FRESH CONCRETE: CHARACTERISTICS OF FRESS CONCRETE
Basic characteristics
of fresh concrete
The following, are some basic characteristics of fresh
concrete, that a civil engineer must know. Fresh concrete, is concrete that is
newly mixed, prepared and/or placed, which has not undergone hardening.
concrete, that a civil engineer must know. Fresh concrete, is concrete that is
newly mixed, prepared and/or placed, which has not undergone hardening.
·
Workability
and Stability
Workability:
Workability is the ease with which concrete can be
transported or moved from the mixer or mixing point, to the final placement
point, the final compacted shape, formwork or mould.
transported or moved from the mixer or mixing point, to the final placement
point, the final compacted shape, formwork or mould.
Properties of workability
Properties of
concrete workability include:
concrete workability include:
- Consistency
- Mobility
- Compactibility
Consistency is
the degree of wetness or fluidity of the concrete mix.
the degree of wetness or fluidity of the concrete mix.
Mobility is how
transferrable the concrete mix is.
transferrable the concrete mix is.
Compactibility is
the degree to which the concrete could be fully contacted. That is, the ease to
which air voids could be expelled out of the fresh concrete.
the degree to which the concrete could be fully contacted. That is, the ease to
which air voids could be expelled out of the fresh concrete.
Workability Tests
Workability tests include:
- Slump Test
- Compacting Factor Test
- V.B. Consistometer
Factors Affecting Workability
i.
Cement and water
Cement and water
ii.
Admixtures
Admixtures
iii.
Aggregates
Aggregates
iv.
Time
Time
v.
Ambient Conditions e.g. humidity, temperature,
wind speed, etc.
Ambient Conditions e.g. humidity, temperature,
wind speed, etc.
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| Slump Types: These are the results you can get from your slump tests |
Collapse slump occurs where the mix is too rich, or there is too
much water in the mix.
much water in the mix.
Shear slump, occurs where the mix is devoid of much cement, in
other words, where the cementitious element is insufficient.
other words, where the cementitious element is insufficient.
True slump occurs where all elements of the mix are in the right
proportion. In other words, cementitious, as well as moisture are in right
quantities.
proportion. In other words, cementitious, as well as moisture are in right
quantities.
Admixtures are substances or chemicals added to concrete to alter
or change some properties of the concrete. Examples of admixtures include:
or change some properties of the concrete. Examples of admixtures include:
- Water reducing agents
- Air-entraining agents
- Retarders, etc.
Water reducing agents and
air-entraining agents are good admixtures for improving concrete workability.
air-entraining agents are good admixtures for improving concrete workability.
Stability:
Stability as a property of fresh concrete, signifies the
uniformity of the concrete mix. If concrete is not stable, it will affect the
concrete in two major ways viz:
uniformity of the concrete mix. If concrete is not stable, it will affect the
concrete in two major ways viz:
Segregation:
Segregation,
is the tendency for coarse aggregates to separate from the fine aggregates.
There is high tendency for segregation to occur in a wet mix, as well as in a
hard mix.
is the tendency for coarse aggregates to separate from the fine aggregates.
There is high tendency for segregation to occur in a wet mix, as well as in a
hard mix.
Bleeding:
Bleeding in
fresh concrete, is a situation where the hard or solid part of the mix, settle
below, while the liquid portion, settle on top of the mix due to different
specific gravities. The wet portion of the mix, later flows out, leaving
behind, a solid mix, devoid of adequate water and cement.
fresh concrete, is a situation where the hard or solid part of the mix, settle
below, while the liquid portion, settle on top of the mix due to different
specific gravities. The wet portion of the mix, later flows out, leaving
behind, a solid mix, devoid of adequate water and cement.
