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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-04-150
Date: July 2006


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Acid (felsic)-igneous rocks: Igneous rocks rich in quartz and the potassium-sodium feldspars. May contain mica, amphibole, etc. Examples are granite and granodiorite. These rocks are often light colored and/or mottled. It is common (except in extrusive volcanic rocks) for the individual grains to be large enough to be easily distinguished by the unaided eye.

Alkali-aggregate reaction (AAR): Any potentially expansive chemical reaction between the aggregate and the hydroxide ions associated with the ions of sodium and potassium in solution in the paste. Can be very deleterious (see Alkali-carbonate reaction and Alkali-silica reaction).

Alkali-carbonate reaction (ACR): A potentially expansive chemical reaction between a carbonate aggregate and the hydroxide ions associated with the ions of sodium and potassium in solution in the paste. The only rock known to so react is an impure dolomitic limestone with a specific internal structure. Can be very deleterious if the forces generated by the reaction exceed the cohesive forces of the concrete. (See chapter 10.)

Alkali-silica reaction (ASR): A potentially expansive chemical reaction between siliceous aggregate and the hydroxide ions associated with the ions of sodium and potassium in solution in the paste. The siliceous rocks involved in this reaction are those with an imperfect crystal structure or those that are not crystalline. Can be very deleterious if the forces generated by the expanding silica gels exceed the cohesive forces of the placement. Deteriorated HCC often exhibits oozing silica gels or the dehydrated and carbonated remains of silica gels. (See chapter 10.)

Analyzer: In a petrographic microscope, a device (located above the objective) that polarizes the light after it has passed through the specimen.

Anhedral: Without bounding crystal faces (see Euhedral and Subhedral).

Anisotropic: The property of not being the same in all directions. Used especially in reference to mineral substances and usually implies that the substance is at least slightly birefringent.

Basic (mafic)-igneous rocks: Igneous rocks rich in the sodium-calcium feldspars, with little or no quartz; often abundant olivine and pyroxenes. Examples are basalts and diorites. These rocks are often very dark colored and fine-grained.

Becke line: In the microscopical test for the relative index of refraction, a bright line that is mostvisible while in motion as the focus is changed. It is the projection of the boundary of two substances of different indices of refraction. If the match between the indices is close, the line may be refracted into rainbow colors (unless monochromatic light is used). As the objective lens is raised from the subject, the Becke line appears to move toward the center of the substance with a higher index of refraction. If the line is refracted into colors, the position (movement) of the rusty red color is taken as the true position.

Beneficiation: The process of improving the chemical properties, physical properties, or both, of an ore or other earth material for use in a manufacturing process or as a construction material.

Bertrand lens: A removable plano-convex lens on a slide located in the tube of a petrographic microscope above the upper nicol (analyzer); used with convergent light from the condenser to form interference figures and thus determine the number of optical axes and the optical sign. The focusable, diaphragmed Bertrand lenses are the most convenient.

Binocular: Pertaining to optical equipment. Allowing the use of both eyes together, either to permit the sensation of relative distance (when two objective lenses are furnished) or to provide a more natural method of viewing the subject. In modern petrographic microscopes, binocular viewing is present to relieve eye strain and cannot provide a sensation of depth. In many microscopes, the prisms necessary to split the image for use by both eyes may make determinations of the properties of the optical axis figures nearly impossible.

Birefringence: The numerical difference in the index of refraction between two rays of light separated from one light source by the properties of the substance. Birefringence can be observed when a nonisotropic substance is placed between crossed nicols. The diffraction colors seen are a product of the birefringence of the substance and its thickness. Also called double refraction. Low birefringence is gray though pale white, 0.005; moderate birefringence includes the first order colors up to 0.030; all above 0.030 is high to very high.

Birefringent: The property of having at least minimal birefringence.

Bleed water: The water produced by bleeding (the rising of water in the concrete – see the definition below for “bleeding”). The water can be seen as a wet sheen on the surface of the concrete. When abundant, it can form puddles.

Bleeding: In fresh concrete, a process whereby the solids of the HCC, including the cement and other fine particles, settle and water rises to the top.

Calcite: Calcium carbonate (CaCO3), a common mineral and a major ingredient of limestones and certain other sedimentary rocks. Often occurs as a vein mineral. Calcite is colorless or white when pure, crystallizes in the rhombohedral class, has a hardness of 3 on the Mohs scale, and dissolves with the effervescence of carbon dioxide in dilute HCl. The transparent forms of calcite exhibit excellent double refraction that can be observed in thick slices without lenses or polarizers. The carbonates have very high birefringence (calcite = 0.172). (See Dolomite.)

