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CIV415
CONCRETE TECHNOLOGY
Chapter V
Fresh Concrete
Assist.Prof.Dr. Mert Yücel YARDIMCI
Spring, 2014/2015
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Fresh Concrete
Fresh concrete is defined as a fully mixed concrete in a rheological
state that has not lost its plasticity.
The fresh concrete stage covers the cement hydration stages I and II.
The plastic state of fresh concrete provides a time period for
transportation, placing, compaction, and surface finishing.
The properties of fresh concrete have a large influence on
construction speed and decision making.
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Workability of fresh concrete
The properties of fresh concrete are short-term requirements in
nature, and should satisfy the following requirements:
1. It must be easily mixed and transported.
2. It must be uniform throughout a given batch, and between batches.
3. It must keep its fluidity during the transportation period.
4. It should have flow properties such that it is capable of completely filling the
forms.
5. It must have the ability to be fully compacted without segregation.
6. It must set in a reasonable period of time.
7. It must be capable of being finished properly, either against the forms or by
means of troweling or other surface treatment.
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Workability of fresh concrete
Compaction plays an important role in ensuring the long-term properties of the
hardened concrete, as proper compaction is vital in removing air from concrete
and in achieving a dense concrete structure.
The compressive strength of concrete can increase with an increase
in the density. Traditionally, compaction is carried out using a
vibrator.
Nowadays, the newly developed self-compacting concrete can reach
a dense structure by its self-weight without any vibration.
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Definition of workability
Workability of concrete is defined in ASTM C125 as the
property determining the effort required to manipulate a
freshly mixed quantity of concrete with minimum loss of
homogeneity (uniform).
The workability of fresh concrete can be defined as “the
amount of mechanical work, or energy, required to
produce full compaction of the concrete without
segregation (Mindess et al. 2003).”
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Measurement of workability
There is no universally accepted test method that can
directly measure the workability.
• The most widely used test, which mainly measures the
consistency of concrete, is called the slump test.
• The second test in order of importance is the Vebe test, which is
more meaningful for mixtures with low consistency.
• The third test is the compacting factor test, which attempts to
evaluate the compactability characteristic of a concrete mixture.
• The fourth test method is the ball penetration test that is
somewhat related to the mechanical work.
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Slump test
If slumping occurs evenly all around, it is regarded as a true slump.
If one-half of the cone slides down along an inclined plane, it is
regarded as shear slump.
Shear slump is caused by insufficient cohesiveness and the
concrete proportions should be adjusted.
The slump test is unreliable for very dry and lean mixes.
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Slump test
https://www.youtube.com/watch?v=lwZf217v5XA
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VeBe test
The test equipment, which was developed by Swedish engineer V. Bahrner, is shown in
Figure.
The Vebe test is a good test,
particularly for very dry mixes.
The cohesiveness of concrete
can be easily distinguished by
Vebe test through the
observation of distribution of the
coarse aggregate after vibration.
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VeBe test
https://www.youtube.com/watch?v=XWJviHtlTsc
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Compacting Factor
It consists of two hoppers and one
cylindrical mold stacked in three levels.
Usually, the range of compaction factor
is from 0.78 to 0.95 and concrete with
high fluidity has a higher compaction
factor.
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Compacting factor
http://youtube.com/watch?v=SRo2vYw_QPI
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Ball penetration test
A concrete with higher consistency leads to a
deeper ball penetration.
Since the measurement is related to the work done
by the hammer penetration, ball penetration
measurement is close to the definition of
workability given by Mindess et al. (2003).
The penetration test is usually very quick and can
be done on site, right in the formwork, provided it
is wide enough.
Weight of the ball is 13.6 kg
The ratio of slump value to penetration depth is
from 1.3 to 2.0.
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Factors affecting workability
• Water content
• Cement content
• Aggregate characteristics
• Admixtures
• Temperature and time
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Through the
influences on
consistency and/or
cohesiveness
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Water content
• The most important factor influencing the workability of
concrete.
• Some of the added water is absorbed on the surface of the
the cement and aggregates. Additional water fills the
spaces among the particles and “lubricates” the particles
by a water film.
• Decreasing the water content will result in a low fluidity.
