Civil Engineering Reference
In-Depth Information
• Sufficient moisture must be available.
Thus by eliminating at least one of these factors the expansion and cracking of concrete
may be prevented.
One approach to preventing expansion due to ASR consists in lowering the alkali
content in the concrete mix to sufficiently low concentrations. It is generally accepted that
an alkali-silica/silicate reaction in concrete made with Portland cement will not occur if
the content of equivalent Na
2
O (defined as Na
2
O
e
=Na
2
O+0.66K
2
O) in the mix does not
exceed 4 or even 3 kg/m
3
. Such low alkali concentrations are usually not achievable with
ordinary Portland cement, but may be achieved if a low-alkali Portland cement (see
section 2.10) is used instead.
The ASR expansion may also be reduced or even prevented completely by replacing a
fraction of the Portland cement in the concrete mix with fly ash or some natural
pozzolanas, provided that they themselves do not introduce too much additional alkali
(Hobbs, 1986, 1994; Nixon
et al.,
1986; Nagataki
et al.,
1991; Blackwell
et al.,
1992;
Shayan
et al.,
1996). Alternatively, this may be done by the use of Portland-fly ash
cement (see section 0.00) or Portland-pozzolana cement (see section 0.00), instead of
ordinary Portland cement. By this measure the amount of alkali in the pore solution is
reduced and the amount of reactive SiO
2
is increased, thus altering the ratio of alkali to
reactive SiO
2
to a value at which the tendency of the concrete mix to expand is reduced or
eliminated. A similar effect may also be obtained by the use of Portland-slag cement (see
section 8.3) in the production of the concrete mix (Bakker, 1981; Chatterji, 1984; Hogan,
1985; Hobbs, 1986, 1987).
The presence of pozzolanic additives or blast furnace slag decreases expansion in most
cases, but in instances where the alkali content in the mineral addition is too high the
reduction in expansion may be insufficient, or the overall expansion may even be
increased. The alkali cations present in fly ash or in the slag are almost completely
incorporated in the glass phase, and enter the pore solution of concrete only in the course
of hydration (Duchesne and Berube, 1994). Generally they tend to be less effective in
raising the OH
−
concentration in the pore solution than are equal amounts of alkalis
released from Portland cement. In calculating the equivalent amount of Na
2
O for mixes
containing mineral additions it has been recommended that one-sixth of the total alkali
content of the fly ash or one-half of that of the slag should be added to the amount
contributed by the Portland cement component (Hobbs, 1986). In long-term experiments
it was shown that fly ashes introduced to concrete mixes made with alkali-reactive
aggregates were effective in preventing expansion with alkali contents in the mix as high
as 7.0 kg Na
2
O/m
3
(Shayan
et al.,
1996).
It has been also recognized that ASR expansion may be prevented by adding
appropriate amounts of a lithium salt to the concrete mix (McCoy and Caldwell, 1951;
Stark
et al.,
1993). Alternatively, the lithium compound may be introduced directly to the
cement (Gajda, 1996). From among different lithium compounds lithium hydroxide
monohydrate (LiOH.H
2
O) was found to be the most suitable for this purpose. The
optimum dosage of the lithium additive ranges between 0.8 and 1.2% LiOH.H
2
O.
An
alkali-carbonate reaction
may takes place if an argillaceous (illitic) dolomitic
limestone is used as concrete aggregate. Here expansion takes place as the consequence
of swelling of the illite constituent, following a de-dolomitization of the dolomitic
