Humanity has amassed great feats of construction and structural marvels. Take the Colossus of Rhodes for example. It stood tall, at 108 feet (Approximately the same as the Statue of Liberty), and was the pride of Ancient Greece. In 226 BC, however, it was destroyed due to natural elements and snapped at its knees.
This incident destroyed the ancient harbor and the buildings around it. The ancient Greeks believed this incident occurred because they offended the God Helios, and refused to rebuild the marvel out of fear. The existence of non-destructive techniques would’ve not only prevented this incident but would have prevented the banishment of a lot of officials and we probably would have experienced the grandeur of the structure for longer, as a species.
Testing of pre-existing structures can often be a long drawn out and invasive procedure, but lack of constant monitoring and testing can lead to catastrophic disasters. To deal with such scenarios, there exists a domain of testing procedures called non-destructive testing. Non-destructive testing uses various apparatus to gauge the current state of the testing subjects in a non-invasive manner.
The Rebound Hammer Test is an uncomplicated Non-Destructive Testing technique wherein the compressive strength of concrete, or an existing structure can be measured and analyzed. The modus operandi of this technique makes it efficient and economical. Extensive structures cannot have intrusive testing performed on them to check longevity without incurring extravagant costs, hence the Rebound Hammer Test poses as a lucrative option in performing structural integrity tests.
Corrosion probability, compressive strength, and homogeneity of concrete are highly crucial properties of concrete that need responsible monitoring to avoid damage due to deterioration. To maintain the resilience of concrete structures, non-destructive testing (NDT) needs to be carried out on the in-situ condition of the reinforced concrete (RCC) structures. The parameters causing such deterioration to RCC structures need to be carefully evaluated and monitored using methods like the Schmidt Hammer Test.
The reconstruction of existing concrete structures and repair of battered buildings are in demand. These structures are often riddled with spalling, leakage, corrosion, carbonation, sulfate attacks, etc. These defects need to be countered in their nascent stages to avoid catastrophic issues in the future.
Complete Guide on Destructive Testing vs Non-Destructive Testing
Rebound Hammer Testing Apparatus (Image Credit: https://spectro.in/Rebound-Hammer-Test.html)
Invented by Ernst O. Schmidt in the 1950s, the Rebound Hammer Test also called the Schmidt Hammer Test has survived technical advancements through the ages and has evolved from analog to digital, with the times. Newer, automated Schmidt Hammer Test apparatus make obtaining rebound numbers less cumbersome and increases efficiency by 90%. They also measure the coefficient of restitution CR (or Leeb Hardness).
WHAT IS THE REBOUND HAMMER TEST
The Schmidt Hammer Test, also called the Rebound Hammer Test is an apparatus for rapid testing of various construction materials like metal, concrete, artificial stone, ceramic products, etc. whether the surface is horizontal, vertical, or inclined; whereby a correction factor would be applied to the rebound intensity for a given inclination.
Rebound Hammer Schematic (Image credit: https://aqcinspection.com/rebound-hammer-test-non-destructive-testing-on-concrete/)
The rebound hammer test assesses the state of concrete by gauging its hardness, indicated by the rebound number. The value of the rebound number is directly proportional to the strength of the concrete. The apparatus of the Schmidt Hammer Test consists of a frame designed to be placed against the test material, with a movable hammer housed inside the frame for impact on the test surface.
The hammer weighs around 1.8 kg and is perfect for field and industry usage as it is easy to repair and maintain. Its small dimensions and light weight make it portable.
This setup is connected to an indicator system that measures the rebound of the hammer after impact on said surface. A resilient element, like a spring, is placed between the frame and the hammer which sets the hammer in motion by the energy stored internally.
The hardness properties of a material can be correlated to other properties of a material experimentally and mathematically. The Indian Standard for the average value of the rebound number/ rebound index for the Schmidt Hammer test (IS:13311 Part-21992) is given as follows:
Prior to testing, the Schmidt Hammer Test apparatus requires calibration. For this purpose, manufacturers supply a calibration anvil that verifies unmitigated operation and appropriate rebound energy values. A sample calibration anvil, e.g., an HM-76B Concrete Test Hammer Calibration Anvil has a hardened alloy steel (66±2 HRC hardness) plate and provides a hammer guide and is intended to provide a minimum rebound reading of 75.
As per ASTM C805/805M standards, the test anvil is made of tool steel of 150mm height and 75mm radius with a hardened impact area. The Brinell hardness number of the test anvil is approximately 5000N/mm².
