This tool solves the AASHTO 1993 empirical equation for the structural number a flexible pavement needs, then checks that number against the layer arrangement you propose. It is built for highway and pavement engineers, design consultants and students working to the AASHTO Guide for Design of Pavement Structures.
The structural number (SN) is a single index for the total load-carrying capacity a
pavement requires. It is not a thickness. Once you know the required SN, it is
converted into real layers using
SN = a₁D₁ + a₂D₂m₂ + a₃D₃m₃, where each a is a layer
coefficient reflecting material quality, each D a thickness, and each
m a drainage coefficient applied to the unbound layers.
The 1993 equation cannot be rearranged to give SN directly. You supply the traffic and support inputs, then adjust the design SN until the right-hand side of the equation balances the left. This page displays both sides and the difference, so you can see the moment they match rather than working blind.
M_R = 2555 × CBR^0.64 (psi).Sections E to H compute the structural number contributed by the asphalt wearing course, asphalt base course, aggregate base and granular subbase. These are summed into the provided SN and compared against the required SN, with the surplus or deficit reported directly and the arrangement drawn as a scaled cross-section.
The layer coefficients on this page are calibrated to centimetre thickness, per the project practice noted in the footer, rather than the inches used in the original AASHTO tables. If you are transferring coefficients from the Guide, convert them before entering.
This implements the 1993 empirical method, which remains in wide use but has been superseded in some agencies by the mechanistic-empirical Pavement ME procedure. Always confirm the required method, coefficients and reliability level against your governing design manual. For concrete pavements, use the rigid pavement design calculator.
The structural number is a single index representing the total load-carrying
capacity a flexible pavement needs. It is an abstract number rather than a
thickness. Once the required structural number is known, it is converted into real
layers using SN = a₁D₁ + a₂D₂m₂ + a₃D₃m₃, where each a is
a layer coefficient reflecting material quality, each D is a layer
thickness and each m is a drainage coefficient for the unbound
layers.
The 1993 equation cannot be rearranged for the structural number directly, so it is solved by iteration. You supply cumulative ESALs, reliability, overall standard deviation, serviceability loss and subgrade resilient modulus, then adjust the design structural number until the right side of the equation matches the left side. This calculator displays both sides and the difference between them so you can see when the two balance.
An ESAL is an equivalent single axle load, the damage caused by one pass of a standard 18 kip single axle. Because pavement damage rises roughly with the fourth power of axle load, mixed traffic is converted into an equivalent number of standard axles rather than counted as vehicles. The cumulative ESALs expected over the design life is the traffic input to the AASHTO equation, and it is dominated by heavy vehicles, not by car volumes.
This calculator uses M_R = 2555 × CBR^0.64, giving the resilient
modulus in psi. It is one of several published correlations, and different agencies
specify different relationships, so check which one your governing design manual
requires. The subgrade modulus has a strong influence on the required structural
number, which is why a reliable CBR value matters.
Reliability is the probability that the pavement will carry the design traffic before reaching its terminal serviceability. A higher reliability produces a more conservative, thicker design. It enters the equation through the standard normal deviate ZR, which becomes more negative as reliability rises: 90% gives −1.282 and 95% gives −1.645. Typical values are 85 to 95 percent for principal arterials and lower for minor roads.
A layer coefficient expresses how much structural capacity one unit of thickness of a material contributes. Asphalt concrete carries the highest coefficient, followed by aggregate base and then granular subbase. Drainage coefficients modify the contribution of the unbound base and subbase to reflect how quickly they drain and how long they stay saturated. Note that in this tool the layer coefficients are calibrated to centimetre thickness rather than the inches used in the original AASHTO tables.
Serviceability is a ride-quality rating on a scale from 0 to 5. Serviceability loss is the difference between the initial serviceability of the newly built pavement and the terminal serviceability at which rehabilitation is judged necessary. Typical initial values are around 4.2 and terminal values around 2.5 for major roads or 2.0 for minor roads. A larger permitted loss means a thinner pavement is acceptable.
Flexible pavement spreads load through successive layers of asphalt, base and subbase, so the design output is a structural number converted into layer thicknesses. Rigid pavement carries load in a concrete slab acting in bending, so the design output is a required slab thickness governed by concrete flexural strength, load transfer and subgrade reaction rather than by a structural number.
Designing and costing the rest of the same road: