Segregation in Concrete - Measurement, Causes & Types

Segregation of concrete can be defined ass the separation of the constituents of the material of concrete. A good concrete is one in which all the ingredients of the concrete are properly distributed to make a homogeneous mixture.

Segregation in Concrete

Types of Segregation in Concrete

There are mainly three types or ways in which segregation can be observed in concrete. They are:

• Type 1: The coarse aggregate will separate out or settle down from rest of the matrix
• Type 2: The cement or the paste matrix will separate away from the coarse aggregate
• Type 3: Water separating out of rest of material that has lowest specific gravity.

Causes of Segregation in Concrete

There are many reasons behind segregation. This mainly depends on the handling, placing and the compacting operations. The chances to segregate are high with the increase in:

• Maximum size of the aggregate
• The amount of coarse aggregate
• Increased Value of Slump

When there is a higher vibration carried out, the coarse aggregate will settle to the bottom and the scum will rise to the surface of the mix. This formation of scum is called as laitance.

The main causes of segregation are:

1. Bad Mixing and Proportioning of the Ingredients
2. Excess Amount of Water
3. Excess Vibration on Too Wet Concrete
4. The concrete discharge from a badly designed mixer or with blades worn out
5. Excess pressing over the concrete layer, during the finishing by using trowels. This will result in the settling down of the aggregates.
6. Conveyance of the concrete mix by Conveyor belts, wheelbarrows are situations that will promote segregation.

Measurement of Segregation in Concrete

The segregation in concrete is very difficult to be tested. But the occurrence can be easily detected.

Flow Test can be used to determine the susceptibility of the concrete mix that is likely to segregate.  In dry mixes, the heavier particles will move way and they will occupy the edges of a flow table. It wet mixes the cement paste tend to move away from the middle and the center of the flow table is left with coarser particles alone.

Flow Test Apparatus

Ring of Cement Paste Separated Out from the Middles - Segregation in Wet Mix

Prevention of Segregation in Concrete

The prevention of segregation is the most important consideration in handling and transporting concrete. The segregation should be prevented and not corrected after its occurrence. The concrete is a non-homogenous composite of materials of widely differing particle sizes and specific gravities, is subjected to a certain amount of internal and external forces while transporting and placing. This will tend to separate the dissimilar constituents.

Segregation can be prevented by ensuring that the direction of fall during the dropping of the concrete is vertical. When the discharge is at an angle, the larger the aggregate is thrown to a farther site of the container that is being charged and the mortar will be collected on the other side, this results in segregation.

Other tips that can be taken to prevent segregation are:

• Proper design of the concrete mix. This will help in having optimum quantity of water
• The mix must not be too wet or too dry
• The mixing speed of the concrete must be correct. Uniformity of the materials is checked in between.
• While transporting, shortest duration is chosen
• Placing the concrete is done fastly at the final position
• A watertight formwork must be employed
• The concrete must not be allowed to flow in undesired direction
• Proper use of vibrator
• Air entraining admixtures help reduction of segregation
• Remixing is recommended if a chance of segregation is found