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Publication Number: FHWA-HRT-04-150
Date: July 2006

Appendix D. AGgregates Used in Hydraulic Cement Concrete

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D.1 OVERVIEW

Usually, the aggregates used in HCC are naturally occurring earth materials that have been crushed, graded, and washed as needed to meet the requirements for the concrete being produced. The amount of beneficiation required will depend on the nature of the aggregate and the requirements of the specifications. Often transporting the aggregate is more costly than obtaining it from the quarry (presumably cleaned and sized). Therefore, aggregate sources near the concrete production plant are often preferred over sources of higher quality material located at a greater distance. The preparation of aggregate specimens for petrographic examination is described briefly in section 5.5.

Not all natural rocks are suitable for use as aggregate, although a wide variety have been successfully used (Mather and Mather, 1991). The material used must pass specific tests as specified in ASTM C 33 and in any specification document provided by the client, customer, or purchaser. In Virginia, the document is Road and Bridge Specifications (Virginia DOT, 2002).

The petrographic description of the aggregate should be guided by ASTM C 125 (standard terminology for concrete) and ASTM C 294 (descriptive nomenclature for aggregates). The procedures given in ASTM C 295 (petrographic examination of aggregates) can be used when a supply of the aggregate is available. Detailed information may be found in ACI 221.1R, Dolar-Mantuani (1983), Forster (1994), Galloway (1994), Mielenz (1994), Meininger (1994), Mullen (1978), Ozol (1978), Price (1978), and Schmitt (1990). Basic texts on petrology, petrography, and mineralogy should be available and familiar to all persons doing this work (see the Reading List). Any person performing petrographic examinations of HCC or aggregates or who is engaged in specifying aggregate properties should carefully study the literature on earth materials and works on concrete aggregates.

The HCC petrographer who generally works with concretes fabricated in a given area, such as a State or group of States, will find that the more familiar he or she becomes with the aggregates from that area, the easier aggregate identification will become. A collection of reference aggregates is very helpful. The aggregates should be identified as to quarry, approximate date quarried, geologic formation, and rock type. The collection should include specimens of concrete containing the aggregate, as well as specimens of the unused aggregate.

The ability of aggregate to withstand the stresses induced during the mixing of HCC is very important and, therefore, aggregates must be able to conform to the abrasion resistance requirements specified in ASTM C 33 (specification for concrete aggregates) as tested in accordance with ASTM C 131 (resistance to degradation of small-size coarse aggregate) or ASTM C 535 (resistance to degradation of large-size coarse aggregate). Commonly, the external characteristics of the aggregate (such as the distribution of the particle sizes, shape, texture, surface coatings, type of fracture, surface area, etc.) are more important to the behavior of aggregates in HCC than is the mineral and chemical composition of the aggregate. Forster (1994) and Galloway (1994) provide excellent explanations of these properties. A summary of some of these testing procedures may be found in ASTM R0030, Manual of Aggregate and Concrete 264 Testing (2002). The fine aggregate (i.e., those passing a No. 4 (4.75-mm) sieve) requires specialized testing (Gaynor and Meininger, 1983).

The particle shape and texture of the aggregates are important characteristics as they can have a significant impact on concrete workability and thus water demand. The particle shape can be evaluated by determining the loose or uncompacted void content by methods such as Virginia Test Method No. 5 (Virginia DOT) or the test method described in Gaynor and Meininger (1983) (subsequently adopted as ASTM C 1252). The preceding methods are specific for use with fine aggregates; however, Hossain, Parker, and Kandhal (2000) report on the development of a similar procedure for coarse aggregates. ASTM C 29 can be used to determine a compacted (by rodding or jigging) or uncompacted (shoveling) void content of coarse or fine aggregate. These tests measure the unforced or forced packability of the aggregate and are thus indirect measurements of the water demand of the aggregate. The void content obtained by these methods is affected by grading, as well as by shape factors, so if one is trying to use the methods to compare the shape of different aggregates, a standard grading must be used. More recently, efforts have focused on developing automated imaging systems to evaluate particle characteristics (Kuo and Freeman, 2000; Masad, Button, and Papagannakis, 2000; Rao and Tutumluer, 2000; Kim, Haas, Rauch, and Browne, 2002; and Rao, Tutumluer, and Kim, 2002).

