Module 3 Properties of Concrete.ppt

Module - 3
Properties of Concrete

Fresh concrete
• Fresh concrete is that stage of concrete in which concrete
can be moulded and it is in plastic state. This is also called
"Green Concrete".
• Another term used to describe the state of fresh concrete
is consistence, which is the ease with which concrete will
flow.
• When wet mix of concrete ingredients begin to set but not
fully set, it is called green concrete.

Fresh concrete
• For fresh concrete to be acceptable, it should:
– Be easily mixed and transported.
– Be uniform throughout a given batch & between batches.
– Be of a consistency so that it can fill completely the forms
for which it was designed.
– Have the ability to be compacted without excessive loss
of energy.
– Not segregate during placing and consolidation.
– Have good finishing characteristics.

Properties of fresh concrete
• Workability
• Setting
• Segregation & Bleeding
• Hydration
• Air Entrainment

Workability
• Workability is the ease with which a concrete can
be transported, placed and consolidated without
excessive bleeding or segregation.
• Also it is the internal work done required to
overcome the frictional forces between concrete
ingredients for full compaction.
• It is the effort required to manipulate a concrete
mixture with a minimum of segregation.

Workability
• WORKABILITY (ASTM C125) - property
which determine the effort required to
manipulate a freshly mixed quantity of
concrete with minimum loss of homogeneity.
• Consistency (slump): easy of flow
• Cohesiveness: tendency to bleed & segregate

Factors Affecting Workability
• w/c ratio
• Amount & type of Aggregate
• Amount & type of Cement
• Weather conditions
– Temperature
– Wind
• Chemical Admixtures
• FA/CA ratio

• Water content or w/c Ratio
– More w/c ratio – more workability; because inter-
particle lubrication is increased.
– High w/c ratio: higher fluidity & greater workability
– Increased w/c ratio: bleeding & slurry will escape
through joints of formwork.

• Amount & Type of Aggregate
– Using smooth & round aggregate increases workability
because they require less water and less lubrication and
hence greater workability
– Workability reduces if angular & rough aggregate is used
because angular aggregates increases flakiness or
elongation thus reducing workability.
– A smooth surface can improve workability, but a rougher
surface generates a stronger bond between paste &
aggregate creating a higher strength.

• Amount & Type of Aggregate
– Greater size of Aggregate: less water is required to
lubricate it, the extra water is available for workability
– Porous aggregates require more water compared to non-
absorbent aggregates for achieving same degree of
workability.
– Crushed stone produces more angular & elongated
aggregates, which have
• higher surface-to-volume ratio
• better bond characteristics but require more cement paste to
produce a workable mixture.

• Aggregate Cement ratio
– More ratio, less workability. Since less cement mean less water, so
the paste is stiff.
• Weather Conditions
– 1. Temperature
• High temperature: evaporation increases, workability decreases.
– 2. Wind:
• If wind is moving with greater velocity, Rate of evaporation also
increase; thus reducing the amount of water and reducing
workability.

• Admixtures
– Chemical admixtures are used to increase workability.
– Use of air entraining agent produces air bubbles which
acts as a sort of ball bearing between particles &
increases mobility, workability and decreases bleeding,
segregation.
– Use of fine pozzolanic materials also have better
lubricating effect & more workability.

• FA/CA ratio
–If amount of sand is more, workability will
reduce because sand has more surface
area and more contact area causes more
resistance.
–Low w/c ratio gives stronger concrete

Slump test
• Slump test is used to determine the workability of fresh
concrete. Slump test as per IS: 1199 – 1959 is followed.
• The apparatus used for doing slump test are Slump cone
and Tamping rod.

Procedure to determine workability of fresh concrete by slump test
• Internal surface of mould is thoroughly cleaned & applied with a light coat of oil.
• Mould is placed on a smooth, horizontal, rigid and non-absorbent surface.
• Mould is then filled in 4 layers with freshly mixed concrete, each approximately
to 1/4th of the height of the mould.
• Each layer is tamped 25 times by the rounded end of the tamping rod (strokes
are distributed evenly over the cross section).
• After the top layer is rodded, concrete is struck off the level with a trowel.
• The mould is removed from the concrete immediately (5-10 sec) by raising it
slowly in the vertical direction.
• Difference in level between the height of the mould and that of the highest point
of the subsided concrete is measured.
• This difference in height in mm is the slump of the concrete.

Slump test
• Slump measured should be recorded in mm of
subsidence of the specimen during the test.
• Any slump specimen, which collapses or shears off
laterally gives incorrect result and if this occurs,
the test should be repeated with another sample.
• If, in the repeat test also, the specimen shears, the
slump should be measured and the fact that the
specimen sheared, should be recorded.

Slump test
• Most widely used
• Simple Apparatus & Simple Procedure
• It indicates behaviour of compacted concrete cone
under Gravitational Force.
• TRUE Slump: slump evenly all around.
• SHEAR Slump: in very lean concrete; ½ of cone
slide down the other

Slump test
• Measures Consistency or wetness.
• Suitable for Medium to High workabilities (slump
25-125mm)
• For very STIFF Mixes, Slump = 0.
• MAX CA not > 38mm.

