Properties of fresh and Hardened Concrete

Properties of Fresh and Hardened
Concrete

PROPERTIES OF FRESH
CONCRETE
 Workability
 Consistency
 Segregation
 Bleeding
 Setting Time
 Unit Weight
 Uniformity

WORKABILITY
It is desirable that freshly mixed concrete
be relatively easy to transport, place,
compact and finish without harmful
segregation.
A concrete mix satisfying these
conditions is said to be workable.

Factors Affecting Workability
 Method and duration of transportation
 Quantity and characteristics of cementing
materials
 Aggregate grading, shape and surface
texture
 Quantity and characteristics of chemical
admixtures
 Amount of water
 Amount of entrained air
 Concrete & ambient air temperature

WORKABILITY
 Workability is the most
important property of freshly
mixed concrete.
 There is no single test
method that can
simultaneously measure all
the properties involved in
workability.
 It is determined to a large
extent by measuring the
“consistency” of the mix.

 Consistency is the fluidity or degree of
wetness of concrete.
 It is generally dependent on the shear
resistance of the mass.
 It is a major factor in indicating the
workability of freshly mixed concrete.
CONSISTENCY

Test methods for measuring consistency
are:
 Flow test → measures the amount of flow
 Kelly-Ball test → measures the amount of
penetration
 Slump test (Most widely used test!)
CONSISTENCY

 Slump Test is related with the ease with
which concrete flows during placement
(TS 2871, ASTM C 143)

10 cm
20 cm
30 cm
The slump cone is filled in 3 layers. Every
layer is evenly rodded 25 times.
Measure the slump by determining the vertical difference
between the top of the mold and the displaced original center
of the top surface of the specimen.

 Segregation refers to a separation of the components
of fresh concrete, resulting in a non-uniform mix
Sp.Gr. Size
Cement 3-3.15 5-80 mm
C.Agg. 2.4-2.8 5-40 mm
F.Agg. 2.4-2.8 < 5 mm
SEGREGATION
 The primary causes of
segregation are differences
in specific gravity and size
of constituents of concrete.
Moreover, improper mixing,
improper placing and
improper consolidation also
lead to segregation.

Some of the factors affecting segregation:
– Larger maximum particle size (25mm) and
proportion of the larger particles.
– High specific gravity of coarse aggregate.
– Decrease in the amount of fine particles.
– Particle shape and texture.
– Water/cement ratio.
SEGREGATION

 Bleeding is the tendency of water to rise to
the surface of freshly placed concrete.
BLEEDING
 It is caused by the
inability of solid
constituents of the
mix to hold all of
the mixing water as
they settle down.
 A special case of
segregation.

Undesirable effects of bleeding are:
• With the movement of water towards the top, the top
portion becomes weak & porous (high w/c). Thus the
resistance of concrete to freezing-thawing decreases.
• Water rising to the surface carry fine particles of
cement which weaken the top portion and form
laitance. This portion is not resistant to abrasion.
• Water may accumulate under the coarse agg. and
reinforcement. These large voids under the particles
may lead to weak zones and reduce the bond
between paste and agg. or paste and reinforcement.
BLEEDING

The tendency of concrete to bleeding
depends largely on properties of cement.
It is decreased by:
 Increasing the fineness of cement
 Increasing the rate of hydration (C3S, C3A and
alkalies)
 Adding pozzolans
 Reducing water content
BLEEDING

MIXING OF CONCRETE
 The aim of mixing is to blend all of the
ingredients of the concrete to form a
uniform mass and to coat the surface of
aggregates with cement paste.