D.2 COARSE AGGREGATE

Coarse aggregates (i.e., those retained on a No. 4 (4.75-mm) sieve) for use in HCC are selected mainly on the basis of durability, size, general shape, mineral composition, economy, and availability. Figures 206 and 207 illustrate aggregate particle shapes that are avoided when economic reasons permit.

The requirements of the proposed HCC placement must be fully considered. A placement that has many reinforcing bars close together will require a much smaller coarse aggregate than one with no reinforcement. Pre-placed aggregate is often very large and always lacking in the finer sizes (Lamberton, 1978; Davis, 1994). In particular cases, the density or mineral composition of the aggregate is important. A high density will make it more difficult to prevent segregation; however, the use of aggregate of high density may be dictated by the availability of aggregate or specific use of the concrete. Refer to any specifying document from the client to determine the fitness of a coarse aggregate to meet the requirements of the HCC (see section E.6).

Figure 206. Shaley particle shape of crushed slate aggregate.

Several coarse aggregate particles of a crushed slate are shown. They are gray and rather flat and angular.

Figure 207. Aggregate particles from a fissile gneiss (a particle shape such as this can cause a high water demand).

Each aggregate has a rough and porous surface. A particle shape such as this can cause a high water demand

D.3 FINE AGGREGATE

Sand (fine aggregate) for use in concrete should be tested for shape and surface smoothness. If the particles have angular shapes with abundant re-entrant angles, the sand has a high void content and a high water demand. It takes more fluid (or cement paste) to surround an angular particle than to surround an equant particle. Among solid shapes, the ratio of the surface area to volume is smallest for spheres and largest for extremely lath-shaped particles and particles with deep re-entrant angles and internal cracks and cavities. If the fluid present is insufficient to coat the surface area, the concrete mixture is harsh and difficult to place and finish. This condition is perceived, during construction, as a need for more water.

It can be difficult to estimate the void content of a fine aggregate from a petrographic examination of a finely lapped slab because the visual contrast between the paste and the sand particles is very low unless the sand is stained or coated. In thin sections, the outline of the sand particles can be easily distinguished from the paste by means of the birefringence of the sand particle. (Very few sand particles have a birefringence as low as that of the paste.) In fluorescent microscopy, where the thin section is impregnated with a fluorescent dye, the outline of the aggregate is emphasized because the aggregate is not illuminated by fluorescence and the porous paste is illuminated. The weak zones caused by high water demand are exceedingly porous, contain very large capillaries, and become brightly illuminated by the fluorescence of the impregnating dye (see figures 168 through 171).

D.4 IDENTIFICATION OF MINERALS AND ROCKS

Often a general identification of minerals or rocks present in an aggregate will suffice for routine purposes. Such identifications can be performed on hand samples with simple tools and hand lenses (FHWA, 1991). The same techniques can be used to examine and identify aggregates in polished slabs of concrete. When required, the exact mineralogical identification of natural and artificial aggregates can be accomplished by determining the optical properties by use of the petrographic microscope when the aggregate is examined in grain mount, thin section, or both; by using various methods of determining the chemicals present (spot tests, EDX, XRF); and by collecting an x-ray diffraction pattern for positive identification.

Such identification is necessary only when the distinction between similar rocks might elucidate the reasons for differences in durability or other behavior when the aggregate in question is used in HCC. There are many excellent books written on these methods, and courses on these methods are taught in the materials engineering and geology departments of numerous universities (see the Reading List).

Most often, exact identification is not necessary. Usually, the most detailed examinations are required when AARs are suspected or carbonate rocks are suspected of causing D-cracking (Schwartz, 1987). For more data on alkali-reactive aggregates, refer to chapter 10 and the associated figures and references; the related ASTM standards (Reading List); and the section on thin-section preparation (section 5.2).

D.4.1 Mineral Identification

Minerals are naturally occurring chemical elements or compounds with well-defined molecular structures. The chemistry and structure of minerals result in their having a unique set of properties that can be used to identify them (table 29). Rock and Mineral Identification for Engineers (Report No. FHWA-HI-91-025) (FHWA, 1991) provides a detailed outline for identifying minerals in hand specimens. These techniques can also be applied in identifying minerals in aggregates in lapped slabs of concrete. In using these characteristics, it can sometimes be misleading to base identification on a single characteristic; a more positive approach is to use the sum of several characteristics. Table 30 provides selected characteristics of common minerals. Figure 208 shows the Mohs scale (a comparative hardness scale).

