Category:450 Bituminous Pavement Design

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Estimate Factors

450.1 Asphalt Binder

Performance grade (PG) asphalt binders are identified by their desired "high temperature" (e.g. "64", "70", "76", etc.) and "low temperature" (e.g. "-22", "-28", "-34", etc.) characteristics. The temperature numbers represent Celsius degrees and are used in 6 degree increments. The high temperature value is associated with controlling rutting, while the low temperature value is associated with resisting cold weather cracking. The following table should be used as a guide to select asphalt binder grades for bituminous mixtures. Occasionally, asphalt grading will be noted with MSCR Grading notations, as per AASHTO M 332. For instance, PG64-22H is a MSCR graded asphalt that is an allowed replacement for PG70-22. Equivalent grading is specified in Sec. 1015.10.3.1.

Asphalt Binder Selection Criteria
Type of Corridor Traffic Level Type of Mix Asphalt Binder
Interstates and Other Freeways All Levels Surface Mixture (SP125 or SMA) and First Underlying Lift PG 76-22
Remaining Underlying Lifts PG 64-22
Other Remaining Major Routes Heavy Volume Surface Mixture (SP125) and First Underlying Lift PG 70-22
Remaining Underlying Lifts PG 64-22
Medium or Low Volume Surface Mixture (SP125 or BP)* PG 64-22
Underlying Lifts PG 64-22
Minor Routes All Levels All Mixtures (Generally BP-1 as Surface Mix) PG 64-22
*Note: Requires JSP to include appropriate smoothness requirements.

The low temperature number is to remain as indicated in the above table unless specifying a Surface Level or BP surface mix in one of the northern districts. To improve thermal crack resistance, a PG58H-28 should be specified on minor routes in the Northwest and Northeast Districts.

The high temperature numbers have been set for the traffic loads and operation speeds normally associated with these corridors. The high temperature numbers are recommended minimums and are not to be reduced; however, may be raised to a PG 70-22 or PG 76-22, when deemed necessary, to accommodate actual or anticipated traffic conditions. Typically, the high temperature number should be raised one increment (6 degrees) when traffic speeds are expected to be in the range of 12 to 45 mph and raised two increments (12 degrees) for extremely slow traffic of < 12 mph. Typical candidates for these high temperature number raises are roadways with AADT greater than 3500 that are in a highly congested, urbanized area, have frequent stop and go traffic, or have steep grades with significantly slow traffic speeds. Use of any other PG asphalt binder must be approved by the State Construction and Materials Engineer.

If a higher type PG asphalt binder is warranted, it shall be used in the surface mixture and the first lift of the underlying mixture.

450.2 Mix Selection

The following table should be used as a guide in selecting the appropriate asphalt mix. See Thickness Determination for additional mix selection discussion regarding minor road spot improvements.

Corridor Designation Traffic Recommended Mix
Interstates and Other Freeways All Traffic Superpave
Remaining Major Routes Total Average 24 Hour Commercial Truck Traffic 600 or greater Superpave
Remaining Major Routes Total Average 24 Hour Commercial Truck Traffic less than 600 BP-1 (Sec 401)(1)(2)
Minor Routes Total Average 24 Hour Commercial Truck Traffic 600 or greater Superpave(2)
Minor Routes AADT > 3500 and Total Average 24 Hour Commercial Truck Traffic < 600 BP-1 (Sec 401)
Minor Routes AADT < 3500 and Total Average 24 Hour Commercial Truck Traffic < 600 BP-1 or BP-2(3)
(1)Note: Requires JSP to include appropriate smoothness requirements.
(2)Note: Consideration should be given to alter mix type for overlays based upon existing conditions.
(3)Note: The selection of which mix to use is left to the district's discretion based upon past field performance.

450.3 Layer Design

Min. Placement Depths and Min. %AC by Volume for Asphalt Treatment Options

Minimum Lift Thickness

The minimum lift thicknesses for asphalt layers should be:

SP250 SP190 SP1251 SP0951 SP0481
4 in. 3 in. 2 in. 1½ in. 1 in.
1 When used as the riding surface. When used as a leveling course below the top lift, the minimum lift thickness should be in accordance with EPG 450.4 Level Course.
PMBB BP-11 BP-21 BP-31
3 in. 2 in. 1½ in. 1 in.
1 When used as the riding surface. When used as a leveling course below the top lift, the minimum lift thickness should be in accordance with EPG 450.4 Level Course.

