How to control segregation of concrete mix during placing when the free fall demands over 1.5 m and without proper placing tools.
- hamerkopbuilders
- Jul 16
- 2 min read
When concrete is poured from a height greater than 1.5m, segregation may occur coz the heavier coarse aggregates (ballast) tend to fall faster than the cement paste and sand (mortar) during high free fall.
When this happens, the concrete mix begins to separate and causing the ballast to collect at the bottom while the mortar remains behind.
This alone leads to honeycombing or weak zones and esp at the base of deep columns.
Concrete pour in column cage
To prevent this from happening, concrete should ideally be placed using tremie pipes, chutes, elephant trunks or by reducing the free fall height from 1.5m and below.
But sometimes when such placement tools AREN’T UNAVAILABLE and the concrete pour height demands over 1.5m, another practical site solution can be applied.
Immersion concrete vibrator
The 1st pour (i.e using buckets) should consist of a SLIGHTLY RICHER MORTAR MIX with REDUCED BALLAST CONTENT. This particular softer 1st layer acts as a CUSHION OR STABILIZER @ the base of the column by reducing the initial impact created by falling concrete mix from the buckets pour and minimizing segregation at the bottom.
Vibrating concrete using an immersion vibrator
The 2nd pour should then contain a HIGHER BALLAST CONTENT than the normal mix. Its purpose is to COMPENSATE for the REDUCED BALLAST CONTENT in the previous 1st pour and serves to restore the OVERALL BALANCE in the 1st concrete mix.
The 3rd and final pour should return to the NORMAL CONCRETE MIX DESIGN PROPORTIONS.
Alignment of column formwork
Dismantling of column formwork
Reinforced concrete column
In simple terms.
• 1st layer has less ballast (cushion / stabilizer)
• 2nd layer has more ballast (compensator to the 1st)
• 3rd layer has the normal mix (standard concrete design)
Since the concrete proportions were intentionally adjusted between pours, proper vibration becomes critical in achieving final uniformity.
Recommended vibration approach.
• In 1st layer, use light / mild vibration
• 2nd layer, use stronger vibration to allowing/push more ballast to penetrate deeper and balance the 1st layer
• 3rd layer, use a normal vibration.
For this temporary unbalanced concrete mix equation to fully redistribute and consolidate properly, vibration will ultimately determine the final result.
Unbalanced mix issue solved.
Work done, SMARTLY and PERFECTLY.
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