Hot Mix Asphalt Frequently Asked Questions
Introduction | Asphalt Basics | Mix Design | Structural Design | Glossary | FAQs | References
General
Q1: Where can I learn more about the design of low volume roads?
A1: The LRRB Report 2002-17REV, Best Practices for the Design and Construction of Low Volume Roads, is a great resource of information pertaining to this subject.
Q2: Who can I contact if I have questions?
A2: Your local State Aid District Office can help; in addition, you can contact the Mn/DOT Office of Bituminous Engineering or the Mn/DOT Pavement Design Unit.
Design Type Selection
Q3: What bituminous specification (2331, 2340, 2350, 2360, or 2360 Combined Gyratory/Marshall) is most appropriate for my project?
A3: If the project contains state/federal funds, Specification 2360 (Combined Gyratory/Marshall) is required (see sidebar). If the project does not contain state/federal funds, the designer has the decision to choose an appropriate specification, however, Specification 2360 (Combined Gyratory/Marshall) is recommended. For additional information, see section 5.4.1.3 of the LRRB Report 2002-17REV.
Q4: Which design type (Marshall or Gyratory) is more economical for my project?
A4: Since gyratory design was introduced in 1998, price differentials between Marshall and gyratory design have become, in most cases, negligible. Mn/DOT’s goal (and the national goal) is to move toward gyratory design mixtures only. However, at the present time the choice is with the designer. The designer should be aware of regional mix design usage or construction activities. Some situations may arise where only gyratory mixtures are being produced and Marshall mixtures are not a reasonable option or vice-versa.
Mix Selection
Q5: What type of bituminous mixture specification is most appropriate for use on a parking lot?
A5: The LV or MV Marshall design mixes are good choices for driveway or parking lot paving. If gyratory design is utilized, Traffic Level 2 or 3 would be the appropriate choice. Asphalt binder grade PG 64-22 utilized with these mixtures provides a durable surface.
Q6: Should I allow the use of Recycled Asphalt Pavement (RAP) in my mixtures?
A6: Performance data shows mixtures containing RAP perform just as well as those without RAP. The decision to use RAP also has economical and environmental benefits. Refer to section 5.4 of the LRRB Report 2002-17REV for more information.
Q7: What maximum aggregate size should I specify for my project?
A7: Asphalt lift thickness may dictate aggregate size. Minimum paving thickness is two times the maximum aggregate size. A 3/4” aggregate is the most versatile aggregate size and most commonly specified aggregate (both in wear and non-wearing courses). The versatility provided by specifying 3/4” aggregate is apparent when RAP is utilized in a mixture. Oftentimes, RAP will contain 3/4” aggregate. If a 1/2” maximum size aggregate is specified by contract and RAP, which contains 3/4” aggregate, is utilized in the mixture there will be a violation of the maximum aggregate size. A 1/2” aggregate will provide a mixture that is finer and tighter in appearance than a 3/4” aggregate and may be more aesthetically appealing than 3/4” aggregate.
Q8: Should I bump up the mixture type (i.e. from LV to MV) if my project contains slow moving or stopping traffic?
A8: If the project contains considerable truck traffic that will be stopping or slow moving at points along the project, it may be beneficial to bump up the mixture type (See PG binder bumping below). However, the designer must consider that it is not economically feasible to require specific mixture types only in areas that contain stopping trucks, in which case, the designer should bump the mixture type for the entire project.
Q9: Should I bump up the PG binder grade (i.e. from PG 58-xx to PG 64-xx) if my project contains slow moving or stopping traffic or heavy truck traffic?
A9: If the project contains considerable truck traffic that will be stopping or slow moving at points along the project, it may be beneficial to bump up the PG binder grade. However, the designer must consider that it is not economically feasible to require specific PG binder grades only in areas that contain stopping trucks, in which case, the designer should bump the mixture type for the entire project.
Q10: How do I select and specify a bituminous mixture?
A10: The designer should see the Bituminous Office web page for design criteria. Mixture selection is based on ESAL’s (equivalent single axle loads) or AADT.
Q11: How do I specify a bituminous mixture for trails and bikeways?
A11: The designer should refer to Mn/Dot’s guidelines for the design and specification of bituminous trails and bikeways or other industry reference, such as MAPA Design Manual.
Binder Selection
Q12: How do I specify bituminous binder material for new pavement sections versus overlay sections?
A12: See Mn/DOT’s PG Binder Guidelines.
Density Decision
Q13: Should I specify maximum density or ordinary compaction on my project?
A13: If the designer is using the 2360 Combined (Gyratory/Marshall) specification, maximum density will typically be the default densification method (see specification 2360.6). If ordinary compaction is desired, this should be stated clearly in the project special provisions. The project engineer must be aware that ordinary compaction methods require the job site inspector to develop a minimum number of roller passes required of the contractor to obtain ordinary compaction. Ordinary compaction requires consistent job site inspection while the contractor’s paving operation is in progress.
Q14: Should I include in my project both the incentives and disincentives portion provided for in the maximum density specifications, or can one or both of them be overridden by the special provisions?
A14: The designer should include both the incentives as well as the disincentives provided to the contractor by the maximum density specification. Visit the Mn/DOT Bituminous Office Web site, which contains a spreadsheet for determining random locations of core samples.
Ride Decision
Q15: Should I specify profilograph or straightedge for determining ride quality on my project?
A15: The designer should review Mn/DOT 2360/2350 to determine testing exclusions that may be applicable to further consideration for the contract requirement.
Q16: Should I include in my project both the incentives and disincentives portion provided for in the ride specification, or can one or both of them be overridden by the special provisions?
A16: The designer should include both the incentives as well as the disincentives provided to the contractor by the ride specification.
Contract Special Provisions
Q17: What should be included for bituminous pavement materials in the contract special provisions?
A17: Anything that differs from the standard specifications must be included in the special provisions. (see Standard Specifications Section 1504 Coordination of Plans and Specifications).