Environmental Engineering Reference
In-Depth Information
The Reaction
Alkali-aggregate reaction can occur between hardened paste of cements containing more
than 0.6% soda equivalent and any aggregate containing reactive silica. The soda equiva-
lent is calculated as the sum of the actual Na
2
O content and 0.658 times the K
2
O content of
the clinker (NCE, 1980). The alkaline hydroxides in the hardened cement paste attack the
silica to form an unlimited-swelling gel that draws in any free water by osmosis and
expands, disrupting the concrete matrix. Expanding solid products of the alkali-silica
reaction help to burst the concrete, resulting in characteristic map cracking on the surface.
In severe cases, the cracks reach significant widths.
Susceptible Rock Silicates
Reactive silica occurs as opal or chalcedony in certain cherts and siliceous limestones and
as acid and intermediate volcanic glass, cristobolite, and tridymite in volcanic rocks such
as rhyolite, dacites, and andesites, including the tuffs. Synthetic glasses and silica gel are
also reactive. All of these substances are highly siliceous materials that are thermodynam-
ically metastable at ordinary temperatures and can also exist in sand and gravel deposits.
Additional descriptions are given in Krynine (1957) (see
Section 5.2.2
for descriptions).
Reaction Control
Reaction can be controlled (Mather, 1956) by:
1.
Limiting the alkali content of the cement to less than 0.6% soda equivalent. Even
if the aggregate is reactive, expansion and cracking should not result.
2.
Avoiding reactive aggregate.
3.
Replacing part of the cement with a very finely ground reactive material (a poz-
zolan) so that the first reaction will be between the alkalis and the pozzolan,
which will use up the alkalis, spreading the reaction and reaction products
throughout the concrete.
Tests to Determine Reactivity
Tests include:
The mortar-bar expansion test (ASTM C227-03) made from the proposed aggre-
gate and cement materials.
●
Quick chemical test on the aggregates (ASTM C289-01).
●
Petrographic examination of aggregates to identify the substances (ASTM C295).
●
3.2.2
Rock Masses
General
The rock mass, often referred to as
in situ
rock, may be described as consisting of rock
blocks, ranging from fresh to decomposed, and separated by discontinuities (see
Section
5.2.7).
Mass density is the basic property. Sonic-wave velocities and the rock quality des-
ignation (RQD) are used as index properties.
Mass Density
Mass density is best measured
in situ
with the gamma-gamma probe (see
Section 2.3.6),
which generally allows for weathered zones and the openings of fractures and small
voids, all serving to reduce the density from fresh rock values.
