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Austroads Flexible Pavement Design: Axle Loads, Design Traffic, Granular & Asphalt Pavements

Step 1: pick the heavy vehicles using the road and enter their axle group loads → ESA per vehicle and design traffic (DESA). Step 2: subgrade CBR and materials → layer thicknesses from the Austroads empirical design chart for granular pavements with a thin bituminous surfacing.

Version: 2026-10-29 09:40Reference: Austroads AGPT02-26, Chapters 7, 8, 10 & 12 (Sections 8.2, 8.3, 12.8)Author: Shahid Pervaiz
👋 You are looking at sample data so you can see what this tool does. Change any value, or click New Project to start your own.

Project

Axle loads & design traffic

Traffic growth and design lane

How do you want to describe the heavy traffic?

Add one row per vehicle type (and load state, e.g. laden and empty as two rows). Enter the two-way daily number of that vehicle in the opening year and the load on each axle group. The axle groups start with the typical layout of each Austroads class (Figure 7.1) at the Standard Load — change them to your survey or legal loads. Light vehicles (classes 1–2) are ignored by the method (Section 7.2).

#Vehicle (Austroads class) & descriptionTwo-way per dayAxle groups & loadsGroupsESA / vehicle (damage factor)ESA / day (two-way)
Legal axle load limits:
Axle group codes (Austroads Section 7.2, Tables 7.7–7.8)
CodeAxle groupTypical useStandard load = 1 ESA
SASTSingle axle, single tyres (2 tyres)Steering (front) axle of most trucks and buses53 kN (5.4 t)
SADTSingle axle, dual tyres (4 tyres)Rear axle of a 2-axle truck; the Austroads Standard Axle80 kN (8.2 t)
TASTTandem axle, single tyres (2 axles, 4 tyres)Twin-steer front of 4-axle rigid trucks89 kN (9.1 t)
TADTTandem axle, dual tyres (2 axles, 8 tyres)Rear of 3-axle rigid trucks (10-wheeler); drive axles of prime movers135 kN (13.8 t)
TRSTTriaxle, single tyres (3 axles, 6 tyres)Trailers on wide single (super single) tyres121 kN (12.3 t)
TRDTTriaxle (tridem), dual tyres (3 axles, 12 tyres)Semi-trailer rear group (18- and 22-wheelers)182 kN (18.6 t)
QASTQuad-axle, single tyres (4 axles, 8 tyres)Heavy trailers on wide single tyres150 kN (15.3 t)
QADTQuad-axle, dual tyres (4 axles, 16 tyres)Heavy-haulage and some low-loader trailers226 kN (23.0 t)

Single-tyre groups: the standard load above is for tyres narrower than 375 mm. Wider tyres carry more for the same damage (Table 7.8), so pick the tyre width in the list next to the code. ESA of a group = (load ÷ standard load)4. Axles less than 2.1 m apart count as one group.

Legal limits are the most a truck may carry, not what it does carry. Overloading is common, so use axle load survey data where you have it. The Pakistan truck types in the vehicle list start at the NHA limits.

Use this when you only have a traffic count and a presumptive traffic load distribution (Sections 7.4.4 and 7.4.7, Appendix D).

Capacity check (Section 7.4.6)

Option 1 — sprayed seal (or thin asphalt) + granular: empirical design (Figure 8.4)

Granular materials (top to bottom)

The first row is the base (needs CBR ≥ 80%). Then list the subbase materials you have, strongest first. Leave a thickness blank and the tool sizes it; type a thickness to fix it and the tool checks it.

LayerMaterialDesign CBR (%)Thickness (mm)Used (mm)Depth to bottom (mm)Cover needed over this material (mm)Check

Selected / improved subgrade (optional, Section 8.3.2)

Material specifications — Option 1

Options 2–5 — mechanistic-empirical design (Section 8.2)

Pick the option above (Option 1, sprayed seal + granular, is Step 2). The buttons set the inputs below and only the panel for that option is shown; you can still change any input yourself, and the highlighted button follows your inputs.

For pavements with structural asphalt (40 mm or more) over granular layers, or full-depth asphalt. The tool models the pavement as elastic layers (granular and subgrade anisotropic, as in CIRCLY), works out the strains under the Standard Axle and the 53 kN single-tyre axle, and checks asphalt fatigue (bottom of each asphalt layer) and subgrade rutting (top of subgrade) against the design traffic. Engine checked against the guide’s CIRCLY results in Appendix K.1–K.4. Subgrade CBR and design traffic come from Steps 1 and 2.

