20.2. Traffic and loading

Traffic is the most important factor in the pavement design. The key factors include contact pressure, wheel load, axle configuration, moving loads, load, and load repetitions.

Traffic is crucial in determining how pavements are designed. It influences various aspects of pavement performance, such as durability and load-bearing capacity. When planning a pavement, engineers must consider multiple factors that traffic presents, which include the pressure that tires exert on the pavement, the weight of vehicles, and how often these loads are applied.

Imagine a sponge (the pavement) being pressed by your hands (the vehicles). The way you press influences how the sponge responds, similar to how traffic impacts pavement. If many hands press down heavily on the sponge often, it will wear faster, just like heavy, frequent traffic affects road durability.

The tyre pressure is an important factor, as it determines the contact area and the contact pressure between the wheel and the pavement surface. Even though the shape of the contact area is elliptical, for sake of simplicity in analysis, a circular area is often considered.

Contact pressure is the force exerted by the tire on the pavement. It is calculated based on the tire pressure and the size of the tire's contact area with the pavement. While the actual contact area is more elliptical, it is often simplified to a circular shape in calculations for easier analysis. Understanding this helps engineers predict how pavement will wear over time under different traffic conditions.

Think of standing on a soft surface, like sand. If you stand on one foot, your weight is concentrated on a smaller area, creating deeper impressions. If you spread your weight by standing on both feet, the imprint is shallower. Similarly, tires exert pressure based on how much weight they carry and the area in contact with the pavement.

The next important factor is the wheel load which determines the depth of the pavement required to ensure that the subgrade soil does not fail. Wheel configuration affects the stress distribution and deflection within a pavement. Many commercial vehicles have dual rear wheels which ensure that the contact pressure is within the limits.

Wheel load refers to the weight supported by each wheel of a vehicle. It is crucial for determining how thick or deep a pavement needs to be to support that weight without damaging the underlying soil (subgrade). The design accounts for different wheel configurations, as they can distribute the weight differently, impacting the pavement's durability. Dual rear wheels help spread the weight more evenly.

Consider a flat-bottomed boat on a lake. If a single person stands on one end, the boat might tip or sink. But if two people stand at opposite ends, it stays balanced. Similarly, wheel load distribution determines how well pavements hold up under heavy vehicles.

The load carrying capacity of the commercial vehicle is further enhanced by the introduction of multiple axles.

Axle configuration refers to the arrangement and number of wheels on a vehicle. More axles generally allow a vehicle to carry heavier loads because they distribute the weight across a wider area of the pavement. This distribution helps prevent localized stress that could damage the pavement structure.

Think of a team of horses pulling a heavy load. If all the weight is on one horse, it might struggle or get hurt. But if several horses share the load, they can pull it more easily. Similarly, multiple axles help distribute a vehicle's weight to protect the pavement.

The damage to the pavement is much higher if the vehicle is moving at creep speed. Many studies show that when the speed is increased from 2 km/hr to 24 km/hr, the stresses and deflection reduced by 40 percent.

The speed at which vehicles travel can significantly affect how pavement endures weight. At slower speeds (creep speed), the same weight can do more damage, leading to greater pressure and deformation over time. Studies indicate that increasing the speed can reduce the stresses experienced by the pavement—meaning that faster-moving loads are less damaging than those moving slowly.

Consider how a heavy person walking slowly on grass leaves deeper footprints than if they were running. The same principle applies to roads and vehicles—moving quickly minimizes long-term impacts.

The influence of traffic on pavement not only depends on the magnitude of the wheel load, but also on the frequency of the load applications. Each load application causes some deformation and the total deformation is the summation of all these. Although the pavement deformation due to single axle load is very small, the cumulative effect of the number of load repetitions is significant.

Regarding pavement design, it’s not just about how heavy a vehicle is, but also how often it drives over a given area. Every time a vehicle rolls over the pavement, it causes slight deformation. While a single pass may not show much damage, the cumulative effect of many vehicle passes can lead to significant wear and tear. This is why modern design practices focus on the total expected traffic load over time.

Think of the wear on a carpet in a hallway. The carpet may look fine at first, but with countless steps taken over time, it eventually wears out. Similarly, repeated traffic on pavements gradually leads to more noticeable deterioration.

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