Carbonation: The process in portland cement concrete and in other HCCs where the calcium ions in solution or in the calcium silicate hydrates react with the carbon dioxide of the atmosphere in the presence of moderate humidity to form calcium carbonate with impurities such as amorphous silica. On the surface of HCC, the production of calcium carbonate by this method can provide a tough surface. In the interior, it can indicate weakness; passageways for water, air,and CO2 to enter the paste; and lowered pH of the paste, thus making the steel more susceptible to corrosion. (See Depassivation.) (Not to be confused with the carbonation of soda water or the coalification of organic materials.)

Coldjoint: A joining between two adjacent placements of HCC material where the earlier placement had hardened when the second was placed. May indicate little or no bond between the two concretes.

Collimate: When pertaining to light rays, to make parallel.

Conchoidal: Said of a fracture surface that is made up of smoothly curved, shelllike surfaces. True of quartz and obsidian and other massive glasses.

Conoscopic lenses: The lenses used to form interference figures. (See Bertrand lens.)

Core: When pertaining to concrete testing, a specimen cut from a hardened placement or a large block of concrete with a diamond-core drill for use as a portion of the material for various testing procedures. It is preferred that the core drill have an internal diameter of at least 100 mm. The core should be as deep as the placement is thick so that the entire thickness of the placement is sampled. When coring bridge decks, a small thickness of concrete is often left in the bottom of the drill hole to prevent the core from falling through the bridge and to provide a bottom for the patching material. A core can be distinguished from a cylinder by the diamond-cut exterior curved surface on which cut aggregate particles will normally be exposed.

Crazing: A fine, very shallow cracking that occurs in the exposed surface of concrete. It may be caused by shallow freezing or minor curing flaws. Crazing often has a very fine pattern, like pattern cracking, except that the individual uncracked central portions are usually less than 50 mm across. Crazing may develop to the point where the surface flakes off to a depth of about 3 mm.

Crossed nicols (polars): The microscopical condition of having the analyzer and the polarizer in the system with their vibration directions at 90 degrees to each other.

Crystallite: A tiny crystal, often acicular, and sometimes too small to exhibit birefringence.

Cubic: Used with reference to the crystal structure of a substance to indicate three crystallographic axes of equal length and mutually perpendicular. Often referred to as an isometric crystal structure.

Cylinder: When pertaining to concrete testing, a specially cast cylindrically shaped specimen of the concrete being fabricated to provide portions of the concrete for various testing procedures. Standard cylinders have a height two times the diameter. In Virginia, the cylinder is usually 100 mm in diameter and 203 mm in height. A cylinder may be distinguished from a core by the fact that the curved surface was cast in a mold with a smooth surface and may show marks of the seams of the mold.

De-dolomitization: Any of several processes in which the dolomite is removed or replaced from within a solid rock; it may take place through movement of ionic solutions in the interior of the aggregate particles or be particularly active on the rock surface. Especially used for the replacement of dolomite by calcite and brucite.

Depassivate: The process of neutralizing or removing the passivation (protection from active corrosion) on the surface of a metal. In concrete, usually used to mean the lowering of the pH of the material or the presence of aggressive chemical species (e.g., Cl) surrounding the reinforcing bars so that the passivating (protective) effect of the high pH of the paste is no longer present.

Design of the mixture: The specified proportioning of the ingredients of the concrete mixture; the document giving this information. The document should indicate the source of each of the ingredients approved for the concrete in question.

Diffraction: The process of bending light as it passes from one medium to another. Examples are the opponent bending of a straight object at the water line and the bending of light by a prism.

Diffraction colors: Those colors caused by the fact that the different colors of light bend at different angles. Examples are rainbow colors and the colors produced by a prism.

Dolomite: Calcium-magnesium carbonate (CaMg(CO3)2), a common mineral in sedimentary rocks such as limestones and dolostones. Much like calcite, but slightly harder and less soluble in acid. The chemical composition grades toward ankerite, with a substitution of iron and manganese for part of the magnesium. The carbonates have very high birefringence (dolomite = 0.185). (See Calcite and Dolostone.) Dolomite is sometimes used to mean a rock composed predominantly of the mineral dolomite.

Dolostone: A rock much like limestone in appearance, but composed mainly of the mineral dolomite. Frequently, the rock is called dolomite.

Effervesce: To bubble, splatter, etc., by the emission of a gas resulting from a chemical action. An example is CaCO3 when acted on by acid.

