The first results of a research program based on a large number of laboratory tests show that the R.I.H.N. can be utilized to investigate the variability of strength values in an apparently homogeneous rock material. Until now the tests have been performed on a marble quarried in Tuscany and used to feed lime kilns after a crushing process. In this area a contact metamorphism involved the rock mass with intensity variable from one zone to another, whose minimum volume can measure even few cubic centimeters. Although the rock mass appears to be homogeneous, the feed subjected to temperatures of about 1100 °C inside the kiln shows a high heterogeneity in response to static stress imposed by overlying material column. In fact, there are some marble pieces that retain their original grain sizes and other ones that are reduced to powder during calcination. This heterogeneity may be found also when lumps apparently similar on the basis of a visual examination are taken at close distance from each other. Preliminary results seem to indicate that such variability in terms of strength is due to local texture which can be determined only by observation of thin-sections using transmitted light microscopy. Two different textures can be recognized: granoblastic and xenoblastic ones. The transition from one texture to another is progressive. In order to examine this inhomogeneity, the following tests are carried out on rock specimens sampled directly from quarry faces: o Point load tests on 544 irregular lumps of various sizes; o Uniaxial Compressive Strength (UCS) tests on 21 core samples, having a diameter of 25 mm and a height – to – diameter ratio of 2. The adopted diameter is smaller than one suggested by standards, to compare the results obtained from UCS and R.I.H.N. tests on samples of the same volume; o R.I.H.N. tests on 128 irregular lumps. Unlike the first two, the R.I.H.N. test is not standardized and consists of breaking a certain number of irregular lumps (four to five) of the same rock material, one by one (charge). A charge of about 25 cm3 is placed into a mortar and impacted by a drop weight. Afterwards, a larger or lower number of blows is given to fresh charges and a graph of percentage of particles smaller than 0,5 mm against number of blows is established. The R.I.H.N. is defined as the number of blows required to produce 25% of particles smaller than 0,5 mm . This test procedure gives an average R.I.H.N. value for the rock material taking into account that four to five heterogeneous charges are used. A new R.I.H.N. test procedure is being adopted, by utilizing the same specimen as charge for sequential tests each conducted at a certain number of blows. In this case, the R.I.H.N. value is representative of the local strength of rock material. Analyzing the statistical distribution of experimental data, it seems that R.I.H.N. and UCS tests can classify the rock material according to textural characteristics, unlikely point load test. The distribution of point load strength index values appears normal with mean value of 3,05 MPa and a high variation coefficient of 36% which highlights the rock material heterogeneity. UCS tests identify two classes of strength, with mean values, respectively, of 64,0 MPa and 99,2 MPa, probably because the tested samples dimensions are of the same order as the volumetric size of “homogeneous zones”, having equal texture. However, because of the limited number of tested samples compared with the other two tests, no further meaningful findings can be made. The statistical distribution of R.I.H.N. test values seems to be bimodal and two classes of impact strength can be identified, with a mean value between 12 and 16 for the first class and between 28 and 32 for the second one. Analyzing microstructure of tested samples, a close relationship between texture and impact strength value is evident. Going from a granoblastic to a xenoblastic texture, R.I.H.N. values increase.
Bandini A., Berry P. (2009). Determination of relationship between textural and mechanical characteristics by means of rock impact hardness number test. UDINE : FIST.
Determination of relationship between textural and mechanical characteristics by means of rock impact hardness number test
BANDINI, ANNALISA;BERRY, PAOLO
2009
Abstract
The first results of a research program based on a large number of laboratory tests show that the R.I.H.N. can be utilized to investigate the variability of strength values in an apparently homogeneous rock material. Until now the tests have been performed on a marble quarried in Tuscany and used to feed lime kilns after a crushing process. In this area a contact metamorphism involved the rock mass with intensity variable from one zone to another, whose minimum volume can measure even few cubic centimeters. Although the rock mass appears to be homogeneous, the feed subjected to temperatures of about 1100 °C inside the kiln shows a high heterogeneity in response to static stress imposed by overlying material column. In fact, there are some marble pieces that retain their original grain sizes and other ones that are reduced to powder during calcination. This heterogeneity may be found also when lumps apparently similar on the basis of a visual examination are taken at close distance from each other. Preliminary results seem to indicate that such variability in terms of strength is due to local texture which can be determined only by observation of thin-sections using transmitted light microscopy. Two different textures can be recognized: granoblastic and xenoblastic ones. The transition from one texture to another is progressive. In order to examine this inhomogeneity, the following tests are carried out on rock specimens sampled directly from quarry faces: o Point load tests on 544 irregular lumps of various sizes; o Uniaxial Compressive Strength (UCS) tests on 21 core samples, having a diameter of 25 mm and a height – to – diameter ratio of 2. The adopted diameter is smaller than one suggested by standards, to compare the results obtained from UCS and R.I.H.N. tests on samples of the same volume; o R.I.H.N. tests on 128 irregular lumps. Unlike the first two, the R.I.H.N. test is not standardized and consists of breaking a certain number of irregular lumps (four to five) of the same rock material, one by one (charge). A charge of about 25 cm3 is placed into a mortar and impacted by a drop weight. Afterwards, a larger or lower number of blows is given to fresh charges and a graph of percentage of particles smaller than 0,5 mm against number of blows is established. The R.I.H.N. is defined as the number of blows required to produce 25% of particles smaller than 0,5 mm . This test procedure gives an average R.I.H.N. value for the rock material taking into account that four to five heterogeneous charges are used. A new R.I.H.N. test procedure is being adopted, by utilizing the same specimen as charge for sequential tests each conducted at a certain number of blows. In this case, the R.I.H.N. value is representative of the local strength of rock material. Analyzing the statistical distribution of experimental data, it seems that R.I.H.N. and UCS tests can classify the rock material according to textural characteristics, unlikely point load test. The distribution of point load strength index values appears normal with mean value of 3,05 MPa and a high variation coefficient of 36% which highlights the rock material heterogeneity. UCS tests identify two classes of strength, with mean values, respectively, of 64,0 MPa and 99,2 MPa, probably because the tested samples dimensions are of the same order as the volumetric size of “homogeneous zones”, having equal texture. However, because of the limited number of tested samples compared with the other two tests, no further meaningful findings can be made. The statistical distribution of R.I.H.N. test values seems to be bimodal and two classes of impact strength can be identified, with a mean value between 12 and 16 for the first class and between 28 and 32 for the second one. Analyzing microstructure of tested samples, a close relationship between texture and impact strength value is evident. Going from a granoblastic to a xenoblastic texture, R.I.H.N. values increase.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
