What is the rebound hammer test? Object, Principle, Apparatus, Procedure, Influence and Interpretation of results
Importance and Rebound Hammer Test :
It is often necessary to test concrete structures after the concrete has hardened to determine whether the structure is suitable for its proposed structure. Such testing should ideally be done without damaging the structure, which is called a Non-destructive Test on concrete. Rebound Hammer is one of the methods of testing for the strength of concrete. Rebound hammer is used for checking the compressive strength of concrete (as per IS-13311(Part-2)/92) at a certain age of concrete.
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| Rebound Hammer Test |
The object of the test:
i) assessing the likely compressive strength of concrete with the help of suitable correlations between rebound index and compressive strength,
ii) assessing the uniformity of concrete,
iii) assessing the quality of the concrete in relation to standard requirements, and
iv) assessing the quality of one element of concrete in relation to another.
NOTE - The rebound hammer method can be used with greater confidence for differentiating between the questionable and acceptable parts of a structure or for relative comparison between two different structures.
A principle of the test:
When the plunger of an instrument is pressed against the surface of the concrete, the spring controlled the mass rebound and the extent of such rebound depends upon the surface hardens of concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound is read off along a graduated scale and designation as the rebound number or rebound index.
Apparatus:
Rebound hammer consists of a spring-loaded steel hammer that when released strikes a steel plunger in contact with the concrete surface. The spring-loaded hammer travels with a consistent and reproducible velocity. The rebound distance of the steel hammer for the steel plunger is measured on a linear scale attached to the frame of the instrument.
It consists of a spring-controlled mass that slides on a plunger within a tubular housing.The impact energy required for rebound hammers for different applications is given in Table 1.
Checking of apparatus:
It is necessary that the rebound hammer is checked against the testing anvil before the commencement of a test to ensure reliable results. The testing anvil should be of steel having Brinell hardness of about 5000 N/mm2. The supplier/manufacturer of the rebound hammer should indicate the range of readings on the anvil suitable for different types of rebound hammers.
The procedure of Obtaining Correlation Between Compressive Strength of Concrete and Rebound Number:
The most satisfactory way of establishing a correlation between the compressive strength of concrete and its rebound number is to measure both the properties simultaneously on concrete cubes. The concrete cube specimens are held in a compression testing machine under a fixed load, measurements of rebound number taken and then the compressive strength determined as per IS 516: 1959. The fixed load required is of the order of 7 N/mm2 when the impact energy of the hammer is about 2.2 Nm. The load should be increased for calibrating rebound hammers of greater impact energy and decreased for calibrating rebound hammers of lesser impact energy. The test specimens should be as large a mass as possible in order to minimise the size effect on the test result of a full-scale structure. 150 mm cube specimens are preferred for calibrating rebound hammers of lower impact energy ( 2.2 Nm ), whereas for rebound hammers of higher impact energy, for example, 30 Nm, the test cubes should not be smaller than 300 mm. If the specimens are wet cured, they should be removed from wet storage and kept in the laboratory atmosphere for about 24 hours before testing. To obtain 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 and the strength of dry tested cubes on which rebound readings are taken. A direct correlation between rebound numbers on wet cubes’ and the strength of wet cubes is not recommended. Only the vertical faces of the cube as cast should be tested. 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 20 mm and should be not less than 20 mm from each other. The same points must not be impacted more than once.
Procedure:
The instrument firmly held perpendicular to the test surface and gradually pushed toward the test surface until the hammer impacted. After impact, the pressure is maintained on the instrument. The rebound number is read on the scale to the nearest whole number and recorded.
For testing, a smooth, clean and dry surface is to be selected. If the loosely adhering scale is present, this should be rubbed off with a grinding wheel or stone. Rough surfaces resulting from incomplete compaction, loss of grout, spalled or tooled surfaces do not give reliable results and should be avoided.
The point of impact should be at least 20 mm away from any edge or shape discontinuity.
For taking a measurement, the rebound hammer should be held at right angles to the surface of the concrete member. The test can -thus be conducted horizontally on vertical surfaces or vertically upwards or downwards on horizontal surfaces. If the situation demands, the rebound hammer can be held at intermediate angles also, but in each case, the rebound number will be different for the same concrete.
Rebound hammer test is conducted around all the points of observation on all accessible faces of the structural element. Concrete surfaces ‘are thoroughly cleaned before taking any measurement. Around each point of observation, six readings of rebound indices are taken as the 2nd average of these readings after deleting outliers as per IS 8900: 1978 becomes the rebound index for the point of observation.
