1 INTRODUCTION Reinforced cement concrete is one of the most widely used modern materials of construction. It is comparatively cheap and readily available and has a range of attractive properties and characteristics that makes it suitable for a variety of building and construction applications. It is also used in a range of exposure conditions. The long-term performance of RCC is usually assessed against two main criteria, serviceability and durability. Serviceability refers to the ability of the concrete to resist changes in its microstructure and properties, particularly where such changes may adversely affect the serviceability of the element perhaps the most obvious consequence of a lack of durability in reinforced concrete is the corrosion of the steel reinforcement, a topic that has been widely studied and reported. Corrosion of steel reinforcement in a concrete is an electrochemical process that requires access of an electrolyte and oxygen to steel. Protective measures against corrosion rely on minimizing or preventing the corrosive electrochemical process. Four types of protective measures as under can be identified: a) Impeding access of deleterious materials water, oxygen, salts, carbon dioxide etc. to the steel surface. b) Slowing the electrochemical process through use of inhibitors. 1
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1 INTRODUCTION
Reinforced cement concrete is one of the most widely used modern materials of
construction. It is comparatively cheap and readily available and has a range of attractive
properties and characteristics that makes it suitable for a variety of building and
construction applications. It is also used in a range of exposure conditions. The long-term
performance of RCC is usually assessed against two main criteria, serviceability and
durability. Serviceability refers to the ability of the concrete to resist changes in its
microstructure and properties, particularly where such changes may adversely affect the
serviceability of the element perhaps the most obvious consequence of a lack of
durability in reinforced concrete is the corrosion of the steel reinforcement, a topic that
has been widely studied and reported.
Corrosion of steel reinforcement in a concrete is an electrochemical
process that requires access of an electrolyte and oxygen to steel.
Protective measures against corrosion rely on minimizing or preventing
the corrosive electrochemical process. Four types of protective
measures as under can be identified:
a) Impeding access of deleterious materials water, oxygen, salts,
carbon dioxide etc. to the steel surface.
b) Slowing the electrochemical process through use of inhibitors.
c) Modifying the electrode through cathodic protection.
d) Providing coatings to the steel reinforcement.
1.1 CORROSION
The deterioration of a material, usually a metal, that results from a reaction with its
environment. Corrosion is the primary means by which metals deteriorate. Most metals
corrode on contact with water (and moisture in the air), acids, bases, salts, oils,
aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode
when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas,
and sulfur containing gases.
Steel embedded in concrete is normally protected from corrosion due to the presence of a
passive film on the surface of the metal. This films in the highly alkaline environment of
1
hydrated cement, with a PH in excess of about 13 and as long as the passive state is
maintained, the steel will not corrode, to ensure long term corrosion protection to the
steel, the concrete mass must be sufficiently impermeable so as to limit the transport of
species such as water, chloride ions, oxygen, carbon dioxide and other gasses through the
concrete to the depth of the reinforcement. The presence of critical levels of these
species, which are usually carried into the concrete in solution in water, either change the
nature of the concrete of alter the condition of the embedded steel. In either case,
corrosion of the steel can then initiate. Should corrosion of embedded steel in concrete
occur, physical damage to the concrete mass is likely to follow, steel corrosion products
are quite voluminous with an expansion factor of 2-10 times and typically precipitate at
the interface between the steel and the concrete. The swelling caused by this generates
stresses of sufficient magnitude about 3-4 MPa to exceed the tensile capacity of the
concrete and as a result the concrete cracks in tension. Such cracks usually run from the
bar to the nearest adjacent surface, which may be the edge of a column or precast element
or the surface of a slab or beam. Once cracking has occurred, unsightly rust staining of
the surface is often observed and further swelling usually leads to delamination of the
element or sapling of pieces of concrete from the surface, by this stage, the structure
would be in a serious state of distress, and remedial action would be in a serious state of
distress and remedial action would be necessary to extend its life. Corrosion induced
damage to RCC often necessitates early repair and occasionally complete replacement of
the structure or element well before its design life is reached. Worldwide, the costs
associated with such remedial work are massive and are expected to increase in the future
at an alarming rate. What has also become evident is that while the repair of RCC may
make good the surface deterioration of the problem. The circumstances that lead to the
initial onset of corrosion often survive in adjacent of more deeply buried regions and may
reveal themselves at some time in the future.
1.2 WHY DO METALS & ALLOYS CORRODE
Excepting for noble metals like gold and platinum etc. other occur in nature only as
compounds e.g. oxides, carbonates, & sulphides etc. and not as metals. This is because
such compounds are more stable compared to metal. In other wards compounds of metals
2
have lower energy as compared to metal itself. By spending energy the ore is converted
into metal which is then processed to yield a component or a structure. It is unstable
because it is energy rich and tends to revert back to more stable lower form namely a
compound like oxide or carbonate. This process is what we call corrosion.