Figure 209 illustrates the use of crystal habit, color, and luster to identify pyrite in a rock. Cleavage traces can be used to differentiate between feldspars and quartz (minerals of similar hardness) (figure 210). Hardness (figure 211), as well as acid solubility, is used to differentiate between fine-grained limestones and quartzites.

If the optical properties are needed to help in identifying the minerals (or rocks), grain mounts or thin sections can be made for study. Data on the optical properties of minerals can be found in a number of sources (e.g., Larsen and Berman (1964), Winchell and Winchell (1964), Heinrich (1965), and Deer, et al. (1992)). MacKenzie and Guilford (1980) provide excellent photomicrographs of the major rock-forming minerals. The measurement of the refractive index is often useful in distinguishing between minerals of similar characteristics, such as chalcedony (microfibrous silica with a refractive index of 1.537) and quartz (microcrystalline, with a refractive index greater than 1.54), both of which may form chert particles (figures 212 and 213). Figures 214 and 215 show the identification of quartz grains dispersed in a calcic schist.

Using the specimen illustrated in chapter 14, a petrographer may combine information derived from both microscopy and x-ray powder diffraction to identify minerals within an aggregate. The x-ray powder diffraction patterns are unique for each crystalline phase; they are produced independent of each other; and, in a mixture, the pattern intensity is proportional to phase abundance. Qualitative and quantitative analysis of aggregate phase composition may then be made by x-ray powder diffraction (figure 216) with the aggregate being composed of albite (24 percent), anorthite (4 percent), quartz (12 percent), epidote (22 percent), hornblende (19 percent), augite (3 percent), and chlorite (15 percent). The SEM/EDX data on texture and bulk chemistry may then be used to identify the mineral constituents in the aggregate of this pavement (figure 217) where the minerals adjacent to the reaction area are identified as albite ((Na,Ca)Al(Si,Al)3O8) and chlorite ((Mg,Fe)Al3(Si,Al)4O10(OH)2).

