Pavement Thickness Calculator
AASHTO Method (USA)
The AASHTO method for flexible pavement design is based on the following formula:
Where:
- W₁₈: ESALs (Equivalent Single Axle Loads), the cumulative traffic load over the pavement’s design life (number of 18-ton axle loads).
- Zᵣ: Standard normal deviate corresponding to the desired reliability level (e.g., -0.8416 for 80% reliability).
- Sₒ: Overall standard deviation accounting for uncertainty in traffic estimates and pavement performance (typically 0.45 for flexible pavements).
- SN: Structural Number, representing the required pavement structural capacity.
- ΔPSI: Change in pavement serviceability index (difference between initial and terminal serviceability, typically ΔPSI = 1.7).
- M_R: Resilient modulus of the subgrade soil (in psi). A common correlation is , where CBR is the California Bearing Ratio of the soil.
Once SN is calculated, layer thicknesses can be derived as:
Where:
- a₁, a₂, a₃: Layer coefficients for surface, base, and sub-base layers.
- D₁, D₂, D₃: Thickness of the surface, base, and sub-base layers, respectively.
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CBR Method (India, UK, Global)
The most commonly used formula for the CBR method is:
Where:
- T: Total pavement thickness (in mm).
- k: Empirical constant (usually based on local standards or data).
- P: Wheel load (in Newtons).
- CBR: California Bearing Ratio of the subgrade, expressed as a percentage.
- n: Exponent (often between 0.3 and 0.5, based on local standards).
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Westergaard Method (Concrete Pavement)
The Westergaard method for rigid pavement design is used to calculate the required slab thickness for concrete pavements:
Where:
- h: Thickness of the concrete slab (in mm).
- P: Applied wheel load (in Newtons).
- ν: Poisson’s ratio of concrete (typically around 0.15–0.20 for concrete).
- E: Modulus of elasticity of concrete (in MPa).
- k: Modulus of subgrade reaction (in kPa/m).
- Z: Critical stress in the concrete slab (depends on load location and slab geometry).