The idea of placing concrete pavement in two lifts is not new. By the turn of the last century, pavement engineers could recognize the potential advantages of using different concrete mixtures for base and surface layers, and a two-lift paving process called “Granitoid Concrete Streets” was patented in 1906. The Granitoid process used normal ready-mixed concrete with inexpensive aggregates for the first 5 inches, followed by a top lift that contained crushed granite rock and less fine aggregate, to produce a very hard and durable surface. Some Granitoid streets built in 1909 and 1910 are still in service today in Minnesota and North Dakota.
In the 1950s, '60s, and '70s, a two-lift technique often was used to facilitate the placement of welded wire mesh reinforcement in concrete highway pavements . After placing a first concrete layer approximately half of the finished pavement thickness, crews would set the wire mesh on the wet concrete between dowel baskets. Then, before the first layer stiffened, a second layer of the same concrete mix was placed on top of the mesh, and a paving machine finished the surface. According to a 2004 report by the Center for Portland Cement Concrete Pavement Technology, this process was used successfully in several states.
After 1970, concrete pavement design trends moved away from mesh reinforcement in favor of shorter panels of plain pavement, dowelled or undowelled depending on traffic volumes. Eliminating the mesh also eliminated the need for two-lift construction in most cases. In the years since, relatively few two-lift concrete pavements have been built in the United States, typically for test or demonstration projects.
European tour piques interest
In 2006, the Federal Highway Administration (FHWA) organized one of its periodic scan tours, “Long-Life Concrete Pavements in Europe and Canada,” which gave participants the chance to observe the techniques and performance of two-lift concrete pavements firsthand. The scan team included representatives of FHWA, state DOTs, the National Cooperative Highway Research Program , academia, and the consulting, cement, and concrete pavement industries.
The scan tour's objective was to study the materials, design strategies, construction methods, and maintenance practices that European countries have used to extend the useful life of concrete highway pavements up to 40 years and more. Because long-life pavements require less frequent repair, rehabilitation, and reconstruction, they help to improve highway safety and ease traffic congestion—two of the main goals of the U.S. highway community.
One technique the scan team observed was two-lift concrete paving. In Austria, Germany, Belgium, and the Netherlands, two-lift highway pavements have become standard. Two main reasons account for its popularity:
- First, it allows significant amounts of recycled aggregate to be reused in concrete for the base lift. Densely developed countries have long discouraged stockpiling or disposal of waste materials and have tended more toward beneficial reuse.
- Second, it allows more efficient and economical use of specialized concrete mixes to produce desirable surface characteristics in the thinner top lift. For example, exposed-aggregate finishes have been used for the surface lift to reduce tire noise on pavements in many urban areas.
Kansas' two-lift pavement trial
After the tour, scan team member Andrew Gisi, of the Kansas DOT (KDOT), worked with FHWA to arrange construction of a demonstration two-lift concrete pavement in the state. The project's goal was to test the viability of using standard paving equipment in a two-lift process as an alternative to the jointed plain pavement design most common in the United States. The demonstration would comprise several test sections totaling about 4 miles long, built along Interstate 70 west of Abilene.
Koss Construction Co., Topeka, Kan., was engaged to construct the test pavement. Before beginning the I-70 project , the contractor opted to build a smaller-scale test near Pleasanton to help crews become familiar with the two-lift process. Koss consulted with Dr. Hermann Sommer , an Austrian paving expert who provided training and advice on its details and features. Koss Construction President David Howard says this training helped clear up some confusion about the two-lift technique and led to some modification of the process. “We used a set retarder and wax curing compound for the initial training section, but the curing compound didn't work too well, so we switched to plastic sheeting for the I-70 test pavements,” he says.
The test pavement was placed on an approximately 6-inch-thick base of recycled concrete produced from the existing concrete pavement. The 16-inch bottom lift featured a standard KDOT mix using locally available limestone aggregates. The bottom lift concrete was treated with admixtures to add stiffness and prevent segregation. The 1.6-inch top lift contained hard-wearing rhyolite aggregate imported from Oklahoma, to provide a durable surface layer.
