When concrete is in its plastic state it is known as fresh concrete, it can be easily molded as a durable structural member, the potential strength and durability of fresh concrete of a given mixing ratio are highly dependent on its degree of compaction, it is also known as green concrete.
Therefore it is very important that the stability of concrete should be such that it can be easily transported, placed and completed sufficiently quickly to achieve the required strength and durability.
The primary 48 hours are crucial for the efficiency of a concrete structure, it controls the behavior and impacts final strength, elastic modulus, creep, and durability.
When the concrete is mixed and ready for placement in the formwork is called fresh concrete.
Properties of Fresh Concrete:
Workability of Concrete is defined as the process in which concrete can be easily laid in the formwork, filling the formwork with a thorough flow and compacted.
A practical concrete can have a uniform colour, the aggregates shall be lubricated with ample cement paste so that the concrete could be simply poured and fill the corners of the formwork easily.
While in hardened concrete it will be harder to insert and will have to be forced by rodding to fully fill the formwork.
The main factor that controls the workability is the water-cement ratio and the other factors that affect the ability to work.
Segregation of Concrete is defined as the separation of the component material of concrete, the segregation has three properties in fresh concrete, i.e. workability, segregation, and bleeding.
A good solid is one that shows the uniform distribution of components.
There are three types of isolation:
(i) Coarse aggregates settle into the matrix.
(ii) Matrix paste is separated from coarse aggregates.
(iii) Water gets separated from the rest of the ingredients.
Bleeding is a special form of segregation in which water comes on top of concrete also known as water gain, while it is more problematic especially in wet mixes.
If along with water, a certain amount of cement also comes to the surface, it forms a cement paste on the top surface of the fresh concrete and this formation of cement paste on the surface is known as “laitance”.
In this case, the top surface of the slab and pavement will not have good wearing quality.
This laitance produces dust in the summer and rainy season, due to the high water content and the absence of aggregate, the top surface develops high shrinkage cracks.
If a laitance is formed on a particular lift, a plane of weakness will form and the bond with the next lift will deteriorate and it can be avoided by completely removing the fan before placing the next lift.
As water moves upstream, water vessels can form that weaken the bond between the aggregate, the steel and concrete, which may cause corrosion of the reinforcement.
After the fresh concrete is placed into forms, the concrete undergoes a volumetric contraction in a plastic state (earlier than the concrete units), it is called plastic shrinkage,
It is known as plastic shrinkage due to the form while the concrete remains to be plastic, i.e. not set.
Rapid drying of the surface of plastic concrete causes it to shrink and crack, they are rarely near the edges of a slab because those locations are usually free to move.
This can begin 30 minutes after pouring concrete or during finishing.
When the solid changes its state from plastic to solid-state and this process is called setting, the period which solidifies to change its state is called the setting time.
Setting time depends on the types of cement and it can be increased or decreased by adding the mixture to the fresh concrete.
It is not recommended to keep concrete at temperatures above 40 ° C without proper precautions, the preparation, placement, and curing of concrete in hot climate pose particular issues.
The excessive temperature of the cement results in increased evaporation of the mixing water because of increased temperatures, larger demand for water and larger volume changes leading to cracks.
7.Water Cement Ratio:
The quantity of water to cement quantity by weight known as the water-cement ratio, the strength and quality of concrete depend upon this ratio.
The quantity of water is often expressed at the rate of cement per liter, i.e. if the water required for a bag of cement is 30 liters, then the water-cement ratio is equal to 30/50 = 0.6.