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A cast fit for Holyrood
10:00 01 Jan 2002
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Defenders of the Scottish Par-
liament building's design argue that it will leave behind a rich architectural legacy for the nation. It will also leave behind a rich legacy of another kind - for Laing O'Rourke's precast business Malling Products.
The firm became involved in the Scottish Parliament building through parent company Laing O'Rourke, which won the contract to build the concrete frame. Malling was working to incorporate as many precast elements as possible into the frame when the contractor was asked to consider precasting the Canongate Wall.
Many stories and rumours abound about the design of the Canongate Wall and the bamboo motifs on the Member of Scottish Parliament (MSP) offices' boundary wall. The late architect Enric Miralles, who schemed the Scottish Parliament building, hinted at his intentions in rough sketches. This was essentially
all that Malling was left to
work with.
Director David Shillito says: "We were given one drawing for the Canongate Wall from which to evolve the precast panels and the finishes." That drawing was a combination of a rough sketch, a montage of images that had been cut out of magazines or from photos taken of the surrounding landscape - and some poetry!
"This was not the usual clarity and precision we have come to expect from architect drawings," says Shillito. "It frightened the living daylights out of us when we first saw it and were asked to comment on the precasting
possibilities."
Shillito invited Gary Lucas of specialist precast moulding manufacturer Patterns and Moulds to join him at the next meeting in the hope that they could find a solution to the problem. "We did not want to say 'yes' straight away and find out later that it was impossible to precast," Shillito adds.
The challenge was not how to cast or finish the concrete, but how to make bespoke moulds that were curved along an asymmetric axis in both planes, indented with large and small pockets in a random order to receive pieces of natural stone.
Normally, this process would have taken an eternity. But the technology that was deployed by Malling took only a fifth of the time that conventional mould-making using teams of carpenters would have done. The cost of the finished concrete was also much cheaper, at around £4,600/m2. "It would have cost at least £10,000/m2 using traditional methods and taken at least three weeks to make each master mould," Shillito says.
The machine used by Malling is known as a router. CNC (computer numeric controlled) routers have been used in the aerospace and car manufacturing industry for many years. They are programmed to cut and accurately form three-dimensional shapes.
Specialist equipment
Shillito originally wanted to see such a machine in action at the Jordan Formula 1 factory, but was disappointed not to receive an invitation to visit. As it turned out, Patterns and Moulds had hired a specialist company with a CNC machine to cut the intricate bamboo motifs on MDF board for the MSP wall panels.
"When the sketch for the Canongate Wall was developed by architect RMJM into a CAD model, we knew that the master mould could be made using a CNC machine driven by specialist software," Shillito explains. "So Patterns and Moulds bought a router and we were able to agree a price for casting the wall."
The architect sent an accurate 3D model of each panel in CAD to Patterns and Moulds. The CAD image was then transposed to a CAD/CAM system that changes the image to an MC9 file, a format suitable for driving the router. The information from the MC9 file tells the router what cutter diameter to use, what depth and curvature to cut to, and what line to follow. It takes a technician about four days to programme the router.
The precast panels were 3m by 4.5m in size, but the router could only tool panels up to 3m by 1.5m, so the 3D image had to be split into thirds.
The biggest drawback of the system was the quality and accuracy of the CAD drawings that were sent. The information input for a CNC machine works to an accuracy of five decimal places, so if any adjoining lines on the CAD image are out by more than 0.5mm the machine will not function and reports an error. The software produces the tool path to drive the cutter and sends positioning signals similar to that of a stereoscopic image of contour lines on an Ordnance Survey map. The cutter has directional information along the X, Y and Z axes, plus the B and C rotational axes. The B axis is rotary on plan and C is rotational on elevation.
"In this way, we can programme the machine to cut a perfect sphere if required," Shillito says.
