Technical Questions

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How can segregation of the concrete between the mixer and its final point of placement be prevented?

Q. How can segregation of the concrete between the mixer and its final point of placement be prevented?

 

A. A mixture that is proportioned so that it contains the maximum amount of coarse aggregate and just enough mortar of the proper consistency to fill the spaces between the coarse aggregate particles and enough fines to minimize bleeding will not segregate. Because of delivery at an angle from the vertical, segregation occurs principally at locations where the concrete is transferred from one device to another, as from mixer to hopper, from hopper to buckets, belts, chutes, or pumps, and from these to the final point of deposit. Swinging baffles and flexible drop chutes of sufficient length should be used so that the concrete will fall vertically, resulting in remixing rather than in separation. Concrete should be placed in layers about 500 mm (20 in.) thick or deep with vibration following to avoid separation from appreciable lateral flowing of the concrete, especially if slumps exceed 65 to 75 mm (2.5 to 3 in.). If concrete is delivered by a conveyor that stacks the concrete and coarse aggregate separately, a vibrator should be used at the delivery point to keep the concrete uniform.

 

References: SP-1(02); ACI 304R-00

Topics in Concrete: Concrete Fundamentals; Placing

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