Segregation of Concrete
Separation of coarse aggregate from cement paste due to over-vibration, drop height >1.5 m, or low cohesion.
Segregation is the separation of coarse aggregate from the cement paste-fine aggregate mixture in fresh concrete. The result is uneven distribution of materials with concentrated coarse aggregate in some zones and excess paste in others. Per IS 456:2000 + IS 10262 mix design, segregation is unacceptable in concrete because it produces non-uniform strength, shrinkage cracks, and aesthetic defects. Common causes: improper mix design, over-vibration, drop height >1.5 m, prolonged transport time, and high water content.
Mix design causes: (a) Insufficient fine aggregate — too few fines to coat coarse aggregate. (b) Gap-graded aggregate — voids in size distribution create channels for separation. (c) High coarse aggregate content. (d) Insufficient cement content. (e) Excessive water (high w/c). (f) Inappropriate aggregate maximum size for placement geometry. Site practice causes: (1) Drop height >1.5 m during placement — concrete bounces and aggregate separates. (2) Over-vibration — extended vibration dewaters fines and concentrates aggregate. (3) Prolonged transport time (>90 minutes per IS 4926 RMC) — slump loss and aggregate settlement. (4) Excessive admixture dosage — over-dispersion of paste. (5) Pumping over long distances or vertical heights — friction in pipe causes paste-aggregate separation.
Identification on site: visible aggregate concentration in some areas, smooth paste-rich zones in others; drop height tests showing concrete fall as separated streams of stone and slurry; cube tests showing wide variability in 28-day strength. Mitigation: (a) Proper mix design with adequate fines and cement (IS 10262 trial verification). (b) Avoid drop height >1.5 m — use chutes, conveyor belts, or pump for placement. (c) Vibrate until cement paste rises (just visible) — stop. (d) Strict transport time limit (60-90 minutes). (e) Adjust admixture dosage if over-dispersing. (f) For pumped concrete, use appropriate mix design with smaller maximum aggregate size and higher fines. The most-overlooked aspect: site practice is often the cause, not mix design — even a properly-designed mix segregates when poured from 4-5 m height. Pre-pour ITP audit must verify drop heights and placement methods.
- Quality control during fresh concrete placement
- Mix design optimization for placement geometry
- Pumped concrete and tall placement applications
- Pre-stressed concrete and high-flow mixes
- Forensic analysis of strength variability in finished concrete