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113 11.2.2. Special Considerations the overall test matrix for the aggregate foam material. All cylinder dimensions specify diameter followed by height. Overall, testing the aggregate foam proved challenging for three reasons: 11.3.1. Density and Dimension 1. The combined crushable and aggregate properties of the Measurements material did not readily fit into standardized test methods. 2. The large size of the foam pieces required large test fixtures The original, as-received aggregate foam material had to capture continuum-type properties of the loose fill; this pieces graded to between 0.4 and 2.4 in., with a loose fill den- precluded the use of some desirable tests. sity of 11.2 pcf. For a small sample of pieces measured, the 3. The dual-mode material behavior did not allow compre- average size was 1.9 in. hensive definition by the material models of the available As previously discussed (Section 11.2.2), the as-received modeling software. aggregate size precluded some testing that might otherwise have been conducted. For other tests, the aggregate was bro- ken down into smaller pieces and sifted through a 1-in. grid, 11.3. Testing Effort giving it a gradation of 0.4 to 1.0 inches. The testing effort for the aggregate foam material involved All tests, therefore, were conducted on either the original several mechanical and environmental tests. Table 11-1 depicts 2.4-in. material, or on the reduced 1.0-in. material. Table 11-1. Test matrix for aggregate foam material. Test Properties Detail Number of Tests Characterized Laboratory Tests Hydrostatic Compressive strength Per ASTM D2850 5 psi 1 Triaxial (u) 6 x 10.5" cylinder 10 psi 2 Compression Shear strength (u) Reduced aggregate size Test Effects of confining Pieces graded for 0.4" to 1.0" 20 psi 2 pressure on strength 0.24%/min compression rate 60 psi 2 Maximum compression of 15 to 25% Confining pressures of 5, 10, 20, 60, 100 psi 2 and 100 psi Total: 9 Confined Compressive strength Version 1: Original Aggregate Size 2 Cylinder (u) Non-standard Compression Compressive stress Pieces graded for 0.4" to 2.4" Test strain curve 12.375 x 9.5" confining cylinder Extrapolated: 3 in./min compression rate volumetric energy Maximum compression of 75% capacity Version 2: Reduced Aggregate Size Fresh 1 Non-standard Pieces graded for 0.4" to 1.0" 12.375 x 7.5" confining cylinder Conditioned 2 3 in./min compression rate Maximum compression of 45% Environmental Durability to freeze Version 1: Per ATSM C 666/C 666M- 2 Chamber Tests thaw cycles in fully 03 (modified) immersed conditions Trays of material, 10 gallons total 50 freezethaw cycles Material compression tested thereafter Version 2: Per AASHTO T 103 2 Particles separated by gradation into different sieve sizes Particle size distribution changes measured 50 freezethaw cycles

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114 Figure 11-6. Confined cylinder test for aggregate foam showing the test fixture (left), pre-test specimen (top) and post-test specimen (bottom). 11.3.2. Confined Cylinder 180 Compression Tests 160 Confined cylinder compression tests were performed on both the original and the reduced size aggregate foam. The 140 tests were conducted by pressing a 12-in. diameter platen into a 12.375-in. diameter cylinder at a fixed rate of 3 in./min (Fig- 120 ure 11-6). The 12-in. diameter was chosen as being greater Stress (psi) 100 than six times the characteristic dimension of the material, which as a rule of thumb helps to ensure continuum material 80 behavior. The material was poured loosely into the cylinder without packing. Due to the irregular nature of the aggregate, 60 initial depth measurements were approximate. The load data for replicate tests was remarkably consis- 40 tent, despite the random nature of the selected aggregate pieces for each test. However, the gradation size of the aggre- 20 gate appeared to have a substantial effect on the loading and the 0 energy absorption. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Figure 11-7 illustrates the load history for both gradations. Compressive Strain (in./in.) The general load curve shape followed an exponential form; as Original Gradation - Loading a result, the energy absorption also followed an exponential Reduced Gradation - Loading trend (Figure 11-8). As shown, the reduced gradation material only obtained a compression of about 45%, whereas the orig- Figure 11-7. Confined cylinder test average load inal larger gradation reached 75% at the same load. This led histories for aggregate foam material.

