Category:450 Bituminous Pavement Design
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.
|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. 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 76-2, 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 (20 to 70 km/h) and raised two increments (12 degrees) for extremely slow traffic of < 12 mph (20 km/h). Typical candidates for these high temperature number raises are roadways with ADT 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||Superpave|
|Minor Routes||Total Average 24 Hour Commercial Truck Traffic > 400||Superpave|
|Minor Routes||ADT > 3500 and Total Average 24 Hour Commercial Truck Traffic < 400||BP-1 (Sec 401)|
|Minor Routes||ADT < 3500 and Total Average 24 Hour Commercial Truck Traffic < 400||BP-1 or BP-2*|
|*The selection of which mix to use is left to the district’s discretion based upon past field performance.|
450.3 Layer Design
[pwname:\\MoDOT\Documents\CADD_Standards\Seed Files\Design - English\exhibit_3d_portrait.dgn seed files] Minimum Lift Thickness
The minimum lift thicknesses for asphalt layers should be:
|SP250||SP190||SP125||SP95 and BP-2||BP-1||PMBB|
|3 in.||2¼ in.||1¾ in.||1¼ in.||1½ in.||3 in.|
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 SP125 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 SP95||BP-3|
|1 in.||¾ in.||½ 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.
Payment for asphaltic concrete by the square yard (m2) 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 (Mg). 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 operations engineer to obtain more reliable results. When payment is by the ton (Mg), estimate factors should be shown on the plans with a note "For Information Purposes Only." When payment is by the square yard [m2], 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.