Selected / improved subgrade (optional, any option, Section 8.2.2)

Modelled in 5 sublayers whose modulus rises from the subgrade to min(150 MPa, 10 × CBR, Esubgrade × 2t/150) (Equations 40–41). Rutting is checked at the top of this layer and at the top of the in situ subgrade. It also serves as the working platform the guide asks for under full-depth asphalt on weak subgrades (Section 3.2.5).

Option 5 — heavily bound cemented subbase (between the asphalt and the granular layer)

Option 2 — lightly bound cemented base (directly under the seal or asphalt)

Only the cracked (post-fatigue) behaviour is modelled, as the guide requires (Section 8.2.4): no fatigue check of this layer, but a minimum thickness and a minimum support (granular subbase at least 150 mm with a top sublayer of at least 150 MPa) to keep it micro-cracked only.

Asphalt layers (top to bottom) — options 3, 4 and 5

LayerMaterialThickness (mm)Design modulus E (MPa)Binder, mixBitumen volume Vb (%)LiftsSize this layer
Asphalt design modulus helper (Table 6.15, Equations 22–24) & WMAPT (Appendix B)
Use this modulus for

WMAPT from monthly mean air temperatures (Appendix B)

Enter the average air temperature of each month (mean of daily maximum and minimum) for a weather station near the project. Works for any country.

Fill from city:
Material specifications

Compare pavement options for this traffic

Designs five alternatives for the current traffic, subgrade and reliability (Table 8.3 Step 32: compare acceptable designs): sprayed seal + granular (Step 2), seal + lightly bound base, asphalt over granular, full-depth asphalt, and asphalt over a cemented subbase. Asphalt uses the modulus and Vb of your bottom asphalt layer. Enter your own rates to see an approximate construction cost per km (leave blank to skip costs).

Whole-of-life cost assumptions (edit to your agency’s practice)

Options to consider
Untick options that do not suit the project (e.g. sprayed-seal options where an asphalt wearing course is required). The comparison table, costs, observations, figure and report then cover only the ticked options; the others are listed as not considered.

Rates (your prices, in place)

MaterialUsed inUnitRate
Asphalt (dense graded)Options 3, 4, 5 (and thin asphalt in option 1)m³
Granular base / subbase (crushed rock, gravel)Options 1, 2, 3, 5m³
Cemented subbase (heavily bound)Option 5m³
Lightly bound cemented baseOption 2m³
Sprayed seal (DST / TST)Options 1, 2m²
Improved / selected subgradeAny option, when tickedm³

Rates in the currency above, per unit of finished work in place. Costs update as soon as you change a rate; the pavement designs do not need to be re-run.

Abbreviations: ESA = equivalent standard axle (a single axle with dual tyres carrying 80 kN); DESA = design traffic in ESA over the design period; HVAG = heavy vehicle axle group; TLD = traffic load distribution; CBR = California bearing ratio; NHA = National Highway Authority (Pakistan), whose legal axle load limits can fill the loads.

