Wunna the most commonly used non-destructive testin’ method for concrete is rebound testin’. This method is really popular with testin’ units ‘cause it’s lightnin’ fast, doesn’t damage the structure much, and it’s real fast ta detect. It’s also a common way for construction units ta check how strong concrete is.
In practice, rebound hammer test results have been controversial, and some areas have only accepted core drillin’ 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 concrete test hammer is not adequately addressed; on the other hand, the influencin’ factors of the rebound method are not sufficiently familiar. At the samtime, it is impossible ta have an intuitive understanding of the possible effects of various factors. In this paper, we gonna present a systematic analysis of the factors that may influence the spring back test results and provide practical advice for the test personnel.
That 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 ta tha concrete through tha 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 ta the position L before the snapping hammer is decoupled is the rebound value in the traditional sense.
Langry makes a series of rebound hammers, with the225 models being used for testing regular concrete and the 450 (or 550) model being used for testing high-strength concrete. Tests have shown that when the medium-sized schmidt rebound hammer strikes, the surface of high-strength concrete is difficult to undergo significant plastic deformation, and the rebound values are close at various strengths, making it hard to distinguish. Therefore, high-strength concrete requires a high-energy concrete rebound hammer to make the surface of the concrete have obvious plastic deformation and energy consumption. During use, it 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, while a small part of the energy is consumed by the friction during the movement of the impact hammer and pointer, the air resistance overcome by the impact hammer, the vibration of the concrete component, and the movement of the impact rod on the concrete surface. Normally, the latter accounts for a small proportion of energy consumption and can be ignored.
For thin-walled and small components that vibrate during impact, most of the impact energy will be consumed by the component vibration, resulting in a significant decrease in rebound value. Such components should be avoided as much as possible durin’ rebound testin’. For example, when testing the rebound of a thin floor, the reduction factor of this part should be considered. During the impact process, pressure should be applied slowly after the impact rod contacts the concrete surface to prevent excessive movement during the impact process, which causes energy consumption.
Durin’ long-term use of the schmidt concrete test hammer, the impact rod, and the pointer will accumulate a lot of dust, and the energy consumed by friction cannot be ignored. If the concrete hammer test machine is not properly maintained, the rebound value will decrease significantly. Therefore, the concrete test hammer tool should be maintained after more than 2000 impacts (about 12 components). It is worth emphasizing that the determination of whether to maintain the rebound hammer cannot be based solely on the unqualified rebound rate: the hardness of the steel anvil is relatively high, the energy returned by the impact is relatively high, and the proportion of friction loss is relatively low. If the rate is low due to friction energy loss, it has already had a significant impact on the results of actual rebound testing.
The main role of determination:
1. Test the processin’ accuracy of the rebound hammer itself;
2. Test the stability of the rebound hammer;
3. Test whether the rebound hammer is worn;
4. Test whether the impact energy meets the specification requirements. From this point of view, the determination value is the basic data for a routine inspection of the workin’ performance of the rebound hammer, but routine maintenance cannot be ignored to ensure that the rebound hammer is in the best workin’ condition.
The common curing methods for concrete include standard curing, natural curing, and steam curing. When concrete is cured in a humid environment or in water, the hydration process is better, and the early and later strength is higher than that of dry-cured concrete. However, the surface hardness decreases due to softening by water.
Although steam curing causes early strength growth of concrete, the surface hardness also increases. After excluding the influence of surface hardness and carbonation depth, the rebound value and strength relationship of steam-cured concrete is basically consistent with that of naturally cured concrete. Therefore, the standard specifies that steam-cured concrete should be naturally cured for more than 7 days after leaving the mold and the concrete surface should be dry. This standard still applies. On the other hand, the rebound value may be lower if the testing is done within 7 days after steam curing. This factor should be considered during actual testing.
When concrete is in a humid state, the surface moisture content is high, and the surface hardness of the concrete is softened, resulting in a lower rebound value. The lower the strength of the concrete, the greater the weakening effect of the humid state on the rebound value. The author once conducted a comparative test on the C25 concrete lining of a certain tunnel, and the estimated strength was less than 10Mpa, while the actual core drilling test strength was around 30Mpa. Therefore, the rebound method should be used with caution for low-strength components in humid basements or tunnels, and the rebound testing should be conducted in a dry state after 7 days of dehumidification to ensure a dry surface. It is difficult to avoid the influence of humidity on the rebound value only by pumping water or temporarily drying the surface locally.
Naturally, cured concrete components undergo carbonation as the surface reacts with atmospheric carbon dioxide to form high-hardness calcium carbonate. This process results in a higher surface hardness than the interior of the concrete, leading to higher rebound values. In accordance with regulations, carbonation depth is considered a factor in adjusting strength estimates. Extensive testing has shown that after a carbonation depth of 6mm, there is no significant increase in rebound energy, so a uniform correction of 6mm is applied.
It is worth noting that false carbonation can occur during the detection process, which seriously affects the accuracy of concrete testing. The use of acidic isolating agents (such as motor oil) or inadequate curing of the concrete, and incomplete hydration of the cement, can lead to a lack of calcium hydroxide on the surface of the concrete, resulting in non-alkaline conditions. In such cases, the use of phenolphthalein reagent to detect carbonation depth can result in significant errors.
Da standard strength measurement curve is applicable from 14 days to 1000 days. Components dat exceed dis age range cannot be used directly because dey exceed da actual age range of da concrete test blocks used to establish da strength measurement curve. Da regulations provide an accurate method to solve dis problem, which is to perform core drilling correction.
Considering da aging of concrete, a correction factor less than 1 is multiplied by da strength values of concrete at different ages. Comparative tables of some projects have shown dat dis method tends to produce conservative results. Considering dat da durability of aged concrete is significantly reduced, appropriate conservative calculations are beneficial for da long-term use of buildings.
Existing buildings often have a plastered surface that needs to be polished on-site to achieve a concrete surface. Ordinary polishing methods cannot guarantee a good concrete surface and there will always be many pits. It is almost impossible to arrange 16 measuring points in each measurement area in an orderly manner. Without violating the requirements of the regulations for the distance between measuring points, the measuring points in the measurement area can be arranged in a slightly disorderly manner. The layout of the measuring points should avoid concrete pits, unevenness, or stains. At the same time, it should be noted that if the surface dust is not cleaned after polishing, the rebound value will be generally low, which 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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