Behaviour of concrete
The behaviour of concrete depends
on several processes, i.e. Physical, Chemical and Biological. These processes
bring changes in material composition and performance due to transport of water
and dissolved deleterious agents within the concrete. Moisture and its transport
within the pores and cracks of concrete control the physical and chemical
processes that lead to structural deterioration.
1.
Physical process
Physical processes lead to gradual
deterioration of concrete, and govern its long-term behaviour.
Cracking: Concrete cracks whenever tensile
strains exceed its tensile strain capacity. Cracks may occur in green concrete
due to plastic shrinkage, settlement of forms and support movements. The
hardened concrete cracks due to loading, drying shrinkage, chemical and thermal
effects. The reason for crac-king of concrete are given in the table.
Abrasion: The movements of person and
traffic on concrete surfaces cause abrasive wear. Industrial floor and bridge
deck slabs are subjected to abrasive wear. In the case of hydraulic structures,
bridge piers and abutments, water flowing against surfaces causes wear due to
suspended particles.
Frost
& de-icing salts:
The transition of water from liquid state to solid state due to icing involves
an increase in volume by about 9%. In the porous concrete, (he freezing of
water induces splitting forces. Several cycles of freezing and thawing of water
may result the in spelling of concrete. The frost resistance of concrete
depends upon several parameters, such as age of concrete, composition,
aggregate type, pore size distribution, rate of cooling and drying between
freeze-thaw cycles.
2.
Chemical process
Chemical processes govern the rate
of decomposition of concrete, and thus its durability. The reaction involves
movement of reaction substances within concrete or from atmosphere to concrete.
The process depends on the nature of chemicals, pore structure and ambient
temperature as well as characteristic of concrete.
Acid
attack: Acid
attack involves conversion of calcium compound to calcium salts after attacking
acid. The structure of the hardened concrete destroyed by acid attack, the rate
of deterioration depends not only on the strength of the reactants but also
upon the solubility of the resultant salts and their transport. The acids
destroy concrete by converting hardened concrete, and its pore system.
Impermeability of concrete is of little consequence in this case.
Sulphate
Attack: Sulphate
attack on only aluminate compounds, calcium and hydroxyl of hardened Portland
cement forming ettringite and gypsum. In the presence of sufficient water,
these reactions of delayed ettringite formation cause expansion of concrete
leading to irregular cracking. The cracking of concrete provides further access
to penetrating substances and to progressive deterioration. The effects of
sulphate on concrete depend upon the severity of attack, accessibility (Permeability
and Cracking), presence of water and susceptibility of cement- Concrete can be
protected against sulphate attack by limiting the aluminates between 3 to 8%.
Blended cements perform better than
Ordinary Portland Cement, when subjected to sulphate attack. Pozzolanaic
materials such as fly ash, silica fume, rice husk ash provide moderate
resistance.
Alkali
attack: Alkalis
react with silica containing aggregates and not with cement. The pore solution
in concrete is lime-saturated and contains potassium and sodium ions. Free
alkalis present in cement dissolve in the mixing water and forming a caustic
solution, which attack the reactive silica in the aggregate. The alkali silica
gel so formed swells in the presence of moisture, and exerts osmotic pressure
on the concrete internally. This may result in pattern cracking and loss of
strength, particularly in thin section. Besides alkali-silica reactivity,
carbonate minerals may also cause deterioration of concrete due to alkali
attack. However, alkali-carbonate reactivity is mil as common as alkali-silica
reactivity.
3.
Biological process
Plant roots penetrating cracks and
other weak spots may cause mechanical deterioration of concrete; the resulting
bursting forces may widen the existing cracks and cause spalling of concrete.
In the case of sewers and biogas plants, the hydrogen sulphide produced in the
anaerobic conditions may be oxidized in the aerobic conditions and form
sulphuric acid, which attack concrete above the water level.
Environmental
factors
The service life of the concrete
structures depends on the environmental factors as well. The nature, intensity
and timing of environmental influences affect the behaviour of materials. The
permeability of concrete, concrete cover, structural form, type and location of
reinforcement and nature of cement and aggregates determine the response of
concrete to environmental influences.
1.
Exposure conditions
There is no standard way of
classifying climate to define the response of concrete and reinforcement. The
general guideline for classification of exposure conditions are as given below:
Mild
Conditions: The
mild conditions, where the relative humidity does exceed 60% for most part of
the year (not more than 3 months). Moderate Conditions : These conditions
include interiors of building with high relative humidity, or subjected to
corrosive vapors. Submerged structures or structures coming in contact with
flowing water or regions of heavy rainfall without heavy condensation of
aggressive gases come under moderate conditions.
