Concrete is the most extensively used construction material in the construction of various structures such as bridges, buildings

and precast products pipes, poles, sleepers etc. From the past and even at the present times, too much emphasis is on concrete
compressive strength rather than environmental factors, which is known as durability.
This is one of the main reasons for serious deterioration of concrete structures that is prevalent today.
Maintenance, repair and strengthening of constructed facilities/infrastructures is presently the most significant challenge facing
the concrete industry
Definitions:

1 DEFECTS: These are the flaws that are introduced through poor design, poor workmanship before a structure begins its design
life or through inadequate operation and maintenance during its service life .

2 REPAIR: Process of reconstruction and renewal of the existing buildings, either in whole or in part .
OR
To bring back the position of the structure to its previous condition so it gives performance same as previously. It doesn’t cover
the strength aspect of the structures.

Some examples of repair:

Decoration of structure, Painting, White Washing.
Checking the wiring of building.
Replastering of any wall if required.
Repairing of damaged flooring Repair of door and window .
Checking or repairing of pipe line connections, gas line connections and plumbing serveries.
Relaying disturbed roof tiles.
3 Renovation: Process of substantial repair or alteration that extends a building’s useful life.
4 Remodeling: Essentially same as renovation – applied to
residential structures.
5 Rehabilitation: An upgrade required to meet the present
needs – being sensitive to building features and a sympathetic
matching of the original construction or the process of
repairing or modifying a structure to a desired useful condition.
or
Rehabilitation of a building means returning a building or a
structure to a useful state by means of repair, modification, or
alteration. It is related to the strength aspect of structures. To
Bring back the position and condition of the structure by
considering the strength aspect.
Some of the examples of Rehabilitation

To fill the wide cracks using some suitable material .

Injecting epoxy like material in to cracks in walls,columns,beams, etc.
Removal of damaged portion of masonry and reconstructing it using rich mortar mix. Addition of reinforcing mesh on both sides
of the wall.

6 Restoration: The process of re-establishing the materials, form and appearance of a structure.

7 STRENGTHENING: The process of increasing the load-resistance capacity of a structure or portion.

8 RETROFITTING: The process of strengthening of structure along with the structural system, if required so as to comply all
relevant codal provisions in force during that period.

9 DEMOLITION: The process of pulling down of the structure not deemed to be fit for service.

Need for Repair and Rehabilitation of Structures: The extent of deterioration to concrete structures globally is occurring at an
alarming rate. It is now being confirmed that even if the structural design abides by all the specific building code requirements like
the concrete quality, cover etc., there is still an acceptable high risk of deterioration of concrete and corrosion of reinforcement.
Steel corrosion is found to be most severe cause of deterioration of reinforced concrete that can create cracks, spalls the concrete
cover, reduce the effective c/s area of the reinforcement and lead to collapse.
3 Renovation: Process of substantial repair or alteration that extends a building’s useful life.
4 Remodeling: Essentially same as renovation – applied to residential structures.
5 Rehabilitation: An upgrade required to meet the present needs – being sensitive to building features and a sympathetic
matching of the original construction or the process of repairing or modifying a structure to a desired useful condition.

Rehabilitation of a building means returning a building or a structure to a useful state by means of repair, modification, or
alteration. It is related to the strength aspect of structures. To Bring back the position and condition of the structure by
considering the strength aspect.
CRACKING- Cracking in concrete is inherent. Classification of cracks
(Based on width)
Type of structure and nature of cracking is the major concern.
Cracks in the concrete does not always mean that the structure is
unusable. Type Width
Structural Cracks: Structural cracks are those that may occur
due to deficient designs, overloading, abnormal vibrations, use
of inferior quality materials, foundation placed on
Thin < 1 mm
uncompacted/loose soils, adoption of improper construction
practices, poor workmanship, etc
Medium 1-2 mm
Non-Structural Cracks- These cracks occur due to the Wide > 2 mm
internally induced stresses in building material or due to the
temperature induced movement of the materials. These cracks
mar the appearance of the structure and at time may give a
feeling of instability.
Internal stress in Building component:
Compressive
Tensile
Shear
Building material
1. Masonry, Concrete, Mortar
2. Weak in tension/shear
3. Causing tension/shear crack
COMMON SIGHT OF CRACK: CLASSIFICATION OF CRACKS

