What Is Rebound Hammer Test?
The Rebound Hammer Test on concrete is a Non–destructive testing method of concrete that provides a convenient, fair, and rapid indication of the compressive strength of the concrete.
The concrete Rebound Hammer Test is also known as the Schmidt Hammer Test that consists of a spring-controlled mass that slides on a plunger within a tubular housing. The rebound hammer test is carried out on hardened concrete.
The operation of the Schmidt Hammer Test is shown in the figure below. During the test, when the plunger of the rebound hammer is suppressed against the surface of concrete, a spring–controlled mass with constant energy is made to hit the concrete surface to rebound back.
The force of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as Rebound Number (rebound index). A concrete with lower strength and lower stiffness will absorb more energy to yield in a lower rebound value (rebound index).
Objectives of Rebound Hammer Test
As per the code IS: 13311(2)-1992, the test has the following objectives:
- To know the compressive strength of the concrete by relating the rebound index and the compressive strength.
- To determine the uniformity of the concrete.
- To check the quality of the concrete based on the standard specifications.
- To relate one or more concrete elements with others in terms of quality of concrete.
These test methods can also be used to differentiate the acceptable and questionable parts of the concrete structure or to compare two different structures based on their strength.
Rebound Hammer Test IS Code is IS 13311-1 (1992): Method of Non-destructive Test
Rebound Hammer Test Procedure
The procedure for rebound hammer test on hardened concrete starts with the calibration of the rebound hammer.
1) The rebound hammer is tested against the anvil made of steel having a Brinell hardness of about 5000 N/mm2.
2) After the rebound hammer is calibrated for accuracy on the test anvil, the rebound hammer is held at the right angle to the surface of the concrete structure for taking the readings.
3) The test thus can be conducted horizontally on a vertical surface and vertically downwards or upwards on horizontal surfaces as shown in the figure below.
Figure 2. Positions of Rebound Hammer on Concrete Structure
4) If the rebound hammer is held at an intermediate angle, the rebound number (rebound index) will give differentiated results for the same surface and concrete.
5) The impact energy required for the hammer test is different for different applications. Approximate Impact energy levels are mentioned in the below table for different applications.
The following rebound hammer test results are obtained from the graph,
Table 1. Impact Energy in Rebound Hammers for Different Applications as per IS: 13311(2)-1992.
|Sr. No.||Applications||Approximate Impact Energy for Rebound Hammer in Nm|
|1.||For Normal Weight Concrete||2.25|
|2.||For small and impact-resistant concrete parts||0.75|
|3.||For mass concrete testing E.g. in roads, hydraulic structures, and pavements||30.00|
Read More: Tensile Strength Of Concrete – Test & Result
Rebound Hammer Test Result Calculation pdf
Correlation to Compressive Strength of Concrete and Rebound Number (Rebound index)
The most preferable & efficient method of obtaining the correlation between the compressive strength of concrete and rebound number is to test the concrete cubes using a compression testing machine as well as using a rebound hammer simultaneously.
How to Calculate Compressive Strength From Rebound Number
To Calculate compressive strength from rebound number first, the rebound number of the concrete cube is calculated. Then the compressive strength of the cube is tested on the compression testing machine. The fixed load necessary is of the order of 7 N/mm2 when the impact energy of the hammer is about 2.2 N.m.
The impact load has to increase for calibrating rebound hammers of greater impact energy and decreased for calibrating rebound hammers of lesser impact energy. The test should be as large a mass as possible in order to minimize the size effect on the result of a full-scale structure.
Minimum 80-100 dry concrete cube specimens are required for calibrating the rebound hammers of lower impact energy (2.3Nm), whereas for rebound hammers of higher impact energy, for e.g.30 Nm, the test cubes of concrete should not be smaller than 300mm.
The entire concrete cube specimen should be kept at room temperature at least for about 24 hours after taking it out from the curing period, before testing it with the rebound hammer.
To establish and get a correlation between rebound numbers and strength of wet cured and wet tested cubes, it is necessary to establish a correlation between the strength of wet tested cubes & the strength of dry tested concrete cubes on which rebound readings are taken.
