What are the Properties of Fresh Concrete?

Table of Contents

Properties of Fresh Concrete

When concrete is in its plastic state it is known as fresh concrete. Fresh concrete can be easily moulded to a durable structural member. Following are the properties of fresh concrete.

Workability
Segregation
Bleeding
Plastic shrinkage
Setting
Temperature
Water Cement Ratio

Workability

The term workability indicates the ease or difficulty with which the concrete is handled, transported and placed. The amount of water present in concrete should be in the proper ratio.

Generally, a higher water-cement ratio is required for good workability. The actual function of water is to lubricate the concrete. The concrete which is easy for handling and placing is workable concrete.

There are various tests to measure the workability of the concrete mixture. The tests such as flow test, compacting test and slump cone test is done to determine the workability of the concrete mixture.

Segregation

Segregation is the separation of the constituents materials of concrete. Segregation is of three types, firstly the coarse aggregates separating out from concrete.

What are the Properties of Fresh Concrete?

Secondly, the paste of cement is separating out from coarse aggregates and thirdly, water is separating out from the rest of the material.

This type of concrete is weak due to a lack of homogeneity and having a deduction of desirable properties.

Dropping of concrete from heights as well as discharging of concrete from badly designed mixer shows a tendency for segregation.

Due to excessive vibration of concrete coarse aggregates try to settle down called segregation.

Bleeding

Bleeding is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete.

The water rising to the surface carries with it particles of sand and cement which on hardening form a scum layer popularly known as laitance.

Bleeding takes place in a highly wet mix, badly proportioned and insufficient mixed concrete.

Plastic shrinkage

After the fresh concrete has been placed in forms, concrete undergoes a volumetric contraction while it is in a plastic state(before the concrete set). This is known as plastic Shrinkage.

Plastic shrinkage is so-called because of the form while the concrete is still plastic, i.e. has not set. Rapid drying of the surface of the plastic concrete cause it to shrink and crack. They rarely occur near the edges of a slab as in those locations the concrete is usually free to move.

It can start after 30 minutes of concrete pouring or during finishing.

Setting

When concrete changes its state from plastic to a hardened state, this process is called the setting. And the duration that concrete takes to change its state is called setting time.

Setting time depends on the type of cement and it can be increased or decreased by adding admixture in concrete.

Temperature

Concrete is not recommended to be placed at a temperature above 40°C without proper precautions as laid down in IS:7861(part -1).

Special problems are encountered in the preparation, placement and curing of concrete in hot weather.

High-temperature results in rapid hydration of cement, increased evaporation of mixing water, greater mixing water demand, and large volume changes resulting in cracks.

Water Cement Ratio

The ratio of the amount of water to the amount of cement by weight is termed the water-cement ratio. The strength and quality of concrete depend on this ratio.

The quantity of water is usually expressed in litre per bag of cement. if water required for one bag of cement is 30 litres, the water-cement ratio is equal to 30/50 = 0.6.