Efflorescence: A surface encrustation caused by the evaporation of solutions seeping out onto the surface of rock or concrete.

Euhedral: Completely bounded by its own regularly developed crystal faces. (See Anhedral and Subhedral.)

Exudation: Something oozed out, usually a fluid solution.

Feldspar: A mineral of one of the two major feldspar groups: the potassium-sodium group and the sodium-calcium (plagioclase) group. The feldspars are aluminum silicates and are common in low-temperature veins and in types of igneous rocks. They are monoclinic or triclinic, have two optical axes, and range in color from colorless through various pastels to dark gray. They have nearly perfect cleavage in at least two directions at close to 90 degrees to each other. They are often euhedral or subhedral, even when intergrown with other minerals and each other. The birefringence is moderate, generally between 0.006 and 0.011. They can be recognized by their cleavage and intergrowths.

Grading: Said of an aggregate and used to describe the distribution of the sizes of the particles therein. Well-graded aggregate has a large variety of particle sizes and fills a space well. Poorly graded aggregate or gap-graded aggregate, has particular sizes missing. When used by a geologist, good grading indicates a definite sorting of grain sizes and excellent grading indicates that the various grain sizes are well separated and deposited in different portions of the sediment.

Greenschist facies or Greenstone facies: The stage of metamorphism in which greenstones occur. The rock is rich in chlorite, actinolite, epidote, or all three, occasionally with significant quantities of feldspars and quartz. (See Greenstone.)

Greenstone: A compact rock formed by the metamorphism of basic to ultrabasic igneous (sometimes volcanic) rock. The metamorphism has proceeded to the greenschist facies.

Groundmass: The phase of a rock that is so fine-grained that individual minerals cannot be identified. Commonly said of the interstitial material surrounding the larger crystals in an igneous rock or of the unidentifiable mudlike matrix of a sedimentary rock.

Hexagonal: Used with reference to the crystal structure of a substance to indicate three crystallographic axes: of equal length, at 120 degrees to each other, and all perpendicular to a fourth axis.

Hornblende: A dark-colored mineral of the amphibole group of monoclinic silicates, all of which exhibit cleavage parallel with the long axis at about 56 and 124 degrees. Birefringence is about 0.023. Occurs in acid-igneous rocks.

Igneous: Used to indicate rock that has formed by cooling or the escape of fluids from molten portions of the earth’s crust either at or near the surface or at great depth. Examples are granites, lavas, and diorites. They occur in massive formations, flows, veins, dikes, and sills. The term includes those vein rocks that are formed from hydrothermal, ion-rich fluids and that produce a large variety of minerals and often gigantic crystal sizes (i.e., pegmatites). On occasion, used to indicate rock (which may be truly metamorphic) that seems to have the characteristics of igneous rock.

Index of refraction: The ratio of the speed of light in a vacuum to the speed of light in the substance under consideration.

Index of refraction oil: Immersion oil used as a standard medium to which to compare subject substances. Prepared sets of such oils are commercially available. Because the index of refraction of these oils may change with time, they should be periodically checked using a refractometer or solid standards.

Insoluble impurities: The substances found in the interstices of a carbonate rock material that cannot be dissolved by warm, dilute HCl. These substances may include clay, quartz, micas, feldspars, and pyrites.

Interference color: The highest order of color seen in a mineral when viewed with the petrographic microscope with crossed nicols. The highest colors will be seen when the two crystallographic axes that have the greatest difference in refractive power are parallel with the stage and when the axes are at 45 degrees to the nicols. The interference color is a product of the birefringence and the thickness of the substance. Charts of the interference colors are available in most books on the use of the petrographic microscope. These charts show 0.000 birefringence as black, increasing to gray, white, yellow, and orange, through the spectrum. When the spectrum starts to repeat, the light is more brilliant and the color bands are increasingly blurred. At about the sixth repeat (the sixth order), the light is a brilliant white in which no specific color can be distinguished. This is the typical appearance of the carbonate minerals at 30 mm in thickness. In a random view of any mineral, the interference colors seen will depend on the thickness, angle of viewing, and angle to the polarization of the nicols. Some particles will be banded from very low to the highest color produced by the substance. Other particles of the same substance when viewed with only equivalent crystallographic axes parallel with the stage will show almost no interference and will be seen as nearly black.

Interference figure: A pattern of light and shadow, and bands of interference colors produced on the back lens of the objective in the petrographic microscope by the use of the accessory convergent lens in the condenser. The figure can be viewed as projected on the back lens surface or with the Bertrand lens. The interference figure provides data concerning the crystal structure and the spread of the optical axes. With the gypsum plate or quartz wedge accessories, the optical sign of the mineral can be determined.

