During a concrete pour, you notice segregation occurring in the mix. What is the most likely cause and immediate corrective action?
Correct Answer
D) Excessive vibration; reduce vibrator use
Segregation (separation of aggregate from cement paste) is often caused by over-vibration. Reducing vibrator use and ensuring proper placement techniques will help maintain mix homogeneity.
Why This Is the Correct Answer
CORRECT_ANSWER - Segregation occurs when the coarse aggregate separates from the cement paste, creating an uneven mix. Excessive vibration is the most common cause of segregation during concrete placement, as over-vibration causes the heavier aggregates to settle and the lighter cement paste to rise. The immediate corrective action is to reduce vibrator use and ensure vibrators are used properly - inserted vertically, moved systematically, and not left in one spot too long.
Why the Other Options Are Wrong
Option A: Pump pressure too high; reduce pumping rate
While high pump pressure can cause issues with concrete placement, it typically results in problems like line blockages or excessive pressure buildup rather than segregation. Segregation is more directly related to how the concrete is consolidated after placement, particularly through vibration methods.
Option B: Mix is too wet; add cement to stiffen
Adding water to concrete during placement is never acceptable as it significantly weakens the concrete strength and durability. A dry mix would cause poor workability but not segregation - segregation specifically refers to separation of materials, not lack of moisture.
Option C: Mix is too dry; add water immediately
This is the correct answer.
Memory Technique
Remember 'VIB-RATE = SEPARATE' - excessive vibration causes segregation by literally shaking the components apart.
Reference Hint
ACI 318 Building Code Requirements for Structural Concrete, Chapter on Concrete Placement and Consolidation, or Florida Building Code Chapter 19 - Concrete
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