Test For A Rebound Hammer : Methods, Outcomes & Uses
2BHK Kharghar Rebound Hammer : The compressive power of concrete can be assessed using the rebound impact test. Here is everything you need to know.
The non-destructive rebound impact test can be used to rapidly and accurately identify the concrete’s compressive strength. Schmidt hammers, also referred to as rebound hammers, are composed of a mass that descends a plunger inside of a circular frame under the direction of a spring.
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Test For A Rebound Hammer : What is a rebound hammer?
Swiss Hammers, Schmidt Hammers, and bounce hammers are a few other terms for a concrete test hammer. They are versatile instruments used to assess the calibre of solidified concrete. Concrete test tools have a lot more to give than simply determining the compressive strength in-place.
When evaluating a concrete structure or pavement, strength is not the only element to take into account. The hammers are movable and easy to use, and given that they have been in common use for so long, it is likely that their effectiveness and results have been thoroughly investigated.
Exactly who invented the return hammer?
The concrete test mallet is attributed to Zurich, Switzerland construction engineer Ernst O. Schmidt, who created it in 1954. The device was developed for industrial use by Antonio Brandestini, who founded the Swiss company Proceq.
The return hammer test: What is it?
The rebound hammer test excels at gathering information about the calibre and homogeneity of concrete in both freshly constructed and existing structures quickly and efficiently. A simple testing method gives a general impression of the state of the concrete, highlights areas with decreased strength, and identifies areas that have been harmed by freezing or fire. Using correlation data from laboratory trials makes it simpler to obtain precise estimates of compressive strength.
The test hammer’s piston strikes hardened concrete, loading a spring system until it triggers and discharging the hammer mass. When a mass strikes a piston, a preset amount of energy is transferred from the bulk to the concrete. As the mallet mass moves along an indicator, the recoil number (R-value) is recorded using a straightforward linear scale. Schmidt found that these surface hardness numbers might represent relative strength when compared to results from laboratory experiments.
What does the return hammer exam measure?
According to the Indian law IS: 13311(2)-1992, the return hammer test is intended to achieve the following objectives:
- To determine the connection between the compressive strength and bounce index in order to determine the compressive strength of the concrete.
- Assess the concrete’s consistency.
- To evaluate the concrete’s grade in accordance with the standards.
- Contrast an element’s properties with those of another element.
It is possible to differentiate between the parts of the building that are okay and those that are troublesome using the rebound mallet method.and to contrast the advantages of two various designs.
Concrete impact mallet test execution
The calibration of the rebound hammer is the first step in the rebound hammer test on concrete building process. The return hammer is put to the test against a steel test anvil for this reason. To take the measurements, it is then held at a right angle to the concrete structure’s surface. The rebound hammer test can be performed directly upwards or downwards on a horizontal surface, as well as laterally on a vertical surface.
What to consider when performing the return hammer test
- The concrete’s surface needs to be fresh, spotless, and smooth.
- Any stray fragments on the concrete’s surface should be eliminated with a grinding wheel or stone before performing an impact test.
- On rough surfaces, such as those brought on by insufficient compression, grout loss, or spalled or tooled concrete surfaces, the rebound impact test is not advised.
- It is advised that any edge or shape discontinuity be at least 20 centimetres from the point of contact of a rebound mallet on the surface of concrete.
- Six measurements of the rebound number are taken at each testing location, and the average of these readings is used to determine the rebound index for the reading’s corresponding place of observation on the concrete surface.
Techniques for selecting and arranging trial sites
- Wetness, surface quality, age, carbonation depth, and proximity to aggregate, steel support, and air gaps are all factors that directly affect rebound figures, so care must be taken when selecting and setting up test sites.
- Rebound measurements on concrete with a high wetness level are lower than those on the same concrete when it is dry. If testing compressive strength by correlation with cores or cylinders, the test area should be wet with water a few hours prior to collecting rebound data. The association sample evaluation should be done under saturated, surface-dry (SSD) circumstances.
- Testing on textured or squishy surfaces necessitates surface preparation because the rebound number measures surface stiffness. The surface can typically be manually smoothed with a carborundum stone, though some areas might require a surface polisher. Rebound testing leaves a few small dimples on the concrete’s surface and causes little damage to it.
How should the return mallet be used?
Although the rebound hammer is a very user-friendly piece of testing apparatus, in order to obtain precise and reliable findings, many prerequisite conditions must be met before testing can commence.
Test For A Rebound Hammer: Getting the test area ready
Inspectors should be very conscious of the following considerations before performing a rebound impact test:
- The measuring surface must have holes that are at least 150 millimetres in circumference.
- If the concrete’s surface has a lot of texture or has broken cement on it, it should be ground. (until it is flat).
