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.Top lift
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.WEB EXTRA
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