Fresh Concrete
Materials of Construction
Dr. TALEB M. AL-ROUSAN
Introduction
Long term properties of hardened
concrete (strength, volume, stability, and
durability) are seriously affected by its
degree of compaction.
 Consistency or workability of fresh
concrete should be such that concrete
can be properly compacted, transported,
placed, and finished easily and without
segregation.
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Concrete Composition
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Cement Paste (25%- 40% of concrete volume)
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Aggregates (60%-75% of concrete volume)
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Cement (25%- 50% of cement paste)
Water
Fine (30% – 45% of aggregate volume)
Coarse
Air (2% - 8% of concrete volume)
 Mineral Admixtures
 Liquid Admixtures
Workability
Represents the ability of concrete to be
mixed, handled, transported, and placed
with a minimum loss of homogeneity.
 Workability is expressed in terms of
consistency, mobility, and compactibility.
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Workability
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Workability: The amount of useful internal work
necessary to produce full compaction.
 Internal work: is the energy required to overcome the
internal friction between the individual particles in the
concrete.
 In practice, additional energy is required to overcome
surface friction between concrete and form work or the
reinforcement.
 Consistence: the ease of which material will flow.
 Consistence (in concrete): means degree of wetness.
 Consistency (general): degree of fluidity.
Workability Cont.
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To obtain good strength maximum possible density is
vital to achieve.
 Compressive strength increase with increasing density
(i.e. reducing air voids).
 5% air voids can lower strength by 30%.
 Voids in hardened concrete:
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Bubbles of entrapped air (its volume depends on grading of fine
aggregates and degree of wetting).
Spaces left after excess water has been removed (its volume
depends on w/c).
Thus for any method of compaction there is an optimum water
content of the mix at which the sum of volume of voids will be a
minimum, and the density will be maximum.
Factors Affecting Workability
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Water content (increase workability since acting as
lubricant).
Aggregate type and grading
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Finer particles require more water to wet their larger specific
surface.
Irregular shape & rougher texture of angular aggregates
demand more water than rounded aggregates.
Porosity & absorption.
Light weight aggregates lower workability.
High ratio of volumes of coarse aggregates to fine aggregate
can result in segregation and in a lower workability (harsh
mix).
Too many fines lead to higher workability
Factors Affecting Workability Cont.
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Aggregate/ Cement ratio:
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for constant w/c , workability increase as the aggregate
cement ratio is reduced because the amount of water relative
to the total surface of solids is increased.
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Presence of admixtures: air entrainment reduce water
requirement.
 Fineness of cement: the finer the cement the greater
the water demand.
 Time
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Fresh concrete stiffens with time (different than setting).
Stiffening of concrete is measured by loss of workability with
time (Slump Loss).
Temperature:
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High temperature reduce workability and increases slump
loss.
Cohesion & Segregation
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Concrete should not segregate (i.e. ought to be cohesive).
Segregation: Separation of the constituents of a
heterogeneous mixture so that their distribution is no
longer uniform.
Primary cause : Difference in size of particles and some
times different in S.G of mix constituents; method of
handling and placing.
Controlled by: Suitable grading; and care in handling,
transporting, and placing, use of air entrainment.
Forms:
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Coarser particles tends to separate out (happens in dry mixes).
Separation of grout (occurs in wet mixes) from the mix.
Improper use of vibrators (Over-vibration) cause
segregation.
Bleeding
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Also known as water gain.
Form of segregation in which some of the water in the
mix tends to rise to the surface of freshly placed
concrete.
Cause: Inability of solid constituents in the mix to hold
all the water when they settle downwards.
Expressed quantitatively as the total settlement
(reduction in height) per unit height of concrete.
As a result of bleeding, the top of every layer of
concrete placed become too wet, and if water is
trapped, a porous and weak layer of non-durable
concrete will result.
Bleeding Cont.
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Bleeding on the top surface cause a weak wearing
surface. This can be avoided by:
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Delaying finishing untill bleeding water evaporates
Using wood floats
Avoiding overworking the surface
If evaporation of water from the surface of concrete is
faster than the bleeding rate, plastic shrinkage
cracking may result.
