Segregation and Bleeding of Concrete: Causes, Effects and Prevention

What is Segregation?

Segregation is the separation of the constituent materials of concrete — primarily the separation of coarse aggregate from the mortar (cement + sand + water). Fresh concrete is a mixture of particles of widely varying densities and sizes, all held together by the cohesiveness of the cement paste. When this cohesion is overcome, heavier and larger particles (coarse aggregate) settle down or move to the periphery while the finer, lighter mortar remains on top or in the center.

Think of it like a thick soup where the heavy bits sink to the bottom if you don’t stir it — concrete does the same thing if it’s too wet, dropped from too great a height, or over-vibrated.

Good Concrete Uniform distribution ✓ Strong, durable

Segregated Concrete Mortar/Paste layer Coarse agg sinks

Bleeding Concrete Water / Laitance layer Water rises (bleeding) channels form

Figure 1: Comparison of Good Concrete, Segregated Concrete, and Bleeding Concrete

Causes of Segregation

Excessive water content (high W/C ratio): The primary cause. Too much water reduces the viscosity of the paste, which can no longer hold coarse aggregates in suspension during transport and placing.
Inappropriate aggregate grading: Gap-graded or poorly graded aggregates with insufficient fine particles leave the mix with low cohesiveness.
Dropping concrete from excessive heights: Dropping concrete more than 1–1.5 m causes coarse aggregates to bounce away from the mortar when they hit the existing concrete surface. Particularly problematic in column formwork.
Over-vibration: Extended or intense vibration (beyond what’s needed for compaction) causes coarse particles to sink and light material to rise — the opposite of the desired effect.
Insufficient cement content (lean mix): Low paste volume means insufficient binder to coat all aggregate surfaces and hold them together.
Large maximum aggregate size: Very large particles (above 40 mm in normal work) are harder to keep uniformly distributed.
Improper handling and transport: Long transport distances, poor roads, or excessive chute angles.

Effects of Segregation on Hardened Concrete

Honeycombing: Pockets of coarse aggregate without surrounding mortar, creating visible voids. Structural weakness and direct exposure of reinforcement to the environment.
Non-uniform strength distribution: Zones rich in mortar are weak in compression; zones with only coarse aggregate have poor binding. Structural performance becomes unpredictable.
Increased permeability: Voids and mortar-deficient zones form channels for water, chloride, and sulphate penetration.
Poor appearance: Visible defects on formed surfaces; aggregate-rich patches alternate with paste-rich streaks.
Construction joints and lifts: Segregated concrete at the top of a pour creates a poor interface with the next pour, weakening the construction joint.

Prevention of Segregation

Use well-graded aggregates (IS 383 grading limits)
Keep W/C ratio as low as workability permits; use admixtures instead of extra water
Limit free drop height to 1.0–1.5 m maximum (use drop pipes, tremie pipes, or elephant trunks for deep pours)
Avoid over-vibration — vibrate until air bubbles stop rising, then withdraw immediately
Use VMA (Viscosity Modifying Admixtures) for flowing concretes
Maintain adequate cement content (≥ 300 kg/m³ for normal RCC)
Design mix with adequate fine aggregate proportion

What is Bleeding?

Bleeding (also called water gain) is a specific form of segregation where excess mixing water rises to the surface of freshly placed concrete before it sets. It occurs because water is the lightest constituent of concrete and is displaced upward by the downward settlement of heavier solid particles (aggregate and cement).

Bleeding is actually a natural, mild, and even slightly beneficial phenomenon in small amounts. A thin layer of water on the surface evaporates and the concrete below remains intact. The problem arises with excessive bleeding.

Causes of Bleeding

High water content (high W/C ratio): More free water than needed → excess water squeezed upward by settling solids.
Very coarse or poorly graded sand: Coarse sand has fewer fine particles to block upward water movement through pore channels.
Low cement content: Less cement surface area to retain water by adsorption.
Coarse cement (low Blaine): Finer cement retains more water; coarser cement bleeds more.
Over-vibration and over-mixing
High ambient temperature in some cases (though higher temp actually reduces bleeding by accelerating setting)

Effects of Bleeding on Concrete

Harmful effects:

Weak top surface: The bleed water, on evaporating or being remixed by finishing tools, creates a high-W/C zone at the top — extremely weak and dusty. This is why surface abrasion resistance is poor in over-troweled slabs.
Laitance formation (described below)
Weakening below reinforcement: Bleed water accumulates under horizontal rebar bars. When it evaporates, a water-void forms right below the bar — a porous, weak zone that compromises bond between steel and concrete and provides a pathway for corrosion.
Bleeding channels: Connected pore channels left by water movement path reduce impermeability dramatically, increasing permeability to chlorides and sulphates.
Settlement cracking: When bleed water is still present and concrete is partially set, differential settlement causes plastic shrinkage cracks — particularly over reinforcement bars.

