Agenda - Council - 02/11/2020 - Laserfiche WebLink

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Figure 2.7 shows the dry and wet tensile strength values of the three mixtures. There is a much more noticeable impact on these strength values due to the increase of air voids than there was on the TSR. Generally speaking, a doubling of the air voids from 7 to 14 corresponds with a reduction of 52% and 58% in the dry and wet tensile strengths respectively. These reduced strength values could be expected to contribute to a less durable pavement. It should also be considered that the tests were conducted on newly prepared mixtures; pavements in the field degrade and loose strength and durability over time. A pavement that has initial TSR value near the borderline, will most likely reach a compromised state sooner than one that has a higher initial TSR value. Tensile Strenght, psi 120 100 80 60 40 20 0 Tensile Strength vs. Air Voids • Dry Strength ■Wet Strength 7% 8% 9% 10% 11% 12% 13% 14% Air Voids, % Figure 2.7. Tensile Strength vs. Air Voids Asphalt Pavement Analyzer (APA) Test "Wet" (Mn/DOT Modified). The asphalt pavement analyzer (APA) device, as shown in Figure 2.8, was used to experimentally evaluate the mixture's susceptibility to rutting. The "dry" test consisted of applying 8,000 cycles of 100 lbf strokes to the lab -compacted gyratory specimens at 137 °F (58°C), which is approximately equivalent to 1,000,000 ESALS. The "wet" test consisted of applying a number of cycles until the mixture reached a set rutting value while immersed in water. A mixture's susceptibility to rutting is typically dependant upon the binder (content and stiffness) as well as the gradation of the mixture and air void content. Higher rut depths indicate a softer mixture where lower rut depths indicate a stiffer mixture; typical level three mixtures evaluated by Mn/DOT have rut depths between 6 — l0mm. B-13