Pavement That Performs

Engineering asphalt that aces the test

By Emily Adams

As the world has changed, so have the performance demands on our roadways.

Steel belted radial tires, which became standard in the mid-1970s, concentrated vehicle weight into a smaller contact patch. Allowable freight weights increased, and population growth brought more stop-and-go traffic and time spent idling at red lights.

Across the country, and in Texas specifically, that added strain surfaced as a sharp rise in rutting and stripping, which led directly to the creation of the Strategic Highway Research Program, or SHRP.

Part I of this series, published in the Summer 2026 issue of Texas Asphalt, looked at what SHRP and decades of follow-up work changed about the materials themselves. This second installment covers the engineering decisions built on top of them, starting with how mixture performance is measured, moving into how entire pavement structures are designed for the toughest jobs in the state, and ending with how that same standard of quality is reaching the local roads Texans drive on every day.

Much of that story starts with a shift away from something called volumetric mix design, which checks the proportions of a mixture—how much binder, how much aggregate, and how much space is left for air, all expressed as a percentage of the whole. If those proportions fall within an approved range, the mixture passes, regardless of how it performs once it is built into a road.

Measuring Mixture Performance

The shift toward performance-based thinking for asphalt binder is now happening at the mixture level.

Volumetric design was never quite as simple as a fixed recipe. Dr. Amy Epps Martin, a professor at Texas A&M University who has led much of the state’s Balanced Mix Design work, explained that even the older Marshall and Hveem systems, which TxDOT once used in a modified form, tried to balance two competing needs.

Engineers would add as much binder as a mixture could handle for good cracking resistance and durability, right up until a rutting test showed there was too much. When the Superpave system was born from SHRP research, an early attempt to build direct performance testing and distress prediction into mix design proved challenging for routine practice. So for decades, Texas and most other states have leaned on volumetric checks alone.

That approach has limits. A mixture can pass every volumetric check and still leave open questions about how it will perform in the field.

“These checks don’t consider the quality of the materials,” Epps Martin said. “Only the quantities.”

That gap matters more today because Texas mixtures increasingly include reclaimed asphalt pavement, known as RAP. Reusing that material is cost effective and sustainable, but RAP stockpiles vary widely in quality and consistency—a passing volumetric result does not guarantee a mixture with recycled material will resist cracking as it ages.

Balanced Mix Design adds direct mixture performance testing on top of thevolumetric checks, verifying how well a mixture resists cracking and rutting before it reaches the road.

When a mixture passes volumetrics but fails that balanced performance check (often because it is carrying more RAP than the unique mixture components can support) engineers have a few tools available. They can adjust the aggregate gradation to make room for more binder, use a softer virgin binder to offset stiff , aged binder already present in the RAP, and more accurately account for how much of that old binder is still active rather than assuming all of it is available.

Texas has moved toward this deliberately. An early special specification piloted the approach on select test sections starting in 2019, and a newer special specification tied to the current TxDOT spec book now requires performance verification outright.

Most states doing this work follow an approach that combines volumetric checks with performance verification, defined under a shared American Association of State Highway and Transportation Officials (AASHTO) framework with four approaches. A newer national effort is organizing that framework into three tiers, letting states advance at their own pace for each type of pavement distress rather than moving in lockstep.

One unsolved challenge is RAP stockpile quality. Epps Martin was candid that the industry still lacks a simple, reliable way to test a RAP pile’s age and consistency before it goes into a mixture.

TxDOT recently issued new guidance for managing RAP stockpiles as one step toward closing that gap. The more confidently contractors can demonstrate consistent, well-managed stockpiles, the more comfortable agencies become specifying higher RAP contents.

There is also a newer wrinkle worth watching. As RAP use grows, some of what gets recycled today already contains previously recycled asphalt, a challenge the industry has started calling re-recycling.

Epps Martin does not want engineers or contractors to see any of this as intimidating.

“You just need knowledge and education and to jump in and try it,” she said. Texas is working toward a permanent specification that could allow mixtures with up to 35 percent RAP, provided performance is adequate based on laboratory testing. It’s a shift she considers one of the clearest wins balanced mix designs offer the industry.

Engineering in Action

From the raw ingredients, the next logical step is ensuring that engineering plays out where the stakes are highest.

Some of the most demanding pavement conditions in Texas sit wherever heavy trucks concentrate: energy sector routes, border crossings, freight corridors, and intersections.

