Alberta road rehabilitation combines technologies
Road rehabilitation combines technologies
By Andy Bateman
Today’s road engineers have an impressive array of methods at their
disposal to rehabilitate deteriorated roads and even rescue failed
|The Roadtec RX 120 is a one-machine cold in place reclaiming and mixing unit.|
Today’s road engineers have an impressive array of methods at their disposal to rehabilitate deteriorated roads and even rescue failed pavements. The challenge is selecting the best rehabilitation solution, given each road’s unique characteristics such as original construction, repair history, current condition and traffic loading. Aggregates & Roadbuilding visited an unusual rehabilitation project in Alberta which combined full depth reclamation, Reclaimed Asphalt Pavement (RAP) paving and seal coating.
At the time of the visit, a 12.9 km stretch of Secondary Road 766 was being rehabilitated for owner Alberta Transportation as part of a wider $6 million rehabilitation plan. This two lane rural road runs generally parallel to and west of Highway 2, Alberta’s arterial route between Calgary and Edmonton. The rehabilitation section was located near the town of Olds and included 9.7 km and 3.2 km sections north and south of Highway 27 respectively. Both the paved surface and road structure were in poor condition, with extensive alligator, reflective cracking and rutting. For rehabilitation contractor West-Can Seal Coating Inc., an additional headache was the variable pavement depth, the net result of inconsistent original construction and subsequent excavate & fill, grader laid cold patch or multiple chip seal repairs.
|A Reimer portable cement mixer was teamed with a modified furnace tank to add 1% Portland Cement to the base mixture.|
Dave Rose is sales manager for West-Can and explains the three phases of the rehabilitation process: “Phase One involved full depth reclamation and the placement of a 150 mm base lift. The base lift consisted of crushed and screened reclaim material, to which was added controlled percentages of foamed asphalt cement and Portland cement. Phase Two involved the placing of an 80 mm lift of imported milled RAP, followed by its immediate recovery and mixing with 20 mm of the Phase One base lift. The resulting blended mixture was then placed in a 100 mm lift by a second paver and compacted. In Phase Three, double chip seal was applied to improve skid resistance and provide a water seal to the rehabilitated road.” Rose adds that the project was another example of the company’s RAP donor and recipient concept, with surplus milled RAP from Highway 2, the donor road, utilised to supply secondary road 766 as the recipient road.
On site, the Phase One process kicked off with the application of Portland cement to the existing road surface by a Reimer portable cement mixer. Some distance behind the cement application, the recycling train was headed by a B-train double tanker carrying 40 tonnes of asphalt cement and pushed by a Roadtec RX 120 recycling machine. The average cut made by the RX 120 itself was a relatively deep 150 mm, with the operator making frequent adjustments to recover as much of the existing asphalt and base material as possible without picking up any subgrade. Milled material in the recycler’s cutter drum was lifted by dual bucket elevators and discharged onto a Deister 5 by 10 double deck screen fitted with 51 mm screen cloths on the top deck and 37.5 mm on the second deck. From there, material retained by both decks was conveyed to the front of the RX 120 where it was crushed by a hammer mill, deposited back onto the pavement and recovered by the cutter drum. Meanwhile, minus 37.5 mm material passing the screen’s bottom deck was carried on a transfer conveyor with integral weigh scale to a dual shaft pug mill. Just ahead of the pug mill, nozzles introduced mists of hot asphalt cement and cold water, triggering the familiar foaming reaction. The resulting asphalt foam and feed material were blended into a homogenous mixture by the pug mill before being deposited in a windrow behind the RX 120. Some way back, usually about 150m, a Barber Greene BG650 windrow elevator discharged the mixture into the hopper of a Caterpillar AP1055B paver equipped with a four sensor Topcon sonic averaging ski. To allow for bulking, the paver laid the base lift up to 40 per cent thicker than its stipulated 150mm compacted thickness. Behind the paver, a Caterpillar 634C dual steel drum roller in the breakdown position made four to six passes in vibratory mode while a Ferguson SP1130 pneumatic roller in the secondary position made ten to fourteen passes. Once these passes had been completed, the 634C looped back and made a single pass in static mode to finish the mat. Support equipment included a 14 000 litre water tanker and a Caterpillar IT28G fitted with a loader bucket. Rose reported average daily production could reach 1.6 to 2.0 lane kilometres per day for the recycling train operating with a crew of eighteen to twenty, including traffic control.
|A Barber Greene BG650 windrow elevator discharged the mixture into the hopper of a Caterpillar AP1055B paver.|
For Phase Two, the RAP milled from Highway 2 was first moved to a stockpile pit where it was passed through a trommel screen to remove any material larger than 25 mm. The screened RAP was then hauled to the secondary road 766 jobsite, where trucks discharged directly into the hopper of the Caterpillar AP1055B paver laying the 80 mm thick RAP lift. Directly behind the paver, the RX 120 reclaimed all of this RAP lift together with 20 mm of the Phase One base lift and deposited a windrow of the resulting blend. This time, the BG650 windrow elevator was teamed with a second asphalt paver, a Caterpillar AP1055D, to recover the windrow and pave a 100 mm lift of the blended material. The compaction train behind the second paver was the same Caterpillar 634C dual steel drum roller and Ferguson SP1130 pneumatic roller duo used in Phase One.
|The paver laid the base lift up to 40 per cent thicker than its stipulated 150 mm compacted thickness to allow for bulking.|
Supervisor Craig Fischer details some of the site specific job requirements and how they were met. “The Phase One mix design called for just one per cent Portland cement to be incorporated into the base lift mixture. To do this, we cut an old furnace tank in half and towed it behind the portable cement mixer. In operation, cement blown into the tank fell through a V-shaped slot cut into the tank’s base and the resulting miniature cement windrow was picked up by the recycler’s cutter drum along with the rest of the reclaimed road material. Phase One also had three mix designs to recognise variations in both the depth and the asphalt content of the existing pavement. The reclaimer’s blending computer was a useful tool to set and maintain asphalt cement content, while the use of a windrow allowed local adjustments to be made to the volume of material being fed to the paver. If, for example, there was excess material in the windrow, the wheel loader could quickly remove some of the surplus and transfer it to a local stockpile. Other locations required extra material for profile correction and here the loader could add windrow material as needed. Separate reclaiming and paving also meant neither process was impacted by any interruption or delays in the other, unlike set ups where the reclaimer discharges directly into the paver hopper. In this regard, the windrow has a similar purpose to a surge pile in an aggregate operation, allowing a downstream plant to operate independently from a supplying upstream plant, at least for a while.”
Rose is of the view that the rehabilitation solution applied here offers several advantages over traditional pulverising: “The process and equipment used on this project provides improved control of the cut depth, improved sizing of reclaimed material and the ability to deal with variable amounts of material. In addition, the weigh scale on the recycler allows the asphalt cement addition rate to be based on the actual weight of recycle material entering the pug mill rather than the estimated or calculated weight of other methods. The travelling public will also appreciate the fact that the rehabilitated surface can be reopened immediately for normal use.”
Manufacturer’s literature describes the Roadtec RX 120 as a one-piece full-specification cold in- place recycling machine. Larger than its stable mate the RX 110, the RX 120 features a 3.5 m wide cutter with optional cutter extensions up to 4.2 m, dual bucket elevators, a screen deck, a crusher, a conveyor with a weigh bridge, a blending computer, a twin shaft pugmill and a liquid pump to create a one-machine cold in place reclaiming and mixing unit.
West-Can Seal Coating Inc is a sister company of Ontario based Seeley & Arnill Construction Limited. n