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Author(s)
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GEOC!HEMICALMETHODS FOR THE IDENTIFICATION
OF ASR GEL
G. D. Guthrie
J. W. Carey
Transportation Research Board
LosAlamos
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GEOCHEMICAL METHODS FOR THE IDENTIFICATIONOF Adi!!d * 1999
Q s-in
George D. Guthrie, Jr. and J. William Carey
Geology and Geochemistry Group, Mail Stop D462
Los Alamos National Laboratory
Los Alarnos, New Mexico 87545 U.S.A.
(505) 665-6340 ([email protected])
(505) 667-5540 ([email protected])
(505) 665-3285 (FAX)
July 1998
.
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.
ABSTRACT
This paper presents a geochemical method for staining various products of the
alkali-silica reaction.
The method is based on both the composition of ASR gel and one
of its properties (the ability to exchange cations with a fluid). The stained concrete can
be observed in normal light and serves as both a rapid field screening method and a usefil
aid for detailed petrographic examinations.
INTRODUCTION
Similarities between concrete and earth materials (rocks) abound. In addition to
the obvious links (such as the use of earth materials as aggregates), concrete and rocks
share the characteristic that their physical properties (strength, durability, permeability,
flexibility, etc.) are controlled by complex chemical reactions that evolve continuously
during the material’s existence.
The similarities go even deeper, because many of the
chemical reactions in concrete have natural analogues and/or produce materials closely
resembling earth materials.
The recognition of similarities between concrete and earth materials is not new.
Indeed, geoscientists
and engineers have traditionally worked together on many concrete
problems; for example, many concrete petrographers were trained as geologists and
developed their skills in the identification of chemical reactions in rocks.
In this paper, we present a geochemical method for the identification
of gel-like
reaction products associated with the alkali-silica reaction (ASR) (1-3). By
“geochemical,” we refer to the fact that the method evolved from a chemical staining
technique used to discriminate earth materials that are compositionally
similar to ASR
gels. However, we also use “geochemical” to emphasize the concept that the method is
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based on the geochemical properties of the ASR gels. Specifically, the properties of earth
materials (and solids in general) depend on the types of atoms (chemical composition)
and their arrangement (atomic structure).
The chemical method we outline exploits the
structure and composition of ASR gels, namely their open structure (allows cation
exchange with a fluid) and their composition (which is characteristically
alkali cation potassium).
enriched in the
Although ASR gels may additionally contain appreciable
amounts of the alkali cation sodium, most ASR gels contain relatively more potassium
than sodium (4), possibly due to the predominance
of potassium as an alkali contaminant
in most cements (5). The technique we outline would require modifications
in order to be
effective on gels highly enriched in sodium and depleted in potassium (such as those
produced by potassium-poor
cements or by reaction in a mortar bar experiencing a NaOH
soak).
Two stains are used in the treatment process.
One stain-a
solution of sodium cobaltinitrite or Na3Co(N02)6-reacts
saturated aqueous
with soluble potassium to
produce a yellow precipitate, thereby staining K-rich ASR gel (1,2). The other stain-a
saturated aqueous solution of one of several rhodarnine compounds-reacts
with
components in concrete other than K-rich ASR gel, staining them pinkish.
this stain is in part to highlight the regions of yellow-stained
high-contrast
The use of
ASR gel (by providing a
background) and in part to identifj other degradation products.
We have
evaluated several rhodamine compounds (including rhodarnine B [C28H3 1N203Cl; CAS
#81-88-9], rhodamine B base [C28H30N203; CAS #509-34-2], and rhodarnine 6G
[C28H31N203C1; CAS #989-38-8]) (1,3), and each has a different staining pattern (as
described below under Observations).
