2. 2
Lab. Objective
Fresh Concrete
Concrete workability
Slump test
Tools & Equipment's
Procedures
Observations & Recording
Objectives:
To measure the slump of a freshly mixed concrete as a part of the concrete quality control process.
4. 4
Fresh Concrete
Concrete is called fresh when the setting and hardening process has not started.
When concrete is its plastic state, it is known as fresh concrete.
Fresh concrete can be shaped and poured which means it can be transported or pumped and used to fill molds and formwork.
Fresh concrete can be easily molded to a durable structural member.
It can be prepared on the spot and may give a wide range of properties from easily available raw materials.
5. 5
Factors Affecting Fresh Concrete
Factors Affecting Fresh Concrete:
Temperature.
Water Cement Ratio / normal consistency
Aggregate properties (gradation/quality)
Cement properties (admixtures/ fineness/quality/type)
Segregation
Bleeding
Plastic shrinkage
Setting Time
Hydration process
Workability
6. 6
Factors Affecting Fresh Concrete - Temperature
• Concrete is not recommended to be placed at a temperature below 4°C and above 35°C without proper precautions.
• Special problems are encountered in the preparation, placement, and curing of concrete in hot weather.
• The First 24 to 72 hours after placing fresh concrete are of extreme importance.
• If the temperature of concrete is not controlled and it goes beyond the maximum range during hydration, then internal stresses are
produced, and cracks are formed in concrete. Also it will slow down the time needs for concrete to gain its strength.
• Also if the temperature falls below the minimum temperature, then concrete takes a long time to set, and the hydration process
slows down.
So we can control the temperature effect by
• Admixtures
7. 7
Factors Affecting Fresh Concrete – Water Cement Ratio
It is the ratio of cement and water utilized in the preparation of concrete.
The quantity of water used to mix concrete is very important.
If the percentage of water used is less than the proper amount, then there will not be a sufficient quantity of water to hydrate
cement. It will result in weak, porous concrete and lower workability.
If the percentage of water used is more than the proper amount, then there will be decrement in strength and higher workability.
Too much water results in the segregation of aggregates and gives porous concrete of low strength and low density.
A certain minimum proportion of water is necessary to hydrate the cement completely. To create the concrete sufficiently workable to
be placed in position, some more water is needed.
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Factors Affecting Fresh Concrete – Water Cement Ratio
The water-cement ratio shouldn’t be allowed to exceed the specified
limits for various types of concrete and should usually be kept as low as
the methods of placing will allow.
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Factors Affecting Fresh Concrete – Segregation of Concrete
Segregation is usually caused by excessive vibration of cement. The different materials that make up the concrete mix
have different weights, so while the concrete is in a liquid state, there is a tendency for heavier materials to settle to the
bottom, and the lighter slurry to rise to the top.
More specifically, it implies the separation of coarse aggregates from the mortar in which they are composed due to
differences in the size, density, shape, and other properties of the material.
Segregation
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Factors Affecting Fresh Concrete – Segregation of Concrete
:
Due to the separation, the hive (honeycomb) is made into concrete, and it
basically affects the strength of the concrete and its porosity.
During concrete work, isolation in concrete can occur on-site, and this
affects the durability of your structures.
In fine concrete, all the ingredients are properly distributed and form a
homogeneous mixture.
If a concrete sample exhibits a tendency to separate coarse aggregates from
the rest of the material, it indicates the separation in the concrete.
Harmful Effects Due to the Segregation of Concrete:
• To comb the concrete honey.
• The high permeability of concrete.
• Low compressive strength.
• Poor finishing of surfaces.
Isolation means the separation of newly designed solid ingredients
from each other in a non-uniform mixture.
Honeycomb
12. 12
Factors Affecting Fresh Concrete – Bleeding of Concrete
Bleeding in concrete is sometimes referred to as water gain. This is a special form of separation, in which some
water from the concrete flows to the surface of the concrete, with the lowest specific gravity among all concrete
components.
Excessive bleeding is seen in thin members such as roof slabs or road slabs, and when the concrete is placed in
sunny weather.
Prevention of bleeding in concrete:
1. Bleeding can be reduced with appropriate proportions and
uniform and complete mixing.
2. Finely divided pozzolanic materials, making a long way to
cross the water.
3. The air-penetrating agent is very effective for reducing
blood.
4. Fine cement or cement with low alkali content.
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Factors Affecting Fresh Concrete – Plastic Shrinkage
If water is removed from the concrete before it sets, the volume of the concrete is reduced by the
amount of water removed. This volume reduction is known as plastic shrinkage.
This is the shrinkage that the fresh concrete undergoes until it sets completely.
Water may be removed from the plastic concrete by
evaporation or by being absorbed by dry surfaces such as soil
or old concrete or by the dry wooden formwork.
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Factors Affecting Fresh Concrete – Setting Time of Fresh Concrete
When concrete changes its state from fresh to hardened then this process is called setting. The
time required to complete this process is known as the setting Time of Concrete.