Isometric: The crystallographic or optical property of a substance having three crystallographic axes of equal length at mutual right angles. An isometric mineral has no birefringence unless it has been distorted or subjected to stress.

Isotropic: The property of being crystallographically the same in all directions. Includes isometric and amorphous.

Laitance: A layer of weak material containing cement, calcium hydroxide, and aggregate fines brought to the surface of concrete by bleed water. The amount is increased by a high water-cement ratio, overworking, or improper finishing (see ACI 116).

Lapping: The process of producing a smooth surface by grinding away projecting portions.

Limestone: A common sedimentary rock mainly composed of the mineral calcite (CaCO3). Includes the lithified chemically deposited benthonic (deep water, bottom) type, as well as the indurated collections of shells and fragments of shells. Usually restricted to meaning only oceanic rocks, but can include the fresh–water limestone, travertine, as well. May be restricted to rocks mainly composed of calcite, but is sometimes used to include the other carbonate rocks, especially dolostones.

Lithology: Rock type, including mineralogy, structure–intrinsic parting, fissibility, and grain size.

Macrocrack: Cracks that can be seen with the unaided eye (often large enough to be seen when one is riding over a pavement or bridge).

Metabasalt: A basaltic, fine-grained, basic igneous rock, often extrusive (volcanic lavas, near surface sills, and dikes, etc.) that has undergone metamorphism. Will frequently include beds of metamorphosed volcanic debris such as ash, lapilli (pebble sized), and bombs. (See Greenstone.)

Metamorphic: Pertaining to a rock formed by the processes of metamorphism. The source rock material may be sedimentary, igneous, or earlier metamorphic.

Metamorphism: A natural process that, over the stretch of geologic time, can transform a rock from one appearance, crystal structure, and composition to another. The process can include the stresses of the folding and faulting crust, subduction at the edges of crust plates, pressures of burial at depth, and recrystallization and chemical changes caused by the various solutions and temperatures found in the earth’s crust. The word metamorphism includes the low-grade, low-temperature processes by which fluids can rearrange the ions, atoms, and minerals of a rock to form new minerals and recrystallize minerals in new forms.

Micrite: The very fine-grained crystalline component of limestones that is thought to have been deposited as a chemical precipitate (calcium carbonate) and is considered to be a lithified ooze. The individual particles are completely anhedral and generally less than 4 µm in diameter.

Microcrack: Cracks that cannot be clearly seen or measured without magnification. They exist on all concrete surfaces, but are extremely difficult to see on rough surfaces.

Modal (from classical petrology): Used with analysis or determination to indicate the data collection and calculations necessary to make a mathematical determination of the relative abundance of the various component solids or discrete substances that make up the whole. There is no implication of the determination of the three-dimensional spatial arrangement, shape, and size of the individual portions of the phases. When the determination is made optically, from points, chords, or areas on a plane through the substance, the results will be in percentage by volume.

Monoclinic: Used with reference to the crystal structure of a material to indicate three crystallographic axes of not necessarily equal length: two are mutually perpendicular and the third is not.

Monocular: Pertaining to optical equipment. Allowing the use of the equipment by one eye only.

Nicol or Nicol prism: In a petrographic microscope, a device that polarizes light. The lower nicol (located in the substage) polarizes the light before it reaches the specimen. The upper nicol (located above the objective) polarizes the light after it has gone through the specimen. (Also known as Polar or Polarizer.)

Nonisotropic: (See Anisotropic.)

Ocular: Pertaining to optical equipment. The lens assembly nearest the eye.

Optical sign: An optical property of birefringent crystal substances. It is one of the properties by which substances can be classified and identified. (See Interference figure.)

Orthorhombic: Used with reference to the crystal structure of a substance to indicate three crystallographic axes of unequal length and mutually perpendicular.

Passivate: To render the surface of a metal chemically inactive, usually by the formation of an oxide film. In concrete, passivation of the surface of the steel reinforcing bars is produced by the high pH of the cement paste.

Paste: The portion of a concrete that is not aggregate, consisting of hardened cement hydration products, residual unhydrated cementitious materials, and air voids.

Phase (as used in chemistry): A distinct, mechanically separate component of a heterogeneous whole. The term may or may not refer to the physical state of the component (i.e., gas, liquid, or solid). Also used with analysis or determination in the same sense as modal.

Phenocrysts: The relatively large conspicuous crystals, surrounded by groundmass, finer crystals, or natural glass (such as obsidian). Found in particular igneous rocks.

