rock strength schmidt’s (rebound) hammer (l-type)

FACULTY OF CIVIL & ENVIRONMENTAL

ENGINEERING

DEPARTMENT OF GEOTECHNICAL &

TRANSPORTATION ENGINEERING

ENGINEERING GEOLOGY & GEOPHYSIC LABORATORY

REPORT

SUBJECT CODE BFC 21303

TEST CODE & TITLE LAB 2 ROCK STRENGTH SCHMIDTS (REBOUND)

HAMMER (L-TYPE)

COURSE CODE BFF

TESTING DATE 17 JANUARY 2011

STUDENT NAME MUHAMMAD RIDHWAN BIN KAMARUDIN (DF100038)

SECTION/GROUP SECTION 1

GROUP MEMBER NAMES 1. MUHAMMAD IKHWAN BIN ZAINUDDIN (DF100018)

2.MUHAMMAD ZAMIR BIN SAMEON (DF100065)

3. MUKHLIS BIN ADAM (DF100080)

4. MUHAMMAD NUH BIN AHMAD ZAIRI (DF100093)

5. HANISAH BINTI HAMZAH (DF100052)

LECTURER/ INSTRUCTOR/

TUTOR NAME

IR. AGUS BIN SULAEMAN

REPORT RECEIVED DATE 24 JANUARY 2011

MARKS ATTENDANCE,

DISCIPLINE &

INVOLVEMENT

/15%

DATA ANALYSES /20%

RESULT /20%

DISCUSSION /25%

CONCLUSION /20%

TOTAL /100%

EXAMINER COMMENT

RECEIVED STAMP

ROCK STRENGTH

SCHMIDTS (REBOUND) HAMMER (L-TYPE)

(LAB 3)

OBJECTIVE:

This experiment deals with determination of rock strength when a certain load implied on the rocks.

Students should be able conducted the experiment, understanding the theory and recognize the rock

strength on different types of rocks in Malaysia.

LEARNING OUTCOMES:

a) To determine a rock strength on different types of rock formation in Malaysia.

b) To evaluate the physical properties of rocks for civil engineering application.

c) To understand the theory rock test.

THEORY:

Rebound hammer test is undertaken using Schmidts hammer L-type (N-type for concrete material). Test

procedure is simple and equipment is portable and easy to operate. Test can be undertaken on site and

the number of test is unlimited. Test can be carried out on the surface of irregular block or, on core

samples does not involve destruction of sample (minimize sample usage). Index value obtained is

rebound number (R), which is an indicator on the degree of hardness of rock surface being tested.

Rebound hammer test is frequently used in estimating the compressive strength of joint surface in rock.

The value of R can be used to estimate the compressive strength of rock using the following equation

(Franklin, 1989):

Log10 JCS = 0.00088()(R) + 1.01

Where, JCS (MPa) is the compressive strength of rock surface; is unit weight of rock (kN/m3). The

term JCS means joint compressive strength, which implies the surface strength of joints (fracture planes)

in rock. This means that rebound hammer test can also be used to estimate the surface strength of joint

in addition to rock block sample. For fresh rock (weathering grade I), JCS is approximately equals to the

UCS of the rock material. In other words when rock is not weathered, its surface compressive strength is

approximately equals to the strength of its material composition (usually measured by UCT).

EQUIPMENT AND MATERIALS:

1. Rebound Hammer (L-type), consisting of a spring-loaded piston, or hammer, which is projected

against a metal anvil in contact with the rock surface. The hammer must travel with a fixed and

reproducible velocity. The rebound distance of the piston from the steel plunger is measured in a

linear scale attached to the frame of the instrument and is taken as an empirical measure of rock

hardness.

2. Steel Base - A steel base of minimum mass of 20 kg to which specimens are securely fastened.

Rock core specimens may be tested in a steel cradle with a semi cylindrical machined slot of the

same radius as the core, or firmly seated in a steel V-block.