The system Malling devised for creating the moulds and casts at Holyrood typically ran as follows. A 50mm-deep MDF panel is laid out below the cutter head. The router cuts to a depth of 25mm into the MDF panel to form the relief patterns. The machine cuts each 3m by 1.5m section in 14 hours and it can take up to two days to complete the whole panel.
The router head rotates at 25,000rpm and the cutting tool is changed depending on the process stage. The first stage is the rough cut using 16mm to 25mm cutter diameters, when a large amount of material is removed. When the router gets down to the finishing stages, the machine makes hundreds of passes over the panel to bring the cut surface to a smooth finish, using ball-end cutter heads.
Then the MDF panel is waterproofed and varnished, and set into the casting frame. The latex rubber - a thermosetting two-part polymer resin - is poured over the MDF. It has the consistency of treacle and fills all the depressions to finish level across the frame.
Once the latex hardened it was lifted out and sent to Malling's factory in Grays, Essex. Here, the latex rubber mould was placed in the precast mould and the release agent was applied. The smaller stones that form the recesses and windows in the panels were placed in position. The larger stones were positioned on site in Edinburgh. "Every piece of stone had to be carefully cut to size and shape and placed in the correct recess in the panel before concreting," Shillito says.
The Scottish Parliament concrete - a pale grey mix using Ordinary Portland cement, crushed Derbyshire limestone fines and coarse aggregates - was then poured into the mould and left to harden.
The 250mm-thick precast panel, which was reinforced with mesh, was left to cure in the mould for 24 hours. When the panel was removed, some surfaces had to be sand-blasted and others left with a lightly polished finish. "We had to mask all the fair-face non-blasted surfaces with plastic insulating tape and fill up the stone rebates with weak mortar so that during blasting it would not get eroded, but was easy to remove afterwards. That was the most difficult part of the precasting work at Grays," says Shillito.
Surface quality
An important issue on the Scottish Parliament Building was the surface quality and preciseness of panel installation. Many of the panels had patterns on the surface that continued on to adjacent units. The position tolerances that were allowed were +/- 10mm for fixing tolerance, and +/- 6mm for manufacturing tolerances (a collective tolerance of +/- 12mm).
"We could have had ugly edges of discontinuity as wide as 15mm to 20mm, which was quite permissible. With the CNC routed moulds we achieved a manufacturing accuracy to within 1mm of exactness," says Shillito. Although the panels on the Canongate Wall were heavy (some were 18t), a positional tolerance of only +/- 2mm was achieved.
Efficiency gains
Investment in a CNC machine is not to be undertaken lightly. The efficiency gains and the reduction in labour cost must be offset against the investment of £140,000 for the machine and £25,000 for the software. The capital cost has to be recovered within a five-year period because by then the machine will be obsolete.
But Malling is more than happy with its investment. "We can say with confidence that the investment was well worth the savings it made on this project alone, never mind the next five years," says Shillito.
Content with the success of the CNC venture, Shillito is now looking to bring further automation into its precast production operations.
"We are looking at a special bar bending machine that can fabricate flat and bespoke mesh panels using a combination of bar sizes from 6mm to 25mm, in any arrangement, including cut-outs," he says. "The machine would make the panel on a 'just-in-time' basis and place it in the mould as the mould is moved around the factory on a conveyor."
The bending machine that Malling wishes to purchase will be programmed to make cages for circular and rectangular columns and bespoke three-dimensional precast cages. At present, UK precasters cannot compete with Europe on sandwich panel construction, faade panels, or composite precast slabs because they do not have automated production processes, fast throughput and low labour costs. In Scandinavian countries they have even educated their architects and structural engineers to use standard precast products and design with standard joint details.
"Unlike mainland Europe we have a market that is design-led, where every building is unique and every precast panel is bespoke," says Shillito. "That's why our market remains a small specialist market, which is
very labour intensive and very inefficient. If we wish to compete with the best in Europe we
must invest in automation and educate our designers about standardisation."
