Effect of segregation and bleeding on quality of concrete

Segregation in concrete: Segregation means the detachment of ingredients in concrete. In concrete, the following types of segregations mainly occur:-

1. Detachment of coarse aggregate from the concrete mixture,
2. Detachment of cement pastes from the concrete throughout its plastic phase.
3. Detachment of water from the concrete mix (Bleeding in concrete)

Concrete is formed by mixing cement, fine and coarse aggregates. In a standard quality concrete, is all the ingredients are grouped properly to develop a uniform mixture. Segregation in concrete is primarily occurred due to variations in specific gravities of the ingredients.

Specific gravity of Cement remains among 3.1-3.6g/cc, and for aggregate it remains among 2.6-2.7g/cc. Because of this variation, the aggregate is detached from the matrix and segregation in concrete occurred.

There are various other factors to create segregation in concrete :-

1. Moving concrete mixes for long distances.
2. Weak mix ratio, where adequate matrix does not exist to unite the aggregates.
3. When concrete falls from over 1m.
4. Vibrating concrete for a prolonged period.

Guidelines to reduce segregation in concrete:-

Segregation is managed properly with exact mix ratio.

Handling, placing, transporting, compacting and finishing of concrete in perfect manner.

With the addition of air entraining agents, admixtures and pozzolanic materials in the mix, the segregation is controlled to some extent.

Bleeding in concrete: Bleeding is a type of segregation in which existing water in the concrete mix is forced upwards owning to the settlement of cement and aggregate. Since specific gravity of water is low, the water may proceed upwards. Bleeding normally occurs in the wet mix of concrete.

Due to greater amount of water cement ratio, the bleeding is found in concrete. If the water-cement ratio is higher, the concrete becomes weak and as a result excessive bleeding happens.

The bleeding in concrete is not a cause of great concern when the rate of evaporation of water is identical to the rate of bleeding. Normal bleeding improves the workability of concrete.

When the concrete becomes completely plastic, bleeding is not injurious. However, concrete still remains in the plastic stage and it is subsidized and compacted in due course.

How bleeding impacts the stability of concrete :-

1. Since water is pushed upwards in bleeding, sometimes with this water, specific amount of cement proceeds together with water to the concrete surface. If the top surface is worked up with the trowel, the aggregate comes downward and cement paste is developed at the top surface and it is known as ‘Laitance in concrete.’ As Laitance is developed, the wearing strength of structure is reduced and the longevity of structure is hampered.
2. While directing to the top from bottom, water produces continuous channels. Because of these channels, concrete turns out to be porous and facilitates water to move, that develops water voids in the matrix and decreases the bond among aggregate and the cement paste.
3. If water is accumulated at the top surface of concrete, the surface finishing is deferred.
4. Concrete becomes porous and its consistency is affected.
5. Excessive bleeding results in rupturing the bond among the reinforcement and concrete.

Remedies to control the bleeding :-

1. Bleeding in concrete is managed with the inclusion of minimum water content in the concrete mix.
2. Allowing the application of air en-training admixtures in the mix.
3. By providing more cement in the mix.

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Published By
Rajib Dey
www.constructioncost.co
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Segregation of Concrete- Causes and prohibition

Concrete is a paste of sand and cement which helps to bind the building, but sometimes this concrete gets segregated due to some reasons which can be prevented easily.

According to popular belief, concrete and cement is not the same thing; cement is actually just a part of concrete. So basically concrete or Portland cement concrete is a compound material of fine and coarse aggregate bonded together with a fluid cement or cement paste that become hardens over time. It is distinguished from other, non-cementations types of concrete all binding some kind of clumped together, including asphalt concrete with a bitumen binder which is frequently used for road surfaces and polymer concretes that use polymers as a binder. When clumped is mixed together with dry Portland cement and water, the mixture forms a fluid slurry which can be easily poured and molded into shape, then the cement ill reacts with the water and other ingredients to make a hard matrix that binds the materials together into a durable stone-like material that can be used in many ways.

Concrete is a popular building material which often known for its toughness; Concrete is made up with three basic things which are: water, aggregate (rock, sand or gravel) and Portland cement. It is known as a very versatile and reliable material though some construction faults and imprudence can lead to the growth of defects in a concrete structure. These failings can be seen as per poor construction practices, poor quality control or for poor structural design and detailing.

