appendix c: stone and rock properties - lyell collection · appendix c: stone and rock properties...

Appendix C: Stone and rock properties

In this Appendix some information on stone and rock properties is given. Firstly, the properties of some cur- rently produced natural building stones from the British Isles and secondly, more generalized data on the properties of some types of rock that are used as building stones. It is strongly emphasised that all the information given in this Appendix must be regarded as providing only a general appreciation of stone and rock properties, and should not be relied upon for specific test values for particular stones and rocks or sources of them.

C1. Properties of some British building stones

The reported properties of some 183 natural building stones from the British Isles are given in Table C1. The data for these are reproduced from the Natural Stone Directory, No. 9 1994 1995 (Tel: 01903 821082) by permission of the publisher. The number beneath the stone name in the first column of Table C1 is the Natural Stone Directory reference number of the quarry owner or quarry operator. All stones from the Directory having at least one result for the physical or strength tests listed below have been included in Table C1.

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 grouped by county, arranged in alphabetical order (e.g. Avon, Cambridgeshire, Cheshire, etc). Finally, the rock types are listed alphabetically (e.g. for Cumbria: granite, limestone, sandstone, slate).

In some cases in compiling Table C1, where the geological information on a particular stone was lacking it has been made good, and occasionally where an item of data was clearly incorrect it has been omitted. Where 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 quarries produce more than one variety of stone (e.g. the various Ancaster stones in Lincolnshire). Conversely, one variety of stone can be produced by several different quarries (e.g. the Purbeck stone in Dorset).

Because stone properties are liable to vary, the test results in Table C1 should not be taken as an indica- tion of present production for particular quarries; instead prospective users should seek up-to-date information from the suppliers. In particular, the data in the Table should not be used for contractual or other such purposes. Also, it should be remembered that stone properties are likely to vary within a quarry, both laterally and with depth, and with the degree of alter-

ation or weathering, and may show anisotropy, so that the single value given in the Table can only be a pointer to what in reality is a range of values. The following quantitative physical and strength properties are tabu- lated in Table CI.

Bulk density. A piece of stone consists of solids, water and air (Fig. C1). The bulk density is the total mass of the solids and water, divided by the total volume of the 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 ' condition as supplied to the user. For porous stones, this 'normal' condition is very variable because it depends on the amount of water in the stone. This variability must be borne in mind when consulting the values of bulk density listed in Table C1.

Relative density. (Formerly called specific gravity.) The mass of the solids, divided by the mass of an equal volume of water. Relative density is dimensionless. However, because the volume of 1 Mg of water is 1 m 3, the relative density can be thought of as having units of Mg/m 3, which is useful when comparing it with bulk density.

Porosity. The volume of pore space (Fig. C1) in the stone divided by the total volume of the solids and pores. Porosity is dimensionless and usually expressed as a percentage.

Water absorption. The mass of water required to saturate the stone divided by the mass of the solids. Water absorption is dimensionless and usually expressed as a percentage.

Compressive strength. The compressive load required to cause failure of an unconfined cylindrical or cubical specimen of the stone, divided by the cross-sectional area of the specimen perpendicular to the axis of loading (Fig. C2). The units are, therefore, force per unit area, which in the SI system is MN/m 2. Compressive strength is simply called 'strength' in the stone industry. Sometimes the compressive strength has been given for stone tested both dry and wet, and this is so indicated in Table C1. Also, for some rocks the compressive strength has been given for stone tested both perpendicular to the cleavage and parallel to the cleavage (cleavage is called 'grain' in the stone industry), and this is indicated in Table C1 by 'perp' and 'para' respectively.

