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PREPLACED AGGREGATE CONCRETE/ PREPACKED CONCRETE

Produced in TWO STAGES

[ I ] Coarse Aggregate is Placed and Compacted in Forms – Voids Being 30 to 35 % of the Overall Volume to be Concreted.

[ II ] Voids are Filled with Flowing Mortar

Aggregate in Concrete is Gap Graded

Sieve Size Cumulative % Passing

C.A. F.A. C.A. F.A.

150 2.36 100 - * - * 100

75 1.18 67 –100 – 100 98

38 600 40 – 62 – 97 72

19 300 6 –4 – 9 34

13 150 1 – 1 – 1 11

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Optimum Packing of Aggregate – Greatly Advantageous (Theoretically) Not Necessarily Achieved in Practice.

COARSE AGGREGATE MUST BE :

Free from Dirt/Dust as the Impairs the Bond

Thoroughly wetted/Inundated before Mortar Intrusion

Free from Stagnant Water – Algae Growth Occurs on Aggregate

COMPOSITION BY WEIGHT

Cement 2 Parts OPC

Pozzolana 1 Part Finely Divided Fly Ash – Reduces Bleeding & Segregation and Improves Fluidity of the Mortar

F.A. 3 – 4 Parts Fine Sand

Water Adequate So as to form Fluid Mixture

Intrusion Aid 1 % of (Cement & Pozzolana) [Improves Fluidity of Mortar & to Keep Solids in Suspension: i.e. delays stiffening of Mortar]

Contains Small Quantity of Aluminum Powder causing slight expansion

INTRUSION

Mortar Pumped through Slotted Pipes

Pipe Dia. 35 mm @ 2 m c/c Starting from the Bottom of the Mass

Intrusion Against Gravity

Pipes are gradually Withdrawn

Pumping Over Long Distances Possible

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Water Expelled

Slotted Pipes

ECONOMICS

Pre-Placed Aggregate Concrete is Economical in Cement Content i.e. 120 – 150 Kg per M3 of Concrete

Strength is Limited by High W/C Ratio, adopted for Sufficient Plasticity of Mortar

Strength Adequate for Usual Applications and it is more Uniform in Properties – as Segregation is Eliminated

Resultant Concrete is

Free of Segregation

Dense

Impermeable

Durable

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PLACEMENT

Pre-Packed Concrete is Placed in:

Locations Not Easily Accessible.

Sections containing Large No. of Embedded Items Precisely Located e.g. Nuclear Shields.

Danger of Segregation of Heavy C.A. especially STEEL AGGR. in Nuclear Shields is Eliminated: As C.A. and F.A. are pre-placed Separately.

Pozzolanas Should not be used in Nuclear Shields because they Reduce Density of Concrete.

Due to reduced Segregation – Suitable for Underwater Construction.

Specialized Systems exists for large Concrete Placement Under Water

CHARACTERISTICS & PROPERTIES

Free of segregation

Dense

Impermeable

Durable

Drying Shrinkage Lower than Ordinary Concrete :

# Due to POINT-TO-POINT Contact of C.A – Without Clearance for Cement Grout/

Paste necessary for Ordinary Concrete.

# This Contact Restrains the Actual Amount of Shrinkage that can be Realized

.

# Occasional Shrinkage Cracks Can Develop

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USES

Mass Construction – Where Temperature Rise needs to be Controlled :

By Pre-Cooling C.A. by Circulating Refrigerated Water Round the AggregateThe Water is Later Displaced By the Rising Water.

Pre-Heat C.A. by Steam in Cold Weather.

Construction of Water Retaining and Large Monolithic Structures – Due to Reduced Shrinkage.

• Repair Work

• To Provide Exposed Aggregate Finish : Special Aggregates Placed against Surfaces – Exposed by Sand Blasting or Acid Wash.

LIMITATIONS

• Numerous Practical Difficulties

• Skilled and Experienced Workmen Required.

• Specialized Equipment Required.

References

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