Test Hammer Calibration Anvil (Image credit: https://www.globalgilson.com/test-hammer-calibration-anvil)
The manufacturers of the hammer provide pre-set correlation curves using standardized concrete cube specimens. Due to dissimilarities in a testing environment and material is not recommended to use these pre-set correlation curves and instead to carry out another correlation procedure.
Some of the resulting correlation curves drastically deviate from those provided by the manufacturer.
Selection of test surface:
The structures/parts to be tested are required to be at least 100mm thick and fixed. Support should be provided for smaller specimens. The form material and finish of the surfaces need to be considered. A screed finish surface and a formed finish surface would have a lower rebound number than a trowelled surface. Hence finished surfaces are often avoided and testing is carried out on the underside.
The test surface should be of an appropriate size in order to reduce the size effect in the results of a full-scale structure.
Preparation of test surface:
The test surface should follow the following requirements:
· The test surface should be thoroughly free of debris, moisture, and grease.
· The surface should be freed of loose particles with a grinding wheel or stone prior to testing.
· Cracked and delaminated surfaces (Spalling), un-compacted concrete, unsealed formwork (leading to grout loss) and tooled surfaces should not be used for testing.
· The impact point of the hammer should be at least 20 mm away from edges and shape discontinuities.
The condition of the surface to be tested greatly impacts the rebound number, as follows:
· Wet concrete surfaces provide lower rebound numbers than dry surfaces.
· Carbonated layers increase rebound numbers.
· Frozen concrete should be avoided. At 0°C, wet concrete provides high rebound numbers.
· Impact angle and direction should be carefully considered and kept uniform in comparative readings.
· The molded type, cement type, type of coarse aggregate, and the specimen’s age, also greatly affect the rebound number.
The temperature of the rebound hammer itself affects the test results and must be taken into consideration. When at -18°C, the hammer might provide a reduction in rebound number by 2 or 3 units.
Local inhomogeneity affecting the rebound hammer test a)large gravel, b) voids, c)gravel aggregate, d) bleeding, e)reinforcing bars close to the surface (Image credits: Civil Engineering and Architecture 1(3): 66-78, 2013 http://www.hrpub.org DOI: 10.13189/cea.2013.010303)
Operation of the Schmidt Hammer:
The Rebound Hammer test uses a spring accelerated mass that slides along a guide bar to be impacted on one end of a steel plunger. Light pressure can be applied to the steel plunger to release it from its locked position.
The other end of the steel plunger is then compressed against the concrete perpendicularly, until the hammer impacts. The button provided on the side of the instrument can be depressed to lock in the position of the plunger.
(Image credits: https://www.fprimec.com/estimate-concrete-strength-using-rebound-hammer)
The resulting rebound number on the scale can be rounded off to the nearest whole number. Multiple readings should be recorded from each test area (approximately 6-10 readings). A distance between impact tests should be maintained (25 mm) for an accurate reading.
Rebound Hammer Test requires different impact energies for different applications, the values of which are given below:
Inference of Results:
· The Schmidt Hammer Test correlates the compressive strength of the material tested to the rebound index number indicated by the test apparatus. This rapid testing method provides a convenient indication of compressive strength.
· Measuring both compressive strength and rebound index simultaneously on concrete cubes provides an adequate correlation between the two data.
· As per IS: 516-1959, a compression testing machine under fixed load can be used to test these specimen concrete cubes, and the rebound number is used to indicate compressive strength. The impact energy of the hammer in this case is adjusted to 2.2 Nm with a fixed load of 7N/mm2. Rebound hammers with greater impact energy will require greater load and vice versa.
· A graduated scale is used to measure rebound distance which is called the rebound index/the rebound number.
· The test can be conducted in horizontal, vertical, and even intermediate positions, as long as all the iterations follow a uniform orientation.
· The resulting graph below indicates a curve wherein the X-axis represents the rebound number, and the Y-axis represents the cube compressive strength in N/mm2. The three curves in the graph represent position A, position B, and position C as shown in the position diagram, respectively.
· The resulting graph hence gives us the rebound number which provides a 95% reliability in indicating the quality of the concrete present.
Rebound hammer test graph (Image credits: https://aqcinspection.com/rebound-hammer-test-non-destructive-testing-on-concrete/ )
Read SNT-TC-1A - Personnel Qualification and Certification in Non-destructive Testing
WHO INVENTED THE REBOUND HAMMER
The inventor of the Schmidt Hammer Test was a German scientist and engineering genius, Ernst O. Schmidt. A recipient of the Grashof Commemorative Medal, which is one of society’s highest awards for merit in the field of engineering, Ernst O. Schmidt is also the inventor of the Schmidt number.