Table 29. Keys to mineral identification.
  • Crystal form, habit: Characteristic shape in which the mineral grows
  • Cleavage: Tendency to split along planes defined by crystal structure
  • Hardness (Mohs scale, see figure 207)
  • Luster: Appearance of reflected light off of the surface (vitreous,dull, waxy, metallic)
  • Acid solubility: Effervescence
  • Color, streak (on ceramic plate)
  • Density
  • Optical properties: Grain (immersion) mounts, thin sections
Table 30. Selected properties of common minerals (FHWA, 1991).
MineralHardnessCleavageOther
Pyrite6 to 6.5NoneBrassy, fool’s gold; weathers easily to give iron stain; common accessory mineral in many rock types
Hematite5.5 to 6.5None (in massive form)Red-brown; common accessory in many rocks; cement in many sandstones
Magnetite6None (in granular form)Black; magnetic; common accessory mineral in many rock types
Limonite5 to 5.5NoneYellow-brown; earthy; may appear softer than 5; formed by alteration of other iron minerals
Fluorite41 planeCommon accessory mineral in limestones and dolostones; translucent to transparent
Calcite33 planes at 75°Very common; occurs in many rock types; chief mineral in limestone; vigorous reaction with dilute HCl
Dolomite3.5 to 43 planes at 74°Common; with calcite in dolomitic limestone or dolostone (> 50 percent dolomite); vigorous reaction with dilute HCl only when powdered
Apatite51 plane, poorCommon minor accessory mineral in all rock classes
Gypsum24 planes, 1 perfectCommon mineral, especially in limestones and shales; may occur in layers
Quartz7NoneSilica; very common; may occur in many rock types; glassy; translucent to transparent; may be colored; very resistant to weathering; chief mineral in sandstones
Chert7NoneCryptocrystalline (microscopic crystals) variety of quartz; appears massive to naked eye; common in limestones or in complete layers associated with limestones; light tan to light brown
Chalcedony7noneMicrofibrous silica with water inclusions; tan to brown; varieties: flint (dark brown to black), jasper (red), agate (banded)
Orthoclase62 planes at 90°A feldspar; very common in many rock types; white to gray to red-pink; translucent to transparent; distinguished from quartz by cleavage
Plagioclase62 planes at 94°A feldspar; very common in many rock types; appears similar to orthoclase; distinguished by the presence of thin, parallel limes on cleavage faces because of crystal structure (twinning)
Olivine6.5 to 7NoneTransparent to translucent; olive green; glassy; common accessory mineral in the darker igneous rocks
Garnet6.5 to 7.5NoneRed to red-brown; translucent to transparent; common accessory mineral in metamorphic and some igneous rocks, also in sands and sandstones
Zircon7.5NoneUsually colorless to brown; usually translucent; common accessory mineral in igneous rocks and some metamorphic rocks, also in sands and sandstones
Pyroxene(group)5 to 72 planes at 87° and 93°Most common in darker igneous rocks; usually green to black; translucent to transparent; most common mineral: augite
Amphibole (group)5 to 62 planes at 56° and 124°Most common in metamorphic rocks and darker igneous rocks; usually dark green to brown to black; translucent to transparent; most common mineral: hornblende; distinguished from pyroxenes by cleavage
Clay minerals(group)2 to 2.51 planeUsually fine-grained; earthy; often derived from weathering of feldspars; montmorillonite is the swelling clay that expands with the absorption of water; illite is the common clay mineral in many shales
Talc11 planeVery soft; greasy; cleavage may be hard to see because of fineness of particles; commonly white to pale green; usually in metamorphic or altered igneous rocks
Serpentine2 to 5 (usually 4)NoneMassive to fibrous; greasy to waxy; various shades of green; found in altered igneous or metamorphic rocks; fibrous variety is the source of asbestos
Muscovite2 to 2.51 planeA mica; perfect cleavage allows splitting into thin, clear transparent sheets; usually light yellow to light brown; common in light-colored igneous rocks and metamorphic rocks
Biotite2.5 to 31 planeA mica; perfect cleavage allows splitting into thin smoky transparent sheets; usually dark green to brown to black; found in light- to medium-colored igneous rocks and metamorphic rocks
Chlorite2 to 2.51 planeSimilar to micas; usually occurs in small particles so cleavage produces flakes; flakes are flexible, but not elastic as are the micas; usually some shade of green

Figure 208. Mohs comparative hardness scale.

The scale plots 10 materials in sequence from least to greatest hardness. They are talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond. The first nine material names fall along a rising straight line.  The much steeper line between the last two (corundum and diamond) shows the step to the mineral diamond as being much more than the other increments because diamond is much harder. The graph also makes reference to five common methods of scratching materials: fingernail is positioned between talc and gypsum; penny is between gypsum and calcite; knife and window glass are in sequence between apatite and orthoclase; and a steel file is between orthoclase and quartz.

Figure 209. Cubic habit, brassy yellow color, metallic luster of pyrite.

In this magnified view, the cubic pyrite stands out in an earthy yellow from the other formations around it.

Figure 210. Granite aggregate exhibiting cleavage traces at right angles in orthoclase (right box) and no cleavage in quartz (left box) (scratch test with metal tool leaves metal (arrows) on both quartz and orthoclase, which are harder than the metal).

The portion with cleavage has a rippled surface. The other portion has smooth surface. Scratch test with metal tool leaves metal on both quartz and orthoclase, which are harder than the metal.

Figure 211. Metal tool scratches upper limestone (calcite) aggregate and leaves metal on lower quartzite particle.

A section of concrete is shown with a piece of limestone coarse aggregate, a piece of quartzite coarse aggregate, and air-entrained mortar between the two larger aggregate particles.

Figure 212. Immersion mount of chert in plane polarized light.

Here small particles of the mineral chert are immersed in a liquid of a selected index of refraction, mounted between glass slides, and light is passed through the specimen and observed using a petrographic microscope.

Figure 213. Immersion mount of chert with crossed polars, showing microcrystalline texture (refractive index matches that of quartz).

The crossed polars darkens most of the object but accentuates certain crystals. Refractive index matches that of quartz.