Koss Construction set up a large plant onsite, with two mixing drums to produce concrete for both the bottom and top lifts. Color coding was used to ensure the proper mix was delivered to the spreaders placing each lift. For the bottom lift, a belt spreader placed concrete on the base, followed closely by a slipform paver to form the pavement profile. The bottom lift concrete was stiff enough to walk on almost immediately after it was placed.
A second spreader and slipform paver were used to place the top lift concrete, following within 200 feet of the first equipment. This spacing is important, because two-lift paving is a “wet-on-wet” process that requires the lifts to bond without intermingling. The top lift concrete was a higher-slump mix, so the top-lift paver used a lower vibration rate to achieve a consistent layer.
The four I-70 test sections received different surface textures, which included longitudinal tining, grooving, Astroturf drag, and exposed aggregate, to allow KDOT to compare their performances. For the exposed aggregate section, the top lift mix contained finer sand and smaller rhyolite aggregate. The surface was sprayed with a retarder to keep the concrete from setting, and then covered with plastic sheeting to cure for about five hours. After the curing period, the sheeting was removed, and the surface was broomed to remove the surface paste and expose the aggregate.
Subsequent sound tests performed on all four of the textured surfaces showed lower tire-pavement noise levels than conventional concrete pavements.
The test successfully demonstrated the viability of the two-lift technique. “We lost some production speed [compared to conventional highway paving],” Howard says, “but not as much as we'd expected. We're used to working on large projects, so we had enough equipment to keep the work moving. It's not a big step to go from single-lift to two-lift paving if it's beneficial.”
The benefits are mostly environmental, achieved by using resources efficiently and improving long-term performance.
In two-lift concrete paving, the bottom lift represents by far the largest concrete component. When freed from the need to provide a durable, high-performance finished surface, bottom-lift concrete can incorporate recycled, low-cost, and even marginal materials that are unsuitable for a single-lift mix.
For example, in some parts of the United States (including Kansas), locally available aggregates lack the durability or resistance to polishing needed for long-life concrete pavements. Some aggregates are subject to deterioration from chemicals used to control snow and ice. It may be feasible to use such aggregates in the bottom lift, however, if they are protected from attack by a more durable and less permeable surface layer.
Another option is to use recycled portland cement concrete (RCC) as aggregate. As in the I-70 test sections, RCC is often used in the United States as a base layer in highway pavements, but seldom as part of the concrete mix. RCC has been used as a concrete aggregate in Austria for more than 30 years, however, and such pavements have performed well in test roads here.
Two-lift paving also may open the way to the use of high-volume fly ash mixtures in concrete pavement. Though common in other applications, high-volume fly ash mixes present some constructability problems that have limited their acceptance in the paving industry. With placement less critical and no finishing required, bottom lift concrete could accommodate larger proportions of fly ash as a cement replacement, and thus reduce the carbon footprint of concrete pavements considerably.
With a relatively thin surface layer, two-lift concrete paving can incorporate materials that would be either too expensive or otherwise impractical for a full-depth pavement. By treating only the top lift, you can improve pavement performance without adding much to the overall project cost.
For example, you can reduce concrete permeability and improve its finishing characteristics by adding supplementary cementitious materials to the mix. Less permeable concrete is less subject to deterioration and more likely to provide a smoother ride for longer.
Varying surface treatments also can affect pavement performance, such as increasing skid resistance, controlling tire noise, and improving safety by minimizing the splash and spray generated by traffic. Exposed aggregate is one option used in Germany and Austria; experimenting with other top lift textures and techniques should yield other effective noise control choices.
Pervious concrete, which allows water to drain through pavement, offers several environmental benefits, such as reduced runoff and enhanced water quality. It also reduces noise and splash and spray. To date, it's been used mostly for low-speed pavement applications, but research into its feasibility as a top-lift material is under way.
An experimental two-lift pavement project planned for later this year in Missouri will include in the top-lift concrete a “smog-eating” titanium dioxide additive that serves to trap and decompose air pollutants through a photocatalytic reaction. This additive is believed to be effective only near the surface, and would be prohibitively expensive for a full-depth pavement as well. If the experiment proves successful, it could signal another major environmental benefit.
— Hooker is a freelance writer based in Oak Park, Ill. This article first appeared in the March issue of CONCRETE CONSTRUCTION, a sister magazine of PUBLIC WORKS.