ASTM C1723-10
(Guide)Standard Guide for Examination of Hardened Concrete Using Scanning Electron Microscopy
Standard Guide for Examination of Hardened Concrete Using Scanning Electron Microscopy
SCOPE
1.1 This guide provides information for the examination of hardened concrete using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX). Since the 1960s, SEM has been used for the examination of concrete and has proved to be an insightful tool for the microstructural analysis of concrete and its components. There are no standardized procedures for the SEM analysis of concrete. SEM supplements techniques of light microscopy, which are described in Practice C856, and, when applicable, techniques described in Practice C856 should be consulted for SEM analysis. For further study, see the bibliography at the end of this guide.
This guide is intended to provide a general introduction to the application of SEM/EDS analytical techniques for the examination and analysis of concrete. It is meant to be useful to engineers and scientists who want to study concrete and who are familiar with, but not expert in, the operation and application of SEM/EDS technology. The guide is not intended to provide explicit instructions concerning the operation of this technology or interpretation of information obtained through SEM/EDS.
It is critical that petrographer or operator or both be familiar with the SEM/EDX equipment, specimen preparation procedures, and the use of other appropriate procedures for this purpose. This guide does not discuss data interpretation. Proper data interpretation is best done by individuals knowledgeable about the significance and limitations of SEM/EDX and the materials being evaluated.
1.2 The SEM provides images that can range in scale from a low magnification (for example, 15×) to a high magnification (for example, 50 000× or greater) of concrete specimens such as fragments, polished surfaces, or powders. These images can provide information indicating compositional or topographical variations in the observed specimen. The EDX system can be used to qualitatively or quantitatively determine the elemental composition of very small volumes intersecting the surface of the observed specimen (for example, 1-10 cubic microns) and those measured compositional determinations can be correlated with specific features observed in the SEM image. See Note 1.
Note 1—An electronic document consisting of electron micrographs and EDX spectra illustrating the materials, reaction products, and phenomena discussed below is available at http://netfiles.uiuc.edu/dlange/www/CML/index.html.
1.3 Performance of SEM and EDX analyses on hardened concrete specimens can, in some cases, present unique challenges not normally encountered with other materials analyzed using the same techniques.
1.4 This guide can be used to assist a concrete petrographer in performing or interpreting SEM and EDX analyses in a manner that maximizes the usefulness of these techniques in conducting petrographic examinations of concrete and other cementitious materials, such as mortar and stucco. For a more in-depth, comprehensive tutorial on scanning electron microscopy or the petrographic examination of concrete and concrete-related materials, the reader is directed to the additional publications referenced in the bibliography section of this guide.
1.5 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with the use of electron microscopes, X-ray spectrometers, chemicals, and equipment used to prepare samples for electron microscopy. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: C1723 − 10
StandardGuide for
Examination of Hardened Concrete Using Scanning Electron
Microscopy
This standard is issued under the fixed designation C1723; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope composition of very small volumes intersecting the surface of
the observed specimen (for example, 1-10 cubic microns) and
1.1 This guide provides information for the examination of
thosemeasuredcompositionaldeterminationscanbecorrelated
hardened concrete using scanning electron microscopy (SEM)
with specific features observed in the SEM image. See Note 1.
combined with energy-dispersive X-ray spectroscopy (EDX).
NOTE 1—An electronic document consisting of electron micrographs
Since the 1960s, SEM has been used for the examination of
and EDX spectra illustrating the materials, reaction products, and phe-
concrete and has proved to be an insightful tool for the
nomena discussed below is available at http://netfiles.uiuc.edu/dlange/
microstructural analysis of concrete and its components.There www/CML/index.html.
are no standardized procedures for the SEM analysis of
1.3 Performance of SEM and EDX analyses on hardened
concrete. SEM supplements techniques of light microscopy,
concrete specimens can, in some cases, present unique chal-
which are described in Practice C856, and, when applicable,
lengesnotnormallyencounteredwithothermaterialsanalyzed
techniques described in Practice C856 should be consulted for
using the same techniques.
