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Required Asphalt Thickness from SN

Mediumstructural numberlayer thickness
The required SN is 4.0. The base is 8 in (a2 = 0.14, m2 = 1.0) and the subbase is 6 in (a3 = 0.11, m3 = 0.90). With a1 = 0.44, the required asphalt thickness is most nearly:
  1. A3.2 in
  2. B4.0 in
  3. C8.0 in
  4. D5.2 in
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Answer: D. 5.2 in

Equation: SN = a1·D1 + a2·D2·m2 + a3·D3·m3. Solve it for the one unknown, D1. Values: required SN = 4.0 is the design target. Base: D2 = 8 in, a2 = 0.14, m2 = 1.0. Subbase: D3 = 6 in, a3 = 0.11, m3 = 0.90. Asphalt: a1 = 0.44 and D1 unknown. Step 1: base contribution, 0.14 × 8 × 1.0 = 1.12. Step 2: subbase contribution, 0.11 × 6 × 0.90 = 0.59. Step 3: the asphalt must supply the rest, 4.0 − 1.12 − 0.59 = 2.29. Step 4: D1 = 2.29 / 0.44 = 5.2 in. Using 4.0 / 0.44 = 9.1 in ignores the lower layers; 4.0 in is a common thickness but gives SN = 3.47, short of the target.
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