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Lab Report GIS and RS 2012-GE-56
11
 LAB: 02 
Statement:
To Determine The Uniaxial Compressive Strength Of Rocks Using L-Type & N-Type Schmidt Rebound Hammer
Designation:
 ASTM D5873 ISRM
What is The Uniaxial Compressive Strength?
The strength of rock or soil sample when crushed in one direction without lateral strain.
Scope:
To study how to determine the strength of hardened concrete using a Schmidt Rebound Hammer and to study the internal mechanism of this hammer.
Principle:
Its principle is very easy; the stone sample is struck by the elastic energy stored in a spring. The rebound of a steel mass, as the result of the propagation of the energy not absorbed by the stone, is indicated in a scale.Its fast and inexpensive character allows the acquisition of a large number of data; the values obtained in smooth surfaces are in general bigger than in irregular surfaces.Results are more coherent in homogenous stones.
Apparatus:
 Schmidt Rebound Hammer
 Rock Samples
 Calibration anvil
 Steel base
 Abrasion stone
Related Theory
Generally three types of test are used for measuring the uniaxial compressive strength.
 UCS Test and Tri-axial
 Point Load Index Test
 Schmidt Rebound hammer
Tri-axial test is the most accurate test and then UCS is near to accuracy. Point load test is less accurate then UCS test and Schmidt hammer have the lowest accuracy.

12
Schmidt Hammer:
In 1948 Ernst Schmidt invented a device which made nondestructive compressive strength testing feasible. A Schmidt hammer is a device to measure the elastic properties or strength of concrete or rock. Today Schmidt hammers are in use throughout the world for estimating strength of concrete. The European standard for Schmidt hammer testing is EN 12504-2. The test hammer hits the concrete with a spring-driven pin at a defined energy, and then measures the rebound (in rebound units). Its rebound is dependent on the hardness of the concrete and is measured by test equipment. When conducting the test the hammer should be held perpendicular to the surface which in turn should be flat and smooth. Note that the Schmidt hammer does not work well for small samples and will make marks. By reference to the conversion tables, the rebound value can be used to determine the compressive strength. Schmidt hammers are available from their original manufacturers in several different energy ranges.
Types of Hammer:
Schmidt hammers are available from their original manufacturers in several different energy ranges. These include:
 Type L-0.735 Nm impact energy:
 Strength of rock near to 100MP we use this type.
 It is used to measure UCS of rocks.
 Type N-2.207 Nm impact energy:
 Strength of the rock more than 100MP e.g 200MP or 250 MP
 It is used to measure UCS of concrete.
Rebound Number:
It is the test menu which allows testing & storage & processing of results.
Calibration:
It is the calibration menu of the instrument.
Set curve:
It is the menu which allows the operator to define a correlation curve based on his needs & experimental experience.

13
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. Here 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.
Following is UCS scale from which quality of rocks can be finding:
Abrasion stone:
It is used to smooth the surface before we take the reading.
Procedure:
1) Hold the instrument firmly so that the plunger is perpendicular to the test surface.
2) Gradually push the instrument toward the test surface until the hammer impacts.
3) After impact, maintain pressure on the instrument and, if necessary, Average Rebound No. Quality of Concrete >40 Very good hard layer
30-40
Good layer 20-30 Fair
<20
Poor concrete 0 Delaminated UCS Quality <60 Poor
60-90
Fair 90-150 Good
150-220
Very good >220 excellent

14
4) Depress the button on the side of the instrument to lock the plunger in its retracted position.
5) Estimate the rebound number on the scale to the nearest whole number and record the rebound number.
6) Take ten readings from each test area. No two impact tests shall be closer together than 25 mm (1 in.).
7) Examine the impression made on the surface after impact, and if the impact crushes or breaks through a near-surface air void disregard the reading and take another reading.
Observations & Calculations:
 For quartzite sample:
 Mass of quartzite = 795gm
 Weight in kN= 7.7989×10-3
 Diameter of quartzite sample = 0.0546m
 Length of quartzite sample = 0.1384m
 Radius of Quartzite sample= 0.0273m
 Volume of sample= V=πx2 l
Putting values we get
V= (3.1416)(0.0273)2(0.1384)
V=3.2×10-4m3
So density= 7.7989×10-3/3.24×10-4 So density = 24kN/m3 UCS will be equal to (by graph) = 41MPa
 For concrete sample:
 Mass of concret= 8.860 kg
 Length of concret = 0.1546 m
 Weight of concret block= 0.0869kN
As concret sample was a perfect cube so all sides are equal in length
 Radius of concret block= 0.773m
As Volume of sample= 13 Serial no Rebound no 1 30
2
28 3 27
Average
28.33

15
 Volume= (0.1546)× (0.1546)× (0.1546) Volume =3.69×10-3m3
Density = 0.086/3.69×10-3 Density = 23.5kN/m3 UCS will be equal to (by graph) = 43MPa. Serial no. Rebound no. 1 36
2
32 3 20
4
30 5 37
6
32 7 29
8
37 9 30
10
32 Average 34.5
Result based on Rebound No:
 The quartzite rock is of poor quality.
 And concrete sample have good layer quality.
Precautions:
 Keep the hammer vertically.
 Carefully note the readings of weighing machine.
 Cores should be of at least NX size (X54.7 mm) for the L-type hammer and preferably T2 size (X84 mm) for the N-type.
 Block specimens should be at least 100mm thick at the point of impact.
 On the other hand, the test may be stopped when any ten subsequent readings differ only by four (corresponding to SH repeatability range of 72).
 Specimens should be intact (free of visible cracks), petrographically uniform and representative of the rock mass domain (identified from cores or exposures) being characterized.
 Test surfaces, especially under the plunger tip (impact points), should be smooth and free of dust and particles.

16
 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.
Comments:
 Hold the button carefully after completely press the Schmidt hammer.
 Schmidt hammer should be perpendicular to the rock; otherwise the reading will not be correct.
 Don’t press the hammer again on the same place of sample, which we used before.
References:
 http://www.astm.org/Standards/C805.htm
 http://en.wikipedia.org/wiki/Schmidt_hammer
 http://www.google.com.pk/imgres?hl=en&client=firefox- a&hs=ORc&sa=X&rls=org.mozilla:en- US:official&biw=1366&bih=638&tbm=isch&prmd=imvns&tbnid=dWkNpxykCOhtgM: &imgrefurl
 https://docs.google.com/viewer?a=v&q=cache:DNUbNkqyqlIJ:www.tj- jwdz.com/en/ASTM%2520C%2520805_C%2520805M%2520EN.pdf+Standard+Test+Method+for+Rebound+Number+of+Hardened+Concrete+1&hl=en&gl=pk&pid=bl&srcid=ADGEESgYtV286unTZOR1AfGk21vAsb2759d6uPFPSejuRqmvKbmUG4j3VYWbSCHqsBAQQq69Y97vfOvHVE9T2tIbkEfhRNXX_ZfLt_b4CSLhkzq- tzdr1BpomHo6rRlYtwoepy1xCJQE&sig=AHIEtbSLVC2eH2jKibq4DkI6KFilsj63sw
 http://civil-online2010.blogspot.com/2010/09/schmidt-rebound-hammer-test.html
 http://www.astm.org/Standards/D5873.htm
 http://www.proceq.com/products/concrete-testing/concrete-test-hammer/original- schmidt.html
 http://www.humboldtmfg.com/test_hammers.html
 Lab o2 completed 

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