Ideal Value of Slump
• Dry sample, slump 25-50 mm.
• Wet concrete, slump 150-175 mm.
• Slump depends on
– properties of aggregates
– Temperature
• Super-plasticizer produce flowing concrete by
increasing the slump of concrete (175-225 mm).

Compaction Factor Test
• This method is based on inverse approach of workability definition, i.e. it
measures the degree of compaction done by a standard work, which
results from falling through the hoppers.
• This test is more sensitive to low workable mixes than the high workable
ones.
• Compacting factor = (Weight of partially compacted concrete) / (Weight
of fully compacted concrete)
• Largest value for the CF Value is 1.
• More workable the mix is, the nearest the CF is to 1.

Compaction factor test
• CF of fresh concrete is done to determine the workability of
fresh concrete by compacting factor test as per IS: 1199 –
1959.
• Apparatus: Compacting factor apparatus.

Procedure of compacting factor test
• Sample of concrete is placed in the upper hopper up to the brim.
• The trap-door is opened so that the concrete falls into the lower hopper.
• The trap-door of the lower hopper is opened and the concrete is allowed
to fall into the cylinder.
• The excess concrete remaining above the top level of the cylinder is
then cut off with the help of plane blades.
• The concrete in the cylinder is weighed. This is known as weight of
partially compacted concrete.
• The cylinder is filled with a fresh sample of concrete and vibrated to
obtain full compaction. The concrete in the cylinder is weighed again.
This weight is known as the weight of fully compacted concrete.

Compacting Factor Test
• Concrete Behaviour under action of external force.
• More accurate than slump test for concrete mix of
Medium & Low workability. CF: 0.9 - 0.8
• More sensitive & gives more consistence.

Compacting Factor Test
• CF lies between 0.8 – 0.92.
• Useful for dryer mixes for which the slump test is
not satisfactory.
• Sensitivity of the CF is reduced outside the normal
range of workability and is generally unsatisfactory
for CF > 0.92 .

Workability of Fresh Concrete by Vee -Bee Consistometer
• To determine workability of fresh concrete by
using a Vee -Bee consistometer as per IS:
1199 – 1959.
• Apparatus: Vee-Bee consistometer.

Procedure of Vee-Bee consistometer
• A conventional slump test is performed, placing the slump cone inside
the cylindrical part of the consistometer.
• The glass disc attached to the swivel arm is turned and placed on the
top of the concrete in the pot.
• The electrical vibrator is switched on and a stop-watch is started,
simultaneously.
• Vibration is continued till the conical shape of the concrete disappears
and the concrete assumes a cylindrical shape.
• When the concrete fully assumes a cylindrical shape, the stop-watch is
switched off immediately. The time is noted.
• The consistency of the concrete should be expressed in VB-degrees,
which is equal to the time in seconds recorded above.

Segregation
• It refers to a separation of the components of fresh
concrete, resulting in a non-uniform mix.
• It is separation of CA from mortar, caused by
– settling of heavy aggregate to the bottom
– separation of aggregate from the mix due to improper
placement.
• In segregation, concrete ingredients are divided
and rearranged by order of density.

Segregation
• The heaviest aggregates go down to the
bottom while the mortar goes up to the
surface.
• This can be due to excessive vibration &
falling from greater height, etc.
• In good concrete, all ingredients are properly
distributed to make a homogeneous mixture.

Segregation
• Segregation can occur:
– Inside the concrete mixer for too long mixing
– During the transport for shaking
– During placing by falling from greater height
– During the pouring for too long vibrating

Segregation
• Factors that increase segregation are:
– Larger maximum particle size (25mm) and
proportion of larger particles; badly proportioned
mix
– High specific gravity of CA.
– Decrease in the amount of FA.
– Particle shape & texture.

Segregation
• Factors that increase segregation are:
– w/c ratio
– Insufficient mix concrete with excess water content
– Dropping of concrete from heights as in the case of
placing concrete in column concreting
– When concrete is discharged from a badly designed
mixer, or from a mixer with worn out blades
– Conveyance of concrete by conveyor belts, wheel
barrow, long distance haul by dumper, long lift by skip
and hoist

Segregation
• Segregation may be of 3 types
– CA separating out or settling down
– Paste separating away from CA.
– Water separating out from the rest of the material
• Only comparatively dry mix should be vibrated.
• Too wet a mix if excessively vibrated; then concrete
gets segregated due to settlement of CA.
• Vibration is continued just for required time for
optimum results.

Bleeding
• Bleeding is defined as the appearance of water on the
surface of concrete after consolidation but before setting.
• Since mixing water is the lightest component of the
concrete, this is a special form of segregation.
• It occurs because aggregates settles into the mix and
releasing their mixing water.
• Some bleeding is normal for good concrete.

Bleeding
• It is mainly observed in
– highly wet mix
– badly proportioned & insufficiently mixed concrete.
– In thin members like roof slab or road slabs and when
concrete is placed in sunny weather.
• If bleeding becomes too localized, it results in
"craters".

