302 - Aggregate Base Courses

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Aggregate Base Course



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Aggregates from dense graded series 21AA, 21A and 22A will be used for aggregate bases as per subsection 902.05 of the Standard Specifications for Construction. 22A is to be used under areas that will be surfaced with Hot Mix Asphalt (HMA) surfaces such as mainline, shoulders and approaches (23A is not to be used for this work).

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Placing and Compacting

Some cross sections are designed with a “step” in the subbase and a different aggregate depth in the shoulder area, making this a difficult section to build. The aggregate base will be placed with methods and equipment that will not cause rutting or distortion to the existing subbase or subgrade and will not cause segregation or contamination of the new aggregate material.

Place and compact the aggregate material on a prepared subgrade or subbase and shape as detailed on the plans within specification tolerances. Compaction results are best when pursued in conjunction with placement and the aggregate has uniform, optimum moisture. An additive may be used at the Contractor’s option to facilitate compaction and maintenance of the aggregate surface (refer to the Density Control Handbook to determine the optimum moisture and for performing density tests).

The aggregate base must be compacted to the required density and have surface stability prior to surfacing. Surface stability is generally considered achieved when a tire produces rutting less than d inch (10 mm) in depth. Sufficient tests must be made prior to surfacing to ensure achievement and preservation of density requirements, especially when the aggregate base is left to either dry out or become saturated by rainfall.

Rutted, raveled or dried out surfaces must be scarified, recompacted and regraded to original plan grade and cross section. Saturated areas must be allowed to dry prior to surfacing. Aggregate bases which are within two percentage points of being saturated should not be surfaced. Moisture content is determined by the one point cone chart (refer to the Density Control Handbook).

Remember that the aggregate base must be rolled after final trimming in order to compact any loose gravel and minor fill areas left by the trimmer or grader.

Prior to surfacing, the completed aggregate base should be checked for conformity to the plan elevation and slope by string line or other method and the results recorded. Density tests and depth checks must be taken after final trimming. Job control density tests and depth checks in the sampling and testing frequency guidelines are minimal for acceptance and record keeping purposes. Additional density tests and depth checks are recommended for tighter job control and a higher level of quality control when field conditions warrant these controls.

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Payment by Square Yard (Square Meter)

The following criteria should be adhered to with payment by the square yard (square meter):

  • Longitudinal - Measure along the surface and parallel to the centerline of the area built.
  • Transverse - Plan width ± any changes.
  • Depth - Taken after final trimming to the nearest 1/4 inch (6 mm), at a minimum of 400 linear feet (120 m) of traffic lane width or two lanes if simultaneous construction. For irregular areas, intersections, crossover widening strips, etc., take one depth check for each 1200 square yards (1,000 m2) of area or fraction thereof.

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Depth Check Deficiency

Depth Deficiencies
>1 inch
(25 mm)

Additional depth checks will be taken 25 feet (8 m) each way from the original and the average of the three depth checks will be recorded for that location.

If avg. >1 inch
(25 mm)

Additional depth checks at 25 feet (8 m) intervals each way will be taken until achieving a depth of less than 1 inch (25 mm) deficient.

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Average Depth

In determining the average, depths greater than plan thickness will be considered as plan thickness. Depths greater than 1-3/4 inches(45 mm) deficiency will not be used for averaging. To be within specifications the average of the three depth checks must be no more than 1 inch (25 mm) less than plan thickness.

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Payment by Weight

When payment is by weight, refer to weighing material and mixtures in subsection 109.01B2 and Section 302.03 of the Standard Specifications for Construction. Aggregate will be weighed to the nearest 100 pounds (50 kg). Pay weights will be the scale weight of the aggregate unless the moisture content is more than 6 percent. Moisture tests will be made at the start of weighing operations and at any time thereafter to determine any change in moisture content. The adjustment to the pay weight will be according to the following formula:

Pay Weight = Weight of wet aggregate x (100 + allowable % of moisture) / (100 + actual % of moisture)

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