What is Rebound Hammer Test?
The Rebound Hammer test is a method of non-destructive testing for concrete that provides a fast and convenient indication of its compressive strength. The test is conducted using a device called a Schmidt hammer or rebound hammer, which consists of a spring-controlled mass that moves on a plunger within a tubular housing. When the plunger is pressed against the concrete surface, the mass strikes it with a constant energy and rebounds back. The degree of rebound, which reflects the surface hardness of the concrete, is measured using a graduated scale, and this value is called the Rebound Number or rebound index.
A lower rebound value indicates that the concrete has low strength and stiffness, which means that it absorbs more energy before yielding. This method allows for a quick assessment of the quality of the concrete surface, which can help in determining whether further testing or analysis is necessary. Overall, the Rebound Hammer test is a useful and efficient tool for evaluating the strength and quality of concrete.
Fig.1.Operation of the rebound hammer
Objective of Rebound Hammer Test
The Indian code IS: 13311(2)-1992 specifies the objectives of the rebound hammer test, which is commonly used for testing the strength of concrete. One of the primary objectives of this test is to determine the compressive strength of the concrete by establishing a correlation between the rebound index and the compressive strength.
In addition to this, the rebound hammer test is also used to assess the uniformity of the concrete and evaluate its quality based on the standard specifications. This allows engineers and construction professionals to gauge the strength and overall quality of concrete elements in a structure, and make informed decisions based on this information.
Moreover, the rebound hammer test method can be used to compare different parts of a structure or even different structures based on their strength. By identifying acceptable and questionable areas of a structure, construction professionals can determine the most suitable course of action to ensure the structural integrity and safety of the building.
In summary, the rebound hammer test is a valuable tool for assessing the strength and quality of concrete structures. Its objectives include determining compressive strength, assessing uniformity and quality, and comparing different parts of a structure or structures.
Principle of Rebound Hammer Test
The rebound hammer test method is utilized to assess the hardness of a concrete surface by measuring the rebound of an elastic mass that strikes the surface. The basic principle behind this test is that the rebound of the mass is directly proportional to the hardness of the concrete surface. To perform this test, the plunger of the rebound hammer is pressed against the concrete surface, and the spring-controlled mass rebounds. The amount of rebound is an indicator of the concrete surface’s hardness, which can then be correlated to the compressive strength of the concrete.
The rebound value, also known as the rebound number or rebound index, is recorded by reading it off a graduated scale. This value represents the amount of rebound of the mass and is used to determine the compressive strength of the concrete. The compressive strength can be read directly from the graph provided on the body of the hammer, which correlates the rebound value to the compressive strength of the concrete.
Overall, the rebound hammer test method provides a non-destructive way of assessing the hardness and strength of a concrete surface. By measuring the rebound of an elastic mass and correlating it to the compressive strength of the concrete, this test method can provide valuable insights into the quality of concrete structures.
Procedure for Rebound Hammer Test
The procedure for conducting a rebound hammer test on a concrete structure begins with calibrating the rebound hammer. This is done by testing the rebound hammer against a test anvil made of steel with a Brinell hardness number of approximately 5000 N/mm2. Once the accuracy of the rebound hammer is verified on the test anvil, it is then held at a right angle to the surface of the concrete structure in order to take readings. The test can be conducted horizontally on vertical surfaces, or vertically upwards or downwards on horizontal surfaces, as shown in the figure below. It’s important to note that if the rebound hammer is held at an intermediate angle, the rebound number will vary for the same concrete.
Fig.2.Rebound Hammer Positions for Testing Concrete Structure
The rebound hammer is used in various applications, and the required impact energy for rebound hammer testing differs depending on the specific application. Table-1 provides approximate impact energy levels for different applications. This information helps users determine the appropriate impact energy level to use based on their specific testing requirements.
