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ASR. The greater the proportion of Class C fly ash used in a mix, the greater
the ASR control benefit.
The concentration of soluble (available) alkali and not the total alkali content
is critical for the reaction. Studies have shown that if the acid soluble alkalicontent
is in excess of 5.73 lb./cu. yd., then it can cause cracking, provided
that reactive aggregates are present. (This is approximately equivalent to 4.21
lb./cu. yd. as water-soluble alkali.) For high-calcium Class C fly ash, the
amount of alkali in the ash affects the effectiveness of expansion reduction.
Another study by EPRI (22) indicated that for high-calcium (22.5% CaO)
moderate-alkali (2.30% Na 2 Oeq) fly ash, the amount of fly ash required to
control expansion due to ASR varies significantly from one aggregate to
another. In the case of the extremely reactive aggregate, between 50%-60% of
fly ash would be required to reduce expansion under the 0.10% level. For less
reactive aggregate, a lower fly ash replacement level is required. For highcalcium
(21.0% CaO) high-alkali (5.83 Na 2 Oeq) fly ash, it still contributed in
reducing ASR expansion; however, an expansion higher than 0.10% level
occurred. Therefore, it is necessary to test the amount of alkali in the fly ash
prior to incorporating it in the concrete to control ASR.
The following aggregates and their mineralogical constituents are known to
react with alkalis:
• Silica materials - opal, chalcedony, tridymite and cristobalite
• Zeolites, especially heulandite
• Glassy to cryptocrystalline rhyolites, dacites, andesites and their tuffs
• Certain phyllites
Low-calcium (ASTM C618, Class F) fly ash is most effective in reducing
expansion caused by alkali-silica reactions where the fly ash is used at a
replacement level of approximately 20 to 30%. Once the replacement
threshold has been reached, the reduction in expansive reaction for a given
cement alkali level is dramatic. Additionally, the greater the proportion of
cement replaced with Class F fly ash, the greater the ASR reduction. In some
cases where silica fume, a very fine material that is high in reactive SiO 2 , is
used in concrete for high strength, adding Class F or Class C fly ash to create
a “ternary blend” can significantly reduce ASR susceptibility without
diminishing high concrete performance. The actual reaction mechanism on
alkali-aggregate reaction and the effect of fly ash is not fully understood today
and will require more research to find a satisfactory explanation.
Soundness of aggregates or the freedom from expansive cracking is one of the
most important factors affecting the durability of concrete. At early ages,
unloaded concrete cracks because of two reasons: thermal contraction and
drying shrinkage. When concrete hardens under ambient temperature and
humidity, it experiences both thermal and drying shrinkage strains.
We Energies 52
Coal Combustion Products
Utilization Handbook