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Reaction  Alkali-Aggregate
Reaction
In most concrete, aggregates are more or less chemically inert.
However, some aggregates react with the alkali hydroxides in concrete,
causing expansion and cracking over a period of many years. This
alkali-aggregate reaction has two forms—alkali-silica reaction
(ASR) and alkali-carbonate reaction (ACR).
 Alkali-silica
reaction (ASR) is of more concern because aggregates
containing reactive silica materials are more common. In ASR, aggregates
containing certain forms of silica will react with alkali hydroxide
in concrete to form a gel that swells as it adsorbs water from the
surrounding cement paste or the environment. These gels can swell
and induce enough expansive pressure to damage concrete
Typical indicators of ASR are random map cracking and, in advanced
cases, closed joints and attendant spalled concrete. Cracking due
to ASR usually appears in areas with a frequent supply of moisture,
such as close to the waterline in piers, near the ground behind
retaining walls, near joints  and
free edges in pavements, or in piers or columns subject to wicking
action. Petrographic examination can conclusively identify ASR.
ASR can be controlled using certain supplementary cementitious
materials. In proper proportions, silica fume, fly ash, and ground
granulated blast-furnace slag have significantly reduced expansion
due to alkali-silica reactivity. In addition, lithium compounds
have been used to reduce ASR. Although potentially reactive aggregates
exist throughout North America, ASR distress in concrete is not
that common because of the measures taken to control it. It is also
important to note that not all ASR gel reactions produce destructive
swelling.
 Alkali-carbonate
reactions (ACR) are observed with certain dolomitic
rocks. Dedolomitization, the breaking down of dolomite, is normally
associated with expansion. This reaction and subsequent crystallization
of brucite may cause considerable expansion. The deterioration caused
by ACR is similar to that caused by ASR; however, ACR is relatively
rare because aggregates susceptible to this phenomenon are less
common and are usually unsuitable for use in concrete for other
reasons. Aggregates susceptible to ACR tend to have a characteristic
texture that can be identified by petrographers.
Click here for more on ASR/ACR test methods.
Prevention of Alkali-Silica
Reaction in New Concrete
Follow the steps in the flowchart below to determine if potential
for ASR exists and to select materials to control ASR. For more
information move your mouse over the individual flowchart boxes.
(Source: IS413
and IS415).
Effect of Cement Fineness on ASTM C1260 Expansion
(SN2963)
The Accelerated Mortar Bar Test, ASTM C1260
or CSA A23.2-25A, is a widely used test to detect alkali-silica
reactive aggregates. Mortar bars are cast with the aggregate under
investigation and the specimens are stored in 1N NaOH solution at
80oC. The expansion at 16 days after casting is taken as an indication
of potential reactivity. ASTM C1260 requires the use of portland
cement meeting ASTM C150. In this research,
sponsored in part by a PCA Education Foundation Fellowship, the
effect of portland cement fineness on ASTM C1260 expansion in conjunction
with other potentially influential factors, such as alkali content
of clinker, aggregate reactivity, and immersion solution concentration,
was studied. The results show that mortar bar expansion increased
with higher cement fineness regardless of cement alkali, aggregate
reactivity, or soak solution normality. More.
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