RILEM Recommended Test Method:AAR-1.1—Detection of Potential Alkali-Reactivity—Part 1: Petrographic Examination Method

Abstract

Petrographic analysis should always be the first step in the assessment of the potential alkali- reactivity of concrete aggregates as stated in RILEM AAR-0 (Outline guide to the use of RILEM methods in assessments of aggregates for potential alkali-reactivity).

Appendices

Annex A: Definitions

Systematic rock classification and nomenclature reflects mineral content, structure and texture/fabric as well as occasionally petrogenesis, compliant with nomenclature conventions as published in the literature. The text of this AAR-1.1 procedure adheres to IUGS classification schemes for igneous rocks by Le Maitre et al. [16] and metamorphic rocks by Fettes and Desmons [17], and (for lack of an IUGS-approved document) the British Geological Survey (BGS) classification scheme for sedimentary rocks by Hallsworth and Knox [18].

1.1 Andesite

Andesite is a fine-grained volcanic rock of intermediate composition. Potentially alkali-reactive constituents include the high-temperature silica polymorphs cristobalite and tridymite, interstitial glass in the matrix, or devitrified glass, opaline silica or chalcedony from low-temperature alteration.

1.2 Argillite, Argillaceous

Argillite is a diagenetically altered or very low-grade metamorphic, very fine-grained rock consisting of >50 vol. % siliciclastic fragments, of which >75 vol. % is smaller than 32 µm, i.e., silt or clay. The rock represents indurated and lithified muds and oozes, typically displays conchoidal fracture but has negligible fissility. With development of cleavage, however, argillaceous rocks grade into slate and higher grade rocks (see entry ‘Slate’). An argillite rich in alumina is known as ‘pelite’, their metamorphic equivalent as ‘metapelite’. Potentially alkali-reactive constituents of these rocks are microcrystalline quartz. For the non-metamorphic equivalent of argillite, see entry for ‘Mudstone’.

1.3 Basalt

Basalt is a fine-grained volcanic rock of mafic composition, chemically slightly poorer in silica than andesite. Potentially alkali-reactive constituents include the high-temperature silica polymorphs cristobalite and tridymite, interstitial glass in the matrix, or devitrified glass, opaline silica or chalcedony from low-temperature alteration.

1.4 Chalcedony

Chalcedony is a fine-grained fibrous silica variety, commonly observed in chert/flint, limestone and other sedimentary lithologies. In thin-section, chalcedony has a ‘feathery’ appearance, with normally length-slow elongation. The alkali-reactivity of chalcedony is generally attributed to its fine grained nature and poor crystallinity, possibly also moganite content.

1.5 Chert, Flint

Chert is a non-systematic name for a fine grained sedimentary rock consisting of chalcedonic, opaline, cryptocrystalline and/or microcrystalline silica, occurring as contiguous beds or layers, whereas flint is a nodular equivalent. Mineralogically, chert and flint consist mostly of very fine grained quartz, sometimes with accessory silica minerals chalcedony, opal and moganite. Chert and flint may furthermore contain minor amounts of clay minerals, sulfides, brownish to brick-red stains of iron (oxy-) hydroxides, etc. In fluorescence petrography, chert/flint particles often reveal internal porosity, especially along an outer rim called ‘cortex’. The silica in chert/flint generally has a poorly developed crystalline structure, and usually contains hydrous species.

1.6 Clays, Clay Minerals

Clays represent a large group of minerals with broadly variable chemical composition, yet sharing a layered structure composed of (alumino-) silicate sheets, similar to micas. Concrete damage from expanding clay minerals is not regarded as alkali-silica reaction, even when alkalis may have contributed to the swelling.

1.7 Cristobalite

Cristobalite is a high-temperature SiO₂ polymorph with a lower density than quartz, normally stable >1470 °C until melting at 1730 °C. After cooling down to ambient temperatures, cristobalite may survive in certain felsic volcanic rocks, as recrystallization to α-SiO₂ quartz (stable below 573 °C). Alternatively, cristobalite may form from the devitrification of glass, as e.g., the snowflakes in natural volcanic glass obsidian. Cristobalite is metastable and quick to engage in deleterious ASR.

1.8 Cryptocrystalline

Fine-grained rock or mineral texture inhibiting routine petrographic assessment of individual grains in thin-section using an optical microscope, i.e., smaller than ~10 µm. Cryptocrystalline materials do exhibit defined peaks in X-ray diffraction, whereas truly amorphous materials only reveal a wide ‘glass bulge’ due to increased background noise and absorption.

