Rebound hammer test for Hardened concrete?

We use rebound hammer to calculate the compressive strength of concrete. Concrete construction is generally gives trouble free service throughout its intended design life. However, we require these test , because of structural deficiency, material deterioration. The most common being when we fail to follow the construction specification or when the facility is expose to harsher service environment than the planning and design stages.

Check this post to have an idea about “different tests for hardened concrete“….

Physical damage can also arise from fire, explosion as well as from restraints, both internal and external against structural movement. Except in extreme cases, most of the structures require restoration to meet its functional requirement by appropriate repair techniques. One of many factors connected with the quality of concrete is its hardness.

Early methods involve measurements of the size of indentation caused by a steel ball either fixed to a pendulum or spring hammer, or fired from a standardized testing pistol.

Later, however note the height of rebound of the mass from the surface. Although it is difficult to justify theoretical relationship between the measured values from any of these methods and the strength of a concrete, their value lies in the ability to establish empirical relationships between test results and quality of the surface layer. Unfortunately these are subject to many specific restrictions including concrete and member details, as well as equipment reliability and operator technique.

Fundamental principle

The Schmidt rebound hammer is principally a surface hardness tester. It works on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. There is little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer

Rebound test equipment and operation

  • Rebound hammer weighs less than 2 kg, and has impact energy of approximately 2.2 Nm.
  • The spring-controlled hammer mass slides on a plunger within a tubular housing.
  • The plunger retracts against a spring when pressed against the concrete surface and this releases the spring automatically when in full tension causing the hammer mass to impact against the concrete through the plunger.
  • When the spring-controlled mass rebounds, it takes with it a rider which slides along a scale and is visible through a small window in the side of the casing.
  • The rider can hold in position on the scale by depressing the locking button.
  • The equipment is very simple to use, and may operate in horizontal, vertical, upwards or downwards direction.
  • Press the plunger strongly and steadily against the concrete at right angles to its surface, until the spring-loaded mass is triggerd from its lock position.
  • After the impact, the scale index is read while the hammer is still in the test position. Alternatively, press the locking button to retain the reading , or record the results automatically by attaching a paper recorder.
  • The scale reading is the rebound number
  • Normally we use 20–60N/mm2 strength range for concrete testing.
  • Electronic digital reading equipment with automatic data storage and processing facilities is also widely available.
  • For low strength concrete rebound hammer with enlarged area is also available.

Note : Rebound number is an arbitrary measure since it depends on the energy stored in the given spring and on the mass used.

Compressive strength and rebound number – correlation

The most satisfactory way of establishing a correlation between compressive strength of concrete and its rebound number is to measure both the properties simultaneously on concrete cubes.

Concrete cube testing

  • Place the concrete cube specimens in a compression testing machine under a fixed load. Then measure rebound number and determine the compressive strength 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 .
  • decreased for calibrating rebound hammers of lesser impact energy.
  • The test specimens should be large in order to minimize the size effect on the test result of a structure.
  • 150mm cube specimens are preferred for calibrating rebound hammers of lower impact energy (2.2Nm).
  • 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.

Key points we should keep in mind

  • Use only the vertical faces of the cubes as cast for testing.
  • At least nine readings should be taken on each of the two vertical faces accessible in the compression testing machine.
  • The points of impact must not be nearer an edge than 20mm and 20mm from each other.
  • The same points must not be impacted more than once.

Thus ,we can correlate the results obtained from compression testing method and the rebound number obtained for the same cube.

In this way, we can relate rebound number with compressive strength. Thus it is possible for us to calculate the compressive strength for practical cases.

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