What is Bleeding of Concrete: Causes, Effects, and Solutions

Bleeding in concrete is one of the most common challenges faced in the construction industry. The global construction industry is moving towards higher performance and precision. But bleeding remains a highlighted concern. It occurs when water in the mix rises to the surface. This results in a weaker top layer of concrete and reinforcement bonds.

Bleeding is a natural phenomenon that occurs in the hydration process. Minor bleeding is often considered inevitable. In modern construction, effectively controlling bleeding is essential for sustainability and efficiency. This enables the delivery of high-quality concrete. 

As technology improves, so does the ability to predict and manage issues like bleeding. Let us look at specific improvements, how they affect concrete, and how to avoid bleeding.

What is the Bleeding of Concrete?

Bleeding in concrete is defined as the process where free water in the mix rises to the surface. It is due to the settlement of heavier particles like cement and aggregate. It manifests as a layer formed on the surface of freshly placed concrete before it has set. This phenomenon is also known as “water gain,” “weeping,” or “sweating.”

The primary reason for its cause is the gravitational settlement of solid material in the concrete mix. Bleeding is a form of segregation. It separates the particles from the water. The rising water often carries cement particles to the top surface. It forms a weaker layer called “laitance.”

Some bleeding can be beneficial as it helps control plastic shrinkage and cracking. In other cases, excessive bleeding can delay the finishing process. This is mainly due to ‌surface water, which needs to evaporate before finishing. 

The premature finishing can trap bleed water, forming blisters under the surface. The bleeding water can reduce the bond between aggregate and paste, thus weakening the concrete.

Accumulated water under reinforcing bars can weaken the bond between the steel and paste. It leads to corrosion. If the rate of evaporation is higher than the bleeding rate, the surface may dry out and crack. The higher water-to-cement ratio leads to increased bleeding. In the case of fine aggregates, their type and amount affect the rate of bleeding. 

Causes of Concrete Bleeding

Water-to-Cement Ratio

A high water-to-cement ratio increases bleeding as there is excess water in the mix. Inadequate mixing can lead to segregation and bleeding. Poor aggregate quality results in bleeding due to rounded or poorly graded aggregates. 

Admixtures

Excessive admixture results in overuse of chemicals. Bleeding can also occur due to higher slumps present in the concrete.

Fines Content

Insufficient fine aggregates and certain types of cement can influence the bleeding rate. Taller concrete pours exhibit increased pressure, and higher temperatures cause increased evaporation. The rapid evaporation can also occur due to the windy conditions. All of these can result in the bleeding of concrete. 

Vibration

Excessive vibration during compression leads to segregation. Delayed finishing of the surface and excessive troweling can bring more water to the surface, resulting in water accumulation. Hence, bleeding occurs. 

Aggregate Sizes

When compared to larger aggregates, angular aggregates reduce bleeding. Finer cement particles improve water retention by reducing bleeding. 

Effects of Concrete Bleeding

• Surface weakness occurs on the top, resulting in weak and porous surfaces where the surface is dusted easily. 
• Bleeding reduces the overall strength of the concrete and increases its permeability, making it susceptible to water ingress.
• Due to differential settlement, the surface is prone to cracking. Bleeding creates a non-uniform strength distribution throughout the concrete. 
• The top surface takes a longer time to set compared to the bottom, which is a result of segregation caused by bleeding. 
• Early shrinkage can occur, leading to cracking. Further, it leads to the weakening of joints between successive layers of concrete. Therefore, there are visible streaks of sand that can appear on the surface. 
• Voids can form under the aggregate, which reduces the strength and bonding between the cement paste and the aggregates.
• Problems such as surface scale and flake-off require regular maintenance and curing time. Bleeding results in the long-term durability of the concrete being compromised. Also, water channels are formed, leading to weak spots. It results in the concrete mix losing its perfection, causing blisters and voids to form.
• The surface is vulnerable to erosion and thermal cracking due to fluctuating temperatures. This leads to a decrease in the concrete’s load-bearing capacity.
• Bleeding causes an increased alkali-silica reaction and higher carbonation. Further adhesion of coatings and finishes is reduced, leading to surface spalling.

Solutions for Concrete Bleeding

• Lowering the water-to-cement ratio can minimise bleeding. Also, the slump of the mixture should be reduced to minimise the amount of free water present. 
• Using finer cement and increasing fines in the sand can increase the surface area for hydration.
• Use supplementary cementitious material like fly ash and slag to reduce bleeding. The use of microfiber can slow the migration of water to the surface. This helps keep the consistent ratio of water to cement.

• Adding air-entraining admixtures can create tiny air bubbles within the mix, thereby improving workability and reducing bleeding possibilities. 
• Using silica fume and other pozzolanic materials, such as calcium hydroxide, can increase the density and reduce the rate of bleeding within the concrete.

How to Check Bleeding in Concrete?

The bleeding of concrete can be analysed through various processes. Some are listed below:

Concrete Layer: A visible layer of freshly placed concrete is found on the top layer of the water surface. 

Bleeding Test: A filter press is used to measure the capacity of the concrete to retain water under pressure.

Water-Cement ratio: Ensure that the water-cement ratio is appropriate, as higher ratios can cause increased bleeding. 

Aggregate Size: Check the aggregate size and its grading before use, as poorly grated aggregates can cause bleeding. 

Slump Test: Perform slump tests to ensure the concrete has the correct consistency. 

Air Entrainment: Use air-entraining admixtures to reduce bleeding by creating microscopic air bubbles that help reduce bleeding.

Concrete Height: Be aware that all taller concrete pours can cause more bleeding due to increased pressure. Therefore, make sure that it is of adequate height.

Temperature and Wind Conditions: Monitor the temperature during mixing and placing. Consider the wind conditions during placement, as they can accelerate evaporation. 

Compaction and Vibration: Ensure proper compression and appropriate vibration technique to minimise the void and reduce the segregation.

Surface Evaporation: Use evaporation retarders to slow down the rate of evaporation of bleed water. 

Permeability and strength: Measure permeability and bond strength between the concrete and reinforcement.

Conclusion

The issue of bleeding remains a critical focus in the construction industry. While traditional construction practices often see bleeding as an unavoidable challenge, it drastically affects the concrete design. Bleeding is a natural part of concrete curing but can negatively impact structural integrity. Technically, bleeding is a symptom of improper ratios and other effects caused by the materials. Looking ahead, the focus on sustainability brings bleeding back into the spotlight. 

Eliminating bleeding plays a crucial role in creating long-lasting and high-performance concrete. This shift underscores a growing commitment to quality control. More technical challenges, including bleeding, can be resolved with the right approach and innovative technology. 

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