The non-destructive testing of concrete strength refers to the method of measuring the relevant physical parameters of concrete through testing instruments, and then calculating the strength of the tested concrete based on the correlation between these physical parameters and the compressive strength of concrete. The strength equation of its correlation is obtained by conducting failure tests on the same concrete standard test block, and then conducting regression analysis and mathematical processing on the test data, which is commonly known as the strength curve.

Non destructive testing can be divided into non destructive testing methods and semi destructive testing methods. Since the local damage caused by the semi damage detection method does not pose a threat to structural safety, from a macro perspective, the semi damage detection method can also be classified as non damage detection.

Non destructive testing methods

The non-destructive testing method is to measure the relevant physical quantities based on the relationship between the compressive strength of concrete and other physical quantities of concrete, without affecting the performance of the structure or component concrete. Then, according to the strength measurement curve, the standard strength conversion value of concrete is calculated, and the fixed value or characteristic strength of the standard value of concrete strength is finally obtained based on statistical principles. This type of method includes rebound method, ultrasonic pulse method, ray absorption and scattering method, maturity method, etc.

One is the rebound method

The rebound method refers to the method of using a rebound instrument to test the compressive strength of ordinary concrete structures or components. The implementation process is as follows: a spring driven heavy hammer is used to hit the surface of the concrete through a rebound rod (transmission rod), and the distance at which the heavy hammer rebounds (x in Figure 1) is measured. The rebound value (the ratio of rebound distance to the initial length of the spring) is used as an indicator related to strength, Convert the strength value of the concrete component from the established regression equation or calibration curve. The detection principle is shown in Figure 1.

The rebound method is easy to operate, fast and economical, and has considerable accuracy, so it is widely used in the field of concrete testing. However, there are many factors that affect the accuracy of rebound method strength measurement, such as instrument standard status, operating methods, on-site conditions, component selection, measurement area and point layout, and calculation methods. Therefore, in order to improve the accuracy of detection, it is necessary to strengthen the analysis of these influencing factors, and reasonably formulate and select formulas.

At the same time, it should be noted that the current research results of rebound method in China are mainly applicable to ordinary concrete. At the same time, when measuring the strength of concrete in on-site structures or components, the rebound strength value only represents the surface quality of concrete. Therefore, when using rebound method, it is necessary to ensure that the surface quality of concrete components is basically consistent with the internal quality.

The second is ultrasound method

The ultrasonic method is a method of estimating the strength of concrete by measuring the average speed of ultrasonic propagation within a distance measurement, as shown in Figure 2. In engineering, the method of establishing statistical strength curves related to the relationship between ultrasonic velocity and compressive strength of concrete in specimens is commonly used to detect and evaluate the mechanical properties of concrete.

There are many factors that affect the measurement of ultrasonic velocity in concrete, such as the cross-sectional size, temperature and humidity of the specimen, reinforcement, aggregate, water cement ratio, age, pouring direction, and internal defects. Therefore, ultrasonic velocity is a comprehensive indicator that reflects its composition. This requires that when establishing a calibration curve, the technical conditions should be as close as possible to the actual testing environment.