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.
2.0 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.
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). 4.0 EQUIPMENT AND MATERIALS
1. Rebound Hammer (L-type)
2. Steel Base
3. Calibration Anvil
4. Abrasive Stone
1. Prior to each testing sequence, the hammer is calibrated using a calibration test anvil supplied by the manufacturer. 2. The calibration is placed anvil in the core holder and conduct ten readings on the anvil. 3. The correction factor is calculated by dividing the manufacturer’s standard hardness value for the anvil by the average of the ten readings taken on the anvil.
1. Drill core specimens should be NX (54 mm) or larger core art at least 15 cm in length. Block specimens should be 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, should be have a smooth, flat test area at least 15 cm in diameter. 2. Samples should be representative of the rock to be studied. Samples is obtained 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, sampling and testing rock material weakened is avoided 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. Simple field tests are conducted to quickly assess a rock material’s suitability for the rebound hammer test method. Rock is scratched very soft with a fingernail and peeled 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.
6.2 SPECIMEN PREPARATION
1. For a block or core specimen, its length is determined 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, its diameter is determined by taking the average...