Bleeding
• Upper layers will become too rich in cement with a
high w/c ratio causing a weak, porous structure.
• Salt may crystallize on the surface which will affect
bonding with additional lifts of concrete.
• This formation should always be removed by
brushing and washing the surface.
• Also, water pockets may form under large
aggregates and reinforcing bars reducing the bond.

Bleeding
• Due to bleeding, water comes up & accumulates at surface.
• Sometimes, along with this water, certain quantity of cement also
comes to the surface.
• When the surface is worked up with the trowel, the aggregate
goes down & cement and water come up to the top surface.
• This formation of cement paste at the surface is known as
“Laitance”.
• In such a case, the top surface of slabs and pavements will not
have good wearing quality.
• This laitance formed on roads produces dust in summer and mud
in rainy season.

Bleeding
• Bleeding may be reduced by:
– Increasing cement fineness or cement with low
alkali content
– Increasing Rate of hydration.
– Using air-entraining admixtures.
– Reducing the water content.
– proper proportioning; & uniform & complete
mixing
– Rich mixes are less susceptible to bleeding than
lean mixes.

Bleeding
• Water while traversing from bottom to top, makes
continuous channels.
• If w/c ratio > 0.7, bleeding channels will remain continuous
and un-segmented.
• These continuous bleeding channels are responsible for
causing permeability of the concrete structures.
• While mixing water is in process of coming up, it may be
intercepted by aggregates where it gets accumulated below
the aggregate.
• This accumulation of water creates water voids and reduces
the bond between the aggregates and the paste.
• This is more pronounced in flaky aggregates.

Bleeding
• Similarly, the water that accumulates below the
reinforcing bars reduces bond between
reinforcement & concrete.
• Poor bond between aggregate & paste or
reinforcement & paste due to bleeding can be
remedied by re-vibration of concrete.
• The formation of laitance & consequent bad effect
can be reduced by delayed finishing operations.

Advantage of Bleeding
• Bleeding is not harmful if Rate of evaporation of
water from the surface = Rate of bleeding.
• Removal of water, after it had played its role in
providing workability, from the body of concrete by
way of bleeding will do good to the concrete.

Advantage of Bleeding
• Early bleeding when the concrete mass is fully plastic, may
not cause much harm, because concrete being in a fully
plastic condition at that stage, will get subsided and
compacted.
• It is the delayed bleeding, when the concrete has lost its
plasticity, which causes undue harm to the concrete.
• Controlled re-vibration may be adopted to overcome the bad
effect of bleeding.

Hydration in concrete
• Concrete derives its strength by hydration of cement
particles.
• Hydration of cement is not a momentary action but a
process continuing for long time.
• Rate of hydration is fast initially, but continues over a very
long time at a decreasing rate.
• In field & in actual work, a higher w/c ratio is used, since the
concrete is open to atmosphere, the water used in the
concrete evaporates and the water available in the concrete
will not be sufficient for effective hydration to take place
particularly in the top layer.

• If hydration is to continue, extra water must be
added to refill the loss of water on account of
absorption & evaporation.
• Therefore, curing privides a favorable environment
during the early period for uninterrupted hydration.
• The desirable conditions
– a suitable temperature and
– ample moisture.

• Concrete, while hydrating, releases high heat of
hydration.
• This heat is harmful from the point of view of
volume stability.
• Heat of hydration of concrete may also shrinkage in
concrete, thus producing cracks.
• If heat generated is removed by water curing, the
adverse effect due to the generation of heat can be
reduced.

Air entrainment
• It reduces
– density of concrete
– strength.
• Its effects on both plastic & hardened concrete:
– Resistance to freeze–thaw action in the hardened concrete.
– Increased cohesion, reducing the tendency to bleed and
segregation in the plastic concrete.
– Compaction of low workability mixes including semi-dry
concrete.
– Stability of extruded concrete.
– Cohesion and handling properties in bedding mortars.

• Main properties of HC are concerned with its
– strength
– Stress-Strain Characteristics
– Shrinkage & Creep Deformation
– Response to temperature Variation
– Permeability & Durability
• Of these, strength of concrete is of greater importance
because the strength is related to structure of hardened
cement paste & gives an overall picture of quality of
concrete.
Hardened Concrete

Strength of Concrete
• Properties are in reference to IS:1343-1980.
• Strength of concrete is required to calculate the
strength of the members.
• For pre-stressed concrete applications, high
strength concrete is required for following reasons.
– To sustain the high stresses at anchorage regions.
– To have higher resistance in compression, tension, shear
and bond.
– To have higher stiffness for reduced deflection.
– To have reduced shrinkage cracks.