Vertical Straight Uniform Structural crack Non structural

Horizontal Toothed throughout crack
Diagonal Stepped Narrow at Incorrect design Internal induced
Map one end Faulty construction stress in building
pattern and material
Overloading
gradually
Random widening at
the other
INVESTIGATION RELATING TO CRACKS LIMITATION OF CRACK WIDTH

• Whether the crack is old or new. 1. For members in water storage units, sewage
units, chemically hazardous atmosphere cracks
• Pattern of the crack. are not permitted.

• Soil condition, type of foundation used, 2. In severe atmosphere up to 0.1mm crack

movement of ground if any. width is permitted.

•Observation on the similar structure in the 3. Moderate atmosphere upto 0.2mm crack
same width is permitted.
locality.
4.In mild atmosphere width of crack is
•Study of specification, construction method permitted upto 0.3mm .
and
climatic conditions.
PERMISSIBLE CRACK WIDTH AS PER ACI
CAUSES FOR THE OCCURANCE OF
EXPOSURE MAXIMUM ALLOWABLE CRACK
CONDITION CRACK WIDTH IN
mm Crack may develop due to :-

Dry air, protective 0.41

1. Structural deficiency resulting from
membrane design deficiency or construction
Humidity, moist air 0.30 deficiency and overloading.

2. Temperature and shrinkage effects.

Sea water and 0.15 3.Settlement of ground.
seawater spray, wetting
and drying 4. Faulty workman ship and poor
construction practice .
Water retaining 0.10
structure
1.STRUCTURAL DEFICIENCY RESULTING FROM DESIGN
DEFICIENCY OR CONSTRUCTION DEFICIENCY AND FLEXURAL CRACKS IN BEAMS
OVERLOADING
• Occurs due to flexural steel deficiency.
CRACKS OCCUR DUE TO:
• Shear, flexural and torsional steel deficiency. • Occurs at maximum bending moment region
• Abrupt curtailment of reinforcement bars.

• Overloading of member.

• Improper anchorage

FLEXURAL CRACKS IN CANTILEVER BEAMS

• Occurs due to shear steel deficiency.

•Occurs in maximum shear region.

 PREVENTIVE MEASURES

• Special care need to be taken

while designing and
detailing.

• Requires continuous investigation.

• Damages from unintentional

construction overloads can
be prevented only if designer
provide information on load
limitations and the construction
personnel heed to these
limitations.
2 SHRINKAGE AND TEMPERATURE
EFFECT:
CRACKS DUE TO SHRINKAGE

•Show up in two basic location in most walls,

approximately mid point of long section wall ,
across door or window head.
•Uniform in width.
• Excessive water content within the concrete.
• Higher water content results in greater
SHRINKAGE CRACK IN WALL MASONRY
shrinkage .
•On exposure, concrete loses some of its
original water and shrink.
PREVENTIVE MEASURES
• Minimise the use of rich
concrete mix.
• Use lean cement mortar in
masonry works.
• Allow adequate time for
curing .
TEMPERATURE EFFECT
• Volume changes .
•Volume relation to temperature is expressed
by coefficient of thermal expansion/contraction.
•Volume change induces stress.
•PREVENTIVE MEASURES
•Adequate insulating or terracing
treatmemt.
•Painting top roof with reflective finish
such as white wash. THERMAL CRACKS IN WALL
•Introducing of expansion and MASONRY
contraction joint at apropriate locations.
3 CRACK S DUE TO SETTLEMENT:
•Uneven settlement can be a major structural
problem in small residential building.
•Vertical distortion or cracking of masonry walls,
wrapped interiors and exterior opening.
•Occurs early in life of building.
SETTLEMENTS ARE CAUSED DUE TO
• Soil consolidation under footing.
• Loss of moisture.
• Water table level.
• Faulty drains, leaking water mains. Building settlement due to cut and fill
• Soil compaction or movement due to
vibration .
PREVENTIVE MEASURES
• Under reamed pile foundation.
• Foundation design for uniform distribution of
pressure.
• SBC is not exceeded.
• Soil should be well compacted.

