The Paving Process
For Bitumen Paving Process the road surface is first cleaned with brushing or high-pressure wash. Thermoplastic road markings may be removed or masked, and drains, utility covers and other areas may be masked to prevent adherence of the slurry. The slurry or micro surfacing mixture is prepared in a mobile mix paver “Slurry Paver” which holds all the ingredients (aggregate, emulsion, water, fillers, additives and optionally fiber and pigment). These ingredients are delivered in the correct proportions into a helical or paddle mixer, for example a pug mill, to produce the slurry which is discharged into a spreader box. Generally aggregate and cement enter the mixer first, followed by water then emulsion. Chemical additives are injected into the water line. The proportions of the ingredients are maintained constant independent of the forward speed or discharge rate of the machine, either by a common clutch shaft driving volumetric pumps for water, emulsion, additive and the filler feeder and aggregate conveyor or by electronic controls. Aggregate delivery may be volumetric via an aggregate gate, or by load cells on the conveyor. Proper calibration is vital for successful application.
The spreader box in Bitumen Paving Process
The function of the box is to receive and contain the mix, evenly distribute the material, agitate the mix to prevent segregation, meter the material onto the road surface, control the transverse and longitudinal profile, and apply the final texture. The box is not rigidly attached to the paver, rather it travels along the road surface on skids, pulled by the paving machine. These adjustable skids on the sides and under the box support it and keep it level to control the longitudinal profile. Augers may be used in the box to spread the slurry across the road surface. With some fine slurries no augers are needed, but with coarser slurries quite intense agitation may be required to prevent segregation and to distribute the material across the road surface, and the box may contain paddle type agitators as well as the augers.
Rubber strips on the front and rear of the box, and sometimes also on the sides, help contain the mix. Adjustable flexible or rigid strips “strike offs” across the rear of the spreader box control the thickness and transverse profile. They can be raised or lowered at different points across the road. The box may be split to better follow the camber of the road. A rigid (metal) strike off is used produce a horizontal profile for a first “leveling course” or “scratch course”. A flexible strike off made from polymer or rubber is used for the final course. Some pavers have a secondary strike off of softer material or burlap which can remove small drag marks and provide a consistent texture to the final seal. The width of the spreader box maybe fixed or variable. Special V-shaped spreader boxes and strike offs are used for deep rut-fill which are designed to produce a profile higher in the center, to allow for compaction under traffic.
Quality assurance and quality control
The paver has to be calibrated before use. The aggregate may be damp which affects its bulk density and the delivery using volumetric gate type feeders. In many cases a test strip will be laid. During construction the consumption of materials is carefully monitored as a double check on the metering systems. Modern pavers have integrated data-loggers which provide a record of material consumption. The quality of ingredients may be tested during construction. The slurry may be sampled for bitumen determination and performance tests may be performed with site materials or using the slurry discharged from the paver.
The fresh seal should have an even color and texture with little or no run off of emulsion from the sides. Workers may correct small imperfections and drag marks in the seal before it cures using squeegees or brooms.
Water
Water addition rate controls the consistency of the slurry and may be adjusted on the paver as required.
Filler and pigments
A screw feeder is used to supply filler to the mixer. Bags of cement or lime are usually added by hand to the filler hopper. Some slurries can be formulated without filler, or lime slurry has been used to avoid handling the powders. Pigments in powder form may be added via the filler feeder or a dedicated feeder. In some systems the pigment is incorporated in the emulsion. Hydrated lime filler rarely exceeds 0.5%; cement may be used up to 2.5% (expressed on aggregate).
Fibers
Fibers are used in some high performance products to improve wet consistency and sometimes the final strength. They can be pre-chopped and added via a hopper or the machine may incorporate a chopper and fiber feeder. Cellulose might be used to increase viscosity or glass or polymer for extra durability.
Bitumen Paving Process Chemical additives
Chemical additives may be added to retard or (less often) to accelerate the curing process. They are stored in a small tank on the paver and dosed into the water line. They may be added undiluted or as a water solution.
Handwork
It may be necessary to apply slurry by hand in order to completely cover irregularly shaped road areas such as intersections and cul-de-sacs. Slurry may also be applied from small mixing equipment and spread by hand on small areas like cycleways and footpaths. Squeegees are most often used to distribute the slurry over the surface. In the case of quick-set slurry and micro surfacing, the mixture may need to be stabilized to allow enough time for handwork – which is done by the addition of break retarders.
Sanding
Sand may be used to avoid damage of the seal and pick up on tires if traffic is allowed onto the surface before full cure, and specially to protect against turning or cross traffic.
Bitumen Paving Pre-treatment of the road with water or tack.
A spray bar on the paver can spray water onto the road surface before the spreader box to help wet out dust on dry road surfaces and cool the surface. Some advanced equipment may have an integrated emulsion distributor to apply a tack coat before the slurry, or tack may be applied separately. Stabilized bases should be preferably treated with a primer seal before application of slurry.
Batch and continuous pavers
Batch pavers are used on smaller jobs and in urban areas because they are more maneuverable. Typically, they hold about 4-8 cubic meters of aggregate and are truck mounted. Larger batch equipment up to 10 cubic meters may be trailer mounted. Once the material in the truck is exhausted it must return to the stockpile and refill. Generally, the contractor will utilize two or more pavers to provide more or less continuous paving at the job site. Continuous run pavers can be refilled by nurse trucks without stopping and are used on larger jobs. Compact continuous machines suitable for urban work are available and batch pavers can be retro-fitted with special nurse feeders for continuous operation.
Compaction
Rolling is not usually required but may be specified for low trafficked areas like parking lots and airports, or for deep rut-fill projects. Pneumatic tired rollers are used once the seal is stable enough to take the weight. One or two passes are sufficient. In some cases, slow moving traffic is allowed initially to accelerate the curing process.
Sweeping
for Bitumen Paving Process in All of the aggregate is incorporated into the surfacing and sweeping of the finished seal is not normally required, but may be specified for airport jobs or in response to early raveling of the seal.
Multiple layers
In micro surfacing a “scratch course” (correction or levelling layer) may be applied and allowed to cure under traffic before placement of the wearing course. Usually 12 hours or more is used between applications. The scratch course should be swept before covering to avoid drag marks.
Bitumen Paving Process Fog seal
A fog seal may be applied to prevent early raveling in cold weather paving.