• If the water content is too small, the concrete will become
too dry to mix and place.
• Too much water will reduce cohesiveness and leads to
segregation and bleeding, and reduces the concrete
strength.
• The water content in a concrete is determined by w/c or
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w/b and cement or binder content.
Cement content
Cement content influences the workability of concrete in two ways.
• For given w/c ratio, the larger the cement content, the higher
the total water amount in the concrete; hence, the consistency
of concrete will be enhanced.
• Cement paste itself plays the roles of coating, filling, and
lubrication for aggregate particles.
In normal concrete, a considerably low cement content tends to
produce a harsh mixture, with poor consistency and, subsequently,
poor finishability.
High cement content implies that more lubricant is available for
consistency improvement.
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Cement content
With an increase of the cement content at a low w/c ratio, both
consistency and cohesiveness can be improved. Under the same w/c
ratio, the higher the cement content, the better the workability.
Increasing the fineness of the cement particles will decrease the
fluidity of the concrete at a given w/c ratio, but will increase the
cohesiveness.
Concretes containing a very high proportion of cement or very fine
cement show excellent cohesiveness but tend to be sticky.
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Aggregate Characteristics
Aggregates can influence the workability of concrete through their need for surface
coating and their friction and mobility during mixing, placing, and compaction.
Aggregate Properties Influencing Workability
•
•
•
•
Maximum aggregate size,
Aggregate/cement ratio,
Fine aggregate/coarse aggregate ratio,
Aggregate shape and texture
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Aggregate Characteristics
For a given w/c ratio, as the maximum size of aggregate
increases, the fluidity increases.
Very fine sands or angular sands will require more paste for a
given consistency;
They will produce harsh and unworkable mixtures at water
contents that might have been adequate with coarser or wellrounded particles.
In general, to get a similar consistency of concrete, more
water is needed when crushed sand is used instead of
natural sands.
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Aggregate Characteristics
The aggregate/cement ratio influences the paste requirement.
A higher aggregate/cement ratio implies more aggregates and less
cement paste.
The concrete consistency decreases with aggregate/cement ratio
increase due to less cement paste being available for lubrication.
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Aggregate Characteristics
Fine aggregate/coarse aggregate ratio also affects the cement
paste requirement.
With an increase of the fine aggregate/coarse aggregate
ratio, concrete contains more fine aggregates and less
coarse aggregates. The total surface area of the aggregates
increases, which leads to a higher demand on the cement
paste for surface coating.
As a result, the consistency of concrete decreases and the cohesiveness
improves.
Increasing the fine aggregate/coarse aggregate ratio is the most effective
measure to increase the cohesiveness of concrete.
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Aggregate Characteristics
The shape and texture of aggregate particles can affect the
workability of concrete through the influence on paste
requirement, particle moving friction, and moving ability.
Cubical, irregular, granular, and rough aggregates require
more coating cement paste and have higher friction than
spherical, glassy, and smooth aggregates.
As a general rule, the more spherical the particles, the more
workable is the concrete.
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Temperature and time
Freshly mixed concrete stiffens with time due to loss of water by
• evaporation of the mixing water.
• absorption by the aggregate.
• consumption in the formation of hydration products.
The stiffening of concrete is effectively measured by a loss of
workability with time, known as slump loss, which varies with
• richness of the mix,
• type of cement,
• temperature of the concrete,
• initial workability.
A high temperature reduces the workability and increases the slump loss because the
hydration rate is higher and the loss of water is faster at a higher temperature.
In practice, when the ambient conditions are unusual, it is best to perform actual site tests
to determine the workability of the mix.
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Segregation and bleeding
SEGREGATION
Cohesiveness is an important
characteristic of the workability.
A proper cohesiveness can ensure
concrete to hold all the ingredients in a
homogeneous way without any
concentration of a single component,
and even after the full compaction is
achieved.
An obvious separation of different constituents in concrete is called
segregation
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Segregation and bleeding
Segregation can be defined as concentration of
individual constituents of a heterogeneous (nonuniform)
mixture so that their distribution is no longer uniform.
In the case of concrete, it is the differences in the size and weight of
particles (and sometimes in the specific gravity of the mix
constituents) that are the primary causes of segregation, but the
extent can be controlled by the concrete proportion, choice of
suitable grading, and care in handling.