Scope and limitations of this tool
Design philosophy of the five options

Method, assumptions & references

  • ESA of an axle group = (L / SL)4 (Equation 37). Standard Loads SL: SADT 80, TADT 135, TRDT 182, QADT 226 kN (Table 7.7); single-tyre groups by tyre width (Table 7.8): SAST 53/58/71, TAST 89/98/119, TRST 121/132/162, QAST 150/164/201 kN.
  • Initial daily heavy vehicles in the design lane n = AADT × DF × %HV/100 × LDF (Equation 30). In the vehicle mix, n = Σ vehicles per day × DF × LDF.
  • Cumulative growth factor CGF = ((1 + 0.01R)P − 1) / (0.01R), CGF = P when R = 0 (Equation 31). Cumulative HV NDT = 365 × CGF × n (Equation 32), or the year-by-year sum capped at capacity (Equations 33–34, Section 7.4.6).
  • Axle groups NDT,HVAG = NDT × NHVAG (Equation 35); design traffic DESA = ESA/HVAG × NDT,HVAG (Equation 38).
  • Cover over a material, DESA 105–108 (Figure 8.4): t = [219 − 211(logCBR) + 58(logCBR)2] log(DESA/120). DESA 103–105 (Figure 12.2): t = max{100, 0.475[219 − 211(logCBR) + 58(logCBR)2] log(14 DESA)}. Materials with CBR ≥ 30 use the CBR 30 line; that cover is the minimum base thickness (CBR ≥ 80 base).
  • Mechanistic-empirical design (Step 3, Section 8.2): linear elastic layered analysis under the Standard Axle (4 × 20 kN, 750 kPa, radius 92.1 mm, spacing 330/1470/330 mm) and the 53 kN single-tyre axle (2 × 26.5 kN, 800 kPa, radius 102.4 mm, 2130 mm); asphalt isotropic (ν 0.4); granular and subgrade anisotropic (Eh = Ev/2, ν 0.35 / 0.45, f = Ev/(1+ν); in situ subgrade ν 0.45 cohesive or 0.35 non-cohesive, as selected); granular in 5 sublayers (Equations 42–43, Tables 6.4/6.5); asphalt fatigue N = SF/RF × [6918(0.856Vb + 1.08)/(E0.36με)]5 (Equation 25, SF 6, RF Table 6.17) summed over the traffic load distribution; subgrade N = (9150/με)7. Heavily bound cemented subbase: isotropic (ν 0.2) before cracking with fatigue N = RF × (K/με)12 for laboratory values (Equation 47, K from Equation 14 capped by Equation 13, RF Table 6.9) or N = (K/με)12 for presumptive values (Table 6.10, Equation 48, no reliability factor); after cracking cross-anisotropic, Ev = min(600, Table 6.8, 4 × support within 240–600), ν 0.35, with total allowable traffic from Equations 53–55 (needs 175 mm asphalt cover). Engine checked against the CIRCLY results of Appendix K.1–K.3 (strains within 0.5%, asphalt damage within 1–2%; cemented damage reproduced exactly from the guide’s strains, the 12th power turns a 0.7% strain difference into about 9%). Lightly bound cemented base (Sections 6.6, 8.2.4): cracked behaviour only, Ev = min(600, Table 6.8 under asphalt, 4 × support within 240–600), Eh = Ev/2, ν 0.35; minimum thickness max[200, 250 + 35 log(DESA/5×106) − 0.25(Eunder − 150)] (Equation 44, less any asphalt, not below 200 mm) and minimum support (subbase ≥ 150 mm, top sublayer ≥ 150 MPa); checked against Appendix K.4 (subgrade strain 827 µε, minimum thickness 255 mm). Selected / improved subgrade (Section 8.2.2): 5 sublayers, top Ev = min(150, 10 × CBR, Eunder × 2t/150) (Equation 40), ratio from Equation 41, Eh = Ev/2, ν 0.45; rutting checked on top of it and on the in situ subgrade; over 2 m it is taken as the subgrade. Full-depth asphalt warnings follow Section 3.2.5 (design CBR below 5%: very difficult to build; in situ CBR above 10% needed at construction). Lean-mix concrete is not included.
  • Pakistan NHA legal axle load limits (National Highways Safety Ordinance 2000 axle load regime, as published by NHA): front single axle 5.5 t, single axle with dual tyres 12 t, tandem 22 t, tridem 31 t (1 t = 9.80665 kN). Gross weights follow from these, e.g. 22-wheeler 5.5 + 22 + 31 = 58.5 t. Check the current NHA notification before use.
  • Improved subgrade design CBR = min{15, lab CBR, CBRbelow × 2(thickness/150)} (Equation 56).
  • A thin DGA/SMA surfacing counts towards the total cover over the in situ subgrade but not towards the base; sprayed seals count for nothing (Section 8.3.2).
  • The guide’s worked examples read values off the chart; this tool uses the chart equations and the rounding you pick, so results can differ from hand-read values by a few mm.
  • Thicknesses are minimum design values and take no account of construction tolerance (Section 8.1). CBR is not the only test of a granular material (Section 6.2.1). Asphalt fatigue of thin surfacings is not covered by this chart (Section 8.3).
  • Source: Austroads (2026) Guide to Pavement Technology Part 2: Pavement Structural Design, AGPT02-26, Sydney. This free tool is independent and not endorsed by Austroads; check designs against the guide and your road agency’s supplement.

How to use the Austroads Flexible Pavement Design tool

The tool designs a pavement to the Austroads Guide (AGPT02-26) in three steps — traffic, then the empirical granular design, then the mechanistic design — and can compare five pavement options for the same traffic. The calculations run in your browser; your design inputs are not uploaded (the site only counts page visits anonymously).

