lab 3

Lab # 3 STATEMENT: To determine the uniaxial compressive strength of rocks using L-type & N-type Schmidt Rebound Hammer.

SCOPE: ASTM D5873: This test method covers the testing apparatus, sampling, test specimen preparation, and testing procedures for determining the rebound hardness number of rock material using a spring-driven steel hammer. This test method is best suited for rock material with uniaxial compressive strengths ranging between approximately 1 and 100 MPa. The portable testing apparatus may be used in the laboratory or field to provide a means of rapid assessment of rock hardness or to serve as an indicator of rock hardness.

APPARATUS: 1. Schmidt Hammer 2. Rock Samples

RELATED THEORY: 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 fig. The hammer weighs about 1.8 kg and is suitable for use both in a laboratory and in the field. 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.

Fig. Schmidt Hammer 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

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).

Uniaxial compressive Strength: The strength or resistance shown by the material up to the point of complete failure in action of the stress applied in the specific single axis. The compressive strength of a material is that value of uniaxial compressive stress reached when the material fails completely. The compressive strength is usually obtained experimentally means of a compressive test. The apparatus used for this experiment is the same as that used in a tensile test. However, rather than applying a uniaxial tensile load, a uniaxial compressive load is applied. As can be imagined, the specimen (Usually cylindrical) is shortened as well as spread laterally. A Stress–strain curve is plotted by the instrument and would look similar to the following.

by

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.

Calculations & Result: Diameter Length Mass(kg) Volume(m3) (cm) (cm)

Sample Quartzit e

0.795

5.46

13.8

Density(g/cm )

Reboun d No.

Specific weight (KN/m3)

2.45

28

24.53

3

0.000324

UCS of Quartzite = 41MPa

Sample

Length Mass(kg) (cm)

Concret e

8.86

15.46

Volume(m )

Density(g/cm )

Reboun d No.

0.003375

2.63

29

3

3

Specific weight (KN/m3) 26.2

UCS of Concrete=48MPa

PRECAUTIONS: Schmidt hammer should be perpendicular to the rock. Otherwise the reading will not be correct.

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.

REFERENCES:   

http://www.astm.org/Standards/C805.htm http://en.wikipedia.org/wiki/Schmidt_hammer http://nonideal2.blogspot.com/2012/09/to-determine-ucs-of-given-rock-sample.html