1. DURABILITY OF THE CONCRETE
The two main structural engineering materials that are now used for civil engineering
applications are reinforced concrete (using carbon steel reinforcement) and structural
steel. These materials have been shown to be very flexible in the construction industry
and have proven, or at least known, performance in a wide range of service
environments. However, in some conditions neither material are regarded as inherently
durable and it is common, in these instances, to modify or provide additional
protection to improve the durability and achieve the design life. This is particularly so
for
civil engineering structures where long (>100 year) is often a basic design requirement.
THREATS TO DURABILITY:-
•So2 attack
•Carbonation
•Steel Corrosion
•Acid attack
•Chloride attack
•Alkali aggregate
PROTECTION AGAINST DUEABILITY:-
SO2 RESISTING CONCRETE:-
•Sulfate attack can be 'external' or 'internal'. is the more common type and typically
occurs where water containing dissolved sulfate penetrates the concrete.Due to which
the composition and microstructure of the concrete will change .These changes may
vary in type or severity but commonly include:
•Extensive cracking
•Expansion
•Loss of bond between the cement paste and aggregate
•Alteration of paste composition, with monosulfate phase converting to ettringite
and, in later stages, gypsum formation. The necessary additional calcium is
provided by the calcium hydroxide and calcium silicate hydrate in the cement paste
OTHER SOURCES OF SULFATE:-

2. Other sources of sulfate which can cause sulfate attack include:
•Seawater
•Oxidation of sulfide minerals in clay adjacent to the concrete - this can produce
sulfuric acid which reacts with the concrete
•Bacterial action in sewers - anaerobic bacterial produce sulfur dioxide which
dissolves in water and then oxidizes to form sulfuric acid
•In masonry, sulfates present in bricks and can be gradually released over a long
period of time, causing sulfate attack of mortar, especially where sulfates are
concentrated due to moisture movement
•Internal sulfate attack
Occurs where a source of sulfate is incorporated into the concrete when mixed.
Examples include the use of sulfate-rich aggregate, excess of added gypsum in the
cement or contamination.
Protection against internal sulfate attack
Proper screening and testing procedures should generally avoid internal sulfate attack.
CORROSION:-
CARBONATION:-
Carbon dioxide, CO2 is a gas form in the atmosphere, it penetrates in the concrete
pores. This penetration is more fast when concrete is more porous. But, this gas can
be dissolved in the water which is in some pores. It can then react with cement and
form carbonates. This reaction decreases the concrete pH, down to a value close to
9,5.
So, carbonation starts on concrete surface, and concerns some thickness (called
carbonation depth) of this material.
STEEL CORROSION:-
CAUSES:-
•Before being placed in a framework, a reinforcement is rusted, because it was initially
exposed to atmosphere.
•When a freshly-mixed concrete is placed around this steel,.
MECHANISM:-
•
When a freshly-mixed concrete is placed around this steel, the mixing water penetrates
through the rust pores, where it gradually forms hydrated calcium ferrite (4.CaO. Fe2

3. O3 13H2 O). Moreover, this water reacts with steel and forms on it a thin layer of iron
and calcium hydroxides, respectively [Fe(OH)2] and [Ca(OH)2].
•All these products in the vicinity of steel raise the pH of concrete pore solution, up to
about 13. It should be noted that in contact with an initial rust, cement hydration of is
disturbed : a transition zone is locally formed, and concrete is more homogeneous, far
from this zone.
PROTECTION:-
So, the concrete mixing water makes it possible to form around steel, some products,
which protect it by passivation. More precisely, under atmospherically induced rust,
reinforcement is covered with a thin protective layer of white products, containing
ferrite and of hydroxide of calcium.
Such a protection vanishes, if the pore solution disappeared (e.g. when large cracks
reach reinforcements) or does not correspond any more to sound concrete.
CHLORIDE ATTACK:-
Corrosion caused by chloride attack can dramatically affect mild. steel and even lower
grades stainless enclosures. To help select ... misery for all. Typically the corrosion is
caused by chloride attack from salt water
IN Reinforced Concrete :
In the case of reinforced concrete the realisation that the material may not always be
durable in some conditions has occurred over the last 25 years or so as the impact of
chlorides (from whatever source)on the long-term performance of structures has
become apparent. The problems can be particular lyacute on highway structures, such
as bridges, and has led to the development of a range of repair strategies to avoid
demolition and reconstruction of these structures. The problems of corrosion arising
from the use of de-icing salts can often occur within a relatively small fraction of the
intended design life (15 to 20 years out of an intended 120 years). The resultant
maintenance, repairs and monitoring of such structures is:
• Difficult
• Expensive
• Increasing unacceptable
CHLORIDE PROTCTION:

