Properties of Fresh Concrete
Concrete remains in its fresh state from the time it is mixed until it sets. 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.
The fresh concrete has the following procedure.
- Settlement & Bleeding
- Plastic shrinkage
- Loss of 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.
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 workabilities, 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 but the slump test, together with an assessment of properties like stone content, cohesiveness and plasticity, gives a useful indication.
3. Settlement and Bleeding
Cement and aggregate particles have densities about three times that of water. In fresh concrete they consequently tend to settle and displace mixing water which migrates upward and may collect on the top surface of the concrete. This upward movement of mixing water is known as bleeding; water that separates from the rest of the concrete is called bleed water.
4. 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.
5. Slump Loss
From the time of mixing, fresh concrete gradually loses 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.