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Canada's “Rock to Road”
Magazine
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Superpave debuts on Ontario 400 Series highway By Andy Bateman, Engineering Editor MTO Contract 20000077 marks the first time that hot mix asphalt designed to full Superpave specifications has been utilised on an Ontario 400 series highway. Aggregates and Roadbuilding visited the project in early October for this report. The $13.3 million contract extends the four lane divided platform of Highway 404 some 2.9 km further north from its current limit at Davis Dr. near Newmarket to Herald Rd. The Central Division of the recently merged Lafarge Canada Inc. and Warren Paving & Materials Group Ltd. has been subcontracted by general contractor Carillion Construction Canada Inc. to complete asphalt paving on this contract. Both Superpave and Marshall mix design asphalts are represented on this stretch while Superpave mixes are being utilized on the southbound lanes. Paving is already underway on the northbound lanes utilising conventional mixes.
Roadtec SB-2500 Shuttle Buggy was key to preventing segregation of the coarse 25 mm Superpave mix on the Highway 404 paving project. The two 3.75 m wide southbound lanes, together with acceleration and deceleration lanes for Herald Rd. and Davis Dr., will receive three lifts of asphalt totalling 180 mm, including two Superpave base lifts of 90 mm and 50 mm followed by a surface lift of 40 mm. Superpave asphalt quantities total some 10 000 tonnes, with 6500 tonnes of 25 mm SP and 3500 tonnes of 19 mm SP mix. At the time of Aggregates & Roadbuilding’s visit, the mix design for the 40 mm surface lift was being finalised and is expected to be either Dense Friction Course (DFC) or Stone Mastic Asphalt (SMA). By comparison, the northbound lanes will have the same overall asphalt thickness of 180 mm, but in this case the two lower lifts of HL8 and lower Heavy Duty Binder Course (HDBC) will each be 50 mm thick and utilise 5828 Performance Grade Asphalt Cement (PGAC). The two upper lifts will be upper HDBC and DFC mixes, each 40 mm thick, and utilising 6428 PGAC. Granular base materials are similar on both northbound and southbound lanes with 625 mm of Granular B Type 1 and 150 mm of Granular A.
Conventional triaxle and live-bottom trailers hauled the first Superpave asphalt laid on a 400 series highway job in Ontario. Mix Design Paul Lum, director of manufacturing for Lafarge/Warren’s Asphalt & Paving Performance Group, explains that the two Superpave mixes, known simply as 25 mm SP and 19 mm SP (for Superpave) were designed to full Superpave specifications by Ashwarren Engineering Services utilising a Troxler Superpave gyratory compactor. The mix number designation refers to the Nominal Maximum Size (NMS) of the aggregate in the mix, where NMS is defined as one size larger that the first sieve to retain more than 10 percent of the sample. The mixes were designed under the maximum density line (MDL), resulting in relatively coarse mixes with a higher stone ratio. Where an aggregate gradation closely matches the MDL, this indicates that the stone skeleton is packed into a tight volume, with very little volume remaining to allow for asphalt cement and air voids. A deviation away from the MDL provides more volume in the mix for asphalt cement and air voids. The next step in thedesign process is to determine the lift thickness, typically calculated as a minimum 3 times multiple of the NMS of the aggregate in each mix. In this application, the compacted lift thickness of the lower base mix was adjusted upwards to 90 mm to ensure that a minimum compaction density of 91.5 per cent could be achieved. These numbers were agreed between the contractor and the MTO for this initial contract, as End Result Specifications (ERS) for Superpave mixes in Ontario are still in the development stage. The 19 mm SP was similarly designed as a 50 mm thick lift. A comparison with conventional mixes underlines the relative coarseness of these Superpave mixes; the 25 mm SP mix has 65 per cent stone retained and the 19 mm SP mix 55 per cent, compared to 50 per cent for HL8 and 48 per cent for HDBC.
