Fresh concrete and it's properties

Fresh Concrete:

Fresh concrete is that stage of concrete in which concrete can be moulded into any shape and it is in plastic state. Sometimes it is also known as Green concrete. Fresh concrete means the wet mix of concrete before being set. The relative quantities of cement, sand, aggregates and water mixed together control the properties in the wet state.
During this time the concrete is handled, transported, placed and compacted. Properties of concrete in its fresh state are very important because the influence the quality of the hardened concrete.

Properties of Fresh Concrete:

1. In its fresh or green state it can be easily handled and moulded into any shape and size according to the specifications.
2. Concrete can be sprayed on and filled into fine cracks for repairs by the guniting process.
3. Concrete can be pumped and can be laid in difficult positions.
4. Fresh concrete shrinks on drying i.e. undergoes plastic shrinkage.
5. Concrete can be handled, transported, placed and compacted.
6. Consistency
7. Workability
8. Segregation

Watch video on segregation

9. Bleeding

Watch video on bleeding

10. Laitance
11. Slump loss

Consistency

Consistency of a concrete mix is a measure of the stiffness or sloppiness or fluidity of the mix.
For effective handling, placing and compacting the concrete, consistency must be the same for
each batch. It is therefore necessary to measure consistency of concrete at regular
intervals. Slump test is commonly used to measure consistency of concrete.
Consistency is the aspect of workability related to the flow characteristics of fresh concrete. It is an indication of the fluidity or wetness of a mix and is measured by the slump test.

Workability

The workability of a concrete mix is the relative ease with which concrete can be placed, compacted and finished without separation or segregation of the individual materials.
Workability is not the same thing as consistency. Mixes with the same consistency can have
different work-abilities, if they are made with different sizes of stone – the smaller the stone the more workable the concrete. It is not possible to measure workability directly.
Good workability is required to produce concrete that is both economical and high in quality.Fresh concrete has good workability if it can be formed, compacted, and finished to its final
shape and texture with minimal effort and without segregation of the ingredients. Concrete with poor workability does not flow smoothly into forms or properly envelop reinforcing steel
and embedded items, and it is difficult to compact and finish.

Plastic Shrinkage

If water is removed from the compacted concrete before it sets, the volume of the concrete is reduced by the amount of water removed. This volume reduction is called plastic shrinkage.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 form work.

Slump Loss

From the time of mixing, fresh concrete gradually losses consistency. This gives rise to the
problems only if the concrete becomes too stiff to handle, place and compact properly.
Slump loss in concrete is caused due to the following reasons:
▪ Hydration of cement (generating more heat)
▪ Loss of water by evaporation
▪ Absorption of water by dry aggregates
▪ Absorption of water by surfaces in contact with the concrete.

Laitance

A weak, milky layer of cement and aggregate fines on a concrete surface that is usually caused
by an over-wet mixture, overworking the mixture, improper or excessive finishing or
combination.
Laitance is caused by bleeding of concrete. In case of bleeding, only water accumulates at the
top of the surface, but in case of laitance, along with water certain quantity of cement also
comes to the surface, forming a thin layer of cement paste at the surface. This formation of
cement paste at the surface is known as laitance.