escape, heat bridges, etc., in the areas inspected. In other words, problematic or unproblematic areas of the structure being investigated are revealed in a non-destructive manner.
The emissivity coefficient must be known (generally between 0.90 and 0.95 for structural material) when examining a structure using the infrared thermal imaging method. Lower or higher emissivity coefficients is a factor influencing the accurate temperature measurement with thermal cameras. Thermal cameras are also affected by other parameters along with emissivity.
Among these parameters are the environmental temperature of the place where the thermal camera is used, wind velocity of the environment, relative humidity, the distance between the camera and the object to be tested, the angle of the camera, the time and season the test will be conducted.
Infrared thermal imaging is used in three different methods. The first one is used to detect the heat transfer from a medium where a heater is used to the outer medium. The second is used where there is a heat flow between two surfaces with different ambient temperatures.
Finally, the third is used to inspect the heat differences with the application of heat radiation to both sides of the object where there is no temperature differences or heaters in place. It is possible to obtain both qualitative and quantitative data using thermal cameras. Analysis can be conducted using relevant software.
2. Superficial Concrete Test Hammer Method
One of the oldest non-destructive test methods, the concrete test hammer method, is used to measure the hardness of the material. Hardness is one of the most important mechanical properties when it comes to distinguishing between materials.
Hardness is the resistance of the material against an object which tries to penetrate the material surface. Superficial hardness was developed by Ernst Schmidt in 1948, known as the Schmidt test hammer. The working principle of this method involves the measurement of the rebound of a spring-loaded mass impacting on a surface. The hardness of the material increases as the rebound increases.
3. Flat-Jack Method
The flat-Jack method is known as the in-situ stress test method. The method allows for the identification of a number of mechanical properties of a structural wall. Compressive strength, elasticity modulus, and Poisson's ratio are among the mechanical properties measured with the flat-jack method. The flat-jack testing tool allows for the measurement of the change in length in response to the power applied to an element.
The flat-jack test mechanism involves two measuring methods. The first method uses a double flat-jack and is able to define compressive strength, elasticity modulus, and Poisson's ratio. Whereas the second method uses a single flat-jack and is able to determine the current compressive stress of the wall.
The mechanism involves a compressor, manometers, flat-jacks, comparators, and pins. Flat-jacks are used in order to apply compression to the surface, while the comparator measures the replacement and the pins locate the comparator.
4. Ultrasound Test Method
The ultrasound velocity test method is a non-destructive method that gives information on the quality of the concrete, its internal structure, its porosity, compressive strength, and the depth and orientation of cracks. It operates on the principle of the measurement of ultrasonic pulse velocity.
In the ultrasound test, the material surface is contacted with two piezoelectric transducers without any void between them and the surface. The first transducer sends the ultrasound waves, and the second receives these ultrasound waves. The time of transmission and velocity of the ultrasound waves are then measured.
If the density of the material is poor, and the material has cracks, then the diffusion of the soundwaves occurs. Therefore, the ultrasound pulse velocity is low. When the material is robust or has a reduced number of pores, it offers a better strength. Thus, the ultrasound pulse velocity is higher. However, this test is not sufficient by itself to define the strength and must be combined with other techniques.
5. Radioactive Method
The radioactive test mechanism consists of an electromagnetic radiation source and a sensor. The sensor measures the time required for the radiation to reach the other side of the material. When the sensor is in the form of a unique photographic film, then it is called radiography, and when the sensor converts the radiation into electrical waves, then it is called radiometry.
6. Penetration Resistance Method
This is a test method that operates on the principle of the measurement of penetration depth of a probe or a nail shot at the concrete using a gun known as the Windsor probe.
Windsor probe was developed in the US in 1964 and is similar to the concrete test hammer. It is used in order to have an idea about the strength of the concrete and identifies the concrete's resistance against penetration. This test method is affected by the aggregates used in the concrete due to its operating principle.
7. Endoscopic Method
The elements of the bearing system used in structures are rather large and hard to investigate. This method is preferred when the material used in a bearing system is to be defined and when it cannot be inspected visually or identified using a borehole sample.
This method involves drilling a hole of 1 cm in diameter in the structure and obtaining images of the structure with the use of a cable and a camera attached to it, thus, making it possible to identify the material used. This method is especially important for the diagnosis of the bearing system of historic structures.
8. Ground-Penetrating Radar Method
In the ground-penetrating radar method, electromagnetic waves are sent through a medium, while the time between the receiver and the transmitter is recorded and the target area is scanned. Thus, it is possible to reveal unknown characteristics due to the physical discontinuity of the medium.
This method does not allow for the exploration of the mechanical properties of the material. However, it is able to define the physical characteristics which cannot be identified with visual inspection or with the use of borehole sample.
the ground-penetrating radar method is used to reveal the unknown characteristics of a stratum due to the physical discontinuity of the medium.
the endoscopic method is used when the material used in a bearing system is to be defined and when it cannot be visually inspected or identified using a borehole sample.
The flat-jack method is very useful in identifying the mechanical properties such as, compressive strength, elastic modulus, etc, very accurately.