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110 CHAPTER 11 Aggregate Foam Arrestor Concept 11.1. Concept Description Because the aggregate foam material has a closed-cell micro- structure, it has an inherent resistance to moisture. However, 11.1.1. System Overview the presence of water during freezethaw cycling could present An aggregate foam arrestor concept has been proposed. The a more aggressive erosion hazard for the material. Two poten- arrestor would use rough-broken foam aggregate made from tial methods have been identified to handle precipitation (Fig- recycled glass (Figure 11-1). The foamed, or aerated, glass ure 11-4). material has nominally 80% void space by volume. Its closed- cell microstructure makes it resistant to water absorption 1. Drainage Approach. This approach would allow water and degradation. The aggregate comes in a variety of gra- drainage downward through the bed. The bed would be dations and is currently used in different civil engineering designed to prevent standing water within it using normal applications for light fill construction, insulation, and frost civil engineering design practices. protection of road foundations and terraces (43). For the eval- 2. Waterproof Approach. In this approach, layers of geo- uation effort, fragments were graded to fall between 0.4 and plastic and geo-textile materials would be employed at the 2.4 in., with a loose fill density of 11.2 pcf (Figure 11-2). top and bottom of the bed. The upper and lower plastic An arrestor using the aggregate foam would be constructed layers would be sealed at the edges to produce a watertight by creating a basin and filling it with the material (Figure 11-3). envelope around the fill material (a practice currently in An engineered turf would serve as a top cover layer for the use at some landfills). Precipitation in this design would bed, which can serve several purposes: run off to the perimeter of the arrestor bed. 1. Prevent material dispersion due to jet blast; 11.1.2. Performance Considerations 2. Mitigate material spraying during overrun by an aircraft tire, thus limiting engine ingestion hazard; Mechanically, each piece of material is composed of crush- 3. Regulate water drainage and potential ice crust formation able glass foam, which has fairly conventional properties. in winter; and However, when acting as a continuum, many irregular and 4. Act as a structural component to prevent lightweight land loose-fitting pieces would exhibit flowing aggregate behav- vehicles from penetrating the arrestor bed. ior. While the material would have compressive and shear strength properties, the loose aggregate would not exhibit This simple fill-and-cover construction would likely pro- tensile strength because nothing binds the loose pieces together. duce lower manufacturing and installation costs than the block Overall, the aggregate foam concept falls somewhere in between construction methods used at present. This potential advan- a conventional foam block system and a conventional hard tage is offset by the possibility that the material could settle over aggregate system. time and result in altered arresting performance; potential Falling between these two different material categories, mechanisms of settling could include: the aggregate foam concept presented multiple issues for evaluation: 1. Consolidation, and 2. Mechanical/chemical degradation and shrinkage/break- Overall dynamic response characteristics, ing of aggregate pieces. Required density/strength for effective arresting,