SEManalysis.Forfurtherstudy,seethebibliographyattheend
1.4 This guide can be used to assist a concrete petrographer
of this guide.
in performing or interpreting SEM and EDX analyses in a
This guide is intended to provide a general introduction to
manner that maximizes the usefulness of these techniques in
the application of SEM/EDS analytical techniques for the
conducting petrographic examinations of concrete and other
examination and analysis of concrete. It is meant to be useful
cementitious materials, such as mortar and stucco. For a more
toengineersandscientistswhowanttostudyconcreteandwho
in-depth, comprehensive tutorial on scanning electron micros-
are familiar with, but not expert in, the operation and applica-
copyorthepetrographicexaminationofconcreteandconcrete-
tion of SEM/EDS technology. The guide is not intended to
related materials, the reader is directed to the additional
provide explicit instructions concerning the operation of this
publications referenced in the bibliography section of this
technology or interpretation of information obtained through
guide.
SEM/EDS.
It is critical that petrographer or operator or both be familiar 1.5 Units—The values stated in SI units are to be regarded
with the SEM/EDX equipment, specimen preparation asstandard.Nootherunitsofmeasurementareincludedinthis
procedures,andtheuseofotherappropriateproceduresforthis standard.
purpose.Thisguidedoesnotdiscussdatainterpretation.Proper
1.6 This standard does not purport to address all of the
data interpretation is best done by individuals knowledgeable
safety concerns, if any, associated with the use of electron
about the significance and limitations of SEM/EDX and the
microscopes, X-ray spectrometers, chemicals, and equipment
materials being evaluated.
used to prepare samples for electron microscopy. It is the
1.2 The SEM provides images that can range in scale from responsibility of the user of this standard to establish appro-
alowmagnification(forexample,15×)toahighmagnification priate safety and health practices and determine the applica-
(for example, 50 000× or greater) of concrete specimens such bility of regulatory limitations prior to use.
as fragments, polished surfaces, or powders.These images can
2. Referenced Documents
provide information indicating compositional or topographical
variations in the observed specimen. The EDX system can be
2.1 ASTM Standards:
used to qualitatively or quantitatively determine the elemental
C125Terminology Relating to Concrete and Concrete Ag-
gregates
This guide is under the jurisdiction ofASTM Committee C09 on Concrete and
Concrete Aggregates and is the direct responsibility of Subcommittee C09.65 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Petrography. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Current edition approved Oct. 1, 2010. Published November 2010. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C1723-10. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1723 − 10
C294Descriptive Nomenclature for Constituents of Con- 4. Description of Equipment
crete Aggregates
4.1 The principles of the electron system of the scanning
C295GuideforPetrographicExaminationofAggregatesfor
electron microscope, the interactions of the electron beam and
Concrete
the specimen under examination, and the detection systems
C457Test Method for Microscopical Determination of Pa-
used for the examination are based on concepts that need
rameters of the Air-Void System in Hardened Concrete
understanding if the resulting image and other analytical
C856Practice for Petrographic Examination of Hardened
information obtained are to be best resolved and understood.
Concrete
An abbreviated discussion is provided here. A more compre-
C1356 Test Method for Quantitative Determination of
hensive understanding can be obtained from texts devoted to
Phases in Portland Cement Clinker by Microscopical
this subject (1,2).
Point-Count Procedure
4.1.1 SEM Optics:
4.1.1.1 An electron beam is generated in a column consist-
3. Terminology
ing of an electron gun and multiple electromagnetic lenses and
3.1 Definitions of Terms Specific to This Standard: apertures.Theelectronbeamisgeneratedbyheatingafilament
so that it emits electrons. The most common filament for
3.1.1 BSE, n—backscatter electrons; these are high-energy
general SEM work is tungsten, but other filaments can be used
electronsemittedbackfromthespecimensurface.Elementsof
for increased brightness.The electrons are accelerated towards
higheratomicnumberwillhavestrongeremissionsandappear
the specimen by an applied potential and then focused by
brighter.
lenses and apertures. The energy of the electron beam influ-
3.1.2 brightness, n—the amount of energy used to produce
ences resolution, image quality, and quantitative and qualita-
an X-ray.
tive X-ray microanalyses.
