## AASHTO Guide for Design of Pavement Structures

### Structural Number (SN)

• SN = a1D1 + a2D2m2 + a3D3m3
• (AASHTO 1993 - Design of Pavement Structures, p. II-35)

### Layer Coefficients

#### a1 asphalt concrete surface course

• “Figure 2.5 provided a chart that may be used to estimate the structural layer coefficient of a dense-graded asphalt concrete surface course based on its elastic (resilient) modulus (EAC) at 68°F. Caution is recommended for modulus values above 450,000 psi. Although higher modulus asphalt concretes are stiffer and more resistant to bending, they are also more susceptible to thermal and fatigue cracking.” (p. II-17)

#### a2 base coefficient

• “The following relationship may be used instead of Figure 2.6 to estimate the layer coefficient, a2, for a granular base material from its elastic (resilient) modulus, EBS
• a2 = 0.249(log10EBS) - 0.977
• log10 is common log, or just log on the Windows Calculator
• log 10 = 1 → 101 = 10
• log 100 = 2 → 102 = 100
• log 1000 = 3 → 103 = 1000
• log 10000 = 4 → 104 = 10000
• EBS = k1θk2 = Elastic (resilient) modulus for Base material
• k1
• typical values 3000 to 8000
• “Each agency is encouraged to develop relationships for their specific base materials (e.g. MR = k1θk2) using AASHTO Method T 274; however, in the absence of this data, values given in Table 2.3 can be used.” (AASHTO 1993 - Design of Pavement Structures, p. II-20)
• θ stress state
• typical values 5 to 30
• “Note, EBS is a function of not only moisture but also the stress state (θ). Values for the stress state within the base course vary with the subgrade modulus and thickness of the surface layer. Typical values for use in design are:” (AASHTO 1993 - Design of Pavement Structures, p. II-20)
• k2
• typical values 0.5 to 0.7

#### a3 subbase coefficient

• a3 = 0.227(log10ESB - 0.839)
• ESB = k1θk2 = Elastic (resilient) modulus for subbase material
• “For aggregate subbase layers, ESB is affected by stress state (θ) in a fashion similar to that for the base layer. Typical values for k1 range from 1500 to 6000, while k2 varies from 0.4 to 0.6. Values for the AASHTO Road Test subbase material were (13).” (AASHTO 1993 Design of Pavement Structures, p. II-22)
• “As with the base layers, each agency is encouraged to develop relationships for their specific materials; however, instead of this data, the values in Table 2.3 can be used.” (ibid p. II-22)
• “Stress states (θ) which can be used as a guide to select the modulus value for subbase thicknesses between 6 and 12 inches are as follows:  