Generally, concrete suffers
from more than one causes of deterioration, which is generally seen in the form
of cracking, spalling, loss of strength, etc. It is now accepted that the main
factors influencing the durability of concrete is its impermeability to the
ingress of oxygen, water, carbon dioxide, chlorides, sulphates, etc.
Concrete, under ideal
conditions, protects embedded steel against atmospheric influences by denying
access to aggressive elements, such as moisture, air, chlorides, sulphates and
chemical fumes.
A detailed investigation of
deteriorated structures is essential before planning its remedial measures. The
investigations involve initial inspection, condition survey for cracks and
other defects, sampling, measurement of concrete cover and assessing the
material strength. The intensity of damage can be assessed on the data
collected through various investigations including Non Destructive techniques.
Cracks
Before hardening |
After hardening |
|
Drying
Plastic shrinkage
Plastic settlement |
Physical
Crazing
Aggregate shrinkage |
|
Constructional
Formwork movement
Sub-grade movement |
Chemical
Carbonation of concrete
Corrosion of steel Alkali - aggregate reaction |
|
Physical
Early frost damage
|
Thermal
Early thermal contraction
Insulation effects Freeze - thaw cycles |
|
|
Structural
Design loads
Creep Over loading |
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.
Conclusion and recommendation
Durability of concrete structures should be considered
as a significant aspect of structural design. Concrete technology plays a
significant role in ensuring durability of concrete structures. A designer
should be aware of the constructional aspects of structures, as well as, in
order to foresee durability problems due to any peculiarities of structural
loads, layout as well as environment.
The following recommendations may be adopted to ensure
safe and durable structures with trouble free long service life.
Concrete protects steel from corrosion only under
controlled conditions. Good quality concrete mix with the lowest water cement
ratio compatible with practical placement and finishing techniques should be
used. Concrete should be properly placed, consolidated and cured. Over
stressing of structures should be avoided.
Application of flexible surface coatings to avoid
concrete surfaces, which can effectively control the ingress of chlorides,
sulphates, carbon dioxide, oxygen and moisture, can be considered as an
effective corrosion control measure. However, the coatings should be applied
before structural deterioration occurs, and not afterwards, to be effective.
Exercising adequate care at every stage of planning,
analysis, design and construction for the expected exposure conditions can
effectively control corrosion of rebar. Corrosion retardant steels, coatings
for concrete and cathodic protection enhance durability of structures. However,
there is no substitute for well designed and well compacted concrete cover of
adequate thickness Concrete, under Meal condition, protects embedded steel to
ensure durable structures.
The performance of structures should be monitored
regularly from the stage of commissioning. Assessment of damage is the first
step in a structural repair project. A successful program of damage assessment
is often the key to cost effective repair system.
The response of the structural system to the changes
due to repair must be understood for successful rehabilitation program. It is
not possible to generalize rehabilitation schemes; each system has merits and
demerits. Depending upon the type of structure and the nature of distress,
various techniques in suitable combination may be adopted.
Trained supervision, workable specifications, speedy
site decisions and enough working area should be made available for
satisfactory, timely and efficient repairs.
For structure in non-corrosive environment, uniting is
usually adequate for rehabilitation: surface coating may be necessary to
protect concrete in corrosive environments.
A holistic approach should be adopted for structural
systems, wherein structural design and durability are considered together
rather than as separate entities.
Design for
durability of concrete structures
Control of deflection
The procedure for control of
deflection is to control span to effective depth ratio .it assumes that the
deflection of beam and slab will depend on the following factors.
1. The span/effective depth ratio
2. Type of supports as to whether simply supported , fixed or continuous
3. Percentage of tension steel or the stress level in the steel level at
service loads if more than the necessary steel is provided at the section.
4. Percentage of compression steel provided.
Span/effective depth ratio
to be used for beams and slabs with span less than 10m are given in the table
below
Design for limit state of deflection
Excessive deflection of beams and slab is not only an eyesore in itself but it can also cause cracking of
partion .As given in IS 456(2000) the commonly accepted limits of allowable
deflection are
1. A
final deflection of span/250 for the deflection
of horizontal bending members like slabs
and beam due to all load so as to be noticed by
the eye.
2. A deflection of span/350 or 20mm which is
less for these members after the construction of the partitions and finishes
etc,to prevent damages to finishes
and partitions.
The allowable crack width in concrete depends on the environmental
conditions to which it is exposed. According to IS456(2000)the values are shown
below
The three aspects of cracking are of importance. They are
1. Effect of cracking on the appearance
2. Corrosion
3. Stiffness of the beam
Structural cracking can be classified according to the cause of
cracking.These cracks may belong to any one of the following categories.
1. Flexural cracks.
2. Diagonal tension cracks
3. Spitting cracks along with reinforcement due
to bond and anchorage failure
4. Temperature and shrinkage cracks.
Method of limiting crack width
Under
service load the crack width in concrete should not be excessive. According to
IS456 (2000), under normal condition crack width at the surface of concrete
should not exceed 0.3mm for sake of appearance. In moderate exposure should be
limited to 0.2mm and severe exposure to 0.2mm for corrosion resistance.
Table
The 0.3mm of crack width can be generally met
by good practice of detailing reinforcement.
Method of crack control
To
control the crack width the important factors to be considered are the
following
1. Maximum and minimum spacing of
reinforcements
2. Maximum and minimum area of steel in the
member
3. Curtilment of reinforcement bars
4. Anchorage
5. Cover to reinforcement.
6. Maximum and minimum sizes of steel to be
used for the various types of steel in the member.
Bar spacing rules for beams
Considering factor 1.above namely bar spacing rules, the major
parameters that affect crack width in concrete beams are as follows
1. The distance of crack from the nearest
reinforcement bar spanning the crack
2. Distance from neutral axis of the cross
section
3. Mean strain at the level of the section
considered.
These
parameters can be related to the distribution of the reinforcement in the beam
in terms of the following factors
1. Maximum horizontal bar spacing
2. Minimum vertical and horizontal bar spacing
3. Arrangement of the side reinforcements for
members whose depth is larger than 750mm.
4. Corner distance to the nearest steel.
Minimum
bar spacing rule for beam
The diameter of a round
bar shall be its nominal diameters, and in the case of bars shall be its
nominal diameters, and in the case of deformed bars or crimped bars, the
diameter shall be taken as the diameter of a circle giving an equivalent
effective area.
The bar spacing should not be less than the diameter of the largest
bar and not less than the maximum size of aggregate plus 5mm