Note elongated quartz grains (white-gray in right image) set apart from the predominant calcite crystals by their low birefringence and high negative relief. Undulose extinction of quartz (arrow in left image) indicates strain of crystal and suggests increased susceptibility to ASR.

Figure 214. Thin section of marble (calcic schist) in plane polarized light.

Here light is passed through a very thin section of rock, showing the individual mineral grains and their relative color and shape.

Figure 215. Thin section of marble (calcic schist) in crossed polarized light.

This photo is the same as figure 213. There are elongated quartz grains (white-gray in polarized image) set apart from the predominant calcite crystals by their low birefringence and high negative relief. Undulose extinction of quartz seen in the plane polarized light image indicates strain of crystal and suggests increased susceptibility to A S R.

Note elongated quartz grains (white-gray in right image) set apart from the predominant calcite crystals by their low birefringence and high negative relief. Undulose extinction of quartz (arrow in left image) indicates strain of crystal and suggests increased susceptibility to ASR.

Figure 216. Quantitative XRD analysis to determine mineral composition of aggregate shown in figures 182 through 184.

This is an example of the output graph that shows percentages of anorthite, augite, albite, hornblende, epidote, chlorite, and quartz. The phase concentration is proportional to the phase pattern intensity.

Example XRD composite pattern (plotted with degrees of the angle 2-theta along the horizontal axis) can be used for both qualitative and quantitative analysis.

Figure 217. Coupling the knowledge of the mineral present (figure 216) with the elemental EDX maps shown here allows identification of the minerals present at specific locations within the fine-grained rock.

The E D X maps are shown here, which are related to figure 215 differentiate between silicon, sodium, aluminum, potassium, magnesium and titanium. This allows one to identify the minerals present at specific locations within the fine-grained rock.

D.4.2 Rock Identification

Rocks are assemblages of one or more minerals. They are classified according to their origin, mineralogy (mineral assemblage), and texture (fabric: size, shape, arrangement, and orientation of individual component grains). There are three origin groupings:

Rock and Mineral Identification for Engineers (Report No. FHWA-HI-91-025) (FHWA, 1991) provides a detailed outline for identifying rocks in hand specimens. The key elements to rock identification are given in table 31. Many excellent texts and reference books are available on the identification and classification of rocks. Examples are Pettijohn (1975), Hyndman (1972), and Williams, et al. (1954). However, it is recommended that the nomenclature in ASTM C 294 be used in reports to facilitate communication between the petrographer and engineers.

Classification of igneous rocks is based on grain size and mineral assembly. Grain size is indicative of cooling rate; thus, coarser grains indicate slow cooling at depth (plutonic), whereas very fine crystals or glassy textures indicate rapid cooling of volcanic rocks. The rocks can be broken down into three basic groups based on predominant mineral assemblage:

Table 32 provides a simple classification chart for igneous rocks. Texts should be reviewed for details in classifying igneous rocks (e.g., Hyndman, 1972, Williams, et al., 1954). Coarser grained rocks can usually be adequately classified in hand specimens with the use of lowpowered magnification. If more detail is needed, thin sections can be examined. MacKenzie, Donaldson, and Guilford (1982) provide an excellent collection of photomicrographs of igneous rocks in thin sections.

Sedimentary rocks are grouped into carbonates, classified by calcite-dolomite ratio and noncarbonates. Table 33 provides a simple classification for carbonate rocks. Initial identification of carbonate rocks is accomplished with scratch hardness testing (figure 211) and acid solubility. Acid etching of lapped surfaces will help in differentiating dolomite from calcite in mixtures of the two, as dolomite is less soluble. Because of the fine-grained nature of most carbonates, thin-section examination is often called for. A collection of photomicrographs of carbonate rocks can be found in Adams, et al. (1984). When detailed differentiation between the carbonate minerals is needed, staining with Alizarin Red solutions, followed by examination of the thin section, or an acetate peel are useful (see Heinrich, 1965; Adams, et al., 1984). Many carbonate rocks in the central States are susceptible to D-cracking and various petrographic means for differentiating among them have been developed. These are discussed in section E.5. Alkali-carbonate reactions are discussed in chapter 10and further information on identifying ACR rocks is given in section E.4.4

