The strength and durability of
concrete structure must go hand in hand. Durability is the ability of a
structure to resist weathering action, chemical attack and abrasion, while
maintaining minimum strength and other desired engineering properties. The commonly observed processes that are
responsible, individually or together, for the deterioration of concrete are
carbonation, alkali chloride aggregate reaction (AAR), attack, initiated
corrosion of reinforcement, sulfate decalcification or leaching and frost or
freeze-thaw action. It is generally accepted that under the optimum conditions
of effective blending components, transportation, placing, and curing, the
addition of mineral admixtures to concrete improves its resistance toward the
deteriorating agents.
When mineral admixtures, such as
fly ash (FA) or blast furnace slag (BFS) are used, the strength of concrete can
be considered as a result of three principal factors, first accounting for the
reduction in the quantity of cement (dilution), second heterogeneous nucleation
(physical) and third pozzolanic reaction (chemical). The net result is higher
long-term strength and durability of the structure. The structures satisfying the
requirement of cost, service life, strength and durability require the use high
performance concrete (HPC). Judicious choice of chemical and mineral admixtures
reduces the cement content and that results in economical HPC.
The carbonation refers to the
precipitation of calcite (CaCO,) as well as other CO,-based solid phases,
through the reaction of penetrating atmospheric COz with the calcium ions in
the pore solution. The main consequence of carbonation is the drop in the pH of
the pore solution of concrete so that the passive layer that usually covers and
protects the reinforcing steel against corrosion becomes unstable. The
continuous diffusion of CO, inside concrete may also lead to decomposition of
calcium silicate hydrate (C-S-H), the principal strength giving phase in
concrete. The consequences are loss of strength, shrinkage, cracking and
increase in the porosity of concrete. In concrete with mineral admixture, where
the amount of calcium hydroxide (CH) is reduced due to pozzolanic or
cementitious reaction, the carbonation is dependent on permeability and the
resultant lower permeability hinders the ingress of COz:
The aggregate containing certain
dolomitic or siliceous minerals react with soluble alkalies in concrete and
sometimes result in detrimental expansion, cracking and the premature loss of serviceability
of concrete structures affected. This phenomenon is known as alkali aggregate
reaction or AAR. All kinds of concrete structures may be affected, although
structures in direct contact with water, such as dams and bridges, are
particularly susceptible to AAR. The mineral admixtures replacing cement, such
as BFS and FA, mitigate or eliminate AAR in concrete.
Under marine conditions, chloride
ions penetrate through porous concrete and build up around the reinforcement
and the alkalinity (pH) of the surrounding pore solution falls substantially.
At that stage, the protective ironoxide film around reinforcing bars
depassivates and cracks, exposing the steel. The exposed steel gets corroded in
the presence of water and oxygen, resulting in the formation of expansive
corrosion products (rust) that occupy several times the volume of the original
steel consumed. The expansive corrosion products create tensile stresses on the
concrete surrounding the corroding steel reinforcing bar, leading to cracking
and spalling of concrete cover. The addition of FA and BFS to concrete inhibits
corrosion of reinforcement, improving the resistance toward chloride
penetration and reducing the quantity of free (soluble)chloride in concrete. Besides
delaying the initiation, the corrosion propagation period is also extended.
The deterioration of concrete due
to external sulfate attack is a commonly observed phenomenon, when structures
are exposed to sulfate solutions or built in sulfate bearing soil and/or ground
water. All commonly obtained water soluble sulfates are deleterious (Mg > Na> Ca) to concrete, but the
effect is severe when it is associated with Mg cations. The concrete with
mineral admixtures, exposed to Na, SO, environment, in general, shows lower
expansion.
it is attributed to the lower content of CH and the formation of
secondary C-S-H due to pozzolanic/ cementitious reactions, with the addition of
FA and BFS. The lower availability of CH in hardened concrete is believed to
create a negative effect, during magnesium sulfate attack. However, that is
often offset by the reduced permeability and densification caused by the use of
mineral admixtures. The decalcification described by dissolution of otesta lss
usually CH (also called portlandite) and C-S-H in hydrated cement systems
exposed to water. It results in surface deposits of CaCO3 termed
efflorescence, and secondary precipitations of monosulfate, ettringite and
calcite, deep within concrete.
The efflorescence or surface
deposits develop on new constructions with Portland cement concrete masonry
units, including bricks and tiles, which have been bonded with Portland cement.
It is normally not damaging but aesthetically undesirable. It has negative
effect on the compressive strength of concrete. The use of mineral admixtures,
combined with adequate curing, decreases the permeability of concrete leaching
and decalcification. It is relevant to hydraulic structures, and radioactive
disposal facilities, wherein long-term stability must be guaranteed.
