Overlay Design for Rigid Pavements

Rigid Pavement Overlay Design: FWD Tests, Formulas and Example

CIVILEASE · PAVEMENT ENGINEERING

Rigid pavement overlay design starts with the existing concrete road. Assess its slabs, joints and foundation, select the overlay system, then check whether a trial thickness can carry future traffic.

Technical references checked 12 October 2026. Educational guide; the governing project standard determines the final design method.

An overlay is a new pavement layer placed over an existing road to improve its condition or capacity. This guide covers overlays on existing rigid pavements: bonded concrete, unbonded concrete and asphalt over concrete. It explains the role of falling weight deflectometer (FWD) tests, deflection-basin AREA, concrete stiffness, joint load transfer and fatigue.

Overlay typesDesign procedureFWD formulasFatigue exampleThickness selectionFAQs

Which overlay is suitable for a concrete pavement?

System	How it works	Main design concern
Bonded concrete overlay	The new concrete bonds to the existing slab so the layers act together.	Sound existing concrete, surface preparation, bond and compatible joint detailing.
Unbonded concrete overlay	A separation layer reduces interaction between the new and old concrete.	Interlayer performance, support, drainage and movement of the existing pavement.
Asphalt overlay on concrete	An asphalt layer resurfaces and strengthens the existing system.	Reflection cracking at joints, pre-overlay repairs and the chosen asphalt design method.

Bonded and unbonded concrete overlays have different structural assumptions. An asphalt overlay also requires its own analysis; the same thickness equation should not be applied to all three systems. [1] [2]

Rigid pavement overlay design: step-by-step procedure

1. Survey pavement condition and identify the cause of distress

Map cracking, joint spalling, faulting, pumping, surface damage and drainage problems. Distinguish isolated defects from widespread slab deterioration. Covering damaged concrete without diagnosing the cause can allow distress to reappear through the new surface. [2]

2. Assemble the existing pavement and traffic inputs

Prepare a project data sheet before calculations begin:

• Slab thickness, concrete properties, base layers and construction history.
• Joint spacing, dowels, shoulder support and previous repairs.
• Axle-load distribution, heavy-vehicle traffic, growth and design period.
• Temperature conditions, drainage and seasonal foundation behaviour.
• Design reliability, performance limits, available elevations and construction constraints.

3. Plan FWD testing for the questions you need to answer

Use interior slab tests for stiffness evaluation, joint or crack tests for load transfer, and appropriate corner tests for loss of support. Record the applied load, loading-plate size, sensor offsets, temperature and test location. Repeat measurements help identify inconsistent data. FWD testing supports structural evaluation; it does not replace the condition survey. [3]

4. Backcalculate properties and check the model fit

Select a model compatible with the pavement and foundation. Compare its predicted basin with the measured deflections and investigate implausible parameters. Keep the backcalculated dynamic support value distinct from the support input required by the selected design method; any conversion must follow that method. [4]

5. Select the overlay system and prepare trial designs

Choose the system before calculating thickness. Document how the existing slab contributes to the new pavement, how the interface is represented, and how joints and cracks will be treated. Then evaluate trial thicknesses against traffic, environmental loading and the selected performance criteria.

FWD deflection-basin and stiffness formulas

AREA parameter: use the actual sensor spacing

The AREA parameter is the basin area normalised by the central deflection, so its unit is length, not square millimetres. For four sensors at 0, Δr, 2Δr and 3Δr, the trapezoidal expression is:

$$\mathrm{AREA}=\frac{\mathrm{\Delta r}}{2}[1+2\frac{D_1}{D_0}+2\frac{D_2}{D_0}+\frac{D_3}{D_0}]$$

D₀ is the central deflection; D₁, D₂ and D₃ are the deflections at successive offsets. Use one unit for all deflections. For the traditional 0, 12, 24 and 36 inch configuration, Δr/2 = 6 inches, giving AREA₃₆ in inches. Its exact metric offsets are 304.8, 609.6 and 914.4 mm; do not silently substitute 300, 600 and 900 mm into a calibration developed for another configuration. [5]

Worked example: a 300 mm-spaced sensor array

Assume deflections of 0.20, 0.18, 0.16 and 0.14 mm at offsets of 0, 300, 600 and 900 mm.

$$\mathrm{AREA}=150[1+2\times 0.9+2\times 0.8+0.7]=765\mathrm{mm}$$

This is the normalised basin integral for that array. It is not an overlay thickness or a direct estimate of remaining life. Use only a backcalculation calibration compatible with the chosen sensor positions.

