VDOT Mixes & Guidelines

VDOT Asphalt Specifications & Mix Design History

The Virginia Department of Transportation (VDOT) specifications, standards, and test methods have been the standard in Virginia for more than 50 years. During that time, VDOT has changed its nomenclature for asphalt mixes several times, typically following a major rewrite of its asphalt specifications and mix design criteria. The last change came in 2000 when VDOT adopted the Superpave Performance Grading System and Superpave Mix Design System. Superpave mixes are the standard today; however, many private and public entities still specify earlier versions of VDOT specifications and the corresponding asphalt mixes.

Common VDOT Dense-Graded Asphalt Mixes

The table shows the most commonly used/specified VDOT dense graded asphalt mixes under current specifications and their equivalent under earlier versions of VDOT specifications.

* – VDOT specifications no longer include the BM-37.5 mix

Current VDOT Mix Designations

BM-37.5*

BM-25.0

IM-19.0

SM-19.0

SM-12.5

SM-9.5

SM-9.0

Marshall VDOT Mix designs

BM-3

BM-2

IM-1

SM-3

SM-2

SM-1

Pre-1987 VDOT Mix Designation

–

S3, S4

Normal Maximum Aggregate Size

37.5 mm (1.5 inch)

25.0 mm (1 inch)

19.0 mm (3/4 inch)

12.5 mm (1/2 inch)

9.5 mm (3/8 inch)

Most Commonly Specified/Used Asphalt Mixes

VDOT specifications include a wide range of asphalt mixes for various applications. These mixes include dense graded, open graded and gap grade mix technologies as well as a variety of surface mixes, intermediate mixes, base mixes and special application mixes. Specific information and guidance on using each of these mixes can be found by clicking the link at the end of this section.

However, the most commonly used mixes for all types of asphalt pavements are dense graded mixes. Also, for each mix use type (surface, intermediate & base) there is one particular mix that is specified for over 90% of all applications. Listed below are excerpts from the VDOT Materials Division’s Manual of Instructions giving the descriptions and binder requirements of these “standard” VDOT asphalt mixes.

VDOT “Standard” Surface Mix

Surface mixes serve as both functional and structural layers of pavement. Surface mixes are directly exposed to traffic and the environment. They must provide a smooth, stable, safe (i.e., skid-resistant) riding surface and promote drainage of surface water. In addition, they prevent excessive water from entering the underlying HMA layers, bases, and subgrade. The surface layer typically contains the highest-quality materials. In most instances, only one surface mix lift will be placed on a project.

SM-9.5A This mix is a ‘fine’ to ‘medium’ (3/8 inch (9.5 mm) nominal maximum aggregate size) surface mix generally placed at 1 ½ inches (40 mm) in thickness. The ‘A’ designation corresponds to the Performance Graded (PG) asphalt binder of PG 64S-22 required in the mix. SM-9.5A mixes usually result in low water permeability values. This mix tends to be less susceptible to segregation than the SM-12.5 mix. SM-9.5A surface mixes can be considered the desired surface mix and are recommended for most final surface applications.

VDOT “Standard” Intermediate Mix

The intermediate mix, sometimes called the binder course, consists of one or more lifts of structural asphalt concrete placed below the surface layer. Its purpose is to distribute traffic loads so that stresses transmitted to the pavement foundation do not cause permanent deformation of that layer.

Additionally, it facilitates the construction of the surface layer. Designed with larger aggregates, the intermediate layer is intended to resist rutting and intercept top-down fatigue cracking. In most instances, only one intermediate mix lift will be used on a project.

IM-19A – This mix is a ‘coarse’ (3/4 inch (19.0 mm) nominal maximum aggregate size) mix generally placed at 2 inches (50 mm) in thickness. This mix can handle public traffic during construction for an extended period of time and allows for the later application of a surface mix to provide a final wearing surface. The ‘A’ designation corresponds to the Performance Graded (PG) asphalt binder of PG 64-22 required in the mix.

VDOT “Standard” Base Mix

As the name implies, the base mix is the base asphalt layer for the pavement structure. Its major function is to provide the principal support of the pavement structure. The base mix uses the largest aggregate particles to resist rutting and bottom-up fatigue cracking. Unlike surface mixes, more than one lift of base mix may be placed on a project to obtain the designed base layer thickness.

BM-25.0A – This mix is a 1-inch (25.0 mm) nominal maximum aggregate-size mix, generally placed at a thickness of 3 inches (75 mm) or greater. Depending on the aggregate gradations and placement procedures, this base usually does not require placing an intermediate course to provide a smooth wearing surface. Public traffic should not be permitted on this material for extended periods of time without restrictions. The ‘A’ designation corresponds to the Performance Graded (PG) asphalt binder of PG 64S-22 required in the mix.

VDOT Guidelines for the Specification and Use of Asphalt Mixes

VDOT has many different mix types, disciplines, and PG binder grades in its specifications, enabling it to tailor asphalt mixes for a wide range of applications, traffic loads, and performance requirements. Guidance for the asphalt industry on mix types, binder grades, application rates, and functions has been developed and is included in Section 605 of the VDOT Materials Division’s Manual of Instructions. The following is the introduction from the manual outlining the purpose of the section. The complete Section 605 can be accessed by clicking on the link at the bottom of the page.

The guidelines provided herein are intended to help the user recommend mix types for asphalt overlays of flexible and rigid pavement and for new construction, based on expected traffic and environmental conditions. These guidelines should be used as part of, or in conjunction with, an engineering analysis of the pavement section. These guidelines are not intended to address pavement distress mechanisms, structural inadequacy of the pavement, existing pavement defects or other types of pavement deficiencies. It is the user’s responsibility to conduct an analysis/evaluation of existing or expected pavement conditions before using this guide. Failure to do so could significantly affect the performance and service life of the selected materials and mixes.

These guidelines are applicable to VDOT projects. While the guidelines could be used for non-VDOT work under similar conditions, experience and engineering judgment should be exercised in such applications.

These guidelines indicate the general highway conditions under which each mix should be used. Generally, a single mix type is used for all lanes in a single direction of a roadway. The asphalt binder type ESAL (Equivalent Single Axle load) range is based on an expected service life and is used in conjunction with the mix type’s nominal maximum aggregate size in this guide. Traffic speed, vehicle types, and volume should also be considered when selecting a mix type. These considerations may warrant using a stiffer binder. Experience and judgment should be used to select the appropriate mixes. Each District may implement a simple guide chart to eliminate those.

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