Evaluation and Prospect of On-site Inspection Methods for Building Structures

On-site inspection　　of building structures can be divided into on-site inspection of concrete structures and on-site inspection of steel structures according to different structures and materials. This article attempts to analyze and look forward to its current status and development trends.

　　1. On-site detection method of concrete structure

　　The macro performance test method of concrete structure is "specimen test". This method uses the measured value when the specimen is destroyed as the basis for judging the performance of concrete, which is more intuitive and is called destructive test. Since the quality, stress condition and various conditions of the concrete in the specimen and the concrete in the structure cannot be completely consistent, it is not very suitable for on-site testing of building structures.

　　In the 1930s, non-destructive testing methods for concrete were developed, such as the rebound method and the ultrasonic pulse method, which allow on-site testing without damaging the concrete.

　　1.1 Rebound method

　　The rebound method is to use a rebound hammer to hit the concrete surface. The change in the rebound energy of the instrument's hammer reflects the elastic and plastic properties of the concrete. The surface hardness of the concrete is measured to infer the compressive strength. It is a non-destructive test method commonly used in on-site inspection of concrete structures, and China has formulated specifications.

　　The main advantages of the rebound method are: simple instrument structure, easy to master the method, high detection efficiency, low cost, and fewer influencing factors. However, there are still certain shortcomings: the rebound value is affected by the carbonization depth and test angle, and the type of stone also has an impact on it. Different corrections must be made to the rebound value. For concrete in areas with quality questions, other methods must be used for further testing.

　　1.2 Ultrasonic pulse method

　　The ultrasonic pulse method is used to detect the strength of concrete, which is to test the propagation parameters of ultrasonic waves in concrete, find out the relationship between the compressive strength of concrete and these parameters, and determine its compressive strength.

　　Concrete is an anisotropic multiphase composite material with widely distributed interfaces between mortar and aggregate and various defects inside. This makes the propagation of ultrasound in concrete much more complicated than in a uniform medium, resulting in reflection, refraction and scattering phenomena, and a large attenuation. Therefore, although the ultrasonic pulse method for testing concrete strength can detect problems inside the concrete, the quantitative relationship between the test instrument, transducer and concrete strength and the ultrasonic propagation speed is affected by the raw material properties and mix ratio of the concrete; the influence of the temperature and moisture content of the test specimen, etc. Only by comprehensively considering various factors and conditions and establishing a special curve with a high degree of fit can a relatively satisfactory accuracy be obtained when used.

　　1.3 Ultrasonic rebound comprehensive method

　　The ultrasonic-rebound comprehensive method is a non-destructive testing method based on the relationship between ultrasonic propagation and rebound value and the compressive strength of concrete. It uses sound velocity and rebound value to comprehensively reflect the compressive strength of concrete.

　　The ultrasonic rebound comprehensive method has overcome the shortcomings of evaluating concrete strength with a single indicator to a certain extent. It corrects the influence of gravel and test surface from the test results. For the comprehensive analysis of multiple indicators, it can more comprehensively reflect the effects of various factors related to concrete strength and improve the test accuracy.

　　1.4 Core drilling method

　　The core drilling method is different from the previous three methods. It uses a special coring machine to directly drill cylindrical concrete core samples from the structure or component being tested, and infers the compressive strength of concrete based on the compressive test strength of the core sample. It is a relatively intuitive and reliable method for testing concrete strength. Since it is necessary to take samples from the structure and cause local damage to the original structure, it is a semi-destructive test method for on-site testing.

　　The core drilling method is adopted by many countries. Russia, the United States, the United Kingdom, Japan, France and other countries have formulated their own standards, and the International Organization for Standardization has also proposed a corresponding international standard draft (ISO/DIS7034).

　　1.5 Extraction method

　　The pull-out test is also a semi-destructive detection method. It uses a metal anchor to be embedded in the unhardened concrete casting component, or drills a hole in the hardened concrete component and embeds an expansion bolt. Then the tension when the anchor or expansion bolt is pulled out is tested. The pull-out strength of the concrete is determined by the projected area of ​​the frustum-shaped concrete block when it is pulled out, and the compressive strength of the concrete is calculated from this.

　　The pull-out method has been widely used in the United States, Canada, Denmark and other countries. The International Organization for Standardization (ISO) has proposed a draft international standard (ISO/DIS8046) for determining the pull-out strength of hardened concrete.

