In my last article, I spoke in depth about the importance of interlayer bonding, why it is often overlooked and the factors that impact tack coat application. Now, I will recap why poor tack coat habits are so hard to break and share official findings by asphalt experts on tack coat best practices. Today, more than ever, departments and agencies are driving tremendous improvements in this area and exploring how better interlayer bonding can improve performance, promote sustainability and drive down the overall cost.
Poor Tack Coat Practices
Problem 1: The current low-bid process of road building encourages the use of inferior materials, such as tack coats that track, removing valuable bonding strength from the road.
As the graph above illustrates, tack coat is one of the lowest cost items used in roadway construction, however, it is not uncommon to see an inferior tack coat product selected solely based on price.
Problem 2: The road builder currently sees waiting for an emulsion to fully set as optional.
Thirty minutes should be standard for tack coat setting under normal conditions but virtually every commodity tack coat listed in ASTM D3628 (MS-1, HFMS-1, SS-1, SS-1H, CSS-1 and CSS-1H) is not capable of fully setting within that time frame under normal conditions. The ASTM standard is so far behind modern technology that it promotes failing options for the engineer, agency, road builder and taxpayer before the project even begins.
Problem 3: Lack of QA enforcement.
Since the removal and spreading of tack coat can create a mess, many times, tack application is either avoided altogether or applied minimally (outside of wheel path locations). Considering the wheel path is the most critical location for tensile and shear stresses according to this NCHRP Report, this makes poor tack coat practices an engineering issue, rather than an aesthetic one.
Problem 4: Flexible pavements are often over designed to compensate for poor bonding.
This NCAT study sought to calibrate the strength values under the belief that too much conservatism is built into the original values. Of the 14 sections that were analyzed in the study, only two had strength values at or below those recommended by AASHTO. These two sections were trenched and the attributed cause of low strength was debonding in both cases! The study concluded that for most AC pavements (>5 inches thickness), AASHTO’s conservative approach results in 18.5 percent of surplus thickness added to the pavement. Think of the cost associated with this additional thickness over the years for every pavement. Improving tack coat application accomplishes the same goal at a tiny fraction of the cost.
What the Experts Say
Asphalt Institute (AI) has been educating its members on proper tack coat usage and practices that improve pavement bonding since at least 2015. Earlier this year, AI’s Regional Engineer, Dave Johnson, P.E., joined us for a webinar on the Role of Bond Strength in Road Performance to further this position.
Federal Highway Administration (FHWA) launched their own Tech Brief in conjunction with AI to reinforce AI’s message and bring awareness to the vital role that tack coats play in pavement construction.
National Cooperative Highway Research Program (NCHRP) released Report 712: Optimization of Tack Coat for HMA Placement, which presents proposed test methods for measuring the quality and performance characteristics of tack coat in the laboratory and the field, and includes a training manual presenting proposed construction and testing procedures for tack coat materials. One of the most valuable (and alarming) findings in this report is the impact of reduced coverage on bond strength, a common measure of tack coat quality. The report revealed that reducing tack coat coverage from 100 to 50 percent of the application area resulted in a 50 to 75 percent loss of bond strength! In fact, the same study showed that doubling the tack coat (in thickness) doesn’t really improve bond strength when incomplete coverage or damage/removal of the tack coat occurs during the construction process. In other words, partial coverage or removal of the tack coat prior to placing the AC layer on top of the tack coat cannot be remedied by overcompensating with a thicker layer of tack coat.
The findings become even more compelling when combined with other studies — such as this presentation by Asphalt Institute and this report published by the Institution of Highways and Transportation — proving that a relatively minor bond loss (10 to 30 percent) reduces fatigue life by as much as 50 to 70 percent!
All of this demonstrates that there is widespread knowledge and agreement of a major tack coat problem, and an ongoing effort to address it.
Model Agencies Making Change
While there is still much room for improvement regarding tack coat in our industry, there are a few states and cities that have prioritized interlayer bonding as a best practice.
Florida D.O.T. was one of the first to recognize the importance of interlayer bonding. That early focus eventually led to a mandate for non-tracking tack coat usage, product specification and vetting system designed to prevent adhesive removal and extend the life of pavement overlays and structures.
Texas D.O.T. has prioritized interlayer bonding through its TXDOT’s Top 10 lists. The Texas Asphalt Pavement Association (TXAPA) has also hosted non-tracking tack coat workshops in the past. Additionally, Texas A&M Transportation Institute (TTI) performed an extensive investigation designed to screen for inferior tack coat products – products that have no chance of resisting adhesive removal. The findings from the investigation led to a pre-approved list of products known as Tracking Resistant Asphalt Interlayer (TRAIL).
In Nashville, a 2010 flood washed away many of the city’s roadway surfaces due to delamination between layers, and a campaign to improve bonding was instituted. It was determined that all future roadways would be built with a superior bond, which means zero tolerance for adhesive removal. Even today, non-tracking tack performance must be demonstrated to the City of Nashville, or the bonding agent is not allowed.
Tack Coat is Key to Road Sustainability
As the asphalt industry continues to prioritize sustainability as a best practice, finding ways to make roads last longer will be vital. A longer pavement life will inevitably reduce the taxpayer burden associated with premature failure, including maintenance costs, wear and tear on vehicles and delays due to closed lanes. By focusing on the quality of tack coats to ensure a better interlayer bond, we can reduce common problems such as slipping, shoving and cracking, ultimately resulting in a stronger, longer-lasting pavement. It’s time we, as an industry, accept how critical interlayer bonding is to our roads and do something about it — together.