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
• 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Pipes are gradually Withdrawn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
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
• 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
USES
• Mass Construction – Where Temperature Rise needs to be Controlled :
– By Pre-Cooling C.A. by Circulating Refrigerated Water Round the Aggregate – The Water is Later Displaced By the Rising Water.
– Pre-Heat C.A. by Steam in Cold Weather.