For getting a clear idea you can see the below videos: Rebound Hammer Test
Influence Of Test Conditions
The rebound numbers are influenced by a number of factors like types of cement and aggregate, surface condition and moisture content, age of concrete and extent of carbonation of concrete.
Influence of Type of Cement
Concretes made with high alumina cement can give strengths 100 per cent higher than that with ordinary Portland cement. Concretes made with super sulphated cement can give 50 per cent lower strength than that with ordinary Portland cement.
Influence of Type of Aggregate
Different types of aggregate used in concrete give different correlations between compressive strength and rebound numbers. Normal aggregates such as gravels and crushed rock aggregates give similar correlations, but concrete made with lightweight aggregates require special calibration.
Influence of Surface Condition and Moisture Content of Concrete
The rebound hammer method is suitable only for close texture concrete. Open texture concrete typical of masonry blocks, honeycombed concrete or no-fines concrete is unsuitable for this test. All correlations assume full compaction, as the strength of partially compacted concrete bears no unique relationship to the rebound numbers. Trowelled and floated surfaces are harder than moulded surfaces and tend to overestimate the strength of concrete. A wet surface will give rise to underestimation of the strength of concrete calibrated under dry conditions. In structural concrete, this can be about 20 per cent lower than in an equivalent dry concrete.
Influence of Curing and Age of Concrete
The relationship between hardness and strength varies as a function of time. Variations in the initial rate of hardening, subsequent curing and conditions of exposure also influence the Relationship. Separate calibration curves are required for different curing regimes but the effect of age can generally be ignored for concrete between 3 days and 3 months old.
Influence of Carbonation of Concrete Surface
The influence of carbonation of concrete surface on the rebound number is very significant. Carbonated concrete gives an overestimate of strength which in extreme cases can be up to 50 per cent. It is possible to establish correction factors by removing the carbonated layer and testing the concrete with the rebound hammer on the non-carbonated concrete.
Concrete Age:
The concrete should be 14 to 56 days old.
Surface condition:
The surface of the concrete at which the test has to be done must be smooth, dry and free of honeycomb.
Interpretation of results:
What Is A Rebound Hammer?
Concrete Test Hammers are also known as Swiss Hammers, Schmidt Hammers, or Rebound Hammers. They are versatile tools for assessing the quality of hardened concrete. Although commonly used to estimate in-place compressive strength, concrete test hammers can provide much more value. The complete evaluation of a concrete structure or pavement is about more than just strength. The hammers are portable and easy to use, and their long history of widespread use means that performance and results are well understood.
Who Invented The Rebound Hammer?
The concrete test hammer was invented in 1954 by Ernst O. Schmidt, a civil engineer in Zurich, Switzerland, and commercially developed by Antonio Brandestini through Proceq, the Swiss company he founded. Proceq continues to be a leader in the production and advanced development of this technology, but test hammer devices are available from numerous manufacturers.
Early Design of the Schmidt Hammer
An early design of the Schmidt Hammer
Photo Credit: www.schmidtundpartner.ch/wir/
When To Use The Rebound Hammer
Testing hardened concrete in place is often necessary to determine the suitability of a structure for its intended use. Existing structures must be assessed for structural integrity after years of exposure to harsh environmental conditions and other stresses. Nondestructive evaluations of buildings and pavements still under construction catch deficiencies in materials while there is still time to fix them.
Rapid and efficient collection of concrete uniformity and quality data in both new and existing construction is the test hammer's strong suit. A simple testing program creates an overall picture of concrete quality, highlights areas with lower strength, and isolates areas damaged by freezing or fire.
What is the Rebound Hammer Test?
A straightforward analysis of test hammer data allows quick, cost-effective, and informed decisions on structural suitability. A broad assessment of a structure with a test hammer is a cost-effective way to decide if a more in-depth testing program is needed. Reliable compressive strength measurements are possible using correlation data from laboratory tests. It is important to note that ASTM C805 states that rebound values cannot be used as the basis to accept or reject concrete.
Pushing the test hammer's piston against the surface of hardened concrete loads a spring mechanism until it trips and releases a hammer mass. The mass strikes the piston, which impacts the concrete with a defined amount of energy. As the hammer mass bounces back, a sliding indicator records the rebound number (R-value) on a simple linear scale. Schmidt found that these measurements of surface hardness can indicate relative strength when compared to laboratory test results.
