Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

Appendix C: Stone and rock properties

In this Appendix some information on stone and rock ation or weathering, and may show anisotropy, so that properties is given. Firstly, the properties of some cur- the single value given in the Table can only be a pointer rently produced natural building stones from the British to what in reality is a range of values. The following Isles and secondly, more generalized data on the prop- quantitative physical and strength properties are tabu- erties of some types of rock that are used as building lated in Table CI. stones. It is strongly emphasised that all the information given in this Appendix must be regarded as providing Bulk density. A piece of stone consists of solids, water only a general appreciation of stone and rock properties, and air (Fig. C1). The bulk density is the total mass of and should not be relied upon for specific test values for the solids and water, divided by the total volume of the particular stones and rocks or sources of them. solids, water and air. The units are, therefore, mass per unit volume, which in the Systeme International d'Unites (SI) is Mg/m 3. Bulk density is simply called 'weight' in the stone industry, and is quoted for stone in its 'normal' C1. Properties of some British condition as supplied to the user. For porous stones, this building stones 'normal' condition is very variable because it depends on the amount of water in the stone. This variability must The reported properties of some 183 natural building be borne in mind when consulting the values of bulk stones from the British Isles are given in Table C1. The density listed in Table C1. data for these are reproduced from the Natural Stone Directory, No. 9 1994 1995 (Tel: 01903 821082) by Relative density. (Formerly called specific gravity.) The permission of the publisher. The number beneath the mass of the solids, divided by the mass of an equal stone name in the first column of Table C1 is the Natural volume of water. Relative density is dimensionless. Stone Directory reference number of the quarry owner However, because the volume of 1 Mg of water is 1 m 3, or quarry operator. All stones from the Directory having the relative density can be thought of as having units of at least one result for the physical or strength tests listed Mg/m 3, which is useful when comparing it with bulk below have been included in Table C1. density. The stones in Table C1 have been grouped by country, in the order England, Scotland, Wales, North- ern Ireland and Eire. For England, the stones are then Porosity. The volume of pore space (Fig. C1) in the grouped by county, arranged in alphabetical order stone divided by the total volume of the solids and (e.g. Avon, Cambridgeshire, Cheshire, etc). Finally, the pores. Porosity is dimensionless and usually expressed as rock types are listed alphabetically (e.g. for Cumbria: a percentage. granite, limestone, sandstone, slate). In some cases in compiling Table C1, where the Water absorption. The mass of water required to geological information on a particular stone was lacking saturate the stone divided by the mass of the solids. it has been made good, and occasionally where an item Water absorption is dimensionless and usually expressed of data was clearly incorrect it has been omitted. Where as a percentage. a piece of information was not available the entry NA is given in the Table. None of the data has been confirmed independently. It should be noted that some individual Compressive strength. The compressive load required quarries produce more than one variety of stone (e.g. the to cause failure of an unconfined cylindrical or cubical various Ancaster stones in Lincolnshire). Conversely, specimen of the stone, divided by the cross-sectional one variety of stone can be produced by several different area of the specimen perpendicular to the axis of load- quarries (e.g. the Purbeck stone in Dorset). ing (Fig. C2). The units are, therefore, force per unit Because stone properties are liable to vary, the test area, which in the SI system is MN/m 2. Compressive results in Table C1 should not be taken as an indica- strength is simply called 'strength' in the stone industry. tion of present production for particular quarries; Sometimes the compressive strength has been given for instead prospective users should seek up-to-date infor- stone tested both dry and wet, and this is so indicated in mation from the suppliers. In particular, the data in Table C1. Also, for some rocks the compressive strength the Table should not be used for contractual or other has been given for stone tested both perpendicular to such purposes. Also, it should be remembered that the cleavage and parallel to the cleavage (cleavage is stone properties are likely to vary within a quarry, both called 'grain' in the stone industry), and this is indicated laterally and with depth, and with the degree of alter- in Table C1 by 'perp' and 'para' respectively. Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

432 APPENDIX C: S'IONE AND ROCK PROPERTIES

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APPENDIX C: STONE AND ROCK PROPERTIES 453

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454 APPENDIX C: STONE AND ROCK PROPERTIES

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APPENDIX C: SI'ONE AND ROCK PROPERrlIES 455

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456 APPENDIX C: STONE AND ROCK PROPERrlIES

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APPENDIX C: STONE AND ROCK PROPER'lIES 45/

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458 APPENDIX C: S'IONE AND ROCK PROPERTIES

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APPENDIX C: STONE AND ROCK PROPER~IIES 459

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460 APPENDIX C: STONE AND ROCK PROPERTIES

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APPENDIX C: STONE AND ROCK PROPER'lIES 461

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462 APPENDIX C: STONE AND ROCK PROPERTIES

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APPENDIX C: STONE AND ROCK PROPER'lIES 463 Load Air W t Pores Water

Solids I f '~ L

Fig. C1. Constituents of stone. Fig. C3. Measurement of flexural strength.

