What Is Segregation in Concrete?
Segregation is the separation of concrete's ingredients — coarse aggregate settling out, cement paste and water rising — producing non-uniform, weak concrete. It happens during handling (dropping concrete from height, over-vibration, poor mix design) and shows up as honeycombing, sand streaks, and variable strength. Segregated concrete is defective concrete; prevention is in the handling, not the mix alone.
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How it happens
Dropping concrete more than 4-6 feet (through rebar cages, down tall wall forms) lets aggregate fall through the paste. Over-vibration liquefies the mix and sinks the stone. Over-wet mixes segregate with minimal provocation. Long chutes, sharp bends in pump lines, and stockpiled aggregate that segregated before batching all contribute. The result: rock pockets with no paste (honeycomb), paste-rich weak zones, and cold joints where segregated layers meet.
Prevention in placement
Limit free-fall distance (use tremies, elephant trunks, or pump hoses to the point of placement), vibrate enough to consolidate but not to segregate (withdraw the vibrator slowly, don't use it to move concrete horizontally), and design cohesive mixes with proper aggregate gradation. ACI 309 covers consolidation practice; the crew's technique matters as much as the mix design.
Assessment and repair
Honeycombing discovered at form removal gets assessed: surface honeycomb (shallow, no exposed rebar) is patched with approved repair mortar; deep honeycomb with exposed or corroded rebar needs engineered repair (remove unsound concrete, clean steel, form-and-pour or shotcrete). Extensive segregation through a structural member can require removal and replacement — the special inspector's call, documented with photos and sounding.
The bottom line
Segregation separates aggregate from paste during bad handling — limit drop heights, vibrate correctly, and design cohesive mixes. Repair honeycomb by depth: patch shallow, engineer deep.
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