305 - HMA Base Crushing and Shaping
The age of the equipment being used by the Contractor will have an effect on the amount of additional crushed material through bulking, particle size and gradation. The amount of bulking of the crushed material for older equipment models has historically been approximately 20 percent of the crushing depth. These older crushing machines will generally crush to a larger particle size and coarser gradation. The result is a higher bulking factor and more excess material which must be removed.
The introduction of newer crushing machines in recent years resulted in smaller particle size and a finer gradation. This improvement in crushing results in little or no bulking of the crushed material. Thus, a lesser amount of additional crushed material is produced. The requirements for automatic grade referencing equipment for the final trimming should be reviewed and emphasized to the grading contractor.
The maximum efficient depth for crushing HMA pavement and aggregate base is 6 inches (150 mm). Crushing depths in excess of 6 inches (150 mm) should be done in stages. The initial stage is crushing 6 inches (150 mm) of HMA pavement. Subsequent stages include windrowing the crushed HMA pavement on the shoulder, crushing the underlying layer and mixing together with the windrowed material. An alternative is to mill the excess HMA pavement from the grade and pay for it as Material, Surplus and Unsuitable, Remove. Remember that the specifications require a range of 1 inch to 2 inches (25 mm to 50 mm) of existing aggregate base to be crushed with the HMA pavement. Therefore, the thickness of the HMA pavement after cold-milling should range from 4 inches to 5 inches (100 mm to 125 mm).
Grading the crushed material should be referenced to either grade stakes or the existing pavement surface elevation at the centerline in order to attain plan grade or cross section. Excess crushed material should be windrowed on the shoulder for removal by the Contractor. The crushing depth and grading requirements should be monitored to guard against excessive generation of material. If additional material is required to obtain the section required, make sure that a well graded homogeneous material results.
Compaction of this material type is moisture sensitive and adequate moisture must be present. Final trimming with automated grade control apparatus is accomplished after compaction requirements have been met. The Contractor is responsible for the condition of the crushed base and for maintaining the base in an uncontaminated, smooth, compacted condition, true to line, grade and cross section until placement of the next layer.
Windrowed material should be removed from the shoulder as soon as possible. Until removal, the windrowed material will be delineated at 1500 feet (400 m) intervals with lighted Type II barricades. Drainage trenches will be cut through the windrowed material to avoid ponding of runoff and saturation of the base. Upon removal of excess material, the shoulder will be graded to a safe, usable condition.