Custom-built hopper mounted over an asphalt sampler receives HMA sample for testing. The Performance Grade Asphalt Cement utilised in the 19 mm SP upper base lift was bumped, or adjusted upwards, by one sixdegree Celsius increment from 5828 to 64 28, to allow for heavy truck traffic. The design loading of the pavement is 6 million 80 kN Equivalent Standard Axle Loadings (ESALs), as this section of the 404 is near several aggregate operations at Stouffville and Herald Rd. is expected to be a popular access point for loaded trucks heading south to Toronto. The grading of the 90 mm thick 25 mm SP lower lift remained at the 5828 grading that is applied to applications in Ontario’s Zone 3 climatic region. Production and Paving In terms of production, laydown and compaction, the Lafarge/Warren team has found that the treatment and handling of Superpave mixes are generally similar to other mixes used in highway paving applications. The 25 mm SP mix has been produced successfully by both batch and drum type plants at Lafarge’s Maple asphalt plant facility. Here, the batch plant is an Astec 3.6 tonne plant and the drum mix plant is a 400 tph Standard Haven counterflow drum mix plant with four 200tonne capacity product silos. Doug McDonald, General Manager of Asphalt Production in Toronto, reports that plant changes included modification of the screen sizes on the cold feed scalping plants and the screens on top of batch plant to accommodate the 40 mm stone. Separate stockpiles were created on site to handle the different sizes of coarse and fine aggregates, to minimize segregation and achieve consistent mix production. "Basically, the asphalt plants were capable of handling the 25 mm SP mixtures with very little changes to our normal operating procedures," said McDonald.
Roadtec Shuttle Buggy feeding a Caterpillar AP-1055B track paver on 404 extension. Conventional triaxle trucks and live bottom trailers hauled the mix to the job site, with typical travel times of 45 minutes. On arrival, the trucks discharged into a Roadtec SB 2500 Shuttle Buggy, where the asphalt was remixed and discharged into the hopper insert of a Caterpillar AP 1055B tracked paver. The hopper helped to maintain a continuous flow of material to the paver’s 1020B ExtendaMat screed, which laid a 115 mm thick uncompacted mat to provide a compacted thickness of 90 mm. The mix temperature behind the screed was between 135 and 140 degrees Celsius or some 10 degrees cooler than the production temperature. Keeping close behind the paver, an IngersollRand DD110HF doubledrum compactor made two passes to complete breakdown compaction, operating in both high frequency (3350 vpm) and high amplitude (0.55 mm) mode. A Caterpillar PS300B pneumatic roller followed, making six passes, while surface finishing was completed by a Bomag AW 164AD 12 tonne doubledrum unit operating in static mode and making between four and six passes.
Ingersoll-Rand DD-110HF compactor makes two passes in high frequency and amplitude mode to complete breakdown rolling sequence. In Lum’s view, the SB 2500 is a necessary part of the paving train for these coarse mixes, as it remixes the top size aggregate, minimizes segregation and produces a more consistent mix temperature. Trials completed before the start of the job confirmed that segregation would occur if the mix were discharged directly from the truck into the paver hopper. On the paver itself, the centreline kickback augers were installed as a further precaution to minimize any tendency to midlane streaking. Regarding compaction, the rolling patterns of the primary and secondary compactors were adjusted to avoid the pushing or roll down of the edge of unconfined mat. The DD110HF compactor kept between 150 mm and 200 mm away from the edge during its first pass and rolled to the edge on its second pass. The PS300B was kept a similar distance from the edge throughout its cycle, due to its 23 tonne maximum operating weight and the kneading action of its seven rubber tires. No adjustments were made to the pattern of the lighter AW 164AD finishing roller which travelled to the edge of the mat for all of its passes. In another first, this contract was the first live demonstration of a new material sampler. This consisted of a custombuilt hopper mounted over an asphalt sampler, with the assembled unit set up in the back of a pick–up truck. Concerns had been raised at the planning stage that the traditional plate method would not yield representative samples on the thick lifts, and this innovative solution was developed by the Brian Samulewitsch, Lafarge Warren’s quality control manager. During the sampling process, the truck pulled up alongside the SB 2500 and the Shuttle Buggy’s discharge conveyor was swung over the hopper. A sample of some 60 kg of material was discharged directly into the hopper and subdivided into four samples by a Gilson Quartermaster asphalt sampler. A slide arrangement facilitated removal of the samples from under the hopper that were then boxed, labelled and sent to the respective testing laboratories.
Caterpillar PS-300B pneumatic roller in the secondary position stayed 150 mm to 200 mm from the edge of the unconfined mat to avoid pushing. Quality Control on this contract is being done by Lafarge/Warren’s Toronto laboratory while Peto MacCallum Ltd. is responsible for Quality Assurance on behalf of the MTO. Golder Associates is the referee laboratory. Prior to the start of the contract, correlation testing was completed between the Quality Control and Quality Assurance laboratories to verify that similar values were being obtained for air void content on control mixes. The laboratories delivered close results and the correlation is being monitored throughout the contract. In addition to Paul Lum, key Lafarge/Warren’s Central Division personnel on the project include Mike Greco, general manager, Construction, Rick Logozzo, project manager, and Dan Gilles, superintendent.
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