3. Calibration Anvil - The standard calibration block used to calibrate the rebound hammer.

4. Abrasive Stone - A medium-grained texture silicon carbide or equivalent material.

CALIBRATION

1. Prior to each testing sequence, calibrate the hammer using a calibration test anvil supplied by the

manufacturer for that purpose.

2. Place the calibration anvil in the core holder and conduct ten readings on the anvil.

3. Calculate the correction factor by dividing the manufacturers standard hardness value for the anvil

by the average of the ten readings taken on the anvil.

SAMPLING

1. Drill core specimens shall be NX (54 mm) or larger core art at least 15 cm in length. Block

specimens shall have edge lengths of at least 15 cm. Rock surfaces tested in place, including

natural outcrops or prepared surfaces such as tunnel walls or floors, shall have a smooth, flat test

area at least 15 cm in diameter.

2. Samples shall be representative of the rock to be studied. Obtain samples by direct sampling of

subsurface rock units with core borings or by sampling blocks of rock material from outcrops that

correlate with the subsurface rock unit of interest. At surface outcrops, avoid sampling and testing

rock material weakened by weathering or alteration or is otherwise unrepresentative of the rock

material of interest.

3. The rebound hammer is generally unsuitable for very soft or very hard rock. Conduct simple field

tests to quickly assess a rock materials suitability for the rebound hammer test method. Scratch

very soft rock with a fingernail and peel with a pocket knife. An intact specimen of very hard rock

breaks only by repeated, heavy blows with a geological hammer and cannot be scratched with a

common 20d steel nail.

SPECIMEN PREPARATION

1. For a block or core specimen, determine its length by taking the average of four lengths measured

at four equally spaced points on the circumference and record to the nearest 5 mm.

2. For a block or core specimen, determine its diameter by taking the average of two diameters

measured at right angles to each other approximately midway along the length of the specimen and

record to the nearest 5 mm.

3. Report the moisture condition of the block or specimen.

4. The test surface of all specimens, either in the laboratory or in the field, shall be smooth to the touch

and free of joints, fractures, or other obvious localized discontinuities to a depth of at least 6 cm. In

situ rock shall be flat and free of surface grit over the area covered by the plunger. If the surface of

the test area is heavily textured, grind it smooth with the abrasive stone.

PROCEDURE

1. Place the steel base on a flat, level surface that provides firm, rigid support, such as a concrete

floor.

2. Securely clamp rock core specimens in a steel cradle with a semi cylindrical machined slot of the

same radius as the core, or firmly seat into a steel V-shaped block. Securely clamp block specimens

to the rigid steel base in such a manner as to prevent vibration and movement of the specimen

during the test.

3. For tests conducted on specimens in the laboratory, orient the instrument within 5 of vertical with

the bottom of the piston at right angles to and in firm contact with the surface of the test specimen. A

guide may be used to ensure the rebound hammer is positioned for optimum performance. Position

the hammer not less than one diameter from the edge of the specimen.

4. For tests conducted in situ on a rock mass, the rebound hammer can be used at any desired

orientation provided the plunger strikes perpendicular to the surface tested. The results are

corrected to a horizontal or vertical position using the correction curves provided by the

manufacturer.

5. Before conducting the tests, ensure the hammer is at the same temperature as the test specimens

by exposing it to the ambient environmental conditions of the test area (indoors or outdoors) for at

least 2 h.

6. Compress the hammer spring by gradually depressing the plunger until the hammer is triggered and

impact occurs.

7. Read and record the height of the plunger rebound to the nearest whole number, as measured on

an arbitrary scale of 10 to 100 divisions located on the side of the hammer, before restoring the

piston to its original extension. Repeat this procedure at ten representative locations on the

specimen. Test locations shall be separated by at least the diameter of the piston and only one test

may be taken at any one point.

