It is crucial 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 approach, 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 methods 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 approach.
The use of IR cameras for tomographic images allows for data on the cracks and humidity content of the structural elements. It is possible to detect the existence of metallic materials in the bearing elements using radar images. The endoscopy method, on the other hand, does not offer detailed information when compared to other methods.
This article describes the non-destructive test approaches used in the assessment of the historical structures. Through the discussed non-destructive test approaches, one can obtain data on the substance 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.
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 substances 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 frameworks, rainwater and wastewater drainage system issues, and structural cracks.
Thus, the area to be examined is defined. This non-destructive test approach, which uses thermal cameras, offers 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. In other words, problematic or unproblematic areas of the structure being investigated are revealed in a non-destructive manner. This approach allows for the detection of thermal issues, humid sections, gas escape, heat bridges, etc., in the areas inspected.
Lower or higher emissivity coefficients is a factor influencing the accurate temperature measurement with thermal cameras. The emissivity coefficient must be known (generally between 0.90 and 0.95 for structural substance) when examining a structure using the infrared thermal imaging method. 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.
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. Infrared thermal imaging is used in three different methods.
It is possible to obtain both qualitative and quantitative data using thermal cameras. Analysis can be conducted using relevant software. 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.
One of the oldest non-destructive test approaches, the concrete test hammer approach, 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.
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. The flat-jack testing instrument allows for the measurement of the change in length in response to the power applied to an element. Compressive strength, elasticity modulus, and Poisson’s ratio are among the mechanical properties measured with the flat-jack method.
The flat-jack test mechanism involves two measuring approaches. Whereas the second approach uses a single flat-jack and is able to determine the current compressive stress of the wall. The first approach uses a double flat-jack and is able to define compressive strength, elasticity modulus, and Poisson’s ratio.
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.
The ultrasound velocity test approach is a non-destructive approach that gives information on the standard of the concrete, its internal framework, 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 substance 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 substance is poor, and the substance has cracks, then the diffusion of the soundwaves occurs. as a result, the ultrasound pulse velocity is low. When the substance is robust or has a reduced number of pores, it offers a better strength. However, this test is not sufficient by itself to define the strength and must be combined with other techniques. Thus, the ultrasound pulse velocity is higher.
The radioactive test mechanism consists of an electromagnetic radiation source and a sensor. 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. The sensor measures the time required for the radiation to reach the other side of the material.
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. This test method is affected by the aggregates used in the concrete due to its operating principle. It is used in order to have an idea about the strength of the concrete and identifies the concrete’s resistance against penetration.
The elements of the bearing system used in frameworks are rather large and hard to investigate. This approach 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 approach involves drilling a hole of 1 cm in diameter in the framework and obtaining images of the framework with the use of a cable and a camera attached to it, thus, making it possible to identify the substance used. This approach is especially crucial for the diagnosis of the bearing system of historic frameworks.
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 approach 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 approach is used when the substance 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.