ASTM C295/C295M:19 pdf free download – Standard Guide for Petrographic Examination of Aggregates for Concrete.
This will ultimately destroy the concrete because such aggregates cannot be protected by adequately air- entrained mortar. Finely porous aggregates near the concrete surface are also likely to form popouts, which are blemishes on pavements and walls.
5.8 Petrographic examinations may also be used to determine the proportions of cubic, spherical, ellipsoidal, pyramidal, tabular, flat, and elongated particles in an aggregate sample or samples. Flat. elongated, and thin chip-like particles in aggregate increase the mixing water requirement and hence decrease concrete strength.
5.9 Petrographic examination should identify and call allenlion to potentially alkali-silica reactive and alkali-carbonate reactive constituents, determine such constituents quantitatively, and recommend additional tests to confirm or refute the presence in significant amounts of aggregate constituents capable of alkali reaction in concrete. See Speciticalion C33/C33M. Alkali-silica reactive constituents found in aggregates include: opal, chalcedony, cristobalite, tridyrnite, highly strained quartz. microcrystalline quartz, cryptocrystalline quartz, volcanic glass. and synthetic siliceous glass. Aggregate materials containing these constituents include:
glassy to cryptocrystalline intermediate to acidic volcanic rocks, some argillites. phyllites. graywacke. gneiss. schist, gneissic granite, vein quartz, quarizite, sandstone, chert, and carbonate rocks containing alkali reactive forms of silica. Criteria are available for identifying the minerals in the list above by their optical properties or by XRD (2).(3). Criteria are available for identifying rocks by their mineral composition and texture (4). Examination in both reflected and transmitted light may he necessary to provide data for these identifications. X-ray microanalysis using energy-dispersive x-ray spectrometers with scanning electron microscopy (SEM/EDX) or wavelength-dispersive x-ray spectrometers in electron micro- probes (EMPA/WDX) may provide useful information on the chemical composition of minerals and rocks. Potentially deleterious alkali-carbonate reactive rocks are usually calcareous dolomites or dolomitic limestones with clayey insoluble residues. Some dolomites essentially free of clay and some very fine-grained limestones free of clay and with minor insoluble residue, mostly quartz, are also capable of some alkali- carbonate reactions, however, such reactions are not necessarily deleterious.
5.10 Petrographic examination may be directed specifically at the possible presence of contaminants in aggregates, such as synthetic glass, cinders, clinker, or coal ash, magnesium oxide, calcium oxide, or both, gypsum. soil, hydrocarbons. chemicals that may affect the setting behavior of concrete or the properties of the aggregate, animal excrement, plants or rotten vegetation, and any other contaminant that may prove undesirable in concrete.
5.11 These objectives, for which this guide was prepared, will have been attained if those involved with the evaluation of aggregate materials for use in concrete construction have reasonable assurance that the petrographic examination results wherever and whenever obtained may confidently be compared.