8. If a specimen breaks during rebound testing, energy is absorbed during breakage and,

consequently, the rebound reading will be lower than had it not broken. Any individual impact test

that causes cracking or any other visible failure shall cause that test and the specimen to be

rejected.

9. Some factors that may affect the results of the test include:

i. Rock at 0 C or less may exhibit very high rebound values.

ii. Temperature of the rebound hammer itself may affect the rebound number. The hammer and

materials to be tested should be at the same temperature.

iii. For readings to be compared, the direction of impact, horizontal, upward, downward, and so forth,

must be the same.

iv. Different hammers of the same nominal design may give rebound numbers differing from one to

three units and therefore, tests should be made with the same hammer in order to compare

results. If more than one hammer is to be used, a sufficient number of tests must be made on

typical rock surfaces to determine the magnitude of the differences to be expected.

RESULT AND ANALYSIS

1. Using the data from the ten readings obtained in procedure no. 7, discard readings differing from the

average of ten readings by more than seven units and determine the average of the remaining

readings. Record in Table 1.

2. To calculate the rebound hardness number (HR) of the tested rock material, multiply this average by

the correction factor determined in calibration no. 3 and record the results to the nearest whole

number.

1. Estimate the compressive strength of rock using the following chart:

2. Estimate the compressive strength of rock using the following equation (Franklin, 1989):

Log10 JCS = 0.00088()(R) + 1.01

Where:

JCS (MPa) = compressive strength of rock surface

= unit weight of rock (kN/m3)

B = HAMMER REBOUND / DURETE A CHOC / PRELLHARTE

Table 1: Calibration Table

Sample GRANITE

Re

bo

un

d

Nu

mb

er

(R) 1 74

2 76

3 76

Total 226

Average Value, R 75.33

Table 2 : Rebound Number Table for Core Sample

SAMPLE GRANITE AVERAGE

VALUE, R

COMPRESSIVE

STRENGTH (MPa),

JCS

Re

bo

un

d N

um

ber

(R)

1 34 38 70.15

2 36 41 81.66

3 32 35 60.26

4 40 46 105.20

5 42 49 122.46

6 42 49 122.46

7 44 52 142.56

8 42 49 122.46

9 44 52 142.56

10 40 46 105.20

11 40 46 105.20

12 42 49 122.46

13 42 49 122.46

14 39 45 100.00

15 40 46 105.20

Total 599 692 1630.29

Average 39.93 46.13 108.69

Sample 1

Use (unit weight of rock) = 25 kNm3

Log10 JCS = 0.00088()(R) + 1.01

= 0.00088(25)(38) + 1.01

= 70.15 Mpa

Average for Core Sample Compressive Strength, JCS

= 1630.29

15

= 108.69 Mpa

Table 3 : Rebound Number Table for irregular Sample

Sample 1 (Upper Side)


Log10 JCS = 0.00088()(R) + 1.01

= 0.00088(25)(60) + 1.01

= 213.80 Mpa

Average for Upper Side Compressive Strength, JCS

= 2890.78

15

= 192.72 Mpa

Sample 1 (Left Side)


Log10 JCS = 0.00088()(R) + 1.01

= 0.00088(25)(57) + 1.01

= 183.67 Mpa

Average for Left Side Compressive Strength, JCS

= 2729.14

15

= 181.94 Mpa

SAMPLE

UPPER SIDE (Pos. B) LEFT SIDE (Post. A)

GRANITE AVERAGE

VALUE, R

COMPRESSIVE

STRENGTH (MPa),

JCS

GRANITE AVERAGE

VALUE, R

COMPRESSIVE

STRENGTH (MPa),

JCS

Re

bo

un

d N

um

ber

(R)

1 56 60 213.80 56 57 183.67

2 46 55 165.96 56 57 183.67

3 58 60 213.80 68 57 183.67

4 52 60 213.80 60 57 183.67

5 42 49 122.46 60 57 183.67

6 46 55 165.96 68 57 183.67

7 56 60 213.80 68 57 183.67

8 58 60 213.80 56 57 183.67

9 48 58 193.20 54 57 183.67

10 40 46 105.20 58 57 183.67

11 68 60 213.80 60 57 183.67

12 58 60 213.80 48 54 157.76

13 54 60 213.80 58 57 183.67

14 62 60 213.80 64 57 183.67

15 52 60 213.80 60 57 183.67

Total 796 863 2890.78 894 852 2729.14

Average 53.07 57.53 192.72 59.60 56.80 181.94

QUESTION AND DISCUSSION

i. Describe generally the differences between index and direct test.

