Hollowcore Slab Extruder

Overview

A hollowcore slab extruder casts prestressed concrete floor planks in one continuous pour. It is a self-propelled machine that sits on the side rails of a long prestressing bed, straddling a row of steel strands that have already been pulled to high tension along the full length of the bed. As the machine creeps forward it takes in stiff, almost dry concrete, forces it around a set of cores that form the hollow voids, vibrates it solid, and trowels the cross-section to shape. Behind the machine it leaves a finished plank with the strands cast into it. When the bed is full the plank is cured, the strands are cut, and the long casting is saw-cut into floor units of the lengths a building needs.

The work splits across nine subsystems. The Concrete Hopper and Feed takes the concrete charge and meters it down, the Auger and Mandrel Forming Section forces it around the cores and forms the voids, and the Vibration and Compaction System system compacts it. The Drive and Traction System system pulls the machine along the bed, the Slipform Shoe and Side Forms shoe shapes the finished surfaces, and the Prestress Strand Pass-Through lets the machine pass over the tensioned strands cleanly. The Electric Power Unit unit feeds the motors, the Control Panel panel keeps casting speed and feed in balance, and the Machine Frame and Rail Wheels carries it all on the rails.

Feeding the concrete

Hollow-core planks are dry-cast, which means the mix has almost no slump and will not flow under its own weight. It has to be pushed. The Concrete Hopper and Feed holds a charge dropped in by an overhead skip, and two vertical Feed Screw augers drag that stiff concrete down out of the hopper and deliver it to the forming section at a steady rate. The screws turn through the Feed Screw Gearbox, driven by the Feed Screw Motor, at low speed and high torque because the mix resists being moved. A Level Paddle in the hopper signals when the charge runs low so the operator calls the next skip before the feed starves and leaves a void in the plank.

Forming the voids

The heart of the machine is the Auger and Mandrel Forming Section. A row of helical Forming Auger screws turns inside a hard Forming Chamber Liner, and each auger drags concrete back along the machine as the whole machine advances forward. That back-pressure is what packs the dry mix solid. Trailing behind each auger is a floating Core Mandrel that holds one void open while the concrete sets around it, and a Compaction Tube that shapes and consolidates the concrete around that void. The augers run on hardened Auger Shaft shafts carried on Ball Bearing units and sealed against grout by Oil Seal rings. All of the augers are driven in step from the Auger Drive Gearbox, which splits the torque of the Auger Drive Motor motors across every shaft so the cores form evenly across the width of the slab. A slab 1200 mm wide carries four to seven cores depending on its depth.

Compaction

Dry-cast concrete traps a lot of air, and the only way to drive it out is vibration. The Vibration and Compaction System system runs high-frequency vibrators inside the cores and along the edges. A Mandrel Vibrator sits inside each core mandrel and consolidates the concrete around the void, while a Side Form Vibrator on each side compacts the webs and edges. Each one is driven by a Vibrator Motor spinning a Eccentric Weight weight, and the eccentric setting tunes how hard the unit shakes. Good compaction sets the strength of the plank, so this system runs continuously while concrete is being placed.

Driving along the bed

The machine moves itself. The Drive and Traction System system has powered Driven Rail Wheel wheels that grip the bed rail, turned by the Traction Motor through the Traction Gearbox at a slow creep of one to two and a half metres per minute. The forward speed has to match the rate the augers can pack concrete, so a Casting Speed Sensor reports the travel speed and the control trims it against the feed. A Holding Brake locks the machine when it stops so it cannot roll back into the fresh plank. As the machine advances it has to pass cleanly over the tensioned strands: the Prestress Strand Pass-Through holds each strand on its line with a Strand Entry Comb and a row of Strand Guide Roller rollers so the auger cannot drag the steel off position.

Finishing and control

The Slipform Shoe and Side Forms shoe trails behind the forming section and sets the final shape. The Top Forming Shoe forms the flat deck, the Side Form plates form the keyed edges that grout to the next plank on site, and the Top Screed Plate floats the top smooth. Slab depth is set by raising or lowering the forming section on its Slab Depth Adjuster screws. Power reaches the motors through the Electric Power Unit unit, where Variable Frequency Drive drives set each speed and a Trailing Cable Reel pays out the supply cable along the bed, backed by a Power Supply and interface Relay units. The Control Panel panel ties it together: a Machine Controller balances feed, auger, and traction speed against the chamber Pressure Sensor reading, an LCD Panel shows the operator the casting state, and a hard-wired Emergency Stop Circuit circuit cuts everything at once. The result is a long, dense, prestressed plank that needs no formwork under it and spans far further than a solid slab of the same weight.