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115 180 54 area comes into contact with the material, but because the Original Gradation - Loading material continually hardens as the compression increases. This 160 Original Gradation - Energy Absorption 48 trend is dissimilar from a typical block of crushable foam, which 140 42 reaches a fairly constant plateau strength for the majority of the compression range. The energy absorption for a crushable Energy Absorption (psi) 120 36 block increases linearly with compression; for the aggregate foam, the increase is exponential. This depth-varying property Stress (psi) 100 30 posed an interesting alternative for arrestor applications. 80 24 11.3.3. Hydrostatic Triaxial Tests 60 18 The hydrostatic triaxial tests evaluated the aggregate foam 40 12 performance at different confining pressures to determine if any strength increase or bulking deformation took place. Due 20 6 to the aggregate mode of behavior, lateral bulging of the spec- 0 0 imens was anticipated. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 The largest practical specimen size for testing was a 6-in. Compressive Strain (in./in.) diameter cylinder. The reduced gradation material was devel- Figure 11-8. Confined cylinder test stress and oped to satisfy the rule of thumb for six times the nominal energy absorption for original gradation particle dimension across this width. All hydrostatic triaxial aggregate foam material. specimens used the 1-in. graded material. The specimens were cylinders (6 10.5-in.) placed between platens using Sorbothane caps at the top and bottom (Fig- to a 39% decrease in energy absorption for the reduced grada- ure 11-9, left). The specimens were fitted with flexible mem- tion material. brane sleeves before immersion in a pressurized vessel of The gradation effect may be due to two geometric differ- water. While at this hydrostatic pressure, the specimens were ences between the specimens. First, in the larger gradation compressed axially until reaching 15 to 25% compression. As material, the particles may simply have more size variation. shown on the right in Figure 11-9, the specimens bulked lat- If the reduced gradation sizing was more uniform (smaller erally as anticipated. size range), then the packing ratio may have been higher, A wide range of hydrostatic pressures was explored in order leading to less empty space between the particles. Second, to determine the overall effects on the material. The upper some of the larger pieces had concave surface features that range was chosen as 100 psi because little was observed in the may have bolstered the void percentage of the original gra- way of new trends by that pressure. Because the hydrostatic dation material. Once broken into smaller 1-in. pieces, the specimen data had more scatter than the confined cylinder particles may have become rounder, which would, again, data, a total of nine tests were conducted. reduce the empty space between the particles. In either case, Figure 11-10 compares the hydrostatic triaxial data for the or in a combined case, a reduction in the void space between three main material candidates: glass foam block, aggregate pieces would lead to earlier compaction of the material and glass foam, and hard engineered aggregate. The data points in less energy absorption. the figure represent the top center for the Mohr's circles at the It is clear from these tests that the aggregate foam gradation different confining pressure conditions. Of the three materi- would need to be chosen carefully for an arrestor application. als, the only material exhibiting a change in behavior is the Further, the intended gradation would need to be protected aggregate foam, which has a distinct transition occurring at during the installation process and over the life of the bed. compressive stress of 20 psi. This 20-psi stress value would Repetitive vehicle overruns, jet blast, or other physical loads that occur for a test specimen at a confinement pressure of about might induce compaction or fragmentation of the pieces would 15 psi. need to be avoided. If gradation shifts occurred over time, the Prior to the transition point, the trend-line fit to the aggre- bed properties could change and lead to unanticipated mechan- gate foam has a steep slope and a very low initial value (near ical performance. zero); these characteristics are analogous to the hard aggregate Another important observation from the confined cylinder material. As the compressive stress drops to zero, so does the tests is that the material would, in effect, act as a depth-varying shear strength of the material. This is consistent with intuitive compressible material. Deeper tire penetrations would lead to observations since lower confinement pressures allow the an increase in vertical load, not simply because a larger surface material pieces to roll and flow past one another more readily.

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116 Sorbothane Bulking Cap Post-Test Membrane Around Specimen Figure 11-9. Hydrostatic triaxial compression test of aggregate foam pre-test (left) and post-test (right), 100 psi confining pressure. After the transition point, the trend-line fit to the aggre- Because the transition point occurred at a fairly low confine- gate foam has a shallow slope and a much higher initial value ment pressure (15 psi), an important simplification became (6 psi); these characteristics are analogous to the foam block possible. By observing the confined cylinder load data from material, albeit with lower values. The pressure dependence of Figure 11-8, only about 3 to 5% of the total material energy the material decreases substantially, producing solid-like absorption would occur by this point. Figure 11-11 illustrates behavior, where the individual pieces are compressed together the relative energy capacity of the material, divided by behav- too much to permit flow. ioral regime. The clear dominance of the solid regime from an 30 25 Glass Foam Block Peak Shear Stress, q (psi) Aggregate Foam - Late 20 Stage Hard Aggregate 15 Trend - Glass Foam Block 10 Trend - Aggregate Foam - Early Stage 5 Trend - Aggregate Foam - Late Stage Trend - Hard Aggregate 0 0 10 20 30 40 50 60 70 80 Compressive Stress, p (psi) Figure 11-10. Comparison of hydrostatic triaxial data for three primary material candidates.

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117 particles, but minimum compression of the material. As such, minimal energy dissipation would have occurred. Solid Regime Aggregate Finally, the turf cover layer of the concept could not be Regime 95% scaled in thickness, mass, or strength, which would have 5% made it disproportionate to the small wheel. Making a larger wheel and strut assembly for the pendulum was considered, but ultimately not feasible. Larger wheels would produce more significant loads on the pendulum mass; the wheel size for the pendulum was chosen as the upper feasible limit without causing bouncing or path deviation of the mass. Figure 11-11. Approximate compaction- based energy dissipation in aggregate foam, divided by behavioral regime. 11.3.5. Environmental Tests Two sets of environmental tests were conducted to deter- mine the necessity for weatherproofing the aggregate foam energy absorption standpoint eventually led to a modeling material. decision to use a solid crushable foam model to simulate the The first test involved subjecting 10 gal of 1-in. graded material. material to freezethaw testing in various sized sieves, per AASHTO T103. The particles were fully immersed in water 11.3.4. Pendulum Tests and subjected to 75 freezethaw cycles. The relative degrada- tion of the different sized pieces was recorded. It was con- The pendulum tests that were conducted for the other can- cluded that the particles generally eroded in size during the didate arrestor concepts were omitted for the aggregate foam course of the testing. However, this test did not ultimately material. It was determined that the ungraded material would provide a great deal of insight regarding the performance be required for such tests. Having a nominal 2-in. particle impact of the degradation. size, this presented substantial scaling issues for the pendu- The second test also subjected 10 gal of 1-in. graded material lum's 1/3-scale tire form. The wheel form was only 5 in. in to freezethaw testing. The test methodology was non- width, leaving it at just over two particles wide. standard, but similar in nature to ATSM C 666/C 666M-03. Further, the effective penetration depth of the wheel The aggregate foam was placed in two open plastic tubs and would likely have been about 7.5 in., or half the diameter. fully immersed in water (Figure 11-12). The specimens were This would likely have created a spraying of the upper surface then subjected to 50 freezethaw cycles. After the test, the Figure 11-12. Aggregate foam environmental test specimens before (left) and during (right) test.