302 Asphalt Concrete Base

Description (302.01)

This work consists of constructing a base course of aggregate and asphalt binder, which is mixed in a central plant, and spread and compacted on a prepared surface. This item is similar to 301 and requires the same level of field inspection and documentation.

The requirements of Item 401 apply, except as modified by this specification.

 

Composition (302.02)

The Contractor shall furnish aggregate for the mix that conforms to the following gradation:

 

Sieve Size

Total Percent Passing

2 inch (50 mm)

100

1-1/2 inch (37.5 mm)

85 to 100

1 inch (25.0 mm)[1]

68 to 88

3/4 inch (19.0 mm)[1]

56 to 80

1/2 inch (12.5 mm)[1]

44 to 68

3/8 inch (9.5 mm)[1]

37 to 60

No. 4 (4.75 mm)

22 to 45

No. 8 (2.36 mm)

14 to 35

No. 16 (1.18 mm)

8 to 25

No. 30 (600 m)

6 to 18

No. 50 (300 m)

4 to 13

No. 200 (75 m)

2 to 6

[1] Provide aggregate to retain a minimum of 7 percent of the material on each of these sieves. This requirement applies to the gradation of the JMF and the mix production according to Item 403.

 

The Contractor may use reclaimed asphalt concrete pavement (RAP) or reclaimed asphalt shingles (RAS) as per 401.04. Asphalt concrete is sampled and tested by the Contractor at the asphalt plant. Contractor tests are used for pay if they are verified by District testing. See 403.06 for sampling details.

 

Spreading and Finishing (302.04)

The Contractor must notify the Engineer at least 24 hours before the start of paving. Spreading equipment must conform to 401.12 specifications. The Contractor must submit, in writing, the model of asphalt spreader to be used, provide a certification statement that all required modifications have been made, and provide a signature. The compacted thickness should be a minimum of 4 inches and a maximum of 7-3/4 inches.

The minimum air temperature required for paving 302 base is 40 F (5 C). The minimum mixture temperature, when delivered to the paver, is 250 F (120 C). When using warm mix asphalt, the minimum temperature is 230 F (110 C). The mixture temperature should be checked a minimum of four times per day or more if necessary. The temperature should be documented in the project records.

 

Hauling

Hauling must conform to specification 401.11.

The Contractor must use trucks for hauling asphalt concrete that have tight, clean, smooth metal beds from which the entire quantity of mixture is discharged smoothly into the spreading equipment.

If transporting hot asphalt concrete at prevailing air temperatures below 50 F (10 C), or if the length of haul exceeds 20 miles (32 km), ensure that all truck beds are insulated to maintain a workable mix temperature and ensure that all covers are fastened to exclude the wind. Do not exceed a distance of 50 miles (80 km) from the asphalt concrete plant to the paving site, unless specified by the Department.

 

Compaction

Compaction must conform to specification 401.16.

Compact the mixture uniformly using a combination of both steel and Type I pneumatic tire rollers which conform to 401.13. Do not use a spreading rate that exceeds the total of the specified capacities of the rollers in use.

Ensure that the compacted depth of any one layer is between 4 and 7-3/4 inches and the temperature of the mixture, when delivered to the paver (not to the job site), is a minimum of 250 F (120 C).

 

Example: Roller Capacity and Placement Rate

A contractor is using one 3-wheel roller, one vibratory roller with 66-inch drums (both vibrating), and one Type I pneumatic tire roller to compact a 5-inch thick mat using material with a Laboratory conversion factor of 2.0 tons per cubic yard.

From Table 401.13-1, the following is the capacity of the rollers the Contractor will use:

         Three wheel = 700 sq yd/hr

         Vibratory roller = 2 drums x 66 in x (15 sq yd/in of width) = 1,980 sq yd/hr

         Type I Pneumatic Roller = 1,000 sq yd/hr

         Maximum roller capacity = 700 + 1,980 + 1,000 = 3,680 sq yd/hr

[(3,360 sq yd/hr) X (9 ft2/sq yd) X (5 in) X (1 ft/12 in)] X (1 yd3/27 ft3) = 511.11 yd3/hr.

511.11 yd3 X 2.0 tons/Yd3 = 1,022.22 tons per hour maximum placement rate.

 

Spreading and Surface Tolerances (302.05)

Spread the mixture at a rate calculated using the specified thickness and the compacted width of the pavement course being placed, and the weight-to-volume conversion factors established in 401.21. Maintain the actual rate of spreading the mixture within a tolerance of 5 percent.

Do not exceed 3/8 inch (10mm) in surface variation from the edge of a 10-foot (3 m) straightedge. Surface variations include bumps and depressions. If using Item 302 Asphalt Concrete Base as a subbase for a rigid pavement or base, do not exceed a variation of 1/4 inch in 10 feet (6 mm).

 

Example: Determining the Required Placing Rate per Station (RPRS)

A contractor is placing a 5-inch uniform mat of asphalt concrete pavement whose Lab conversion factor is 2.0 tons/cubic yard. The required placing rate (RPRS), in tons of material per station, for a 12-foot wide mat is:

RPRS = [Volume of asphalt in cubic yards] x [Lab Conversion Factor]

= [{100 ft per station X 12 ft X (5 in 12 in/ft)} 27 ft3/ yd3] X 2.0 tons/yd3

= 37.04 tons/station

 

Documentation Requirements - 302 Asphalt Concrete Base

1.      Document condition of base material at the time of paving (example: primed 304, clean and dry concrete, etc.).

2.      Obtain JMF for the project.

3.      Obtain letter from Contractor stating paver to be used and certifying that all modifications have been made as required.

4.      Document tack or prime used along with source and quantity used versus required.

5.      Write location (station), date, and time on asphalt plant tickets. Tickets should be totaled daily, initialed, with the calculator tape attached.

6.      Document lift thickness, mat width, weather conditions, surface tolerance checks, equipment problems, mat problems (segregation, tearing, tenderness, etc.), spreading rate, roller coverage, and any other issues or observations made during paving operations.

7.      Obtain and document temperature of the mix at project site and place this information on ticket of load checked. This should be done a minimum of four times daily or any time temperature is in question.

8.      Document the kind of rolling equipment and maximum tons per hour allowed. See example, Roller Capacity and Placement Rate.

9.      Calculate and document the quantity of material placed for payment.

10.  Document on form CA-FP-4 or other approved form.