Civil Engineering Reference
In-Depth Information
Portland Cement Content
Petrographic Analysis
Petrographic analysis uses microscopic techniques
described in ASTM C 856 to determine the constituents of
concrete, concrete quality, and the causes of inferior per-
formance, distress, or deterioration. Estimating future per-
formance and structural safety of concrete elements can be
facilitated. Some of the items that can be reviewed by a
petrographic examination include paste, aggregate, fly
ash, and air content; frost and sulfate attack; alkali-aggre-
gate reactivity; degree of hydration and carbonation;
water-cement ratio; bleeding characteristics; fire damage;
scaling; popouts; effect of admixture; and several other
aspects. Almost any kind of concrete failure can be ana-
lyzed by petrography (
St. John, Poole, and Sims 1998
).
However, a standard petrographic analysis is sometimes
accompanied by “wet” chemical analyses, infrared spec-
troscopy, X-ray diffractometry, scanning electron
microscopy with attendant elemental analysis, differential
thermal analysis, and other analytical tools.
The Annex to ASTM C 856 (AASHTO T 299) describes
a technique for field and laboratory detection of alkali-
silica gel. Using this method, a uranyl-acetate solution is
applied to a broken or roughened concrete surface that has
been dampened with distilled or deionized water. After
one minute, the solution is rinsed off and the treated sur-
face is viewed under ultraviolet light. Areas of gel fluo-
resce bright yellow-green. It must be recognized, however,
that several materials not related to ASR in concrete can
fluoresce and interfere with an accurate indication of ASR
gel. Materials that fluoresce like gel include: naturally flu-
orescent minerals, carbonated paste, opal, and some other
rock ingredients, and reactions from fly ash, silica fume,
and other pozzolans. ASTM C 856 includes a prescreening
procedure that gives a visual impression to compensate
for the effects of these materials. However, this test is con-
sidered ancillary to more definitive petrographic examin-
ations and other tests. In addition, the toxicity and
radioactivity of uranyl acetate warrants special handling
and disposal procedures regarding the solution and
treated concrete. Caution regarding potential eye damage
from ultraviolet light also merits attention.
The Los Alamos method is a staining technique that
does not require ultraviolet light or uranyl-acetate solu-
tion. Instead, solutions of sodium cobaltinitrite and rho-
damine B are used to condition the specimen and produce
a dark pink stain that corresponds to calcium-rich ASR gel.
It should be noted that these methods can produce evi-
dence of ASR gel without causing damage to concrete. ASR
gel can be present when other mechanisms such as
freeze-thaw action, sulfate attack, and other deterioration
mechanisms have caused the damage. These rapid meth-
ods for detecting the presence of ASR gel are useful but
their limitations must be understood. Neither of the rapid
procedures is a viable substitute for petrographic examina-
tion coupled with proper field inspection (
Powers 1999
).
The portland cement content of hardened concrete can be
determined by ASTM C 1084 (AASHTO T 178) standard
methods. Although not frequently performed, the cement
content tests are valuable in determining the cause of lack
of strength gain or poor durability of concrete. Aggregate
content can also be determined by these tests. However,
the user of these test methods should be aware of certain
admixtures and aggregate types that can alter test results.
The presence of supplementary cementitious materials
would be reflected in the test results.
Supplementary Cementitious Material
and Organic Admixture Content
The presence and amount of certain supplementary
cementitious materials, such as fly ash, can be determined
by petrographic techniques (ASTM C 856). A sample of
the supplementary cementitious material used in the con-
crete is usually necessary as a reference to determine the
type and amount of the supplementary cementitious
material present. The presence and possibly the amount of
an organic admixture (such as a water reducer) can be
determined by infrared spectrophotometry (
Hime,
Mivelaz, and Connolly 1966
).
Chloride Content
Concern with chloride-induced corrosion of reinforcing
steel has led to the need to monitor and limit the chloride
content of reinforced concrete. Limits on the water-solu-
ble chloride-ion content of hardened reinforced concrete
are given in
ACI 318
. The water-soluble chloride-ion
content of hardened concrete can be determined in accor-
dance with procedures outlined in ASTM C 1218. In addi-
tion, ASTM C 1152 can be used to determine the
acid-soluble chloride content of concrete which in most
cases is equivalent to total chloride.
Many of the above tests for chloride-ion content also
extract chloride-ions from the fine and coarse aggregates
that generally do not contribute to corrosion of reinforcing
steel. ASTM PS 118 (to be redesignated as ASTM C 1500) is
a standard for the analysis of aggregate for water-
extractable chloride (Soxhlet method). It is used when
chloride contents have been found to be significantly high
in aggregates, concretes, or mortars when tested by either
ASTM C 1152 or C 1218. Because ASTM PS 118 does not
pulverize the aggregates as other tests do, it theoretically
measures more closely the chloride-ions available for cor-
rosion.
ACI 222.1
is also a Soxhlet procedure that tests
chunks of concrete for water-extractable chloride. The true
meaning of results from the Soxhlet procedures is still
being debated.