The above minimum lift thicknesses are recommended to ensure adequate field density can be achieved and a quality product can be provided. The minimum lift thicknesses do not account for surface irregularities in the roadway. Additional asphalt quantities should be included with single lift asphalt overlays to account for the surface irregularities. The additional asphalt quantities added should be in accordance with the average rutting depth measurements from the roadway as provided in Table 402.1.2.1.

Central Office Construction and Materials may recommend thicknesses greater than the above minimums but projects should not be designed for thicknesses less than the above minimums. If using the minimum lift thickness criteria results in the project exceeding its budget, then a finer gradation asphalt mixture should be used and designed for its minimum lift thickness or greater (for example, if 2 in. of BP-1 exceeds project budget, use 1.5 in. or greater of BP-2).

450.4 Level Course

Existing surfaces, both asphaltic and portland cement concrete, should be prepared either by milling or leveling course before the first full-thickness lift of asphalt is placed. Milling is the preferred method of leveling, however PMBP or SP asphaltic concrete may be used for spot wedging and for leveling course as per the standard specifications, providing the minimum thickness is not less than the following:

BP-1 and SP125 BP-2 and SP095 BP-3 and SP048
1¾ in. 1¼ in. ¾ in.

The level course can be one design level lower than the lift above it. For example, an SP level C design may be used to level under an SP level B overlay. A BP mix may be used to level under an SP Level C overlay.

450.5 Payment

Payment for asphaltic concrete by the square yard should only be used for full depth asphaltic concrete pavements on alternate bid projects. All other payment for asphaltic concrete should be by the ton. For preliminary design, estimate factors may be used to compute quantities for bases, flexible type pavements, seal coats, etc. For final design, the designer should request estimate factors for anticipated rock formations from the District Construction and Materials Engineer to obtain more reliable results. When payment is by the ton, estimate factors should be shown on the plans with a note "For Information Purposes Only." When payment is by the square yard, estimate factors should not be shown on the plans.

450.6 Automatic Screed Control or Established Grade Reference

The standard specifications require the use of automatic screed controls with the sensor following a traveling reference plane. In some instances, such as a badly warped pavement or very poor riding condition, the use of an established grade reference may be desirable. If the established grade reference is desired, a special provision must be included in the contract requiring its use for the initial pass of the paver when placing the first continuous layer. The special provision may be modified to require the established grade reference for a portion of a project if necessary.

450.7 Commercial Mixtures

The standard specifications permit the use of approved commercial mixture for plant mix bituminous pavement and plant mix bituminous base course when so specified in the contract. Commercial mixtures are not permitted where Section 403 or Superpave mixtures are specified. Care should be exercised when setting up small quantities of asphaltic concrete in that it may be more desirable, all factors considered, to use plant mix bituminous pavement so that a commercial mixture can be permitted.

450.8 Asphalt Over Rubblized Pavement

Asphalt Over Rubblized Pavement.jpg

Rubblization is the in-place breaking of an existing Portland Cement Concrete (PCC) pavement into an aggregate base for new hot mix asphalt. When major asphalt rehabilitation over existing concrete pavement is considered the existing pavement is rubblized prior to the asphalt overlay. If the existing pavement consists of asphalt over concrete the existing asphalt should be removed prior to rubblization.

When rubblization is used, the asphalt is paid for by the ton of mix necessary to obtain the thickness indicated by Construction and Materials. The plan quantity should be established with consideration for thickness in excess of that specified to account for irregularities in the existing pavement. Similar to unbonded overlay the contractor is required to establish the existing roadway profile and set the final overlay profile. For this reason the bid item for contractor staking should be included in the contract.

There must be adequate subgrade support under old rigid pavements. Dynamic cone penetrometer (DCP) testing of the unbound layers under the PCC pavement, performed during the condition surveys, must indicate that adequate support exists for rubblization to occur.

450.9 Safety EdgeSM

Shaping the edge of pavement to a 30-degree bevel is an effective strategy for mitigating the negative effects associated with vertical edge drop-off. Refer to Std. Plan 401.00.

As with conventional paving, the graded material adjacent to the Safety EdgeSM should be brought flush with the top of the pavement following paving. If this material should settle or be otherwise displaced in the future, however, the Safety EdgeSM would continue to provide a durable height transition to smoothly conduct vehicles back to the paved road.

Safety EdgeSM must be included in all single-lift overlay projects when the finished surface will be 21 ft. wide or greater. This includes shoulder paving unless the paved width of each shoulder exceeds 4 ft. When using the Safety EdgeSM, the quantity of asphalt mix will increase by approximately 2 percent.

Articles in "450 Bituminous Pavement Design"

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