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Start with the sample. The page opens with sample data so you can see what the tool does. Load Sample Data brings it back; New Project clears it. Hover over any box for a short explanation.
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Step 1 — traffic. Enter the design period, heavy vehicle growth, direction factor and lane distribution factor (pick a typical LDF from Table 7.3). Then choose how to describe the heavy traffic:
  • Vehicle mix & axle loads: + Add vehicle, pick an Austroads class or a Pakistan NHA truck type, enter how many pass per day (both directions) and the load on each axle group. The codes (SAST, TADT, TRDT…) are explained in the key under the table; Set all axle loads to these limits fills the NHA legal loads. Enter laden and empty trucks as separate rows.
  • AADT, %HV, NHVAG & ESA/HVAG: when you only have a count and presumptive load data.
The result is the design traffic DESA, used by every option below.
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Option 1 — sprayed seal (or thin asphalt) + granular, Step 2. Enter the subgrade design CBR, the surfacing (sprayed seal such as DST/TST, or DGA/SMA under 40 mm), the rounding (layers are rounded up; every check is made against the unrounded chart value, so rounding never reduces the cover) and the minimum layer thickness. List your materials with their design CBR: base first (CBR ≥ 80%), then subbases, strongest first. A weaker base, a negative thickness or asphalt of 40 mm or more is refused (use Step 3 for thicker asphalt). Below CBR 2% the chart is extrapolated and the result is shown as QUALIFIED, not OK. Leave thicknesses blank to let the tool size them, or type your own to check them. Tick improved subgrade for select fill or lime-stabilised subgrade. The chart (Figure 8.4 / 12.2) and the cross-section show the result.
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Step 3 — common inputs for options 2 to 5. The subgrade CBR and design traffic come from Steps 1–2. Set the project reliability (95% is common for highways), the total granular thickness and the base standard. In the asphalt layer table give each layer’s thickness, design modulus E and bitumen volume Vb (11% typical). For E, open the modulus helper: choose binder and mix size, enter WMAPT (or click Fill from city: Lahore, or type 12 monthly temperatures and Calculate WMAPT), speed and air voids, then Apply modulus (it records the binder and mix on the layer). In the Lifts column enter how many construction lifts a layer is laid in, or leave it blank for the fewest lifts within the Table 6.11 range; the structural thickness does not change.
Subgrade below your minimum CBR? Set NHA practice: minimum design CBR (default 15%). When the subgrade is weaker, Steps 2 and 3 show a box comparing NHA practice (improve first) with Austroads (no structural minimum; thicker layers, and Section 3.2.5 for full-depth asphalt), and Add improved subgrade turns the layer on in both steps.
Weak subgrade? Tick selected / improved subgrade and give its thickness and design CBR (select fill, capping, lime stabilisation). It is modelled in 5 sublayers (Section 8.2.2), rutting is checked on top of it and on the in situ subgrade, and it works with every option below. For full-depth asphalt on a subgrade below CBR 5% the guide says construction is very difficult without it (Section 3.2.5) — the tool warns you.
Use only one cemented layer at a time: tick either the heavily bound cemented subbase (option 5) or the lightly bound cemented base (option 2), never both. Ticking one unticks the other. For options 3 and 4 leave both unticked.
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Option 2 — sprayed seal + lightly bound cemented base. Click the Option 2 button at the top of Step 3 (it ticks lightly bound cemented base and removes the asphalt layers), then enter its thickness and keep soaked CBR ≥ 30% ticked. Set the granular subbase to at least 150 mm. The Lightly bound base checks table shows the minimum thickness (Equation 44) and the support checks.
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Option 3 — asphalt over granular. Click Option 3 (both cemented options off, a granular thickness such as 300 mm kept) and enter the asphalt layers. Tick size on the layer to be sized and click Find minimum thickness of the selected layer: it tries the layer in 5 mm steps from the Table 6.11 minimum for its mix size and reports the lowest passing tested thickness. Use the modulus helper’s Apply modulus so each layer records its binder and mix; choose each layer’s Binder, mix in the table (typing a new E keeps the binder). A layer with no declared binder, or a multigrade / A10E binder, is QUALIFIED (Equation 25 is for conventional binders). Thicknesses whose lifts cannot fit Table 6.11 are skipped by the search. If you change an input while a search runs, the search is cancelled.
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Option 4 — full-depth asphalt. Click Option 4: the granular thickness is set to 0 and the asphalt sits directly on the subgrade. Use Find minimum thickness again.
