Calculator Inputs
Example Data Table
This illustrative scenario shows one possible study setup for a flexible pavement screening check.
| Parameter | Example Value | Comment |
|---|---|---|
| Design Life | 20 years | Planning-level roadway analysis period. |
| Initial ADT | 12,000 vehicles/day | Starting traffic volume. |
| Truck Percentage | 18% | Heavy vehicle share of ADT. |
| Annual Growth Rate | 3% | Moderate long-term growth. |
| Truck Factor | 0.65 ESAL/truck | Average axle damage per truck. |
| Reliability | 90% | Common screening target. |
| Terminal Serviceability | 2.5 | Typical major road screening threshold. |
| Subgrade CBR | 5 | Used only if MR is omitted. |
| Base / Subbase MR | 30,000 / 15,000 psi | Illustrative layer stiffness values. |
| Possible Adopted Section | 6 in / 8 in / 8 in | Illustrative surface, base, and subbase trial section. |
Formula Used
1) Traffic ESAL Estimate
2) Growth Factor
3) Flexible Pavement Structural Number Equation
4) Layer Structural Number
5) Staged Layer Sizing Logic
How to Use This Calculator
- Enter design life, traffic, truck share, growth, and truck factor, or provide a direct ESAL.
- Set reliability, standard deviation, and terminal serviceability to match your planning assumptions.
- Enter subgrade CBR or, preferably, a tested subgrade MR.
- Enter base and subbase modulus values, layer coefficients, and drainage coefficients.
- Set minimum layer thicknesses and the upward rounding increment you want for adopted construction values.
- Click the calculate button to generate the required structural number and a trial section.
- Review the plotted thickness graph, the provided structural number, and any warnings shown above the form.
- Export the result using the CSV or PDF download buttons for reporting or further comparison.
FAQs
1) Is this a final construction design?
No. It is a screening calculator built around flexible pavement design relationships. Final thickness must still reflect local manuals, frost, drainage, climate, shoulder support, materials, construction quality, and agency detailing requirements.
2) Should I enter ESAL directly?
Use direct ESAL when a traffic study already exists. Otherwise, the page estimates cumulative ESAL from ADT, truck share, truck factor, growth, directional split, lane distribution, and design life.
3) Why are both CBR and MR included?
MR is preferred because the structural equation uses resilient modulus directly. CBR is helpful during early studies. When MR is blank, the calculator converts CBR into a planning-level subgrade modulus estimate.
4) Why are adopted thicknesses larger than theoretical values?
Theoretical thickness is the raw mathematical result. Adopted thickness is rounded upward and checked against your minimum layer limits, which makes the section easier to specify and construct.
5) What do drainage coefficients change?
They change the effective structural contribution of the unbound base and subbase. Better drainage can improve structural benefit, while poor drainage can reduce support and push the required thickness upward.
6) Can I model a pavement without subbase?
Yes. Set subbase modulus, layer coefficient, or drainage coefficient to zero, and keep minimum subbase thickness at zero. The calculator then places the remaining required structure in the upper layers.
7) Does this page design rigid concrete pavements too?
No. This page sizes flexible pavement layers. Rigid pavement design uses different inputs, such as slab thickness, modulus of rupture, k-value, drainage, and joint load transfer characteristics.
8) What units are used?
Thickness calculations use inches, and modulus values use psi. The result section also shows millimeter conversions so you can move quickly from conceptual sizing into drawings and reports.