Differential settlement caused due to variable soil

Possible Causes of Damage : Swelling of formwork Mechanism:
Pre-Construction stage: • Formwork absorbs moisture from concrete or the
• Poor Design atmosphere, which results in swelling of form.
• Poor Design Detailing • Crushing of wale in the formwork also causes movements of
• Poor Deflection Estimations forms. These result in cracks in the concrete while setting.
• Faulty Design of Rigid Joints in Precast Elements Preventive Measures :
• Faulty Design Estimations at changes in section Coating of the formwork with moisture resistant material.
Preventive Measures: Using unyielding lateral ties with good end anchorage
Through careful design by experienced design engineers
Internal settlement of cracks Mechanism:
Construction stage : Differential settlement between the surface and the interior
• Local settlement of Subgrade Mechanism volume of the concrete suspension causes surface cracks.
• Pouring fresh concrete some-times may cause subgrade Concrete on the surface sets faster than the interior
below it to compress or settle. suspension.
• Uneven stresses thus created cause cracks in the concrete. Preventive Measures :
Preventive Measures :’ Surface cracks can be cured and closed by delayed finishing.
• Pour concrete on compacted subgrade to prevent cracking. Curing of concrete must start immediately after casting to
If the subgrade is not compacted, the soil, and concrete delay setting of the surface concrete.
above it, will settle and cause the slab to crack. Good compaction will also help prevent this defect.
• Most rental companies have equipment available to
properly compact the subgrade, and it is well worth the
investment.
 Vibrations Mechanism:
Vibrations due to indiscreet walking over concrete and
Setting shrinkage Mechanism: dumping construction materials, etc., can also lead to cracking
While setting the concrete shrinks giving rise to surface Preventive Measures :
cracks resembling the scales of the alligator. Workers have to be trained in avoiding such carelessness
Preventive Measures : Adding Excess water to Concrete mix: Water that is added to
Good and timely curing will help avoid this type of damage. increase slump decreases durability.
Excess water added during finishing causes scaling, crazing and
dusting of concrete.
Improper curing: Curing is the most abused aspect of concrete
construction.
Improper curing causes cracking and surface disintegration.
It may also lead to structural cracking.
Curing of concrete, if not started soon after its placement,
results into setting of the surface concrete that leads to
differential settlement.
Improper timing of finishing of concrete: Finishing the surface
too soon, i.e., toweling when the bleed water is still there leads
to formation of cold joints.
Inadequate number of joints: Inadequate number of
contraction joints or fails to make expansion joints wide
enough to accommodate the temperature expansion results in
severe damage.
Post-Construction stage : Corrosion of Reinforcement Causes:
Temperature Stresses Corrosion of reinforcement bars can be due to: Entry of
Corrosion of steel moisture through cracks, availability of oxygen and moisture at
Aggressive action of chemicals rebar level, carbonation and entry of acidic gaseous pollutants
Weathering action that reduce the pH of concrete, ingress of chloride ions, relative
Overloading humidity & electrochemical action.
Moisture effects Preventive measures:
Natural disasters Seal the crack before it reaches the reinforcement bar
Fire Protect against corrosive chemical action by
Temperature Stresses Causes: i. Keeping structures clean
Cracks in concrete can be produced due to temperature ii. Painting
stresses due to: iii. Prevent from absorbing moisture
i. Difference in temperature inside and outside the building iv. Provide bituminous or zinc coatings.
ii. Variation in the internal temperature due to heat of v. Encase using fibre wrapping systems
hydration Proper finishing
Mechanism: The temperature difference within concrete
structure results in differential volume change.
When the tensile strain due to differential volume change
exceeds the tensile strain capacity of concrete, it cracks.
Preventive measures:
The finishing of the surface should be such that it reflects solar
radiation and not absorbs it.
Good concrete mix with low heat of hydration
Allowing for movements by using properly designed
contraction joints
Early Frost Damage:
When fresh concrete is exposed to extremely low
temperatures, the free water in the concrete is cooled below
its freezing point and transforms into ice, leading to a decrease
in the compressive strength of concrete. When freezing takes
place after an adequate curing time, the decrease in
compressive strength does not occur.
Early thermal contraction:
Fresh concrete undergoes temperature rise due to cement
hydration. When concrete is cooling to the surrounding ambient
temperature in a few days, the concrete has very little
tensile strength.
Weak tensile strength + thermally contracting concrete = tension
cracks
Plastic Deformation Shrinkage Cracks:
• Plastic shrinkage cracks appear in the surface of fresh
concrete soon after it is placed.
• These cracks appear mostly on horizontal surfaces, and are
usually parallel to each other 1-3 feet apart, shallow and not
reaching the perimeter of the slab.
Mechanism of shrinkage Cracks:
• Rapid loss of water from the surface of concrete before it
has set causes these cracks.
• It is critical when rate of evaporation of surface moisture
exceeds the rate at which rising bleed water can replace it.
Water receding below the concrete surface forms menisci
between fine particles of cement and aggregate causing a
tensile force to develop in the surface layer.
Settlement (subsidence) Mechanism:
Plastic settlement is caused due to bleeding, which refers to
the migration of water to the top of concrete and the
movement of solid particles to the bottom of fresh concrete.
Causes of Plastic Deformation:
• Poor construction practices
• Low sand content and high water content
• Large reinforcement bars
• Poor thermal insulation
• Restraining settlement due to irregular shape
• Excessive, uneven absorbency
• Low humidity
• Insufficient time between top-out of columns and
placement of slab and beam
• Insufficient vibration
• Movement of formwork

