The New Mexico Department of Transportation (NMDOT) specifications, standards and test methods have been the standard in New Mexico for more than 50 years. During that time NMDOT has changed their nomenclature for asphalt mixes several times, typically following a major rewrite of their asphalt specifications and mix design criteria. The last change came in 2000 when NMDOT adopted the Superpave Performance Grading System and Superpave Mix Design System. Superpave mixes are the standard today; however, there are still many private and public entities specifying earlier versions of NMDOT specifications and corresponding asphalt mixes.
The table below shows the most commonly used/specified NMDOT dense graded asphalt mixes under current specifications and their equivalent under earlier versions of NMDOT specifications.
|Current NMDOT Mix Designations||Marshall NMDOT Mix Designs (1987-2000)||Pre-1987 NMDOT Mix Designation||Nominal Maximum Aggregate Size|
|SM-9.0||SM-1||S3, S4||9.5 mm (3/8 inch)|
|SM-9.5||SM-2||S5||9.5 mm (3/8 inch)|
|SM-12.5||SM-2||12.5 mm (1/2 inch)|
|SM-19.0||SM-3||19.0 mm (3/4 inch)|
|IM-19.0||IM-1||19.0 mm (3/4 inch)|
|BM-25.0||BM-2||25.0 mm (1 inch)|
|BM-37.5*||BM-3||37.5 mm (1.5 inch)|
* – NMDOT specifications no longer include the BM-37.5 mix
Most Commonly Specified/Used Asphalt Mixes
NMDOT specifications include a wide range of asphalt mixes for a wide range of 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 of the use of each of these mixes can be found by clicking on 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 exerts from NMDOT Materials Division’s Manual of Instructions giving the descriptions and binder requirements of these “standard” NMDOT asphalt mixes.
NMDOT “Standard” Surface Mix
Surface mixes serve as both functional and structural layers of the pavement structure. Surface mixes are directly exposed to traffic and the environment. They must provide a smooth, stable, safe (i.e. skid resistance) riding surface, and promote surface water drainage. In addition, they serve to prevent the entrance of excessive quantities of water into the underlying HMA layers, bases and subgrade. The surface layer normally 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) thickness. The ‘A’ designation corresponds to the Performance Graded (PG) asphalt binder of PG 64-22 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.
NMDOT “Standard” Intermediate Mix
The intermediate mix, sometimes called 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 will not result in permanent deformation of that layer.
Additionally, it facilitates the construction of the surface layer. Designed with larger aggregates, the intermediate layer is intended to provide resistance to rutting and to 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) thickness. This mix can handle public traffic during construction for an extended period of time and allows for 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.
NMDOT “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 provide resistance to rutting and to 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 3 inches (75 mm) or greater thickness. Depending on the aggregate gradations and placement procedures, this base does not usually require placement of an intermediate course to provide a platform for placement of 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 64-22 required in the mix.
NMDOT Guidelines for the Specification and Use of Asphalt Mixes
NMDOT has many different mix types, disciplines and PG binder grades in their specifications that enable them to tailor asphalt mixes for a wide range of applications, traffic loading and functionality. Working with the asphalt industry description and guidance on mix types, binder grades, application rates, and function have been developed and are included in Section 605 of the NMDOT 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.
Current Section 605 – ASPHALT CONCRETE MIX SELECTION GUIDELINES
The guidelines provided herein are intended to aid the user in recommending mix types for asphalt overlays of flexible and rigid pavement, and new construction based on specific traffic and environmental conditions expected. 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 responsibility of the user to conduct an analysis/evaluation of the existing or expected pavement conditions prior to using this guide. Failure to do so could significantly affect the performance and service life of the materials and mixes selected.
These guidelines are applicable to NMDOT projects. While the guidelines could be used for non-NMDOT work with similar conditions, experience and engineering judgment should be exercised for such application.
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 in the selection of a mix type. These considerations may warrant the use of a stiffer binder. Experience and judgment should be used in selecting the appropriate mixes to be used. Each District may implement a simple guide chart to eliminate those.