Zerdly

Rigid Pavement Thickness Calculator

Estimate the required concrete slab thickness for a rigid pavement from the AASHTO 1993 rigid-pavement design equation. Enter the design traffic, reliability and concrete/subgrade properties and the tool solves the implicit equation for slab depth D, reported in millimetres and inches.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the design traffic W18 (cumulative ESALs) and the reliability R (%); the tool maps R to the standard normal deviate ZR.
  2. Enter or accept the concrete and support defaults: So (0.35), ΔPSI (2.0), pt (2.5), Sc' modulus of rupture (650 psi), Cd (1.0), J (3.2), Ec (4,000,000 psi) and k (100 pci).
  3. Read the required slab thickness D — the millimetre value is emphasised, with the native inch value alongside.

How it works

The AASHTO 1993 rigid equation relates log10(W18) to the slab thickness D through 7.35·log10(D+1) − 0.06, a serviceability term log10(ΔPSI/3.0)/(1 + 1.624×10⁷/(D+1)^8.46), the reliability term ZR·So and a concrete-strength term (4.22 − 0.32·pt)·log10 of a ratio involving Sc', Cd, J, D, Ec and k. Because D cannot be isolated, the tool bisects on [1, 30] inches until the right-hand side equals log10(W18); when a trial D is too thin to give a valid (positive) log argument, the lower bound is raised. The result is converted from inches to millimetres (× 25.4).

Worked example

Worked example. For W18 = 5,000,000 ESALs, R = 95% (ZR = −1.645), So = 0.35, ΔPSI = 2.0, pt = 2.5, Sc' = 650 psi, Cd = 1.0, J = 3.2, Ec = 4,000,000 psi and k = 100 pci, the equation balances at D ≈ 9.59 in ≈ 244 mm — a typical highway concrete slab thickness.

Common mistakes

  • Entering SI values in the imperial fields — Sc', Ec are in psi and k is in pci; use the CBR-to-k converter to obtain k, and note 650 psi ≈ 4.48 MPa.
  • Using the flexible-pavement So of 0.45 — rigid pavements use a lower overall standard deviation of about 0.35.
  • Setting J for the wrong joint type — undoweled/aggregate-interlock joints use J ≈ 3.2, doweled joints use lower J (≈2.5–3.1), which reduces the required thickness.

Frequently asked questions

What is the modulus of subgrade reaction k?

k (in pci — pounds per cubic inch) describes how stiffly the subgrade/subbase supports the slab, from a plate-load test or estimated from CBR. Higher k gives more support and a thinner required slab. Use the CBR to Modulus of Subgrade Reaction Converter to get k.

Why is the result in inches as well as millimetres?

The AASHTO rigid equation is imperial-native and solves D in inches, so the tool shows both: the emphasised millimetre value for metric use and the inch value that the equation actually produces.

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