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The Hollowcore Production Cycle

There have been during the long life span of prestressed hollow core slabs several methods to produce the slabs. Today two basic technologies form the majority of the production; the slipforming and the extrusion. Both technologies base on long casting beds where the products are formed on top of 70 to 200 meters long moulds. The daily production capacity may vary from 400m2 to 2000m2.

Conventional layout consists of batching and mixing plant concrete conveying and several parallel casting beds with pre-stressing abutments at the ends of the casting lines. (see the illustration on next page There are also some improved layouts. Some of them have moving moulds (casting beds) and/or stationary sawing stations. The target is to increase the productivity and improve the safety and working environment.

The basic workflow in these factories is as follows:

  • Preparing the casting beds
  • Pulling and atttaching the pre-stressing wires or strands
  • Pre-stressing
  • Forming the slab around the reinforcement
  • Marking and identifying the slabs
  • Making the openings etc.
  • Curing of the long slabs
  • Cutting of the slabs to delivery lengths
  • Drilling the drainage holes
  • Lifting the slabs to stockyard stacks
 
Pre-tensioning
The pulling and pre-tensioning of the wires or strands is one of the first steps in the production. There are couple of different materials to be used: 7wire-strands and 4 to 6 mm wires. However the majority of the manufacturers are using 7-wire strands giving the 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 pre-stressing force might be up to 250 tons or with the heaviest sections even higher.
 
The principles of product forming systems

Extrusion is known for the high compaction capabilities and excellent structural capacity of the hollow core slab. One of the advantages in the modern extrusion is the silent and automatic operation. The key to an optimum compaction on dry cast is the frequency of vibration since the absence of water makes it more dificult for the concrete particules to flow. Some of the Extruders manufactured today do not use the high frequency vibrators, instead they operate based on pressing extrusion screws, which usually requires more water to achieve the same level of compaction than the Hollow Core Extruders based on high frequency of vibration.

The second form of production is the Slipforming process. This is a relatively simple principle where concrete is flowing on to the product in two to three phases (layers) each phase is compacted typically by vibrators. The Slipform system is used mainly for shallower hollow core cross sections and various other cross sections like inverted T-beams. However, there are slipformers, which can form 0,7m hollow core sections.

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

 
Marking

There is an increasing need to make different kind of openings and cut-offs to the slabs. These are most effectively marked on the top of the slabs by automatic plotters. In addition, the plotter may simultaneously print the identification data on the sides of the slabs. The plotter uses today certain inkjet type ink.

A new development is to insert RFID transponders to the slabs. These help identify the slabs and provide information regarding for example the reinforcement used, the date made etc. This can be picked up using electronic readers, both in the stock-yard or the finished building.

 
Cutting of the slabs

The slabs will be cut in final lengths after releasing the strands from abutments which can be done after 8 to 20 hours from the casting. The concrete must have reached compressive strength of 35 MPa or some other value, which may have been defined in the national codes.

The cutting is made by circular diamond blade saw. Conventionally the saw is moving on casting bed rails. The positioning is in many cases still manual but the automatic operation is becoming more popular. In some of the newest systems the sawing is made in stationary sawing stations in order to control the noise and saw dust slurry in one concise area.

 
Lifting and transporting

The cut slabs are lifted on delivery stack and transported to stockyard. Lifting is made by bridge cranes using a suitable lifting beam with clamps to grab on the slab. Special slabs must have hooks for proper handling. The weight of large slabs might be up to 14 tons and length up to 22 meters.

 
Maintaining the casting beds

The casting beds (moulds) are the basis of production quality. All the defects on beds are directly copied to the products. It is therefore very important that the beds are properly cleaned and oiled by suitable machinery after each casting. Typically such a maintenance machine has multifunction versions which, for example, will simultaneously pull the new strands or wires along the beds prior to preparation for stressing.

 
Waste handling

The hollow core production unfortunately does create some waste in form of concrete sludge and pieces of concrete. The target of all manufactures should be to minimize the waste and maximize the re-use of the waste materials. There are specialized solutions to take care of the sludge and prepare it for recycling.

 
Production planning

In order to manage the production and ensure timely deliveries to the site, there has to be adapted a well-planned shipping order and truck loads system which optimises utilisation of the production capacity and includes a reliable system for the tracking of each slab. This would generally involve experienced staff using an advanced production planning software system.

 
Setting up a new Factory
If you are planning to set up a new factory please contact us and use following list as a basic check list for what should be considered in a factory planning:

Product specifications, what is really needed:

  • Thicknesses
  • Lengths
  • Cuttings and openings (askew ends, corner cuts, holes etc.)
  • Marking (Identification)
  • Requirements of standards
  • The required loading capacities

General info

  • Capacity / day / year
  • Available raw materials / supply
  • Operation in stockyard / truck loading
  • Expansion plans
  • Working hours
  • Environmental requirements
  • Low cost investment or low labour need?
  • Production planning, how to do?

Technical specifications

  • Mixing plant suitable for homogenous zero slump concrete
  • Concrete transportation fast and reliable
  • Cranes heavy enough
  • Lifting beams and clamps suitable for all production
  • Casting beds are basis of product quality
  • Basic layout or advanced layout?
  • Does the layout allow continuous and safe operation?
  • Are the drainage holes needed?
  • Safety requirements
  • Maintenance arrangements
  • Do not try to squeeze to much the factory area
  • How to handle the waste materials?
 

Information courtesy of the IPHA.