4. For the most part the approach is to attempt to improve the resistance of the concrete to
detrimental affects of chlorides by restricting the transport of chlorides through the
concrete to the steel reinforcement. In broad terms this is achieved by one or more of
the following:
• Increasing the thickness of cover to the reinforcement, thereby increasing the time
for chloride transport to the steel.
• Altering the specification of the mix to include cement alternatives that result
in a concrete that is less permeable to chlorides; thereby increasing the time for
chloride transport to the steel.
• Improving the overall quality of the concrete to ensure a low permeability to
chlorides.
• Treatment of the surface (using coatings or impregnation materials) to prevent
the ingress of water and chlorides into the concrete; such methods require future
maintenance.
Alkali-Aggregate Reaction :-
In most concrete, aggregates are more or less chemically inert. However, some
aggregates react with the alkali hydroxides in concrete, causing expansion and cracking
over a period of many years. This alkali-aggregate reaction has two forms—alkali-silica
reaction (ASR) and alkali-carbonate reaction (ACR).
Alkali-silica reaction (ASR) :
In ASR, aggregates containing certain forms of silica will react with alkali
hydroxide in concrete to form a gel that swells as it adsorbs water from the
surrounding cement paste or the environment. These gels can swell and induce
enough expansive pressure to damage concrete .
Alkali-carbonate reactions (ACR) :
Dedolomitization, the breaking down of dolomite, is normally associated with
expansion. This reaction and subsequent crystallization of brucite may cause
considerable expansion. The deterioration caused by ACR is similar to that
caused by ASR; however, ACR is relatively rare because aggregates susceptible
to this phenomenon are less common and are usually unsuitable for use in
concrete for other reasons.
ACID ATTACK:-

5. Concrete is susceptible to acid attack because of its alkaline nature. The
components of the cement paste break down during contact with acids.
Most pronounced is the dissolution of calcium hydroxide which occurs
according to the following reaction:
2 HX + Ca(OH)2 -> CaX2 + 2 H2O
(X is the negative ion of the acid)
Factors Affecting Acid Attack:-
The decomposition of the concrete depends on
•The porosity of the cement paste,
•On the concentration of the acid, Acids such as nitric acid, hydrochloric acid and
acetic acid are very aggressive as their calcium salts are readily soluble and removed
from the attack front.
•SULPHURIC ACID
Sulphuric acid is very damaging to concrete as it combines an acid attack and a sulfate
attack.
• The solubility of the acid calcium salts (CaX2) and on the fluid transport through the
concrete.
•Insoluble calcium salts may precipitate in the voids and can slow down the attack.
calcium salt, due to their low solubility, inhibit the attack by blocking the pathways
within the concrete such as interconnected cracks, voids and porosity
CONCLUSION:-
The best means of maximizing the probability that concrete will be durable is to
produce concrete that will provide the desired service for the desired service life in the
environment in which it will be placed and used. Every concrete mixture should be
proportioned in accordance with exposure conditions, construction considerations, and
structural criteria. Exposure to freezing and thawing, sulfates, deicing chemicals,
acids,varying moisture conditions, and abrasive loadings should all be considered when
selecting materials and proportions.
PREPARED BY:-SANA ADNAN.
ROLL # CE-107.