Remedial Measures
• Use the largest possible coarse aggregate.
• Ensure the coarse aggregate is evenly graded.
• Use less water in the concrete mix (but beware the effect on
workability and finishability
Construction Movement: Form movement
Causes for deterioration of concrete structures :
PHYSICAL CAUSES
1.SHRINKAGE 2.CRAZING
CHEMICAL CAUSES:
1. Acid Attack 2. Aggressive water Attack Alkali carbonate rock reaction 3. Alkali silica reaction 4. Alkali Aggregate reaction 5.
Sulphate Attack
THERMAL CAUSES
1. FREEZING AND THAWING 2. TEMPERATURE
STRUCTURAL CAUSES
1. IMPROPER DESIGNS 2. ACCIDENTAL LOADINGS 3.CREEP

PHYSICAL CAUSES
SHRINKAGE :Shrinkage is defined as the volume changes in concrete due to loss of moisture from concrete due to evaporation
or by hydration of cement.
Shrinkage can be classified in to following categories
1. Plastic shrinkage
2. Drying shrinkage
3. Autogenous shrinkage
4. Carbonation shrinkage
1. Plastic shrinkage: The concrete will exhibit bleeding to some degree between placing and setting time is called plastic
shrinkage. Bleeding is the appearance of moisture on the surface of concrete; caused by the settling of the heavier
components of the mixture. Usually, the bleed water evaporates from the concrete surface.
 HARDENED STATE OF CONCRETE:
Physical Cause
1. Aggregate Shrinkage:
Mechanism :
• Some rocks exhibit the property of absorbing water with
attendant change in dimension.
• The shrinkage that occurs as the aggregate dries up is called
aggregate drying shrinkage.
• Change in volume of aggregate induces cavities and leads to
shrinkage, weakening of compressive strength.
Remedial Measure :
Choose aggregate which do not have these problems.

