With the rapid development and maturity of nondestructive testing technology, the role of nondestructive testing technology in building engineering is increasingly obvious. It is not only one of the detection and analysis methods of engineering accidents, but also a tool for engineering quality control and reliability monitoring in the process of structure use. It can be said that it has its place in the whole process of construction, acceptance and use.
According to its principle, nondestructive testing of concrete strength can be divided into semi-destructive method, non-destructive method and comprehensive method. The common methods are rebound method, ultrasonic method, pull-out method, core drilling method and ultrasonic rebound synthesis method. In recent years, although there is no new breakthrough in basic theory, some achievements have been made in the research of detection methods, data processing and evaluation technology. Not only the detection range is wider, but also the detection accuracy is improved. For example, at present, recycled concrete can be detected by rebound method, and even freeze-thaw concrete can be evaluated by ultrasonic-rebound comprehensive method. Compared with other methods, the comprehensive method has made great progress, which indicates that the comprehensive method will be the main research direction of detection methods in the future. The latest research on data processing and evaluation technology mainly includes: using inverse regression model to introduce interval estimation, and introducing artificial neural network (ANN) technology which is mature in the field of nonlinear data analysis.
First, the basic theory and testing instruments of nondestructive testing of concrete strength.
The nondestructive testing method of concrete strength must be based on the relationship between concrete strength and appropriate physical quantities. In order to find physical quantities that are closely related to the strength of concrete and can be directly measured on structures or components by nondestructive methods, regression methods and deduction methods are often used. Although deductive method is more universal than regression method, the former is still widely used because of the lack of research on the relationship between strength and physical quantity in the past. In recent years, with the development of basic science, it has laid a foundation for the study of the theoretical relationship between concrete properties and physical quantities.
At present, the commonly used nondestructive testing methods mostly calculate the strength of concrete through its stress-strain characteristics, compactness and porosity. Therefore, it is necessary to establish the theoretical relationship between stress-strain characteristics of concrete and void ratio and strength. So far, from the theoretical research results, we can know that the strength of concrete is not only a function of elastic properties, but also a function of plastic properties and experimental conditions. In order to improve the accuracy of nondestructive testing, these two factors must be reflected at the same time. At the same time, the research results also show that although porosity is the main factor affecting the strength when measuring the strength of concrete with material density or porosity index, it is not enough to reflect porosity only, and the potential strength and pore structure of materials must be taken as important reference factors to improve the detection accuracy. This points out the direction for nondestructive testing methods based on porosity, such as ray method and infiltration method. Although the research of basic theory is difficult and slow, and few people have studied it in recent years, it is an indispensable part of the overall research of nondestructive testing technology and should be paid enough attention.
With the development of detection means and electronic technology, nondestructive testing instruments have also developed to a new level. At present, the research trends of test instruments at home and abroad mainly include the following trends: diversification, intelligence, specialization, miniaturization, integration and instrument intensification of sensing systems. The research of testing instruments is also the basis of the development of nondestructive testing technology. At present, the development level of China's electronic industry is enough to provide all kinds of advanced instruments, but how to closely combine electronic technology with testing technology is a problem we need to solve at present.
Second, the nondestructive testing method of concrete strength
In recent years, the research on nondestructive testing methods of concrete strength has made some progress. Below we will explain it from several aspects: semi-destructive method, non-destructive method and comprehensive method.
1. Semi-breakage method
Semi-failure method is to directly carry out failure experiments on structures or components, or directly drill core samples for failure experiments without affecting the bearing capacity of structures or components. Then, according to the correlation between the test value and the standard strength of structural concrete, it is converted into a standard strength conversion value, and the estimated value or characteristic strength of the standard strength value is calculated accordingly. These methods include pull-out method, core drilling method and blasting method. This method is characterized by obtaining the actual anti-failure ability of structural concrete through local destructive tests, so it is intuitive and reliable, and the test results are easily accepted by people. Its disadvantage is that it causes local damage to the structure and needs to be repaired, which is not suitable for large-scale comprehensive inspection.
Because the strength of pull-out method is often discrete, the reliability is not as good as that of core drilling method, and the test results of target shooting method are obviously affected by aggregate, so core drilling method is the most widely used semi-failure method in engineering at present. Therefore, the Technical Specification for Testing Concrete Strength by Core Drilling Method (CECS03:08) was formulated. But it is not suitable for large-scale use in the same structure, because it will cause local damage to the structure or components. Therefore, the combination of core drilling method and other nondestructive testing methods is advocated at home and abroad. On the one hand, the uniformity of concrete is detected by nondestructive method to reduce the number of drilling cores, on the other hand, the detection results are corrected by nondestructive method to improve reliability. Therefore, in recent years, there is little research on testing concrete strength by drilling core method alone, and more research focuses on the combination of drilling core method and other nondestructive testing methods.
In recent years, with the improvement of people's requirements for structural safety and earthquake resistance, the reinforcement ratio of members is getting higher and higher, and the spacing between steel bars is getting smaller and smaller. For example, the spacing between longitudinal steel bars in concrete beams and columns and stirrups in dense areas is mostly below 100mm, and some of them are below 75 mm According to CECS03:88, it is difficult to avoid not taking steel bars from core samples, and the influence of steel bars on the compressive strength of core samples is a complicated problem that has not been solved, which greatly affects the detection accuracy and reliability of core drilling method. Therefore, it is the main research direction of core drilling method to study the detection of structural concrete with small diameter core samples. The experimental study shows that it is feasible and effective to detect commercial concrete and high strength concrete with small diameter core samples, which can be popularized and applied in engineering practice.