Table 31. Keys to rock identification.
  • Constituent grain size
  • Easily visible: Medium- to coarse-grained
  • Mineral assembly (relative amounts)
  • Fabric (grain orientation and distribution)
  • Hardness
  • Acid solubility (carbonates effervesce)
  • Color

  • Not easily visible: Fine-grained
Table 32. Igneous rock classification.
Rock (coarse-grained/ fine-grained)Minerals (primary)Minerals (accessory)
Granite/RhyoliteQuartz, OrthoclaseMica, Plagioclase, Pyroxene, Amphibole
Trachyte/SyeniteOrthoclase, PlagioclaseMica, Pyroxene, Amphibole
Diorite/AndesitePlagioclase, AmphiboleMica, Pyroxene
Gabbro, Diabase/ Basalt (Trap)Plagioclase/PyroxeneBiotite, Magnetite
PeridotitePyroxene, OlivineMagnetite, Chromite

Noncarbonate rocks are classified based on the size, shape, and composition of the constituent grains. A simple classification scheme is presented in table 34. Thin-section examination may be needed to provide more detailed information about the composition of the rock, in particular, the matrix or cementing material. Adams, MacKenzie, and Guilford (1984) provide a collection of photomicrographs of clastic sedimentary rocks.

Table 33. Classification of carbonate rocks.
CaCO3 (> 90 percent)CaCO3 (50 to 90 percent)CaMg(CO3)2 (50 to 90 percent)CaMg(CO3)2 (> 90 percent)
LimestoneDolomitic limestoneCalcitic dolomiteDolomite
HCl: Readily effervescentGrades toward ->
Depends on amount of insoluble material
HCl: Slowly effervescent
Scratched with a knife blade: Relative hardness depends on internal porosity (compactness), nature, and amount of insoluble constituents
Most contain some component of noncarbonate constituents (e.g., fine quartz, chert, clay (argillaceous material), pyrite, gypsum)
Table 34. Classification of clastic sedimentary rocks.
TextureComposition
Quartz > 90 percentQuartz < 90 percent of framework grains
Coarse-grained > 30 percent > 2 mmConglomerate Framework grains rounded or Breccia Framework grains angular
Medium-grained 0.06 to 2 mmQuartz sandstoneGraywacke (> 25 percent feldspar, dark mica matrix)Arkose (> 15 percent feldspar, other than dark mica matrix)
Fine-grained < 0.06 mm> 66 percent framework is quartz silttSiltstone
> 50 percent clay-sizedShale or mudstone

The classification of metamorphic rocks is based on composition, grain size, and fabric. They are usually foliated to some degree, but the extent is dependent on the mineral assemblage and the degree of force exerted on the rock. Simple classifications are given in table 35. Details of classification systems can be found in many texts and other reference materials (e.g., Hyndman, 1972; Williams, et al., 1954; Yardley, et al., 1990). Yardley, et al. (1990) provides photomicrographs of the major metamorphic rock types and textures.

Table 35. Classification of metamorphic rocks.
Fine-grained

Coarse-grained
Slate: Laminated, parallel cleavage; mineral grains not distinguishable; metamorphosed shale
Phyllite: Layered; micaceous minerals distinguishable; metamorphosed argillite
Schist: Thinly layered, nearly parallel cleavage; quartz and feldspar with abundant mica, chlorite, or amphiboles
Gneiss: Foliated; alternating layers of mica, amphiboles with granular quartz and feldspar
Metaquartzite: Metamorphosed quartz sandstone
Metagraywacke: Metamorphosed graywacke
Hornfels: Equigranular; massive; high-temperature alteration by contact metamorphism
Marble: Medium- to coarse-grained carbonate

D.4.3 Identification of Alkali-Silica Reactive Aggregates

ASR is discussed in chapter 10. Aggregate examinations should identify known reactive rock types and the petrographer should look for the presence of reactive constituents (table 36). In some cases (e.g., opal, which may occur as coatings or filling voids), as little as 0.5 percent is sufficient to cause deleterious expansion. These examinations will require preparation of immersion mounts, thin sections, or polished sections, and can be supplemented by XRD (see figures 212 through 218). More detail can be found in ACI 221.1R, ASTM C 295, and other ASR-related references. Van Epps and Erlin (1990) describe a case study of an aggregate with opal coatings.