1.9 Diorite

Diorite is a coarse-grained rock type rich in plagioclase, and poor in quartz and K-feldspar. In addition, diorites typically contain pyroxene and other dark minerals. Related rock types are anorthosite and monzonite. Some diorites behave deleteriously due to retrogradation of pyroxene to form amphibole plus micro-crystalline quartz, or to late-stage veining.

1.10 Dolomite

Dolomite is a double-carbonate mineral of idealized composition CaMg[CO₃]₂. Alternatively, dolomite is the name for a sedimentary rock containing more dolomite than calcite Ca[CO₃] or other non-carbonate minerals. Fine-grained dolomite may be prone to dissolution under high-pH conditions as in concrete (ACR), which however does not cause expansion. Finely disseminated quartz, opal or chalcedony present in dolomite rocks have been shown deleterious by ASR.

1.11 Glass (Natural, Industrial)

An amorphous material of natural or industrial origin, with a broad possible range of chemical compositions. Natural glass of volcanic origin occurs as (dark coloured) obsidian of intermediate to mafic composition, light coloured vesicular pumice of felsic composition, or as matrix in andesite, basalt, rhyolite, tuff, and other volcanic rock types. Natural glass of dynamo-metamorphic origin occurs in pseudotachylite. Devitrification occurs by slow recrystallization of the disordered glass structure into cristobalite, e.g., the snowflakes in snowflake obsidian. The alkali-reactivity of natural glass is attributed to its amorphous, non-crystalline structure, rendering it thermodynamically unstable. Especially weathered and partially devitrified/altered/hydrated glasses are known to be prone to deleterious ASR.

1.12 Gneiss

Gneiss is a medium to coarse grained high-grade metamorphic rock, with characteristic alternating dark/light coloured bands at millimetre- to metre scale. The banding is caused by differences in mineral content, lighter bands being richer in quartz, feldspars and light micas (e.g., muscovite), darker bands richer in amphibole (e.g., hornblende), dark micas (e.g., biotite) and/or pyroxene (e.g., augite). Ortho-gneiss is derived from an igneous precursor, para-gneiss from a sedimentary one. The lower-grade equivalent of gneiss is schist, towards higher grade gneiss transforms into granulite. The alkali-reactivity of gneiss is attributed to strained quartz from (tectonic) deformation, and/or poorly (micro-, crypto-) crystalline interstitial quartz.

1.13 Grain, Subgrain

An individual granule consisting of a single mineral, e.g., quartz, K-feldspar, plagioclase. Grains of intensely deformed quartz may be divided into multiple subgrains of yet smaller size to reduce local crystal-structural deformation. Thus, the presence of subgraining is considered relevant for alkali-reactivity.

1.14 Granite, Granodiorite

Granite is a coarse to medium grained felsic igneous rock of plutonic (=deep-seated) origin containing mainly quartz, K-feldspar and plagioclase, a granodiorite is richer in plagioclase, poorer in K-feldspar. The alkali-reactivity of granite and granodiorite is attributed to strained quartz from deformation (rather common along intrusive margins), or poorly (micro-, crypto-) crystalline quartz.

1.15 Granulite

Granulite is a fine grained metamorphic rock with a characteristic granular texture, of higher grade than gneiss, only lower than eclogite. Granulites may show signs of partial melting. The alkali-reactivity of granulite is attributed to strained quartz by deformation, or poorly (micro-, crypto-) crystalline interstitial quartz.

1.16 Greywacke

Greywacke is a poorly sorted sandstone, characterised by a very fine grained matrix embedding coarser sedimentary detritus including rock fragments, that may also contain dark organic matter. The alkali-reactivity of greywacke is attributed to very fine grained quartz with a large accessible surface area and/or the presence of poorly (micro-, crypto) crystalline quartz.

1.17 Hornfels

Hornfels is a fine-grained contact-metamorphic rock formed at high temperature and shallow depth, characteristically without a foliation or banding (as implied by ‘-fels’). A generic name for coarse-grained isotropic rocks lacking foliation or banding is granofels. The alkali-reactivity of hornfels is attributed to very fine grained quartz with a large accessible surface area and/or poorly (micro-, crypto-) crystalline quartz.

