The basic principle of the rebound method to detect the compressive strength of concrete: there is a certain relationship between the hardness of the concrete surface and the compressive strength of the concrete. There is a certain relationship between hardness. In this way, the surface hardness of concrete can be tested by concrete resiliometer ,combined with the carbonation depth of concrete, so as to indirectly measure the strength of concrete.

The rebound method has been widely used in my country to detect the compressive strength of concrete. It has been proved that the compressive strength of concrete estimated by the rebound method is of great significance for dealing with engineering quality problems.

One. Quantity of sampling components

For components to be tested in batches, the number of random inspections should not be less than 30% of the total number of components in the same batch, and the number of components should not be less than 10. When the number of components in the inspection batch is greater than 30, the number of sampling components can be adjusted appropriately, but it must not be less than the minimum sampling quantity specified in the current relevant national standards.

Two. Survey area layout requirements

1. For general components, the number of survey areas should not be less than 10.

The number of inspected members is more than 30 and it is not necessary to provide the deduced strength of a single member, or the member whose dimension in one direction of the shear member is not greater than 4.5m and the dimension in the other direction is not greater than 0.3m; the number of test areas can be appropriately reduced, but not less than 5 cases;

2. The distance between two adjacent measurement areas should not be greater than 2m, and the distance between the measurement area and the end of the component or the edge of the construction joint should not be greater than 0.5m, and should not be less than 0.2m;

3. The measuring area should be selected on the side of concrete pouring where the hammer can be placed in the horizontal direction. When this requirement cannot be met, the hammer can also be placed on the concrete pouring surface or bottom in a non-horizontal direction;

4. The measurement area should be arranged on two symmetrical measurable surfaces of the component. If it cannot be arranged on the symmetrical measurable surface, it can also be arranged on the same measurable surface, and should be evenly distributed. The survey area should be arranged in the important parts and weak parts of the components, and the embedded parts should be avoided;

5. The area of the survey area should not be larger than 0.04 square meters;

6. The surface of the measurement area should be the original concrete slurry surface, which should be clean and smooth, and there should be no loose layer, laitance, grease, coating, honeycomb, and pockmarked surface;

7. Thin-walled and small components that vibrate when being bombarded should be fixed;

8. The survey area should be marked with a clear number, and it is advisable to draw a schematic diagram of the survey area layout and describe the appearance quality on the recording paper.

Thrid. Measurement of rebound value and carbonization depth

1. When measuring the rebound value, the axis of the rebound hammer should always be perpendicular to the concrete testing surface, and the pressure should be applied slowly, accurate readings, and quick reset.

2. Each measuring area should read 16 rebound values, and the rebound value reading of each measuring point should be accurate to 1. The measuring points should be evenly distributed within the survey area, and the net distance between two adjacent measuring points should not be less than 20mm; the distance between the measuring point and the exposed steel bars and embedded parts should not be less than 30mm. The measuring point should not be on air holes or exposed stones, and the same measuring point should only be hit once.

3. After the rebound value is measured, the carbonization depth value should be measured on a representative measurement area. The number of measurement points should not be less than 30% of the number of component measurement areas, and the average value should be taken as the value of each measurement area of the component. Carbonization depth value. When the extreme difference of carbonation depth value is greater than 2.0mm, the carbonation depth value should be measured separately in each measurement area.

Four. Calculation of schmidt hammer test results

1. When calculating the average rebound value of the survey area, 3 maximum values and 3 minimum values should be removed from the 16 rebound values in the survey area, and the remaining 10 rebound values should be taken as the arithmetic mean value;

2. When detecting the side of concrete pouring in a non-horizontal direction, the average rebound value of the measurement area should be corrected for the angle;

3. When detecting the concrete pouring surface or pouring bottom surface in the horizontal direction, the average rebound value of the measurement area should be corrected for the detection surface;

4. When the rebound hammer is in a non-horizontal direction and the test surface is the non-poured side of the concrete, the rebound value should be corrected for the angle first, and the corrected rebound value should be corrected for the pouring surface.

Five. Concrete strength conversion and estimation

The strength value of concrete at the current age of the survey area obtained from the average rebound value and carbonation depth value of the survey area through the strength measurement curve or the strength conversion table of the survey area.

Six. About drill core sampling correction

1. For non-pumped concrete that meets the following conditions, the strength of the measured area should be converted according to the regulations:

(1) The cement, sand, admixtures, admixtures, and mixing water used in the concrete meet the relevant current national standards;

(2) Adopt ordinary molding process;

(3) Use templates that comply with current national standards;

(4) Steam curing out of the pool after natural curing for more than 7 days, and the concrete surface is dry;

(5) The natural conservation age is (14-1000) days;

(6) The compressive strength is (10.0-60.0) MPa.

2. Correction method

Drill concrete core samples on the component: the number of core samples should not be less than 6, the nominal diameter should be 100mm, and the ratio of height to diameter should be 1. Core samples should be drilled in the measurement area, and each core sample should only process one test piece. When correcting test blocks with the same conditions, the number of test blocks should not be less than 6, and the side length of the test blocks should be 150mm. 

Seven. Verification of rebound instrument

The verification period of the rebound hammer is half a year. When the rebound hammer has one of the following conditions, it should be tested by the legal metrological verification institution according to the industry standard:

1. Before the new hammer is used;

2. Exceeding the validity period of the verification;

3. The difference between the rebound value displayed by the digital rebound hammer and the pointer direct reading value is greater than 1;

4. After maintenance, the rate setting value on the steel anvil is unqualified;

5. Suffer severe impact or other damage.

The series of rebound hammers produced by Langry NDT fully meet the standards and are suitable for the detection of concrete with various strengths. Among them, the 225-type medium-sized rebound hammer is used for ordinary concrete testing, and the 450-type (or 550-type) high-strength rebound hammer is used for high-strength concrete testing. The test found that the surface of high-strength concrete is difficult to undergo large plastic deformation under the impact of a medium-sized rebound hammer, and the rebound values at different strengths are close to each other, making it difficult to distinguish. Therefore, high-strength concrete needs to use a high-energy rebound instrument to make the concrete surface have obvious plastic deformation and energy consumption. During use, they should be differentiated and not mixed.

In the future, Langry will continue to strictly control product quality, increase investment in production research and development, and deeply cultivate the research and development of integrated solutions for main structure detection.