Oxford County’s $1.5 million rehabilitation of a 9.1 km section of County Road 2 west of Woodstock marks the latest refinement of a technique that has been developed over the last three years. Tony Decoo, County construction manager explains, "Oxford County Road 2 was originally constructed in the late 1960’s and consisted of mesh reinforced concrete panels some 7 m wide and 200 mm thick, laid on between 50 and 100 mm of granular A base. About ten years later, the road was rehabilitated for the first time by surface grinding, the addition of 750 mm widening strips and a 75 mm lift of HMA overlay across the full width."

Apart from local repairs, no significant work was done on the road for a further ten years. By 1998, the surface asphalt was in poor overall condition and significant reflective cracking had occurred due relative movement between the underlying concrete panels, and joints had also heaved at a number of joint locations. The County issued Requests for Proposals (RFP’s) from the industry, seeking rehabilitation solutions. Ideally, these would prevent any future surface damage from movement of the concrete slabs, while reusing as much of the existing pavement material as possible. In response, a number of treatments were proposed including surface overlay, full depth rehabilitation by cold in place recycling (CIR), selective "piano key" joint repair as well as pulverizing. "We concluded," states Decoo, "that the concrete slabs had to be broken up to ensure that the reflective cracking cycle would not reoccur some time again in the future. However, concrete breaking with multiple hammers seemed impractical, so we took a closer look at rubbilizing the concrete with pavement breakers." (See sidebar on rubbilizing).

After further discussion, Decoo opted for the rubbilizing proposal submitted by Cayuga Materials & Construction. It involved leaving the concrete and converting into a coarse granular subbase by the rubbilization process. In addition, the RAP generated by the milling of the surface asphalt was utilised as granular base. During that first job in 1999, the RAP was temporarily graded into windrows on each side of the road, while the exposed concrete was rubbilized. The RAP was then graded back over the rubbilized surface and the job completed by the paving of two lifts of virgin hot mix asphalt on the compacted base materials.

Decoo was pleased with the results of the rubbilizing operation, but noted that the   rubbilized concrete/granular RAP base combination still required two lifts of virgin asphalt. Subsequent discussions focused on reducing the quantity of virgin material, and the resulting modified method was used successfully on a 9 km section of Road 2 last year. This time, instead of being windrowed, the milled RAP was taken to a nearby sand and gravel pit, where it was stockpiled, crushed and reused in a cold recycled base asphalt mix. The rubbilized concrete was then paved with a single 100 mm lift of recycle mix with both a conventional paver and a Midland paver that has an onboard pug mill. Some two weeks later, when the cold recycled mix had reached its target moisture and compaction targets, the surface was finished with a scratch coat of HL8 and a top lift of HL4.

On this year’s contract, Cayuga Materials & Construction Co. Ltd. subcontracted Miller Paving Ltd. to complete asphalt milling, recycling, mixing and asphalt paving, while International Resonant Breaking (IRB), Glenmont, NY, completed the concrete breaking operation. The general method is similar to previous efforts, with an added refinement – the 70 mm thick lower lift of cold recycled asphalt utilised McAsphalt’s CMS 2S emulsion which enabled the mixed product to be stockpiled for at least 24 hours.  This ensured that there was always inventory of cold mix recycled product available to cover any exposed rubbilized concrete, in the event of plant breakdowns or other unscheduled stops.

At 2.5 per cent, the emulsion content of the lower lift was slightly higher than the 2 per cent of CSS 1 emulsion utilised in the 50 mm upper lift. The mixes were otherwise similar, with both containing 80 per cent RAP, 20 per cent granular A aggregate and 3 per cent water.