2. Nondestructive method
Based on the correlation between concrete strength and some physical quantities, nondestructive method tests these physical quantities without affecting any performance of concrete in structures or components, and then calculates the standard strength conversion value of the tested concrete according to the correlation, and accordingly calculates the estimated value or characteristic strength of the standard strength value. There are rebound method, ultrasonic method, curing method and so on. This method is characterized by convenient test and low cost, but the reliability of its test results depends on the correlation between the measured physical quantity and strength. Therefore, a strict correlation formula or calibration curve must be established before testing. Because this correlation is often affected by many factors. Therefore, the established related formulas have their limitations, and when the conditions change, they should be revised accordingly to ensure the reliability of the calculation results.
Among nondestructive testing methods, rebound method has become one of the most widely used nondestructive testing methods in China because of its simple instrument structure, simple method, good correlation between test values and concrete strength under certain conditions and low testing cost. Therefore, in recent years, most of the research focuses on the related fields of rebound method. At present, the rebound method can be used not only for the detection of conventional concrete structures, but also for the detection of recycled concrete. According to the different replacement rate of recycled coarse aggregate, the researchers used ordinary concrete rebound instrument to test the compressive strength of recycled concrete blocks. The results show that the rebound method can be used to detect the compressive strength of recycled concrete. In addition, through the comparative analysis of various regression models, a unified formula of strength curve of recycled concrete is obtained. Quantitative analysis shows that the test accuracy meets the requirements of concrete quality control. At the same time, in order to reflect the influence of the replacement rate of recycled coarse aggregate, the researchers also give the strength curve and regression equation classified by the replacement rate of recycled coarse aggregate. Finally, the national unified curves of recycled concrete and ordinary concrete are compared. The results show that the surface hardness of recycled concrete is less than that of ordinary concrete, but with the increase of compressive strength, it increases faster than that of ordinary concrete. At the same time, the rebound method is applied to the detection of prestressed steel cylinder concrete pressure pipeline. Through comprehensive comparative analysis, it is proved that rebound method can be used to detect the strength of pipeline concrete. But at present, the rebound method is still in the experimental stage, which can't really solve the practical engineering problems. In addition, the research on rebound correction is also very active. At present, the rebound results are mainly corrected by core drilling method, and some scholars think that the rebound correction by core drilling method can be regarded as a comprehensive method of core drilling and rebound. Therefore, the research results in this area will be introduced in the next part.
3. Integrated approach
The so-called comprehensive method is to use two or more nondestructive testing methods to obtain multiple physical parameters, establish the comprehensive correlation between strength and multiple physical parameters, and comprehensively evaluate concrete strength from different angles. Because the comprehensive method adopts many physical parameters, it can comprehensively reflect various factors that constitute the strength of concrete and offset some factors that affect the correlation between strength and physical quantity, so it has higher accuracy and reliability than the nondestructive testing method of single physical quantity. At present, the commonly used synthesis methods include ultrasonic-rebound synthesis method, core drilling-rebound synthesis method and so on. Among them, the ultrasonic-rebound synthesis method has been widely used in China, and the corresponding technical regulations have been formulated (CECS02:08).
In recent years, with the continuous improvement of people's requirements for test results, the comprehensive method has been paid more and more attention. At present, the ultrasonic-rebound comprehensive method can not only detect the strength of negative temperature concrete, predict the early strength of concrete, but also evaluate the compressive strength of freeze-thaw concrete. Borehole rebound synthesis method has a good application in detecting existing structural concrete and commercial concrete. At the same time, in the study of the correction coefficient η in the drilling rebound synthesis method, it is found that it obeys the lognormal distribution, which provides the corresponding theoretical basis for eliminating abnormal data in practical engineering. At present, there are other studies on comprehensive methods, such as "rebound-ultrasonic-pull-out" comprehensive method to detect the strength of concrete, and core peeling method to detect the bond strength of repaired concrete on site. In order to better understand the development status of various commonly used detection methods, the following table compares them:
Third, data processing.
1. Data fusion idea and interval estimation based on inverse regression model
In order to further improve the accuracy of the test results, the estimated results of rebound method become the acceptance criteria of the project. The latest research not only introduces the idea of data fusion and inverse regression model, but also gives the confidence interval of concrete strength of building structures under a certain degree of confidence by using the confidence test theory, and gives the criteria for judging whether the concrete strength is qualified or unqualified, forming a new system for testing the concrete strength of building structures by inverse regression rebound method. Compared with the traditional regression model and point estimation method, the new method not only improves the accuracy of test results, but also conforms to the engineering practice, and can make quantitative judgment on concrete strength.
2. Artificial neural network technology
In view of the limitation of regression method in the data analysis of concrete strength nondestructive testing, the latest research introduces the mature artificial neural network (ANN) technology in the field of nonlinear data analysis. The results show that artificial neural network is suitable for processing this kind of data, which makes up for the deficiency of regression method in some aspects. Table 2 lists the details and substantive differences between regression method and neural network method.
In addition, the high-dimensional data difficult to be processed by regression method can be used as a way to improve the error of artificial neural network. Therefore, the introduction of artificial neural network will add new related variables to the data analysis of concrete strength nondestructive testing, so as to study the related problems of concrete strength nondestructive testing and data processing more deeply or widely. However, at present, the application of artificial neural network in nondestructive testing data processing of concrete strength is less, and there are still some problems that need to be further improved. The processing results can provide reference and supplement for the processing results of regression method.
Four. conclusion
1. From the current research results, the comprehensive method has become the main research direction because of its own advantages. At the same time, with the further improvement of people's requirements for testing technology, it is believed that the comprehensive method will be the main trend of nondestructive testing of concrete strength in the future.
2. In terms of data processing, with the increasing demand for the accuracy of test results. It is sometimes difficult to meet our requirements by using traditional regression models and point estimation methods. Therefore, finding more effective and accurate data processing methods is the main direction of future research.
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