Drying Shrinkage
Mechanism:
• On exposure to the atmosphere, concrete loses some of its
original water through evaporation and shrinks.
• Normal weight concrete shrinks from 400 to 800
microstrain. One microstrain is equal to 1 X 10-6 in./in.
• If unrestrained, results in shortening of the member
without a build-up of shrinkage stress.
• If the member is restrained from moving, stress build-up
may exceed the tensile strength of the concrete. this over
stressing results in dry shrinkage cracking
A typical plastic shrinkage cracks occurred due to:
Drying shrinkage:
Rapid evaporation of water from the surface of concrete. The loss of moisture after setting is called drying shrinkage. It
Occurs within few hours after placing concrete while still it is in is the long term change in volume of concrete.
plastic and before it has attained sufficient strength. If this shrinkage could take place without any restraint, there
These cracks occur almost entirely on horizontal surfaces would be no damage to the concrete.
exposed to atmosphere. The combination of shrinkage and restraints causes tensile
These cracks are parallel to one another are spaced 0.3m to stresses that can ultimately lead to cracking.
0.1m apart and width varying from 0.1mm to 3mm.
A drying shrinkage cracks occurred due to :
Elimination of plastic shrinkage cracks:
Plastic shrinkage cracks can be eliminated by following These cracks is caused by physical loss (evaporation) and
measures chemical loss(hydration) of water during the hardening process
Reduce the time between placing and finishing. If there is and exposure to unsaturated air.
delay cover the concrete with polythene sheets.
Minimize evaporation by covering concrete with fog spray and Reduction in volume of concrete can cause cracks if it is
curing compounds. restrained and its tensile strength exceeded.
Erect temporary roof to protect concrete from hot sun.
These cracks appears at about 7-10 days after concreting and
about 80% of drying shrinkage take place in about a year.

Controlling drying shrinkage cracks :

Use of minimum water content.
Use of highest possible aggregate content.
Providing adequate and early curing.
3. Autogenous shrinkage :
Autogeneous shrinkage cracks occurred due to :

If no movement of water to or from set paste of concrete is allowed, then the shrinkage developed is known as autogeneous
shrinkage.

This shrinkage is caused by the loss of water consumed or used in the hydration of cement.

Controlling Autogenous shrinkage cracks:

Consider a higher content of supplementary cementitious material like Fly ash and Ground granulated blast furnace Slag (GGBS)
in the concrete mix.
Keep the surface of the concrete continuously wet; conventional curing by sealing the surface to prevent evaporation is not
enough and water curing is essential.
Consider addition of shrinkage-reducing admixtures more commonly used to control drying shrinkage,
Consider addition of saturated lightweight fine aggregates

4. Carbonation shrinkage :
Carbonation is the reaction of CO2, which is present in the atmosphere with hydrated cement. The CO2 in presence of moisture
forms carbonic acid that reacts with calcium hydroxide - Ca(OH)2, a product of hydration to form Calcium Carbonate (CaCO3).

