Pavements & traffic · Pavement design
Asphalt overlay from FWD deflections calculator
Falling-weight deflectometer bowls at each test point give the surface modulus (from D0), the subgrade modulus (from D900) and the curvature function D0 − D200. Characteristic values are taken over the section, the existing pavement modulus is back-calculated by Odemark–Boussinesq so that it reproduces the characteristic D0, and the asphalt overlay is sized so the subgrade strain under the Standard Axle meets the Austroads criterion for the design traffic.
Standard: Austroads AGPT05 rehabilitation concepts · Boussinesq back-calculation (E0, Esg at 900 mm) · Odemark equivalent thickness · subgrade criterion N = (9300/µε)^7
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Test points: chainage (m), D0, D200, D900 (µm) at 40 kN | 0, 720, 480, 120 50, 810, 540, 130 100, 650, 430, 115 150, 900, 610, 145 200, 760, 500, 125 250, 840, 570, 140 300, 690, 450, 118 350, 870, 600, 150 | |
| FWD load | 40 | kN |
| Plate radius | 150 | mm |
| Existing pavement thickness | 350 | mm |
| Design traffic | 3000000 | ESA |
| SAR7/ESA | 1.6 | |
| Overlay asphalt modulus | 3000 | MPa |
| Odemark factor f | 0.9 | |
| Subgrade Poisson's ratio | 0.45 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Test points | 8 | |
| Characteristic D0 (mean + 1.645 SD) | 927 | µm |
| Characteristic curvature D0 − D200 | 295 | µm |
| Subgrade modulus (10th percentile, from D900) | 76 | MPa |
| Back-calculated existing pavement modulus | 240 | MPa |
| Subgrade strain now (Standard Axle) | 1,066 | µε |
| Allowable now / required SAR7 | 3,850,402 / 4,800,000 | |
| Overlay thickness for the subgrade criterion | 3 | mm |
| Recommended overlay | 40 mm asphalt |
- ✗ Existing pavement carries the design traffic without overlay
- ✗ Curvature D0 − D200 ≤ 200 µm (asphalt not fatigue-critical) — 295 µm
- ✓ Overlay ≤ 150 mm (beyond this consider reconstruction) — 40 mm
Method, formulas and assumptions
- Surface modulus E0 = 2(1 − ν²)σ0·a/D0 and subgrade Esg = (1 − ν²)σ0·a²/(r·Dr) at r = 900 mm (Boussinesq, ν = 0.35 and 0.45).
- Existing pavement modulus found so that the Odemark two-layer deflection matches the characteristic D0; overlay sized with he = f(t(Ea/Esg)^⅓ + H(Ep/Esg)^⅓) and the on-axis Boussinesq strain under one 40 kN dual-tyre set at 750 kPa.
- Austroads AGPT05 uses deflection-reduction charts or a full mechanistic analysis with temperature and seasonal corrections; check the overlay's own fatigue with CIRCLY.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.