　　1.6 Ultrasonic pulse method

　　The ultrasonic pulse method is the most widely used method for detecting internal defects and operations of concrete. When there are defects or damage in structural concrete, the ultrasonic pulse will produce diffraction when passing through the defect. The propagation speed of sound is lower than that of defect-free concrete of the same type of material, and the sound time is longer. Reflection occurs on the defect interface, so the energy is significantly attenuated, the amplitude and frequency are significantly reduced, the waveform of the received signal is flat, and even distorted. Therefore, the defects and damage of concrete can be identified. This can provide a reliable basis for identification for structural reinforcement and maintenance in project acceptance, accident handling and reliability appraisal of existing buildings.

　　It should be noted that only ultrasonic testing instruments with waveform display can be used for flaw detection, while digital sound velocity meters without waveform display can only provide sound time and sound velocity as the only basis for judgment, which can easily lead to misjudgment.

　　1.7 Steel bar location and steel bar corrosion detection in concrete structures

　　1.7.1 Detection of steel bar position

　　When conducting reliability diagnosis on existing concrete structures and evaluating the construction quality of new concrete structures, it is required to determine the location and condition of the steel bars. When using the core drilling method to test the concrete strength, the steel bar position detection is often performed to avoid the steel bars. The commonly used electromagnetic induction method is more suitable for the situation where the reinforcement is sparse and the concrete cover is not too thick. When the steel bars are located in the same plane or in different planes with a large distance, the measured results are more satisfactory.

　　1.7.2 Detection of steel bar corrosion

　　If the concrete is of poor quality, the working environment is bad or other reasons cause various cracks in the structure, it will cause the steel bar to rust. The steel bar rust will cause the concrete protective layer to crack and peel off, and the effective area of ​​the steel bar to weaken, which will directly affect the bearing capacity and service life of the structure. When conducting structural appraisal and reliability diagnosis of the built structure, the steel bar rust must be detected.

　　The half-cell method is used to measure the potential difference between the steel bar surface and the probe to determine the possibility and degree of steel bar corrosion.

　　2. On-site detection method of masonry structure

　　Masonry structures mainly refer to brick masonry. The strength of masonry is determined by the strength of bricks and mortar or the strength of masonry test blocks made during construction. The traditional testing method is to directly cut samples from the masonry structure and conduct compressive strength tests. However, the characteristics of masonry structures make sampling difficult, and the disturbance during sampling will cause significant damage to the samples, thus affecting the test results. Therefore, the on-site in-situ non-destructive or semi-destructive test methods of masonry structures are naturally valued, and are widely studied and applied in engineering practice. [page]

　　2.1 Indirect determination of masonry strength

　　The strength of masonry is directly related to the strength of mortar and bricks. The compressive strength of masonry can be determined by the strength grade of mortar and bricks. The indirect measurement method is to use special instruments and special test methods to measure a certain strength index of mortar and bricks or a physical parameter related to material strength, and thus indirectly measure the strength of masonry.

　　(1) Impact method. Based on the basic principle that the work consumed when an object is broken is proportional to the new surface area created during the breaking process, and the empirical formula between the surface area increment per unit work and the compressive strength established in advance, the strength of the mortar or brick sample is calculated.

　　(2) Rebound method. The basic principle of testing brick and mortar strength is the same as the rebound method of concrete strength testing, except that a special mortar rebound tester is used. Because the surface hardness of bricks has a good correlation with strength, this method is highly accurate, simple and applicable.

　　(3) Push-out method. The push-out method is also called the top-pushing method or the shearing method, specifically the single-brick single-shearing method. That is, the mortar on the top and two sides of a single brick is removed, leaving only the bottom surface, and it is "pushed out" with a special small jack. At the limit state, the shear strength of the bond between the brick and the mortar is measured, and the compressive strength is deduced based on the relationship between the shear strength and the compressive strength.

　　In addition, there are methods such as the cylinder pressure method and the point load method that measure the strength of masonry by measuring the strength of mortar. A lot of research has been done on these methods, and certain results have been achieved.

　　2.2 Direct determination of masonry strength

　　(1) Sampling test method. It mainly includes cutting method and coring method. The cutting method cuts large specimens, which are greatly disturbed during transportation, resulting in large discreteness of test results. It consumes a lot of manpower and financial resources and is limited to the handling of quality accidents in large masonry projects and the calibration of other methods. The coring method is to conduct compression and shear tests on core samples, which also greatly disturbs the masonry, and its test results are not consistent.