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432 APPENDIX C: S'IONE A N D 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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458 APPENDIX C: S'IONE A N D ROCK PROPERTIES

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APPENDIX C: STONE AND ROCK PROPER' l IES 461

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462 A P P E N D I X C: S T O N E A N D R O C K P R O P E R T I E S

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APPENDIX C: STONE AND ROCK PROPER'lIES 463

Air

Water

Solids

t Pores

Load W

I f

'~ L

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

Flexural strength. The load required to cause failure in a beam-shaped specimen of stone using quarter-point loading (Fig. C3). The test is usually done with the load applied perpendicular to the bedding where this is applicable. The flexural strength is given by:

3 W L

4bd 2

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 1.5d and L is made 10d. The units are, therefore, force per unit area, which, in the SI system, is MN/m 2. An earlier version of the test using half-point loading (Fig. C4) is called transverse strength and is given by:

3 W L

2bd 2

and these results are indicated in Table C1 by 'tran'. If the specimen in the transverse strength test fails at its centre, the transverse strength is equal to the flexural strength. Sometimes the transverse strength is referred to as the 'modulus of rupture'.

Details of how to carry out these tests are given at the appropriate places in the main chapters of the book, particularly Chapter 9 or in Appendix B.

Discussion. It will be noticed in Table C1 that for some stone types the same test results are quoted for a number of different quarries (e.g. the slates from Cumbria). It is extremely unlikely that these are independent deter- minations. What seems to be more likely is that a single set of test results has been quoted by several quarries. The reader should be aware of this possibility when using Table C1.

Load

j

t

Load

t Fi~. C2. Measurement of compressive strength.

Area




1

Load W

1 I I

L r I

Fig. C4. Measurement of transverse strength.

The most commonly reported property of building stones in Table C1 is the bulk density. The reason for this is not technical but commercial. Stone is sold by the tonne (1 Mg = 1 tonne); each lorry laden with stone is weighed on a weighbridge as it leaves the quarry and the

mass of stone is determined by subtracting the mass of 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 aggregate because of the necessity of demonstrating that the aggregate complied with national and local specifi- cations (Smith & Collis 1993). By contrast, in the stone industry, quantitative physical and strength properties are much less importance. Indeed, many of the quarries listed in the Natural Stone Directory do not give quantitative data for their stones. Table C1 shows that, with the exception of bulk density, physical and strength properties of building stones are sparsely reported.

After, bulk density, the property most commonly 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.



APPENDIX C: STONE AND ROCK PROPER'lIES 465

to be of direct relevance to the load-bearing use of stone in masonry construction (see example, Fig. C5). For the porous stones for which compressive strengths are given both wet and dry, it will be seen that the dry strength is greater than the wet strength. One reason for this is that when a porous stone is dried there is a contribution to the strength from suction, over and above the intrinsic mineral strength of the rock (West 1994). As might be expected, flexural or transverse strength is listed only for those stones for which this property is relevant to their use, such as cladding and paving, and cantilever stairs (see example, Fig. C6).

The range of compressive strength of the building stones listed in Table C1 is from 14MN/m 2 for the Stamford Freestone, an oolitic limestone, to 314 MN/m 2 for the Hillend Black, a quartz dolerite; a factor of over 20.

The compressive strength of a building stone can be of crucial importance in the selection of a stone for structural purposes in a large, complex building. For example, Fig. C7 shows the front elevation of a masonry building, the front of which, at ground level, is supported on widely spaced free-standing pillars so as to provide a walkway behind, giving access to set-back shop fronts. The pillars not only have to support their own weight, but have to support the weight of all the

masonry above, together with some of the floor loading. Therefore, the compressive strength of the stone selected for the pillars must be greater than the total compressive stress induced in them by their own weight plus the much greater weight of the superincumbent load. Further discussion of the importance of compressive strength is given in Chapter 9.

By contrast, for small, simple buildings, the compressive strength of even the weakest building stones can be more than adequate for masonry construction purposes, as the following example shows. The strength of the Beer stone, a chalk, is 17 MN/m 2 and its bulk density is 2.4Mg/m 3. A one-metre cubical block of this stone exerts a pressure on the base of 0.0235 MN/m 2. Theo- retically, therefore, it would be possible to build an unmortared, monolithic structure from such blocks up to 723 m high before the compressive stress in the base course exceeded the compressive strength of the stone.