Setting time depends on the type of cement and it can be increased or decreased by adding
admixture in concrete.
15. 15
Factors Affecting Fresh Concrete – Hydration of Fresh Concrete
When cement comes in contact with water, a chemical reaction begins. This reaction
is known as hydration.
If the mixing water dries out too rapidly before the cement has fully hydrated, the curing
process will stop, and the concrete will not harden to its intended strength.
Hydration occurs more rapidly at higher air temperatures.
Hydration itself also generates heat.
This heat of hydration can be helpful during cold-weather construction but harmful during
hot-weather construction.
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Factors Affecting Fresh Concrete – Workability Concrete
The diverse requirements of transportability, compatibility, mobility, stability, mix-ability, playability, and finish
ability of fresh concrete mentioned above are collectively referred to as workability.
The workability of fresh concrete is thus a composite property. It is difficult to define precisely all the aspects of
the workability in a single definition.
Workability can be defined as that Property of freshly mixed mortar or concrete that determines the ease and
homogeneity by which it could be mixed, placed, compacted, and completed.
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Factors Affecting the Workability of Concrete
Nine important Factors that affect the workability of concrete :
1- water content of the concrete Mix
2- Size of Aggregate
3- Shape of Aggregate
4- Grading of Aggregate
5- Using Admixtures
6- Temperature
7- Type and components of Cement
8- Time
9- Wind
18. 18
Factors Affecting the Workability of Concrete
1- Water Content of the Concrete Mix:
Water content will have important influences on the workability in given volume of concrete. The higher the
water content per cubic meter of concrete, the higher will be the fluidity of concrete, which affect the
workability.
Water requirement is mainly associated with absorption by aggregates surface & filling up the voids between
aggregates.
However, adding extra water to concrete mix can be disadvantageous, as given below:
- Strength of the concrete may get reduced.
- More quantity of water comes out from the surface of concrete resulting into bleeding.
- Cement slurry also escapes through the joints of formwork resulting into loss of cement from concrete
2- Size of Aggregate:
Workability is mainly governed by maximum size of aggregates. Water and paste require, will not be less if a
chosen size of aggregates for concrete is bigger. Consequently, for a given quantity of water content & paste,
bigger size aggregate will give higher workability.
On site, the maximum size of aggregate to be used will depend upon the many factors such as, the handling,
mixing and placing equipment, thickness of section and quantity of reinforcement. Later two are very important.
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Factors Affecting the Workability of Concrete
3- Shape of Aggregates:
The shape of aggregates seriously influences the workability of concrete. Angular, flaky & elongated aggregate
reduces the workability of concrete.
Rounded or sub rounded aggregates increase the workability due to reduction of surface area for a given volume
or weight. Therefore, excess paste is available to give better lubricating effect. Rounded shape aggregate has less
frictional resistance and give a high workability as compared to angular, flaky or elongated aggregates
4- Grading of Aggregates:
Grading of aggregate will have a huge influence on workability. The better the grading, the less is the amount of
void in concrete so well graded aggregates should be used. When total voids are less in concrete, excess paste is
available to give better lubricating effect.
With excess amount of concrete paste present in the mixture, it becomes cohesive & fatty that prevents
segregation of particles & least amount of compacting efforts is required to compact the concrete.
For a given workability, there is one value of coarse / Fine aggregate ratio, which needs the lower water content
20. 20
Factors Affecting the Workability of Concrete
5- Using Admixtures:
This is one of the commonly used methods to enhance workability of concrete. Chemical admixtures such as
Plasticizer and superplasticizers greatly improve the workability.
Air entraining agents are also used to increase the workability. Air entraining agents creates a large number of
very tiny air bubbles. These bubbles get distributed throughout the mass of concrete and acts as rollers and
increases workability.
Mineral admixtures like Pozzolanic materials are also used to improve workability of concrete
6- Temperature:
In hot weather, if temperature increases, evaporation rate of mixing water also increases and hence fluid viscosity
increases, too. This phenomena affects the flow ability of concrete and due to fast hydration of concrete, it will
gain strength earlier which decreases the workability of fresh concrete.
7- Type and Composition of Cement:
There are also effect of type of cement or characteristics of cement on the workability of concrete. The cement
with increase in fineness will require more water for same workability than the comparatively less fine cement.
The water demand increased for cement with high Al2O3 (Aluminum oxide) or C2S (Carbon disulfide) contents.
21. 21
Factors Affecting the Workability of Concrete
8- Time:
Fresh concrete stiffens with time and loss workability though it is not exactly settling or getting strength at all.
After mixing concrete, some water is absorbed by aggregate, some may be lost by evaporation and some may be
spent for initial chemical reactions.