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Option 5 — asphalt + heavily bound cemented subbase. Click Option 5 (it ticks the cemented subbase), then enter its thickness and choose the material (presumptive Table 6.7/6.10 values, or laboratory modulus, binder content and flexural strength — Equation 14 needs 3–5% binder, 3000–5000 MPa and 1.0–1.5 MPa). Tick post-cracking phase to count the life after the cement cracks — allowed only with at least 175 mm of asphalt above it; otherwise the cemented layer must last the whole design traffic. The results then show both phases and the total allowable traffic.
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Reading the Step 3 results. Pavement OK means every damage factor (design ÷ allowable) is 1 or less and the case is inside the method’s validated range; the governing check is named. QUALIFIED means the numbers pass but an input is outside what the method covers (for example the Austroads example traffic distribution instead of your vehicle mix, asphalt under 40 mm, a non-conventional binder, Equation 14 limits, lightly bound base under 105 ESA, full-depth asphalt on CBR below 5%) — it is not a design approval. Below 105 ESA asphalt fatigue is shown but does not govern (Section 12.8.1). Choose the subgrade Poisson’s ratio: 0.45 cohesive, 0.35 non-cohesive. The tables give the strains, the damage by axle group type and the layer model. The granular layer appears as 5 analysis sublayers — a calculation step, not construction layers.
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Compare all five options. At the end of Step 3, choose the road type (local, collector, main road / highway, freeway / motorway, urban): each option is then rated typical use, possible with conditions or not typical for that road and your traffic, following AGPT02 Section 2.2. Enter your prices in the Rates table (asphalt, granular, cemented, lightly bound and improved subgrade per m³; sprayed seal per m²) (asphalt, granular, cemented, lightly bound per m³; seal per m²), the carriageway width and currency, then click Compare pavement options. Set the whole-of-life assumptions too (analysis period, discount rate, reseal interval for the sealed options, overlay interval and thickness for the asphalt options). You get the options table, a Costs per km bill of quantities (each layer’s thickness, quantity per km, unit, rate and amount, then construction, maintenance present value and whole-of-life cost — it updates as soon as you change a rate), an Observations box explaining what the results mean (for example when option 5 has no advantage over option 3, or why the cheapest option to build may not be the best), and a figure of the five cross-sections at the same scale. Thicknesses are the lowest passing tested candidates (5 mm steps), not an optimisation of every layer. Options that are qualified or not buildable are marked and left out of the typical-use list and the cost ranking. Use this option loads an option into Step 3 for its full results. The five options are compared automatically when the page opens and again whenever you change an input (traffic, CBR, materials, reliability); the button re-runs it by hand. Step 3 above shows one option in full detail — the one its inputs describe (use Use this option to switch). Untick options in Options to consider to leave them out; add routine maintenance and salvage value for the full Equation 67 present worth. Each row shows the traffic basis of the option, and the box Choosing between the options lists the Chapter 10 criteria (whole-of-life cost first, but not alone).
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Scope and limitations. Open Scope and limitations of this tool at the end of Step 3 for the supported ranges and what the tool does not cover; the same text is printed at the end of the report. Equations and material sources used under the Step 3 results lists every equation and input source for the case.
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Vehicles from a spreadsheet. Export Vehicles (.csv) shows the format; edit it and use Import Vehicles (.csv) (replaces the list). Columns: Class (3–12, c, or pk6, pk10, pk14, pk18t, pk18d, pk22), Description, PerDay (two-way), AxleGroups (e.g. SAST 53; TADT 135; TRDT 182, loads in kN; wide single tyres SAST-W1 / SAST-W2).
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Save your work. Save Project (.json) downloads all inputs to your computer; Load Project opens it again. With Autosave in this browser ticked, the page also remembers your last inputs on this computer.
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Print or report. Print / Save as PDF prints this page. Generate Report builds a design report: report information (guide edition, tool build, input checksum, traffic data source, time, review status “not reviewed”), traffic, Step 2, the selected Step 3 design exactly as calculated (pass, qualified or fail, with all its messages), then the generated alternatives and the option comparison with its figure. It opens in a new tab, or inside the page if pop-ups are blocked, with its own Print / Save as PDF and Download buttons.
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Inputs are never uploaded. Thicknesses are minimum design values without construction tolerance. Check results against AGPT02 and your road agency’s requirements before use.

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