Radius of relative stiffness and concrete modulus

For a homogeneous elastic plate on a Winkler foundation:

$$\ell ={\frac{E_c{h}^3}{12(1-{{\mu }_c}^2)k}}^{\mathrm{1/4}}$$

$$E_c=\frac{12(1-{{\mu }_c}^2)k{\ell }^4}{h^3}$$

Here h is slab thickness, ℓ is the relative stiffness radius, μ_c is Poisson’s ratio and k is the foundation reaction modulus. The fourth power of ℓ is essential. This relationship belongs to the stated plate-foundation model. [6]

Unit check: if h and ℓ are in mm and E_c is in MPa, use k in MPa/mm. Convert 50 MPa/m to 0.050 MPa/mm. Using 50 directly produces a result 1,000 times too large.

For h = 300 mm, ℓ = 1,000 mm, μ_c = 0.15 and k = 0.050 MPa/mm, the equation gives E_c ≈ 21,722 MPa. These assumed numbers illustrate units; they are not a recommended material specification.

Why k cannot be calculated as Pℓ²/D

That expression has the wrong dimensions for a foundation reaction modulus. Backcalculation includes the plate response, load geometry and normalised deflection coefficients. Use the documented AREA or best-fit implementation rather than inserting load and one deflection into an incomplete expression. [4]

Concrete stiffness is not concrete strength

E_c describes stiffness; compressive and flexural strength describe failure resistance. Establish strength using appropriate material tests or a justified correlation. If a particular empirical model states E_c = 5000√f_c with both values in MPa, its algebraic inversion is:

$$f_c={\left(\frac{E_c}{5000}\right)}^2$$

The exponent is 2, not 0.5. This algebra does not establish that the correlation is suitable for a particular aged pavement. Do not substitute an estimated compressive strength for the flexural strength required by the design model without justification.

Joint load transfer efficiency (LTE)

$$\mathrm{LTE}=100\frac{d_{\mathrm{unloaded}}}{d_{\mathrm{loaded}}}\%$$

Identify the loaded and unloaded slabs explicitly; arbitrary sensor labels D₁ and D₂ can reverse the ratio. For 0.16 mm on the unloaded slab and 0.20 mm on the loaded slab, uncorrected LTE = 80%. Some procedures apply a slab-bending correction, so report the convention, sensor positions and temperature. [3]

Fatigue life and cumulative fatigue damage

The stress ratio compares the tensile flexural stress used by the design model with the corresponding concrete flexural strength:

$$\mathrm{SR}=\frac{\sigma }{f_{\mathrm{mr}}}$$

For illustration, the PCA-type relationships reproduced in IRC:58-2015 are shown below. They are historical model equations, not a statement that this edition governs every current overlay project. Check the project’s required standard, stress definition and cracking mode.

For SR < 0.45: the model treats fatigue life as unlimited. This does not mean a real pavement is immune to other deterioration.

For 0.45 ≤ SR ≤ 0.55:

$$N={\left[\frac{4.2577}{\mathrm{SR}-0.4325}\right]}^{3.268}$$

For SR > 0.55:

$${\log }_{\mathrm{10}}N=\frac{0.9718-\mathrm{SR}}{0.0828}$$

N is the allowable number of repetitions at that stress ratio within the model. For several load or stress groups, Miner’s rule sums their consumed fatigue fractions: [7]

$$\mathrm{CFD}=\sum _{i=1}^m\frac{n_i}{N_i}$$

n_i is the expected repetition count for group i. Organise groups and top-down or bottom-up checks according to the adopted procedure. CFD is a damage index; it is not automatically a percentage of calendar life remaining.

