To determine the UCS of a given rock sample by Schmidt hammer.

Scope:-

To determine the UCS of a given sample to classify rocks.

Apparatus:-

• Schmidt hammer
• Rock specimen

Schmidt hammer:-

A Schmidt hammer, also known as a Swiss hammer or a rebound hammer, is a device to measure the elastic properties or strength of concrete or rock, mainly surface hardness and penetration resistance. The hammer measures the rebound of a spring loaded mass impacting against the surface of the sample. The steel hammer impacts the surface of a concrete with a steel plunger using a predetermined amount of energy and measures the distance that the hammer rebounds. This value is correlated to a compressive strength. The Schmidt rebound hammer is shown in. The hammer weighs about 1.8 kg  and is suitable for use both in a laboratory and in the field. A schematic cutaway view of the rebound hammer is shown in . The main components include the outer body, the plunger, the hammer mass, and the main spring. Other features include a latching mechanism that locks the hammer mass to the plunger rod and a sliding rider to measure the rebound of the hammer mass. The rebound distance is measured on an arbitrary scale marked from 10 to 100. The rebound distance is recorded as a “rebound number” corresponding to the position of the rider on the scale.

The Schmidt rebound hammer is shown in . The hammer weighs about 1.8 kg  and is suitable for use both in a laboratory and in the field. A schematic cutaway view of the rebound hammer is shown in . The main components include the outer body, the plunger, the hammer mass, and the main spring. Other features include a latching mechanism that locks the hammer mass to the plunger rod and a sliding rider to measure the rebound of the hammer mass. The rebound distance is measured on an arbitrary scale marked from 10 to 100. The rebound distance is recorded as a “rebound number” corresponding to the position of the rider on the scale.

Operational Principle:-

The rebound hammer test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. The Schmidt hammer consists of a spring-loaded piston which is released when the plunger is pressed against a surface. The impact of the piston onto the plunger transfers the energy to the material. The extent to which this energy is recovered depends on the hardness of the material, which is expressed as a percentage of the maximum stretched length of the key spring before the release of the piston to its length after the rebound.

Types of Schmidt hammer:-

There are different types of Schmidt hammer classified on the basis of several different energy ranges but often used are

• L Type Schmidt hammer
• N Type Schmidt hammer
• NR & LR Type Schmidt hammer

L Type Schmidt Hammer:-

Hammer is designed for testing thin-walled structural components with a thickness of less than 4" (100mm) or rock cores. This hammer features an impact of 0.74 Nm, 1/3 less energy than the Type N hammers. Include a conversion table with a (N/mm2) scale. L-type hammer has greater sensitivity in the lower range and gives better results when testing weak, porous and weathered rocks.

Type N Schmidt Hammer:-

Type N Hammer is designed for testing concrete items 4" (100mm) or more in thickness, or concrete with a maximum particle size less than or equal to

 

1.25" (32mm).It is designed for testing concrete within a compressive strength range ft-lbs (2.207 Nm).

Type NR & LR Schmidt Hammer:-

With this model Rebound values are recorded as a bar chart on a paper strip. One roll of paper strip offers room for 4000 test impacts.

APPLICATIONS:-

The hammer can be used in the horizontal, vertically overhead or vertically downward positions as well as at any intermediate angle, provided the hammer is perpendicular to the surface under test. The position of the mass relative to the vertical, however, affects the rebound number due to the action of gravity on the mass in the hammer. Thus the rebound number of a floor would be expected to be smaller than that of a soffit and inclined and vertical surfaces would yield intermediate results. Although a high rebound number represents concrete with a higher compressive strength than concrete with a low rebound number, the test is only useful if a correlation can be developed between the rebound number and concrete made with the same coarse aggregate as that being tested. Too much reliance should not be placed on the calibration curve supplied with the hammer since the manufacturer develops this curve using standard cube specimens and the mix used could be very different from the one being tested.

RANGE AND LIMITATIONS:-

1. Smoothness of the test surface:-

Hammer has to be used against a smooth surface, preferably a formed one. Open textured concrete cannot therefore be tested. If the surface is rough, e.g. a trowelled surface, it should be rubbed smooth with a carborundum stone.

2. Size, shape and rigidity of the specimen:-

If the concrete does not form part of a large mass any movement caused by the impact of the hammer will result in a reduction in the rebound number. In such cases the member has to be rigidly held or backed up by a heavy mass.

3. Age of the specimen:-

For equal strengths, higher rebound numbers are obtained with a 7 day old concrete than with a 28 day old. Therefore, when old concrete is to be tested in a structure a direct correlation is necessary between the rebound numbers and compressive strengths of cores taken from the structure. Rebound testing should not be carried out on low strength concrete at early ages or when the concrete strength is less than 7 MPa since the concrete surface could be damaged by the hammer.

4. Surface and internal moisture conditions of concrete:-

The rebound numbers are lower for well-cured air dried specimens than for the same specimens tested after being soaked in water and tested in the saturated surface dried conditions. Therefore, whenever the actual moisture condition of the field concrete or specimen is unknown, the surface should be pre-saturated for several hours before testing. A correlation curve for tests performed on saturated surface dried specimens should then be used to estimate the compressive strength.

5. Type of cement:-

High alumina cement can have a compressive strength 100% higher than the strength estimated using a correlation curve based on ordinary Portland cement. Also, super sulphated cement concrete can have strength 50% lower than ordinary Portland cement.

6. Carbonation of the concrete surface:-

In older concrete the carbonation depth can be several millimeters thick and, in extreme cases, up to 20 mm thick. In such cases the rebound numbers can be up to 50% higher than those obtained on an un-carbonated concrete surface.

Procedure:-

• Press the Schmidt hammer against a stone surface.
• At a given moment, the spring loaded mass is automatically impelled against the plunger and the rebound of the mass is indicated on the graduated scale by a pointer. The reading obtained is related to the initial tension of the spring and is called the Schmidt hardness number or Rebound number (R).
• As the result of each measure is dependent on the direction of the pressure exerted, the reading must be taken from the appropriate curve attached to each hammer to convert rebound in tension.

 

Precautions:-

• Hammer has to be used against a smooth surface, preferably a formed one. Open textured concrete cannot therefore be tested.
• When conducting the test the hammer should be held at right angles to the surface which in turn should be flat and smooth.

Comments:-

• It is a portable device and easy to handle.
• Being a portable it can be used to determine the in situ strength of the rocks.