None of the rhodamine compounds, however,
appears to affect the areas stained by sodium cobaltinitrite (i.e., the bonajde
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METHODS
Preparing the Concrete Specimen
The staining method described below applies to a concrete surface under
investigation
for ASR. We have evaluated several standard methods for preparation of
the concrete surface, including bush hammering in the field, fracturing of a core or hand
sample, and sawing/polishing
to produce a flat surface for petrographic examination
or
for preparation of a thin section. The staining method is most effective on a fi-actured
surface, probably because fracturing induces the least amount of chemical change to the
ASR gel and some fractured surfaces contain a higher proportion of ASR gel than
surfaces produced by bush hammering, sawing, or polishing.
Nevertheless,
the staining
method works on most prepared surfaces, provided ASR gel is present. In general,
application of chemicals that might interact with the ASR gel (e.g., sawing/polishing
lubricants) should be minimized.
Preparing the Staining Solutions
The two compounds used in this method to stain concrete samples for the
identification
of ASR gel are saturated aqueous solutions of sodium cobaltinitrite
rhodamine compounds.
and
Saturated solutions of each compound are made by adding the
solid compounds to pure water, ensuring that excess solid remains in the solution
following equilibration
(e.g., after several minutes of agitation).
Sodium cobaltinitrite is very soluble, and the solution can be stabilized for longer
periods of time by the addition of a small amount of acetic acid (6) (which produces no
noticeable difference in staining of ASR gel).
.
Each of the rhodamine compounds that we have evaluated appears to dissolve
slowly in water and is less soluble than sodium cobaltinitrite.
Treating the Concrete Surface
Treatment of the concrete surfaces is done following a simple procedure:
.
pre-rinse the concrete with pure water to wet the surface and to remove any
loose dusts;
●
apply sodium cobaltinitrite solution and allow it to remain on the concrete
surface for -30-60
●
seconds;
rinse the treated surface thoroughly with pure water;
At this stage, one can observe regions of yellow staining indicating the presence
of areas with soluble (exchangeable) potassium.
Some stained regions will be intensely
yellow, whereas some will be less intensely stained, giving a rough indication of relative
variations in K-content in the gel. Staining will intensi~
as the core surface dries. At
any stage following the rinse, the treatment may continue as follows:
●
apply rhodamine compound and allow it to remain on the concrete surface for
-30–60 seconds;
●
rinse the treated surface thoroughly with pure water.
The choice of rhodarnine compound affects the staining pattern, as discussed
below. Each of the compounds stains some degradation products in the concrete pink.
Early on we focused on rhodamine B (1), but we now consider rhodamine B base to be
the most generally usefhl of the rhodamine compounds, because rhodamine B and
rhodamine 6G are more aggressive stains that are not as specific for gel products.
All of
these rhodamine compounds will leave the K-rich ASR gel stained yellow, although some
ASR gels that contain minor amounts of K may stain both yellow and pink, resulting in
an orangish color.
We examined the effect of differing staining sequences, including the
simultaneous
application of both stains. The results are best when following the
procedure outlined above. The staining (especially that due to the rhodarnine
compounds) fades over the course of weeks to months, but the samples can be re-stained
following the same procedure.
As with the use of any chemical, one should avoid contact and follow applicable
local, state, and federal requirements with respect to disposal of waste.
OBSERVATIONS
We have examined numerous concrete structures of various ages and degrees of
field-distress.
Cores were taken fi-om a variety of structures including highway slabs on
grade, bridge decks, sidewalks, and curbing.
Some of the core material was fresh, but
archived materials stored under normal room conditions for up to several years could also
be effectively examined with the staining methods.
Initially, the presence or absence of
ASR was determined by petrographic techniques, which was then followed by application
of the stains. As we refined our staining methodology,
it became possible to combine the
staining and petrographic techniques as described below.
Observations
cobaltinitrite
of stained concrete were made following application of Na-
and the rhodamine compounds and allowing the samples to dry. The
following discussion will consider the staining results for the two compounds separately,
as very distinct behavior was found for the Na-cobaltinitrite
and rhodamine compounds.