Flexural strength. The load required to cause failure in and these results are indicated in Table C1 by 'tran'. a beam-shaped specimen of stone using quarter-point If the specimen in the transverse strength test fails at its loading (Fig. C3). The test is usually done with the centre, the transverse strength is equal to the flexural load applied perpendicular to the bedding where this is strength. Sometimes the transverse strength is referred to applicable. The flexural strength is given by: as the 'modulus of rupture'. Details of how to carry out these tests are given at the 3WL appropriate places in the main chapters of the book, 4bd 2 particularly Chapter 9 or in Appendix B. where W is the maximum load, L is the span, b is the breadth, and d is the depth of the specimen; b is made Discussion. It will be noticed in Table C1 that for some 1.5d and L is made 10d. The units are, therefore, force stone types the same test results are quoted for a number per unit area, which, in the SI system, is MN/m 2. of different quarries (e.g. the slates from Cumbria). It is An earlier version of the test using half-point loading extremely unlikely that these are independent deter- (Fig. C4) is called transverse strength and is given by: minations. What seems to be more likely is that a single set of test results has been quoted by several quarries. 3 WL The reader should be aware of this possibility when 2bd 2 using Table C1.

Load Load

Area

j t t Fi~. C2. Measurement of compressive strength. Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

464 APPENDIX C: STONE AND ROCK PROPERTIES

Load mass of stone is determined by subtracting the mass of W the unladen lorry from the gross mass. However, the customer is usually not interested in the mass but needs to know the volume of stone that is being supplied. This can be simply obtained by dividing the mass of the stone by the bulk density. In the aggregates industry, quantitative physical and strength properties are of paramount importance. With- out them there would be difficulty in marketing an 1 aggregate because of the necessity of demonstrating that I 1I the aggregate complied with national and local specifi- L r I cations (Smith & Collis 1993). By contrast, in the stone industry, quantitative physical and strength properties Fig. C4. Measurement of transverse strength. are much less importance. Indeed, many of the quarries listed in the Natural Stone Directory do not give quan- The most commonly reported property of building titative data for their stones. Table C1 shows that, with stones in Table C1 is the bulk density. The reason for the exception of bulk density, physical and strength this is not technical but commercial. Stone is sold by the properties of building stones are sparsely reported. tonne (1 Mg = 1 tonne); each lorry laden with stone is After, bulk density, the property most commonly weighed on a weighbridge as it leaves the quarry and the listed is compressive strength, probably because it is seen

rig. C5. Use of stone requiring compressive strength. Ta'Pinu Fig. C6. Use of stone requiring flexural strength. Ta'Pinu Basilica, Gozo, Malta GC. Basilica, Gozo, Malta GC. Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

APPENDIX C: STONE AND ROCK PROPER'lIES 465 to be of direct relevance to the load-bearing use of stone masonry above, together with some of the floor load- in masonry construction (see example, Fig. C5). For the ing. Therefore, the compressive strength of the stone porous stones for which compressive strengths are given selected for the pillars must be greater than the total both wet and dry, it will be seen that the dry strength is compressive stress induced in them by their own weight greater than the wet strength. One reason for this is that plus the much greater weight of the superincumbent when a porous stone is dried there is a contribution to load. Further discussion of the importance of compres- the strength from suction, over and above the intrinsic sive strength is given in Chapter 9. mineral strength of the rock (West 1994). As might be By contrast, for small, simple buildings, the compres- expected, flexural or transverse strength is listed only for sive strength of even the weakest building stones can be those stones for which this property is relevant to their more than adequate for masonry construction purposes, use, such as cladding and paving, and cantilever stairs as the following example shows. The strength of the (see example, Fig. C6). Beer stone, a chalk, is 17 MN/m 2 and its bulk density is The range of compressive strength of the building 2.4Mg/m 3. A one-metre cubical block of this stone stones listed in Table C1 is from 14MN/m 2 for the exerts a pressure on the base of 0.0235 MN/m 2. Theo- Stamford Freestone, an oolitic limestone, to 314 MN/m 2 retically, therefore, it would be possible to build an for the Hillend Black, a quartz dolerite; a factor of unmortared, monolithic structure from such blocks up over 20. to 723 m high before the compressive stress in the base The compressive strength of a building stone can course exceeded the compressive strength of the stone. be of crucial importance in the selection of a stone for Table C1 also shows that natural building stones in structural purposes in a large, complex building. For the British Isles are derived from formations of a wide example, Fig. C7 shows the front elevation of a masonry range of geological age, ranging from Cambrian slate to building, the front of which, at ground level, is sup- Cretaceous chalk (see geological column, Chapter 2). ported on widely spaced free-standing pillars so as to However, certain geological systems are more important provide a walkway behind, giving access to set-back sources of building stone than others; particularly note- shop fronts. The pillars not only have to support their worthy are the Carboniferous for its sandstones and own weight, but have to support the weight of all the limestones, and the Jurassic for its oolitic limestones.