Strength of Concrete
• Strength
–compressive strength 2000-8000 psi
–tensile strength 200-800 psi
–flexural strength
–compression >> tension

Factors Affecting Strength
• Curing conditions, humidity
• temperature
• w/c, (inversely related) Abram’s law
• Air Content, (inversely related), short and long term
• Aggregate Characteristics, roughness, grading,
mineralogical.
• Cement Type, Composition, Fineness,
• Cement Content (directly related)
• Strength Porosity relationship
• Mixing Water

Compressive Strength
• It is indexing property as concrete has its maximum
strength against compression.
• In RCC structures, concrete mainly resist
compression
• Concrete is classified into different grades on the
basis of its 28-days
• 28-days compressive strength is the basic & main
background for designing the concrete mixes

Compressive Strength
• Compressive strength of concrete is given in terms
of the characteristic compressive strength of 150
mm size cubes tested at 28 days (fck).
• Characteristic strength is defined as the strength of
the concrete below which not more than 5% of the
test results are expected to fall.
• This concept assumes a normal distribution of the
strengths of the samples of concrete.

Factors affecting strength of concrete
• w/c Ratio & degree of compaction:
– Since concrete is a brittle material, its porosity governs
its strength.
– Compressive strength decreases with increase in
porosity
– Porosity of concrete is affected by the gel/space ratio in
concrete
– Gel/space ratio is the ratio of the solid products of
hydration to the space available for these hydration
products.

• w/c Ratio & degree of compaction:
– Abrams’ law states that “assuming full
compaction, and at a given age and normal
temperature, strength of concrete can be taken
to be inversely proportional to the water/cement
ratio”
– Degree of compaction also affects the strength
significantly

• Age:
– With an increase in age, the degree of hydration
increases so that strength increases
– Increase in the strength of concrete (at same w/c
ratio) with increase in early age (from 1 to 28
days) of concrete
– As a general rule the ratio of 28-day to 7-day
strengths lies between 1.3 and 1.7 and generally
is < 1.5

• Combined effect of age and temperature, i.e. “maturity”:
– Since hydration of cement is function of both the time and the
temperature of hydration, so the gain in strength of concrete is also
jointly affected by these two factors
– The concept of “maturity” is used to explain the combined effect of
age and temperature on the strength of concrete
– Maturity is defined as function of product of curing time, at, and
curing temperature, T, as follows:
– Maturity (˚C × days) = ∑ at (T + 10) where at is curing time in days
and T is the curing temperature in ˚C

• Effect of cement
– The effect of Portland cement on concrete strength
depends on the chemical composition and fineness of
the cement. Cement content also affects the strength of
concrete
– Cements having higher C3S contents gain strength more
rapidly, but have slightly lower strengths at later ages
– Since fineness of cement affects the rate of hydration it
also governs the strength of concrete

• Effect of aggregate/cement ratio
– For a constant w/c ratio, a leaner mix (having more
aggregate and less cement) leads to a higher strength
– This is because if paste have a smaller proportion in
volume of concrete (leaner mix), then total porosity of the
concrete is lower, and hence its strength is higher

• Effect of aggregate properties
– Influence of aggregate properties on strength is of
secondary importance
– Aggregate parameters that are most important are Shape
& Texture and MSA
• Effect of admixtures
– Any admixture which can reduce the w/c ratio or the
porosity of the concrete increases the strength of
concrete

Tensile Strength
• Tensile strength of concrete is much < compressive
strength, because of ease with which cracks can
transmit under tensile loads.
• Tensile strength of concrete is measured in 3 ways:
– Direct tension
– Splitting tension
– Flexural tension
• Tensile strength of the same concrete tested in:
• Direct tension < splitting tension < flexural tension

Tensile Strength
• Tensile strength of concrete can be expressed as:
– Flexural tensile strength: It is measured by testing beams
under 2 point loading
– Splitting tensile strength: It is measured by testing
cylinders under diametrical compression.
– Direct tensile strength: It is measured by testing
rectangular specimens under direct tension.

Tensile Strength
• In absence of test results, Code recommends to
use an estimate of the flexural tensile strength from
compressive strength by the following equation.
• fcr = 0.7 (fck)1/2
• Here, fcr = flexural tensile strength in N/mm2
• fck= characteristic compressive strength of cubes in
N/mm2

Tensile Strength
• It is difficult to test concrete in direct (uniaxial)
tension because of the problem of gripping the
specimen satisfactorily and because there must be
no eccentricity of the applied load
• Therefore, direct tensile test is not standardized
and rarely used.
• Modulus of Rupture test & Splitting test are
commonly used to determine the tensile strength of
concrete

Bond Strength
• The strength of bond between concrete & steel
reinforcement is called as bond strength of concrete
• Bond strength develops primarily due to friction and
adhesion between steel reinforcement and concrete
• In general, bond strength is approximately proportional to
the compressive strength of concrete up to about 20 MPa
(3000 psi).
• For higher compressive strengths of concrete, increase in
bond strength becomes progressively smaller and
eventually negligible.

Impact Strength
• Impact strength of concrete is of importance
– in driving concrete piles,
– in foundations for machines exerting impulsive loading,
– when accidental impact is possible, e.g. when handling
precast concrete members
• Impact strength varies from 0.50 to 0.75 of the
compressive cube strength
• Impact strength of water-stored concrete is lower
than when the concrete is dry.

