Fresh concrete in concrete technology for civil engineers

Concrete details

1. UNIT-2
FRESH CONCRETE

2. Workability
•A theoreticalwater/cement ratio calculated from the considerations is not
goingto give an ideal situation for maximum strength
•Hundred percent compaction of concrete is an important parameter for
contributing to the maximum strength
•Lack of compaction will result in air voids whose damaging effect on
strengthand durability
•To enable the concrete to be fully compacted with given efforts, normally
a higher water/cement ratiothan that theoretical considerations may be
required
•This is to say the function of water is also to lubricate the concrete so that
concrete can be compacted with specified effort forthcoming at the site
of work
•The lubrication required for handling concrete without segregation, for
placing without loss of homogeneity,for compacting with the amount of
efforts forth coming and to finish it sufficiently easily, the presence of a
certain quantity of water is of vital importance

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•The quality of concrete satisfying the requirements is
termed as workable concrete.
•Every job requires a particular workability.
•A concrete which is considered workable for mass concrete
foundation is not workable for concrete to be used in roof
construction.
•Concrete considered workable when vibrator is used, is not
workable when concrete is to be compacted by hand.
•Road research laboratory U.K., who have extensively
studied the field of compaction and workability defined
workability as “the property of concrete which determines
the amount of useful internal work necessary to produce
full compaction”

4. Factors Affecting workability

5. Water

6. MIX Proportions

7. Size

8. Shape

9. Grading

10. Admixtures

11. Measurement of workability

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32. Flow table test

33. Segregation

34. Segregation

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36. Bleeding

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39. Batching
•The measurement of materials for making
concrete is known as batching .
•There are two types of batching:
➢Volume batching
➢Weigh batching

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46. Mixing

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54. Mixing time
•Concrete mixers are generally designed to run at a speed of
15 to 20 revolutions per minute. For proper mixing, It is seen
that about 25-30 revolutions are required in a well designed
mixer.
•In the site the normal tendency is to speed up the outturn of
concrete by reducing the mixing time. This results in poor
quality of concrete.
•On the other hand if the concrete is mixed for a comparatively
longer time, it is uneconomical from the point of view of rate
of production of concrete and fuel consumption.
•Therefore it is of importance to mix the concrete for such a
duration which will accureoptimum benefit.

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•It is seen from the experiments that the quality of concrete in
terms of compressive strength will increase with increase in
time of mixing, but for mixing time beyond two minutes, the
improvement in compressive strength is not very significant.
•Concrete mixer is not a simple apparatus. Lot of
considerations have gone as input in the design of mixer
drum. The shape of the drum, the number of blades,
inclination of blades, the space between the drum and the
blades are important to give uniform mixing quality and
optimum time of mixing.
•Generally mixing time is related to the capacity of mixer. The
mixing time varies between 1 ½ to 2 ½ minutes. Bigger the
capacity of the drum more is the mixing time.

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57. Compaction of concrete

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67. Admixtures

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69. Functions

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76. Plasticizers

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79. Superplasticizers

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82. Retarders

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•Fly ash is finely divided residue resulting from the combustion
of powdered coal and transported by the flue gases and
collected by electrostatic precipitator.
•In U.K. it is referred as pulverisedfuel ash(PFA).
•Fly ash is the most widely used pozzolanicmaterial all over
the world.
•Fly ash was first used in large scale in the construction of
Hungry Horse Dam in America in the year 1948 in the
approximate amount of 30% by weight of cement.
•In India, Fly ash was used in RihandDam construction
replacing cement upto15%.

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92. Effect of fly ash on fresh concrete
•Good fly ash with high fineness, low carbon content,
high reactive forms only a small fraction of total fly
ash collected.
•The fly ash collected in chambers I&II are generally
very coarse.
•They can be called as cola ash rather than fly ash.
•Such fly ash (coal ash)are not suitable for use as
pozzolanand they do not reduce the water demand.

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95. Durability of concrete
•Sufficiently cured concrete containing good quality of fly
ash shows dense structure which offers high resistivity to
the infiltration of deleterious substances.
•Published data reports that concrete with fly ash shows
similar depth of carbonation as that of concrete with out
fly ash, as long as the compressive strength level is same.
•It is also recognisedthat the addition of fly ash
contributes to the reduction of the expansion due to
alkali-aggregate reaction.
•In conclusion it may be said that although fly ash is an
industrial waste, its use in concrete significantly improves
the long term strength and durability and reduce the
heat of hydration.

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