Building Materials, Third Edition

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tends to pull the heavy solid particles downward, the lighter water being displaced upward.
This upward migration of water known as bleeding ceases either when the solid particles
touch each other and cannot settle any more, or when the concrete stiffens due to cement
hydration and prevents further movement. It can be reduced by grinding cement fine, or by
using air entraining agents, dispersing agents, puzzolanas and by imparting vibration. It is a
particular form of segregation in which some of the water comes out on the surface of concrete.
Mixes which bleed excessively are those which are harsh and not sufficiently cohesive. Basically
this is caused by the lack of very fine materials in mix. Obviously this is because of the coarse
sand or when the sand content has been kept low. The causes of bleeding are highly wet mix,
insufficient mixing, and thin sections (slabs) cast in sunny weather—being more in flaky
aggregate and more in the first hours of concreting. The ill effects are reduced bond between
aggregate and cement, and between cement and reinforcement.
Bleeding can be checked by the use of uniformly graded aggregates, puzzolana—by breaking
the continuous water channel, or by using—entraining agents, finer cement, alkali cement, and
a rich mix.

v—
—™ defined as cement and water slurry coming on top and setting on the surface is very
dangerous since the top surface will weather out fast with larger shrinkage cracks. If laitance
is formed in a lift, it should be removed before next lift is placed.

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Delayed vibration of concrete, already placed and compacted, is known as revibration. When
successive top layers are vibrated, some vibration is transferred to bottom layers. It is beneficial
since the quality of concrete improves as the entrapped air and water escape and also the
rearrangement of particles take place.

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The true elastic curve for concrete in compression
(Fig. 10.36) can be plotted by applying and
releasing load until the set at zero loads becomes
constant. By subtracting the-set from the total
deformation the elastic deformation for a given
load is ditermined. Since mortar and concrete have
no elastic limit, the modulus of elasticity for
concrete must be the slope of the stress-
deformation curve at zero stress.
The elastic modulus of concrete in compression,

Ec = 5000 fck, where fck is the characteristic

strength of concrete. The elastic modulus of
concrete varies from 14 × 10^3 to 40 × 10^3 N/mm^2.
Its value increases with the density and to some
extent with age.

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