Shear & Fatigue Strength
• Shear Strength
– Shear strength of concrete is taken approx = 20 % its
compressive strength.
• Fatigue Strength
– Strength of concrete against cyclic or repeated loading is
called as its fatigue strength

Stress-Strain Behaviour of Concrete
• After an initial linear portion lasting up to about 30 – 40% of
the ultimate load, the curve becomes non-linear, with large
strains being registered for small increments of stress.

• The non-linearity is primarily a function of the coalescence (union
of diverse things into one body) of micro-cracks at the paste-
aggregate interface.
• The ultimate stress is reached when a large crack network is
formed within the concrete, consisting of the coalesced micro-
cracks and the cracks in the cement paste matrix.
• The strain corresponding to ultimate stress is usually around
0.003 for normal strength concrete.
• The stress-strain behaviour in tension is similar to that in
compression.

• Descending portion of the stress-strain curve, or the post-
peak response of the concrete, can be obtained by a
displacement or a strain controlled testing machine.
• In load controlled machines, a constant rate of load is
applied to the specimen.
• Thus, any extra load beyond the ultimate capacity leads to a
catastrophic failure of the specimen.
• In a displacement controlled machine, small increments of
displacement are given to the specimen.

• Decreasing load beyond the peak load can also be
registered.
• The strain at failure is typically around 0.005 for normal
strength concrete.

• Post peak behaviour is a function of stiffness
of the testing machine in relation to the
stiffness of the test specimen, and the rate of
strain.
• With increasing strength of concrete, its
brittleness also increases, and this is shown
by a reduction in the strain at failure.

Creep in concrete
• Concrete creep is defined as deformation of
structure under sustained load.
• It is time-dependent deformation that occurs
due to prolonged application of stress.
• Long term pressure or stress on concrete can
make it change shape.

Creep in concrete
• This deformation occurs in the direction the force is
being applied.
• Like a concrete column getting more compressed, or
a beam bending.
• Creep does not cause concrete to fail or break apart.
• Creep is factored in when concrete structures are
designed.

Factors Affecting Creep
• Aggregate
• Mix Proportions
• Age of concrete

Influence of Aggregate
• Aggregate undergoes very little creep. It is really
the paste which is responsible for the creep.
• Aggregate influences the creep of concrete through
a restraining effect on the magnitude of creep.
• The paste which is creeping under load is
restrained by aggregate which do not creep.

Influence of Aggregate
• The stronger the aggregate the more is the restraining effect
and hence the less is the magnitude of creep.
• The modulus of elasticity of aggregate is one of the
important factors influencing creep.
• Higher the modulus of elasticity the less is the creep.
• Light weight aggregate shows substantially higher creep
than normal weight aggregate.

Influence of Mix Proportions
• Amount of paste content and its quality is one of the most
important factors influencing creep.
• A poorer paste structure undergoes higher creep. Therefore,
it can be said that creep increases with increase in w/c ratio.
• In other words, creep is inversely proportional to the
strength of concrete.
• Hence, all other factors which are affecting the w/c ratio are
also affecting the creep.

Influence of Age
• Age at which a concrete member is loaded will
have a predominant effect on the magnitude of
creep.
• Quality of gel improves with time, hence such gel
creeps less, whereas a young gel under load being
not so stronger creeps more.

Creep in concrete
• Creep of concrete is directly proportional to stress (stress
should be < 1/3rd of characteristic compressive strength).
• Creep coefficient = ratio of Ultimate Creep Strain to the
Elastic Strain at the time of loading.

Shrinkage in concrete
• Shrinkage of concrete is defined as contraction due
to loss of moisture.
• There are 2 types of shrinkage:
– Plastic shrinkage
– Drying Shrinkage
• The property of swelling when placed in wet
condition, and shrinking when placed in drying
condition is referred as moisture movement in
concrete.

• Plastic shrinkage
– Hydration of cement causes a reduction in the volume of
cement + water to an extent of 1% of the volume of dry
cement.
– This contraction is Plastic Strain & is aggravated due to
loss of water by evaporation from the surface of concrete,
particularly in hot climates & high winds. This results in
surface cracking.

– The aggregate particles or the reinforcement comes in the
way of subsidence due to which cracks may appear at the
surface or internally around the aggregate or reinforcement.
– In case of floors and pavements where the surface area
exposed to drying is large as compared to depth.
– When this large surface is exposed to hot sun & drying wind,
the surface of concrete dries very fast which results in plastic
shrinkage.

– Sometimes even if the concrete is not subjected to severe drying, but
poorly made with a high w/c ratio, large quantity of water bleeds and
accumulates at the surface.
– When this water at the surface dries out, the surface concrete
collapses causing cracks.
– high w/c ratio, badly proportioned concrete, rapid drying, greater
bleeding, unintended vibration etc., are some of the reasons for
plastic shrinkage.
– It can also be further added that richer concrete undergoes greater
plastic shrinkage.

– Plastic shrinkage can be reduced mainly by preventing the rapid loss
of water from surface.
– This can be done by covering the surface with polyethylene sheeting
immediately on finishing operation; by fog spray that keeps the
surface moist; or by working at night.
– Use of small quantity of aluminium powder is also suggested to
offset the effect of plastic shrinkage.
– Similarly, expansive cement or shrinkage compensating cement also
can be used for controlling the shrinkage during the setting of
concrete.