I I It I i Foorsmayspani 6-8 m on to facade and have a total load - 10 kN/m 2

IL Shop fronts set back to give walkway behind ff ~~ ff free-standing pillars

Fig. C7. Front elevation of masonry building with front supported on pillars. Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

466 APPENDIX C: STONE AND ROCK PROPERTIES

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APPENDIX C: STONE AND ROCK PROPERlIES 46/ C2. Generalized properties of strength (see below). The tensile load is applied by building stones means of metal caps that are cemented to the ends of the specimen using a cement which is stronger than the The generalized properties of different rock types used as tensile strength of the stone. Because of difficulties with building stones are given in Table C2. This has been the direct tensile test, the indirect tensile strength test is compiled from ranges of values given by Winkler (1973), often carried out instead. The values for the range of supplemented by some data from other sources. Again, tensile strength for various rock types given in Table C2 the information given is for a general appreciation only are from Farmer (1968). and should not be used for contractual or other such purposes. The headings in Table C2 are the same as Indirect tensile strength. A disc-shaped specimen of those in Table C1, except for the following additions. stone is compressively loaded at diametrically opposed surfaces over a small arc of contact (Fig. C9a). Although Coefficient of thermal expansion. The increase in length the loading is compressive, the specimen fails in tension per unit length per degree Celsius rise in temperature. (Fig. C9b) because it has been found that most rocks in biaxial stress fields fail in tension at their uniaxial tensile strength when one principal stress field is tensile and the Modulus of elasticity. For rocks that behave elastically, other compressive. This test is also called the Brazil test. the modulus of elasticity is the stress divided by the The tensile strength is given by: strain, usually measured axially during unconfined com- pression (see above). Because strain is dimensionless, the modulus of elasticity has the same dimensions as stress 0.636P/Dt (MN/m 2) which in the SI system is GN/m 2. The modulus of elasticity is sometimes referred to as 'Young's modulus'. where P is the compressive load at failure (N), D is the diameter of the test specimen (mm), and t is the thick- ness of the test specimen (mm) (Brown 1981). Direct Tensile strength. The tensile load required to cause and indirect tensile tests on the same rock give closely failure of an unconfined cylindrical specimen of the similar results. stone, divided by the cross-sectional area of the speci- The tensile strength of stone may also be determined men perpendicular to the axis of loading (Fig. C8). by carrying out the point load strength test (International The units are, therefore, force per unit area, which in Society for Rock Mechanics 1985), which is an index the SI system is MN/m 2. Sometimes called the direct test, and then empirically correlating the point load tensile strength to distinguish it from the indirect tensile strength index obtained with the tensile strength. An advantage of the point load strength test is that it can be carried out on pieces of core or irregular lumps of rock Load rather than on prepared specimens.

t' Discussion. The coefficient of thermal expansion and the modulus of elasticity of building stone can be important in the following circumstances. Ideally, if two or more types of stone are to be used in juxtaposition in the same large structure (see example, Fig. C10), then to prevent differences in displacement occurring due to temperature change and due to loading, the coefficients of thermal expansion and the moduli of elasticity of Area the different stones should be chosen to be of similar magnitude. Where this is not possible, the consequences of the differences must be allowed for in the design of the structure. These considerations also apply to the use of stone and concrete together, and to the use of stone cladding on a different substrate. These matters are dealt with further in Chapter 9. Flexural strength of stone is ( l clearly of importance in assessing the suitability of a particular stone for cladding and in deciding on panel 4, thickness. Use of the indirect tensile strength test in testing the condition of masonry construction has been Fig. C8. Measurement of direct tensile strength. described by Beckmann (1994). Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

468 APPENDIX C: STONE AND ROCK PROPERTIES

Load P

(b) Fig. C9. Measurement of indirect tensile strength.