Segregation of concrete is the division of cement paste and groups of concrete from each other during handling and placement. Separation also occurs due to over-vibration or compression of concrete where cement paste comes to the top and aggregates settles at the bottom. This separation affects strength and endurance in structures. While in a good concrete, all concrete separates are equally coated with sand and cement paste and creates a homogeneous mass.

While handling, transporting and placing for the jerks and vibrations the cement-sand paste may gets separated from rough aggregate. But it can be mixed again properly before depositing but it is recommended not to use a concrete where initial setting is over.

Reasons of Segregation of Concrete:

• Using of high water-cement ratio in concrete makes concrete segregation and it happens when concrete is mixed at site by unskilled workers.
• Extreme vibration of concrete with mechanical needle vibrators creates heavier particles resolve at bottom and lighter cement sand paste comes on top.
• When concreting is done from high for underground foundations and rafts, which causes concrete to separate.

Way to prevent Segregation of Concrete: When the depth of concrete is more than 1.5 meters it should be located through temporary inclined chutes. The delivery end of chute should be as close as possible to the point of deposit. When Segregation in deep foundations and rafts of thickness is more than 1 meter, there is chance of presence of segregated concrete near bottom or in center without proper supervision. This segregation can be detected by advanced method of testing such as ultrasonic testing.

Source www.theconstructor.org

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Published By

Rajib Dey

www.constructioncost.co

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Tips to reduce bleeding in concrete

Concrete is formed by mixing cement, fine aggregates and course aggregates. If process for mixing, curing and placing is inaccurate, several issues may arise. Bleeding in concrete is one of the vital issues. Bleeding means a type of segregation, in which water emerges out of concrete.

Bleeding in concrete happens if the course aggregates is like to settle down and free water in the mix moves up to the surface and produces a paste of cement on the surface called “laitance”. This upward transition of water at the time of penetrating from bottom to top, creates continuous channels. If the applicable water cement ratio is 0.6 or more, the bleeding channels will be continuous.

Penetration in the structure occurs due to these continuous bleeding channels. While upward transition is going on, the water is stored underneath the aggregates and forms water voids and minimizes the bond among the aggregates and paste.

In the same way, the water that is stored underneath the reinforcing bars minimizes the bond among the reinforcement and concrete. When bleeding remains at normal rate, it is not harmful but high rate of bleeding can lead to damage of bond.

Reasons for bleeding: Bleeding in the concrete mix is caused by segregation. For segregation, heavy aggregate particles are settled down and owing to this water moves up to the surface and develops a layer. This upward transition of water also conveys the fine particles of cement with it. The top surface of slabs and pavements will not contain superior wearing quality.

Bleeding mostly operates on the surface of concrete, when water to cement ratio is greater. The rate of bleeding is based on the type of cement utilized, quantity of fine aggregate.

Impacts Of Bleeding:

1. Because of bleeding, the consistency of concrete is lost.
2. Due to bleeding, permeability in concrete occurs.
3. The water stored underneath the reinforcing bars, decreases the bond among the reinforcement and concrete.
4. While bleeding is going on, the storage of water develops a water voids and minimizes the bond among the aggregate and cement paste.
5. Because of bleeding, pumping ability of concrete is decreased.
6. Bleeding raises the water-cement ratio at the top.
7. The storage of water at the top leads to delayed surface finishing.

Some useful steps to minimize bleeding:

1) Put in minimum water content in the concrete mix, chemical admixtures should be applied to minimize the quantity of water to maintain desired workability.
2) Design the concrete mix correctly.
3) Apply fly ash or other additional cementitious materials.
4) Air entraining admixtures is useful for minimizing the bleeding.
5) Put in extra cement in the mix.
6) The amount of fine aggregate should be raised when sand is coarser (fineness modulus of 2.5 to 2.8 is recommended) in mix and lessen aggregate proportionately.

There should be perfect precautions throughout formation of mix and method of mixing to lessen bleeding as it leads to weak structure and an interruption in construction cycle.

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Published By
Rajib Dey
www.constructioncost.co
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