Severe
conditions:
Exposure to slightly acidic liquids, saline or oxygenated water, corrosive
gases and aggressive soils constitute severe conditions for concrete
structures.
Very
severe Conditions:
Exposure to seawater spray, corrosive fumes, industrial atmospheric and severe
freezing conditions can be categorized as severe conditions of exposure.
Extreme Conditions: These include
tidal zone and direct contact of liquid or solid aggressive chemicals.
2.
Temperature and humidity
The ambient temperature and
humidity influence the rate of chemical reactions. An increase in temperature
of 10°C. the rate of reaction is approximately doubled. The main parameters for
determining the aggressiveness of atmosphere are moisture, ambient temperature
and aggressive substances available in moisture. Carbonation of concrete lakes
place rapidly, when the relative humidity is around 50-60%. The rate of
corrosion is maximum, when relative humidity is 90-95%. The rate of corrosion
is independent of humidity, in the presence of chloride.
Water: Water is essential for most of the
processes leading to concrete deterioration. Constant wetting and drying is
more detrimental to concrete than submerged conditions. The concentration of
aggressive substances in the pore structures increases as a result of cyclic
wetting and drying leading to corrosion. The splash zone and tidal zone of
marine structures are more prone to corrosion than submerged zone.
Aggressive
elements:
Aggressive elements in nature
include water and air. The usual substance present in water and their actions
detrimental to concrete are listed below.
·
Oxygen
dissolved in water is essential for corrosion of embedded steel
·
Carbon
dioxide leads to carbonation of concrete and subsequently reduction in its
ability to protect embedded steel
·
Chlorides
cause corrosion of embedded steel
·
Acids
in water dissolve cement and change its pore structures leading to further
deterioration
·
Alkalis
in water promote reactivity with silica aggregates
·
Sulphates
react with cements and cause its expansion
·
Aggressive
fumes from industrial processes may attack concrete.
Marine
conditions: Marine
conditions are more severe than those occurring on land. Seawater contains
MgCI2, MgS04, CaSO4, KCI, K2SO4. The mean concentration of these salts is about
35 gm/L. Apart from these salts, sea water also contains' dissolved oxygen and
carbon dioxide to add to corrosive process. The marine, environment may be
classified in four zones according to exposure conditions:
·
Marine
Atmosphere Zone: In this zone,
concrete is not exposed to sea water directly, but comes in contact with
salt-laden mist.
·
Splash
Zone: This zone lies above high tides but is subjected to direct wetting by sea
waves and spray.
·
Tidal
Zone: The zone between high and low tide is termed tidal zone.
·
Submerged
Zone: Concrete in the submerged zone or on the sea beds.
Causes
of deterioration:
Concrete normally provides
excellent corrosion protection to embedded reinforcement. The high alkalinity
of concrete, i.e. above pH 12.5, results in the formation of protective oxide
film on steel bars. However, unless concrete is well compacted and dense, it is
susceptible to carbonation, and looses its capacity to protect reinforcement.
Some of the causes for deterioration of concrete structures are discussed here.
Design
and construction defects
Design of concrete structures,
including detailing of reinforcement, governs the performance of structures to
a considerable degree. Structures that are correctly designed and have good
workmanship develop narrow cracks, as compared to poor design/workmanship.
The quality of form work also helps
in quality of concrete. The beam-column junctions are particularly prone to
defective concrete, if reinforcement detailing is improper or fabricated
carelessly.
Concrete cover is also very
important parameter, which help in protection of reinforcement from corrosion.
It is essential to ensure adequate concrete cover, depending upon the
aggressiveness of the environment. Cracks in reinforcement concrete structure
can also result from design deficiencies.
Poor
quality materials
The specified quality of materials
should be ensured by frequent tests on cement, aggregates and water.
Alkali-aggregate reaction and sulphate attack results early deterioration.
Salinity in sand causes deterioration of concrete and reinforcement corrosion.
Clayey material in fine aggregate weakens the mortar-aggregate bond, and
reduces concrete strength.
Inadequate
supervision
It is essential to ensure dm the
minimum specification of concrete mix and construction practice are satisfied.
Environment
The root causes of deterioration in
aggressive environment are the development of cracks and high porosity and
permeability of concrete. The design of structures should consider
environmental factors as well and not strength alone.
Corrosion
of reinforcement
Due to protection loss of concrete
protection, steel bars embedded in concrete are also prone to electro-chemical
effects. Corrosion affects structures in two ways. Firstly, the product of
corrosion occupy a larger -volume than that of the steel destroyed and exert
pressure on surrounding concrete causing cracking and spalling. Secondly, die
area of effective steel reduces due to corrosion or migration of ions, and in
course of time, area of steel may not be adequate to resist me imposed loads.