3.1.3 charging, n—thebuildupofelectronsonthespecimen 4.1.1.2 Theelectronbeamisfinelyfocusedthroughelectro-
at the point where the beam impacts the sample. Charging can magnetic lenses and apertures. A smaller beam size improves
alter the normal contrast of the image (usually becomes resolution, but decreases signal intensity.
brighter)andmaydeflectthebeam.Coatingthespecimenwith 4.1.1.3 Electron systems operate under vacuum. Specimens
athinlayerofconductivematerial(suchasgoldorcarbon)can should be prepared to minimize alteration or damage when
minimize this effect. they are exposed to the vacuum (See 5.1.3). Variable pressure
scanning electron microscopes, low vacuum scanning electron
3.1.4 contrast, n—the difference in intensity of the energy
microscopes (LVSEM), and environmental scanning electron
produced by varying elements when excited.
microscopes (ESEM) permit the examination of samples con-
3.1.5 dead-time, n—the time of finite processing during
taining some moisture under low vacuum. The ESEM also
which the circuit is “dead” and unable to accept a new pulse
allows analysis of organic materials. Even in an ESEM,
from the X-rays.
however, some drying occurs.
4.1.2 Signal Generation and Detection:
3.1.6 EDX (energy-dispersive X-ray spectroscopy), n—the
4.1.2.1 Theinteractionoftheelectronbeamwiththesample
interaction of the electron beam with atoms in the sample
generates several types of signals that can be utilized for
produces characteristic X-rays having energies and wave-
imaging and X-ray microanalysis. The intensities of these
lengths unique to atoms.
signals are measured by detectors. The signals allow the
3.1.7 live-time, n—howtheacquistionofX-raydataistimed
examination and determination of properties such as surface
when the rate of X-ray events between measurements are
topography, elemental composition, and spatial distribution of
compared. Opposite of dead-time.
components.Signalintensitiesaregenerallyusedtoprovidean
3.1.8 K, L, or M peaks, n—characteristic X-ray intensities image on a screen.
4.1.2.2 The signals that are produced when the electron
detected for elements.
beam strikes the specimen surface are secondary electrons
3.1.9 raster, n—to scan as when the beam from the filament
(SE), backscattered electrons (BSE), and X-rays.
sweeps back and forth over the sample
4.1.2.3 To generate an image, the electron beam is moved
3.1.10 SE, n—secondary electrons; these are low-energy
repeatedly across the specimen to form a raster. The magnifi-
electrons emitted when the specimen is hit with the beam and
cation is the ratio between the size of the raster and that of the
associated with the topography of the same.
screen image.
4.1.2.4 Images produced by secondary electrons are most
3.1.11 SEM, n—scanning electron microscope.
commonlyutilizedfortopographicalimaging.TheSEintensity
3.1.12 stage, n—platform upon which the specimen is
depends mainly on the angles between the electron beam and
placedwithinthevacuumchamberthatcanberemotelymoved
thespecimensurfaceandbetweenthespecimensurfaceandthe
in various directions.
detector. The SE intensity is relatively insensitive to the
specimen composition.
3.1.13 working distance, n—thedistancebetweenthedetec-
tor and the sample. Each SEM will have an optimun distance
in which X-rays can be collected for EDX.
