The reliability of major civil engineering structures has an important impact on society, the economy, and the safety of people's lives and property. Correctly assessing the actual behavior of the structure is a prerequisite for determining reliability, and these require the use of to building structure inspection technology.
Structure detection methods can generally be divided into two categories, namely static detection methods and dynamic detection methods. This article attempts to analyze and prospect its current situation and development trends.
Static detection method The static detection method is a traditional detection method. The data of this method is more accurate, but it is limited for large structures due to their large volume and many components, and some parts cannot be detected.
Rebound method The rebound method uses a rebound instrument to hit the concrete surface. The change in the rebound energy of the instrument's weight reflects the elastic and plastic properties of the concrete. The surface hardness of the concrete is measured to calculate the compression resistance. strength. The advantages of the rebound method are simple instrumentation, high detection efficiency, and low cost. However, it also has certain shortcomings. The rebound value is affected by the carbonization depth and the test angle. Different corrections are required for the rebound value, and it consumes a lot of manpower and time.
Radar method The radar method uses the form of wide-band short pulses to send high-frequency electromagnetic waves from the ground to the ground through the transmitting antenna, and then returns to the ground after being reflected by the concrete with electrical differences, and is received by the receiving antenna. When the transmitting and receiving antennas move synchronously along the survey line at a fixed distance, a radar image reflecting the distribution of underground concrete quality defects at the survey line can be obtained. When the uniformity of the concrete is poor, such as honeycombs, overhead phenomena, etc., the electrical difference between this area and the surrounding concrete increases, and the reflected waves are enhanced; when it is complete and dense, the properties are relatively uniform and the reflected waves are very weak. In this way, the quality of concrete can be detected.
Impact echo method The impact echo method is a technology based on transient stress waves applied to non-destructive testing. When the stress wave propagates in concrete and encounters defects and bottom surfaces, it will produce reciprocating reflections and cause Response to tiny displacements on both sides of concrete. Receiving this response and performing spectral analysis yields a spectrogram. The prominent peaks on the spectrum chart are caused by the back-and-forth reflection of stress waves between the concrete surface, bottom surface and defects. The presence and depth of defects can be determined based on the frequency peak. The impact-echo method is a new non-destructive testing method that can be used to measure structural concrete thickness. Particularly suitable for single-sided structures. However, due to the complexity and diversity of concrete structures, thickness detection is complicated.
Vertical reflection method The vertical reflection method uses high-power high-frequency sound waves to transmit pulse signals into the concrete, and then uses an acceleration (or velocity) detector to receive the signal. The offset between transmission and reception is almost zero. a detection method. Using the waveform characteristics of vertical reflection and processed by a variety of signal technologies, the presence or absence of defects and their depth can be determined.
Rayleigh surface wave method The Rayleigh surface wave method uses the secondary waves generated by the mutual interference and superposition of longitudinal waves and shear waves to form a curve shape that propagates along the surface of the medium to determine anomalies in the medium. The Rayleigh surface wave curve propagating in a uniform continuous medium should be smooth and continuous. If there are discontinuous discontinuities or non-uniform anomalies in the medium, the curve will be interrupted and a "zigzag" shape will appear.
Infrared thermal imaging infrared detection technology is a newly developed non-destructive testing method that is effective in identifying the bonding quality of building exterior walls. It uses infrared radiation to detect and measure the surface of objects or materials. The exterior wall has bonding defects such as peeling and hollowing, which appear as "hot spots" on the infrared thermal image. The detection results are intuitive and reliable. The infrared thermal image characteristic map of the exterior wall is analyzed and theoretical calculations are performed. The bonding quality of the exterior wall can be determined. It has the advantages of non-contact, long-distance, real-time, fast, and full-field measurement, but the instrument cost is high.
Photometry With the combination of digital image processing technology, photometry is increasingly used in structural testing. The main features are high test accuracy and full-field measurement, but the requirements for on-site test conditions are relatively high. Including holographic interference method, speckle method, moiré method, etc. Holographic interference method is to form an interference fringe pattern by comparing the interference measurements of two or more waves (at least one of these waves is a holographic reproduction wave), and measure physical quantities through the interpretation of the interference fringe pattern.
The speckle method uses light with good coherence to illuminate the rough surface of an object to form randomly distributed light and dark points (speckles) in the space in front of the surface, which move with the deformation of the object's surface, and record two dislocations before and after the object deforms. Speckle pattern, comparing the changes of speckle pattern before and after deformation, can detect the displacement or strain of each point on the object surface with high precision. Moire detection technology uses the relative changes of the specimen grid and the reference grid to form an optical moire pattern to detect the surface displacement or strain of the object. It has been widely used in deformation analysis of engineering structures. With its combination with digital image processing technology, the detection speed and analysis accuracy have been greatly improved.
The laser detection method is also a new detection method, and the laser detection system has many advantages. First, it can detect multiple detection points at a single location. Secondly, the laser system has no specific target requirements, and the traditional method of entering inaccessible links to complete the task has become a thing of the past. Finally, the laser detection system is easy to install and can quickly obtain detection results. The detection accuracy of laser instrument is very high, easy to operate, and by combining with computer, the results can be obtained more easily and accurately.
