Pavement Design and Materials
Flexible design (AASHTO 1993)
The design equation relates traffic (W18, in 18-kip ESALs), reliability (ZR × S0), change in serviceability (ΔPSI), and drainage to the required structural number SN. Layer thicknesses then satisfy SN = a1D1 + a2D2m2 + a3D3m3, where a is the layer coefficient, D the thickness in inches, and m the drainage coefficient (applied to base and subbase, not the surface). Subgrade support is characterized by resilient modulus Mr, often estimated from CBR.
Rigid design
Slabs are sized using ESALs, modulus of subgrade reaction k, PCC flexural strength, drainage, and load transfer; joint spacing, dowel bars, and base type control performance.
Materials
Superpave mix design selects an N-design gyration level from traffic, targets 4% air voids at N-design, and enforces minimum VMA with a dust-to-binder ratio of roughly 0.6–1.2. PG binder grades read as high-temperature performance grade then low: PG 64-22 means 64°C rutting resistance and −22°C thermal cracking resistance. Field compaction acceptance is typically 92% of maximum theoretical specific gravity (Gmm).
Traps
- Reporting ESALs as truck counts.
- Applying m to the HMA surface layer.
- Reversing PG numbers (the second number is the low temperature).
- Confusing layer coefficient a with drainage coefficient m.
Worked example
Required SN = 3.5. With a1 = 0.44 and D1 = 4.0 in, the surface contributes 1.76, leaving 1.74. For a base with a2 = 0.40 and m2 = 1.0, D2 = 1.74/0.40 = 4.35 in, rounded up to 5 in.