That said, if Malling continues with its present development plans, it will soon be able to
offer bespoke precast panels like those on the Canongate Wall at a price that others in Europe will struggle to match. o
liament building's design argue that it will leave behind a rich architectural legacy for the nation. It will also leave behind a rich legacy of another kind - for Laing O'Rourke's precast business Malling Products.
The firm became involved in the Scottish Parliament building through parent company Laing O'Rourke, which won the contract to build the concrete frame. Malling was working to incorporate as many precast elements as possible into the frame when the contractor was asked to consider precasting the Canongate Wall.
Many stories and rumours abound about the design of the Canongate Wall and the bamboo motifs on the Member of Scottish Parliament (MSP) offices' boundary wall. The late architect Enric Miralles, who schemed the Scottish Parliament building, hinted at his intentions in rough sketches. This was essentially
all that Malling was left to
work with.
Director David Shillito says: "We were given one drawing for the Canongate Wall from which to evolve the precast panels and the finishes." That drawing was a combination of a rough sketch, a montage of images that had been cut out of magazines or from photos taken of the surrounding landscape - and some poetry!
"This was not the usual clarity and precision we have come to expect from architect drawings," says Shillito. "It frightened the living daylights out of us when we first saw it and were asked to comment on the precasting
possibilities."
Shillito invited Gary Lucas of specialist precast moulding manufacturer Patterns and Moulds to join him at the next meeting in the hope that they could find a solution to the problem. "We did not want to say 'yes' straight away and find out later that it was impossible to precast," Shillito adds.
The challenge was not how to cast or finish the concrete, but how to make bespoke moulds that were curved along an asymmetric axis in both planes, indented with large and small pockets in a random order to receive pieces of natural stone.
Normally, this process would have taken an eternity. But the technology that was deployed by Malling took only a fifth of the time that conventional mould-making using teams of carpenters would have done. The cost of the finished concrete was also much cheaper, at around £4,600/m2. "It would have cost at least £10,000/m2 using traditional methods and taken at least three weeks to make each master mould," Shillito says.
The machine used by Malling is known as a router. CNC (computer numeric controlled) routers have been used in the aerospace and car manufacturing industry for many years. They are programmed to cut and accurately form three-dimensional shapes.
Specialist equipment
Shillito originally wanted to see such a machine in action at the Jordan Formula 1 factory, but was disappointed not to receive an invitation to visit. As it turned out, Patterns and Moulds had hired a specialist company with a CNC machine to cut the intricate bamboo motifs on MDF board for the MSP wall panels.
"When the sketch for the Canongate Wall was developed by architect RMJM into a CAD model, we knew that the master mould could be made using a CNC machine driven by specialist software," Shillito explains. "So Patterns and Moulds bought a router and we were able to agree a price for casting the wall."
The architect sent an accurate 3D model of each panel in CAD to Patterns and Moulds. The CAD image was then transposed to a CAD/CAM system that changes the image to an MC9 file, a format suitable for driving the router. The information from the MC9 file tells the router what cutter diameter to use, what depth and curvature to cut to, and what line to follow. It takes a technician about four days to programme the router.
The precast panels were 3m by 4.5m in size, but the router could only tool panels up to 3m by 1.5m, so the 3D image had to be split into thirds.
The biggest drawback of the system was the quality and accuracy of the CAD drawings that were sent. The information input for a CNC machine works to an accuracy of five decimal places, so if any adjoining lines on the CAD image are out by more than 0.5mm the machine will not function and reports an error. The software produces the tool path to drive the cutter and sends positioning signals similar to that of a stereoscopic image of contour lines on an Ordnance Survey map. The cutter has directional information along the X, Y and Z axes, plus the B and C rotational axes. The B axis is rotary on plan and C is rotational on elevation.
"In this way, we can programme the machine to cut a perfect sphere if required," Shillito says.
The system Malling devised for creating the moulds and casts at Holyrood typically ran as follows. A 50mm-deep MDF panel is laid out below the cutter head. The router cuts to a depth of 25mm into the MDF panel to form the relief patterns. The machine cuts each 3m by 1.5m section in 14 hours and it can take up to two days to complete the whole panel.
