THMACO Exceeds Expectations: I-95 Ultra-Thin Bonded AC Overlay

David T. Lee, P.E., Vice President, Virginia Asphalt Association

For many years, Virginia’s approach to pavement maintenance involved installing traditional asphalt concrete overlays or inlays and the use of Latex Modified Emulsion Treatments (LMET) at a point when the existing surface had nearly reached the end of its service life or beyond. In the late 2000s, Virginia recognized this method alone was not always the best approach and investigated several relatively new preservation treatments. One such method was Ultra-Thin Bonded AC Overlays, otherwise known as Thin Hot Mix Asphalt Concrete Overlays (THMACO). THMACO is a unique process that provides a highly functional wearing course with a thin layer of plant-produced asphalt concrete. At the time, the material was almost exclusively placed with a spray-bar paver. This unique paver-mounted tacking system provided an impervious layer with a substantial bond to the original surface. The surface was then “seated” with relatively few passes of conventional compaction equipment. The thin application, the integrated tacking/paving system, and the light compaction requirements resulted in a very rapid operation.

I-95 in Greensville

I-95 in Greensville County was completed in December of 2008 as part of a pavement preservation study. The THMACO project covered 17.14 miles in each direction, for a total of 68.56 lane miles, with the southern termini occurring at the North Carolina state line. This same pavement section was rehabilitated in 1998 and 1999 with asphalt concrete overlays ranging in thickness from 18.5 inches to 6.5 inches, with the thicker sections being placed on crack and seated jointed concrete pavement (JCP) and the thinner sections being placed on 8 inches of continuously reinforced concrete pavement (CRCP). In an effort to protect the significant investment made in this section of pavement, VDOT began to look at potential pavement preservation treatments in 2007 while the pavement was still in good overall condition.

VDOT, in conjunction with the Virginia Transportation Research Council (VTRC), performed a cost analysis comparing typical asphalt overlay alternatives. These alternatives included SM-9.5D, SMA 12.5, and THMACO. Additionally, VDOT evaluated using a latex modified emulsion treatment (i.e., microsurfacing) overlay, which was eventually installed on the adjoining sections for comparison purposes. For THMACO, the per-mile cost included a ¾-inch (85psy) overlay for two lanes (25-feet) and fog sealing for the 10-ft and 3-ft outside and inside shoulders, respectively. The SM 9.5D and SMA 12.5 estimates assume a 1 ½-inch application rate for 36-ft of roadway. One of the many benefits of THMACO is the thin application rate. This is evidenced by the need for wider coverage on the SM-9.5D and SMA 12.5 due to the thicker minimum application rates and the requirement to avoid unsafe edge drops. An alternative would have been a mill and inlay, but that would have required the additional cost for milling. This analysis concluded that the THMACO was the best alternative from an initial cost and equivalent uniform annual cost (EUAC) perspective. Based on the EUAC analysis, the THMACO only needed five years of service life to hit the breakeven point. At ten years, the THMACO system approached half the EUAC of the other alternatives. The final 2008 pavement preservation project generally consisted of asphalt patching, crack sealing, applying THMACO, fog sealing the shoulders, and replacing associated traffic engineering items at an awarded contract value of $5,336,453.45.

I-95 THMACO March 2009


So how has it performed? Over 12 years later, and the THMACO is still in service and performing very well. From VDOT’s Pavement Management System, the Critical Condition Index (CCI), as measured in 2020, ranges from 59 to 95, with an average rating of 84. VDOT categorizes pavements with a CCI of 90 and above as Excellent and a CCI of 70–89 as good. Only one small section less than one mile long on the southbound side has a rating barely in the poor category (59), while 5 of the 15 sections still have ratings in the excellent category.

As numerous research projects have shown, smooth pavements typically last longer. VDOT considers interstate ride quality to be excellent when the IRI is less than 60 in/mi. Despite the thin application, the placement contractors improved initial ride quality (i.e., 2008) by 18–35 percent over the original surface, with final surface IRI values ranging between 40 and 50 in/mi. Today, this smoothness continues with 2020 IRI values ranging from 46 to 76 in/mi and an average IRI of 56. Still in the excellent ride category!

I-95 THMACO March 2009 Near Rte. 631 Overpass

What Can We Learn?

While real estate is all about “location, location, location,” pavement preservation is all about “timing, timing, timing” with the proper application. This project application represented a proactive departure from VDOT’s traditional overlay or “mill and fill” approach by intervening with an effective preservation treatment just before end-of-life indicators had developed. By VDOT’s CCI criteria, the existing pavement in 2007 was still in good condition and did not need a more traditional pavement maintenance treatment. Working from a structurally sound existing platform facilitated the placement of an extremely safe and smooth new wearing course that would effectively seal and preserve a substantial in-place investment.

I-95 THMACO March 2021

Today we have methods that make THMACO even more attractive as a preservation treatment. Alternative materials, such as high-performance tacks and newer paver systems, allow placement of these treatments with or without a true spray bar paver. While THMACO will not completely replace the more traditional pavement preservation treatments (i.e., thin surface seals, crack sealing, etc.), clearly this project proves it is a great tool when used correctly. In a letter to VDOT management in 2009, Michael Sprinkel, Associate Director with the Virginia Transportation Research Council (VTRC), summarized it this way, “…We can see no reason to be anything but optimistic about the prospects for the THMACO system for preserving high-priority pavements in Virginia.”

Moving Forward

So why haven’t we see more THMACO as a preservation treatment in Virginia? One reason is the overwhelming systemic issues surrounding the lack of appropriate funding levels for pavement maintenance in the past. Today $400M to $450M is programmed to be spent each year on pavement maintenance paving. These projects are spread over 120,000 lane miles—the third-largest state-maintained system in the US. And while a significant annual investment, VDOT’s projections on long-term pavement condition expects at these funding levels 18 percent of the interstate, high-volume primary system, and high-volume secondary system (i.e., 3,500 vehicles per day and higher) to be in poor or very poor condition (i.e., CCI less than 60). Likewise, 25 percent of the lower-volume primary system and 40 percent of the lower-volume secondary system to be in poor or very poor condition. This lack of funding continues to drive pavement managers to stretch the limited available dollars as far as possible to meet annual performance targets. This compounds the push-pull of treating pavements in good condition (as demonstrated by the success on I-95), where preventive maintenance treatments are effective with those in fair or poor condition that needs immediate attention. This drives decision-making towards delaying or even potentially avoiding pavement preservation on pavements in good to excellent condition. While one can argue that the opposite should be true, the reality is that many decision-makers will tend to spread the available funds as thin as possible to avoid the inevitable massive failure.

Fortunately, this project and others show that VDOT is willing to pursue methods to appropriately spend the available funds in a prudent and effective manner. Tom Tate, District Materials Engineer with VDOT put it best when he said, “As one can see from our experience on I-95 in Greenville Co., THMACO is clearly a great pavement preservation tool that, when used appropriately, can prove to save money and avoid unnecessary and untimely maintenance activities.”

I-95 THMACO March 2021 Near Rte. 631 Overpass