Factors directly affecting rebound numbers include surface texture, moisture, age, carbonation depth, along with the proximity of aggregate, steel reinforcement, and air voids, so care must be exercised when selecting and preparing test areas. The rebound number measures surface hardness, so textured or soft areas require surface preparation before testing. A carborundum stone for manual grinding of the surface is often adequate, but some areas may need a surface grinder for preparation. Damage to the concrete surface from rebound testing is minimal, often leaving just minor dimples in the surface.
What are the Types of Concrete Test Hammers?
Two different versions of concrete test hammers generate different impact energies:
Type N test hammers have an impact energy of 1.63ft-lbf (2.207Nm) that allows testing a wide range of concrete elements and structures. Type N hammers are used on concrete with thicknesses greater than 4in (100mm) and in-place strengths of 1,450 to 10,152psi (10-70mPa). They have limitations for applications where concrete cross-sections are thinner or on "green" early-age concrete with lower strengths.
Type L rebound hammers test concrete in similar compressive strength ranges as Type N models but generate a lower impact energy of 0.54ft-lbf (0.735Nm). These models provide more repeatable values when testing green concrete or areas less than 4in (100mm) thick. They are Ideal for precast pipe or checking early-age concrete for form removal or post-tensioning operations. Type L hammers also reduce cosmetic and structural damage to concrete with thin cross-sections. One Type L model is the Type L Silver Schmidt Hammer can be fitted with a Mushroom Plunger accessory that allows accurate rebound tests on low strength or green concrete with compressive strength as low as 725psi (5mPa).
Strategies for Effective Concrete Assessment
Performing multiple rebound tests at different locations on a structure helps piece together a picture of overall concrete quality, uniformity, and relative strength. Areas with low rebound numbers can be slated for closer examination and testing with Ultrasonic Pulse Velocity Tester measurements, Windsor Probe penetration tests, or other non-destructive testing equipment techniques. Drilled cores can be taken in critical areas to measure compressive strength directly, while zones showing higher strengths can come under less scrutiny.
If the goal is to estimate in-place concrete strength, the testing laboratory must establish the relationship between R-values produced by a particular instrument and compressive strength. ASTM C805 and American Concrete Institute (ACI) 228.1R provide detailed guidance for this procedure using drilled cores or concrete cylinder samples. Cores or cylinders are clamped between the platens of compression testing machines; then rebound hammer impact tests are performed on the specimens just before breaking them for compressive strength values.
Researchers have found that combining two different testing techniques produces more reliable compressive strength estimates. The SONREB (SONic-REBound) method uses an algorithm to correlate impact data from the test hammer and compression wave velocities from ultrasonic pulse velocity (UPV) instruments with existing compressive strength test results. The method provides correction values that allow either device to estimate strength accurately. ASTM C597 describes procedures for using pulse velocity meters on concrete, but there is not yet an ASTM or AASHTO test procedure for the SONREB method.
Many rebound hammers still include a chart converting rebound numbers to a specific compressive strength in psi or mPa. This information does not account for differences in mix design, surface moisture, and other factors and is only useful to indicate relative strengths between different test locations.
Advances in Technology Produce Better Results
More sophisticated rebound hammers have followed innovations in contemporary electronic instrumentation and software on multiple platforms.
Modern test hammers like the Schmidt Hammers capture traditional R-value impact energy measurements with accelerometers, then process, calculate, and store results to eliminate manual recording. GPS positioning, notes, images, and audio comments can also be stored to enhance the testing program's details. Wi-fi and cellular technology and cloud-based connectivity allow immediate distribution of test results to stakeholders, right from the job site.
The most advanced units like the Silver Schmidt Concrete Test Hammer use optical encoders to determine new "Q" impact values. Measurements from these encoders are unaffected by internal friction or impact angle, enhancing repeatability and extending the range of concrete strengths that can be tested. Using correlation curves developed by the operator, Q-values are effective on ultra-high performance concrete (UHPC) with compressive strengths up to 17,405psi (120MPa).
Rebound hammers for geological and geotechnical applications test the strength and consistency of rock formations. These unique models evaluate age, weathering, and strength in rock formations or estimate penetration rates for tunnel boring machines.
Whether you select a simple or advanced model, the concrete test hammer is versatile, reliable, and efficient; a valuable tool and a time-saver to help in the complete assessment of hardened concrete structures.
We hope this blog article has helped you understand the rebound hammer test, to read more visit our concrete test hammers and their usefulness as part of a complete nondestructive evaluation of hardened concrete structures. Please contact our team of testing experts today for help with your applications
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