The router head rotates at 25,000rpm and the cutting tool is changed depending on the process stage. The first stage is the rough cut using 16mm to 25mm cutter diameters, when a large amount of material is removed. When the router gets down to the finishing stages, the machine makes hundreds of passes over the panel to bring the cut surface to a smooth finish, using ball-end cutter heads.
Then the MDF panel is waterproofed and varnished, and set into the casting frame. The latex rubber - a thermosetting two-part polymer resin - is poured over the MDF. It has the consistency of treacle and fills all the depressions to finish level across the frame.
Once the latex hardened it was lifted out and sent to Malling's factory in Grays, Essex. Here, the latex rubber mould was placed in the precast mould and the release agent was applied. The smaller stones that form the recesses and windows in the panels were placed in position. The larger stones were positioned on site in Edinburgh. "Every piece of stone had to be carefully cut to size and shape and placed in the correct recess in the panel before concreting," Shillito says.
The Scottish Parliament concrete - a pale grey mix using Ordinary Portland cement, crushed Derbyshire limestone fines and coarse aggregates - was then poured into the mould and left to harden.
The 250mm-thick precast panel, which was reinforced with mesh, was left to cure in the mould for 24 hours. When the panel was removed, some surfaces had to be sand-blasted and others left with a lightly polished finish. "We had to mask all the fair-face non-blasted surfaces with plastic insulating tape and fill up the stone rebates with weak mortar so that during blasting it would not get eroded, but was easy to remove afterwards. That was the most difficult part of the precasting work at Grays," says Shillito.
Surface quality
An important issue on the Scottish Parliament Building was the surface quality and preciseness of panel installation. Many of the panels had patterns on the surface that continued on to adjacent units. The position tolerances that were allowed were +/- 10mm for fixing tolerance, and +/- 6mm for manufacturing tolerances (a collective tolerance of +/- 12mm).
"We could have had ugly edges of discontinuity as wide as 15mm to 20mm, which was quite permissible. With the CNC routed moulds we achieved a manufacturing accuracy to within 1mm of exactness," says Shillito. Although the panels on the Canongate Wall were heavy (some were 18t), a positional tolerance of only +/- 2mm was achieved.
Efficiency gains
Investment in a CNC machine is not to be undertaken lightly. The efficiency gains and the reduction in labour cost must be offset against the investment of £140,000 for the machine and £25,000 for the software. The capital cost has to be recovered within a five-year period because by then the machine will be obsolete.
But Malling is more than happy with its investment. "We can say with confidence that the investment was well worth the savings it made on this project alone, never mind the next five years," says Shillito.
Content with the success of the CNC venture, Shillito is now looking to bring further automation into its precast production operations.
"We are looking at a special bar bending machine that can fabricate flat and bespoke mesh panels using a combination of bar sizes from 6mm to 25mm, in any arrangement, including cut-outs," he says. "The machine would make the panel on a 'just-in-time' basis and place it in the mould as the mould is moved around the factory on a conveyor."
The bending machine that Malling wishes to purchase will be programmed to make cages for circular and rectangular columns and bespoke three-dimensional precast cages. At present, UK precasters cannot compete with Europe on sandwich panel construction, faade panels, or composite precast slabs because they do not have automated production processes, fast throughput and low labour costs. In Scandinavian countries they have even educated their architects and structural engineers to use standard precast products and design with standard joint details.
"Unlike mainland Europe we have a market that is design-led, where every building is unique and every precast panel is bespoke," says Shillito. "That's why our market remains a small specialist market, which is
very labour intensive and very inefficient. If we wish to compete with the best in Europe we
must invest in automation and educate our designers about standardisation."
That said, if Malling continues with its present development plans, it will soon be able to
offer bespoke precast panels like those on the Canongate Wall at a price that others in Europe will struggle to match. o