Table-1: Impact Energy for Rebound Hammers for Different Applications As per IS: 13311(2)-1992
| Sl.No | Applications | Approximate Impact Energy for Rebound Hammer in Nm |
| 1 | For Normal Weight Concrete | 2.25 |
| 2 | For light weight concrete / For small and impact resistive concrete parts | 0.75 |
| 3 | For mass concrete testing Eg: In roads, hydraulic structures and pavements | 30.00 |
Points to Remember in Rebound Hammer Test
In order to conduct a successful rebound hammer test on a concrete surface, it is important to ensure that the surface is smooth, clean, and dry. If there are any loose particles present on the surface, they should be removed by using a grinding wheel or stone prior to conducting the test. It is not recommended to conduct the rebound hammer test on rough surfaces, as this may be an indication of incomplete compaction, loss of grout, or a spalled or tooled concrete surface.
It is also important to consider the placement of the point of impact when conducting a rebound hammer test. The point of impact should be at least 20mm away from any edge or shape discontinuity on the concrete surface. This will help to ensure that accurate readings are obtained during the test.
During the test, six readings of the rebound number should be taken at each point of testing. These readings should then be averaged to determine the rebound index for the corresponding point of observation on the concrete surface. By following these guidelines, a successful and accurate rebound hammer test can be conducted on a concrete surface.
Correlation between compressive strength of concrete and rebound number
The most suitable method for determining the correlation between compressive strength of concrete and rebound number involves testing the concrete cubes using both a compression testing machine and a rebound hammer simultaneously. The rebound number of the concrete cube is first recorded, and then the compressive strength is tested using a compression testing machine. For calibrating rebound hammers of different impact energies, the fixed load required is typically around 7 N/mm2 when the impact energy of the hammer is 2.2 Nm. The load should be increased for calibrating rebound hammers with greater impact energy and decreased for calibrating rebound hammers with lesser impact energy. It is recommended to use large test specimens to minimize the size effect on the test result of a full-scale structure. For calibrating rebound hammers with lower impact energy (2.2 Nm), 150mm cube specimens are preferred, while for rebound hammers with higher impact energy (e.g., 30 Nm), the test cubes should not be smaller than 300mm.
To ensure accurate results, the concrete cube specimens should be kept at room temperature for approximately 24 hours after being removed from the curing pond before testing them with the rebound hammer. It is necessary to establish a correlation between the strength of dry-tested cubes and the strength of wet-tested cubes on which rebound readings are taken to obtain a correlation between rebound numbers and the strength of wet-cured and wet-tested cubes. It is not recommended to establish a direct correlation between rebound numbers on wet cubes and the strength of wet cubes. Only the vertical faces of the cubes as cast should be tested, and at least nine readings should be taken on each of the two vertical faces accessible in the compression testing machine when using the rebound hammers. The points of impact on the specimen must not be nearer an edge than 20mm and should be not less than 20mm from each other. The same points must not be impacted more than once.
Interpretation of Rebound Hammer Test Results
The correlation between compressive strength and rebound number can be used to assess the strength of a structure. Typically, an increase in rebound number is associated with higher compressive strength, although other parameters such as the type of cement, type of aggregate, surface condition and moisture content of the concrete, curing and age of concrete, and carbonation of the concrete surface can also affect the rebound number. By considering these factors, the relationship between rebound number and compressive strength can provide valuable information about the structural integrity of the concrete.
Fig.3.Relationship Between Cube Strength and the Rebound Number
The rebound index is a useful indicator of the compressive strength of concrete, but it is limited to a shallow depth from the surface. It may not accurately reflect the internal cracks, flaws, or heterogeneity that may exist across the cross-section of the concrete. Table-2 provides information on the quality of concrete based on the average rebound number.
Table.2. Quality of Concrete for different values of rebound number
The estimation of concrete strength using the rebound hammer method is not considered highly accurate, with a potential accuracy range of ± 25 percent. However, if the relationship between rebound index and compressive strength is determined through tests conducted on core samples taken from the structure or standard specimens made with the same concrete materials and mix proportion, the accuracy of results and confidence in the estimates can be significantly improved.
Advantages and Disadvantages of Rebound Hammer Test
The Rebound Hammer Test is a non-destructive testing method used to evaluate the uniformity properties of a surface. One of its advantages is that the apparatus is easy to use, making it a popular choice for engineers and technicians. Additionally, the equipment used is inexpensive, which makes it accessible to a wide range of users. This test is also commonly used in the rehabilitation of old monuments.