Table C1 also shows that natural building stones in the British Isles are derived from formations of a wide range of geological age, ranging from Cambrian slate to Cretaceous chalk (see geological column, Chapter 2). However, certain geological systems are more important sources of building stone than others; particularly note- worthy are the Carboniferous for its sandstones and limestones, and the Jurassic for its oolitic limestones.

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

IL f f ~ ~ f f

Fig. C7. Front elevation of masonry building with front supported on pillars.

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



466 A P P E N D I X C: S T O N E A N D R O C K P R O P E R T I E S

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APPENDIX C: STONE AND ROCK PROPERlIES 46/

C2. Generalized properties of building stones

The generalized properties of different rock types used as building stones are given in Table C2. This has been compiled from ranges of values given by Winkler (1973), supplemented by some data from other sources. Again, the information given is for a general appreciation only and should not be used for contractual or other such purposes. The headings in Table C2 are the same as those in Table C1, except for the following additions.

Coefficient of thermal expansion. The increase in length per unit length per degree Celsius rise in temperature.

Modulus of elasticity. For rocks that behave elastically, the modulus of elasticity is the stress divided by the strain, usually measured axially during unconfined com- pression (see above). Because strain is dimensionless, the modulus of elasticity has the same dimensions as stress which in the SI system is GN/m 2. The modulus of elasticity is sometimes referred to as 'Young's modulus'.

Tensile strength. The tensile load required to cause failure of an unconfined cylindrical specimen of the stone, divided by the cross-sectional area of the specimen perpendicular to the axis of loading (Fig. C8). The units are, therefore, force per unit area, which in the SI system is MN/m 2. Sometimes called the direct tensile strength to distinguish it from the indirect tensile

Load

t'

( l 4,

Area

Fig. C8. Measurement of direct tensile strength.

strength (see below). The tensile load is applied by means of metal caps that are cemented to the ends of the specimen using a cement which is stronger than the tensile strength of the stone. Because of difficulties with the direct tensile test, the indirect tensile strength test is often carried out instead. The values for the range of tensile strength for various rock types given in Table C2 are from Farmer (1968).

Indirect tensile strength. A disc-shaped specimen of stone is compressively loaded at diametrically opposed surfaces over a small arc of contact (Fig. C9a). Although the loading is compressive, the specimen fails in tension (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 other compressive. This test is also called the Brazil test. The tensile strength is given by:

0.636P/Dt ( M N / m 2)

where P is the compressive load at failure (N), D is the diameter of the test specimen (mm), and t is the thickness of the test specimen (mm) (Brown 1981). Direct and indirect tensile tests on the same rock give closely similar results.

The tensile strength of stone may also be determined by carrying out the point load strength test (International Society for Rock Mechanics 1985), which is an index test, and then empirically correlating the point load 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 rather than on prepared specimens.

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 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 clearly of importance in assessing the suitability of a particular stone for cladding and in deciding on panel thickness. Use of the indirect tensile strength test in testing the condition of masonry construction has been described by Beckmann (1994).




Load P

(b)

Fig. C9. Measurement of indirect tensile strength.

Table C2 shows that the compressive strength of rocks is about ten times the tensile strength. This difference provides an explanation of the traditional practice of using stone in masonry construction so that it carries

compressive rather than tensile loads. This is discussed further in Chapter 9.

The tallest self-supporting, masonry structure in the world is the Washington Monument (Figs C l l & C12),

Fig. CI0. Use of two different stones together. Douai Abbey, Woolhampton, Berkshire, UK. (The individual blocks shown are 230 mm square.)



APPENDIX C: STONE AND ROCK PROPERqlES

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

469

Description Rock mass Sample Compressive weathering strength grade ( MN/m 2)

Saturated bulk density (Mg/m 3 )

Water absorption (%)

Fresh granite I Fresh 262 Partially stained granite II Stained rim of block 232 Partially stained granite II Whole sample II 90% stained 163 Completely stained granite II Completely stained II block 105 Weakened granite III-IV Rock core of III block 46 Weakened granite III IV Rock core of IV block 26