MIXING OF CONCRETE
 Ready-Mix concrete: In this type
ingredients are introduced into a mixer
truck and mixed during transportation to
the site.
• Wet – Water added before transportation
• Dry – Water added at site
 Mixing at the site
• Hand mixed
• Mixer mixed

Mixing time should be sufficient to produce
a uniform concrete. The time of mixing
depends on the type of mixer and also to
some properties of fresh concrete.
 Undermixing → non-homogeneity
 Overmixing → danger of water loss,
brekage of aggregate particles
MIXING OF CONCRETE

CONSOLIDATING CONCRETE
Inadequate consolidation
can result in:
– Honeycomb
– Excessive amount of
entrapped air voids
(bugholes)
– Sand streaks
– Placement lines (Cold joints)

VIBRATION OF CONCRETE
 The process of compacting concrete
consists essentially of the elimination of
entrapped air. This can be achieved by:
– Tamping or rodding the concrete
– Use of vibrators

VIBRATORS
 Internal vibrator: The poker is immersed
into concrete to compact it. The poker is
easily removed from point to point.
 External vibrators: External vibrators
clamp direct to the formwork requiring
strong, rigid forms.

Internal Vibration
d
1½ R
Vibrator
Radius of Action

Internal Vibrators
Diameter
of head,
(mm)
Recommended
frequency,
(vib./min.)
Approximate
radius of
action, (mm)
Rate of
placement,
(m3/h)
Application
20-40 9000-15,000 80-150 0.8-4
Plastic and flowing
concrete in thin
members. Also used for
lab test specimens.
30-60 8500-12,500 130-250 2.3-8
Plastic concrete in thin
walls, columns, beams,
precast piles, thin slabs,
and along construction
joints.
50-90 8000-12,000 180-360 4.6-15
Stiff plastic concrete
(less than 80-mm
slump) in general
construction .
Adapted from ACI 309

Systematic Vibration
CORRECT
Vertical penetration a few inches
into previous lift (which should not
yet be rigid) of systematic regular
intervals will give adequate
consolidation
INCORRECT
Haphazard random penetration of
the vibrator at all angles and
spacings without sufficient depth
will not assure intimate combination
of the two layers

 To aid in the removal of trapped air the
vibrator head should be rapidly plunged into
the mix and slowly moved up and down.
Internal Vibrators
 The actual completion
of vibration is judged by
the appearance of the
concrete surface which
must be neither rough
nor contain excess
cement paste.

External Vibrators
 Form vibrators
 Vibrating tables (Lab)
 Surface vibrators
– Vibratory screeds
– Plate vibrators
– Vibratory roller
screeds
– Vibratory hand floats
or trowels

 External vibrators are rigidly clamped to the
formwork so that both the form & concrete are
subjected to vibration.
 A considerable amount of work is needed to
vibrate forms.
 Forms must be strong and tied enough to prevent
distortion and leakage of the grout.
External Vibrators

 Vibrating Table:
used for small
amounts of
concrete
(laboratory and
some precast
elements)
External Vibrators

CURING OF CONCRETE
 Properties of concrete can improve with age as
long as conditions are favorable for the continued
hydration of cement. These improvements are
rapid at early ages and continues slowly for an
indefinite period of time.
 Curing is the procedures used for promoting the
hydration of cement and consists of a control of
temperature and the moisture movement from
and into the concrete.

 Hydration reactions can
take place in only
saturated water filled
capillaries.
CURING OF CONCRETE
 The primary objective of curing is to keep
concrete saturated or as nearly saturated as
possible.

Curing Methods
1. Methods which supply additional water to
the surface of concrete during early
hardening stages.
– Using wet covers
– Sprinkling
– Ponding

Curing Methods
2. Methods that prevent loss of moisture from
concrete by sealing the surface.
– Water proof plastics
– Use liquid membrane-forming compounds
– Forms left in place

3. Methods that accelerate strength gain by
supplying heat & moisture to the concrete.
– By using live steam (steam curing)
– Heating coils.
Curing Methods

Hot Weather Concrete
 Rapid hydration  early setting  rapid loss of
workability
 Extra problems due to
– Low humidity
– Wind, excessive evaporation
– Direct sunlight
Solutions
– Windbreaks
– Cooled Concrete Ingredients
– Water ponding (cooling due to evaporation)
– Reflective coatings/coverings

Cold Weather Concrete
 Keep concrete temperature above 5 °C to
minimize danger of freezing
Solutions
– Heated enclosures, insulation
– Rely on heat of hydration for larger sections
– Heated ingredients --- concrete hot when placed
– High early strength cement