Table 36. Alkali-silica reactive constituents.
Volcanic GlassesSilica (SiO2) Minerals
Felsic (> 66 percent SiO2)Cristobalite, tridymite
Opal
Chalcedony
Intermediate (> 0 percent SiO2)Quartz
Cryptocrystalline, microcrystalline, strained, granulated

D.4.4 Identification of Alkali-Carbonate Reactive Aggregates

ACRs are described in chapter 10, which discusses the characteristics of susceptible rocks. Because the rocks are fine-grained and texture is an important component in differentiating the expansive rocks, thin-section examinations are invariably called for. Examples of expansive ACR textures are shown in figure 219 and can be compared with the non-expansive textures shown in figure 220. A variety of other methods available for evaluating the potential expansion of ACR rocks are discussed in ACI 221.1R, Ozol (1994), and Famy and Kosmatka (1997).

Figure 218. Thin section of strained quartzite: Crossed polars.

The photo is a magnified view using crossed polars. It consists of jagged patches of white and black in several bands. The white patches are predominant

Field is 15 mm across.

Figure 219. Alkali-reactive microtexture in four carbonate rocks.

Four magnified images show the typical texture and spacing between individual crystalline phases in reactive A C R rock.

Figure 220. Nonreactive microtextures of carbonate rocks (examples are shown to illustrate the difference between these crystalline textures and the partially crystalline reactive textures shown in figure 219).

Two magnified images show the difference between individual crystalline textures and the partially crystalline reactive textures shown in figure 218. They are close together with little or no fine material or spacing between them.

D.5 AGGREGATE CONCERNS IN D-CRACKING

D-cracking that is caused by destruction of the aggregate by cycles of freezing and thawing has been well documented in the literature, and much information concerning it can be easily found (Schwartz, 1987). Both the ACR and D-cracking involve dolomitic limestones whose composition includes a large portion of insoluble material. Dolomites involved in D-cracking are thought to have a particular fine pore structure and/or contain a minor amount of iron, strontium, or both, in their crystal structure (Dubberke and Marks, 1987; Schwartz, 1987). A specific pore structure and iron or strontium do not seem to be necessary conditions for ACR. The crack patterns produced by these two reactions are very different. D-cracking deterioration is first evident at the edges and near the joints of a pavement. The cracking is parallel with the joint or edge (see figure 221 and compare with figures 94-95, 97, 103-104, and 106-107). ACR is an expansive chemical reaction with the alkalis, whereas D-cracking is caused by freezing andthawing of moisture in the particles of particular susceptible dolomites and does not involve alkalis.

Regarding D-cracking aggregates, Schwartz (1987) stated:

D-cracking is a form of portland cement deterioration associated primarily with the use of coarse aggregates in the concrete that disintegrate when they become saturated and are subject to repeated cycles of freezing and thawing. It is defined by a characteristic crack pattern that appears at the wearing surface of the pavement as a series of closely spaced fine cracks adjacent and generally parallel to transverse and longitudinal joints and cracks and to the free edges of the pavement (p. 5).

It is generally accepted that pore size is the most important characteristic of coarse aggregate influencing its susceptibility to D-cracking (p. 10).

It is generally agreed that the brand or composition of cement does not significantly influence D-cracking (p. 11).

In Iowa, Dubberke and Marks (1987) found that D-cracking is not necessarily related to the pore structure of the aggregate and came to the conclusion that the incidence of D-cracking is higher with ferroan-dolomite aggregates than with other compositions, and D-cracking may be caused or hastened by a chemical reaction with deicing salts.

When unsound aggregate particles are situated just below the concrete surface, they can cause popouts (figure 222). Unsound carbonates, shales, and low-density chert particles have been associated with this type of distress (a conical break with the offending particle at the base). ASR of chert and shale particles can also cause popouts.

Figure 221. D-cracking of a pavement caused by destruction of the aggregate by cycles of freezing and thawing (cracking parallel to the joint and wrapping around at the juncture of joints defines D-cracking).

Photo shows the surface of a concrete slab that has two joints (gap or spacing) crossing each other perpendicularly. D-cracking is present. This cracking is parallel with each joint and wraps around at their juncture.

Figure 222. Pavement surface with a flaw caused by freezing and thawing of water trapped in aggregate particle.

