Segregation in Concrete- Types, Causes, Signs and Prevention

Concrete is a strong building material, but it needs proper mixing and setting to attain the required strength. Concrete ingredients must be mixed precisely to achieve optimal stability and durability. But when this balance is disrupted, segregation occurs, compromising the concrete’s quality. 

Let’s explore what segregation means in concrete, its causes, and prevention methods.

What is Segregation in Concrete?

Segregation, an error in the concrete process, occurs when the concrete mixture’s components separate. Heavier components, including sand and gravel, settle at the base, whereas admixtures and cement paste rise to the top. 

This separation occurs because concrete ingredients fail to maintain uniformity, usually resulting from improper mixing ratios or methods. As a result, it leads to poor concrete placement.

Let’s see further why and how this segregation happens.

Types of Segregation in Concrete

Segregation can manifest in various forms, including separation of aggregates at the surface (bleeding), settlement of coarse aggregates at the bottom (settling), or formation of voids and pores with honeycomb-like structures. 

Separation of Coarse Aggregates

Coarse aggregates, like sand and gravel, separate from other materials and settle at the bottom due to weight differences. This happens when mixing is insufficient or the aggregate grade is incorrect. As a result, coarse aggregate separation leads to uneven surfaces, reduced strength, and increased water passage.

Bleeding Segregation

The optimal amount of water should be added to the concrete. It should be in a ratio of 0.4 -0.6 to cement as per standards. When this level is not followed and excess or less amount of water is added to a concrete mix, it can lead to bleeding segregation, which means water from the concrete starts to bleed and rise to the surface while coarse aggregates tend to settle at the base. 

This separation can cause uneven distribution and reduce concrete strength.

Laitance Segregation

Laitance segregation occurs when finer particles separate from coarse aggregates in freshly placed concrete. This happens due to over-vibration, insufficient compaction, incorrect mix , or poor workmanship. As a result, a weak, porous layer forms at the surface, prone to cracking and scaling, which reduces  durability.

This also causes uneven texture and allows water and contaminants to enter, compromising concrete’s strength and appearance.

Grout separation

Grout refers to the cement paste used for filling gaps between sand and gravel particles. Incorrect mixing, water ratio, or vibration causes grout separation, creating gaps between grout and aggregates. This weakens concrete, makes it prone to water damage, and affects its appearance.

Causes of  Segregation in Concrete

Here is the list of factors that contribute to the segregation of concrete:

Uneven Proportion

Mixing ingredients in the wrong quantity can lead to segregation. For instance, if cement, sand, and gravel should be in the ratio of 1:2:4 but instead use 1:1:4. This imbalance leads to uneven distribution, where aggregates either settle at the bottom or rise to the surface. It results in affecting workability and weakening concrete properties.

Insufficient Mixing Time

Insufficient mixing time refers to inadequate blending of concrete ingredients, failing to achieve a uniform mixture. This can lead to segregation, as:

– Cement paste and aggregates don’t fully integrate.

– Materials aren’t evenly distributed.

– Clumps or pockets of unmixed materials form.

Handling of Concrete Mix

Concrete should be handled carefully, using proper force and equipment. When transferring concrete from one place to another, dropping the mix should be prevented. Additionally, excessive vibration should be avoided, as it can push aggregates apart, leading to segregation.

For example, pouring concrete from a height without using a chute or drop chute can cause segregation. Similarly, a poorly maintained mixer can fail to re-blend separated materials.

Placing of Concrete Mix

Proper placement of the concrete mix is crucial to prevent segregation. For this, concrete should be poured as close to its desired position as possible. Pouring from excessive heights should be avoided, and spreaders or finishing tools should be used to avoid segregation.

Vibration of Concrete

Improper vibration of concrete can contribute to segregation. Over-vibration or incorrect vibration techniques can:

– Displace aggregates, settling them at the bottom.

– Push cement paste to the surface.

– Create air pockets and honeycombing.

It can increase the risks of defects in concrete structures.

Chemical Segregation

Even mixing of concrete’s chemical components is crucial. However, differences in density, particle size, and chemistry often cause cement, aggregates, and additives to separate or settle. This leads to chemical segregation, affecting the concrete’s performance.

Biological Segregation

Biological segregation in concrete occurs when living things like microorganisms, insects, or animals contaminate the mix. This happens due to dirty materials, pest problems, or growth of bacteria, algae, or fungi. These biological activities affect how cement sets, causing weaker concrete, discolouration, and cracks. To prevent this, use clean materials and seal concrete properly.

How To Prevent Segregation

• To fix segregation, improve mixing techniques by ensuring thorough blending of ingredients and consider mixing time and equipment.
• Optimise aggregate gradation by selecting aggregates with suitable size and distribution to minimise separation and promote uniform distribution.
• Adjust concrete workability by achieving optimal water content, admixture usage, and temperature control.
• Using segregation-resistant mix designs, which incorporate elements like higher cement content, reduced water-cement ratio, and optimised aggregate proportions, can also help.
• Proper placement and compaction techniques, such as vibratory compaction, tamping, and finishing operations, minimise segregation during placement.
• Reducing transportation and handling mistakes and doing regular checks can also help prevent segregation.

Signs of Segregation in Concrete

1. Signs of segregation in concrete include visible separation of aggregates and cement paste.
2. Aggregate accumulation at the surface or bottom of the concrete indicates segregation.
3. Discolouration, such as streaks or patches, can result from segregation.
4. Honeycomb-like structures, characterised by voids and separated aggregates, are a common sign of segregation.
5. Segregation often leads to reduced strength, increased cracking, and concrete deterioration, making honeycomb structures vulnerable to water penetration and damage.

Conclusion

In conclusion, following standard guidelines and processes prevents concrete segregation. Unchecked segregation reduces concrete strength and increases maintenance costs. Moreover, good quality materials and equipment, and skilled workers are necessary for a strong and durable concrete mix.

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