Index or Indirect Test

Index test is relatively simple and rapid to conduct, but it does not provide fundamental

property. The data obtained is just an indicator on property that being tested. The apparatus used

are normally simple and portable which also allows the test to be conduct at site.

The tests may not require some detailed sample preparation where certain tests are non-

destructive type and does not involve failure of samples (cost saving for sample could be reused).

The data also not suitable for detailed design purposes but it is useful and valuable for preliminary or

pre-feasibility assessments. The tests for Index and Indirect Strength test include:

Point-load index test

Schmidts hammer test (rebound hammer L-type)

Slake durability index test

Seismic velocity test (PUNDIT test)

Brazilian test (inderect tensile test)

Direct or Strength Test

The test procedure requires detailed preparation of sample in terms of standard shapes

and finishing. The sample preparation process is equipment related and it is costly. The testing itself

involving sophisticated and large equipment significant to the detailed testing procedures and may

require complex analysis and this is also costly.

However, the data obtained is the fundamental property and would be the direct

presentation of property being evaluated. The numbers of tests were limited due to its cost of

operation and with this the data obtained can be use for detailed design. The tests for Direct or

Strength test include:

Permeability of rock

Deformation modulus modulus of elasticity E and Poissons ratio, v.

Unconfined compression test (UCT) and Triaxial compression test.

Shear test on discontinuity planes (e.g. joint bedding and foliation).

ii. Explain the discontinuities in rock and their effect on strength.

In rock mechanics, discontinuity or weak plane is commonly used term for rock defects.

Bedding planes in sedimentary rocks, cleavages and schistosities in metamorphic rock are typical

examples of fabric defects. Folds, faults and joints are structural defects (Giani,1992).

Discontinuities are divided into two scales either large-scale discontinuities or small scale

discontinuities.

Large-scale discontinuities are bedding plane, folds, faults and joints. Bedding planes

represent interruptions in the course of sedimentary rock grain deposition, which are therefore

separated by beds or strata. Folds are caused by a bend in the strata of layered rock. Faults are

fractures or fractured zones along which there has been an appreciable shear displacement. Joints

are fractures in rock along which there has been little or no displacement or very slight movement

perpendicular to the joint surface.

Small-scale discontinuities are cleavage planes and schistosities. Cleavage planes are

generated under the influence of tensile stresses, which determine rock splitting along definite

parallel planes. Cleavage is also associated with changes in rock fabric and large folding.

Schistosities are the varieties of foliations that occur in the coarser-grained metamorphic rocks and

are usually the results of the parallel arrangement of platy and ellipsoidal grain within the rock

substance.

For small laboratory rock sample, it is affected by minerals arrangement and how cleavage

such as schistoscity in metamorphic rock. On a larger scale, rock masses are affected by

geological structures and discontinuities like bedding, joint and fault. (Figure 1)

The degree of strength reduction depends on loading orientation with respect to discontinuity

planes. Figure 2 shows the effect of single joint and multiple joint-set under different

inclination of uniaxial compression. In many cases luck exhibits multiple joint sets hence, it is

weakened in all direction (3rd cured in).

In laboratory testing it is important to take note the direction of loading with respect to rock

anisotropy. Strength parameters of a rock mass that exhibits small and large-scale

discontinuities are more appropriately assessed using in-situ large-scale testing like in-situ

shear and plate bearing test.

