Lecture ( 6 )

Sixth Lecture: 31 / 03 / 2015 Course of: Measurement of Rock Properties

1

This method covers procedures for testing aggregate for resistance to abrasion using LOS ANGELES TEST MACHINE.


2

Sample preparation

The test sample shall be washed & oven-dried at 105 C to 110 C to substantially constant mass.

The sample shall be separated into individual size fractions & recombined to the grading of Table 1 & 2 most nearly corresponding to the range of sizes in the aggregate as furnished for the work.

The mass of the sample shall prior to test shall be recorded to the nearest 1 g (m1).


3

The charge shall consist of 12 steel spheres averaging approximately 47 mm (12732 in.) in diameter, each having a mass between 390 & 445 g, & having a total mass of 500025 g.

Place the test sample & the charge in the Los Angeles testing machine & rotate the machine at a speed of 30 to 33 rev/min for 500 revolutions.

The number of revolutions shall be 500 for aggregate smaller than 38 mm. The number of revolutions shall be 1000 for aggregate larger than 19 mm.

Grading No of Steel balls Weight of charge in gm

A 12 5000 25

B 11 4584 25

C 8 3330 20

D 6 2500 15

1 12 5000 25

2 12 5000 25

3 12 5000 25


4

After the prescribed number of revolutions, discharge the material from the machine & make a preliminary separation of the sample on a sieve coarser than the 1.70-mm (No. 12). Sieve the finer portion on a 1.70-mm sieve in a manner conforming to Test Method C 136. Wash the material coarser than the 1.70-mm sieve & oven dry at 1105C (2309F) to substantially constant mass & determine the mass to the nearest 1 g If the aggregate is essentially free of adherent coatings & dust, the requirement for washing after test may be waived, but drying before the test is always required. However, in the case of referee testing, the washing procedure shall be performed.

Valuable information concerning the uniformity of the sample under test may be obtained by also determining the loss after 100 revolutions in the case where 500 revolutions is specified or after 200 revolutions in the case where 1000 revolutions is specified. The loss should be determined without washing the material coarser than the 1.7-mm sieve. The ratio of the loss after 100 or 200 revolutions to lose after 500 or 1000 revolutions, respectively, should not greatly exceed 0.20 for material of uniform hardness. When this determination is made, care should be taken to avoid losing any part of the sample; the entire sample, including the dust of abrasion, shall be returned to the test machine for the final 400 or 800 revolutions required to complete the test.

CALCULATION

Express the difference between the original weight and the final weight of the test sample as a percentage of the original weight of the test sample report this value as a percentage of wear.


5

This method is suggested for the use of the Schmidt impact hammer for the hardness determination of rock.

When performing rebound measurements on core samples, both ASTM and ISRM recommend the use of a heavy specimen holder to prevent damage to the cores on impact. Ideally it should have a semi-circular groove with the same diameter as the core, but it may also be a V-shaped groove.

The device is portable and may be used both in laboratory and field.

Schmidt hammer models are available in different level of impact energy. The L type hammer having an impact energy of 0.74Nm shall be used.

Procedure

(a) Prior to each testing sequence, the Schmidt hammer should be calibrated using a calibration test anvil supplied by the manufacturer for that purpose.


6

(b) Specimens obtained for laboratory tests shall be representative of the rock to be studied. When possible, use larger pieces of rock for the Schmidt hardness tests. The Type L hammer should be used on NX or larger core specimens or on block specimens having an edge length of at least 6 cm.

(c) The test surface of all specimens, either in the laboratory or in the field, shall be smooth and flat over the area covered by the plunger. This area and the rock material beneath to a depth of 6 cm shall be free from cracks, or any localized discontinuity of the rock mass.

(d) Small individual pieces of rock, whether tested in the laboratory or in the field, shall be securely clamped to a rigid base to adequately secure the specimen against vibration and movement during the test. The base shall be placed on a flat surface that provides firm support.

(e) The hardness value obtained will be affected by the orientation of the hammer. It is recommended that the hammer be used in one of three positions: vertically upwards, horizontally, or vertically downwards with the axis of the hammer 5 from the desired position. When use of one of the three orientations is not feasible (e.g. in situ testing in a circular tunnel), the test should be conducted at the necessary angle and the results corrected to a horizontal or vertical position using the correction curves supplied by the manufacturer. The hammer orientation for the test and any corrections applied to non-vertical or non-horizontal orientations should be recorded and reported in the results.

(f) At least 20 individual tests shall be conducted on any one rock sample. Test locations shall be separated by at least the diameter of the plunger. Any test that causes cracking or any other visible failure shall cause that test and the specimen to be rejected. Errors in specimen preparation and testing technique tend to produce low hardness values.


7

Calculation

(a) The correction factor is calculated as:

Correction factor =

(b) The measured test values for the sample should be ordered in descending value. The lower 50% of the values should be discarded and the average obtained of the upper 50% values. This average shall be multiplied by the correction factor to obtain the Schmidt Rebound Hardness.

Reporting results

The following information shall be reported:

(a) Lithologic description of the rock. Source of sample, including: geographic location, depth and orientations.

(b) Type of specimen (core, blasted or broken sample, in situ). Size and shape of core or block specimen.

(c) Date of sampling, date of testing and condition of storage (i.e. exposure to temperature extremes, air drying, moisture, etc.).

