Civil Engineering Reference
In-Depth Information
0.3
Identification of Potentially Reactive Aggregates.
Field
performance history is the best method of evaluating the
susceptibility of an aggregate to ASR. For the most defini-
tive evaluation, the existing concrete should have been in
service for at least 15 years. Comparisons should be made
between the existing and proposed concrete's mix propor-
tions, ingredients, and service environments. This process
should tell whether special requirements are needed, are
not needed, or whether testing of the aggregate or job
concrete is required. The use of newer, faster test methods
can be utilized for initial screening. Where uncertainties
arise, lengthier tests can be used to confirm results. Table
5-8 describes different test methods used to evaluate
potential alkali-silica reactivity. These tests should not be
used to disqualify use of potentially reactive aggregates,
as reactive aggregates can be safely used with the careful
selection of cementitious materials as discussed below.
Materials and Methods to Control ASR.
The most effec-
tive way of controlling expansion due to ASR is to design
mixtures specifically to control ASR, preferably using
locally available materials. The following options are not
listed in priority order and, although usually not neces-
sary, they can be used in combination with one another.
In North America, current practices include the use of
a supplementary cementitious material or blended cement
proven by testing to control ASR or limiting the alkali
content of the concrete. Supplementary cementitious mate-
rials include fly ash, ground granulated blast-furnace slag,
silica fume, and natural pozzolans. Blended cements use
slag, fly ash, silica fume, and natural pozzolans to control
ASR. Low-alkali portland cement (ASTM C 150) with an
alkali content of not more than 0.60% (equivalent sodium
oxide) can be used to control ASR. Its use has been success-
ful with slightly reactive to moderately reactive aggre-
gates. However, low-alkali cements are not available in all
areas. Thus, the use of locally available cements in combi-
nation with pozzolans, slags, or blended cements is prefer-
able for controlling ASR. When pozzolans, slags, or
blended cements are used to control ASR, their effective-
Sudbury aggregate
0.25
0.10% expansion limit for evaluating
effectiveness of supplementary
cementitious materials against AAR
0.2
0.15
0.1
0.05
0
20
30
56
35
50
65
7.5
10
12.5
Control
% Fly ash
% Slag
% Silica fume
Fig. 5-23. Influence of different amounts of fly ash, slag,
and silica fume by mass of cementing material on mortar
bar expansion (ASTM C 1260) after 14 days when using
reactive aggregate (
Fournier 1997
).
ness must be determined by tests such as ASTM C 1260
(modified) or C 1293. Where applicable, different amounts
of pozzolan or slag should be tested to determine the opti-
mum dosage. Expansion usually decreases as the dosage
of the pozzolan or slag increases (see Fig. 5-23). Lithium-
based admixtures are also available to control ASR. Lime-
stone sweetening (the popular term for replacing
approximately 30% of the reactive sand-gravel aggregate
with crushed limestone) is effective in controlling deterio-
ration in some sand-gravel aggregate concretes. See
AASHTO (2001),
Farny and Kosmatka (1997)
, and
PCA
(1998)
for more information on tests to demonstrate the
effectiveness of the above control measures.
Alkali-Carbonate Reaction
Mechanism of ACR.
Reactions observed with certain
dolomitic rocks are associated with alkali-carbonate reac-
tion (ACR). Reactive rocks usually contain large crystals of
dolomite scattered in and surrounded by a fine-grained
matrix of calcite and clay. Calcite is one of the mineral
forms of calcium carbonate; dolomite is the common
name for calcium-magnesium carbonate. ACR is relatively
rare because aggregates susceptible to this reaction are
usually unsuitable for use in concrete for other reasons,
such as strength potential. Argillaceous dolomitic lime-
stone contains calcite and dolomite with appreciable
amounts of clay and can contain small amounts of reactive
silica. Alkali reactivity of carbonate rocks is not usually
dependent upon its clay mineral composition (
Hadley
1961
). Aggregates have potential for expansive ACR if the
following lithological characteristics exist (
Ozol 1994
and
Swenson 1967
):
Fig. 5-22. Polished section view of an alkali reactive aggregate
in concrete. Observe the alkali-silica reaction rim around the
reactive aggregate and the crack formation. (43090)