During Aggregates & Roadbuilding’s visit, which took place on a Monday, the 70 mm thick lower lift was being completed with material that had been produced theprevious Friday, although there is typically only a one-day interval between the material’s production and application. Decoo adds, "Once the exposed concrete is covered with at least one lift of cold recycled asphalt, we have protected the base against water and there is also a suitable driving surface in place. Should it start raining heavily during the rubbilizing operation, we immediately stop the process and cover any exposed rubbilized sections with stockpiled recycle mix. This road carries up to 10,000 vehicles a day and, for both safety and stability reasons, the surface has to be black at day’s end. If for any reason the cold recycled mix is not available, the contractor must utilise HMA instead. The rubbilizing and paving operations are balanced as closely as possible so that the paver has rubbilized surface to work on, but not so much as to require big catch-up job in the event of rain."

Material quantities on this project totalled some 16,000 tonnes of RAP used in the cold recycled mix, 4,000 tonnes of virgin granular A, about 11 000 tonnes of surface hot mix asphalt and 42 000 kg of emulsion.  All of the existing pavement material has been utilised, either as rubbilized subbase or as recycled asphalt.

The process began with surface milling to a depth of 90 mm by Miller’s Wirtgen 2100 milling machine. The exposed concrete was then rubbilized by three of IRB’s resonance breakers working in formation, each rubbilizing a 178 mm wide strip with each pass. During rubbilization, the concrete is broken up, with no pieces larger than about 152 mm remaining. The resonance also breaks the bond between the concrete and reinforcement and pieces of reinforcement occasionally project from the surface.   Average production rates achieved by the three machines working in formation were about 1.3 km per day over the 7 m width in 10 operating hours. (The 750 mm widening strips either side of the concrete slab were added with a granular base, so were not   rubbilized.)

An Ingersoll-Rand SD-100 compactor worked in front of the paver to tighten up the surface for the paver and prevent any pick up of loose concrete. The compactor operator also watched for and removed any pieces of reinforcement projecting from the rubbilized surface.

The cold recycled asphalt was being paved with a Blaw Knox PF-5510 paver, fitted with the Blaw-Kontrol grade and slope system utilising an UltraEye V sonic grade sensor. The PF-5510 lends itself to this type of work, reflecting its European origins where asphalt is typically paved in thicker lifts.

Behind the paver, Miller’s breakdown compactor was an Ingersoll-Rand SD-110, operating in static mode and making three passes. This was followed by a Bomag BW 20R eight-wheeled pneumatic roller, which continued rolling until tire marks no longer appeared in the surface. During a normal rolling cycle, the SD-110 operator has sufficient time to travel back behind the Bomag and finish the surface in low amplitude vibration.

Crushing and asphalt mixing operations were completed in Cayuga’s Embro sand & gravel pit, located some 5 km east of the centre of the job. Here, crushing contractorDon Wiseman utilised a portable spread to crush to milled RAP to minus 25 mm as well as producing the virgin granular A.  Wiseman’s spread included a Cedarapids 22x48 Cedarapids primary jaw crusher and a Nordberg Omnicone 1560 secondary cone crusher. The Omnicone operated in a closed circuit with a 6x20 Simplicity double-deck screen.

During asphalt production, the crushed RAP and virgin aggregate were fed into the two calibrated bins of a Midland pug mill.  From there, the blended material was conveyed to the twin-shaft 9x4 pug mill, where emulsion and water from two nearby tankers was metered into the mix. The system produced cold recycled asphalt at an average rate of 350 tonnes/h. Haulage to the job was  completed by a mixed fleet of triaxle and live bottom trailers, including two new live bottom Raglan units that are recent additions to Miller’s fleet.

In terms of job schedule, surface milling began at the end of June, followed by a July 9 start for the concrete breaking and paving.  The target completion date for lower recycled base asphalt paving was July 18, followed by completion of the upper lift some four to five days later. After a curing period of about fourteen days, the cold recycled asphalt was tested to verify that it had reached the required 96 per cent density figure and that its moisture content is 2 per cent or less.  THE HMA overlay operation took ten days, and, with a further seven days on final shoulder grading work, the overall job was completed by the end of August.

In addition to his duties as construction manager for the County of Oxford, Tony Decoo is a familiar figure in Ontario roadbuilding circles in his role as President of the Ontario Good Roads Association (O.G.R.A.)