In 1948, Ernst O. Schmidt founded an agency for building construction and structural model tests in Basel, Switzerland. He furthered his studies at ETH, Zurich, Switzerland, and then encountered methods for studying structural problems with model tests during his studies at MIT, Boston, USA.
The Rebound Hammer Test was commercialized by a company named Proceq in the 1950s, founded by Antonio Brandestini, an inventor himself. Proceq still designs apparatus for the Rebound Hammer Test models for various test applications, having different impact energies.
The newest Rebound Hammer Test apparatus runs on Android and iOS, providing time-efficient live test reports mingled with the time-proven efficiency of the Schmidt Hammer Test.
WHEN TO USE THE REBOUND HAMMER
Concrete structures can develop cracks due to shrinkage phenomena and lose structural integrity over time which needs to be appropriately assessed. Failure to do so may lead to catastrophic losses to not just the structure, but also to human life and the environment around.
Residual Life Assessment (RLA) is vital for such structures or equipment to avoid critical failure. Such structures and equipment are operated under harsh temperature and pressure conditions and need to be responsibly monitored and maintained as per industry standards.
Testing old structures involves the removal of the top layer of concrete and plaster and performing strength tests on cores obtained henceforth. This process is cumbersome, expensive, and destructive. Hence, in such situations, non-destructive testing methods such as the Rebound Hammer Test are preferred.
ADVANTAGES AND DISADVANTAGES OF THE REBOUND HAMMER TEST
The primary benefit of using the Rebound Hammer Test is the direction of impact while testing can be variable i.e. it can be used in a horizontal, vertical, or angled orientation.
The Rebound Hammer Test also provides the option of testing old structures to estimate their longevity. The apparatus is also affordable and portable compared to other non-destructive testing techniques.
This process is also quick and saves skilled labor hours expended by organizations.
The primary disadvantage of the Rebound Hammer Test is the regular maintenance and calibration required because of the intricate mechanisms that rely on the uniformity of readings.
A relative analysis of the rebound number to the crushing strength needs to be conducted to find the extent of resistance of the surface under load.
The minuscule mechanisms also make it difficult to pinpoint defects in the Rebound Hammer.
OBJECTIVES OF THE REBOUND HAMMER TEST
Non-destructive testing opens various avenues in the field of engineering as various testing can be carried out on different materials, structures, and mechanisms without causing any permanent damage to them.
The simple Schmidt Hammer apparatus can be used to fulfill the following testing objectives:
· Creating a correlation between the compressive strength and rebound index to assess the compressive strength of structure/material under testing.
· To analyze the uniformity of concrete.
· To check if the quality of concrete meets industry standards.
· Creating a comparative analysis of various elements of the concrete.
PRINCIPLES OF THE REBOUND HAMMER TEST
The Rebound Hammer Test is based on the principle that an elastic mass rebound is dependent on the surface hardness of the object it impinges.
The extent of rebound of the spring-controlled mass on pressing the steel plunger against the concrete varies based on the surface hardness of the concrete.
On pushing the plunger, the connection spring between the body and hammer mass gets stretched. By pushing to the limit, the latch inside the hammer is released and the hammer mass is propelled by the energy stored in the spring.
On the rebound, the indicator slide travels with the hammer mass, hence recording rebound distance. The push button lets the user lock in the position above and the scale reading provides the user with the rebound number.
Read Top 3 Important Lessons Technicians Can Learn In NDT Training & Courses
REBOUND HAMMER TEST PRICE
The industry provides a variety of hammers to carry out the Rebound Hammer Test. There are two main types available in the market with variable impact energies. Various types of Schmidt concrete hammers can be purchased in the industry like the Silver Schmidt, Schmidt Live, and the Original Schmidt Hammer.
Models with both analog and digital systems can be found with advanced data handling features. These kits come with peripherals like a case, reference chart, silicon carbide stone, etc. Calibration anvils are also available for purchase to maintain the accuracy and consistency of readings.
A few examples of Schmidt Hammer models available, in different price ranges, are as follows:
(Image credits: IndiaMart)
Appropriate statistical analysis of the data obtained in the rebound hammer tests is vital and ensures uniformity and accuracy of results. Modern non-destructive techniques, especially in this age of limited resources and increasing demand, helps to conserve existing structures, materials, and mechanisms, hence saving time, money, and work hours.
Newer, digital models of the device pose great potential for improvement in the quality of such tools. Digital tools with better data management, voice functions, and simplified usage systems help increase the iterations of the tests, in turn ensuring better quality data.
The rebound hammer test is an excellent Quality Control tool and helps organizations keep up with safety and industry standards.