SM-4.75 Mixtures: A little bit goes a long way, but does it last?

Hari Nair, Senior Research Scientist, VTRC | David Shiells, P.E., Materials Engineer-NOVA, VDOT | Bryan Smith, Asphalt Field Engineer, VDOT

This spring researchers at VTRC will wrap up a multi-year evaluation of VDOT’s newest class of dense-grade asphalt mixtures, the SM-4.75. It’s among the more promising tools that VDOT has for addressing the agency’s goal of 65% or more of secondary roads in fair or better condition. The SM-4.75 incorporates a 4.75 mm nominal maximum aggregate size (NMAS) along with a finer overall gradation that accommodates higher liquid asphalt content, which promotes durability. The fine gradation also permits for very thin application rates (0.75 to 1.0 inch) thereby reducing the quantity and cost of materials, as well as construction time. The thin layers and smooth surface texture makes the SM 4.75 mix well suited for subdivision streets and other low to moderate volume secondary roads (Figure 1). Depending on the expected service conditions, the mixture can also incorporate neat or polymermodified binders.

Despite the theoretical potential, there is limited published research on the field performance of the 4.75 mm dense-graded mix. Research conducted at NCAT has shown that the design of 4.75 mm NMAS mixtures is largely dependent on the characteristics of available fine aggregates. It also determined the mixture’s ability to resist cracking to be a function of both asphalt content and dust content. Research at the FHWA Turner Fairbank accelerated loading facility (ALF) showed that thin 4.75 mm NMAS overlays used as a preservation treatment can delay the age-related top-down cracking, but acknowledged a loss of that benefit once these thin layers become brittle with age.

Figure 1 – Smooth, fine surface texture of SM-4.75 (Sterling Blvd, NOVA)

For several years, researchers informally monitored trial installations of the SM-4.75 mixtures, which began circa 2009 and included the treatment at the Turner-Fairbank ALF. The research to be released this spring picks up as the use of the SM-4.75 became more “mainstream” by 2014 and will provide an evaluation of both laboratory and field performance. It includes laboratory test results and observations related to construction for seven production mixtures between 2014 and 2015. The evaluation of in-service performance attempts to cover representative installations dating back to the first trials (2009).


The laboratory assessment involved a conventional breakdown of asphalt material properties to include mix volumetric and gradation data, as well as a comprehensive binder recovery and grading. It also included a regiment of performance testing starting with Dynamic Modulus and including Repeated Load Permanent Deformation, the Asphalt Pavement Analyzer Rut test, mixture permeability, the Cantabro test for durability, and another suite of tests for cracking susceptibility. The cracking tests include the Semi-Circular Bend (SCB) test from Illinois, the Ideal Cracking (IDEAL CT) and Overlay Test (OT) from Texas, and the Nflex Factor Test from NCAT.

Given the anticipated primary use of SM-4.75 mixtures, good resistance to cracking is particularly important. Improved cracking resistance is also central to VDOT’s research supporting performance based or balanced mixture design. While that research considers several of the cracking tests mentioned previously, the test showing the most promise for production use is the IDEAL-CT test, which produces the CTindex. The results of testing with seven SM-4.75 mixtures are shown in Figure 2. It will be interesting to see if the two mixtures with a higher cracking resistance (i.e., higher CTindex) prove to be superior performers in service. Those mixtures do have higher asphalt and generally lower recycle content.


Since SM-4.75 mixtures are intended for very thin applications, it is important that they are placed on a structurally sound and uniform platform. Thin applications also cool more quickly, so higher air and surface temperatures also improve the chances for trouble-free placement. Figure 1 (from Sterling Boulevard in NOVA) is a good example of good preparation combined with good temperatures. In this instance, a very uniform and smooth SM-4.75 surface was made easier by first milling in a new 3” high modulus high binder (HMHB) base.

A good platform does not guarantee good construction, as evidenced by another SM-4.75 project in Prince William County (Figure 3) that happened later in the same season. In this instance, the contractor attempted to place a 0.75-inch lift of SM-4.75 over a new 2-inch IM 19.0. Cooler temperatures reduced workability and resulted in substantial segregation and raveling. This raveling worsened quickly revealing uneven application rates (down to 0.5” thickness).


Field performance of SM-4.75 mixtures was evaluated through site visits and using condition state data from VDOT’s Pavement Management System (PMS). The PMS chronicles a full life cycle for at least two of the earliest SM-4.75 trials – one from Route 3 in Culpeper and the other from Route 689 in Appomattox County. In both cases, the surface was replaced after approximately seven years of service, although it is not clear that this replacement was triggered by poor condition state. More recent installations, which include several subdivision applications, were surveyed visually by researchers and found to be in good condition (Figure 4). Transverse cracking is present in several cases, but likely reflective of underlying pavement cracks or utility structures.

Figure 3 (left) – SM-4.75 surface (“shadow” in center of both lanes: segregation and raveling) – October placement.
Figure 4 (right) – Subdivision application.


Readers should anticipate a full report on VDOT’s experience with SM-4.75 mixtures by early summer 2019. For now, although some mixtures indicate susceptibility to cracking, this potential can be mitigated through sufficient asphalt content and good platform preparation. Rutting, especially at these thin application rates, does not appear to be a significant risk, and neither does durability. Good surface preparation and mid-season (i.e., warm-weather) placement are also recommended to promote accurate and even thickness.


Jhony Habbouche joined VTRC in March 2019 after completing his graduate studies in Civil and Environmental Engineering Pavement Engineering and Science at University of Nevada Reno (UNR) (Master of Science and Doctorate of Philosophy).

Jhony’s graduate research topics and principal fields of interest range from advanced testing and characterization of engineered paving materials; mechanistic and mechanistic- empirical analyses for pavement structures; full-scale pavement structure testing, evaluation, and numerical modeling; evaluation and benefit-cost analysis of pavement preservation treatments; and computer applications in Civil Engineering.

At UNR, Jhony’s main focus was on the laboratory characterization, field performance, and structural capacity of highpolymer (HP) asphalt binders and mixtures. Jhony expects to continue working on HP paving materials at VTRC as well as other new topics of interest. Jhony will also be helping with and supporting ongoing topics at VTRC such as balanced (performancebased) mix designs, asphalt-treated cold recycled pavements, and implementation of Mechanistic-Empirical Pavement Design Guide (MEPDG) through AASHTOWare® Pavement ME software for the State of Virginia.