Inadequate
understanding of materials
Concrete technology and structural
design should not be separated, but unified in order to obtain durable
structures of adequate safely margin. In most of me cases, ductile material
with low Young's Modules is required in order to control early cracking of
concrete. In the absence of such an ideal material, the use of surface coatings
is recommended for durable structures.
Technological
factors
The techniques of concrete
manufacturing, handling and processing influence the quality of concrete
significantly. The technological factors responsible for structural
deterioration are given here.
·
Characteristics
of concrete making materials and the deleterious substances present in them
·
Concrete
mix proportions
·
Water-Cement
ratio
·
Cement
content of concrete
·
Water
content of the mix
·
Admixtures
·
Workmanship
in mixing, placing, compaction and curing of concrete
The right time measure to be taken
to prevent die corrosion of reinforcement in concrete is during the design and
construction stages of structures. The basic principle of prevention of
corrosion is to maintain the passivity of the embedded steel;it is obvious mat
the permeability of concrete is key to control me various process involved in
the phenomenon.
Low permeability can be achieved by
adopting tow water-cement ratio, adequate cement content, blended cements
suitable admixtures, and proper control on size grading and quality of
aggregates.
Proper compaction and curing of
concrete are also essential. Some of these measures to be considered at the
design and construction stages are discussed here briefly.
Concrete
The durability is governed by the
quality of concrete. The manufacturing process of concrete plays a significant
rote in assuring me structural durability.
Water
cement ratio
Water cement ratio influences the
permeability of concrete, and should be decreased with increasing environmental
aggressivity. Cement content of concrete is of lesser significance than
water-cement ratio for structural durability, provided the mix of adequate
workability. The water-cement ratio should be lying tome range of 0.55 to 0.4.
Depending on the aggressiveness of the environment.
Cement
content
It is possible to obtain the
required strength of concrete by adopting higher grades of cement. According to
IS 456:1999, the minimum cement content for plain concrete must be 220 Kg/Cum
for mild exposure, whereas 300 Kg/Cum reinforced concrete requires minimum
cement content 300 Kg/ Cum for mild exposure conditions. For extreme
environment condition, minimum cement content may go up to 375 Kg/Cum.
Curing
The strength and permeability of
the cover-concrete can only be achieved if concrete is cured adequately. The
exposed surfaces of concrete should be kept continuously wet for at least 7
days from the date of placing concrete for proper curing. However, longer
curing periods, up to 28 days, are recommended for blended cement.
Steel
Steel is prone to corrosion when
not protected adequately. Corrosion mechanism and process are governed by
several parameters and require a multi directional approached to prevent
deterioration of corrosion structures. Some of corrosion prevention methods are
given below;
·
Metallurgical
methods
·
Corrosion
inhibitors
·
Coating
to reinforcement
·
Cathodic
protection
·
Corrosion
retardant steel
·
Coating
to concrete
Cover
concrete
The concrete cover should be dense,
strong, impermeable in order to resist the ingress of deleterious substances.
The IS 456: 1999, specifies concrete cover 20 mm for mild exposure conditions
increasing to 75 mm in extreme conditions.
Planning
and construction details
Architectural planning and
constructional details often determine the durability of structures. Attention
to small and simple details of structural components prevents possible local
deterioration of materials and subsequent effects on structure performance. It
should be noted that, the exposed surface should be of simple profile to avoid
local deterioration. Complex details often lead to maintenance problem later.
Drainage
of water
It is important to note that water
is essential to cause structural deterioration- Properly drained surfaces, with
no possibility of water stagnating, enhance structural durability. The drained
water should not How against the structure at the outlets.
Structural
design
Structural design Structural
cracks, even if they are not detrimental to structural performance under loads,
affect durability Sudden changes in cross section should be avoided.
Differential settlement and thermal effects should be considered in the design
to avoid inexplicable cracking.
Constructional
aspects
During construction, proper
attention should be made at the time of positioning the reinforcement, so that
its usability is to its optimum level.
Accessibility
and maintainability
The designer should consider
accessibility of various structural components, their reparability and
replaceability, and incorporate suitable measures. Lack of accessibility
hampers inspection, and may lead to avoidable excessive repairs at a later
date. Buried components of structures (footing and piles) cannot be reached or
inspected after construction. Such inaccessible components require greater
attention and care at construction stage itself from other components.
Replaceability
Structural components such as
joints, seals, drainage system and water proofing treatments, can be replaced
later on, if necessary. These components should be planned for easy replacement
without damaging the adjacent structural component.
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