About Flexible Pavement Design Calculator (AASHTO 1993)
The flexible pavement design calculator implements the AASHTO 1993 Guide for Design of Pavement Structures equation for flexible (asphalt) pavements. It solves the design equation iteratively for the required structural number SN given the design traffic in 18-kip equivalent single axle loads (W18 ESALs), the reliability level, the overall standard deviation, the serviceability loss, and the subgrade resilient modulus MR.
It then computes the structural number provided by a layer schedule as SN = sum of a_i * D_i * m_i, where a_i is the layer coefficient, D_i the thickness in inches, and m_i the drainage coefficient, and compares the provided SN to the required SN so you can confirm the section is adequate. All inputs use US customary units (MR in psi, thicknesses in inches).
How It Works
- Enter the design traffic W18 (ESALs), the reliability R in percent, the overall standard deviation S0, the initial and terminal serviceability indices, and the subgrade resilient modulus MR.
- The calculator converts reliability to the standard normal deviate ZR and the serviceability indices to the design loss dPSI = p0 - pt.
- It solves the AASHTO 1993 design equation for the structural number SN by bisection, since the equation is monotonic in SN.
- Enter the asphalt, base, and subbase layers with their layer coefficients, thicknesses, and drainage coefficients; the provided SN = sum a_i D_i m_i is computed and compared to the required SN.
Worked Example
A low-volume road carries W18 = 110,000 ESALs at 95% reliability (ZR = -1.645), S0 = 0.45, with serviceability dropping from p0 = 4.2 to pt = 2.5 (dPSI = 1.7) over a subgrade with MR = 5000 psi. Solving the AASHTO 1993 equation gives a required SN of about 3.0. A section of 5 in asphalt (a1 = 0.44), 10 in aggregate base (a2 = 0.14, m2 = 1.0), and 12 in subbase (a3 = 0.11, m3 = 1.0) provides SN = 0.44*5 + 0.14*10 + 0.11*12 = 2.20 + 1.40 + 1.32 = 4.92, which exceeds the required 3.0, so the design is adequate with a margin of 1.92.
Formulas
- AASHTO 1993 flexible pavement design equation
log10(W18) = ZR*S0 + 9.36*log10(SN+1) - 0.20 + log10(dPSI/(4.2-1.5)) / (0.40 + 1094/(SN+1)^5.19) + 2.32*log10(MR) - 8.07- Provided structural number
SN = a1 D1 + a2 D2 m2 + a3 D3 m3 + ...- Design serviceability loss
dPSI = p0 - pt
Standards & References
- AASHTO Guide for Design of Pavement Structures (1993)
- AASHTO reliability ZR table (50% -> 0, 95% -> -1.645, 99% -> -2.327)
- US customary units (MR in psi, thicknesses in inches)
Frequently Asked Questions
What is the structural number SN in AASHTO pavement design?
The structural number is a dimensionless index of the total structural capacity of a flexible pavement. It is the weighted sum of each layer thickness times its layer coefficient and drainage coefficient: SN = sum of a_i * D_i * m_i. A higher SN carries more traffic.
How is reliability converted to ZR?
Reliability R is the probability that the pavement will perform as designed. AASHTO tabulates the corresponding standard normal deviate ZR: for example R = 50% gives ZR = 0, R = 95% gives ZR = -1.645, and R = 99% gives ZR = -2.327. Higher reliability makes ZR more negative and increases the required SN.
What values should I use for S0 and the serviceability indices?
For flexible pavements AASHTO recommends an overall standard deviation S0 of about 0.45 (0.49 with traffic variability). Initial serviceability p0 is typically 4.2 for new flexible pavements, and terminal serviceability pt is 2.5 for major highways or 2.0 for lower-volume roads.
Why is the required SN solved iteratively?
The AASHTO 1993 equation gives W18 explicitly as a function of SN but cannot be rearranged in closed form for SN. Because W18 increases monotonically with SN, the calculator uses bisection to find the SN that exactly carries the design traffic.