Carbonation shrinkage is probably caused by the dissolution of crystals of calcium hydroxide and deposition of calcium carbonate
in its place. The carbonation proceeds from the surface of concrete inwards, but does so extremely slowly.
The actual rate of carbonation depends on the permeability of the concrete, its moisture content and on the CO2 content and
relative humidity of the ambient medium
FREEZING AND THAWING:
Freeze-thaw disintegration or deterioration takes place when Preventive Measures:
the following conditions are present.
(a) Freezing and thawing temperature cycles within the 1. Use of lowest practical water-cement ratio and water
concrete. content.
(b) Porous concrete that absorbs water (water-filled pores and 2. Use of air entrainment.
capillaries) 3. Use of durable aggregate.
4. Adequate curing of concrete prior to exposure to freezing
Mechanism : conditions.
5. Designing the structure to minimize the exposure to
As the temp. of a critically saturated concrete is lowered during moisture.
cold weather, the freezable water held in the capillary pores of
the cement paste and aggregates expands upon freezing. If
subsequent thawing is followed by refreezing the concrete is
further expanded, so that repeated cycles of freezing and
thawing have a cumulative effect.
Concrete hydraulic structures are vulnerable to freezing and
thawing. Exposure in such areas as the top walls, piers,
parapets and slabs enhances the vulnerability of concrete to
the harmful effects of repeated cycles of freezing and thawing.
The use of de-icing chemicals on concrete surfaces may also
accelerate damage caused by freezing and thawing and may
lead to pitting and scaling
 CRAZING :
WEATHERING :
Crazing is the development of fine random cracks on the
It is defined as change in colour, texture, strength,
surface of the concrete caused by shrinkage of the surface
chemical composition, or other properties of a natural or
layer.
artificial material due to the action of weather.
These cracks do not affect the structural integrity of concrete
1. The damage from freezing and thawing is the most
but may lead to subsequent deterioration of the concrete.
common weather related physical deterioration.
2. 2. Alternate wetting and drying, and heating and
The generally observed reasons for appearance of Crazing
cooling may cause cracking in concrete due to
cracks are
weathering
• Poor or inadequate curing.
3. Concrete generally loses strength with increase in
• Too wet a mix, excessive floating, the use of a jitterbug or
temperature about 300 C, damage being greater with
any other procedure which depresses the coarse aggregate and
aggregate having higher coefficient of thermal
produces an excessive concentration of cement paste and fines
expansions.
at the surface.
• Sprinkling cement on the surface to dry up bleed water. This
concentrates fines on the surface.
• Occasionally carbonation of the surface can cause crazing.
HONEYCOMBING ON CONCRETE :
Honeycomb consists of exposed pockets of coarse aggregates
not covered by a surface layer of mortar.
It may also be defined as the hollow spaces and cavities left in
the concrete mass on surface or inside the concrete which is
caused by mortar not filling the space between coarse
aggregates
This may be caused by inadequate compaction.
Presence of excess water in concrete or by leaky forms, which
allow the water to escape.
Types of Honeycomb :
Small size honeycomb – Depth is less than 25mm
Moderate size honeycomb- Deeper than 25mm but steel bars
have not exposed.
Larger size honeycomb- Deeper than 25mm and bars have
come out
Causes of Honeycomb:
1. Poor workability 2. Poor grading of aggregate 3. Grout leak.
4. Movement of formwork. 5. Improper compaction. 6.
Improper cover and placement of rebar
Preventive measures :
To follow good construction practice.
To use workable concrete.
To provide good forms.
POPOUTS ON CONCRETE : . CREEP ON CONCRETE :
A popout is a small, cone shaped cavity or hole in a horizontal Concrete undergoes instantaneous elastic deformation
concrete surface left after a near surface aggregate particle has when subjected to sustained loads with respect to time
expanded and fractured. known as creep
The cavity may range from 6mm to few mm diameter Factors affecting :
1. W/C ratio. 2. Type of aggregate. 3. Admixture 4. Age of
Causes : concrete 5. Type of cement and cement content 6. Mix
They are caused by freezing of water in the aggregate particles proportions. 7. Mixing Time. 8. Humidity. 9. Temperature.
that have internal pore structure which causes to expand. 10. Size of the specimen.
pop outs do not appear during construction but they start to
appear during the first winter and may continue to form for
several years

FOLLOW THESE RULES FOR POPOUTS:

1. Use durable aggregate from a proven source. A limit of 1%
deleterious material by mass of dry aggregate has been found
to minimize difficulties with popouts.
2. Use concrete with the lowest water content and slump
possible for the application.
3. Use air entrained concrete.
4. Do not finish concrete when bleed water is on the surface.
5. Avoid over finishing or hardsteel troweling where not
needed, such as most exterior and garage slabs.
6. Reduce concrete temperature to 10C to 21C
 THERMAL MOVEMENT IN CONCRETE :
Thermal movement is defined as concrete expands or contracts
when change in temperature.
Causes :
Thermal movement due to considerable amount of heat due to
heat of hydration, atmospheric temperature and external fire.
Due to thermal movement, changes in shape and volume of
concrete causes cracks on the concrete structure.
The extent of temperature rise depends on the properties of
cement used and the shape and size of the component.