1.18 Limestone, Chalk, Marl

Limestone is a consolidated sedimentary rock predominantly composed of calcite Ca[CO₃], possibly with subordinate amounts of dolomite. Depending on origin and geological history, limestones may contain variable amounts of fossil remains. Limestone with 33–67 vol. % clay impurities is known as marl, very pure but little consolidated limestone as chalk (from the Cretaceous formation known as Chalk). Silica from sedimentary detritus, diatom frustules, sponge spiculae or certain types of corals may occur finely dispersed throughout the rock, intercalated with the carbonate as chert or flint, or filling vugs as opal or chalcedony. The alkali-reactivity of limestone is attributed, at least in most cases, to the presence of very fine grained and poorly (micro-, crypto-) crystalline silica.

1.19 Microcrystalline

Fine grained rock or mineral texture requiring optical thin-section petrography for reliable identification. In practice, this applies to rocks with grain size less than ~0.1 mm (=100 µm) that cannot be readily identified macroscopically with the unaided eye or a hand lens.

1.20 Moganite

Moganite is a little known silica polymorph occurring frequently in chert and flint, and in other lithologies altered at low temperature (e.g., Parisian Basin sandstone, Mogan Formation-Gran Canary basalt). However, its frequent presence in the most common types of concrete aggregate lithologies renders moganite a very plausible contributor to deleterious AAR.

1.21 Mudstone

Mudstone is a generic name for a sedimentary rock type from an indurated clay-rich precursor, lacking or with poorly developed fissility. Claystone or shale are generally considered only diagenetically compacted and consolidated, shale showing fissility. Slate does have a defined cleavage and may contain newly formed lowest-grade metamorphic minerals (e.g., chlorite, muscovite) identifiable in thin-section, which are macroscopically recognizable in phyllite (e.g., muscovite, biotite). The alkali-reactivity of some mudstones sensu lato is attributed to the presence of fine-grained and/or poorly (micro-, crypto-) crystalline quartz.

1.22 Mylonite, Pseudotachylite, Fault Breccia/Gouge, Cataclasite

Mylonite is a dynamo-metamorphic rock with a thinly foliated structure, formed by ductile deformation under geological conditions. The glassy matrix in “pseudotachylite” results from seismic friction-melting and rapid quenching immediately after. Under brittle geological deformation conditions, “fault breccia” forms (“fault gouge” being its finer counterpart). These rock types are alternatively called “cataclasites” The alkali-reactivity of cataclastic rocks is attributed to the presence of strained quartz, glass matrix (pseudotachylite), and/or very fine grained and poorly (micro-, crypto-) crystalline silica from tectonic crushing.

1.23 Myrmekite

Myrmekite is one type of symplectitic intergrowth of quartz and feldspar or plagioclase, common in granitic rocks including granites, gneisses, and others. Myrmekite can be recognized as vermicular (‘worm-like’) intergrowths of quartz embedded in feldspar, but the opposite has been observed too. As the quartz in myrmekite is fine-grained, it could be prone to develop deleterious ASR, if it can be accessed by the concrete pore solution.

1.24 Opal, Opaline Silica

Opal is a hydrated form of silica (SiO₂·nH₂O) with low density and variable water content, which appears glassy and amorphous in thin-section. Opal varieties include opal-AN and opal-CT, which can be distinguished by X-ray diffraction. The silica in opal-AN is amorphous, whereas in opal-CT the structure bears resemblance to silica polymorphs cristobalite and tridymite. Opaline silica is certainly capable of causing ASR damage in field concrete.

1.25 Particle

An individual aggregate granule composed of single or multiple mineral grains (excluding subgrains). Particles are composed of multiple grains from different minerals, or from a single mineral representing a mono-mineralic particle. The term ‘grain’ is only synonymous with ‘particle’ if a particle consists of one single mineral grain (disregarding inclusions).

1.26 Quartz

Quartz is mechanically hard, lacks cleavage and is chemically stable under normal ambient conditions from pH2 to pH9. Its chemical composition is near to pure SiO₂, and its crystal structure is denoted as α-SiO₂. Micro- or cryptocrystalline as well as strained quartz are considered potentially alkali-reactive.

1.27 Quartzite

Quartzite is a crystalline rock predominantly comprising quartz grains. Commonly quartzite is a metamorphic rock. Some quartzite was formed by sedimentary processes and can be termed ortho-quartzite. The alkali-reactivity of quartzite is attributed to deformed-strained quartz, and/or the presence of very fine grained and poorly (micro-, crypto-) crystalline silica along grain boundaries or interstitial silica.

1.28 Rhyolite

Rhyolite is a fine grained to glassy felsic volcanic rock, richer in silica SiO₂ than andesite and basalt (see respective entries). Potentially alkali-reactive constituents include high-temperature silica polymorphs cristobalite and tridymite, interstitial glass in the matrix, or devitrified glass, opaline silica or chalcedony from low-temperature alteration.