The boldface numbers in parentheses refer to a list of references at the end of
3.1.14 X-ray detector, n—also known as EDX system. this standard.
C1723 − 10
4.1.2.5 Images produced by backscattered electrons are 5.1.2.1 Residualportlandcementparticlesappeardenseand
often used for elemental contrast imaging. The BSE image is angulartosubangular.Aliteusuallyhasatleastonecrystalface
useful for identifying different chemical constituents in con- while belite is usually rounded and sometimes striated. In a
crete. The BSE intensity depends on the average atomic BSE image, residual portland cement particles occur as rela-
number and density of each phase. The BSE intensity also tively bright objects in a matrix of gray cement hydration
depends on the angles between the electron beam and the products.
specimen surface and between the specimen surface and the
5.1.2.2 Calcium-silicate-hydrate is the major hydration
detector.Therefore,someBSEdetectorscanbemanipulatedto
product of portland cement and is usually amorphous or very
observe the sample topography.
poorly crystalline. Its morphology varies depending on the
4.1.2.6 The interaction of the electron beam with atoms in
calcium to silica ratio, water to cementitious materials ratio,
the sample produces characteristic X-rays having energies and
curing conditions, degree of cement hydration, and chemical
wavelengths unique to atoms. Chemical analysis (or micro-
admixtures. At high magnifications, the morphology of
analysis) is performed using an X-ray spectrometer that mea-
calcium-silicate-hydrate varies from very fine fibrous growths,
sures the energies and intensities of the X-rays.The intensities
to sheet-like units, to irregular massive grains.
of X-rays depend upon many factors, including electron beam
5.1.2.3 Portlandite (calcium hydroxide) is a major phase of
currents and accelerating voltages, as well as chemical com-
cement hydration and occurs in variable sizes and shapes
position of the specimen interacting with the electron beam.
including platy hexagonal crystals and sheet-like masses,
4.1.2.7 One important parameter for image quality is the
depending on the orientation. Calcium hydroxide is normally
working distance, the distance between specimen surface and
observedthroughoutthecementpasteandsometimesdevelops
the point where the electron beam exits the electron optics.
along paste-aggregate interfaces. It also sometimes occurs as
Small working distances maximize BSE collection efficiency
secondary deposits in voids and cracks.
and improve the image resolution. Long working distances
5.1.2.4 Ettringite is a primary product of the reactions
improve image depth of field for topographical images but
between calcium aluminates and the sulfate phases in cement.
decrease image resolution. The working distance generally
It has a characteristic acicular shape. Ettringite often also
must be within a predetermined range to perform X-ray
appears as a secondary deposit. Secondary deposits of ettring-
microanalysis.
itearecommonlyfoundinvoidsandcracks.X-raymicroanaly-
sis is sometimes required for its identification. A compound
5. Materials and Features
that has similar morphology is thaumasite (See 5.1.6 on
5.1 Important microstructural features include the size and
secondary deposits). These two compounds can be distin-
shape of individual constituents (including pores), the spatial
guished by elemental analysis. In polished sections (See 6.1)
relationships between these constituents (what constituents are
ettringite may sometimes appear microcrystalline and high
touching or associated with each other), and the volume
magnification (for example, 50,000X) may be needed to see
fractionofeachconstituent.Constituentsaredescribedinmore
individualcrystals.Insomecasesettringitecrystalsmaybetoo
detail by Taylor (3).
small to be identified.
In order to study these microstructural features, it is neces-
5.1.2.5 Calcium monosulfoaluminate usually forms platy
sary to recognize the individual phases which are usually
crystals. Elemental analysis (EDX) may be required for its
recognized by their size, shape, association, backscatter inten-
identification.
sity and elemental composition (See Note 1 for examples).
5.1.3 Aggregates—Descriptive Nomenclature C294 and
These characteristics may sometimes be insufficient to conclu-
Guide C295 outlines methods and information relevant to the
sively identify a phase, or to differentiate between two phases,
identification and classification of aggregates. Microstructural
such as chert and quartz (SeeTerminology C125). In this case,
features of individual constituents within the aggregate can be
othertechniquesmustbeused,suchasXRD
...
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