Some of bleeding water become trapped under
aggregate particles or reinforcement, thus creating
zones of poor bond. This water leaves behind voids
which are oriented in same direction which may
increase permeability of concrete.
Bleeding may increase frost damage.
Bleeding Cont.
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Bleeding is not always harmful, when water
evaporate the water cement ratio decrease
which result in strength increase.
 Tendency of bleeding depends largely on
cement properties (bleeding is lower with finer
cement, high alkali, high C3A, or when NaCl is
added).
 Lower bleeding can be achieved with rich
mixes, addition of Pozzolan, and air
entrainment.
Workability Tests
There is no direct test that measures
workability as defined.
 Slump test.
 Compaction factor test.
 Vebe test.
 Flow table test.
 Ball penetration test.
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Slump Test / ASTM 143- 90a
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Mold is a frustum of a cone (12 in high, 8 in D
base, and 4 in D opening).
Mold placed on a level smooth surface with
small opening up.
The inside of the mold and base are moistened
with water to reduce influence of the variation of
surface friction.
Mold is filled with concrete in three layers.
Each layer is tamped 25 times with a steel rod
(5/8 in D).
Top surface is struck off by screeding.
Slump Test Cont.
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Mold must be firmly held against its base during
the entire operation (facilitated by handles and
foot-rests).
Clean the area around the base of the mold from
concrete which may be dropped during filling.
Immediately after filling, the cone is slowly lifted.
Unsupported concrete will slump.
The decrease in the height of the center of the
slumped concrete is called slump and is
measured to the nearest 1/4 in.
Slump Test Cont.
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True slump: concrete slump evenly all around (See
Figure 5.2 a) (0 – 125 mm).
Shear Slump: One half of the cone slides down an
inclined plane (Test should be repeated). It indicates
lack of cohesion of the mix. ( up to 150 mm)
Collapse Slump (Test should be repeated).(150 – 250
mm)
Advantages: check variation in materials being fed into
the mixer for a daily or hourly bases.
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For example: increase in slump may indicate increase in
water or change in grading of aggregates such as deficiency
in sand.
Very simple test which made it widely spread.
The Slump Test - ASTM C 143 Measures
Consistency
(b) Normal slump or collapse slump (harsh or extremely wet mix)
(c) Shear Slump: Concrete lacks plasticity and cohesion
Notes on the Slump Test
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Recommended slump values depend on types
of construction.
Can be used to test for the uniformity of the
concrete delivered to the project.
Influenced by changes in ambient temperature.
Values lower than requirements can be used if
the concrete can be placed within the forms.
Workability degrees: very low (0 – 25mm); low
(25 – 50mm); medium (50 – 100mm); high (100
– 175mm).
Compaction Factor
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Best test available that uses an inverse
approach to the workability definition (the
amount of work necessary to produce full
compaction).
 Compaction factor Approach: determining the
degree of compaction achieved by a standard
amount of work.
 Measured by density ratio: Ratio by the
density actually achieved in the test to the
density of the same concrete fully compacted.
 Tests developed in UK (BS).
Compaction factor Cont.
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Apparatus (See Fig 5.3 in Text):
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Two hoppers in a shape of a frustum of a cone with hinged
doors at the bottom (one above the other with the lower being
smaller).
One cylinder.
The upper hopper is filled with concrete gently with no
compaction.
 The bottom door of the upper hopper is opened and
the concrete falls into the lower hopper.
 Since the lower hopper is smaller therefore it will be
filled to overflowing.
 The bottom door of the lower hopper is released and
the concrete falls into the cylinder.
 Excess concrete is cut.
Compaction Factor Apparatus
Compaction factor Cont.
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Net mass of the concrete in the known volume
of the cylinder is determined.
 Calculate the density of the concrete in the
cylinder.
 Compaction Factor = Ratio of the density of
concrete in the cylinder to the density of the
fully compacted concrete.
 Fully compacted concrete density is found by
filling the cylinder with concrete at four (4)
layers each tamped or vibrated. Density is
found by dividing concrete mass in the cylinder
to its known volume.