1.3 CHOICE OF PROTECTION
There are many different ways of protecting steel but in general they
fall into two categories; metal coatings and organic coatings. Hot dip
galvanizing is a metal coating obtained by dipping steel or iron into a
bath of molten zinc. The iron and zinc react together to form alloy
layers which are covered by a coating of pure zinc as the work is
withdrawn from the bath. This gives an all over protection, inside and
outside, that resists knocks and abrasion yet has a probable life in
excess of 25 years. These are some of the reasons for choosing hot dip
galvanizing, but the deciding factor may well be financial or economic.
Before the economics of galvanizing can be compared with other
methods of corrosion protection, it is necessary to use the same units
of measurement. (Farm Building Research Team)
3
2 MECHANISM OF CORROSION
2.1 GENERAL
During hydration of cement a highly alkaline pore solution (PH between 13 and 13.8).
Principally of sodium and potassium hydroxides, is obtained. In this environment the
thermodynamically stable compounds of iron are iron oxides and oxy-hydroxides. Thus
on ordinary reinforcing steel embedded in alkaline concrete a thin protective oxide film is
formed spontaneously. This passive film is only a few nanometers thick and is composed
of more of less hydrated iron oxides with varying degree of Fe 2+ and Fe3+. The protective
action of the passive film is immune to mechanical damage of the steel surface. It can
however be destroyed by carbonation of concrete or by the presence of chloride ions, the
reinforcing steel is then depassivated.
2.2 INITIATION AND PROPAGATION OF CORROSION
The service life of reinforced concrete structures can be divided in two distinct phases.
The first phase is the initiation of corrosion in which the reinforcement is passive but
phenomena that can lead to loss of passivity e.g., carbonation or chloride penetration in
the concrete cover take place. The second phase is propagation of corrosion that begins
when the steel is depassivated and finishes when a limiting state is reached beyond which
consequences of corrosion cannot be further tolerated.
2.2.1 Initiation Phase
During the initiation phase aggressive substances (CO2 , chlorides) that can depassivate
the steel penetration form the surface into the bulk of the concrete.
Carbonation: Beginning at the surface of concrete and moving gradually towards the
inner zones, the alkalinity of concrete may be neutralized by carbon dioxide form the
atmosphere so that the PH of the pore liquid of the concrete decreases to a value around 9
where the passive film is no more stable.
Chloride Ions from the environment can penetrate into the concrete and reach the
reinforcement; if their concentration at the surface of the reinforcement reaches a critical
level, the protective layer may be locally destroyed.
4
The duration of the initiation phase depends on the cover depth and the penetration rate
of the aggressive agents as well as on the concentration necessary to depassivate the steel.
The influence of concrete cover is obvious and design codes define cover depths
according to the expected environment class. The rate of ingress of the aggressive agents
depends on the quality of the concrete cover (porosity, permeability) and on the
microclimatic conditions (wetting, drying) at the concrete surface. Additional protective
measures can be used to prolong the initiation phase.
2.2.2 Propagation Phase
Breakdown of the protective layer is the necessary prerequisite for the initiation of
corrosion. Once this layer is destroyed, corrosion will occur only if water and oxygen are
present on the surface of the reinforcement. The corrosion rate determines the time it
takes to reach the minimally acceptable state of the structure bur it should be borne in
mind that this rate can vary considerably depending on temperature and humidity.
Carbonation of concrete leads to complete dissolution of the protective layer Chlorides
instead cause localized breakdown, unless they are present in very large amounts.
Therefore:
Corrosion induced by carbonation can take place on the whole surface of steel in
contact with carbonated concrete.
Corrosion by chlorides is localized with penetrating attacks of limited area surround
by non-corroded areas. Only when very high levels of chlorides are present the
passive film be destroyed over wide areas of the reinforcement and the corrosion will
be of a general nature.
If depassivation due to carbonation or chlorides occurs only on a part of the
reinforcement, a macro cell can develop between corroding bars and those bars that are
still passive (and connected that is already corroding.
In structures affected by electrical fields, DC stray current in the concrete can enter the
reinforcement in some areas i.e. it passes from the concrete to the steel and return to the
concrete in a remote site. The passive layer can be destroyed in those areas where the
current leaves the steel.
On high-strength steel used in prestressed concrete but not with common reinforcing steel
under very specific environmental, mechanical loading, metallurgical and
5
electrochemical conditions, hydrogen embitterment can occur which may lead to brittle
fracture of the material.
2.3 CARBONATION INDUCED CORROSION
2.3.1 Carbonation Of Concrete
In moist environments, carbon dioxide present in the air forms an acid aqueous solution
that can react with the hydrated cement paste and tends to neutralize the alkalinity of
concrete ( this process is known as carbonation). Also other acid gases present in the
atmosphere, such as SO2 can neutralize the concrete’s alkalinity but their effect is
normally limited to the surface of concrete.
The alkaline constituents of concrete are present in the pore liquid (mainly as sodium and
potassium hydroxides) but also in the solid hydration product. Calcium hydroxide is the
hydrate in the cement paste that reacts most readily with CO2. The reaction that takes
place in aqueous solution can be written schematically as