Table C2 shows that the compressive strength of rocks compressive rather than tensile loads. This is discussed is about ten times the tensile strength. This difference further in Chapter 9. provides an explanation of the traditional practice of The tallest self-supporting, masonry structure in the using stone in masonry construction so that it carries world is the Washington Monument (Figs Cll & C12),

Fig. CI0. Use of two different stones together. Douai Abbey, Woolhampton, Berkshire, UK. (The individual blocks shown are 230 mm square.) Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

APPENDIX C: STONE AND ROCK PROPERqlES 469

Table C3. Effect of weathering on some physical properties of granite

Description Rock mass Sample Compressive Saturated Water weathering strength bulk density absorption grade ( MN/m 2) (Mg/m 3) (%)

Fresh granite I Fresh 262 2.61 0.11 Partially stained granite II Stained rim of block 232 2.62 0.35 Partially stained granite II Whole sample II 90% stained 163 2.58 1.09 Completely stained granite II Completely stained II block 105 2.56 1.52 Weakened granite III-IV Rock core of III block 46 2.55 1.97 Weakened granite III IV Rock core of IV block 26 2.44 4.13

built by Robert Mills in 1885, in Washington DC, USA; 2.17MN/m 2 (Allen Howe 1910). Even assuming a it is 169 m high and has the form of a obelisk. It has been factor of safety of 20, it can be seen from the ranges estimated that the stone at the base of the monument of compressive strength listed in Table C2 that all the sustains a maximum vertical compressive stress of rock types for which there are data could provide

Fig. C12. Close-up view of the Monument. The exterior is of Fig. Cll. The Washington Monument, 169 m high, is the tallest white marble from Maryland and Massachusetts with an self-supporting masonry structure in the world (see text). It was interior of granite. Within the obelisk there is a shaft containing designed by the architect Robert Mills and largely constructed eight wrought-iron columns supporting a stairway and elevator. by the US Army Engineers. Construction commenced in 1848 The slight change in colour of the marble about a quarter of the and was completed in 1885, but there was a hiatus in way up marks the level where construction was interrupted. construction from 1854 to 1879. (Photo." D. Newill.) (Photo. D. Newill.) Downloaded from http://egsp.lyellcollection.org/ by guest on October 1, 2021

4/0 APPENDIX C: S'IONE AND ROCK PROPERTIES Table C4. Descriptive terms for rock strength C4. Scale of rock strength Descriptive term Unconfined compressive If the compressive strength of a rock has been strength (MN/m 2) determined, a descriptive term for the rock, in terms of Extremely strong rock >200 its strength, can be derived from Table C4. This Very strong rock 100-200 classification comes from the Geological Society Engi- Strong rock 50-100 neering Group Working Party (1977), and was devised Moderately strong rock 12.5-50 for engineering geology purposes, but there is no reason Moderately weak rock 5.0 12.5 why it should not be used to describe the stength of Weak rock 1.25-5.0 building stones. Very weak rock 0.60 1.25 References

ALLEY HOWE, J. 1910. The Geology of Building Stones. Edward examples of stone more than adequately strong for the Arnold, London, p. 366. base of such a monument. BECKMANN, P. 1994. Structural Aspects of Building Conserva- tion. McGraw-Hill, London, 82-83. BROWN, E. T. 1981. Rock characterization testing and monitor- ing." ISRM suggested methods. Pergamon Press, Oxford, 119-121. FARMER, I W. 1968. Engineering Properties of Rocks. E & FN C3. Effect of weathering Spon Ltd, London, p. 57. FOOKES, P G. 1980. An introduction to the influence of natural Generally speaking, quarry operators working building- aggregates on the performance and durability of concrete. stone quarries will extract unweathered rock. How-ever, Quarter O, Journal of Engineering Geology, 13, 207-209. in some instances partly weathered rocks will be worked GEOLOGICAL SOCIETY ENGINEERING GROUP WORKING PARTY 1977. The description of rock masses for engineering for their attractive colours, or for other reasons. In purposes. Quarterly Journal o[" Engineering Geology, 10, general, the physical properties of rock will decrease in 355 388. quality as the degree of weathering increases. This is INTERNATIONAL SOCIETY FOR ROCK MECHANICS 1985. Suggested shown, for example, by studies of granite from a quarry method for determining point load strength. International on Dartmoor in southwest England reported by Fookes Journal o['Rock Mechanics and Mining Science, 22, 51 60. (1980), and summarized in Table C3. It can be seen that SMITH, M. R. & COLLIS, L. (eds) 1993. Aggregates: sand, gravel the compressive strength of the rock decreases from and crushed rock aggregates for construction purposes 262MN/m 2 to 26MN/m 2, a factor of 10, as the rock (second edition). Geological Society, London, Engineering Geology Special Publications, 9. mass weathering grade increases from I to IV. (The rock STONE INDUSTRIES 1994. Natural Stone Directory, No 9 1994- mass weathering grade scale runs from I: fresh 1995. Herald House, Worthing. unweathered rock, to V: completely weathered rock.) WEST, G. 1994. Effect of suction on the strength of rock. The bulk density and water absorption show similar Quarterly Journal of Engineering Geology, 27, 51-56. trends of reduction in quality with increase in weath- WINKLER, E M. 1973. Stone." Properties, Durability in Man's ering. The weathering of rock is discussed in Chapter 2. Environment. Springer, Vienna, 43 & 46-47.