• Drying Shrinkage
– Loss of free water contained in hardened concrete, does not result in any
appreciable dimension change.
– It is the loss of water held in gel pores that causes the change in the volume.
– Under drying conditions, the gel water is lost progressively over a long time,
as long as the concrete is kept in drying conditions.
– Cement paste shrinks more than mortar and mortar shrinks more than
concrete.
– Concrete made with smaller size aggregate shrinks more than
concrete made with bigger size aggregate.
– The magnitude of drying shrinkage is also a function of the fineness of gel.
– The finer the gel the more is the shrinkage.

• Drying Shrinkage
– Just as the hydration of cement is an ever lasting process, the
drying shrinkage is also an ever lasting process when concrete is
subjected to drying conditions.
– It that take place after the concrete has set & hardened.
– Most of it take place in the first few months.
– Withdrawal of water from concrete stored in unsaturated air voids causes
drying of shrinkage.
– A part of this shrinkage is recovered on immersion of concrete in water.
– Rate of shrinkage decreases with time.
• 14-34% of 20-yr shrinkage occurs in 2 weeks. 40-70% in 3 months & 66-80% in 1
year.

• Shrinkage is effected by
– w/c ratio: directly proportional
– Cement content: shrinkage increases with cement
content but is inter-related to w/c ratio because of
necessity to maintain workability.
– Humidity: shrinkage increases with decrease in humidity
& immersion in water causes expansion.
– If the concrete is placed in 100% relative humidity for any
length of time, there will not be any shrinkage; instead
there will be a slight swelling.

– Type of aggregate: aggregate exhibiting moisture
movement & having low elastic modulus causes large
shrinkage.
– A increase in MSA decreases the shrinkage.
– Grading & shape has little effect on shrinkage.
– Harder aggregate with higher modulus of elasticity like
quartz shrinks much less than softer aggregates such as
sandstone.

– Size & Shape of Specimen: rate & ultimate magnitude of
shrinkage decreases with surface/volume ratio of
specimen.
– Type of cement: rapid-hardening cement shrinks more
than others.
– Admixtures: S increases with addition of Calcium
Chloride & reduces with lime replacement.
– Other factors: steam curing has little effect on shrinkage
unless applied at high pressure.

Durability of Concrete
• Durability is defined as the capability of concrete to
resist weathering action, chemical attack and abrasion while
maintaining its desired engineering properties.
• It refers to duration or life span of trouble-free performance.
• Different concretes require different degrees of durability
depending on the exposure environment and properties
desired.
• For example, concrete exposed to tidal seawater will have
different requirements than indoor concrete.

Durability of Concrete
• Concrete will remain durable if:
– The cement paste structure is dense and of
low permeability
– Under extreme condition, it has entrained air to resist
freeze-thaw cycle.
– It is made with graded aggregate that are strong & inert
– The ingredients in the mix contain minimum impurities
such as alkalis, Chlorides, sulphates and silt

Factors affecting durability of concrete
• Cement content
• Compaction
• Curing
• Cover
• Permeability

• Cement content
– Mix must be designed to ensure cohesion & prevent segregation and
bleeding.
– If cement is reduced, then at fixed w/c ratio the workability will be
reduced leading to inadequate compaction.
– However, if water is added to improve workability, water / cement
ratio increases and resulting in highly permeable material.
• Compaction
– The concrete as a whole contain voids can be caused by
inadequate compaction.
– Usually it is being governed by the compaction equipments used,
type of formworks, and density of the steelwork

• Curing
– It is very important to permit proper strength development aid
moisture retention and to ensure hydration process occur
completely
• Permeability
– It is considered the most important factor for durability.
– It can be noticed that higher permeability is usually caused by
higher porosity .
– Therefore, a proper curing, sufficient cement, proper
compaction & suitable concrete cover could provide a low
permeability concrete.

Types of Durability
• Major Concrete Durability types are:
– Physical durability
– Chemical durability

Types of Durability
• Physical Durability
• Physical durability is against the following actions
– Freezing and thawing action
– Percolation / Permeability of water
– Temperature stresses i.e. high heat of hydration
• Chemical Durability
• Chemical durability is against the following actions
– Alkali Aggregate Reaction
– Sulphate Attack
– Chloride Ingress
– Corrosion of reinforcement

Durability of Concrete – low w/c ratio
• Use of high w/c ratio – permeability – Volume change – cracks –
disintegration – failure of concrete: is a cyclic process in concrete.
• For a durable concrete, use of lowest possible w/c ratio is the
fundamental requirement to produce dense & impermeable concrete.
• In low w/c ratio concretes, there is not enough water available to fully
hydrate all cement particles; only surface hydration of cement particles
takes place leaving a considerable amount of unhydrated cement grains.
• This unhydrated cement grains represent the strength in reserve.
• If for any reason, cracks develop, the unhydrated cement gets hydrated
on getting water through micro-cracks.
• This hydrated product seal the cracks & restore the integrity of concrete
for long-term durability.

