With the improvement of construction market management, the sixteen-character guiding ideology "separation of inspection and evaluation, strengthening acceptance, improvement means and process control" in the unified standard for construction quality acceptance of construction projects has been realized in the concrete application of projects, and the guiding ideology of "improvement means" has also shown an increasingly important position in the acceptance of construction quality. Judging by scientific test data, avoiding reducing the interference of human factors and the influence of subjective evaluation, and increasing the control of test means in the "parallel inspection" of supervision specifications all provide a guarantee for the structural integrity and improvement of the use function of engineering quality acceptance.

Keywords: ultrasonic rebound, concrete, strength, technical application

In 1960s, Romanian Academy of Architecture and Building Economics took the lead in putting forward the ultrasonic rebound comprehensive method to detect concrete strength. Since 1976, China has also carried out a comprehensive and large number of experiments on this method, and in 1988, the Technical Specification for Testing Concrete Strength by Ultrasonic Rebound Comprehensive Method (CECS 02: 88), a standard of China Engineering Construction Standardization Association, was formulated. In 2005, China Research Institute of Building Science revised the Technical Specification for Testing Concrete Strength by Ultrasonic Rebound Comprehensive Method again, and implemented the Technical Specification for Testing Concrete Strength by Ultrasonic Rebound Comprehensive Method CECS 02:2005 on June 5438+February 1 day, 2005. Short for "comprehensive method". In order to implement the comprehensive method, the national standard "Standard for Inspection and Evaluation of Concrete Strength" (GBJ 107-87) stipulates: "When there is doubt about the representativeness of concrete specimen strength at that time, the strength of concrete in the structure or member can be estimated by drilling the specimen from the structure or member or adopting nondestructive testing methods according to the provisions of relevant standards". Code for Acceptance of Construction Quality of Concrete Structure Engineering (GB 50204-2002) also stipulates that "when the strength evaluation of concrete specimens is unqualified, nondestructive or partially damaged inspection and measurement methods can be adopted, and the strength of concrete in structural members can be estimated according to the provisions of current national standards and used as the basis for treatment".

1. Definition and advantages of comprehensive law

Ultrasonic rebound testing technology of concrete strength, referred to as "comprehensive method", is one of the nondestructive testing methods of concrete strength. The so-called comprehensive method is to use two or more nondestructive testing methods (ultrasonic pulse velocity-rebound value) to obtain a variety of physical parameters, establish the comprehensive correlation between strength and various physical parameters, and thus comprehensively evaluate the strength of concrete from different angles. Because the comprehensive method adopts many physical parameters, which can comprehensively reflect various factors that constitute the strength of concrete and offset some factors that affect the correlation between strength and physical quantity, it has higher accuracy and reliability than the nondestructive testing methods of single physical quantity (rebound method and ultrasonic method). Single rebound method and ultrasonic method, the same influencing factor has different effects on different methods, and some are even completely opposite. For example, when the concrete strength grade is the same, and the production technology and mix ratio are basically the same, when the strength of standard concrete specimens is tested by single rebound method and ultrasonic method, the concrete age and concrete humidity are found. For rebound method, with the increase of concrete age, the concrete surface hardens, which makes the rebound value higher, while the surface hardness of wet concrete decreases and the rebound value decreases significantly. For the ultrasonic method, the situation is the opposite. With the increase of age, the interior of concrete tends to be dry gradually, and the propagation speed is low, and the propagation speed of wet concrete is faster than that of dry concrete. If the two single methods are combined, the effects of concrete age and humidity can cancel each other out without consideration. In this way, the comprehensive method makes use of two physical indexes that have the least damage to the component, one is to use the physical indexes inside the component, and the other is to comprehensively reflect the compressive strength of concrete from the surface hardness. Because it is the strength index of concrete in many aspects, it has a good correlation. In addition, the comprehensive method has certain advantages over the semi-damaged core drilling method, which is not suitable for large-scale use in structures because it will cause local structural damage to structures or components. In this way, the comprehensive method of "ultrasonic pulse velocity-rebound value" has become an important development method of nondestructive testing technology for concrete strength.