- Any loose moisture or water that is present on the concrete’s surface must be cleared prior to conducting any testing.
- Never try a frozen solid surface. It is significant to note that evaluating concrete should only be done after it has warmed because frozen concrete tends to have large rebound numbers.
- Direct testing over the steel support should be avoided when the cover width is less than 20 mm. It is best to use a rebar locator to avoid checking on reinforcement with a short depth.
Test For A Rebound Hammer: The return hammer’s path
The tester must take a solid hold of the tool and place the plunger so that it is perpendicular to the surface being tested in order to successfully complete the rebound hammer test. It is also necessary to record the instrument’s inclination in relation to the level to the nearest 45 degrees. If the instrument is facing upwards, a positive angle should be used; if it is looking downward, a negative angle should be used.
Test For A Rebound Hammer : Taking readings
The tester will carefully advance the instrument closer to the surface being tested until the hammer makes impact with the surface after the instrument has been aligned and the angle has been noted. To keep the plunger in the retracted position after the contact, you should keep applying pressure to the tool and pushing the button on the side of the tool.
For the findings to be accurate, each test area must have at least 10 different readings taken. Additionally, there should be at least 50 mm between the sides of the member and the contact sites, and at least 25 mm between the impact locations. The measurement must be discarded and a new reading must be taken if the contact breaks or crushes an air void near to the surface.
Uses for the return hammer test
The method of finding the relative hardness of concrete using the idea of the bounce is where a rebound mallet is most frequently used. Despite this, academics have tried to use the method to assess a number of concrete qualities. Examples of very popular uses include the following:
- Check the consistency of the concrete before it is installed.
- Set boundaries around regions of a building made of poor-quality or deteriorating concrete.
- As a method for determining concrete’s power while it’s still in place
Rebound anvil test advantages and disadvantages
Among these uses, the use of a rebound mallet to assess the strength of concrete has been and is still up for discussion. Researchers disagree about the best method for using a rebound mallet to gauge the weight of concrete.
Test For A Rebound Hammer: Test For A Rebound Hammer: Pros
- The equipment is easy to use.
- Identifies the uniformity and stability of the surface.
- The utilised machinery is fairly priced.
- Used for historic building and construction repair.
Test For A Rebound Hammer: Test For A Rebound Hammer: Cons
- The results are predicated on the neighbourhood.
- The results of the evaluations do not directly affect the surface’s tensile strength or its ability to bend.
- The probe and spring system will require regular cleaning and upkeep.
- It is impossible to pinpoint defects with accuracy.
What influences a return hammer test’s outcomes?
Test For A Rebound Hammer: Type of aggregate
The connection between the compressive strength and rebound number of concrete will change when different types of material are used. Standard relationships in the results are produced by using common stones, such as gravel and crushed rocks. Concrete’s lightweight particles will require customised adjusting in order for the substance to pass the test.
Test For A Rebound Hammer : Cement kind
Concrete made with high alumina cement should have higher compressive strength than concrete produced with normal portland cement. When compared to concrete produced with regular Portland cement, the compressive strength of super sulphated cement is decreased by half.
Type of surface and wetness level
When used on concrete with a tight texture as opposed to concrete with an open texture, the rebound impact test is more precise. Rebound hammers cannot be used to gauge the power of concrete that contains a lot of honeycombs or no particles.
When tested on floating or trowelled surfaces in contrast to testing on moulded surfaces, the power of the substance is overestimated. Testing a wet concrete surface will reveal that the power has reduced. Contrary to when it’s fresh, the concrete’s power can be underestimated by up to 20%.
Concrete age and curing
Over time, there will be changes in the strength-to-hardness ratio of the concrete. This relationship is also influenced by the conditions that concrete is subjected to moisture and permitted to cure in. Concrete that has been matured for three days or more but fewer than ninety days does not experience the effects of ageing. It is necessary to use specially adjusted curves to account for rising quantities of old concrete.
Carbonation on the surface of the concrete
The rebound hammer calculates the enhanced strength of carbonated concrete when used with that substance. It is anticipated to be 50% higher. Therefore, in order to finish the test, you must take the carbonated layer off the concrete and conduct it using a rebound mallet on the uncarbonated layer.
Test For A Rebound Hammer : Observations about the rebound hammer test
- The rebound mallet, which Schmidt invented, provides a quick and reasonably priced method for non-destructively checking concrete.
- Recognizing and taking into account the limitations presented by the testing method is crucial when using the rebound mallet.
- It is crucial to stress that the hammer test should not be viewed as a substitute for the standard compression tests but rather as a method for comparing different types of concrete, assessing how uniformly the concrete used in the structures is, and reducing the number of core samples that need to be taken.
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