PROPERTIES OF FRESH CONCRETE
FRESH CONCRETE:
·
Fresh
Concrete is the plastic state of concrete and in this stage concrete can be
molded into any designed shape in construction.
·
This
is also called as ‘Green Concrete’.
·
The
term commonly used to describe the state of fresh concrete is ‘consistence’, is
the ease with which concrete will flow.
PROPERTIES OF FRESH CONCRETE:
INTRODUCTION:
There are two terms which should be
considered during preparation of concrete –
·
First one is the long term properties of
hardened concrete like strength, volume, stability and durability are seriously
affected by its degree of compaction,
·
Second one is the short-term requirement like workability. Workability of fresh
concrete should be such that the concrete can be properly compacted and also it can be transported, placed, and finished
significantly easily without segregation to attain the expected strength and
durability.
·
The
48 hours are very important for the performance of the concrete structure. It
also controls the various long-term behavior, influence ultimate strength,
elastic modulus, creep and durability.
The properties of fresh concrete are
as follows –
A.
Workability
B.
Consistency
C.
Setting
of Concrete
D.
Segregation
E.
Bleeding
F.
Unit
weight and Yield
G.
Plastic
shrinkage
H.
Hydration
I.
Uniformity
WORKABILITY:
·
The
workability of fresh concrete is the ease with the concrete is transported,
placed and compacted while using the lowest possible water/cement ratio without
excessive bleeding and segregation. The concrete mix satisfying these
conditions is said to be workable.
or
The internal
work done required to overcome the frictional forces between concrete
ingredients for full compaction.
·
Workability should be obtained by the use of
well graded aggregate which has possibly the largest maximum particle size.
·
Workability should be increased by using the
smooth and rounded shaped aggregate rather than irregular shaped aggregate.
·
Using
of air entrained admixture also improved the workability of the concrete mixes
but resulting in loss of strength up to 15%.
Factors affecting the Workability of concrete –
Ø Quantity and characteristics of
cementing materials.
Ø Water cement ratio of the concrete
mix.
Ø Aggregate grading, shape and surface
texture.
Ø Sand and aggregate ratio
Ø Quantity and characteristics of
chemical admixtures and amount of air entrant.
Ø Concrete and ambient air
temperature.
Ø Method and duration of transport.
Measurement of Workability –
There are
various types of tests by which workability can be tested indirectly and
unfortunately, there are no accepted tests which can measure workability
directly.
1.
Slump Test: It gives good result for rich
concrete mixes.
2.
Compacting Factor Test: It is used for low workable
concrete.
3.
Flow Table Test:It is used for high workable
concretes.
4. VeBe
Test:It is used for
low workable concrete like fiber reinforced concrete.
5. Kelly
Ball Test:It is
practical in field test.
CONSISTENCY:
·
The
consistency is the fineness of the concrete or the ease with which the concrete
flows or is a measure of the stiffness or sloppiness of the mix, for
effectively handling, placing, and compacting the concrete.
·
It
refers to the mean degree of wetness of the concrete or fluidity of concrete.
·
Consistency
generally depends on the shear resistance of the mass.
·
It
is a major factor in indicating the workability of freshly mixed concrete.
Test
methods using for measuring consistency are as follows –
1. Flow
Test – By measuring
the amount of flow.
2. Kelly
Ball Test –
Measuring the amount of penetration.
3. Slump
Test – Most
commonly used for measuring the consistency of concrete.
SETTING OF CONCRETE:
·
The
hardening of concrete before its hydration is known as setting of concrete or
hardening of concrete before it gains strength.
·
Setting
is the onset rigidity in fresh concrete. Hardening is the development
ofmeasurable strength; settingprecedes hardening and both are gradual changes
controlled by hydration. Fresh concretelose measurable slump before initial set
andmeasurable strength will be achieved after final set.
·
The
transmission process of changing of concrete from plastic state to hardened
state. Setting of concrete is based or related to the setting time of cement
paste. Thus, cement properties greatly affect the setting time.
Factors
that affecting the setting of concrete are as follows –
1. Cement content
2. Type of cement
3. Fineness of cement
4. Relative Humidity
5. Temperature
6. Water/Cement ratio.
7. Type and quantity of aggregate
8. Admixtures
9. Control heat of Hydration
10. Homogeneous nature of concrete
SEGREGATION:
·
Segregation
of concrete is the separation of some size groups of aggregates from cement
mortar in isolated location resulting in to deficiencies of those materials in
other locations because of differences in size, density shape and other
properties of ingredients in which they are composed.
Conditions favorable for segregation are –
Ø The main reason of segregation in
concrete is the differences in size of particles and sometimes the specific
gravity of the mix constituents differs from each other.Specific gravity of
aggregate is in between 2.6-2.7 g/cc and for cement it is in between 3.1-3.6
g/cc, due to these differences, the aggregate separates from the matrix.
Ø Use of poorly graded aggregate, for
example flakiness.
Ø Bad proportioned of concrete mix.
Ø Using high water cement ratio
(happens when concrete mixed by unskilled worker at site)
Ø Transportation of concrete for long
distance.
Ø Poor compaction of concrete after
placing.
Ø Over compaction of concrete.
Ø Dropping of concrete more than 1 m
height
There are two types of segregation –
Ø First
one - Segregation
due to the separation of aggregate.
Ø Second
one -Segregation
due to the separation of cement slurry.
To
control Segregation
–
Ø
The
concrete mix should be properly designed with most favorable quantity of water
i.e. not too wet or not too dry.