Weathering also affects the strength of rock. Degree of weathering od rock usually evaluated

by site assessment. Table 1 describes degree of weathering of rock mass

(Zone 1 to Zone 6) and rock material (Grade I to Grade VI). Figure 3 can be used to estimate

the strength reduction of the rock. It is important to differentiate between ZONE (rock mass)

and GRADE (rock material). Since weathering grade could only be assessed subjectively

with little information on numerical value of weathering degree, the effect of weathering is

relatively difficult to be included in design. Slake durability test may be used to assessed the

degree of weathering numerically.

Intact rock strength



Single discontinuity

Pair of discontinuties

Many discontinuities

Angle of fracture

Angle of fracture

Angle of fracture

Strength reduced

by discontinuity

Strength reduced by

two discontinuities

Effect of many

discontinuities



Figure 1: Effect on flow cleavage (schistocity) on strength of rock material

Figure 2: The influence of weakness or discontinuity planes on strength e.g joint

orientation with respect to loading axis

Figure 3: Effect of weathering on rock strength Strength Reduction Factor (SRF) chart

Weathering Grade I is a fresh rock SRF = 0, Grade VI is residual soils SRF = 0.001

TABLE 1 : Description of zone and weathering grade of rock (Attewell, 1993).

Weathering Zone

(material grade)

Descriptive

terms

Material description and likely engineering characteristics

Zone 6 (Grade VI) Residual soil. Completely degraded to a soil; original rock fabric is completely absent;

exhibit large volume change; the soil has not been significantly

transported.

Stability on slopes relies upon vegetation rooting and substantial erosion

& local failures if preventive measures are not taken.

Zone 5 (Grade V) Completely

weathered

Rock is substantially discoloured and has broken down to a soil but with

original fabric (mineral arrangement & relict joints) still intact; the soil

properties depend on the composition of the parent rock.

Can be excavated by hand or ripped relatively easily. Not suitable as

foundation for large structures. May be unstable in steep cuttings and

exposes surfaces will require erosion protection.

Zone 4 (Grade IV) Highly

weathered

Rock is substantially discoloured and more than 50% of the material is in

degraded soil condition; the original fabric near to the discontinuity

surfaces have been altered to a greater depth; a deeply weathered,

originally strong rock, may show evidence of fresh rock as a discontinuous

framework or as corestone; an originally weak rock will have been

substantially altered, with perhaps small relict blocks but little evidence of

the original structure.

Likely engineering characteristics are as in Zone 5.

Zone 3 (Grade III) Moderately

weathered

Rock is significantly discoloured; discontinuities will tend to be opened by

weathering process and discolouration have penetrated inwards from the

discontinuity surfaces; less than 50% of the rock material is decomposed

or disintegrated to a soil; rock samples containing discolouration are

noticeably weaker than the fresh undiscoloured rock; an originally weak

rock will comprise relict blocks of substantially weathered material.

Occasionally may be excavated without blasting or cutting (i.e. by block

leverage at the discontinuities); will be relatively easily crushed by

construction plant moving over it in situ, may be suitable as rock

foundation (with some reinforcements); joints may exhibit lower strength

characteristics, so rendering side slopes unstable.

Zone 2 (Grade II) Slightly

weathered

Some disclouration on and adjacent to discontinuity surfaces; discoloured

rock is not significantly weaker than undiscoloured fresh rock; weak (soft)

parent rock may show penetration of discolouration.

Normally requires blasting or cutting for excavation; suitable as a

foundation rock but with open jointing will tend to be very permeable.

Zone 1 (Grade I) Fresh No visible sign of rock material weathering; no internal discolouration or

disintegration.

Normally requires blasting or cutting for excavation; may require minimal

reinforcement in cut slope unless rock mass is closely jointed.

CONCLUSION

Conclude your results of the Schmidts (Rebound) Hammer (L-Type) by rate its significance or

applications in civil engineering or construction industry.