How to Build an Inverted Pavement
BY Sandy Lender
With regional exceptions, pavement systems are typically built with a crushed aggregate subbase atop the prepared and compacted soil layers, followed by base, intermediate and surface pavement layers of varying thicknesses and designs. These systems transfer traffic load to the subgrade. For the purposes of this article, I’ll refer to this typical design as “conventional” and I’ll not veer off course from that basic structure for that definition.
Another pavement system garnering interest in transportation research circles—despite its conceptualization over half a century ago—is the inverted pavement design (IPD). For the purposes of this article and audience, we’ll discuss the inverted asphalt pavement (IAP) concept. This type of system distributes loads laterally and has been shown to offer some carbon footprint “savings” for agencies selecting it, thus it’s on the radar.
The most robust definition I’ve seen for IPD comes from the presentation of Professor Wynand Steyn of the University of Pretoria’s faculty of engineering, built environment and information technology. It’s logical to see his presentation cited often because IPD has history in South Africa. In his presentation titled “Inverted Pavements,” Steyn spelled out:
“An inverted pavement is when the base layer is a high quality granular layer, and the subbase a cement stabilized layer. A thin asphalt layer or seal provides the surfacing. The term “inverted” is used because the strength of the pavement does not decrease with pavement depth, because of the stiff cemented layer. This means that the pavement is not in balance. The idea behind an inverted pavement is that the cemented layer provides an anvil upon which the granular base can be well compacted. This achieves a high quality, dense base. Over time, the cemented layer weakens to an equivalent granular state. The pavement is then in balance.”
As Shane Buchanan of Vulcan Materials stated in his 2010 presentation, “Inverted Pavement Systems,” the “[v]alue of the base is best captured when it is placed near the surface where stresses are the greatest.” The Vulcan team adequately summarized Steyn’s longer definition with: “Inverted pavement moves base to the top where it performs more efficiently.” You can read a writeup of the presentation at https://www.vulcaninnovations.com/public/pdf/4-Inverted-Pavement-Systems.pdf.
More recently, a team of researchers in China prepared a report—published August 2023—titled “Preliminary optimization design of inverted asphalt pavement structure using response surface method” for Construction and Building Materials, Vol. 390, saying IAP uses an “unbound aggregate base (UAB) as a sandwich between a tougher asphalt concrete (AC) layer and a cement-treated base (CTB).”
“The idea behind an inverted pavement is that the cemented layer provides an anvil upon which the granular base can be well compacted.”—Professor Wynand Steyn
The AC is a point to watch, depending on climate, country or region, traffic loading, etc. As with all pavement designs, the mix design and necessary lift thickness are dependent on regional needs. The report at https://www.sciencedirect.com/science/article/pii/S0950061823014952 gives detailed explanation on that concept. Presenters for the Transportation Research Board (TRB) webinar titled “Implementation of Inverted Pavements” Sept. 18, 2023, also spoke to the material selection and quality control of the AC layer and UAB layer to ensure success of the system.
The point, in short, is to design and build the pavement so the materials in each layer don’t experience too much stress—thus don’t fail in their performance(s)—as the pavement changes over its lifetime. What contractors reading AsphaltPro Magazine will be most interested in is how to prepare and compact IAP for bonus-worthy end results. While the paving portion of the operation looks like operations-as-usual for each individual layer, compacting the aggregate layer takes some finesse. It relies on understanding the system you’re building and adhering to best practices throughout the process.
The Layers of IAP
Let’s start the discussion of how to build and compact IAP the way most presenters do—by describing the layers that make up IAP. It’s supported by a prepared and compacted soil subbase.
In the United States, IAP typically begins with 4 to 8 inches of cement-treated base atop the compacted soil. The amount of cement is typically 3 to 5% and this amount is standard across the presentations I’ve read or attended. Imad L. Al-Qadi of the University of Illinois added, during the “Implementation of Inverted Pavements” webinar mentioned above, that this layer could be up to 16 inches, and that’s dependent on region, climate, traffic conditions, and so on.
It relies on understanding the system you’re building and adhering to best practices throughout the process.
The next layer is the 4 to 6 inches of high-quality crushed aggregate. The aggregate quality and angularity play a role in the system’s success. Its moisture content, stockpile management and distance from source-to-site play a role in the carbon footprint or ESG score for the system. Steyn, among others, pointed out the use of single-crushed, durable, hard parent rock is ideal for this layer. Presenters focus on the quality of aggregate when discussing this layer, reminding agencies and contractors to consider availability of high-quality material when deciding on the use of an IAP system at all.