Compaction factor Cont.
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High compaction factor indicate higher
workability that is equivalent to high slump
values.
More sensitive than slump at low workability.
Not very appropriate for dry mixes.
Due to its large size the compaction factor
apparatus is not convenient at site.
Workability degrees: very low (0.75); low
(0.85); medium (0.92); high (0.95).
Vebe Test
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BS 1881: Part 104: 1983
See Fig. 5.4 in text for Apparatus diagram.
Standard slump cone is placed in a cylinder (D=9.5 in ,
H=8 in).
Slump cone is filled in a standard manner.
Cone removed
A disc-shape rider (2.75 kg) is placed on top of the
concrete.
Compaction is achieved using vibrating table.
Compaction is complete when the transparent rider is
covered with concrete and all cavities on the surface
are disappeared (judged visually).
Vebe Test
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The time required for complete compaction is
known as Vebe seconds and its considered as
measure of workability.
 Dry concrete or low workability concrete
required more time.
 Vebe is good workability specially for very dry
mixes.
 The method has the advantage of being
similar to the work done in site (i.e. treatment
of concrete during test is closely related to
method of placing in practice).
Vebe Apparatus
Flow Table
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BS 1881: Part 105: 1984
Apparatus shown in (Fig. 5.5 in Text).
Wooden board covered by a steel plate with total mass
(16 kg).
This board is hinged along one side to a base board.
Each board being square with (700 mm) side.
The upper board can be lifted up to a stop so that the free
edge rises (40mm).
The table top is moistened and a frustum of a cone of
concrete is placed using mold with (8 in high & 8 in
bottom diameter & 5 in top diameter) that is lightly tamped
with wooden tamper at two layers.
Remove excess concrete before lifting the mold.
Remove mold after 30 sec.
Flow Table Cont.
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The table top is lifted and allowed to drop for 15 times
for the specified distance (40 mm). Each cycle should
take 4 sec.
 In consequence, the concrete spread.
 Measure the max. spread parallel to the two edge.
 The average of the two values represent the flow.
 A value of 400 mm indicates Medium workability
concrete.
 A value of 500 mm indicates High workability concrete.
 Concrete should appear uniform and cohesive
otherwise it is considered inappropriate.
Flow Table
Ball Penetration Test
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ASTM C360 -92
Simple field test
 Determines the depth to which a (6 in) diameter metal
hemisphere (30 Lb) will sink under its own weight into
fresh concrete.
 Known also as Kelly Ball (See fig. 5.6).
 As slump test, Kelly ball test is used for routine
checking.
 Simpler than slump test and quicker to perform.
 Can be applied to concrete in wheelbarrow or in forms.
 Depth of concrete being tested should > (8 in) and
least lateral dimension > (18 in).
Kelly Ball
Comparison of Tests
There is no unique relation between the
results of the various tests.
 Fig. 5.8 for pattern of relation between
workability tests.
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General Pattern of Relations between
Workability Tests
Vebe time sec
Vebe time
-sec
Compaction factor
Slump -mm
Slump -mm
Compaction factor
Density (Unit mass or Unit Weight
in Air) of fresh Concrete
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ASTM C138 -92
Density is obtained by weighing the compacted fresh
concrete (by rodding or vibrating) in a standard
container of known volume and mass.
If the density is know, it becomes easy to find the
volume of concrete using the mass of the ingredients.
When the ingredients are expressed as quantities in
one batch, then we can calculate the yield of concrete
per batch.
Yield: volume of concrete in a given batch
Density of Fresh Concrete (r)
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Let mass per batch of:
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W = Water mass per batch
C = Cement mass per batch
Af = Fine aggregate mass per batch
Ac = Coarse aggregate mass per batch
Then the volume of compacted concrete
obtained from one batch (yield) is
V = [(C + Af + Ac + W) / r ]
 Also the cement content (mass of cement per
unit volume of concrete ) is
C/ V = r - [ (Af + Ac + W) / V ]
Volume of Air Voids
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Volume of air voids in concrete (Va)
Va = Vconc – (Vc + Vw + Vagg)