Worked fatigue example

Take flexural stress σ = 2.25 MPa and flexural strength f_mr = 4.50 MPa. Then SR = 0.50 and the middle equation gives N ≈ 762,043 repetitions.

If that one stress group contributes 100,000 repetitions, its fatigue fraction is 100,000/762,043 ≈ 0.131. Additional groups must still be included. This example explains the calculation; it is not a completed overlay design.

How is the required overlay thickness selected?

Follow a documented, system-specific design method. Start with a trial thickness, calculate the structural response, check all required performance criteria, and revise the thickness or details until the design satisfies them.

As a useful comparison, the AASHTO 1993 procedures reproduced in the TxDOT manual distinguish the following concrete-overlay relationships:

Bonded concrete overlay:

$$D_{\mathrm{ol}}=D_f-D_{\mathrm{eff}}$$

Unbonded concrete overlay:

$$D_{\mathrm{ol}}=\sqrt{{D_f}^2-{D_{\mathrm{eff}}}^2}$$

D_f is the required slab thickness for future traffic and D_eff is the effective existing thickness determined by that procedure. These are method-specific, not universal shortcuts. They require the correct condition factors and design assumptions; do not equate D_eff with the measured thickness by default. They are not asphalt-overlay equations. [1]

Repair and construction details belong in the design

Prepare a repair schedule for defective slabs, joints, drainage and verified loss of support. Specify interface preparation or separation, joint layout, load transfer details, curing, level transitions and quality checks. Grouting is a treatment for suitable support defects—not a substitute for replacing badly damaged concrete. [8]

After construction, establish a baseline for ride, surface condition and joint behaviour so subsequent monitoring can identify whether the rehabilitation is performing as intended.

Design review checklist

• Confirm the existing pavement type and the cause of distress.
• Check sensor offsets, repeatability, test load and temperature.
• Use a backcalculation model appropriate to the structure.
• Keep stiffness, strength, dynamic support and design support distinct.
• Use consistent units and the correct stress-ratio equation.
• Include the required axle groups, environmental conditions and cracking modes.
• Specify pre-overlay repairs, interface treatment, joints and drainage.
• Document the governing standard, inputs, assumptions and performance checks.

Frequently asked questions

Can asphalt be placed over a concrete road?

Yes. Asphalt over concrete is a rehabilitation option, but existing joints and cracks can reflect through it. Assess the existing concrete and plan the required repairs and reflection-cracking treatment.

What is the difference between bonded and unbonded overlays?

A bonded overlay relies on the new and existing concrete acting together. An unbonded system uses a separation layer and different structural assumptions. The design method must match the system.

Does FWD testing directly give the overlay thickness?

No. It provides measured pavement responses used to estimate structural properties and joint behaviour. Thickness also depends on future traffic, materials, climate, interface conditions and performance requirements.

Is there one standard minimum thickness for every rigid pavement overlay?

No. A minimum specified for one agency, overlay type or construction method should not be transferred to a different project without checking its governing requirements.

Is remaining life equal to one minus CFD?

Not as a general calendar-life prediction. CFD sums fatigue fractions for specified stresses and repetition counts. Estimating years remaining requires a traffic forecast and the full adopted deterioration model.

Technical references

1. TxDOT Pavement Manual: bonded and unbonded concrete overlays
2. FHWA: treatment selection for concrete pavement distress
3. FHWA-HRT-16-011: deflection testing guidelines, Chapter 2
4. FHWA-HRT-16-009: rigid pavement backcalculation, Chapter 5
5. FHWA-HRT-05-152: AREA definitions and sensor offsets
6. FHWA-HRT-16-009: accessible descriptions of stiffness equations
7. IRC:58-2015, fatigue model; archived primary document
8. FHWA Concrete Pavement Preservation Guide, second edition

References describe specific models and editions. Confirm applicable revisions and agency requirements before using them for a project.

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