Chris Henry, deputy director of TxDOT’s maintenance division, helped lead a new Heavy Duty Pavement Design Guide after portable weigh-in-motion sensors measuring actual truck weights and traffic volumes turned up loads far higher than anyone had assumed. The guide draws on the same principle behind perpetual pavement design: keeping strain at the bottom of the asphalt layer and the top of the subgrade low enough that the structure only needs resurfacing over time, never a full rebuild.

That design work now runs through FPS 23, the newest version of TxDOT’s pavement design software. Ruben Carrasco, TxDOT’s branch manager for pavement analysis and design, worked alongside Henry on the guide, and points to a real gap FPS 23 closes.

Older modeling leaned on standard assumptions that did not always reflect a given district’s needs, producing thickness numbers that worked out mathematically but, in some cases, did not match specific conditions.

FPS 23 includes a companion check called TxME, short for Texas Mechanistic-Empirical, which models how a structure will perform based on materials, traffic, and local climatic conditions.

That same shift toward purpose-built design now reaches intersections, which carry traffic differently than a straight highway. Charles Gurganus, pavement management program manager at the Texas A&M Transportation Institute’s materials and pavements division, explained that a highway sees traffic pass over quickly and repeatedly, while an intersection sees vehicles slow, sit, and accelerate again, often in the same spot for years.

That sustained load, combined with heat radiating down from idling vehicles, calls for its own design approach. TxDOT’s working group on intersections is treating them as their own category, engineered on their own terms rather than as an afterthought to the surrounding roadway.

Designing for 15 Years from Now

This is also where the conversation circles back to a tension in materials design: resisting rutting today versus resisting cracking years down the road.

Darren Hazlett with the University of Texas Center for Transportation Research described binder selection as a bet against a range of future weather, not a single expected temperature.

“Climate is a distribution,” he said, and choosing a binder grade means choosing how far out into that range an engineer is willing to be exposed. He compared it to building in a fl oodplain: design for the average condition, and a pavement sits in something like a two-year floodplain, fine most years and badly exposed in the ones that matter. Pushing the reliability higher starts to resemble a 50- or 100-year floodplain instead.

Miss too high on that range and the pavement leans toward rutting. Miss too low and it leans toward cracking as the binder ages and oxidizes over the years.

This is the sweet spot an engineer must find—a pavement that performs as intended today and that still maintains most of that performance after 15 years. It is part of why high polymer binders, often called HPG binders, are drawing interest on heavy duty projects. A more elastic, resilient binder protects against rutting now without giving up as much resistance to cracking later.

Bringing Consistancy to Local Roads

That same standard of quality does not stop at the state highway system. It reaches the roads most Texans drive on every day.

Cities and counties maintain roughly 70 percent of Texas’s centerline miles— more than TxDOT—yet local specifications vary from one city or county to the next. Many still reference a generic or modified TxDOT specification decades out of date.

That gap costs local agencies real value for their paving dollars and can make new hot mix construction a harder sell against stopgap strategies like seal coating, even when a properly built hot mix road is the better long-term investment.

TXAPA built Specification 341M, now standing for “Modified for Local Agencies,” to close that gap. Unlike TxDOT’s single statewide standard, local agencies have historically had no shared benchmark, so local road quality across Texas varies.

Larry Peirce, TXAPA’s lead voice on local agency issues, frames the value of unifying that in terms of certainty. A contractor working across several cities and counties in the same week may need to adjust to a different specification at every stop, raising the chance of a mismatched mix or a costly production error.

“Uncertainty equals risk, and risk equals cost,” Peirce said. A single, modern standard also means producers can deliver a more consistent mix, and a workforce can operate the same way no matter where a job lands.

341M also gives local agencies a tested framework for using recycled material with confidence, building on the same stockpile guidance now shaping practice statewide.

A Resource Worth Knowing About

Alongside 341M, PAVEXpress is a free, web-based pavement design program built for local agencies, engineers, and architects who need a reliable way to determine the required pavement thickness for a given roadway.

It builds technically sound structural designs based on widely accepted AASHTO standards—without requiring TxDOT’s full design software or years of specialized experience. For a city or county engineer trying to right size a project, it is a practical way to put the same performance-based thinking behind BMD, FPS 23, and 341M to work on a local road.