Slightly beneficial effect: Controlled, limited bleeding can reduce plastic shrinkage cracking by keeping the surface moist longer before initial set. It also helps in hot, windy conditions. The industry recognizes that zero bleeding is not necessarily ideal — a small, controlled amount is acceptable.

Laitance

Laitance is the thin, weak, flaky layer that forms on the surface of freshly placed concrete as bleed water evaporates. It contains: bleed water + fine cement particles + fine aggregate particles that were carried upward by the water. When hardened, laitance is chalky, dusty, and extremely weak — much weaker than the concrete below.

Problem at construction joints: If a new layer of concrete is poured on a surface with laitance, the bond between old and new concrete is very poor — the joint acts as a weak plane. Laitance must always be removed (by wire brushing, grit blasting, or water jetting) before pouring the next lift.

Prevention of Bleeding

Reduce water content; keep W/C ratio low; use superplasticizers
Use fine sand (Zone II or III) rather than very coarse sand
Increase cement content (more surface area to retain water)
Use finer cement (higher Blaine fineness)
Add mineral admixtures: fly ash and silica fume significantly reduce bleeding — silica fume is particularly effective due to its enormous surface area that physically blocks water channels
Avoid over-vibration and over-troweling of surfaces
Do not add water during placing or finishing operations

🎯 Exam Tips (RTMNU)

Segregation = coarse aggregate separates from mortar; Bleeding = water rises to surface — state both definitions clearly and separately.
Laitance is a very commonly asked topic — define it as a “thin, weak, chalky layer of fine material on the surface” and explain why it must be removed before new pours.
Void under rebar from bleeding is a classic point — shows understanding of reinforcement bond and corrosion risk.
Over-vibration causes segregation (not improved compaction) — many students get this backwards.
Maximum free drop height = 1.0–1.5 m — state this limit for prevention of segregation.
Silica fume is the most effective mineral admixture for reducing bleeding — useful for 10-mark comprehensive answers.

✅ Key Takeaways

Segregation = coarse aggregate separates; causes: high W/C, drop height >1.5 m, over-vibration, poor grading.
Effects: honeycombing, weak zones, high permeability, poor bond at joints.
Bleeding = free water rises; causes: high water content, coarse sand, low cement.
Effects: weak top layer, voids under rebar, laitance, bleeding channels, settlement cracks.
Laitance = weak chalky surface layer from bleed water + fines; must be removed before construction joint.
Prevention: reduce W/C, use graded aggregate, limit drop height, avoid over-vibration, use SCMs.

🔗 External Reference: IS 456:2000 – Plain and Reinforced Concrete, Clause 7 (BIS)

❓ FAQs

Q1. What is the difference between segregation and bleeding?

Segregation is the separation of coarse aggregate from the mortar phase of concrete, creating non-uniform distribution of ingredients. Bleeding is the rise of excess mix water to the top surface of freshly placed concrete before it sets. Both are forms of instability in fresh concrete and both compromise the quality of hardened concrete.

Q2. What is laitance and why is it harmful?

Laitance is a thin, weak, chalky layer of fine material (fine cement + fines + water) that forms on the surface of concrete due to bleeding. It is extremely weak because of its high W/C ratio. At construction joints, laitance creates a weak interface between pours and must be completely removed by wire brushing, grit blasting, or water jetting before the next layer is placed.

Q3. How does over-vibration cause segregation?

Vibration temporarily reduces the yield stress of concrete (liquefies it). While moderate vibration expels trapped air and improves compaction, over-vibration keeps the concrete in a liquid-like state long enough for gravity to separate particles by density — heavy coarse aggregates sink and lighter mortar/water rises. This is the opposite of the desired uniform compaction.

Q4. What is the maximum permissible free-fall height for concrete?

To prevent segregation, concrete should not be dropped freely from a height exceeding 1.0–1.5 m. For deeper pours (columns, deep walls), a tremie pipe, drop chute, or elephant trunk must be used to guide concrete to its placement level without free fall.

Q5. How does silica fume reduce bleeding?

Silica fume particles are ultra-fine (0.1–0.2 μm) with enormous surface area. They physically fill the interconnected capillary pore channels through which bleed water travels, blocking the upward water migration. They also rapidly react with Ca(OH)₂, consuming free water in the pozzolanic reaction. A dosage of just 5–10% silica fume can reduce bleeding to near zero.