The surface layer is anywhere from 1 ½ to 3 ¾ inches of asphalt pavement, as gathered from multiple sources. In the United States, this could be up to 6 inches total. The asphalt surface not only provides a smooth ride for the traveling public, but also controls water infiltration, Al-Qadi acknowledged in his presentation. He also noted the South African studies had additional success when using crack seals, waterproof seals and/or patches on the asphalt layer—whatever it takes to maintain an impervious surface.
Build With a Slush Factor
When compacting the unbound aggregate layer, Vulcan’s Buchanan stated in his report that you’re typically going for a minimum of 100% modified Proctor density in a lift of 6 to 8 inches thickness. He explained in the report that the CTB layer between the prepared subgrade and UAB sets up to be a strong, rigid foundation against which the UAB gets compacted.
“This assists the contractor in obtaining the required density in the UAB layer,” Buchanan wrote. “Generally, a minimum UAB layer density of 86 percent of solid density is specified, which equates to 100 to 105 percent of modified Proctor (AASHTO T180) density. The exact relationship will vary based on the mineralogy, surface texture, and grading of the base material being used.”
Al-Qadi confirmed this information, telling the TRB audience to achieve at least 85% of solid density. He shared his advice for meeting the goal, which is to perform the first one to two passes on the aggregate layer with a heavy, steel-wheel roller in static mode to achieve particle interlock. Then perform the next two passes in low vibratory mode. Too much vibration, he said, will destabilize the aggregate.
And that ruins the whole point.
During construction of IAP, the crew will want to compact the UAB until excess fines rise to the top and are expelled as slush. The best practice is to then broom and spread those initial fines into deficient areas.
Al-Qadi offered excellent advice for achieving compaction of this layer:
- Continue to roll the UAB layer until you see no more air bubbles escape during slushing.
- Continue to roll until the expelled water is substantially cleared.
- Continue to roll until you can see the well-knit particle interlock through the surface water.
- Continue to roll until the road surface does not heave under the heavy roller drum.
- Continue to roll until you can perform visual and “ping” tests to your satisfaction.
You don’t want to leave free water atop the UAB layer. Moisture left inside this system is not your friend. The specifying agency should tell you what the optimum moisture content (OMC) is. When you’ve completed rolling, you will broom and clean the surface.
Paving atop the UAB is executed as normal after the surface is clean and dry. In IAP, the “thin” asphalt lift serves as a membrane to keep water out of the aggregate base below and to reduce tension. Because the asphalt layer is the cap on the system to prevent water intrusion, compaction is vital once again.
Erol Tutumluer of the University of Illinois explained during the TRB presentation that the increased deterioration in IAP compared to the conventional/control pavement in an I-25 test project in New Mexico could be blamed on moisture ingress. The aggregate base may have been too wet to begin with. This lesson proves we want to get the moisture out of the UAB but we want to do so without raising the carbon footprint of the construction process. One of the attractions of IAP for agencies is its potential for lowering environmental impact scores but that only holds true when best practices are maintained throughout the production and paving chain and when the system is employed where it makes sense. Both Al-Qadi and Tutumluer reminded the TRB audience that regions experiencing high rainfall and snowmelt might not be ideal for installing an inverted pavement while arid, warmer regions may be perfect candidates to try it out. Again, moisture and dryness are some of the elements to pay attention to, along with the quality of aggregate readily available to keep haul distances reasonable.
Lower GHGs with IAP
If you review the sections titled Benefits (Economics), Benefits (Energy Demand) and Benefits (Product Mix) in Shane Buchanan’s report “Inverted Pavement Systems” from 2010, you’ll see early evidence of the inverted asphalt pavement (IAP) appeal for state departments of transportation. Visit Vulcan Innovations online to read his entire report. https://www.vulcaninnovations.com/public/pdf/4-Inverted-Pavement-Systems.pdf
The team of Imad L. Al-Qadi and Erol Tutumluer also spoke of environmental benefits when using reduced (less than 4%) cement in the cement-treated base (CTB) and various asphalt pavement methods to execute the IAP. Watch a recording of their Transportation Research Board (TRB) presentation Sept. 18, 2023, at https://webinar.mytrb.org/Webinars/Details/1703. Their presentation included far more than what could be included in this article.