UNIFORMITY OF CONCRETE
 Concrete uniformity is
checked by conducting
tests on fresh and
hardened concretes.
Slump, unit weight, air
content tests
Strength tests

PROPERTIES OF
HARDENED CONCRETE
 The principal properties of hardened
concrete which are of practical importance
can be listed as:
1. Strength
2. Permeability & durability
3. Shrinkage & creep deformations
4. Response to temperature variations
Of these compressive strength is the most
important property of concrete. Because;

PROPERTIES OF
HARDENED CONCRETE
Of the abovementioned hardened
properties compressive strength is one of
the most important property that is often
required, simply because;
1. Concrete is used for compressive loads
2. Compressive strength is easily obtained
3. It is a good measure of all the other
properties.

Factors Affecting Strength
 Effect of materials and mix proportions
 Production methods
 Testing parameters

STRENGTH OF CONCRETE
 The strength of a concrete specimen
prepared, cured and tested under specified
conditions at a given age depends on:
1. w/c ratio
2. Degree of compaction

COMPRESSIVE STRENGTH
 Compressive Strength is determined by
loading properly prepared and cured cubic,
cylindrical or prismatic specimens under
compression.

 Cubic: 15x15x15 cm
Cubic specimens are crushed after rotating
them 90° to decrease the amount of friction
caused by the rough finishing.
 Cylinder: h/D=2 with h=15
To decrease the amount of friction, capping
of the rough casting surface is performed.

Cubic specimens
without capping
Cylindrical specimens
with capping

Bonded sulphur capping Unbonded neoprene pads

TENSILE STRENGTH
 Tensile Strength can be obtained either by
direct methods or indirect methods.
Direct methods suffer from a number of
difficulties related to holding the specimen
properly in the testing machine without
introducing stress concentration and to the
application of load without eccentricity.

SPLIT TENSILE STRENGTH
Due to applied compression load a fairly uniform
tensile stress is induced over nearly 2/3 of the
diameter of the cylinder perpendicular to the
direction of load application.

 The advantage of the splitting test over
the direct tensile test is the same molds
are used for compressive & tensile
strength determination.
 The test is simple to perform and gives
uniform results than other tension tests.
σst =
2P
πDl
P: applied compressive load
D: diameter of specimen
l: length of specimen
Splitting Tensile
Strength

Load bearing capacity under twisting moment
The flexural tensile strength at failure or the
modulus of rupture is determined by loading a
prismatic concrete beam specimen.
FLEXURAL STRENGTH
The results
obtained are useful
because concrete
is subjected to
flexural loads more
often than it is
subjected to
tensile loads.

P
M=Pl/4
d
b
c
I =
bd3
12
2
3
σ =
M c
I
=
(Pl/4) (d/2)
bd3/12
=
Pl
bd2
M=Pl/6
P/2 P/2
σ =
(Pl/6) (d/2)
bd3/12
=
Pl
bd2

Factors Affecting the Strength
of Concrete
1) Factors depended on the
test type:
– Size of specimen
– Size of specimen in relation
with size of agg.
– Support condition af
specimen
– Moisture condition of
specimen
– Type of loading adopted
– Rate of loading
– Type of test machine
2. Factors independent of
test type:
– Type of cement
– Type of agg.
– Degree of compaction
– Mix proportions
– Type of curing
– Type of stress situation

STRESS-STRAIN RELATIONS IN
CONCRETE
σult
(40-50%)
σult
εult
σ-ε relationship for
concrete is
nonlinear. However,
specially for
cylindrical
specimens with
h/D=2, it can be
assumed as linear
upto 40-50% of σult

MODULUS OF ELASTICITY OF
CONCRETE
Load carrying capacity with same axis
Due to the
nonlinearity of the σ-ε
diagram, E is the
defined by:
1. Initial Tangent Method
2. Tangent Method
3. Secant Method
ACI → E=15200 σult
½ → 28-D cylindrical comp.str.
(kgf/cm2)
TS → E=15500 W ½→ 28-D cubic comp.str. (kgf/cm2)

PERMEABILITY OF
CONCRETE
 Permeability is important because:
1. The penetration of some aggresive solution may result
in leaching out of Ca(OH)2 which adversely affects the
durability of concrete.
2. In R/C ingress of moisture of air into concrete causes
corrosion of reinforcement and results in the volume
expansion of steel bars, consequently causing cracks &
spalling of concrete cover.
3. The moisture penetration depends on permeability & if
concrete becomes saturated it is more liable to frost-
action.
4. In some structural members permeability itself is of
importance, such as, dams, water retaining tanks.