Durability of Concrete – low w/c ratio
• In low w/c ratio, the quality of hydration product (gel) & the micro-
structure of concrete is superior as compared to when concrete is
prepared with high w/c ratio.
• In concretes with low w/c ratio, the capillaries of inter-connected network
are so fine that water cannot flow through them, i.e., permeability
decreases to such a level that these concretes are almost impervious to
water.
• Chloride-ion diffusion in such concretes are 10-15 times slower.
• Such concretes are less sensitive to carbonation, external chemical
attack & other detrimental effect that cause lack of durability of concrete.
• A low w/c ratio & adequate cover is best way to protect reinforcing steel
against corrosion.

Permeability of Concrete
• Permeability is the ease with which liquids or gases
can travel through concrete.
• Permeability of concrete plays an important role in
durability because it controls the rate of entry of
moisture that may contain aggressive chemicals
and the movement of water during heating or
freezing.
• Higher the permeability lesser will be the durability.

Factor’s Controlling Permeability
• There are 2 major factors which determine the
permeability in concrete.
– w/c ratio
– Compaction and curing.
• It can be seen that permeability increases as w/c
ratio increase.
• Typically, at a w/c ratio of around 0.4, permeability
is practically nill.

• Permeability of cement paste primarily
depends upon its porosity.
• Paste permeability is very low at & < 30%
paste porosity.
• A small increase in porosity above around
30% causes a steep increase in the
permeability.

• For a given w/c ratio, the permeability decreases with AGE
as the cement continues to hydrate and fills some of the
original water space, the reduction in permeability being
faster at the lower w/c ratio.
• Segmentation of capillary pores is another major factor,
which affects the permeability.
• For the same porosity of a paste, permeability will be
LESSER in case of capillary pores being segmented
disrupting the continuity of pores, as compared to the case
of interconnected capillary pores.

• Large size AGGREGATES increases the permeability.
• Increase in the permeability is due to development of micro-
cracks at early stage.
• An air-entrainment < 6% make concrete more impervious.
• Steam curing of concrete using pozolonna has been
reported to DECREASE the permeability due to
– formation of coarser C-S-H gel
– lower shrinkage
– accelerated conversion of Ca(OH)2 into cementing product.

• With AGE, permeability DECREASES because gel
gradually fills the original water filled space.
• For same w/c ratio, permeability of paste with coarser
cement particles is higher than that with finer cement.
• Higher the strength of cement paste, the lower will be the
permeability.
• A durable concrete should be relatively impervious.

Chemical Attack on Concrete
• Chemical attacks on concrete structures causes
deterioration of structure and its durability is affected.
• Life of structure reduces & it can lead to failure of
structures.

Chlorides attacks on concrete structures
• High concentrations of chloride ions cause
corrosion of reinforcement & products of corrosion
disrupts concrete.
• Chlorides can be introduced into the concrete
either during or after construction as follows.
– Before construction
– After construction

• Before construction
– Chlorides can be admitted in admixtures containing
calcium chloride, through using mixing water
contaminated with salt water or improperly washed
marine aggregates.
• After construction: Chlorides in salt or sea water,
in airborne sea spray and from de-icing salts can
attack permeable concrete causing corrosion of
reinforcement.

• Chlorides are discussed in BS8110: Part 1, clause 6.2.5.2.
• The clause limits the total chloride content expressed as a
% of chloride ion by mass of cement to 0.4% for concrete
containing embedded metal.
• It was common practice in the past to add CaCl2 to concrete
to increase the rate of hardening.
• The code now recommends that CaCl2 & chloride-based
admixtures should not be added to reinforced concrete
containing embedded metals.

• Chloride ion penetration results in concrete swelling of 2-2.5 times larger
than observed with water penetration. This causes a decrease in
strength.
• Presence of chloride ions near the reinforcement steel makes it
vulnerable to corrosion.
• If hydroxide to chloride ratio drops below 0.3 near the reinforcement,
passivation is destroyed & corrosion occurs.
• Reduction in w/c ratio which reduces permeability & chloride ion
penetration into concrete prevents corrosion of steel.
• Chloride diffusion through cement blended with pozzolanic materials
(33%) or slag (65%) is very slow as compared to OPC & sulphate-
resisting cements.

Sulphate attacks on concrete structures
• Sulphate are present in most cements and some
aggregates.
• Sulphate may also be present in soils, groundwater
and sea water, industrial wastes and acid rain.
• The products of sulphate attack on concrete occupy
a larger space than the original material and this
causes the concrete to disintegrate and permits
corrosion of steel to begin.

Sulphate attacks on concrete structures
• BS8110: Part 1, clause 6.2.5.3, states that the total water-
soluble sulphate content of the concrete mix expressed as
SO3 should not exceed 4% by mass of cement in the mix.
• Sulphate-resisting Portland cement should be used where
sulphate are present in the soil, water or atmosphere and
come into contact with the concrete.
• Super-sulphated cement, made from blast furnace slag can
also be used although it is not widely available.
• This cement can resist the highest concentrations of
sulphate.

