Richard Silcock looks at the arty side of a motorway project as the contractor works on some elaborate median barrier engraving in Wellington.
Together with upgrading and widening a part of Wellington’s urban motorway, a new concrete central median barrier is being installed featuring engraved Wellington landmarks replicated along part of its fascia.
Lead contractor for the project, Fletcher Construction has tasked Steam and Sand Porirua with the engraving work that will see three kilometres of the new, narrower median barrier given an artistic touch with semi-dimensional engravings of the iconic Beehive, St Gerard’s Monastery, the Brooklyn wind turbine and the War Memorial Carillon.
Steam and Sand’s general manager, Matt Trail, says the flat fascia surface of the new concrete barrier provides the medium for the work, and by using a compressed air driven Quill Falcon abrasive blasting machine exerting 105-115psi they are able to engrave just a thin layer of the concrete surface to produce the simplistic imagery.
“To achieve the outline of the icons and maintain consistency of the shapes we use a 3m x 1.2m x 3mm steel stencil of the icons as a guide,” says Matt.
“It conforms to the shape and size of the barrier fascia and is placed and clamped in place over the surface of the particular section of barrier we are working on.
“Inert fine garnet [sand-like crystals of about 0.5mm in diameter that are specifically sourced and imported from Western Australia], is used to blast away the surface concrete and engrave the design shapes to a maximum depth of up to 4mm. We use pure garnet as the blasting medium as it is free from salt, silica and iron particles and has lower dust levels than iron sand.
“With a skilled operator we are able to achieve a pretty good consistency for each engraving even though the concrete may have some slight imperfections due to the aggregate,” says Matt.
“However we are achieving differences of only 300 microns and are able to create quite a bit of detail.
“The engraved work is then sprayed with an epoxy resin so that it stands out against the grey concrete, and it also acts to protect the exposed concrete from the weather.”
Matt says all the ‘blasting’ work is being done progressively as the new barrier is extruded in place by a team working through the night between the hours of 11pm and 4am when there is low traffic volume.
“On this basis we are able to achieve four to five finished engravings per night,” he says. “The engravings are 50 metres apart and when complete will be near back-to-back on both sides of the barrier.”
The concrete barrier itself is being constructed by Seovic (part of the Fletcher Construction Group) using a special slipform concrete extruding machine.
“Using our Miller Formless M-8800 slipform paving machine that we imported from the States back in 2005, and travelling at around one metre every two to three minutes we are able to create, over the course of a night shift, about 55 metres of barrier in one continuous stream,” says Seovic’s general manager, Paul Houston.
“Each 55 metres requires 30 cubic metres of high strength, dry, ready-mix concrete with a rating of 30MPa. This is delivered on site from the nearby Firth Concrete plant at Kaiwharawhara by their revolving agitator trucks. Tipped into the top of the machine via a conveyor belt, the concrete mix is moulded and compacted by vibration before being extruded in shape onto the motorway surface in a continuous semi-solid form.”
The extruded barrier is then further manually finished and control joints applied with complete hardening and curing of the concrete taking a further four to six hours. As part of the supply contract, Firth tests each batch of concrete to ensure that it meets the NZ Transport Agency’s design specification.
Removal of the old, gravel-filled steel barrier is complex. NZTA’s project manager for the motorway upgrade Glen Prince says replacing the old barrier makes good sense, as the new barrier is much narrower and therefore allows for more usable road space.
“The old barrier was getting to the end of its use-by date as it no longer met today’s safety specifications and maintenance was difficult,” he says.
“This removal work of the old barrier involves peeling away the steel lid, scooping out the gravel, demolishing the steel façade, breaking up and removing a three-metre wide concrete plinth and rebuilding the pavement.
“Exposing the ground beneath the several decades old barrier did present a few surprises however,” says Glen. “Several manholes and stormwater drains were revealed and these needed to be investigated along with the integrity of the drainage pipes. For this reason this work is being carried out in 300 to 400 metre sections.”
At the time of Contractor going to print, over 1.5 kilometres of the old barrier had been removed and over 600 metres of the new barrier extruded in place.
While there has been some controversy over the creation of what some critics say are distracting features and unnecessary expense, the Transport Agency and the Wellington City Council (whose design teams worked together) and the Police say they have no reason to believe the engravings will distract drivers, as the repetitive nature of them will become integrated into the visual surroundings, rather than being singled out as a point of interest. The Transport Agency says the monochromatic dimensional design was assessed by its safety experts who rated it well within their safety and hazard tolerance criteria.
“This, along with a temporary slow speed restriction, will allow motorists the opportunity to become accustomed to the new median layout and the design features,” says Glen.
The barrier work is being carried out as part of the Transport Agency’s $50 million motorway upgrade which also includes the provision of a fourth northbound lane that utilises the western side shoulder space and a hereto unused sub-bridge off-ramp for part of the over-rail bridge section (refer Contractor, July 2015). The engraving work will cost $130,000 and is expected to add interest to the motorway scene for the 95,000 motorists who use the motorway each day.
The motorway upgrade is expected to be completed in March 2016.