However, the Rebound Hammer Test does have some disadvantages. Firstly, the results obtained are based on a local point, which means that they may not be representative of the entire surface being tested. Moreover, the test results are not directly related to the strength and deformation properties of the surface, which can limit its usefulness in certain situations. Another drawback is that the probe and spring arrangement of the equipment will require regular cleaning and maintenance to ensure accurate results. Finally, the test is unable to detect flaws with great accuracy, which can limit its ability to identify defects or weaknesses in the material being tested.
Factors Influencing Rebound Hammer Test
The rebound hammer test is a commonly used non-destructive testing method to evaluate the quality of concrete. This test is influenced by several important factors that can impact the accuracy of the results.
One of the factors that can affect the rebound hammer test is the type of aggregate used in the concrete. Aggregates with different characteristics can result in varying degrees of hardness and can affect the rebound values obtained from the test.
Another important factor is the type of cement used in the concrete. Different types of cement can have varying strengths, which can impact the rebound values.
The surface and moisture condition of the concrete can also influence the results of the rebound hammer test. If the surface of the concrete is uneven or has moisture, it can impact the rebound values obtained from the test.
The curing and age of the concrete can also play a significant role in the rebound hammer test. Concrete that has not been cured properly or is too young may not have reached its full strength potential, which can result in lower rebound values.
Finally, the carbonation of the concrete surface can also affect the results of the rebound hammer test. Carbonation can lead to a decrease in the pH of the concrete, which can affect the rebound values obtained from the test.
Type of Aggregate
When it comes to measuring the compressive strength of concrete, the rebound number is often used as a way to correlate the two factors. However, it’s important to note that this correlation can vary depending on the type of aggregate used. Typically, normal aggregates such as gravels and crushed aggregates are used to achieve standard results.
If lightweight aggregates are used in the concrete mixture, it’s necessary to take special measures to calibrate the testing process. This is because the unique properties of lightweight aggregates can affect the correlation between rebound number and compressive strength. To ensure accurate results, it’s important to adjust the testing process accordingly.
Type of Cement
The compressive strength of concrete made from high alumina cement is expected to be higher than that of Ordinary Portland Cement. On the other hand, the use of supersulphated cement in concrete leads to a significant decrease in compressive strength. In fact, when compared to the strength of concrete made from OPC, the compressive strength of concrete made from supersulphated cement can be reduced by up to 50%.
Type of Surface and Moisture Condition
The rebound hammer test is effective for assessing the strength of close texture concrete, but it may not be as reliable for open texture concrete. Concrete that has high levels of honeycombs or is made of no-fines concrete is not suitable for this test. When testing floated or trowelled surfaces, the strength of the concrete may be overestimated compared to moulded surfaces. It’s important to note that wet concrete surfaces should not be tested using this method, as it can lead to an underestimation of strength. In fact, the strength of wet concrete may be underestimated by up to 20% compared to dry concrete.
Type of curing and age of concrete
The strength and hardness of concrete are interrelated, but this relationship is not constant and can change over time. Various factors can influence this relationship, including the curing conditions of the concrete and its exposure to moisture. However, for concrete that is between 3 days and 90 days old, the age of the concrete does not significantly impact this relationship.
Beyond 90 days, the relationship between strength and hardness becomes more complex, and calibrated curves may be necessary to accurately measure it. These curves take into account the age of the concrete as well as its specific curing and moisture exposure conditions. By using these calibrated curves, it is possible to accurately assess the strength and hardness of concrete, even for concrete that has aged significantly.
Carbonation on Concrete Surface
According to the given context, a concrete surface that has undergone carbonation appears to have a higher strength when tested using a rebound hammer. This higher strength is estimated to be approximately 50% more than the strength of non-carbonated concrete. Therefore, to obtain accurate results, it is necessary to conduct the rebound hammer test on a non-carbonated layer of concrete by removing the carbonated layer from the surface. By conducting the test on the non-carbonated layer, it will be possible to determine the true strength of the concrete without the influence of carbonation on the results.