A direct correlation to rebound numbers on wet concrete cubes and the strength of wet concrete cubes is not recommended. Only the vertical faces (excluding horizontal) of the concrete cubes as cast should be tested.
At least ten 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 of the application must not be impacted more than once.
Interpretation of Test Results
A Rebound hammer test graph is prepared after obtaining the correlation between compressive strength and rebound number (rebound index), the strength of the structure can be assessed.
In general, the rebound number increases as the strength increases and is also affected by a number of parameters i.e. types of cement, types of aggregate, surface condition of the concrete, and moisture content of the concrete, curing, and age of concrete, carbonation of concrete surface, etc.
Moreover, the rebound index is indicative of the compressive strength of concrete up to limited depth from the surface. The internal cracks, flaws, etc., or heterogeneity among the cross–section will not be indicated by rebound numbers. rebound hammer test values should be taken into account.
|Average Rebound Number||Quality of Concrete|
|> 40||Very Good Hard Layer|
|30 to 40||Good Layer|
|20 to 30||Fair|
|< 20||Poor Concrete|
By means of the rebound hammer method, the determination of the strength of concrete cannot be held to be very accurate, and the probable accuracy of prediction of concrete strength in a structure is ± 25 percent.
The correlation between rebound index (rebound number) & compressive strength can be found by tests on core samples obtained from the concrete structure or standards specimens made with the same concrete ingredients and mix proportion, then the accuracy of test results and confidence thereon gets greatly increased.
Read More: Ultrasonic Pulse Velocity Test On Concrete
Points to be kept in mind while performing the Hammer Test
- The concrete surface should be smooth, clean and dry,
- Loose particles should be rubbed off from the concrete with a grinding wheel or stone before start hammering.
- Test should not be conducted on rough surfaces as a result of incomplete compaction, loss of grout, or tooled surface.
- The point of impact of the rebound hammer on the concrete surface is at least 20mm away from edge or shape discontinuity.
- Six readings of rebound numbers are taken at each point of testing and an average of the value of these readings is taken as a rebound index for the corresponding point of observation.
- Apparatus is easy to use.
- Determines uniformity properties of the surface.
- The equipment used in the test is inexpensive.
- Used for the rehabilitation of old monuments.
- The test results obtained are based on a local point.
- The test results are not directly related to the strength and the deformation property of the surface.
- The probe and spring arrangement will require cleaning and maintenance.
- In tests, flaws cannot be detected with accuracy.
Factors Affecting the Rebound Hammer Test:
The important factors that influence Schmidt Hammer Test are mentioned below
- Type of aggregate
- Type of cement
- Surface and moisture condition of the concrete
- Curing and age of concrete
- Carbonation of concrete surface
1. Type of Aggregate
The correlation between the compressive strength of concrete and the rebound number will vary with the use of different aggregates. Normal correlations in the results are obtained by the use of normal aggregates like gravels and crushed aggregates. The use of lightweight aggregates in concrete will require special calibration to undergo the test.
2. Types of Cement
The concrete made of high alumina cement has higher compressive strength compared to Ordinary Portland Cement. The use of super sulphated cement in concrete decreases the compressive strength by 50% compared to that of OPC.
3. Surface and Moisture Condition of Concrete
The rebound hammer test gives the best result for close texture concrete compared with open texture concrete. Concrete containing high honeycombs and no-fines concrete is not suitable to be tested by rebound hammer. The strength is overestimated by the test when testing floated or trowelled surfaces when compared with molded surfaces.
Wet concrete surfaces if tested will give a lower strength value. This underestimation of the strength of concrete structure can go lower to 20% than that of dry concrete.
4. Type of Curing and Age of Concrete
As time passes, the relation between the strength and hardness of concrete will change. Curing conditions of concrete surfaces and moisture exposure conditions of surface also affect this relationship. For concrete with an age between 3days to 90 days in exempted from the effect of age. For concrete having more age, specially calibrated curves are necessary.
5. Carbonation of Concrete Surface
Higher strength is calculated by the rebound hammer on concrete that is subjected to carbonation. It is estimated to be 50% higher. So the hammer tests have to be performed by removing the carbonated layer and testing by rebound hammer over the non-carbonated layer of concrete.
Watch Video: How to Conduct Rebound Hammer Test
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