Geoscience Reference
In-Depth Information
matrix appears more porous and fine cracks filled with
gypsum run subparallel to the outer surface.
Uncarbonated Portland cement concrete is highly
alkaline (pH >12.5) owing to the presence of calcium
hydroxide (Ca(OH)
2
) and alkali hydroxides (NaOH and
KOH) in the hydrated cement. They are therefore relatively
resistant to alkali attack. However, concentrated (>10%)
alkaline solutions will attack the cement matrix of
Portland cement concrete, with dissolution of the cement
matrix typically occurring at a much slower rate than with
acid attack. High-alumina cement (HAC) concrete is more
susceptible to alkali attack than Portland cement concrete.
Alkaline hydrolysis of HAC is discussed on p. 119. In
addition to alkali attack, alkali-bearing solutions can cause
AAR in concrete, which is discussed on p. 110.
to frost attack. Pop-outs have also been caused by
artificial materials that have contaminated the concrete
during production or transport, such as periclase, lime, or
anhydrite (Katayama & Futagawa, 1997).
Pop-outs on concrete surfaces are not usually of
structural significance but they do have implications for
durability of reinforced concrete, as they locally reduce
the depth of cover to the reinforcement. Most complaints
about pop-outs are for aesthetic reasons as they are
unsightly and certain types, such as pyrite (
216
) and
organic matter (
217
), may produce a brown stain on the
concrete surface.
A
LKALI
-
AGGREGATE REACTION
(
AAR
)
AAR has been recognized as a rare, but potentially
damaging form of concrete deterioration since 1940
(Stanton, 1940). There are two main types of deleterious
AAR, the most common being alkali-silica reaction (ASR)
and the second being the rather rare alkali-carbonate
reaction (ACR). Both these types involve the interaction of
reactive aggregate constituents with alkali (sodium and
potassium) hydroxyl ions (from the cement or an external
source) in the presence of moisture, to form deleterious
reaction products (notably alkali-silica gel). The reactions
often cause expansion and cracking that, while not directly
causing structural failure, will reduce the service life of the
concrete by allowing penetration by aggressive agents.
Successful diagnosis of AAR relies on the integrated
assessment of all the evidence gleaned from
documentary, site, and laboratory investigations.
P
OP
-
OUTS
Pop-outs are conical cavities on concrete surfaces that
range from a few millimetres to as much 100 mm in
diameter and up to 50 mm in depth. They are formed by
volume increase of aggregate particles or concrete
contaminants immediately beneath the concrete surface.
Pop-outs typically start as a semicircular cracks that, as
expansion increases, become roughly circular with a
cone of protruding concrete, which may be removed to
form the cavity.
Expansive materials found to cause pop-outs include
naturally occurring aggregate particles such as alkali-
reactive particles, particles containing swelling clays,
pyrite particles, organic matter, and particles susceptible
216
217
216
View of a concrete surface (left) exhibiting a pop-
out caused by a pyritic aggregate particle (black). The
oxidized pyrite has stained the concrete surface
brown (iron oxide); PPT, ×35.
217
Concrete contaminated with particles of lignite
(black, right) that caused pop-outs on exposed
concrete surfaces; PPT, ×35.
