Segregation & Bleeding in Concrete

Segregation is the separation of the coarse aggregate from the mortar in fresh concrete; bleeding is water rising to the surface of freshly placed concrete. They are two different defects with two different directions of movement — aggregate going down, water coming up — and both leave the concrete weaker than it should be.

The difference at a glance

	Segregation	Bleeding
What separates	Coarse aggregate from mortar	Water from the mix
Direction	Heavy stones settle down	Water rises to the top
Result	Non-uniform concrete, honeycomb	Weak, porous top surface (laitance)

Segregation — causes and cure

Segregation happens when the mix is not cohesive enough to hold together:

Over-vibration — vibrating too long drives the stones down and brings paste up.
Excessive drop height — dropping concrete more than about 1.5 m lets the aggregate separate; use a chute or tremie.
A harsh, gap-graded or too-wet mix.

Cure it by using a well-graded cohesive mix, the right water-cement ratio, controlled placing and just-enough vibration.

Bleeding — causes and cure

Bleeding is water finding its way up as the heavier solids settle:

Too much water in the mix (high w/c ratio).
Over-vibration and thin, wet mixes.

The rising water leaves a thin, weak, dusty layer called laitance on the surface. If a slab is finished (trowelled) while it is still bleeding, that weak layer is worked in and the surface stays weak and prone to dusting. Cure bleeding with a lower w/c ratio, a properly graded mix (enough fines), and by waiting for bleed water to evaporate before final finishing.

Why both matter

The weakest part of a poorly placed pour is usually the top, exactly where a slab is walked on and finished. Segregation causes honeycomb and non-uniform strength; bleeding leaves a weak surface. Controlling the mix, the drop and the vibration prevents both at once.

Frequently asked questions

What is the difference between segregation and bleeding in concrete? Segregation is coarse aggregate separating and settling out of the mix; bleeding is water rising to the surface. Aggregate goes down, water comes up.

What causes segregation in concrete? Over-vibration, dropping concrete from too great a height, and a harsh, gap-graded or overly wet mix.

What causes bleeding in concrete? Too much water (a high water-cement ratio) and over-vibration, which let water rise as the solids settle.

How do you prevent segregation and bleeding? Use a well-graded cohesive mix with the right water-cement ratio, limit the drop height, vibrate just enough, and let bleed water evaporate before final finishing.