Waste case study

Aims of this project

The aims of the project were to reduce the number of incorrectly cut concrete slabs and, more particularly, to reduce the high level of waste that results from the incorrect marking of slabs. Hollowcore flooring is made of hollowcore slabs measuring typically 1.2 metres (width) by 150 mm to 400 mm (depth) while the length is cut to customer requirements. The depth of the hollowcore slab is dependant on the relevant load and length of the slab. A series of parallel holes running from end to end minimise the weight of the slab without compromising its strength. Near the bottom of the slab a series of pre-tensioned wire strands run parallel to and beneath the solid part of the hollowcore slab: these strands reinforce the concrete. Both the thickness and the arrangement of pre-tensioned wires depend on the load-bearing requirements of the slab.

Project description

In order to manufacture the product, a frame consisting of a continuous steel plate is raised off the ground. Prior to casting, the bed is cleaned: any remaining concrete or aggregate from previous use is removed. A series of steel wires varying in diameter from 9.3 to 12.5 mm is then threaded between headstocks at either end of the bed. Both the size and the number of wires depend on the specifications of the slab being cast. The wires are then tensioned using a hydraulic power pack. Once the bed has been prepared, batches of dry mix are assembled in the computerised mixing plant. These are transported to the casting machine, which lays a continuous slab of concrete along the length of the bed.

Prior to the implementation of this project, the concrete slab was marked out by hand into individual lengths. This was done manually by drawing a line on the concrete using a scriber. The person marking the slab used a works order produced by Oran Pre-Cast's drawing office to determine the lengths to be marked out. Once the concrete has cured sufficiently, a hollowcore saw is used to cut the slabs to the correct length. In some cases angles need to be marked for cutting at a later stage. These cut outs enable the flooring slabs to fit around pipes and other projections on site: they also allow them to be laid quickly. It is therefore imperative that in addition to the slabs being the correct length, all cut-outs must be placed correctly. The angles can be difficult to mark out, as it is difficult to use the edge of the cast slab as a reference point.

Any mistakes in cutting the slabs would generally be noticed only at the point where they were being installed at the client premises. Normally, rectification of the error on site would not be possible, and the faulty slab would have to be returned to Oran Pre-Cast and a replacement duly manufactured.

Achievements and lessons

The project was conceived in order to eliminate all markingout errors and to ensure that the slabs leaving the factory would be 100% correct. The solution was to use a bed plotter, which followed the casting machine and automatically marked the slab with an ink spray. The data for the bed plotter was directly downloaded from the drawing office PreCAD system. This solution has resulted in zero marking defects although there may still be some concrete failures and damaged slabs. The innovative element of the project was to have a direct link from the CAD system to the bed plotter, thus eliminating human error.

Plant productivity has improved by 20% due to the elimination of error. This ratio is calculated by subtracting the disruption to production schedules caused by having to remake product that has been returned to the plant because of an original manufacturing error. Since Oran Pre-Cast has five pre-cast floor beds, this capacity improvement represents the equivalent of gaining one additional bed.

More information

The implementation of the new system means that the number of truck movements has now been reduced as a result of not having to collect faulty product and return it to Oran Pre-Cast. This has been estimated as a saving of two truck movements a week.

For more information on this project contact: Michelle Melville, Oran Pre-Cast, Deerpark Industrial Estate, Oranmore, Co Galway, telephone 091-794 537 or fax 091-794 586. Email: info@oranprecast.ie. Website: www.oranprecast.ie

Source:  Cleaner Greener Production Programme