Carbonation of concrete structures
• Carbonation is the process by which CO2 from the
atmosphere slowly transforms Ca(OH)2 into CaCO3 in
concrete.
• Due to carbonation, concrete itself is not harmed but
reinforcement gets seriously affected by corrosion.
• Normally the high pH value (alkalinity) of the concrete
prevents corrosion of the reinforcing bars by keeping them
in a highly alkaline environment due to the release of
Ca(OH)2 by the cement during its hydration.

Carbonation of concrete structures
• Carbonated concrete has a pH value of 8.3 while
the passivation of steel starts at a pH value of 9.5.
• Depth of Carbonation in good dense concrete is
about 3 mm at an early stage and may increase to
6–10 mm after 30–40 years.
• Poor concrete may have a depth of Carbonation of
50 mm after say 6–8 years.
• Rate of Carbonation depends on time, cover,
concrete density, cement content, w/c ratio &
presence of cracks.

Acids attacks on concrete structures
• Portland cement is not acid resistant and
acid attack may remove part of the set
cement.
• Acids are formed by the dissolution of CO2 or
SO2 in water from the atmosphere.
• Acids can also come from industrial wastes.
• Good dense concrete with adequate cover is
required and sulphate-resistant cements
should be used if necessary.

Chemical attack precautions in concrete structures

Freeze & Thaw Attack
• The problem of freeze-thaw attack on concrete is not of
great importance to Indian conditions.
• The concrete structures particularly, the one which are
exposed to atmosphere are subjected to cycles of freezing
and thawing and as such suffer from the damaging action of
frost.
• It is very well known that fresh concrete should not be
subjected freezing temperature. Fresh concrete contain a
considerable amount of free water.
• If this free water is subjected to freezing temperature
discrete ice lenses are formed.

• Water expands about 9% in volume during freezing.
• The formation of ice lenses formed in the fresh concrete
disrupt the fresh concrete causing permanent damage to
concrete.
• The fully hardened concrete is also vulnerable to frost
damage, particularly to the effect of alternate cycle of
freezing & thawing.
• The severest conditions for frost action arise when concrete
has more than one face exposed to the weather and is in
such a position that it remains wet for a long period.
• Examples are road kerbs, parapets, concrete members in
hydraulic structures.

• The mechanism of freeze-thaw attack is that the empty
pores which are holding the free water will not be sufficient
to accommodate the increased volume of the ice formed.
• This exerts a bursting pressure on the pore walls.
• Since concrete is weak in tension the pore walls crack
disintegrating the concrete.
• During the next cycle of wetting, more water will enter the
concrete more ice will be formed and concrete crack will
open up further.
• This process of alternative wetting and drying of concrete
will lead to cracking and spalling of concrete, which leads to
the disintegration of concrete structure.

Freeze & Thaw Attack - PREVENTION
• Making concrete impermeable.
• Reducing the size of the pores by using air entraining
agents (reduces the size of the pores which reduces the
chance of ice formation).
• Use of mineral admixtures like silica fume and rice husk ash
to refine the pore structure i.e., Reducing the size of pores
and reducing the porosity of concrete.
• There is a need for a minimum size of water molecule for ice
formation. If the pore size is smaller the critical size then
formation of ice will be reduced.

Resistance to Fire
• Generally, concrete has good properties wrt fire resistance.
• Concrete perform satisfactorily under fire & no toxic fumes
are emitted.
• Length of time over which the structural concrete preserves
structural action is known as Fire-Rating.
• Under sustained exposure to temperature > 350C, a
decrease in strength occurs.
• Leaner mix suffer lower loss of strength than rich ones.
• Flexural strength is affected more than the compressive
strength.

Resistance to Fire
• Loss of strength is lower when aggregate does not contain
silica.
• Low conductivity of concrete improves its fire resistance.
• A light-weight concrete is more fire resistance than ordinary
concrete.
• Calcined material aggregate have a low density leading to
good fire resistance of concrete.
• Modulus of elasticity of concrete is reduced & thermal creep
increases at high temperature.

Concrete in Marine Environment
• Concrete used in marine environment faces simultaneously
following type of deterioration
– Physical
– Chemical
– Mechanical
• Marine environment is divided into 3 zones depending on
their effect on the structure
– Atmospheric Zone
– Tidal Zone
– Submerged Zone

• Atmospheric Zone: where the parts of structure are above
the highest high tide level: chemical & Physical Attack
• Tidal Zone: located between highest high tide level & lowest
low tide level: Chemical, Physical & Mechanical Attack
• Submerged Zone: where the parts of structure always
remain submerged in sea water: Chemical & Physical Attack

• Concrete structures located in tidal zones
deteriorate more severely due to
– mechanical forces
– Alternate wetting & drying cycles which accelerates
chemical action of salts & water on reinforcement steel.
• Prevention
– Slag & pozollanic cements are preferred.
– Mineral additives like granulated blast furnace slag or fly
ash or micro silica can be successfully used with OPC.

Module 3 Properties of Concrete.ppt

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