Optical fiber detection technology is a new technology developed in the late 1990s. It uses external factors to cause changes in characteristic parameters such as light intensity, phase, polarization state, wavelength or frequency when light propagates in the optical fiber. , thereby detecting and transmitting signals to external factors. This new technology has been used in aviation, aerospace and other fields. It uses optical fiber sensors embedded in composite materials to detect the strain inside the structure and detect the damage of the structure. It has fully demonstrated that this is an effective new technology for non-destructive testing. Compared with the traditional strain gauge detection technology currently used in detection, this new technology has obvious advantages and shows great development potential.
Magnetic detection method Leakage magnetic field detection technology is a magnetic detection technology that has developed rapidly in recent years. It uses magnetic sensitive components and electronic instruments to detect and analyze the leakage magnetic field formed by component defects, such as crack depth. and width analysis, the magnetization level of the detection object must reach at least the saturation state, the detection device can scan a large area of ??the detection object, and the detection efficiency is high.
Metal magnetic memory detection method. Compared with traditional non-destructive testing methods, the main advantages of metal magnetic detection methods are: traditional detection methods can only be used to detect existing defects, while metal magnetic methods It can predict the dangerous areas where defects may occur, that is, the areas where maximum stress and deformation are concentrated, so that timely measures can be taken to prevent damage and accidents. Since the self-magnetization phenomenon of the detection object can be used, there is no need for artificial magnetization devices; it can keep the original metal The detection is carried out under the condition, so there is no need to specially clean the detection object, and there is no need to use coupling technology (such as when using ultrasonic detection). Therefore, this method is more suitable for production sites, field conditions and census operations; the detection sensitivity is higher than other magnetic detection methods; the instrument is small in size, light in weight, has an independent power supply and recording device, is easy to carry, easy to use, and has high detection efficiency .
Ultrasonic pulse method engineering concrete structures often produce cracks due to various reasons. The presence of cracks compromises the safety and durability of the structure. Usually, the location of cracks can be found through visual inspection, and the depth of cracks can be detected using ultrasonic method. According to the principles of acoustics, when sound waves encounter the interface of different media during propagation, reflection and transmission will occur. Due to the reflection of sound waves by cracks, when there are defects and damage in structural concrete, ultrasonic pulses will be diffracted when passing through the defects, and the propagation sound speed will be smaller than that of defect-free concrete of the same material, and the sound time will be longer; Reflection occurs on the defect interface, so the energy is significantly attenuated, the amplitude and frequency are significantly reduced, and the signal receiving the waveform is flat or even distorted. The existence of cracks can be found by comparing the received signal amplitude with that of normal parts of the structure. The application of ultrasonic method is particularly prominent in the detection of cracks in underwater parts of structures.
Dynamic detection method Dynamic detection method is the application of vibration inversion theory in engineering. Under excitation modes such as pulsation and starter vibration, by measuring the frequency and vibration shape of the structure and other parameters, The interlayer stiffness is obtained according to the system identification theory.
The basic problem of structural dynamic detection is to identify the current state of the structure based on its dynamic response, which is divided into structural modal parameter identification (natural frequency and mode shape) and structural physical parameter identification stiffness.
Dynamic detection methods can be divided into sinusoidal steady-state excitation, environmental excitation detection methods and local excitation detection methods.
Sinusoidal steady-state excitation Sinusoidal steady-state excitation is an excitation method that uses a certain device to exert stable simple harmonic vibration on the structure. The advantages of sinusoidal steady-state excitation are concentrated excitation energy, high signal-to-noise ratio, and high test accuracy. However, the test requires special excitation equipment, which is expensive and may affect the normal use of the building during the test.
Environmental excitation detection method The tiny vibrations of the ground environment around the building (called ground pulsation) and the flow of the air environment (i.e. wind) can cause vibrations of engineering structures, and the ground that causes vibrations of structures can be detected. Pulsation and wind as environmental excitations. According to the way of excitation, it can be divided into natural pulsation, artificial pulsation, ground vibration and pulsating wind. The advantages of environmental random vibration are: the test is simple, no vibration equipment is required, it is not restricted by the shape and size of the structure, and the test cost is low. But the recording signal-to-noise ratio is low. The test time is long.
For the vibration test of high-rise buildings, natural ground pulsation and pulsating wind are more suitable. Due to the high chance of earthquakes, it is not suitable to use ground vibration as the excitation source.
Local excitation detection of local damage in engineering structures often has little impact on the overall performance of the structure. Coupled with the impact of structural dynamic response measurement, it is very difficult to dynamically detect structural damage that targets the entire structure. Sometimes you don't even get accurate results. The local vibration of the structure reflects the local characteristics of the structure more accurately than the overall vibration of the structure. Therefore, using the local vibration response of the structure helps to accurately identify the local characteristics of the structure. Using detection at the overall level, we first roughly determine the location of structural damage, and then excite it to measure the local vibration response of the structure.
The environmental excitation detection method can better grasp the overall performance of the structure and is easy to implement. The local excitation detection method can accurately grasp the physical parameters of local components of the structure. Structural dynamic detection methods are not limited by the size and concealment of the structure, as long as dynamic response sensors are installed at accessible structural locations. With the improvement of detection instrument technology, the accuracy of results is getting higher and higher. At present, efficient modular and digital structural dynamic response measurement technology has provided solid and effective technical support for structural dynamic detection methods. Although the structural dynamic detection method has fewer application conditions and is more efficient, the reliability of the structural dynamic detection results cannot sometimes be guaranteed due to limitations in the quality and quantity of structural dynamic measurement signals.