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Causes of low concrete rebound strength

The rebound method is to use a spring-driven heavy hammer to bounce the concrete surface through a bounce rod (transmitting rod), and measure the rebound distance of the heavy hammer to calculate the rebound value (the ratio of the rebound distance to the initial length of the spring) ) As an index related to strength, it is a method to estimate concrete strength based on the surface hardness of concrete. The rebound method for testing concrete strength has the characteristics of simple equipment, convenient operation, and rapid testing, and does not damage the concrete structure. It has greater advantages than other testing method.

But sometimes the estimated value of concrete rebound is very uneven and the changes are not regular. There are several situations:


(1) The same engineering design of the same strength grade concrete structure, using the same mix ratio, the rebound value of different parts is very different, and the homogeneity is very poor.

(2) For different engineering designs of the same strength grade of concrete structures, using the same mix ratio, the rebound value is also very different.

(3) The same engineering design of the same strength grade concrete structure, using the same mix ratio, different construction time, the rebound value is also very different, and the homogeneity is very poor.


The main factors causing the low strength of concrete tested by rebound method are:


(1) rebound hammer

The quality of the rebound hammer and its test performance directly affect the accuracy of the concrete strength estimation results. The main factors that affect the test performance of the rebound hammer are: pointer friction, bounce hammer unhook position, bounce tension spring working length, bounce hammer The length of the impact and the jump position of the bounce hammer. Before the rebound test, make sure that the rebound hammer used must be in the standard state within the valid verification period. When the number of tests is large at one time, multiple rebound hammers that have been inspected to the standard state should be equipped, and standard steel drills should be carried with you to prepare for the calibration test at any time during the test process, and replace the rebound tester in time to ensure the accuracy of the test results. During the inspection process, continuous bounce on a large number of components will cause the working status of the rebound instrument to gradually fall below the standard state. For example, continuous long-term bounce will also cause the jump point of the bounce hammer and the hook point of the bounce hammer to deviate , Causing great deviation to the test results. The pointer of the rebound hammer rubs on the pointer shaft for a long time and cannot be maintained or verified in time. Large scratches on the pointer shaft (reading 0-55) will cause excessive friction in this section of the pointer, which is on the concrete member. When bounced, it will directly cause the reading to be significantly lower, resulting in a large deviation.


(2) The surface condition of the inspection component

The rebound method is a surface test technique to estimate the body strength of concrete through the statistical correlation between the rebound value of the concrete surface and the compressive strength of the concrete. That is, the rebound value reflects the thickness of the concrete surface in the range of 10mm-15mm. hardness. Therefore, the state of the concrete surface directly affects the accuracy and rationality of the estimated value. The measurement area should be preferentially selected on the concrete pouring side. The selected measurement area is relatively flat and clean, and there is no honeycomb, pitted surface, laitance and loose layer. If necessary, the loose layer and debris can be removed with a grinding wheel. Of course, the concrete inspection surface should not be over-polished, because the laitance and loose layer on the concrete surface is often a very thin layer. Over-polishing, especially the low-grade concrete, can easily lead to sanding, which can cause deviations in the test values.


(3) Humidity

The moisture content of concrete will affect the hardness of its surface. Water has a softening effect on the surface of the concrete. The higher the moisture content of the concrete surface, the lower the surface hardness. When conducting rebound testing, it is best to conduct testing under the condition of natural air-drying on the concrete surface.


(4) Pouring and curing

In the concrete pouring project, sufficient and uniform vibration can eliminate the internal defects of the concrete and improve the strength of the hardened concrete. Insufficient vibration, the internal defects of the concrete are correct, and the bubbles are difficult to discharge, resulting in low rebound strength. The vibrating time is too long, causing the concrete to have a layered bleeding phenomenon, there are more stones at the bottom of the component, and the rebound value is high. The surface water glue is relatively large, the surface is loose, and the rebound value is low.


Adequate curing after pouring can ensure the normal progress of concrete cement hydration and continuous increase in strength. Insufficient curing or non-curing causes the surface to lose water, the surface cement particles stop hydration, the surface hardness is reduced, and the rebound strength is low. For example, many vertical structures such as walls and pillars were demoulded too early, and the maintenance was insufficient or not maintained, resulting in chalking and whitening of the surface, and the rebound value was significantly lower than that of beams that were demoulded later.

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