PREVENTIVE MEASURES:
Use of pozzolana.
Use of low heat cement.
Pre-cooling of aggregates and mixing water.
Post cooling of concrete by refrigerated water through pipes
embedded in the body of concrete.
Providing joints to relieve the restraints in the structure
CHEMICAL CAUSES:
Acid Attack :
• Concretes made of Portland cement (OPC) are highly
alkaline with pH values normally above 12.5 and are not
easily attacked by acidic solutions.
• As the pH of the solution decreases the equilibrium in the
cement matrix is being disturbed, and the hydrated cement
compounds are essentially altered by hydrolytic
decomposition which leads to the severe degradation of the
technical properties of the material.
• At pH values lower than 12.5 portlandite is the first
constituent starting dissolution.
• If pH decreases to values lower than stability limits of
cement hydrates, then the corresponding hydrate loses
calcium and decomposes to amorphous hydrogel.
• The final reaction products of acid attack are the
corresponding calcium salts of the acid as well as hydrogels
of silicium, aluminum, and ferric oxides.

SULPHURIC ACID ATTACK:

Owing to the poor penetration of sulphuric acid, the chemical
Sulphuric acid attack causes extensive formation of gypsum in
changes of the cement matrix are restricted to the regions
the regions close to the surfaces, and tends to cause
close to the surfaces
disintegrating mechanical stresses which ultimately lead to
spalling and exposure of the fresh surface.
 The chemical reactions involved in sulphuric acid attack on
cement based materials can be given as follows:

Ca(OH)2 + H2SO4 -------CaSO4 .2H2O

3CaO.2SiO2 .3H2O + H2SO----- CaSO4 .2H2O + Si(OH)4

Source water has high sulphur content, both as sulphate or

sulphide, and form hydrogen sulphide, H2S.

the hydrogen sulphide gas comes out of the solution and forms
sulphuric acid in the air space.

