Calculator Inputs
Example Data Table
| Scenario | W18 | R | So | Pt | Sc' | Ec | k | J | Cd | Indicative D |
|---|---|---|---|---|---|---|---|---|---|---|
| Urban arterial | 20,000,000 | 95% | 0.35 | 2.5 | 4.5 MPa | 28 GPa | 40 MN/m³ | 3.2 | 1.0 | ~230–280 mm* |
| Industrial access | 5,000,000 | 90% | 0.35 | 2.5 | 4.2 MPa | 26 GPa | 30 MN/m³ | 3.6 | 0.9 | ~200–260 mm* |
Formula Used
This tool solves for slab thickness D (inches) using the 1993 AASHTO rigid pavement design equation. The equation is evaluated in log space and solved iteratively by bisection.
+ log10(ΔPSI/(4.5−1.5)) / (1 + 1.624×10^7/(D+1)^8.46)
+ (4.22 − 0.32·Pt) · log10( (Sc'·Cd·(D^0.75 − 1.132)) / (215.63·J·(D^0.75 − 18.42 / ( (Ec/k)^0.25 ))) )
- W18: cumulative 18-kip ESALs.
- ZR: standard normal deviate from reliability.
- So: overall standard deviation.
- Sc': concrete modulus of rupture; Ec: concrete elastic modulus; k: modulus of subgrade reaction.
- J: load transfer coefficient; Cd: drainage coefficient; ΔPSI = Po − Pt.
How to Use This Calculator
- Enter your design traffic as total ESALs (W18) for the design life.
- Select reliability and set So based on guidance or local practice.
- Provide serviceability indices (Po, Pt) consistent with your roadway class.
- Enter material properties and support values: Sc', Ec, and k.
- Set joint/load transfer and drainage adjustments: J and Cd.
- Click Calculate Thickness, review results, then download CSV/PDF for records.
Rigid Pavement Thickness: Practical Design Notes
This short technical guide summarizes key inputs used by the calculator and how to interpret results.
Design purpose and context
Rigid pavement thickness is selected to control cracking and faulting under repeated axle loads while meeting long‑term ride quality targets. The calculator applies an AASHTO‑style relationship between cumulative traffic (ESALs), reliability, serviceability loss, slab strength, load transfer, drainage, and subgrade support. It supports rapid what‑if studies during early pavement planning.
Traffic, reliability, and variability
Traffic is entered as design ESALs. For context, light industrial roads may be 1–5 million ESALs, urban arterials often 10–30 million, and major highways can exceed 50 million. Reliability shifts the design toward thicker slabs; 90–95% is common for primary routes, while 70–85% may suit low‑volume facilities. Standard deviation So captures overall uncertainty; 0.30–0.40 is common for preliminary design.
Concrete and support parameters
Concrete flexural strength Sc′ (modulus of rupture) typically ranges 3.8–5.5 MPa, and elastic modulus Ec commonly falls near 25–35 GPa depending on mix and aggregates. The modulus of subgrade reaction k is strongly affected by base thickness and moisture; many projects fall between 50–200 pci (≈14–54 MN/m³). Serviceability uses Po and Pt; Po ≈ 4.5 and Pt ≈ 2.0–2.5 are widely used for highways.
Joint transfer and drainage effects
Joint/load‑transfer factor J reflects dowels, joint spacing, and edge support; lower values indicate better load transfer and can reduce required thickness. Drainage coefficient Cd accounts for how quickly the base drains; values below 1.0 penalize poor drainage, while stabilized, well‑drained bases may justify Cd above 1.0.
Use, checks, and documentation
Use the result as a starting thickness, then validate with local specifications, minimum constructability limits, frost or expansive soil requirements, and fatigue/erosion checks where required. Typical slab thicknesses for many roadway projects fall in the 200–350 mm range, but heavy freight corridors and poor support can push higher. Record assumptions, compare scenarios (e.g., improved base raising k, dowels lowering J), and use the CSV/PDF exports to document design iterations.
FAQs
What does this calculator output?
It estimates required concrete slab thickness using an AASHTO‑style rigid pavement design equation, based on traffic ESALs, reliability, serviceability targets, material strength, support, drainage, and load transfer factors.
Which units should I use?
Use any consistent units. If you enter k in pci, keep Sc′ in psi and Ec in psi for U.S. practice. If you use SI, enter k in MN/m³, Sc′ in MPa, and Ec in GPa; the tool converts internally.
How do I choose design ESALs?
Prefer a traffic study or agency forecast. Convert axle-load spectra to 18‑kip ESALs using approved equivalency factors, then sum over the design period and lane distribution. Add growth and directional factors per your jurisdiction.
What is a typical range for k?
Many roadway projects fall around 50–200 pci (about 14–54 MN/m³). Improved base layers and better drainage generally increase k, while weak, wet, or pumping subgrades reduce it and often require thicker slabs.
Why does reliability change thickness?
Higher reliability reduces the risk of premature distress by shifting the design to a more conservative thickness through the ZR term. Moving from 80% to 95% reliability can add meaningful thickness, especially at high traffic levels.
When should I adjust J and Cd?
Adjust J when dowels, joint spacing, tied shoulders, and edge support change load transfer. Adjust Cd when base drainage quality or time-to-drain changes. Poor drainage (Cd < 1.0) and weak edges typically increase thickness.
Is this a final design tool?
Use it for preliminary sizing and scenario comparison. Final design should follow your local pavement manual, material testing, erosion/fatigue checks where required, constructability minimums, and any frost or expansive soil criteria.