Segregation in Aggregate Base Stockpiles
Aggregate base is placed between the compacted subgrade and the concrete or AC surface layer to provide support and drainage. It is composed approximately 50% coarse and 50% fine aggregate, and its tight compaction is achieved by an even gradation and angular particle shape. Specifications for ¾″ and 1 ½″ maximum base products are provided by Caltrans Section 26.
CAUSES OF SEGREGATION
Segregation of aggregate base can lead to rejection or costly penalties for the producer, and base material is especially susceptible to segregation because of a greater variation in particle size than that of other stockpiled products. Segregation of an aggregate product will likely occur where the ratio of the size of the largest particles to the size of the smallest exceeds 2:1. A ¾″ maximum base product exceeds this ratio and will readily segregate.
Segregation occurs in the formation of a traditional conical stockpile. As the product is transported by conveyor, vibration and motion of the belt causes fine particles to settle on the bottom. Coarse particles at the top of the material stream have a higher velocity as the material reaches the head pulley, and are thrown a greater distance. In addition, the coarser particles hit the front face of the stockpile with a greater momentum, and roll down the outer edge of the pile, creating overrun: an accumulation of particles at the pile’s bottom edge, or toe. Fine particles, which have settled against the surface of the conveyor belt, tend to cling to the belt and drop to the back face of the pile. The resulting stockpile is segregated, with coarse particles settled at the toes, and fine particles in the center portion of the pile.
REMEDIES FOR STOCKPILE SEGREGATION
When loading from a segregated stockpile, the loader operator will attempt to blend material from the fine and coarse portions. This is best accomplished by drawing the material evenly from the edge of the pile perpendicular to the belt, thereby loading material from both the front and back face. Loading exclusively from either the front or back face of the stockpile will result in loads that are overly fine or overly coarse. Careful and even loading of the truck bed will prevent further overrun segregation.
The formation of a continuous layered stockpile will reduce the effects of segregation and eliminate the need to reblend the base before shipment. Each layer of the layered stockpile will have the same segregation characteristics of a large conical pile, but the multiple layers mitigate segregation at the point of loading, and eliminate the need to reblend.
A radial or telescopic conveyor can be used to construct a layered stockpile. If this equipment is not available a dozer may substitute. If delivered by truck, the pile is formed by pushing the material into inclining layers. If delivered by conveyor belt, the pile is formed by pushing it into horizontal layers. To prevent overrun, the operator must be careful not to push the material over the edge of the pile. When building a stockpile with a loader, the pile should not be made so high as to necessitate the loader driving on the material. This will lead to degradation and possibly contamination.
Automated Telescoping Stackers
Slurry seal has a number of advantages; some are listed below:
- Rapid application.
- No loose cover aggregate.
- Excellent surface texture for paint striping.
- Ability to correct minor surface irregularities.
- Minimum loss of curb height.
- No need for manhole and other structure adjustments.
- In many cases, the relatively low cost of the treatment makes it practical to import aggregates for special effects, such as high skid resistance, color contrast and noise reduction.
The slurry is usually applied in a thickness of 3 to 6 mm (1⁄8 to ¼ inches). It comes directly from a traveling mixing plant into an attached spreader box that spreads the slurry by a squeegee-type action. The machine used for production of the slurry seal is self-contained, continuous-flow mixing unit. It is capable of delivering accurately to the mixing chamber predetermined amounts of aggregate, mineral filler (if required), water, and asphalt emulsion. It also discharges the thoroughly mixed materials on to the prepared surface. Certain basic features are common to all batch type slurry machines. They are truck-mounted units with separate storage tanks, bins, and metering systems for emulsified asphalt, water aggregate and mineral filler. The slurry machine has a continuous-flow mixing unit, either single or double pugmill, from which the slurry is discharged into a spreader box. The box is equipped with flexible squeegees and a device for adjustable width. Spreader boxes may be equipped with hydraulically-powered augers to keep the slurry in motion and help keep the mixture uniformly spread across the spreader box width.