A portion of the concrete surface has popped out. The resulting hole is conical shape about the size of a pocketknife with folded blade.

D.6 SPECIAL-PURPOSE AGGREGATES

D.6.1 Skid Resistance

Skid resistance is usually desirable for any aggregates used in a surface of HCC that supports traffic. Skid resistance requires that the surface of an HCC pavement be fabricated with hard, nonpolishing aggregates. This generally means that nonsiliceous carbonate rocks cannot be used. In areas where carbonate aggregate is much less expensive than harder aggregate from more distant sources, two-course construction may be the most economic alternative. Not all types of siliceous aggregate provide the same wearing-surface microtexture as others. Despite their hardness, some quartz and feldspar pebbles and quartzitic and granitic rocks may tend to wear with a rounded surface and to polish (see figure 223). Others with particular zones of weakness (e.g., particular granites and graywackes) wear and then break with a microtexture that creates a very skid-resistant wearing surface (see figure 224) (Webb, 1970).

D.6.2 Lightweight Aggregates

Lightweight aggregates may be specified whenever the weight of conventional aggregates might be a problem. They are frequently used for long-span bridges to alleviate the dead load on the support structures. They may be used when bridge decks require widening and it is considered more economical to widen with the more expensive expanded aggregates than to increase the strength of the support structure.

Figure 223. Traffic-worn rounded surface of feldspar aggregate particle (this will not provide good skid resistance).

A magnified view of pavement surface centers on a 5-millimeter aggregate particle. The surface of the particle is rounded and almost polished. This surface will not provide good skid resistance.

Scale is in millimeters.

Figure 224. Traffic-worn surface of granite aggregate particle (zones of weakness provide an irregular skid-resistant surface).

A magnified view of pavement surface centers on a 6-millimeter aggregate particle. It has zones of weakness to provide an irregular skid resistant surface. This surface is good for skid resistance

Scale is in millimeters.

Manufactured lightweight aggregates are usually shale or slate that has been expanded by treatment at very high heat. The exterior of the particles becomes fluid, and the gases and vapors inside expand to create a very porous substance with a fused exterior shell. Figure 225 shows this sort of aggregate exposed on a lapped slice of HCC.

Figure 225. Lapped slice of HCC containing expanded-shale lightweight aggregate.

The aggregate particles on this specimen are highly porous.

Scale is in millimeters.

These expanded aggregates vary considerably with the source material used and the nature of the heat treatment (temperature, time, oxidation conditions, etc.). Much depends on the depth and continuity of the fused surface of the aggregate particles. If this surface is continuous, the aggregate will have low permeability despite its high porosity.

It is suspected that the chemical composition of the fused layer may have an important effect on the ability of these aggregates to bond with cement paste. Particular expanded aggregate materials produce concretes in which the bond between the aggregate and the paste is extremely good. In other cases, the bond is no better than would be expected of a quartz-pebble aggregate. In examples of a good bond, the aggregate surface does not seem to have an attraction for water, as does quartz and many other highly siliceous materials. These layers of water on the surface of aggregate particles create space along the bond line in the finished concrete that, whether or not it fills with calcium hydroxide, is a zone of weakness in the concrete and a possible channel for water, salt solutions, and other liquid materials. It has been shown that this attraction for water, which lessens the paste-aggregate bond, is most often present in materials that are acidic by nature, whereas the materials that may be considered mafic or alkaline, such as the carbonates and iron-rich minerals,generally have a much tighter bond (Walker, 1972). It has not yet been shown that the chemical composition of the fused layer in expanded aggregate has a direct effect on the properties of the bond. We suspect that such research would probably yield interesting and useful results.

Particular rocks can be used as lightweight aggregates without heat treatment. These may include volcanic ashes, tuffs, and pumices.

D.6.3 Radiation Shielding

Concrete for radiation shielding is designed using heavyweight or special composition aggregates so that the maximum amount of radiation may be contained. These requirements are covered in ASTM C 637 and ASTM C 638. Many of the aggregate minerals used in radiation shielding are opaque, and they cannot be identified with the petrographic microscope. If the exact mineral identification is required, x-ray diffraction and some form of chemical testing or examination with a metallographic microscope will have to be performed.

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