1.29 Sandstone, Siltstone

Sandstone and siltstone are clastic sedimentary rocks. Constituents reflect mineral composition of the provenance area after sedimentary processing; quartz, feldspar, rock fragments and matrix are used for classification and nomenclature. Grain size of sandstone varies from 2 to 0.063 mm, of siltstone from 0.063 to 0.002 mm. Sandstones/siltstones are compacted by overburden loading, and are cemented by neogenic minerals, such as quartz, calcite, iron minerals and clay minerals. The alkali-reactivity of some sandstone/siltstone is attributed to clastic constituents that are themselves reactive, to very fine grained quartz and/or the presence of poorly (micro/crypto) crystalline quartz.

1.30 Slate

Slate is a very fine-grained, low-grade metamorphic rock with well-defined cleavage formed from a clay-rich sedimentary precursor. Newly formed metamorphic minerals (e.g., chlorite, muscovite) are identifiable in thin-section. The alkali-reactivity of slate is attributed to the presence of very fine-grained (micro/crypto) crystalline quartz.

1.31 Tridymite

Tridymite is a high-temperature SiO₂ polymorph, normally stable >870 °C until 1470 °C when it transforms to cristobalite. After cooling down to ambient temperatures, tridymite may survive in certain felsic volcanic rocks, as recrystallization to α–SiO₂ quartz (stable below 573 °C) requires complete reconstruction of the crystal structure and is very slow. Tridymite is metastable and quick to engage in deleterious ASR.

1.32 Tuff

Tuff is a ‘pyroclastic’ rock of volcanic origin, composed of deposited ash particles, consolidated by welding when still hot (welded tuff), or by weathering and alteration at ambient temperature. Ignimbrite is a welded tuff formed from the deposition of particles of pumice, lapilli, glass shards and crystals. The alkali-reactivity of tuff is attributed to the presence of siliceous glass or devitrified glass, cristobalite or tridymite, poorly (micro-, crypto-) crystalline quartz. Opal and/or chalcedony are common products of natural tuff weathering and alteration and also contribute to its alkali-reactivity.

1.33 Undulatory Extinction

Certain types of quartz are observed to have ‘undulatory extinction’ in thin-section petrography, signifying that certain parts of a contiguous individual quartz grain are oriented at an angle to other parts. This quality has been interpreted as an indication that the quartz grain’s crystalline structure is strained, and therefore potentially susceptible to deleterious ASR. However, measurement of undulatory extinction angles to predict alkali-reactivity potential has been found unreliable and is now discouraged. For purpose of reference, some geological literature prefers ‘undulous’ for ‘undulatory’.

Annex B: Estimation of Absolute Error in Counting Analysis

Figures B.1 and B.2 are reproduced with permission from Howarth [19].

Fig. B.1

Lower absolute analytical error, for 95 % confidence limits

Fig. B.2

Upper absolute analytical error, for 95 % confidence limits

The graphs enable graphical estimation of absolute analytical error with 95 % confidence limits for total counts (N) in the range 25–5000, for modal percentages (100 × n/N %; equals vol. %) found by (point) counting assessment of aggregate. In both graphs, the total number of counts is listed along left and right vertical axes (ordinates). Observed percentages (abundances) to 50 vol.% are listed along the lower horizontal (abscissa), observed percentages over 50 vol.% are listed along the upper horizontal. Figure B.1 gives lower bound absolute error, whilst Fig. B.2 gives the upper bound absolute error.

To determine the absolute error for a given observed percentage, first draw a horizontal tie-line connecting the total counts on left and right ordinates. Next, draw a vertical line starting at the observed percentage on the abscissa (lower or upper, as applicable). The curve in the diagram nearest to the point of intersection gives the absolute error. Experienced users may prefer using a transparent triangular geometry template of suitable size.

Example: A petrographic analysis with 1000 total counts classifies 20 counts as chert, which represents 2.00 vol. % of the total sample. According to Fig. B.1, the lower absolute error amounts to 0.8 vol. %, whereas the upper absolute error shown in Fig. B.2 amounts to 1.1 vol. %. Thus, the true value lies between (2.0–0.8) = 1.2 vol. % and (2.0 + 1.1) = 3.1 vol. %. By contrast, relative errors range from 40 % (=[0.8/2.0] × 100 %) to 55 % (dimensionless!).

Upper/lower error asymmetry decreases for more abundant species. Absolute error can be reduced by increasing the total number of particles assessed.