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Segregation and bleeding
Bleeding
is a form of local
concentration of water in some
special positions in concrete,
usually the bottom of the coarse
aggregates, the bottom of the
reinforcement, and the top
surface of the concrete member.
Bleeding can be expressed quantitatively as the total settlement
(reduction in height) per unit height of concrete, and bleeding
capacity as the amount (in volume or weight) of water that rises to
the surface of freshly placed concrete.
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Segregation and bleeding
As a result of bleeding, an interface between aggregates and bulk cement paste
is formed, and the top of every lift (layer of concrete placed) may become too
wet. If the water is trapped by the superimposed concrete, a porous and weak
layer of nondurable concrete may result.
If the bleeding water is remixed during the finishing process of the surface, a
weak wearing surface can be formed. This can be avoided by delaying the
finishing operations until the bleeding water has evaporated, and also by the use
of wood floats and avoidance of overworking the surface.
On the other hand, if evaporation of water from the surface of the concrete is
faster than the bleeding rate, plastic shrinkage cracking may be generated.
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Slump loss
Slump loss can be defined as the loss of
consistency in fresh concrete with elapsed time.
Slump loss is a normal phenomenon in all concretes
because it results from gradual stiffening and setting of
hydrated cement paste, which is associated with the
formation of hydration products such as ettringite and
calcium silicate hydrate.
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Slump loss
Slump loss occurs when the free water from a concrete mixture is
removed by hydration reactions, by absorption on the surface of
hydration products, and by evaporation.
Slump loss should be controlled to an acceptable value, especially
for concrete transported with a long delivery time, to ensure that it
is still placeable and compactable when shipped to the construction
site.
Slump loss can be minimized by using a setting retarder.
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Setting of Concrete
Setting of concrete is distinguished as
the initial and final setting of cement
paste.
The initial setting is defined as the
loss of plasticity or the onset of
rigidity (stiffening or consolidating) in
fresh concrete.
The final setting is defined as the onset point of strength.
It is different from hardening, which describes the development of useful and
measurable strength.
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Setting time
The measurement of the setting time for concrete is very different
from that of cement paste.
For cement paste, it uses the samples made of the water amount
needed for consistency. For concrete, it uses the sieved mortar
from a concrete with different water/cement or water/binder
ratios.
For cement paste, it measures the penetration depth of the Vicat
needle, 1mm in diameter, under a constant weight.
For concrete, it measures the resistance of the mortar to a rod
under an action of the load.
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Setting time
ASTM C 403 (1995) defines two points as the initial
setting time and the final setting time, corresponding
approximately to the point at which the concrete will no
longer be plastic during vibration, and a concrete
strength gain of about 0.7 MPa.
Initial setting and final setting of concrete are determined
by the times at which the penetration resistance reaches
3.5 and 27.6 MPa, when a designated rod penetrates
25.4mm into the mortar, sieved from the fresh concrete.
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Abnormal setting
• False setting
• Flash setting
False setting: If a concrete stiffens rapidly in a short time
right after water is added and restores its fluidity by
remixing and sets normally, it is called false setting.
The main reason causing the false setting is crystallization
of gypsum.
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False setting
In the last procedure in the process of cement production,
gypsum is milled with a clinker by intergrounding. During
grinding, due to clinkers that may be still very hot and
heat generated by friction, the temperature can rise to
about 120◦C, thus causing the reaction
CSH1/2 is called half-water gypsum or plaster. During mixing, when
water is added, the plaster will rehydrate back to two-water gypsum,
form a crystalline matrix quickly, and make the concrete stiffen.
Due to the small amount of plaster in the mix, very little strength will actually develop and
the fluidity can be easily restored by further mixing to break the plaster set.
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Flash setting
It is caused by the formation of large quantities of
monosulfoaluminate or other calcium aluminate hydrates
due to quick reactivities of C3A, without the presence of
gypsum.
This is a rapid set and thus is a more severe
condition than false setting.
Flash setting has been largely eliminated by the addition of 3 to 5%
gypsum to the cement, which can react with C3A and water to form
AFt as a barrier layer of C3A to prevent further reaction of C3A
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