The loss in workability by time depends on various factors like:
Initial workability: if initial workability is high, slump loss will be greater
Property of cement: if alkali content is high and sulfate content is low, slump loss will be greater
Moisture content of aggregate: dry aggregate will absorb more water and workability will decrease
9- Wind:
If wind is moving with greater velocity, the rate of evaporation also increase reduces the amount of water and
ultimately reducing workability
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Factors Affecting the Workability of Concrete
Practices to improve the workability of concrete:
1. Increase water/cement ratio
2. Increase size of aggregate
3. Use well-rounded and smooth aggregate instead of irregular shape
4. Use non-porous and saturated aggregate
5. Increase the mixing time
6. Decrease the mixing temperature
23. 23
Slump Test
Slump Test:
Concrete slump test or slump cone test is to determine the
workability or consistency of concrete mix prepared at the
laboratory or the construction site during the progress of the
work.
Concrete slump test is carried out from batch to batch to check
the uniform quality of concrete during construction. The slump
test is the most simple workability test for concrete, involves
low cost and provides immediate results.
The slump is carried out as per procedures mentioned in ASTM
C143 in the United States and EN 12350-2 in Europe.
Generally concrete slump value is used to find the workability,
which indicates water-cement ratio, but there are various
factors including properties of materials, mixing methods,
dosage, admixtures etc. also affect the concrete slump value.
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Procedure
1. Clean the internal surface of the mold and apply oil.
2. Place the mold on a smooth horizontal non- porous base plate.
3. Fill the mold with the prepared concrete mix in 3-4 approximately equal layers.
4. Tamp each layer with 25 strokes of the rounded end of the tamping rod in a uniform manner over the cross
section of the mold. For the subsequent layers, the tamping should penetrate into the underlying layer.
5. Remove the excess concrete and level the surface with a trowel.
6. Clean away the mortar or water leaked out between the mold and the base plate.
7. Raise the mold from the concrete immediately and slowly in vertical direction.
8. Measure the slump as the difference between the height of the mold and that of height point of the specimen
being tested.
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Observation and Recording:
Part1:
1- Use the table of mix design to prepare the sample.
2- Find the following values: w/c = ………………………..
Temperature= …………………………
Part2:
1-Find the value of Slump = ………………………..
2-Find the Slump type.
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Types of Concrete Slump Test Results
TRUE SLUMP:
concrete mix retains its
shape to a large degree.
Concrete drops evenly
without any
disintegration. This is the
desired test result.
ZERO SLUMP:
Concrete maintains the
shape of the mold. This type
of concrete is too stiff and
has almost no workability.
A zero slump indicates the
very low water-to-cement
portion.
COLLAPSED SLUMP:
Concrete completely
collapses. This slump
indicates that the water-to-
cement portion is too high
and the mix has high
workability.
SHEAR SLUMP:
If one-half of the cone slides
down in an inclined plane.
This is an indication that the
concrete lacks cohesion.
Shear slump indicates that the
result is imperfect, and the
concrete should be tested
again.
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Calorimeter Test
Calorimetry:
Measures the heat generated from the early hydration of cementitious materials. The measurement of the
reaction rate in a calorimeter is continuous and in real time, giving visibility into the behavior of cement
paste, concrete or mortar in a way that traditional testing such as set time or compressive strength tests
could not
hydration of cement:
The hydration of cement is an exothermic reaction that is carried out when cement comes in contact with
water. The reaction will result in several by-products that will contribute to the strength of the concrete or
mortar formed.
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Calorimeter Test
Stages of Hydration of Cement:
Stage 1 - Initial Hydrolysis: The initial dissolution of cement will result in the sort release of heat shown by
the first peak in the calorimetry curve.
Stage 2 - Induction or Dormant Period: After the initial dissolution process, the hydration products are
precipitated on the surface of each cement particle. The layer acts as a protective barrier and temporarily
delays the dissolution of the particle. This slows down the reaction for a period of several hours. This is called
as the Dormant Period.
The existence of the dormant period allows the concrete to be transported to the construction site and
placed and finished in the forms.
Stage 3 - Hydration Acceleration: The end of the dormant period shows the beginning of the setting at which
time the cement starts to react more rapidly with water. This will result in the formation of new hydration
products.
Stage 4 - Hydration Deceleration: This period will undergo formation of hydration products but the rate of
reaction and the dissolution is very controlled and slow.
Stage 5 - Steady State: This is the stage that is for a longer period that is equal to the age of the structure.
The hydration reaction carried out throughout this period at a very slow rate.
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Calorimeter Test - Apparatus
An apparatus that measures quantities of heat of chemical or physical changes as well as
heat capacity.
34. 34
Calorimeter Test - Procedure
Calorimeter test procedure:
1. Store the cement and the mixing water before the test at room temperature (23 oC ± 2 oC).
2. Before the test, make sure that they are at the ambient temperature.
3. Once done, mix 150 g of cement and 60 mL of distilled water.
4. Stir the mixture with a mechanical stirrer for 5 minutes inside the calorimeter.
5. Measure the initial temperature just after mixing the cement with water.
6. Record the temperature every 20 minutes (heat of hydration).
7. Plot the time vs. temperature.