Staining by Na-cobaltinitrite
We found that Na-cobaltinitrite
is a weak stain that requires the presence of freely
available potassium to become fixed to the surface. In ASR-free concrete, the cement
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paste contains potassium at sufficiently low concentrations that no observable staining
appears (Fig. 1a).
Aggregate and sand unaffected by ASR was also found to be not susceptible to
Na-cobaltinitrite.
For example, potassium feldspar crystals in coarse-grained
granite
aggregate were free of any staining. These observations are consistent with the practice
reported in Keith (7), as well as others, in which geologic specimens must first be etched
in HF acid to produce a surface susceptible to Na-cobaltinitrite.
Note that the HF etch has
the effect of producing a silica gel containing potassium on the surface of potassium
silicates, such as feldspar.
Concrete affected by ASR contained regions stained yellow by Na-cobaltinitrite.
The staining was highly localized and, upon petrographic examination, was found in
association with reactive aggregates, fractures, or pockets containing silica gel. The most
obvious examples were gel pockets, in which the white, translucent gel was stained deep
yellow. Petrographic and scanning electron microscopy data showed that such material
was typical of ASR gel and contained the potassium necessary to cause the fixation of
Na-cobaltinitrite
(1).
Reactive aggregates showing the presence of ASR gel were distinctly stained with
Na-cobaltinitrite
(Fig. 1c). In broken pieces of reactive aggregate, the staining was
evident along the rims, within the aggregate, and in the matrix at a small distance from
the interface. Staining was also evident along pop-out surfaces and on the surface of
reactive aggregate.
In many cases, there were distinct yellow bands that could be
associated with reaction fronts progressing into the aggregate.
Reactive sands showing evidence of ASR were also distinctly stained by Nacobaltinitrite.
In this case, staining was not localized to aggregate but occurred in
localized, but pervasive regions of the cement-paste matrix. Because the staining patterns
of aggregate and sands were very distinct, it was possible to determine the relative
importance of sand and aggregate in the reactivity.
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The extent of ASR present in the concrete is correlated with the amount of yellow
staining, providing a rapid visual estimation method.
The severity of ASR was also
evident in the number of reactive aggregate grains affected, the development
of the
reaction in particular aggregate grains (i.e., depth of penetration of the stains), the number
of gel-filled pockets, and the pervasiveness
of staining of the paste-matrix.
However,
additional work is necessary to develop a unique criterion linking specific staining
patterns to modes of deterioration.
Staining by Rhodamine
Compounds
Several different rhodamine compounds were investigated to determine their
relative effectiveness as a guide to modes of deterioration in concrete.
Previously (1), we
reported results using rhodamine B. In this study, rhodamine B base, and rhodarnine 6G
were also investigated. We found that rhodamine B base was the most effective
compound in this group. Rhodamine B base is a non-ionic form of rhodarnine and was
less aggressive than rhodamine B as a staining agent. Consequently,
rhodamine B base
offered superior sensitivity to deterioration mechanisms and provided a better
discrimination
of ASR.
Healthy concrete without visible deterioration generally is not stained by the
rhodamine compounds (Fig. 1a). In such specimens, patches of weak, pink-staining
concrete may occur unassociated with obvious deterioration. Such staining should be
ignored in the application of this method.
Concrete affected by ASR may or may not show staining by rhodamine
compounds. However, it was found that severely ai%ected concrete often displayed
extensive staining by rhodamine B base (Fig. 1d). This staining was found to occur on
Ca-forms of ASR gel and in the matrix itself. The staining was particularly acute for CaASR gel, and rhodamine B base was found to be usefil in locating such gel (1). Ca-ASR
gel is interpreted as a chemically modified form of ASR gel occurring at a distance from
the reactive aggregate source. At some point during the migration of this gel, it exchanges
its alkali content for calcium with the cement paste. It retains its porous, gel-like structure
and is therefore susceptible to staining by rhodamine B base. It lacks potassium, however,
and is not stained by Na-cobaltinitrite.
