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Canada's “Rock to Road”
Magazine
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New slurry seal system promises up to a decade of protection A modified slurry surface treatment developed by MSO Construction Ltd. is appearing on Ontario’s roads for the first time, promising thin layer durability and protection of the underlying asphalt for between seven and ten years. Aggregates & Roadbuilding recently visited a 5.3 km long by 7.3 m wide section of Wellington County Road 42, some 20 km west of Brampton, where the new treatment was being applied.
Bergkamp mobile mixer paver applying slurry surface treatment near Brampton, Ont. After spreading and any hand finishing, traffic can be redirected back onto the surface about one hour after treatment. The existing surface on this two-lane rural road, located between Trafalgar Rd. and Winston Churchill Blvd., was a high float treatment some 8 years old, laid over an emulsified asphalt stabilized base. Local ravelling or breaking up of the surface had caused some loss of aggregate while longitudinal cracking had also occurred in other areas. According to MSO Construction, its Metro-Mat™ slurry system will seal the surface in these applications and protect the existing pavement against further damage from weather and traffic. The system, developed using experience gained in the U.S., is designed to exceed the provincial slurry seal specification (OPSS 337) by combining polymer modified asphalt emulsion and trap rock aggregate. On site, supply trucks ferry raw materials from a materials yard to a mobile continuous mixer paver unit, which mixes and spreads the seal in a single pass. The mix’s emulsion breaks (sets) soon after placing and traffic can normally be redirected back onto the surface about one hour after treatment. MSO’s first step on this $80,000 job was to set up a materials yard at the west end of the job where an inventory of dry aggregates was stockpiled, together with a 27 000-litre tanker of emulsion asphalt. There, the dry aggregates were rescreened on a Powerscreen MkII portable unit fitted with a 4x8 Dillon slotted screen and reloaded onto MSO’s supply trucks, in addition to emulsion and water. The water supply on the project was maintained by tanker, as city water was not available. On reaching the mixer paver unit, the aggregates were discharged into the paver’s front receiving hopper, while emulsion and water were pumped into the paver through quick disconnect hoses. On board the Bergkamp mobile mixer paver, a 163-tonnes/h conveyor refilled its 2.7 m 3 capacity main aggregate hopper, while the 3028 litre emulsion and 2544 litre water tanks were refilled. Mineral filler was stored in a 0.31 m 3 capacity hopper and added to the pugmill by a double pitch reversible auger. All the raw materials were combined and mixed in the paver’s multi-paddle, twin-shaft pugmill. The delivery of aggregate and emulsion into the pugmill was maintained at a constant ratio through a mechanical drive system. Although not used on this job, the paver also incorporates a system for the addition of liquid additives such as accelerator or set retarder. When used, these are stored in a 378-litre stainless steel tank and premixed at a 10:1 ratio before entering the unit’s pugmill. After mixing, the slurry discharged into the rear mounted spreader box where it was evenly distributed across the lane width by twin augers. The unit has twin driver stations, while the operator controls are located at the fixed console at the rear of the unit. Here, the operator has one button control of the mixing operation and spreader box, as well as machine forward/reverse and ground speed. Existing grade, slope and curb reveals are maintained by the unit’s taper control while a 325 hp Cummins in-line six-cylinder diesel engine provides all the necessary mixing and motive power. Underneath the paver, water is sprayed onto the road surface ahead of the spreader box. This prewetting is designed to reduce surface tension and promotes coating of the pavement surface by the slurry. In addition, water sprays directed onto the paver’s wheels reduce slurry pick up during reversing or turning. Overall, the paver was delivering an average production rate of 5 lane km per day, with a site crew of 12, including the materials yard loading crew and traffic control. MSO’s Dan Murphy emphasises that attention to detail in the production and spreading process is essential for good results. On the raw materials side, the sizing of the aggregates is critical to ensure a uniform mix, and their rescreening at the materials yard is insurance against any oversize or foreign material finding its way into the mixture. The correct emulsion temperature is equally important, and experience confirms that the emulsion should have cooled to about 37.7ÁC before mixing. During the mix’s development stage, problems with premature mix setting were traced