Flexible Pavement Structural Number (AASHTO 93)

The structural number, a weighted sum of layer thicknesses. It is how a designer trades asphalt against base, and how an inspector checks whether a substituted section is equivalent. THE LAYER COEFFICIENTS ARE WHAT MAKE THE TRADE VISIBLE. An inch of asphalt is worth roughly three inches of crushed base and four of subbase, so a contractor proposing to substitute base for asphalt has to add three or four times the thickness to stay equivalent. That substitution is often cheaper and often perfectly sound, which is exactly why the arithmetic gets used in the field, and the equivalency is not a judgment call -- it is one division, reported here both ways. The drainage coefficient is the term that quietly punishes bad detailing, and it applies only to the UNBOUND layers. An unbound layer that stays saturated carries a coefficient below one, and the section loses structural number without losing an inch of material: drop a base from 1.0 to 0.8 and eight inches of rock gives up most of a quarter of a point. Edge drains, a daylighted base, and a subgrade that actually drains are worth real thickness, and this is where that shows up as a number rather than as an argument. THE REQUIRED STRUCTURAL NUMBER IS AN INPUT AND THIS DOES NOT SOLVE FOR IT. It comes from the AASHTO 93 design equation and depends on design traffic in equivalent single axle loads, the reliability level, the overall standard deviation, the subgrade resilient modulus, and the allowable serviceability loss. Two of those dominate -- the traffic loading and the subgrade modulus -- and both are commonly assumed rather than measured, and both move the answer by inches of asphalt. A design built on assumed values deserves to be checked against measured ones. Layer coefficients are agency-specific and depend on material quality; using generic values where an agency publishes its own gives a wrong answer with a confident look to it. AASHTO 93 is an empirical method derived from the AASHO Road Test of the late nineteen fifties, and mechanistic-empirical design supersedes it in many agencies and gives different answers. This does not address rigid pavement, layer thickness minimums, construction tolerances, or the subgrade preparation and compaction that decide whether the designed section performs at all. The agency's pavement design manual, the geotechnical investigation, and the pavement engineer govern.

SN = sum over layers of (layer coefficient x thickness x drainage coefficient), with the drainage coefficient applying only to unbound layers; the equivalency for a substitution is the ratio of the two layers' rates.

The AASHTO 93 structural number relation by name. Layer coefficients (asphalt about 0.44, crushed base about 0.14, subbase about 0.11) and drainage coefficients (0.8 to 1.2 in ordinary conditions) are AGENCY VALUES and are entered, not shipped. The required SN comes from the AASHTO 93 design equation -- design ESALs, reliability, standard deviation, subgrade resilient modulus and serviceability loss -- and is NOT solved here. The agency's pavement design manual, the geotechnical investigation, and the pavement engineer govern.

A weighted sum of three layers on coefficients the reader supplies; no agency coefficient table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: ac_thickness_in, ac_coefficient, base_thickness_in, base_coefficient, base_drainage, subbase_thickness_in, subbase_coefficient, subbase_drainage, required_sn, sn_surface, sn_base, sn_subbase, sn_total, sn_margin, base_per_inch_of_ac_in, subbase_per_inch_of_ac_in

• Layer coefficients are agency-specific generic values where an agency publishes its own give a wrong answerthe agency's pavement design manual
• The required SN is entered, not solved the AASHTO 93 equation needs ESALs, reliability, standard deviation, modulus and serviceability lossAASHTO 93
• Flexible pavement only rigid pavement, thickness minimums and construction tolerances are outside itthe agency's pavement design manual