Ø
If
the concrete does not have far to travel and transferred directly from skip or
the wheelbarrow to the final position of in the formwork, the danger of
segregation is small.
Ø
During
placing of concrete more than a height of 1.50 m it is possible to avoid the
dropping of concrete by passing the concrete through a inclined chute, keeping
the inclination between 1:3 to 1:2 for the smooth travel of concrete from top
to bottom, and particularly with changes of direction or by discharging
concrete through an obstacles reduce the possibility of segregation.
Ø
By
using a correct method of handling, transporting and placing of concrete the
chances of segregation can be minimized.
Ø
The
segregation in concrete can be controlled by using air entrain agents,
admixtures and pozzolanic material in the concrete mix.
Ø
The
formwork in which concrete should be poured should be air tight so that leakage
of slurry prevented which leads to the control of segregation.
BLEEDING:
·
Bleeding
of concrete is the tendency of water to rise to the surface of a freshly placed
concrete.
·
It
is because of water has the lowest specific gravity than aggregate and cement
so the solid constituents of the mix is unable to the hold all the mixing water
as they settle down.
·
Predominantly
bleeding observed in a highly wet mix, badly proportioned and insufficiently
mixed concrete.
·
Water
that separates from the rest of the concrete is called bleedwater.
·
Due
to bleeding sometimes certain quantity of cement also comes to the surface and
when the surface is worked up with trowel, the aggregate goes down and the
cement and water comes up to the top of the surface, this formation of cement
paste at the top surface is known as “Laitance”.
·
During
bleeding while water travelling from bottom to top makes some continuous
channels when water cement ratio is more than 0.7. These bleeding channels are
often responsible for permeability of the concrete structure. During bleeding
the bleed water sometimes intercepted by aggregate. The bleeding water likely
to accumulate below the aggregate. This accumulation of water creates water
voids and reduces the bond between the aggregates and the paste.
Bleeding can be prevented by –
1.
Bleeding
can be remedied by re vibration of concrete.
2.
Bleeding
can be reduced by proper proportioning of concrete.
3.
Uniform
and complete mixing of concrete.
4.
Bleeding
can be reduced by the use of finer cement or cement with low alkali content.
Rich mixes are less susceptible to bleeding than lean mixes.
5.
By
using finely divided pozzolanic materials reduces bleeding by creating a longer
path for the water to traverse.
6.
Air-entraining
agent is very effective in reducing the bleeding.
Bleeding of
concrete is not completely harmful if the rate of evaporation of water from the
surface is equal to the rate of bleeding. Removal of water, after it had played
its role in providing workability, from the body of the concrete by way of bleeding
will do good to the concrete.
UNIT
WEIGHT AND YIELD:
The unit
weight of fresh concrete can be determined by weighing a known volume. This is
usually performed before air content isdetermined since there is known volume
concrete. The volume of a batch of concrete can be determined from the
following relationship:
And yield of
concrete mix can be determined from –
PLASTIC
SHRINKAGE:
·
The
reduction in volume of concrete by the volume of water is removed from the
compacted concrete before its set. This volume reduction is called plastic
shrinkage.
·
Water
may be removed from the plastic concrete by evaporation or by being absorbed by
dry surface like soil or old concrete or by the dry wooden form work.
HYDRATION:
·
Concrete
derives its strength by the hydration of cement particles. Hydration of cement
is not a momentary action but a continuing process of long time.
·
Rate
of hydration starts fast but continues over a very long time, at a decreasing
rate in the field and in actual work, even a higher water cement ratio is used,
since the concrete is open to the atmosphere. The water used in the concrete
evaporates and water used in the concrete will evaporates and the water
available in the concrete will not be sufficient for effective hydration to
take place particularly in the top layer.
·
If
the hydration continue extra water must be added by curing to fill the loss of
water due to absorption and evaporation by creating a favorable environment
during the early period for uninterrupted hydration. The desirable conditions
are, a suitable temperature and moisture.
·
During
hydration, concrete releases high heat of hydration which is harmful from the
point view of volume stability and it may also shrinkage in concrete, producing
cracks. The heat reduction can be reduced by throughout water curing.
·
During
hydration there are ways in which concrete the various types of cement
compounds reacts with the water –
In the first,
a direct addition of some molecules of water take place, this being true
reaction of hydration.
In the second type of reaction with water is hydrolysis, in which its
nature can be illustrated using C3S hydration equation –
UNIFORMITY:
·
Concrete
uniformity is checked in fresh as well as in hardened concrete.
·
Slump
test, unit weight, air content test and strength test are conducted.
·
Due
to the heterogeneous nature of concrete there always be some variation. The
variations are grouped as –
-
Within-Batch
variations: Inadequate mixing, non-homogeneous nature.
-
Batch-to-Batch
Variations: Type of materials used, changes in gradation of aggregates, changes
in moisture content of aggregates.
SLUMP
LOSS:
·
From
the time of mixing fresh concrete starts gradually loses its consistency,
resulting in the problems like the concrete becomes too stiff to handle, place
and compact properly.
·
Slump
loss in concrete is cause due to the following reasons –
1.
Loss
of water by evaporation.
2.
Absorption
of water by dry aggregates.
3.
Absorption
of water by surface in contact with concrete.
4.
Hydration
of cement (generation of heat)
Arbitrarily applying the sealer will not produce desirable results. It is also important you apply the seal coat as quickly as working too slowly might cause it to dry up. asphalt repair
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