Civil Engineering Reference
In-Depth Information
13.0 ALKALI-AGGREGATE EXPANSION
Although all aggregates can be considered reactive, only those that
actually cause damage to concrete are cause for concern. Experience has
shown that the presence of an excessive amount of alkalis enhances the
attack on concrete by an expansion reaction. Use of marginal quality
aggregate and the production of high strength concrete may also produce
this effect.
The alkali-aggregate reaction in concrete may manifest itself as
map cracking on the exposed surface, although other reactions may also
produce such failures. The alkali-aggregate reaction, known as alkali-silica
type, may promote exudation of a water gel, which dries to a white deposit.
These effects may appear after only a few months or even years.
Three types of alkali-aggregate reactions are mentioned in the
literature:
•
Alkali-silica reaction.
Alkali-silica reactions are caused
by the presence of opal, vitreous volcanic rocks, and those
containing more than 90% silica.
•
Alkali-carbonate reaction.
The alkali-carbonate reaction
is different from the alkali-silica reaction in forming
different products. Expansive dolomite contains more
calcium carbonate than the ideal 50 % (mol) proportion
and frequently also contains illite and chlorite clay
minerals.
•
Alkali-silicate reaction.
The alkali-silicate reaction
has
not received general recognition as a separate entity. The
alkali-silicate reaction was proposed by Gillott.
[29]
The
rocks that produced this reaction were greywackes, argil-
lites, and phyllites containing vermiculites.
The preventive methods to counteract alkali-aggregate expansion
include replacement of cement with pozzolans or blast-furnace slag and use
of low alkali cement.
14.0 FROST ACTION
Frost action is defined as the freezing and thawing of the moisture
in materials and the resultant effects on these materials. Essentially, three
kinds of defects are recognized:
