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 SCHMIDT’S (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/
IR. AGUS BIN SULAEMAN
TUTOR NAME REPORT RECEIVED DATE
24 JANUARY 2011
MARKS
ATTENDANCE,
/15%
DISCIPLINE & INVOLVEMENT
EXAMINER COMMENT
DATA ANALYSES
/20%
RESULT
/20%
DISCUSSION
/25%
CONCLUSION
/20%
TOTAL
/100% RECEIVED STAMP
ROCK STRENGTH SCHMIDT’S (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 Schmidt’s 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(γ)(R) + 1.01 Where, JCS (MPa) is the compressive strength of rock surface; γ 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 manufacturer’s 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 material’s 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.
4.
5.
6. 7.
8.
9.
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. 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. 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. Compress the hammer spring by gradually depressing the plunger until the hammer is triggered and impact occurs. 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. 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. 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:
B = HAMMER REBOUND / DURETE A CHOC / PRELLHARTE
2.
Estimate the compressive strength of rock using the following equation (Franklin, 1989): Log10 JCS = 0.00088(γ)(R) + 1.01 Where: JCS (MPa) = compressive strength of rock surface γ = unit weight of rock (kN/m3)
Table 1: Calibration Table
Rebound Number (R)
Sample
Table 2
Rebound Number (R)
SAMPLE
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Total Average
1
74
2
76
3
76
Total
226
Average Value, R
75.33
: Rebound Number Table for Core Sample GRANITE
34 36 32 40 42 42 44 42 44 40 40 42 42 39 40 599 39.93
Sample 1 Use γ (unit weight of rock) = 25 kNm3 Log10 JCS
= = =
GRANITE
0.00088(γ)(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
AVERAGE VALUE, R
38 41 35 46 49 49 52 49 52 46 46 49 49 45 46 692 46.13
COMPRESSIVE STRENGTH (MPa), JCS
70.15 81.66 60.26 105.20 122.46 122.46 142.56 122.46 142.56 105.20 105.20 122.46 122.46 100.00 105.20 1630.29 108.69
Table 3 : Rebound Number Table for irregular Sample UPPER SIDE (Pos. B) GRANITE AVERAGE COMPRESSIVE SAMPLE VALUE, R STRENGTH (MPa), JCS
Rebound Number (R)
1 56 60 2 46 55 3 58 60 4 52 60 5 42 49 6 46 55 7 56 60 8 58 60 9 48 58 10 40 46 11 68 60 12 58 60 13 54 60 14 62 60 15 52 60 Total 796 863 Average 53.07 57.53 Sample 1 (Upper Side) Use γ (unit weight of rock) = 25 kNm3 Log10 JCS
= = =
213.80 165.96 213.80 213.80 122.46 165.96 213.80 213.80 193.20 105.20 213.80 213.80 213.80 213.80 213.80 2890.78 192.72
0.00088(γ)(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) Use γ (unit weight of rock) = 25 kNm3 Log10 JCS
= = =
0.00088(γ)(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
LEFT SIDE (Post. A) GRANITE AVERAGE COMPRESSIVE VALUE, R STRENGTH (MPa), JCS
56 56 68 60 60 68 68 56 54 58 60 48 58 64 60 894 59.60
57 57 57 57 57 57 57 57 57 57 57 54 57 57 57 852 56.80
183.67 183.67 183.67 183.67 183.67 183.67 183.67 183.67 183.67 183.67 183.67 157.76 183.67 183.67 183.67 2729.14 181.94
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 nondestructive 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 Schmidt’s 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 Poisson’s 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.
Figure 1: Effect on flow cleavage (schistocity) on strength of rock material Single discontinuity
Intact Intact rock rockstrength strength
Strength reduced by discontinuity Angle of fracture Pair of discontinuties
Intact Intactrock rockstrength strength
Strength reduced by two discontinuities Angle of fracture Many discontinuities
Intact rock strength
Effect of many discontinuities
Angle of fracture
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
Weathering Zone (material grade)
Descriptive terms
Material description and likely engineering characteristics
Zone 6 (Grade VI)
Residual soil.
Zone 5 (Grade V)
Completely weathered
Zone 4 (Grade IV)
Highly weathered
Zone 3 (Grade III)
Moderately weathered
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. 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. 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. 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
Zone 1 (Grade I)
Fresh
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. 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.
TABLE 1 : Description of zone and weathering grade of rock (Attewell, 1993).
CONCLUSION Conclude your results of the Schmidt’s (Rebound) Hammer (L-Type) by rate its significance or applications in civil engineering or construction industry.
