Interpretation of the series of issues related to concrete rebound strength
Release time:
2026-03-10
Did the construction personnel add water to the concrete without authorization during construction, resulting in inadequate physical testing strength.
Q: C55 concrete, all test block inspection reports are qualified, but the rebound tester inspection results show that we have taken 12 sets of data, and the strength is only around C45. Moreover, the carbonation depth is calculated as zero. The concrete pouring has been completed for more than two months. May I ask what happened?
A: The inconsistency between the concrete test block and rebound results can be roughly divided into two reasons:
1. Did the construction personnel add water to the concrete without authorization during construction, resulting in inadequate physical testing strength.
2. The vibration situation during working hours, whether there are any missed or excessive vibrations, and whether the later maintenance is in place. If the cause still cannot be analyzed, it is recommended to perform a core sampling and damage detection. After the general concrete test block is qualified, it does not need to rebound. If the age does not exceed 90 days, the carbonation depth does not need to be calculated.
If slag cement is used with admixtures, it is recommended to rebound again after 90 or 120 days, without considering a rebound period of 45 days. The rebound period of cement with different compositions varies. When the two are inconsistent, the test block should prevail. The location of the rebound hammer is different, and the data is also different. For the strength analysis of high-grade concrete, the method of taking rock cores is more in line with the actual situation.
Q: What is the reason why the C45 concrete standard curing test block is qualified, but the rebound strength is still insufficient?
A: If the rebound strength is not enough, of course, it is because the core was drilled and sampled. After taking the core, the main body strength is determined. It is still necessary to see how the strength is judged.
Firstly, consider whether the rebound method and calculation method are correct. The unqualified concrete strength of the components may be related to curing, and the wall columns need to be demolded and cured the next day. Good maintenance results in high strength, low carbonization depth, and a higher rebound strength.
Please refer to JGJ/T 23-2011 Technical Specification for Testing Concrete Compressive Strength by Rebound Method for details. The rebound value of concrete can correspond to the strength value, which can then be corrected. Carbonization is one of the correction items. Basic idea:
1. Rebound value collection.
2. Collection of carbonization depth values, using the average of three or more points as the carbonization depth value of the component.
3. Calculation of rebound value in the testing area: Exclude 3 maximum and 3 minimum values from 16 points, and take the average of 10 values.
4. According to the rebound value and average carbonization depth of the test area, refer to the table and calculate the strength conversion value for each test area.
5. Estimate the strength of components based on the strength values of each measurement area; If there are less than 10 measurement areas, take the minimum value. If there are not less than 10 measurement areas, calculate the strength value using the formula based on the average and standard deviation.
Note: Two tables (two types of strength curves) are used for cast-in-place concrete and pumped concrete.
Q: How to convert the rebound value of concrete test blocks into the final compressive strength?
A: For example, the design strength of the pumped concrete C50 column, the on-site rebound value (this is one of them, each column has only one measurement zone) is as follows:
42 42 50 42 50 42 41 42
42 40 41 42 51 50 42 42
Six columns were tested on each floor, and the data for the remaining five were also similar.
Calculation basis standard: "JGJ/T 23-2011 Technical Specification for Testing Concrete Compressive Strength by Rebound Method", removing the three maximum values (51, 50, 50) and three minimum values (40, 41, 41), the average value of the remaining ten rebound values is 42.8.
The rebound of the pillar is horizontal and does not require correction.
There are only 6 measurement areas, which is less than 10, so the minimum value of these six measurement areas is selected for evaluation.
There is no data for carbonization depth, set to 0.5; When the carbonization depth is greater than 2.0, core drilling is required
Refer to Appendix A for the "Conversion Table of Concrete Strength in the Measurement Area", which shows 45.7MPa.
If it is for pumping concrete, then make corrections according to Appendix B "Correction of Concrete Strength Conversion Values for Pumping Concrete Test Areas". Assuming a carbonization depth of 0.5mm here, the correction value is 3.0MPa.
The final compressive strength is 45.7+3.0=48.7MPa (97.4%)
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