It was also found that rhodamine B base stains regions of concrete not affected by
ASR (Fig. lb). This maybe the case in severely affected structures. We have also found
that concrete containing no evidence of ASR, but showing other signs of deterioration,
susceptible to rhodamine compounds.
is
In these cases, the staining is associated with
regions of carbonation or regions of generalized paste deterioration in which the
permeability
of the paste has increased.
These observations indicate that, in general, the rhodamine compounds are not
specific for ASR. They appear to identifj semi-permeable
regions that can absorb some
of the stain. Such regions include Ca-forrns of ASR gel, carbonated paste, and paste
rendered semi-permeable
as a result of leaching.
Summary of Observations
It was found that ASR gel is highly susceptible to staining by Na-cobaltinitrite.
In
all of the samples studied, there was adequate potassium in the ASR gel to cause fixation
of the stain. In contrast, hydrated cement paste, sand, and aggregate were found to be
unaffected by Na-cobaltinitrite.
Thus the presence of yellow staining was found to be
highly diagnostic of ASR gel, and the extent of yellow staining was found to be
diagnostic of the degree of reactivity and the abundance of reactive sand or aggregate.
The rhodamine compounds are, in general, very aggressive stains. Although there
is clearly some degree of Ca-specificity as previously suggested in Guthrie and Carey (1),
more recent observations,
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rhodamine compounds maybe taken as a possible indication of a Ca-form of ASR gel or
some form of paste deterioration brought on by leaching and carbonation made possible
by cracking of the concrete by whatever mechanism. Of the rhodarnine compounds
considered, rhodarnine B base is by far the most effective and should be substituted for
rhodamine B in the method described in (1).
ACKNOWLEDGMENTS
This work was supported by the Department of Energy through a contract to Los
Alamos National Laboratories via a Laboratory-Directed
Research and Development
grant.
REFERENCES
1. Guthrie, G. D. and Carey, J. W. A simple environmentally
fkiendly, and chemically
specific method for the identification and evaluation of the alkali-silica reaction.
Cement and Concrete Research, Vol. 27, No. 9, 1997, pp. 1407–1417.
2.
Guthrie, G. D. and Carey, J. W. Detection of alkali-silica reaction swelling in
concrete by staining, patent pending, 1997.
3. Carey, J. W. and Guthrie, G. D. Continuation
of” Detection of alkali-silica reaction
swelling in concrete by staining,” filed 4/14/98, 1998.
4. Natesaiyer, K. and Hover, K. C. Cement and Concrete Research, Vol. 19, 1989, pp.
770–778.
5. Lea, F. M. The Chemistry of Cement and Concrete. Chemical Publishing Company,
Inc., New York, 1971.
6. Budavari, S. The Merck Index, Ilth edition, Merck&
Co., Rahway, NJ, 1989.
7. Keith, M. L. American Mineralogist, Vol. 24, 1939, pp. 561–565.
Figure Captions
Figurel.
Images ofconcrete
base solutions.
cores tieated titisodiw
cobalttitite
mdrhodtine
a) Fractured core from a pavement in Albuquerque
B
(NM)
showing no distress in the field, no staining by sodium cobaltinitrite,
and only
minor staining by rhodamine B base; petrographic examination revealed no
signs of ASR. b) Fractured core from a pavement in Albuquerque
(NM)
showing some distress in the field but no signs of ASR; treated core shows no
staining by sodium cobaltinitrite but significant staining by rhodamine B base.
c) Fractured core horn a pavement in Albuquerque
(NM) showing significant
distress in the field; treated core shows staining by sodium cobaltinitrite
around reactive aggregates and within some air voids, but only minor staining
by rhodamine B base. d) Fractured core from a highway pavement in New
Mexico showing severe distress in the field; treated core shows extensive
staining by both sodium cobaltinitrite and rhodamine B base.
.
Guthrie and Carey Figure 1
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