One of the most commonly used non-destructive testing methods for concrete is rebound testing. This method is popular with testing units because it's light and fast, doesn't damage the structure much, and it's fast to detect. It's also a common way for construction units to check how strong concrete is.
In practice,rebound hammer testresults have been controversial, and some areas have only accepted core drilling results. However, the entire rebound strength curve is established on the basis of thousands of reliable data, and its accuracy is beyond doubt.
In such a circumstance, there are many factors that are at play: on the one hand, the operation of the rebound is not sufficiently standardized, and the maintenance of the rebound hammer is not adequately addressed; on the other hand, the influencing factors of the rebound method are not sufficiently familiar. At the same time, it is impossible to have an intuitive understanding of the possible effects of various factors. In this paper, we will present a systematic analysis of the factors that may influence the spring back test results and provide practical advice for the test personnel.
Under a certain impact energy, the impact rod impacts the concrete surface, the concrete surface produces plastic deformation and consumes a part of the work (the higher the concrete strength, the greater the surface hardness, the smaller the plastic deformation), and the other part of the work is transmitted back to the concrete through the elastic deformation of the concrete. The ejector rod converts kinetic energy into elastic potential energy. The percentage of the ratio of the distance L' of the snapping hammer back to the position L before the snapping hammer is decoupled is the rebound value in the traditional sense.
Among the series of concrete test hammer produced by Langry NDT, the ht225 rebound hammer is suitable for ordinary concrete testing, and the 450 (or 550) high-strength hammer is used for high-strength concrete testing. The test found that under the impact of the medium-sized hammer, it is difficult for the high-strength concrete surface to undergo large plastic deformation, and the spring back values at each strength are close to each other, making it difficult to distinguish. Therefore, high-strength concrete needs to be tested with a high-energy rebound meter, so that the concrete surface has obvious plastic deformation energy dissipation. When used, they should be distinguished and not mixed.
During the whole impact process, the impact energy is mainly consumed by the plastic deformation of the concrete surface. At the same time, a small part of the energy is used by the impact hammer and the friction during the movement of the pointer, the impact hammer overcomes air resistance, and the concrete components are vibrated and impacted. It is consumed by the movement of the rod on the concrete surface. Under normal circumstances, the latter accounts for a small proportion of the energy consumption process and can be ignored.
For thin-walled and small components that vibrate during the bounce, most of the bounce energy will be consumed by the component tremor, and the rebound value will drop significantly. Such components should be avoided as much as possible in rebound detection. If the slab with smaller thickness rebounds, the reduction factor of this part should be considered. During the ejection process, after the ejector rod touches the concrete surface, apply pressure slowly to prevent energy consumption caused by excessive movement during the ejection process.
In the long-term use of the Schmidt rebound hammer, the ejector rod and pointer accumulate a lot of dust and friction consumes a lot of energy. If the concrete rebound hammer is neglected, the rebound value will be greatly reduced. Therefore, more than 2000 times langry rebound hammer bounces (about 12 components) should be maintained once. It is worth emphasizing that the failure rate of the hammer cannot be used as the basis for maintenance: the hardness of the steel anvil is larger, the energy returned by the bombardment is larger, and the proportion of friction loss is relatively low. As a result, the rate setting value is low, and the actual springback detection has already had a significant impact on the results.
The main functions of calibration are:
1. Detecting the machining accuracy of schmidt concrete test hammer
2. Detecting the stability of concrete hammer test machine
3. Checking whether the concrete test hammer tool is worn
4. Detecting whether the impact energy meets the specification requirements. From this point of view, it is clear that the rate setting is the fundamental principle of routine inspection of the working performance of concrete test hammer ht 225, but routine maintenance cannot be ignored to guarantee that the hammer is in the best condition for use.
Commonly used curing methods mainly include standard curing, natural curing and steam curing. When concrete is cured in a humid environment or in water, due to better hydration, the early and later strengths are higher than those produced in dry conditions. However, the surface hardness is reduced due to water softening.
According to the relevant research of the Institute of Building Science, although the early strength of concrete increases too quickly, the surface hardness also increases. Natural care is basically the same. Therefore, the specification stipulates that the specification still applies if the steam curing pool has been naturally cured for more than 7 days. In addition, the concrete surface is dry . On the other hand, within 7 days after the steam curing comes out of the pool, the earlier the detection time, the lower the rebound value is likely to be. In the actual detection, this factor is taken into account.