Sulphuric acid is highly reactive and reacts with calcium

compounds to form gypsum which causes the concrete to
soften, ultimately leading to roof collapse.
HYDROCHLORIC ACID ATTACK : AGGRESSIVE WATER ATTACK :
• The chemicals formed as the products of reaction between • Water has been reported to have extremely low
hydrochloric acid and hydrated cement phases are some concentrations of dissolved minerals.
soluble salts and some insoluble salts. • This soft or aggressive water will leach calcium from cement
• Soluble salts, mostly with calcium, are subsequently leached paste or aggregates and this attack take places very slowly.
out, whereas insoluble salts along with amorphous Aggressive water attack to have a serious effect on hydraulic
hydrogels, remain in the corroded layer. structures, the attack must occur in flowing water.
• Besides dissolution, the interaction between hydro gels may • This keeps a constant supply of aggressive water in contact
also result in the formation of some Fe-Si, Al-Si, Ca-Al-Si with the concrete and washes away aggregates particles that
complexes which appear to be stable in pH range above 3.5. become loosened as a result of leaching of the paste
• Hydrochloric acid attack is a typical acidic corrosion whichPreventive Measures :
can be characterized by the formation of layer structure. • Assessing the nature of water at the site before construction.
Water quality evaluation in structures to monitor the
Preventive Measures : aggressiveness of water at the structure Coating with an non
• By increasing cement content and reducing W/C ratio. By Portland-cement based coating
improving quality of cover concrete.
• By treating the surface with sodium silicate known as water
glass.
• By surface treatment with coal, tar, bituminous paints,
epoxy resins etc
ALKALI REACTION ON CONCRETE: Influencing Factors :
• The reaction of silica and carbonates in aggregates with the • Size of aggregate
alkalis(Sodium or potassium hydroxide) in cement produces • Porosity of aggregate.
a gel, which causes expansion and cracks is called alkali • Alkali content in cement.
reaction Sodium hydroxide penetrates concrete and becomes • Fineness of cement particles
concentrated at an evaporating face, physical damage would
result from crystallisation of sodium carbonate.
(A)Alkali-aggregate reaction (AAR):
• This is also called alkali carbonate reaction. Carbonate rock
aggregates have been reactive in concrete.
• This reaction is apparently limited to reactions with impure
dolomitic aggregates and are a result of either
dedolomitization or rim-silicification reactions.
Causes :
• Visual examination of those reactions that are serious
enough to disrupt the concrete in a structure will generally
show map or pattern cracking and a general appearance,
which indicates that the concrete is swelling.
•
• Alkali-Silica reaction(ASR) :
• The sodium and pottasium alkalis released during the • SULPHATE ATTACK :
hydration of OPC and siliceous constituent in aggregate. • Sulphates found in most of the soils as calcium, potassium,
• The alkali-silica gel formed, imbibes pore fluid causing sodium and magenisum.
expansion. Expansion of gel induces stresses and these • Solid salts do not attack concrete, but when present is
stresses may be ocassionaly base enough to cause cracking solution they can react with hardened cement paste.
and expansion of concrete. • Sulphate attack occurs when pore system in concrete is
Causes : penetrated by solution of sulphates.
• Visual examination of those reactions that are serious enough • Sulphate reaction depends upon
to disrupt the concrete in a structure will generally show map • Concentration of sulphate ions.
or pattern cracking and a general appearance, which • Cations present in the suplhate solution
indicates that the concrete is swelling.
• Petrographic examination may be used to confirm the
presence of alkali-silica reaction.
Preventive Measures :
• Use of low alkali content.
• use of slag cement
• Use of non-reactive aggregates
• Use of silica in concrete mix.
Physical Mechanism :
In addition to two chemical reactions, there may also be a
purely physical phenomenon in which the growth of crystals of
sulphate salts disrupts the concrete.
The damage starts at the egdes and corners and is followed by
progressive cracking and spalling which reduces the concrete to
a friable or even soft state.
The rate of sulphate attack increases with increase in strength
of the solution.
CHLORIDE ATTACK :
Chloride can be introduced in to the concrete by coming into
contact with environment containing chlorides, such as sea
water or deicing salts.
Penetration of the chlorides starts at the surface and then
moves inward.
Chlorides may enter in to concrete from following sources
Cement of the concrete Water mixed in concrete Aggregates
Admixtures added Chloride difussion from atmosphere
Mechanism :
The concentration of chlorides in contact with the reinforcing
steel will cause corrosion when moisture and oxygen are
present.
As the rust layer builds, tensile forces generated by the
expansion of the oxide cause the concrete to crack and
delaminate.
SEA WATER ATTACK :
The marine environment is characterized by wave action,
which imposes shock loads and causes the erosion of concrete
structures by abrasion.
In addition, the concrete is exposed to the aggressive
constituents of sea water and subjected to repeated freeze-
thaw and wet-dry cycles.
The deterioration of concrete structures in such an
environment is both chemical and physical in nature and the
type of the attack may be demarcated into 3 zones depending
on the tidal lines.
Preventive Measures :
Concrete density, cement type and cement content play a vital
role in the resistance of concrete to sea water.
Concrete made with calcium aluminates, super sulfate cements
and other supplementary cementitious materials.
Sufficient cover to reinforcement.
Temperature Variation: Temperature Variation leads to
Volume Changes in concrete. Resulting stresses lead to
cracking, spalling and excessive deflections.
Structural Cause
Accidental overload
Fig. Cracking Modes in Continuous Span
Creep:
Creep is the ‘time-dependent’ part of the strain resulting from
stress. In other words, creep is the increase in strain under
sustained stress.
• Mechanism :
• Under sustained stress and with time, the hydrated cement
gel, the adsorbed water layer, the water held in the gel pores
and the capillary pores yields, flows and readjust themselves,
resulting in shrinkage of concrete.
Causes of creep Influence of aggregate: Stronger aggregate of
high modulus of elasticity and a larger aggregate content in
concrete mix reduces the magnitude of creep.
• Creep is the ‘time-dependent’ part of the strain resulting from
stress.
Mix Proportions: Creep increases with increase in w/c ratio.
Creep is inversely proportional to the strength of the concrete.
Influence of age: In a broad sense, the age at which the
concrete is loaded has predominant effect on creep.
• Cement gel quality improves with time.
• Stresses induced on young concrete will result in large creep.
Effects of Creep :
• Unwanted deflections in reinforced concrete beams
• In columns, creep in concrete will transfer greater load on to
the reinforcing steel bars.