2.61 0.11 2.62 0.35 2.58 1.09 2.56 1.52 2.55 1.97 2.44 4.13

buil t by R o b e r t Mills in 1885, in W a s h i n g t o n DC, USA; it is 169 m high and has the fo rm o f a obelisk. It has been es t imated tha t the s tone at the base o f the m o n u m e n t sustains a m a x i m u m vertical compress ive stress o f

2 . 1 7 M N / m 2 (Allen H o w e 1910). Even as suming a factor o f safety o f 20, it can be seen f r o m the ranges o f compress ive s t rength listed in Table C2 tha t all the rock types for which there are da ta cou ld p rov ide

Fig. Cl l . The Washington Monument, 169 m high, is the tallest self-supporting masonry structure in the world (see text). It was designed by the architect Robert Mills and largely constructed by the US Army Engineers. Construction commenced in 1848 and was completed in 1885, but there was a hiatus in construction from 1854 to 1879. (Photo." D. Newill.)

Fig. C12. Close-up view of the Monument. The exterior is of white marble from Maryland and Massachusetts with an interior of granite. Within the obelisk there is a shaft containing eight wrought-iron columns supporting a stairway and elevator. The slight change in colour of the marble about a quarter of the way up marks the level where construction was interrupted. (Photo. D. Newill.)



4/0 APPENDIX C: S'IONE AND ROCK PROPERTIES

Table C4. Descriptive terms for rock strength

Descriptive term Unconfined compressive strength (MN/m 2)

Extremely strong rock >200 Very strong rock 100-200 Strong rock 50-100 Moderately strong rock 12.5-50 Moderately weak rock 5.0 12.5 Weak rock 1.25-5.0 Very weak rock 0.60 1.25

C4. Scale of rock strength

If the compressive strength of a rock has been determined, a descriptive term for the rock, in terms of its strength, can be derived from Table C4. This classification comes from the Geological Society Engi- neering Group Working Party (1977), and was devised for engineering geology purposes, but there is no reason why it should not be used to describe the stength of building stones.

References

examples of stone more than adequately strong for the base of such a monument.

C3. Effect of weathering

Generally speaking, quarry operators working building- stone quarries will extract unweathered rock. How-ever, in some instances partly weathered rocks will be worked for their attractive colours, or for other reasons. In general, the physical properties of rock will decrease in quality as the degree of weathering increases. This is shown, for example, by studies of granite from a quarry on Dartmoor in southwest England reported by Fookes (1980), and summarized in Table C3. It can be seen that the compressive strength of the rock decreases from 262MN/m 2 to 26MN/m 2, a factor of 10, as the rock mass weathering grade increases from I to IV. (The rock mass weathering grade scale runs from I: fresh unweathered rock, to V: completely weathered rock.) The bulk density and water absorption show similar trends of reduction in quality with increase in weathering. The weathering of rock is discussed in Chapter 2.

ALLEY HOWE, J. 1910. The Geology of Building Stones. Edward Arnold, London, p. 366.

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 Spon Ltd, London, p. 57.

FOOKES, P G. 1980. An introduction to the influence of natural aggregates on the performance and durability of concrete. Quarter O, Journal of Engineering Geology, 13, 207-209.

GEOLOGICAL SOCIETY ENGINEERING GROUP WORKING PARTY 1977. The description of rock masses for engineering purposes. Quarterly Journal o[" Engineering Geology, 10, 355 388.

INTERNATIONAL SOCIETY FOR ROCK MECHANICS 1985. Suggested method for determining point load strength. International Journal o['Rock Mechanics and Mining Science, 22, 51 60.

SMITH, M. R. & COLLIS, L. (eds) 1993. Aggregates: sand, gravel and crushed rock aggregates for construction purposes (second edition). Geological Society, London, Engineering Geology Special Publications, 9.

STONE INDUSTRIES 1994. Natural Stone Directory, No 9 1994- 1995. Herald House, Worthing.

WEST, G. 1994. Effect of suction on the strength of rock. Quarterly Journal of Engineering Geology, 27, 51-56.

WINKLER, E M. 1973. Stone." Properties, Durability in Man's Environment. Springer, Vienna, 43 & 46-47.



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