The aggregate used in slurry seal must be clean, angular, durable, well grades, and uniform.
The three generally accepted gradings used for slurry mixtures are:
- Type I is used for maximum crack penetration. Also, it makes an excellent pretreatment for hot mix overlay or chip seal. It is usually used in low density traffic areas such as light aircraft airfields, parking areas, or shoulders where the primary objective is sealing.
- Type II is the most widely used gradation. It is used to seal; to correct severe raveling, oxidation, and loss of matrix; and to improve skid resistance. It is used for moderate to heavy traffic, depending upon the quality of aggregates available and the design.
- Type III is used to correct surface conditions, as the first course of multicourse applications for heavy traffic, and to impart skid resistance.
Emulsified asphalt used in the slurry mix may be SS-1, SS-1h, CSS-1, or CSS-1h. The recently developed quick-setting (CQS) asphalt emulsion is being used when early opening to traffic is necessary.
Blending the slurry seal materials in varying proportions in the laboratory is a great aid in selecting the proper mixture. Correct blending should produce a slurry with a creamy texture that will flow smoothly in a rolling wave ahead of the strike-off squeegee. This slurry should be a semifluid, homogenous mass with no emulsion runoff.
Just before applying the slurry, the pavement surface should be cleaned of all dirt, dust, mud spots, vegetation, and other foreign matter. A tack coat of diluted emulsified asphalt of the same type and grade specified for the slurry may be required directly ahead of the slurry application. With relatively new asphalt pavements, the tack coat may be omitted. In this case, the surface should be pre-wetted by water fogging. The surface should be damp but with no free water in front of the slurry machine.
Special care must be taken with longitudinal and transverse joints to prevent excessive buildup of slurry (ridging) or to prevent streaking. It is best to make the joint after the first placed lane is either completely cured or is still in a semi-fluid condition. For good appearance and durability, a joint should not be made when the lane to be joined is only partially set, as tearing and scarring may result.
Quite often a drag is pulled behind the spreader box to improve the joint and overall surface appearance. Drags should be changed regularly. Hand squeegees and hand drags are used to improve joints and place the slurry in areas inaccessible to the machine.
Rolling a slurry seal is only needed in those areas where pneumatic-tired rolling will improve durability. Such areas include taxiways, runways, truck terminal yards, and intersections of heavily traveled roads. All of these are subject to power steering turns, brakes, or acceleration forces. For rolling, a 4.5 tonne (5 ton) pneumatic roller with 345 kPa (50 psi) tire pressure will be most effective. Rolling can start as soon as clear water can be pressed out of the slurry mixture with a pieceof paper, without discoloring the paper. In most cases, however, traffic will iron out the slurry and close any hairline cracks of dehydration. Rolling usually is not needed unless the thickness is more than 6 mm (¼″) or unless late season work is involved.
Slurry should be placed only when the temperature is at least 10°C (50°F) and rising and when no rain is expected. A newly placed slurry should not be opened to traffic until it has completely cured. As with rolling, traffic generally can be allowed on the slurry as soon as a clear water can be pressed out of the slurry mixture with a piece of paper without discoloring the paper. The traffic, of course, must be controlled somewhat as quick stops or accelerations and the turning of wheels while parked will cause damage to the slurry.
For slurry seal design the following sources are recommended:
ASTMD 3910 “Standard Practices for Design, Testing and Construction of Slurry Seal”
American Society for Testing & Materials
1916 Race Street
Philadelphia, PA 19103-1108
A105 “Recommended Performance Guidelines for Emulsified Asphalt Slurry Seal Surfaces”
International Slurry Surfacing Association
1101 Connecticut Avenue
Washington, D.C. 20036-4303
“Recommended Guideline for Slurry Seal”
Asphalt Emulsion Manufacturer’s Association
#3 Church Circle, Suite 250
Annapolis, MD 21401-1903