Stable foam. Reliable process.
How an additive can stabilize foamed bitumen and, in turn, improve the process reliability.
Foamed bitumen is an established technology for the production of low-temperature asphalt. But how can the stability of the foam be reliably ensured? The targeted use of an additive can improve the foam’s properties and thus contribute to greater process reliability.
The foamed bitumen process aims to maintain the workability of the asphalt mix at a high level, even when the paving temperatures for low-temperature asphalt are significantly reduced. The process is relatively simple: hot bitumen is foamed by injecting a small amount of water (often in the range of 1% to 3%) together with air or steam.
Foaming occurs as a result of the sudden evaporation of the water, which causes the bitumen to expand. The increase in volume improves the distribution of the bitumen throughout the mix and can thereby optimize the coating of the aggregate. The rheological properties of the bitumen are not permanently altered; rather, the process temporarily improves its workability.
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It Depends on the Process
Foamed bitumen technology has so far been used only to a limited extent in Germany. Yet the cost of a foamed bitumen plant, including a water-injection system, is manageable. The greater challenge lies in the process itself.
Two parameters are crucial for foamed bitumen: the expansion ratio and the half-life. The expansion ratio describes the relationship between the maximum foam volume and the original bitumen volume. The most critical factor here—the foam’s lifespan—is indicated by what is known as the half-life. This describes how long the foamed bitumen remains stable before the trapped water vapor bubbles escape from the bitumen-water-vapor mixture and the foam collapses.
More specifically, half-life is the time taken for the maximum foam volume to decrease by half. If bitumen reaches a maximum volume of 20 L after foaming and decreases to 10 L after 12 seconds, the half-life is 12 seconds. Typical half-lives are often only a few seconds.
If the bitumen foam is not stable, the improved flow properties—and, consequently, the better wettability—are quickly lost. The key to an optimal process lies in stabilizing the bitumen foam for as long as possible. This is precisely where a specially developed additive comes into play, enabling the efficient production of foamed bitumen.
Stability Makes the Difference