Pleochroic properties: The property of a mineral such that it exhibits different colors when viewed at different angles, particularly in plane polarized light. A useful identifying property particularly well exhibited in the minerals biotite, hornblende, and tourmaline.

Polarizer: In a petrographic microscope, a device (located in the substage) for polarizing the light before it reaches the specimen. (Also known as Nichol.)

Pozzolanic materials: Materials that combine with the lime or alkali hydroxides in cement paste to produce cementitious silicate hydrates, includes natural pozzolans, fly ash, silica fume, and ground granulated blast–furnace slag.

Pyrite: A stable, common mineral of the pyrites group, iron sulphide (FeS2). It is opaque, with a golden metallic color and conchoidal fracture. Crystallizes in the cubic system with striated faces; common habits are the cube and pyritohedron. Common in all metamorphic rocks, even those of very low-grade metamorphism. (See Pyrites.)

Pyrites or Pyrite group: A group of common, opaque, iron sulphide minerals; stability and metallic color depend on the ratio of iron to sulphur. Black in color when occurring in an extremely finely divided state. The group includes pyrite, pyrrholite, and marcasite. The black variety is common in very dark–colored limestones, shales, and slates. (See Pyrite.)

305 Quartz: A common mineral, silicon dioxide (SiO2), crystallographically rhombohedral, colorless when pure, a major constituent of the rocks of the earth’s crust. On the Mohs hardness scale, it is number 7. Occurs in six-sided prisms with pyramidal terminations in veins and in anhedral masses in almost all acidic rocks. A major constituent of most beach and river sands. Cleavage is usually visible only in very thin sections. The birefringence is moderate (0.009).

Refraction: The change in a light wave as it passes into a substance of different density or with different optical properties. Related to diffraction, but includes the splitting of a light wave into wo components that travel at different speeds and whose vibration directions are at right angles to each other. Examples are the apparent bending of a straight object at the water line, the bending of a light wave by a prism, and the double refraction of crystals of substances such as calcite. (See Index of refraction.)

Relief: The optical property of the degree of contrast microscopically observed, caused by the difference in the index of refraction between a substance and the medium with which it is surrounded. High relief appears as a rough surface; low relief appears as a smooth surface that is hard to distinguish from the background.

Rhombohedral: Used with reference to the crystal structure of a crystalline substance to indicate a subset of the hexagonal crystal system in which the symmetry is incomplete. The two end terminations may differ, and alternate (thus opposing) prism faces may have different textural characteristics.

Screeding: The process of leveling and smoothing concrete to prepare it for the final surface texture. Often combined with vibration as part of the consolidation process.

Sedimentary: Used to indicate rock that has formed by the collection and usually the induration of materials in solution and particles derived from other rocks by the forces of weathering, gravity, running water, etc. The induration may be purely chemical and can take place on the surface or may be caused by compression and solutions at depth. (See Siltstone.)

Siltstone: A very fine–grained consolidated rock, the particles of which are predominantly between 0.06 and 0.004 mm across and have been removed from other rocks and transported by wind or water to the place of consolidation.

Stalactite: A cylindrical or conical deposit of solids caused by the evaporation of solutions dripping from an overhead structure such as a cave roof or bridge.

Subconchoidal: Said of a fracture surface that is nearly, but less than perfectly, like a conchoidal surface. (See Conchoidal.)

Subhedral: Incompletely bounded by its own regularly developed crystal faces. (See Anhedral and Euhedral.)

Tetragonal: Used with reference to the crystal structure of a substance to indicate three crystallographic, mutually perpendicular axes, two of which are of equal length.

Tourmaline: A mineral of a complex silicate group that contains boron and is crystallographically rhombohedral. They occur in striated columnar crystals in many different colors. They may be found in acid–igneous rocks and veins. The birefringence of the minerals of the tourmaline group is about 0.020.

Triclinic: Used with reference to the crystal structure of a substance to indicate three crystallographic axes with unequal interaxial angles.

Ultraviolet light: Radiation beyond the visible light in the violet end of the spectrum. Wavelengths are shorter than those of visible light and longer than x rays.

Vitreous: Having the luster of broken glass or quartz, glassy.

Vitrified: Converted into a glassy substance by heat and fusion.

Volcanic: Extrusive igneous rocks that are glassy or very fine–grained because of rapid cooling.

Zeolite: A group of hydrous alumino-silicates of Na, Ca, Sr, Ba, and K characterized by easy gain or loss of water of hydration. Also noted for their base exchange capacity.


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