What is segregation of concrete?
Segregation is the separation of a mix into its individual components. In fresh concrete the heavy aggregate sinks while water and cement paste rise, so the concrete becomes far too coarse in one place and far too fine in another.
A good mix is homogeneous: aggregate, sand, cement and water are present in the same proportions everywhere. As soon as those proportions shift locally, the strength shifts with them. Segregated concrete develops honeycombing (voids between the stones with no mortar around them), a weak and porous top layer, and unreliable cover to the reinforcement.
What causes it
- Concrete that is too wet — excess water is by far the most common cause. Water added on site to make placing easier lowers the strength and invites segregation.
- Excessive drop height — free-falling concrete separates: the aggregate bounces and rolls away while the mortar stays behind. Limit the drop and use a chute or a hose.
- Moving concrete sideways with the poker vibrator — a vibrator is for compaction, not for pushing concrete along the formwork.
- Over-vibrating — vibrate too long or too fiercely and the aggregate settles while water rises.
- Poor grading — too little fine material makes the mix prone to separation.
- Vibration during transport — long journeys with a mix that is too fluid.
How to prevent it
- Order the right consistency class and add no water on site.
- Place in layers of roughly 300 to 500 mm and compact each layer separately.
- Keep the drop height small; on tall walls use a placing hose that you pull up as you go.
- Insert the poker vertically, quickly in and slowly out, and let it penetrate a little into the layer below.
- Recognise bleeding (a film of water on the surface) as an early warning and adjust the way you are working.
Related terms
- Fresh concrete
- Consistency class
- Grading
- Formwork
- Concrete cover
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