The three types of machine used to produce prestressed hollowcore floor slabs are extruders, slipformers and flowformers. The conventional method is to run the machines on steel beds up to 200 m long, equipped with stressing abutments. Travel speed will depend on quality of raw materials, type of machine and depth of slab. A speed of 2 m/minute can be considered for budget purposes. More than single shift operation is possible, if the correct curing conditions are available.
Normal production width is 1200 mm. Other widths are used as well e.g. 600 mm, 1.500 mm and 2.400 mm. Normal slab depths go from 120 mm to 500 mm.
Concrete is batched, mixed and transported to the casting machine, which forms the shape of the slab as it moves along the bed. The machine is transferred from bed to bed, using an overhead crane.
Some recently constructed factories have static machines and moving beds, in an effort to improve operational efficiency. In addition, static sawing stations reduce noise and create cleaner factories.
The main operations in the hollowcore business are :
- Drawing, designing and production planning
- Creation of method statement for site
- Preparation of casting beds
- Drawing out and fixing prestressing strands and wires
- Feeding the concrete
- Casting the bed
- Plotting and identifying the slabs
- Creating notches, openings etc in the wet state
- Creating water drainage holes
- Casting in lifting hooks
- Covering (if needed) the slabs, to retain moisture during curing
- Curing the total slab length
- Soft detensioning of the strands or wires
- Cutting the slabs to the required lengths, widths and angles
- Creating stacks of slabs
- Removing stacks to the stockyard
- Delivery to site and erection on the building
The drawing out, fixing and prestressing of wires or strands are the first steps in the production. The choice between wires and strands comes down to local availability and cost. Much of the industry has moved to strand, because of its easier handling. Strands are often stressed individually, but equipment is available for multiple stressing.
Slab Casting Systems
Extrusion is characterised by compaction with very low moisture content of mixes. Extruders are provided either with or without vibration. They are used for light weight cross sections with different thicknesses.
Slipformers feed and vibrate the concrete in two or three stages over moving steel cores. They have the flexibility to make other products of constant cross-section, such as T-beams, lintels etc.
Flowformers cast the concrete without mechanisms or driving units. The concrete flows by gravity and vibration. The process uses plastic quality concrete for versatility of cross section profile options with a smooth, rough or indented surface finish.
Automatic bed plotters, linked to bed optimisation software, are used to accurately mark saw cut lines, notches and openings. In addition, they can write the slab data, both on the top and sides of the slab. RFID transponders can be attached to the slabs, to record data.
Stacks of slabs are removed to the stockyard by crane, sideloader or fork-lift truck.
Additional machines are available to clean and oil beds, to draw out wire or strand, to create vacuum notches and openings and to cast in lifting hooks.
This has become a critical part of any manufacturing business. Specialist software is available from our Associate Members, to draw, design, plan, schedule and trace every slab.
The first step into prestressed hollowcore manufacturing is to consult the Associate Members of IPHA. They have extensive experience of all aspects of hollowcore and will be very pleased to provide you with the kind of practical advice which will save your money and hasten your entry into the business. A full list of Associate Members is available in the Members’ section of the website.