 The permeability of concrete is controlled
by capillary pores. The permeability
depends mostly on w/c, age, degree of
hydration.
 In general the higher the strength of
cement paste, the higher is the durability &
the lower is the permeability.
PERMEABILITY OF
CONCRETE

DURABILITY
A durable concrete is the one which will
withstand in a satisfactory degree, the
effects of service conditions to which it will
be subjected.
Factors Affecting Durability:
 External → Environmental
 Internal → Permeability, Characteristics of
ingredients, Air-Void System...

Structure of “damaged”
Concrete
Macrostructure
Visible cracks in hcp
and aggregates due
to volume changes
(to understand
cause of cracks,
microstructure
should be examined)
Microstructure
 Alkali-silica reaction:
Reaction product forms
at TZ and expands
 Frost action: Water
freezes in capillary
pores and expands
 Sulfate attack: reaction
products form in hcp
and expand

Leaching & Efflorescence
 When water penetrates into concrete, it
dissolves the non-hydraulic CH (and
various salts, sulfates and carbonates of
Na, K, Ca)
 Remember C-S-H and CH is produced
upon hydration of C3S and C2S
 These salts are taken outside of concrete
by water and leave a salt deposit.

Sulfate Attack
 Ground water in clayey soils containing alkali
sulfates may affect concrete.
 These solutions attack CH to produce gypsum.
Later, gypsum and calcium alumina sulfates
together with water react to form “ettringite”.
 Formation of ettringite is hardened cement
paste or concrete leads to volume expansion
thus cracking.
 Moreover, Magnesium sulfate may lead to the
decomposition of the C-S-H gel.

 Seawater contains some amount of Na and Mg
Sulfates. However, these sulfates do not cause
severe deleterious expansion/cracking because
both gypsum and ettringite are soluble in
solutions containing the Cl ion. However, problem
with seawater is the frequent wetting/drying and
corrosion of reinforcing steel in concrete.
 To reduce the sulfate attack
1. Use low w/c ratio→ reduced permeability & porosity
2. Use proper cement → reduced C3A and C3S
3. Use pozzolans → they use up some of the CH to
produce C-S-H
Sulfate Attack

Acid Attack
 Concrete is pretty resistant to acids. But in
high concentrations:
 Causes leaching of the CH
 Causes disintegration of the C-S-H gel.

Carbonation
 Ca(OH)2 + CO2 → CaCO3 + H2O
 Accompanied by shrinkage → carbonation
shrinkage
 Makes the steel vulnerable to corrosion
(due to reduced alkalinity)

Alkali-Agg. Reactions
 Alkalies of cement + Reactive Silica of Aggs
→ Alkali-Silica Gel
 Expansions in volume
 Slow process
 Don’t use aggs with reactive silica or use
cements with less alkalies.

Corrosion
 Electrochemical reactions in the steel rebars
of a R/C structure results in corrosion
products which have larger volumes than
original steel.
 Thus this volume expansion causes cracks in
R/C. In fact, steel is protected by a thin film
provided by concrete against corrosion.
However, that shield is broken by CO2 of air
or the Cl- ions.

Freezing and Thawing
 Water when freezes expands in volume.
This will cause internal hydraulic pressure
and cracks the concrete.
 To prevent the
concrete from this
distress air-entraining
admixtures are used
to produce air-
entrained concrete.

Abrasion
 Aggregates have to be hard & resistant to
wear.
 Bleeding & finishing practices are also
important.

Properties of fresh and Hardened Concrete

Properties of fresh and Hardened Concrete

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