The wet state leads to a high water content on the concrete surface, softening the hardness of the concrete surface and a low rebound value. The lower the concrete strength, the greater the weakening of the springback value in the wet state . The author has carried out a comparative test on the C25 concrete lining of a tunnel. The estimated strength is less than 10Mpa, while the actual core testing strength is about 30Mpa. Therefore, the springback method should be used cautiously for low-strength components in wet basements or tunnels, and the springback test should be carried out on-site when the surface is dry for 7 days after dehumidification. It is difficult to avoid the influence of humidity on the rebound value by only pumping water or local temporary surface drying.
For concrete components under natural curing, calcium hydroxide on the surface interacts with carbon dioxide in the air to form calcium carbonate with higher hardness. This process is carbonization of concrete. The hardness of the carbonized surface concrete is higher than that of the concrete interior, resulting in a higher springback value. In the specification, the estimated strength value is corrected by taking the carbonization depth into consideration. A large number of tests have shown that after the carbonization depth is greater than 6mm, the size of the rebound energy will not increase significantly. Therefore, it is uniformly revised according to 6mm.
It is worth noting that during the carbonization detection process, false carbonization is prone to occur, which seriously affects the detection accuracy of concrete. When an acid release agent (such as motor oil) is used, or the concrete is not well maintained, and the cement is not sufficiently hydrated, it will cause the concrete surface to lack calcium hydroxide and not become alkaline. At this time, the use of phenolphthalein reagent to detect the carbonization depth will produce a large error
"Technical Specifications for Testing Concrete Compressive Strength by Rebound Method" stipulates that the applicable range of the strength measurement curve is 14d~1000d. The reason why the components beyond the age range cannot be used directly is because it exceeds the concrete test used to establish the strength measurement curve. The actual coverage age of the block. The specification also gives an accurate method to solve this problem, that is, perform drill core corrections.
The "Civil Building Reliability Appraisal Standard" also gives the correction method for the rebound age of aged concrete. Taking into account the aging of concrete, the strength values of concrete at different ages are multiplied by a correction factor less than 1. The comparison of some projects shows that the test results of this method are conservative. Considering that the durability of concrete in the aging period is obviously reduced, an appropriate conservative calculation is beneficial to the long-term use of the building.
In the "Technical Regulations for Testing Concrete Compressive Strength by Rebound Method", angle correction can only be performed for non-pumped concrete. According to literature, pumped concrete has a high fluidity. After pouring, there will be more aggregates at the bottom and more cement slurry at the upper part, so the side rebound is more reasonable. However, with the continuous development of pumping technology and admixtures, and strict control of segregation indicators, it is difficult for this situation to occur today. In the process of compiling "Technical Regulations for Testing Pumped Concrete Compression by Rebound Method", the editor-in-chief has concluded through a large number of tests and demonstrations that pumpable concrete is also suitable for angle correction, which is also reflected in the specification.
Therefore, the concrete strength of the floor member can also be detected by the vertical springback correction. This is taking into account the quality of the concrete being pumped at the moment. Therere is no big difference between the bottom and the side, so it is not necessary to make corrections. At the same time, the concrete surface is rough and uneven, and it is difficult to rebound; for existing buildings, the leveling and decoration of the floor surface also makes it difficult to achieve the rebound of the pouring surface.
The core-drilling method is preferred for strength testing of aged building concrete, and the rebound-drilling core correction method is recommended when a large number of cores cannot be drilled. When it is impossible to drill the core at all, the reduction factor in Appendix K of "Civil Building Reliability Appraisal Standard" shall be used.
Existing buildings are often plastered and plastered on the surface, which needs to be polished to the concrete surface on site. Ordinary grinding methods cannot guarantee a smooth concrete surface, and there will always be more potholes. It is almost impossible to arrange 16 measuring points in each survey area in a regular manner. Under the circumstance that the distance between the measuring points is not violated by the specification, the measuring points in the survey area can be properly arranged in disorder. The arrangement of measuring points should avoid concrete potholes, undulations or stains. At the same time, it should be noted that if the surface dust is not cleaned after grinding, the rebound value will be low as a whole. This will affect the test results.
As one of the most common nondestructive testing methods, rebound testing has unparalleled advantages. However, springback is an indirect method based on the strength measurement curve. This makes many inspectors unable to understand the essence of springback detection more deeply, resulting in a lot of misunderstandings. In the case of inaccurate springback detection, it is always attributed to the method itself. To determine the reasons for rebound, inspectors must examine themselves more, accumulate experience in the field, and summarize the factors affecting the rebound. Only in this way can they be more confident in their test results.
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