Transportation Pavement Thickness Design

What Is Pavement Thickness Design?

Pavement thickness design means determining how much thickness is required in each layer of a road, including:

• Asphalt layer
• Base layer
• Subbase layer
• Subgrade support

The purpose is simple:
👉 carry traffic loads
👉 resist deformation
👉 avoid cracks and failure
👉 ensure long life

A properly designed pavement reduces maintenance cost and prevents early failure, especially in high-traffic highways.

Why Pavement Thickness Matters

Wrong thickness = early failure.

💥 Rutting
💥 Cracking
💥 Potholes
💥 Excessive maintenance

Correct thickness ensures the pavement can safely carry Expected traffic load + Environment effect + Material strength for a defined number of years.

Types of Pavements

There are two main pavement types:

✔ Flexible Pavements (Asphalt Roads)

• Made of bitumen layers
• Flexible under load
• Most common worldwide

✔ Rigid Pavements (Concrete Roads)

• Made of Portland cement concrete
• High stiffness
• Strong against heavy loads

This article mainly explains asphalt pavement thickness design based on AASHTO guidelines.

Key Design Parameters You Should Know

To design pavement thickness, we need to estimate many engineering inputs. Here are the important ones explained in simple words.

1. ESALs (Equivalent Single Axle Loads)

ESAL means the total wheel load a pavement will experience during its entire life based on a standard axle load.

More traffic = more ESAL
More ESAL = more thickness

Example categories:

Road Type	ESAL Value
Local Street	0.5 Million
Collector Road	2 Million
Arterial Road	10 Million
Highway	30 Million

2. Subgrade CBR

CBR (California Bearing Ratio) indicates soil strength. Weak soil needs thicker pavement.

Example:

CBR	Support
3	Very weak
6	Medium
10+	Strong

3. Material Strength

Stronger asphalt reduces thickness.
Better base material also reduces thickness.

4. Climate Zone

Climate affects moisture, freeze–thaw, cracking etc.

Climate	Factor
Arid	1.0
Temperate	1.2
Freeze	1.4

Higher factor = more thickness

5. Design Life

Expected life:

• Local streets – 10 to 15 years
• Highways – 30 to 40 years

Longer life = higher thickness

How Thickness Is Distributed Among Layers

Most thickness design divides pavement into:

• Asphalt (wearing + binder)
• Base course
• Subbase course

Typical distribution:

• Asphalt = 55–60%
• Base = 25%
• Subbase = 15%

AASHTO 1993 Pavement Design – Simple Explanation

The calculator uses the world-famous AASHTO 1993 pavement design approach, which converts traffic load into structural number (SN) and converts it into thickness.

SN increases with:
✔ heavy traffic
✔ weak soil
✔ climate factor

Then SN is divided into thickness based on:
✔ material coefficient
✔ asphalt stiffness
✔ base type

Your Online Pavement Thickness Calculator (How It Works)

Your online tool automatically calculates pavement thickness based on inputs like:

• Traffic category
• Design life
• Growth rate
• CBR value
• Asphalt strength
• Base material
• Climate factor

👉 It instantly displays:

• Total thickness in mm
• Asphalt layer
• Base layer
• Subbase layer
• ESALs

This helps engineers and students get quick preliminary numbers without manual calculation.

Recommended CBR vs Thickness (General Guidelines)

CBR	Typical Asphalt Thickness
3	200+ mm
5	150–180 mm
8	120–150 mm
12	100–120 mm

Influence of Traffic Categories

Your calculator already uses traffic presets such as:

• Local roads
• Collector roads
• Arterials
• Highways

Higher traffic = thicker layers.

Material Selection Impact

Material	Impact
Granular base	Normal
Stabilized	Thinner
Lean concrete	Very strong base

Example Output from Calculator

Sample result:

• Asphalt: 140 mm
• Base: 210 mm
• Subbase: 110 mm
• Total: 460 mm

This is typical for medium-volume collector roads.

Key Lessons (Easy Summary)

• Pavement design depends on ESAL, CBR, climate and materials
• More traffic = more thickness
• Weak soil = higher thickness
• Climate zones increase required thickness
• AASHTO 1993 remains globally accepted

Use the Calculator for Concept + Planning

This calculator should be used for:
✔ early feasibility
✔ academic learning
✔ design comparison
✔ project planning

But final design must be approved by a professional civil engineer.

Important Note

“These results are preliminary and based on standardized AASHTO equations. Always verify final design through soil tests, material properties, field evaluation, and local guidelines.”