(d) Orientation of the hammer axis in the test. (e) Method of clamping sample (V-block or clamps). (f) The Schmidt Hardness value obtained as in the Calculations section above.


8


9

SUGGESTED METHOD FOR DETERMINATION OF THE UNIAXIAL COMPRESSIVE STRENGTH OF ROCK MATERIALS

SCOPE This method of test is intended to measure the uniaxial compressive strength of a rock sample in the form of specimens of regular geometry.The test is mainly intended for strength classification & characterization of intact rock.

APPARATUS A) Suitable machine shall be used which have sufficient capacity and capable

of applying load.It shall be verified at suitable time intervals and shall comply with accepted national requirements.

B) A spherical seat shall be removed or placed in a locked position, the two loading faces of machine being parallel to each other. C) Steel plates in the form of discs shall be placed at the specimen ends. The diameter of platens shall be between D and D + 2mm, where D is the diameter of the specimen. The thickness of the platens shall be at least 15mm or (D/3).Surfaces of the discs should be ground and their flatness should be better than 0.005 mm. D) One of the two platens shall incorporate a spherical seat which placed on the upper end of specimen.It should be lightly lubricated.The curvature center of seat surface should coincide with center of top end of specimen.


10

PROCEDURE

a) Test specimens shall be right circular cylinders having a height to diameter ratio of 2.5-3.0 and a diameter preferably of not less than core size approximately 54 mm. b) The ends of specimen shall be flat to 0.02mm and shall not depart from perpendicularity to the axis of specimen by more than 0.001 radian or 0.05mm in 50mm. c)The sides of specimen shall be smooth and free of abrupt irregularities and straight to within 0.3mm over the full length of specimen. d)The use of mapping materials or end surface treatments other than machining is not permitted. e)The average diameter shall be used for calculating the cross-sectional area.The height of the specimen shall be determined to nearest 1.0mm. f)Samples shall be stored for no longer than 30 days in such a way as to preserve the natural water content,as far as possible,and tested in that condition. g)Load on the specimen shall be applied continuously at a constant stress rate such that failure will occur with 5-10 min of loading. Loading rate of 0.5-1.0 MPa/s. h)The maximum load on the specimen shall be recorded in Newtons. j)The number of specimen tested sould be determined from practical considerations but at least five are preffered.


11

CALCULATIONS

The uniaxial compressive strength of the specimen shall be calculated by dividing the maximum load carried by the specimen during the test by the original cross-sectional area.

REPORTING OF RESULTS

a)Lithologic description of the rock. b)Orientation of the axis of loading with respect to specimen anisotropy. c)Source of sample. d)Number of specimen tested. e)Specimen diameter and height. f)Water content and degree of saturation at time of test. g)Test duration and stress rate. h)Date of testing,type of testing machine. i)Mode of failure. j)Uniaxial compressive strength for each specimen in sample.


12

SUGGESTED METHOD FOR DETERMINING DEFORMABILITY OF ROCK MATERIALS IN UNIAXIAL COMPRESSION

SCOPE This method of test is intended to determine stress-strain curves and Youngs modulus and Poissons ratio in uniaxial compression of a rock specimen of regular geometry.The test is mainly intended for classification and characterization of intact rock.

APPARATUS

From (a) to (d) same as Part 1. e)Electrical resistance strain gauge,linear variable differential transformers, compressometers, optical devices or other suitable measuring devices. f)An apparatus for recording the loads and deformations,preferably an X-Y recorder capable of direct plotting of load-deformation curves.


13

PROCEDURE

From (a) to (e) same as Part 1. f)When possible,in situ moisture conditions should be preserved until the time of the test. g)Load on the specimen should be applied continuously at a constant stress rate such that failure will occur 5-10 min of loading. h)Load and axial circumferential strains or deformations shall be recorded at evently spaced load intervals during the test,if not continually recorded. i)It is sometimes advisible for a few cycles of loading and unloading to be performed. j)The number of specimens instrumented and tested under a specified set of conditions shall be governed by practical considerations but at least five are preffered. CALCULATIONS a) Axial strain,a and diametric strain, d may be recorded directly from strain indicating equipment or may be calculated from deformation readings depending upon the type of instrumentation. b) Axial strain is calculated from equation:

0 Where o = original measured axial length. = change in measured axial length.


14

c)Diametric strain is calculated form equation: 0 Where = original unformed diameter of specimen. = change in diameter. In the case of measuring the circumferential strain c ,the circumference is Cd,thus change in circumference ,is Cd.

d) The compressive stress:

e) Figure 1, illustrates typical plot of axial stress versus axial and diametric strains. The curves give the best description of the deformation behavior of rocks having non-linear stress-strain behaviour at low and high stress levels.


15

f) Axial Youngs modulus, E may be calculated using any engineering practice.The most common methods ,listed in Fig.2,are as follows:

g) Poissons ratio 19 may be calculated from the equation:

h) The volumetric strain EO: 2 REPORTING OF RESULTS From (a) to (i) same as Part 1. j) Values of applied load, stress & strain as tabulates results or recorded on chart. k) Young's modulus & Poisson's ratio for each specimen. I) Method of determination of Young's modulus & what axial stress level determined.

123

Lecture ( 6 )

Documents

Transcript of Lecture ( 6 )