to the mixing of emulsion that was still too hot at 79.4ÁC. The preparation of a clean and dry road surface ahead of the paver is also key to a successful and durable surface. Any quantities of foreign material including road kill, leaves or even spilled dry aggregates from the supply truck can cause local pop outs and rework under MSO’s two-year warranty. Metro-Mat’s durability is attributed to the use of polymer modified asphalt and premium aggregates, together with its high application rate. The mix for this contract was designed for normal daylight summer temperatures and included T.J. Pounder CQS - 1H polymer modified emulsion, trap rock from the Aecon Group Inc.’s Marmora Quarry, sized at 100 per cent passing a 9.5 mm sieve. Bagged Type 10 cement was used as the mineral filler here, with the overall mix containing dry aggregate together with 14 per cent emulsion, 1 per cent Type 10 cement, and 6 per cent water. At between 8 and 10 kg/m 2 , the application rate of this mix was some 50–60 per cent higher than the 5.4 to 8.1 kg/m 2 rate required by OPSS 337. In an interesting example of equipment utilisation, the tandem supply trucks used for material delivery have been adapted from their normal application as salt /sand spreaders. For the slurry seal application, the truck’s 7.65 m 3 hopper has been turned round, providing rear rather than front discharge, while a customized gate at the rear discharge point prevents spillage of dry aggregate during transfer to the paver. In addition, a split tank has been added between the hopper and the driver’s cab, providing 2300 litres of emulsion and 2300 litres of water carrying capacity. MSO Construction Ltd. was originally founded in 1922 as the Municipal Spraying and Oiling Co. Ltd, and is a subsidiary of T.J. Pounder (Ontario) Ltd. What is slurry seal? Slurry seal is a homogenous mixture of asphalt emulsion, well-graded fine aggregate, water and mineral filler, applied in a cold fluid state to a prepared bituminous surface. Properly applied, slurry seals can extend the life of structurally sound pavements by filling small cracks in the pavement surface, protecting the existing pavement and improving skid resistance. Ontario Provincial Standard Specification (OPSS) 337 covers the construction specification for slurry seal, including recent amendments that set out the requirements for mixtures utilising polymer modified emulsions and 100 per cent crushed bed rock aggregates. Slurry seal requires warm dry weather for successful application and the specifications limit its application to between May 31 and September 30. In addition, the temperature during application is stipulated as at least 10 Á C and rising, with no forecast of freezing within the 24 hours after application. Further, the weather has to be free of fog, rain or high relative humidity. OPSS 336 covers the closely related process of microsurfacing. The processes of slurry seal and microsurfacing are similar, except that slurry seal is completed in a single coat, while microsurfacing typically involves two coats including a scratch or levelling coat and a surface coat. Modified slurry seal treatment used on Pearson taxiway shoulders In another recent application, polymer modified surface treatment was used to resurface taxiway shoulders on Toronto’s L.B. Pearson International Airport. The job extended from Hotel 5 to Hotel 2 on the Taxiway Hotel, including both north and south shoulders and covering a total area of some 32 000 m 2 . At these locations, loose aggregate was a potential intake hazard to aircraft engines and resulted from severe ravelling and cracking of the 30 year old oxidized surface asphalt. The shoulders were treated with a Metro-Mat mix containing minus 4.75 mm aggregate, applied at a rate of 6 to 8 kg /m 2 to seal the surface and prevent further deterioration. Work was completed at night under tight operating constraints, as only a limited number of Pearson’s runways & taxiways can be closed at any one time due to high air traffic volumes, and then only between the hours of midnight and 6:00 am. In addition, new security measures have been introduced at Pearson following the events of September 11 th , causing further headaches in terms of access for construction personnel, equipment and materials. MSO’s Dan Murphy reports that progress in these difficult conditions was helped by the Airstar balloon lighting system. According to the manufacturers, the Airstar Sirocco is a portable lighting unit that produces a strong glarefree light, covering 360 degrees and an area of up to 2,500 m 2 . It consists of a halogen or metal halide lamp mounted inside a protective cage at the centre of a self-inflating balloon. The balloon inflates in 45 seconds and can reportedly withstand wind speeds of up to 100 km/h, while its shock mounting construction allows the Sirocco to be mounted directly onto construction equipment.
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