There have been during the long lifespan of prestressed hollow core slabs, several methods to produce them. Today, two basic technologies make up the majority of hollowcore production – slipforming and extrusion. Both technologies are based on long casting beds where products are formed on top of 70 to 200 metre-long moulds. The daily production capacity may vary from 400m2 to 2000m2.

Conventional layouts consist of batching and mixing plant concrete, then conveying it to several parallel casting beds with prestressing abutments at the ends of the casting lines. There are also improved layouts; some of them have moving moulds (casting beds) or stationary sawing stations. The target is to increase productivity and improve the safety and working environment.

The basic workflow in these factories is as follows :
  • Preparing casting beds
  • Pulling and attaching prestressing wires or strands
  • Prestressing
  • Forming slab around the reinforcement
  • Marking and identifying slabs
  • Making openings, etc.
  • Curing long slabs
  • Cutting slabs to delivery lengths
  • Drilling drainage holes
  • Lifting slabs to stockyard stacks

 
Pre-tensioning

The pulling and pre-tensioning of the wires or strands are among the first steps in production. There are a couple of different materials to be used: 7wire-strands and 4 to 6 mm wires. However, the majority of manufacturers are using 7-wire strands, which provides opportunity for high tensioning and high-quality products.

The typical tension of each strand is between 1100 to 1300 Mpa. The strands are tensioned individually or in bundles, depending on the capacity required. The number of strands depends on the cross section of the product and the capacity of the load to be imposed on the product. The total prestressing force might be up to 250 tons, or with the heaviest sections, even higher.

The Principles of Product Forming Systems

Extrusion is known for its high compaction capabilities and excellent structural capacity of the hollowcore slab. One of the advantages in modern extrusion is silent and automatic operation.

The key to an optimum compaction on a dry cast is the frequency of vibration since the absence of water makes it more difficult for the concrete particles to flow.

Some extruders manufactured today do not use high-frequency vibrators; instead, they operate based on pressing extrusion screws, which usually requires more water to achieve the same level of compaction than hollowcore extruders based on a high frequency of vibration.

The second form of production is slipforming. This process is a fairly straightforward principle where concrete is flowing onto the product in two to three phases (layers). Each of these phases is compacted typically by vibrators. The slipform system is used mainly for shallower hollowcore cross sections and various other cross sections, such as inverted T-beams. However, there are slipformers which can form 0,7m hollowcore sections.

The production speed for each system varies depending on raw materials, depth of products and machine types. The speeds range between 1 to 3 meters/min.

Marking

There is an increasing need to make different kinds of openings and cutoffs to the slabs. Automatic plotters effectively mark the tops of the slabs. Additionally, the plotter may simultaneously print the identification data on the sides of the slabs. The plotter uses inkjet-type ink.

Sometimes, RFID (radio-frequency identification) transponders are inserted into the slabs. These help identify the slabs and provide information about the reinforcement used, date of manufacture, etc. This information is collected using electronic readers.

Slab Cutting

Eight to 20 hours after casting, the slabs are cut into final lengths after releasing the strands from abutments. The concrete must have reached the compressive strength of 35 MPA or some other value, as defined in national codes.

Circular diamond blade saws make cuts. Conventionally, the saw moves on casting bed rails. The positioning is in many cases still manual; however, automatic operation is becoming more popular. In some systems, cuts are made in stationary sawing stations to control noise and sawdust slurry in one compact area.

Lifting & Transporting

The cut slabs are lifted onto delivery stacks and transported to the stockyard. Lifting is performed by bridge cranes using a suitable lifting beam with clamps to grab onto the slab. Slabs must have hooks for proper handling. The weight of large slabs may range up to 14 tons in weight and length up to 22 meters.

Maintaining Casting Beds

Casting beds (moulds) are the key to production quality. Any bed defects are directly copied to the products. It is, therefore, important that the beds are properly cleaned and oiled by suitable machinery after each casting. Typically, such a maintenance machine has multifunction features which, for example, simultaneously pull the new strands or wires along the beds before preparation for stressing.

Handling Waste

Hollowcore production, unfortunately, does create some waste in the form of concrete sludge and pieces of concrete. The target of all manufacturers should be to minimize waste and maximize reuse of waste materials. There are specialized solutions to take care of the sludge and prepare it for recycling.

Production Planning

To manage production and ensure timely deliveries, there must be a well-planned shipping order and truckload system which optimizes production capacity and includes a reliable system for tracking slabs. This process would involve experienced staff using an advanced production planning software system.