This tool solves the AASHTO 1993 rigid pavement equation for the concrete slab thickness a road needs, then checks that thickness against the pavement structure you propose. It is built for highway and pavement engineers, design consultants and students working to the AASHTO Guide for Design of Pavement Structures.
Like its flexible counterpart, the rigid pavement equation cannot be rearranged to give slab thickness directly. You supply the traffic, support and material inputs, then adjust the trial design thickness until the right-hand side of the equation balances the left. This page shows both sides and the difference, so you can see the moment they converge instead of guessing.
E_c = 57000 × √f′c (psi).S_c = 7.5 × √f′c, with a floor of 700 psi — if the computed value falls below that, 700 is used and the page tells you so.The modulus of rupture, not compressive strength, is what governs rigid design: a slab fails in bending, not crushing.
The subgrade modulus of subgrade reaction is taken as
k_sg = 20 × CBR (pci). This correlation is only applied for
CBR greater than 10 — below that the field reports "use CBR-k chart", because
the linear relationship is not reliable for weak subgrades and you should read k from
the published chart instead.
The composite value is then k_comp = k_sg × f_sb × f_lean,
where the two factors account for the granular subbase and lean concrete layers raising
the effective stiffness seen at the underside of the slab. It is k_comp, not
the raw subgrade value, that enters the thickness equation.
Section F compares the required slab thickness against the slab you have provided, together with the lean concrete and granular subbase beneath it, and reports the surplus or deficit in centimetres. The arrangement is drawn as a scaled cross-section schematic.
This implements the 1993 empirical method, still in wide use but superseded in some agencies by the mechanistic-empirical Pavement ME procedure. Confirm the required method, coefficients and reliability level against your governing design manual before finalising a design. For asphalt pavements, use the flexible pavement design calculator.
The AASHTO 1993 rigid pavement equation relates the required concrete slab thickness to cumulative equivalent single axle loads, reliability, overall standard deviation, serviceability loss, concrete modulus of rupture, concrete elastic modulus, the load transfer coefficient J, the drainage coefficient Cd and the composite modulus of subgrade reaction. It cannot be rearranged for thickness directly, so it is solved by iteration: you adjust the trial slab thickness until the right side of the equation balances the left. This calculator shows both sides and the difference between them.
The modulus of subgrade reaction k measures how stiffly the foundation pushes back under load, expressed as pressure per unit deflection, typically pounds per square inch per inch. It is the rigid pavement equivalent of the resilient modulus used in flexible design. A higher k means a stiffer support and allows a thinner slab, though the effect is much weaker than in flexible design because a concrete slab distributes load over a wide area regardless of what sits beneath it.
The composite k is the effective modulus of subgrade reaction seen at the underside of the concrete slab once a subbase has been placed over the natural subgrade. Adding a lean concrete or granular subbase raises the effective stiffness above the subgrade value alone. This calculator computes the composite k from the subgrade k together with the subbase layers you specify, and it is that composite value, not the raw subgrade value, that enters the thickness equation.
J accounts for how effectively load is transferred across joints and cracks from one slab to the next. Lower values mean better load transfer and therefore a thinner slab. Typical values are around 2.5 to 3.1 for jointed plain concrete pavement with dowelled joints and tied shoulders, rising to about 3.2 or higher where there are no dowels or where the shoulder is asphalt rather than tied concrete.
The modulus of rupture is the flexural tensile strength of the concrete, which is the property that actually governs rigid pavement design because a slab fails in bending rather than in crushing. It is far lower than compressive strength, commonly in the region of 10 to 15 percent of it, and is usually estimated from compressive strength by a published correlation or measured directly by a beam test.
Cd modifies the design to reflect how quickly water drains from beneath the slab and how much of the time the pavement structure is exposed to moisture near saturation. Values above 1.0 represent good drainage and permit a thinner slab, while values below 1.0 represent poor drainage and demand a thicker one. It is a direct multiplier on the computed capacity, so the assumption should match the drainage detailing actually being built.
A rigid pavement carries load in a concrete slab acting in bending, so the design output is a required slab thickness governed by flexural strength, load transfer across joints and the composite modulus of subgrade reaction. A flexible pavement spreads load progressively through asphalt, base and subbase layers, so its design output is a structural number that is then converted into layer thicknesses using layer coefficients. The two use different equations and are not interchangeable.
Reliability is the probability that the pavement carries its design traffic before reaching terminal serviceability, and it enters the equation through the standard normal deviate ZR. Higher reliability gives a more conservative, thicker slab. Typical values are 85 to 95 percent for principal arterials and motorways and lower for minor roads, but the governing agency design manual should always take precedence.