Civil Engineering Reference
In-Depth Information
in-place concrete is insignificant and does not have to be
considered in engineering practice.
Carbonation of paste proceeds slowly and produces
little direct shrinkage at relative humidities of 100% and
25%. Maximum carbonation and carbonation shrinkage
occurs at about 50% relative humidity. Irreversible
shrinkage and weight gain occurs during carbonation.
And the carbonated product may show improved volume
stability to subsequent moisture change and reduced per-
meability (
Verbeck 1958
).
During manufacture some concrete masonry units are
deliberately exposed to carbon dioxide after reaching 80%
of their rated strength; this introduction to carbonation
shrinkage makes the units more dimensionally stable.
Future drying shrinkage is reduced 30% or more
(
Toennies and Shideler 1963
).
One of the causes of surface crazing of concrete is the
shrinkage that accompanies natural air carbonation of
young concrete. More research is needed on the effect of
early carbonation on deicer scaling resistance.
Carbonation of another kind also can occur in freshly
placed, unhardened concrete. This carbonation causes a
soft, chalky surface called dusting; it usually takes place
during cold-weather concreting when there is an unusual
amount of carbon dioxide in the air due to unvented
heaters or gasoline-powered equipment operating in an
enclosure.
Note: Same concrete strength at
time of loading in all cases
press
ure steam-
0
50
100
150
200
250
300
350
Time after loading, days
Fig. 15-29. Effect of curing method on magnitude of creep
for typical normal-density concrete (
Hanson 1964
).
The method of curing prior to loading has a marked
effect on the amount of creep in concrete. The effects on
creep of three different methods of curing are shown in
Fig. 15-29. Note that very little creep occurs in concrete
that is cured by high-pressure steam (autoclaving). Note
also that atmospheric steam-cured concrete has consider-
ably less creep than 7-day moist-cured concrete. The two
methods of steam curing shown in Fig. 15-29 reduce
drying shrinkage of concrete about half as much as they
reduce creep.
Sulfate Attack
Sulfate attack of concrete can occur where soil and
groundwater have a high sulfate content and measures to
reduce sulfate attack, such as use of a low water to
cementing materials ratio, have not been taken. The attack
is greater in concrete that is exposed to wetting and
drying, such as foundation walls and posts. Sulfate attack
usually results in an expansion of the concrete because of
the formation of solids from the chemical action or salt
crystallization. The amount of expansion in severe cir-
cumstances has been significantly higher than 0.1%, and
the disruptive effect within the concrete can result in
extensive cracking and disintegration. The amount of
expansion cannot be accurately predicted.
CHEMICAL CHANGES AND EFFECTS
Some volume changes of concrete result from chemical
reactions; these may take place shortly after placing and
finishing or later due to reactions within the hardened
concrete in the presence of water or moisture.
Carbonation
Hardened concrete containing some moisture reacts with
carbon dioxide present in air, a reaction that results in a
slight shrinkage of the surface paste of the concrete. The
effect, known as carbonation, is not destructive but actu-
ally increases the chemical stability and strength of the
concrete. However, carbonation also reduces the pH of
concrete. If steel is present in the carbonated area, steel
corrosion can occur due to the absence of the protective
oxide film provided by concrete's high pH. Rust is an
expansive reaction and results in cracking and spalling of
the concrete. The depth of carbonation is very shallow in
dense, high-quality concrete, but can penetrate deeply in
porous, poor-quality concrete. Because so little of a con-
crete element carbonates, carbonation shrinkage of cast-
Alkali-Aggregate Reactions
Certain aggregates can react with alkali hydroxides in
concrete, causing expansion and cracking over a period of
years. The reaction is greater in those parts of a structure
exposed to moisture. A knowledge of the characteristics of
local aggregates is essential. There are two types of alkali-
reactive aggregates, siliceous and carbonate. Alkali-aggre-
gate reaction expansion may exceed 0.5% in concrete and
can cause the concrete to fracture and break apart.
Structural design techniques cannot counter the
effects of alkali-aggregate expansion, nor can the expan-
