Rebound Hammer Test: Everything You Must Know

Introduction

The non-destructive rebound hammer test can quickly and accurately determine the concrete’s compressive strength. Schmidt hammers, also referred to as rebound hammers, are composed of a mass that descends a plunger inside a cylindrical casing under the control of a spring.

What is a Rebound Hammer?

Swiss Hammers, Schmidt Hammers, and rebound hammers are a few other names for concrete test hammers. They are versatile instruments used to assess the caliber of hardened concrete. However, concrete test hammers have much more to offer than simply determining compressive strength.

Strength is one of many factors to consider when evaluating a concrete building or pavement. The hammers are portable and straightforward to use, and given that they have been in everyday use for so long, it is likely that their effectiveness and results have been thoroughly investigated.

Who came up with Rebound Hammer?

The concrete test hammer is attributed to Zurich, Switzerland, civil engineer Ernst O. Schmidt, who created it in 1954. The device was developed for commercial use by Antonio Brandestini, who founded the Swiss company Proceq.]

What is Rebound Hammer Test?

The rebound hammer test gathers information about the caliber and homogeneity of concrete in newly constructed and existing buildings quickly and efficiently. A simple testing procedure gives a general impression of the condition of the concrete, highlights areas with decreased strength, and identifies areas that have been harmed by freezing or fire. In addition, using correlation data from laboratory experiments makes it simpler to obtain precise values of compressive force.

The test hammer’s piston strikes hardened concrete, loading a spring mechanism until it trips and releasing the hammer mass. When a group strikes a piston, a predetermined energy is transferred from the assembly to the concrete. As the hammer mass moves along an indicator, the rebound number (R-value) is recorded using a straightforward linear scale. Schmidt found that these surface hardness values might represent relative strength compared to laboratory test results.

What Is the Purpose of the Test?

According to the Indian code IS: 13311(2)-1992, the rebound hammer test is intended to achieve the following objectives:

• To determine the relationship between the compressive strength and rebound index to determine the compressive strength of the concrete.
• To assess the concrete’s consistency.

• To evaluate the concrete’s quality following the standards.
• To contrast an element’s properties with those of another piece.

It is possible to distinguish between the parts of the construction that are acceptable and those that are problematic using the rebound hammer technique. And to contrast the advantages of the two various structures.

Things to consider as you take the Test!

• The concrete’s surface needs to be dry, clean, and smooth.
• Any loose particles on the concrete’s surface should be eliminated with a grinding wheel or stone before performing a hammer test.
• The rebound hammer test is not advised on rough surfaces, such as those brought on by insufficient compaction, grout loss, or spalled or tooled concrete surfaces.
• It is advised that any edge or form discontinuity be at least 20 millimeters from the rebound hammer’s point of impact on the concrete’s surface.
• Six readings of the rebound number are taken at each testing location. The average of these readings is used to determine the rebound index for the reading’s corresponding point of observation on the concrete surface.

Techniques for selecting and preparing testing sites

• Wetness, surface quality, age, carbonation depth, proximity to aggregate, steel reinforcement, and air voids are all factors that directly affect rebound figures, so care must be taken when selecting and setting up test locations.

• Rebound measurements on concrete with high moisture content are lower than those on the same concrete when it is dry. If measuring compressive strength by correlation with cores or cylinders, the test surface should be wet with water for a few hours before collecting rebound data. The correlation sample evaluation should be done under saturated, surface-dry (SSD) conditions.
• Testing on textured or soft surfaces necessitates surface preparation because the rebound number measures surface hardness. The cover can usually be manually smoothed with a carborundum stone, though some areas might require a surface grinder. Rebound testing leaves a few small dimples on the concrete’s surface and causes minor damage to it.

How should the rebound hammer be used?

Although the rebound hammer is a user-friendly piece of testing equipment, to obtain accurate and reliable results, many prerequisite conditions must be met before testing begins.

Getting the test surface ready:

Inspectors should be very aware of the following considerations before performing a rebound hammer test:

• First, the testing surface must have holes at least 150 millimeters in diameter.
• If the concrete’s character has a lot of texture or lose mortar, it should be ground (until it is flat).
• Finally, any free moisture or water on the concrete’s surface must be removed before conducting any testing.
• Never test a frozen concrete surface. It is significant to note that testing concrete should only be done after it has thawed because fixed concrete tends to have high rebound figures.
• Direct testing over the steel reinforcement should be avoided when the cover thickness is less than 20 mm. It is best to use a rebar locator to prevent testing on support with a shallow depth.