Engineering example: detection results of existing problems (comparison of comprehensive methods)

Sample block

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Determination of strength fcu(MPa) error (er)% specimen synthesis method ultrasonic rebound method ultrasonic rebound method II. Engineering application of "comprehensive method" (1) In supervision engineering practice, it is often found that the quality of concrete may be affected due to lax construction control or some accidents in the construction process, and the sampling, production, maintenance and compressive test of reserved test blocks do not conform to the provisions of relevant technical regulations or standards. It is suspected that the strength of reserved test blocks cannot represent the actual strength of structural concrete. If the reserved test block is lost, the same condition maintenance and the standard maintenance are not true, the comprehensive method should be given priority.

(2) When it is necessary to know the strength growth of concrete in the construction process to meet the requirements of structures or components for concrete strength, such as formwork removal, leaving the curing pool, leaving the factory, hoisting, tensioning or releasing of prestressed tendons, and loads during construction, etc. Such as winter construction. , should be based on comprehensive methods.

(3) When the existing structure needs to be repaired, storey added, demolished or affected by disaster factors, the comprehensive method can be given priority to estimate the strength of the original concrete, providing basic strength parameters and other design basis for the reconstruction and reinforcement design.

(4) In addition, the international inspection of building structures has been extended to the inspection in the design reference period, and the comprehensive method based on concrete strength should be adopted.

3. Establish a power curve that suits you.

The relationship curve between compressive strength of concrete block and nondestructive testing parameters is called strength measurement curve. Ultrasonic rebound synthesis method is used to measure the strength curve. Based on the compressive strength, ultrasonic sound velocity and rebound value of concrete specimens, response mathematics is selected to fit the correlation between them. Establish a power curve that suits you. According to the local (or a project) representative concrete raw materials, molding and curing technology and age, a certain number of test blocks are made, and then nondestructive testing is carried out before damage detection, and the strength curve is established. The strength curve thus established

The field adaptability and strength measurement accuracy are better than the unified strength measurement curve. In addition, while each region has established its own strength curve, Baoding has also established its own strength curve.

4. The Technical Specification for Testing Concrete Strength by Ultrasonic Rebound Comprehensive Method (CECS 02:2005) has added some engineering applications.

(1) The rebound hammer and ultrasonic flaw detector used in the comprehensive method must have product certificate and verification certificate. For concrete ultrasonic detectors, both analog and digital ultrasonic detectors that meet the verification requirements of JG/T5004 Concrete Ultrasonic Detector can be used for comprehensive detection. The validity period of rebound instrument verification is half a year, and the validity period of concrete ultrasonic flaw detector verification is one year.

(2) The comprehensive method adds new contents such as ultrasonic leveling and angle measurement:

Angle measurement method: when the concrete to be measured can only provide two adjacent surfaces, although it cannot be measured, the Ding Angle method can be used for detection. That is, a pair of F and S sensors are respectively coupled to two adjacent surfaces of the tested part for point-by-point testing, and the axes of the two sensors form an included angle of 90. For example, detect concrete columns with obstacles such as walls and pipes beside them.

Plane survey method. When the concrete testing part can only provide one testing surface, the flat plate testing method can be used for testing. A pair of F and S sensors are placed on the same surface of the tested structure, and point-by-point detection is carried out at a certain testing distance. For example, detecting structures such as roads, airstrips and tunnel walls. In addition, ultrasonic testing should give priority to face measurement or angle measurement. When the measured component does not have the conditions for opposite measurement or angle measurement, the single-sided flat measurement method can be used. In horizontal measurement, the connecting line between the two transducers should keep an included angle of 40 ~ 50 with the nearby reinforcement line to reduce the influence of reinforcement, and the ultrasonic ranging should be 350 mm ~ 450 mm ... Because the ultrasonic sound speed in the comprehensive strength conversion table is calculated by the regression of the measured longitudinal sound speed, for example, most of the signals received by one-sided flat measurement are surface waves, so the strength conversion table cannot be read directly, and the strength conversion table needs to be modified.

(3) In the revision of the strength curve, the age of the strength curve is prolonged, the strength range is increased, and the contents of high strength concrete and pumping concrete are added. Rule 6. 0. 1:6 o'clock 7 ~ 2000d age (original age 7 ~ 730d, if it exceeds this age, concrete core samples can be drilled for modification); VII. Concrete strength: 10 ~ 70 MPa (original C 10~C50 ~ C50), which makes the time and scope of concrete inspection wider.