What is Fresh Concrete?
Fresh concrete is that stage of concrete in which
concrete can be moulded and it is in plastic state. This is also called “Green
Concrete”. Another term used to describe the state of fresh concrete is
consistence, which is the ease with which concrete will flow.
Properties of Fresh Concrete:
1. Workability of Concrete
Workability is often referred to as the ease with
which a concrete can be transported, placed and consolidated without excessive
bleeding or segregation.
It is obvious that no single test can evaluate all
these factors. In fact, most of these cannot be easily assessed even though some
standard tests have been established to evaluate them under specific
conditions.
In the case of concrete, consistence is sometimes
taken to mean the degree of wetness; within limits, wet concretes are more
workable than dry concrete, but concrete of same consistence may vary in
workability.
Because the strength of concrete is adversely and
significantly affected by the presence of voids in the compacted mass, it is
vital to achieve a maximum possible density. This requires sufficient
workability for virtually full compaction to be possible using a reasonable
amount of work under the given conditions. Presence of voids in concrete
reduces the density and greatly reduces the strength: 5% of voids can lower the
strength by as much as 30%.
Factors affecting concrete workability:
• Water-Cement
ratio
• Amount
and type of Aggregate
• Amount
and type of Cement
• Weather
conditions
a. Temperature
b. Wind
• Chemical
Admixtures
• Sand to Aggregate ratio
2. Concrete Bleeding
Bleeding in concrete is sometimes referred as water gain. It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thin members like roof slab or road slabs and when concrete is placed in sunny weather show excessive bleeding.
click :Lean concrete mix
3. Setting of Concrete
The hardening of concrete before its hydration is
known as setting of concrete. OR The hardening of concrete before it gains
strength. OR The transition process of changing of concrete from plastic state
to hardened state. Setting of concrete is based or related to the setting of
cement paste. Thus cement properties greatly affect the setting time.
Factors affecting setting:
1. Water Cement ratio
2. Suitable Temperature
3. Cement content
4. Type of Cement
5. Fineness of Cement
6. Relative Humidity
7. Admixtures
8. Type and amount of Aggregate
4. Segregation in concrete
Segregation can be defined as the separation of the
constituent materials of concrete. A good concrete is one in which all the
ingredients are properly distributed to make a homogeneous mixture. There are
considerable differences in the sizes and specific gravities of the constituent
ingredients of concrete. Therefore, it is natural that the materials show a
tendency to fall apart.
5. Air Entrainment
Air entrainment reduces the density of concrete and
consequently reduces the strength. Air entrainment is used to produce a number
of effects in both the plastic and the hardened concrete. These include:
1. Resistance
to freeze–thaw action in the hardened concrete.
2. Increased
cohesion, reducing the tendency to bleed and segregation in the plastic concrete.
3. Compaction
of low workability mixes including semi-dry concrete.
4. Stability
of extruded concrete.
5. Cohesion
and handling properties in bedding mortars.
6. Hydration
in concrete
Concrete derives its strength by the hydration of
cement particles. The hydration of cement is not a momentary action but a
process continuing for long time. Of course, the rate of hydration is fast to
start with, but continues over a very long time at a decreasing rate In the
field and in actual work, even a higher water/cement ratio is used, since the
concrete is open to atmosphere, the water used in the concrete evaporates and
the water available in the concrete will not be sufficient for effective
hydration to take place particularly in the top layer.
If the hydration is to continue, extra water must be
added to refill the loss of water on account of absorption and evaporation.
Therefore, the curing can be considered as creation of a favorable environment
during the early period for uninterrupted hydration. The desirable conditions
are, a suitable temperature and ample moisture.
Concrete, while hydrating, releases high heat of
hydration. This heat is harmful from the point of view of volume stability. Heat of hydration of concrete may also shrinkage in concrete, thus producing
cracks. If the heat generated is removed by some means, the adverse effect due
to the generation of heat can be reduced. This can be done by a thorough water
curing.
Click to see: Difference between Nominal mix and Design mix of concrete.
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