The rebound hammer’s direction:

The inspector must take a firm hold of the tool and position the plunger so that it is perpendicular to the surface being tested to complete the rebound hammer test. It is also necessary to record the instrument’s angle about the horizontal to the nearest 45 degrees. If the device is pointing upwards, a positive angle should be used; if it is pointing downward, a negative angle should be used.

Collecting Readings:

The inspector will carefully move the instrument closer to the tested surface until the hammer makes contact with the surface after the device has been aligned and the angle has been recorded. To keep the plunger in the retracted position after the impact, you should keep applying pressure to the tool and pressing the button on the side.

For the results to be accurate, each test region must take at least ten different readings. Additionally, at least 50 mm between the edges of the member and the impact sites and at least 25 mm between the impact locations. Finally, the measurement must be discarded, and a new reading must be taken if the impact breaks or crushes an air void close to the surface.

Uses for the rebound hammer test:

The process of determining the relative hardness of concrete using the concept of the rebound is where a rebound hammer is most frequently used. Despite this, academics have tried to use the method to assess several concrete qualities. Examples of general applications include the following:

• First, check the consistency of the concrete before it is installed.

• Set boundaries around areas of a structure made of poor-quality or deteriorating concrete.

• As a method for determining concrete’s strength while it’s still in place

Rebound hammer test advantages and disadvantages:

Among these applications, using a rebound hammer to assess the strength of concrete has been and is still up for discussion. Researchers have differing views about the best method for using a rebound hammer to gauge the strength of concrete.

Advantages: 

• The equipment is easy to use.
• Identifies the homogeneity and consistency of the surface.
• In addition, the utilized equipment is reasonably priced.
• Used for historic building and structure restoration.

Disadvantages: 

• The results are predicated on the neighborhood.
• The results of the tests do not directly affect the surface’s tensile strength or ability to deform.
• The probe and spring arrangement will require routine cleaning and maintenance.
• It is only possible to pinpoint flaws with accuracy.

What influences a rebound hammer test’s outcomes?

Aggregate Type – The relationship between the compressive strength and rebound number of concrete will change when different types of aggregate are used. Standard correlations in the results are produced using common aggregates, such as gravel and crushed aggregates. However, concrete’s lightweight aggregates will require customized adjusting for the substance to pass the test.

Cement Type – Concrete made with high alumina cement should have greater compressive strength than concrete made with regular portland cement. Compared to concrete made with ordinary Portland cement, the compressive strength of super-sulfated cement is reduced by half.

Moisture Condition And Surface Type – When used on concrete with a close texture as opposed to concrete with an open texture, the rebound hammer test is more accurate. Rebound hammers cannot be used to gauge the strength of concrete containing many honeycombs or no fines.

When tested on floating or trowelled surfaces compared to testing on molded covers, the strength of the material is overestimated. Testing a wet concrete surface will reveal that the power has decreased. Contrary to when it’s dry, the concrete’s strength can be underestimated by up to 20%.

Age And Curing Of Concrete – Over time, there will be changes in the strength-to-hardness ratio of the concrete. This relationship is also influenced by the conditions in which concrete is exposed to moisture and allowed to cure. Concrete that has been aged for three days or more but fewer than ninety days does not experience the effects of aging. Therefore, it is necessary to use specially calibrated curves to account for rising amounts of old concrete.

Carbonation on the concrete’s surface – The rebound hammer calculates the increased strength of carbonated concrete when used with that material. It is anticipated to be 50% higher. Therefore, to complete the test, you must take the carbonated layer off the concrete and conduct it using a rebound hammer on the uncarbonated layer.

Points to consider regarding the rebound hammer test:

• The rebound hammer, which Schmidt invented, provides a quick and reasonably priced method for non-destructively testing concrete.
• Recognizing and considering the testing method’s limitations is crucial when using the rebound hammer.
• 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.

Conclusion:

Concrete’s compressive strength is measured using the Rebound Hammer test, and the higher the rebound height, the stronger the concrete.

In general, it is helpful as a non-destructive testing technique for determining concrete’s compressive strength. A metal plunger tests the concrete’s exterior, and a spring-loaded device calculates the plunger’s rebound.

The test can be carried out on concrete surfaces that are vertically or horizontally oriented, as well as on other surfaces like beams and columns.