Conservation of Historic Stone Buildings and Monuments (1982) / Chapter Skim
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Physical Properties of Building Stone
Physical Properties of Building Stone
Pages 62-86
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From page 62... ...
Variations in physical properties because of compositional and textural inhomogeneities can be seen in small to large quarried blocks or in rock in place and can be more significant in explaining rock deterioration than laboratory tests of the physical properties of selected small samples of rock. Examples of inhomogeneities are intercalated shaley layers, calcite, limonite, or clay cements; thin to thick bedding; mineral variations within beds in sedimentary rocks; foliation; induration; microfractunng; and incipient to open jointing.
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From page 63... ...
Because mineral composition and texture differ according to the varying geologic histories of rocks, the values of physical properties range widely {Table 11. Building stones are polycrystalline mineral aggregates, not single crystals.
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From page 64... ...
64 lo; ~ r I_ 0\ 00 rib U' be m o cn ·_' o C
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From page 65... ...
Porosity Porosity, +, is the ratio of pore volume to bulk volume. Porosities of common building stones are listed in Table 1.
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From page 66... ...
Clay minerals in shale or sandstone reduce permeability markedly because the pores between clay particles are very small; smectite clays in a rock expand by water absorption, further reducing permeability. Figure 1 presents points from measurements on five sandstones in rows labeled by their porosities, which range from 8 to 22 percent;
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From page 67... ...
The percentages marked on the lines represent the proportion of pores larger than the pore radius for a given permeability on the ordinate axis Redrawn from Blatt et al.~.~4
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From page 68... ...
The line on the right is for sandstones in which 10 percent of the pores are larger than those found on the abscissa for a given ,u" on the ordinate; the line on the left is for sandstones in which 10 percent of the pores are smaller for a given A As might be expected intuitively, the permeability of sandstones varies exponentially with porosity and with pore size.
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From page 69... ...
of six sandstones of varying porosity are shown inside the enclosed areas in Figure 3. Specific surface area is high for fine-grained, low-permeability sandstones; it is Tow for coarse-grained, high-permeability sandstones.
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From page 70... ...
The interdependence of ,u and ~ is not clearly understood. The value of s is obviously strongly affected by grain size, so that ,u for a coarsegrained sandstone can be an order of magnitude higher than for a finegrained sandstone, although both have the same ¢.~4 The Kozeny formula has limited use because s and t are difficult to estimate.
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From page 71... ...
Hudec shows that water in the pores weakens rock, making thermal stresses more effective.5 A property like elasticity could be used to reveal the extent of damage from thermal cracking. Modem ultrasonic, acoustic, or mechanical velocitylogging devices can be used to measure the expected decrease in elasticity and could be adapted to measure the weathering of monument stones.
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From page 72... ...
, (5) where or is stress in kilobars; ax is coefficient of expansion in reciprocal degrees Celsius; E is elastic modulus in kb; To and To are final and initial temperatures in degrees C; and v is Poisson's ratio, which can be taken as about 0.25.
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From page 73... ...
(acting horizontally) from thermal expansion caused by a 25° C increase in temperature In granodiorite at Mount Airy, N.C.
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From page 74... ...
Thermal Conductivity Thermal conductivity, K, is the rate at which heat is conducted in millicalories per second through a 1-cm2 area down a temperature gradient of 1° C over 1-cm length. In rocks, K is affected not only by the mineral composition but also by the porosity,- the degree of fluid saturation, and heating.
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From page 75... ...
However, the Mobs hardness of rocks is difficult to determine and is too much affected by friability and surface texture to be really useful. Clearly, the ease of polishing monument stones is related to harness, but stone deterioration is not obviously related to Mobs' scale.
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From page 76... ...
Limestone and rock salt, under respective confining pressures of 1 kb and 0.2 kb or higher, will deform plastically under differential stress above the elastic limit; they can deform by creep to 20 to 30 percent before large cracks form. The uniaxial strengths of porous sandstones, limestones, and shales depend on porosity; the strength of these rocks increases about threefold as porosity decreases from 35 to 1 percent.
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From page 77... ...
Friability is low in dense, igneous rocks and high in porous, sedimentary rocks; it depends on the character of the intergranular bonds, from the weak bonds of a poor cement to the strong ionic bonds of silica tetrahedra. OPTICAL PROPERTIES Color The colors and patterns of monument, facade, and other building stones are important for artistic reasons.
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From page 78... ...
~S ^~ ': _ 4~ \ ~~ Tap Water 50Q-m \ Salt Solution 0.3Q-m 0.1 1 10 100 POROSITY (percent) FIGURE 7 Decrease in resistivity of many crystalline igneous and metamorphic rocks with increase in porosity to about 5 percent, for saltwater and tap water saturating the samples under 4 k bar confining pressure {Bracel.2i
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From page 79... ...
Merrill reported on a new firm that started up an abandoned but formerly successful quarry and lost nearly $1 million because the new operators failed to observe imperceptible defects in the rock in the new quarrying zone.3 He said that, as a consultant, if he were restricted to either field examinations or laboratory tests, he unhesitatingly declares that, with good natural outcrops or quarry openings of Tong standing, he would choose the field examination, no matter how elaborate the other tests might be. At the time of writing, he was probably correct, but today, presumably, careful sampling and complete testing of physical properties can detect small but critical differ
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From page 80... ...
These rocks usually are dense and range in grain size from fine and equigranular, through medium and equigranular or porphyritic, to coarsely granular. Porosity and permeability are usually low, and the granite has high resistance to weathering and corrosion unless it is highly jointed, microfractured, or foliated.
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From page 81... ...
Silica-cemented, fine-grained quartzites do not deteriorate, but if shaTey layers or close jointing occur, they will constitute planes of weakness and high permeability. Marble Both calcitic and dolomitic marble are massive rocks but commonly have moderate intrinsic permeability.
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From page 82... ...
Clayey cement absorbs water, and sandstone containing it is easily broken, either by freezing or because clay minerals form poor intergranular bonds. The reddish-brown, porous sandstone from Seneca, Maryland, used in the Smithsonian building in Washington, D.C., is limonite-cemented and has stood up reasonably well.
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From page 83... ...
Shale Shale is inherently friable in that it is not lithified well enough to resist abrasion. Shale has very low intrinsic permeability because of its clay mineral content, but some shales have pronounced bedding planes and jointing, which provide permeable channels if the shale is under very low confining pressure.
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From page 84... ...
It can also result from zones of small-scare microfracturing, which can form when existing tectonic stresses are concentrated by the quarrying operation until they exceed the strength of the rock and it fails. For those analyzing deterioration processes in particular stones, geophysical techniques could provide useful measurements of the physical condition to supplement laboratory tests of physical properties.
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From page 85... ...
,S so saturation must be accounted for in interpreting velocity studies. In general, to obtain a fuller explanation of each deterioration process in building stone, collaboration will be needed among the following people: the preservationist, who knows where these processes take place, what stones to study because of their architectural and historic significance, and what remedies have been tried; the geologist, who knows the origin and mineral content of the stone and its probable geologic inhomogeneities; the specialist in rock mechanics, who knows the measurement and the significance of physical properties; the geophysicist, who knows exploration techniques and their application to characterizing the extent of stone decay; and the geochemist, who understands the chemistry of weathering and knows what analytical techniques can be used to explain the detenoration process.
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From page 86... ...
15. Torraca, G., 1976, Brick, adobe, stone, end architecturalceramics: Deterioration processes and conservation practices, in Proc.
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