RPD NX
Web-based design and verification of road pavements — the AASHTO 1993 empirical method, the Ivanov rational (deflection) method and the Westergaard rational (rigid multilayer-plate) method, side by side, with a real-time 2D type-section, a 3D model of the package and a built-in AI assistant. Opens in any browser — no installs, no hardware keys.
What it does
RPD NX takes a road pavement from traffic and subgrade all the way to a fully verified, reinforced package — three design methods, real-time drawings and traceable normative reports, entirely in the browser.
Three design methods — AASHTO 1993 empirical (Structural Number), Ivanov rational (maximum deflection) and Westergaard rational (multilayer plate for rigid pavements), computed side by side.
Design traffic in equivalent standard axles — average daily traffic, growth rate, commercial share and lane distribution converted to 8.2-tonne ESALs with the fourth-power damage law and the road-type spectrum.
Layered pavement package — per-layer thickness and material coefficients for wear, binder, base and subbase courses, with each layer switchable in and out of the check.
Real-time 2D & 3D — a type-section that redraws as you type and a navigable 3D model of the pavement package.
Reports and cloud projects — Word and PDF calculation reports, SVG type-sections and .rpdnx projects saved to GeoDropbox, available in seven languages.
The three design methods
AASHTO 1993 — empirical method
Sizes the pavement by its Structural Number and checks the standard 8.2-tonne axles the structure can carry against those that will actually transit the design lane over its service life.
Structural Number — SN = Σ aᵢ · mᵢ · Dᵢ, from each layer thickness Dᵢ with its structural coefficient aᵢ and drainage coefficient mᵢ.
Safety factor — FS = N₈.₂,lim / N₈.₂,calc, verified when FS > 1.
Reliability-driven — reliability R and its normal deviate Z_R, overall standard deviation S₀, serviceability loss ΔPSI and subgrade resilient modulus M_R.
Ivanov — rational method (maximum deflection)
An elastic (Boussinesq) approach for flexible pavements that limits the surface deflection produced by the design wheel load.
Equivalent elastic modulus — built up from the subgrade (E₀ = 65 · CBR^0.65) through each active layer to a single E_eq.
Admissible deflection — f_adm = 0.17 − 0.026 · log₁₀(N), from the end-of-life daily standard axles N.
Safety factor — FS = f_adm / f_d, with the calculated deflection f_d = p · Dp / E_eq from tyre pressure p and footprint Dp.
Westergaard — rational method (rigid pavements)
Models a rigid (concrete) pavement as a multilayer plate on elastic soil, solved with a double Fourier series, to check the maximum slab deflection.
Per-layer stiffness — elastic modulus E, thickness and Poisson ratio for each layer, with the αK coefficient linking modulus to the reaction modulus (K = E / αK).
Subgrade reaction — the reaction modulus K closes the model; deflection is verified at the surface layer.
Safety factor — FS = f_adm / f_d, verified when FS ≥ 1.
How it works
A guided flow takes you from project setup to an exported report in eight steps.
1 · General data — project description, road type (which sets the traffic spectrum), design life and reliability/serviceability parameters.
2 · Geometry & preview — lanes, widths, shoulders and slopes, with the cross-section drawn in real time in 2D and 3D.
3 · Pavement stratigraphy — layer thicknesses and coefficients for wear, binder, base and subbase courses; toggle layers and add geogrid reinforcement.
4 · Subgrade & traffic — subgrade CBR or resilient modulus, average daily traffic (TGM), growth rate and commercial-vehicle share.
5 · Calculation method — choose AASHTO 1993, Ivanov or Westergaard.
6 · Results — safety factor, Structural Number, allowable axles and pass/fail, computed automatically.
7 · Meet verification — when FS < 1, clickable suggestions (thicker layers or a geogrid with a given TBR) that recalculate instantly; when over-designed, an optimisation card proposes trimming material.
8 · Export — Word/PDF reports, SVG sections and .rpdnx cloud projects.
Inputs and outputs
What you enter
Road type and traffic — TGM, growth rate, commercial-vehicle and design-lane shares, trajectory dispersion; standard deviation S₀ for AASHTO.
Design targets — service life, reliability R, initial and final PSI.
Subgrade — CBR, resilient modulus M_R or reaction modulus K.
Layers — per-layer thickness and, by method, structural coefficient a and drainage m, elastic modulus E, Poisson ratio and αK.
Wheel load — tyre inflation pressure and contact footprint.
What you get
Verification — safety factor and pass/fail for the chosen method.
Design quantities — Structural Number, allowable standard axles (N₈.₂,lim), admissible and calculated deflections, equivalent modulus.
Drawings — real-time 2D type-section and navigable 3D package model.
Deliverables — Word/PDF report, SVG section and .rpdnx cloud project, in seven languages.
Reinforcement, AI and optimisation
Bituminous geogrids — add a geogrid interlayer and apply its Traffic Benefit Ratio (TBR) to raise the allowable traffic, straight from the manufacturer's data sheet.
Auto-solver — when a check fails, RPD NX solves for the layer thicknesses or reinforcement needed and offers them as one-click, instantly-recalculated proposals.
Optimisation — when a package passes with margin, it suggests trimming thickness while staying compliant, saving material.
Built-in Gemini assistant — an optional AI layer explains every parameter and result, identifies the governing method and proposes motivated changes to layers and materials.
Who it's for
Road and infrastructure engineers · Pavement designers · Geotechnical engineers · Public-works technicians · Consultants. RPD NX is for anyone who today sizes road pavements with spreadsheets or desktop tools and wants a single web environment that goes from traffic and subgrade to a fully verified, reinforced package with traceable normative output.
Why it's different
Runs directly in the browser — no installation, no hardware key, always up to date.
Three established methods — AASHTO 1993, Ivanov and Westergaard — in one place, side by side.
Faithful to the originals and aligned with the Italian CNR pavement catalogue; every formula and coefficient transcribed verbatim.
Real-time 2D type-section and 3D package model as you design.
Built-in AI that explains results and proposes compliant, motivated changes.
Traceable Word/PDF reports and cloud projects, in seven languages.
Frequently asked questions
Are AASHTO 1993, Ivanov and Westergaard still current?
Yes. They are classic, consolidated methods that remain the standard professional reference and underpin the Italian CNR pavement catalogue.
Why is the reliability deviate Z_R negative?
That is correct: Z_R is the standard-normal deviate for reliability R and is negative for R > 50% (for example −1.282 at 90%), which makes higher-reliability designs more conservative.
Where is the Calculate button?
There isn't one. Results are computed automatically when you open the Results tab, and only when the inputs have changed.
What happens if a check fails (FS < 1)?
A “how to meet verification” card appears with clickable proposals — thicker layers or a geogrid with a given TBR — that recalculate instantly.
What is a geogrid's TBR?
The Traffic Benefit Ratio multiplies the allowable axles gained from a bituminous geogrid; use the value from the manufacturer's technical sheet.
What units does the Structural Number use?
SN is shown in inches (with the centimetre equivalent); you enter layer thicknesses in centimetres.
Ready to start with RPD NX?
Choose your plan and start using RPD NX today. Runs in the browser, always up to date, no installation required.