　　(2) In-situ testing method. It mainly includes the flat top method, the in-situ axial compression method and the in-situ shear method. The flat top method is an in-situ testing method for masonry strength by inserting a flat hydraulic force gauge into the excavated masonry mortar joint. It overcomes the shortcomings of the sampling method, but the equipment is complex, the allowable ultimate strain is small, and the determination of the ultimate strength of the masonry is limited. The in-situ axial compression method is an improvement on the flat top method. Its principle is the same as that of the flat top method. It determines the ultimate compressive strength of the masonry and calculates its standard compressive strength. The disadvantage is that the equipment is heavy and inconvenient to use. The in-situ shear method directly tests the shear strength of the masonry joint on the wall. Due to the restrictions on the test location, its application has certain limitations.

　　(3) Dynamic measurement comprehensive method. The dynamic measurement comprehensive method is the application of vibration inversion theory in engineering. Under the action of pulsation, vibration generator resonance, free release or impact and other excitation modes, by measuring the frequency and vibration mode of the masonry structure, the interlayer stiffness is obtained according to the system identification theory, and the axial compressive strength of each layer of masonry is calculated. Starting from the overall house, this method can not only obtain the strength of the masonry, identify the quality of the house, but also facilitate the safety assessment of the house. With the improvement of detection instrument technology and the optimization of algorithms, the accuracy of the results is constantly improving, and it has great development prospects.

　　(4) Microstructure method. When sound, waves, and rays propagate in a material, they will differ depending on the microstructure of the material, from which the strength of the material can be inferred. The methods used in my country to detect masonry houses include stress wave method and ultrasonic method. The stress wave method is not very accurate when measuring low-strength and high-strength mortar masonry. The ultrasonic method has many influencing factors, so the test results are not ideal and need to be further improved.

　　Steel structure is also an important structural form in my country's building structure. When appraising the built steel structure, the material of the steel structure is checked and the mechanical properties of the structural steel are understood. The most ideal method is to cut samples from the structure and determine its strength by tensile test, but it will damage the structure and affect its normal operation. Therefore, commonly used methods include surface hardness method, ultrasonic method, etc.

　　3.1 Surface hardness method

　　The surface hardness method is measured by the Brinell hardness tester. The strength of the steel is measured by the diameter of the indentation on the steel surface and the hardness standard sample when the steel ball at the end of the hardness tester is pressed, and the strength and yield strength of the steel are converted. This method is widely used in testing because the instrument is simple, easy to use, and basically harmless.

　　3.2 Ultrasonic testing of steel and weld defects

　　Ultrasonic testing of steel mostly uses the pulse reflection method. The ultrasonic pulse is transmitted into the material to be tested. If there is no defect, there will be no defect reflection wave, otherwise there will be partial reflection. Since the density of steel is much greater than that of concrete, in order to detect smaller defects, a higher ultrasonic frequency is required. This method is more conducive to on-site detection than magnetic particle testing and radiographic testing.

　　4. Prospects of structural on-site inspection technology

　　Structural on-site inspection technology has great application value in the detection and handling of engineering quality accidents. Judging from the development status at home and abroad, this technology involves application technologies of multiple disciplines and should be further studied and improved. Efforts and innovations should be made in the following aspects.

　　(1) Testing of new parameters and new performance indicators. With the development of materials science, many new materials are used in engineering and the design of building structures is constantly improving. Some new parameters and new performance indicators can illustrate the reliability of new materials and new structures. It is necessary to continuously study the testing methods of these parameters and indicators to serve engineering practice. This is the current trend of testing technology development.

　　(2) Introduction of new ideas, innovation and improvement of mathematical models. In the study of building structure detection methods, new ideas should be introduced. Not only macroscopic mechanics but also microscopic mechanics should be considered, and the problem should be looked at in depth and comprehensively. The empirical formulas used in existing detection methods have certain limitations. When establishing new mathematical models, more attention should be paid to their boundary conditions, the scope of use should be expanded, and the degree of fitting should be improved.

　　(3) Improvement of measuring instruments . With the popularization and development of computers, it is inevitable to improve measuring instruments. The miniaturization and intelligence of measuring instruments will continuously improve the test accuracy to ensure the needs of on-site testing.

　　(4) Improvement of operating methods. The operating methods of structural testing instruments should be increasingly simplified to make them more suitable for testing the quality of large-scale construction projects.