20.2.5. Repetition of Loads

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.

Traffic affects pavements in two main ways: how heavy the vehicles are (magnitude of wheel load) and how often these loads hit the pavement (frequency of load applications). A heavy vehicle applying weight on the pavement will cause significant wear and tear, but if this vehicle comes over frequently, the damage can be much greater than if it just passed by occasionally.

Think of a sponge. If you place a heavy object on the sponge only once, it might compress a little but can bounce back. However, if you repeatedly push down on it, the sponge gets crushed over time. In the same way, a pavement can tolerate a single heavy load, but many repeated loads will lead to more significant damage and deformation.

Each load application causes some deformation and the total deformation is the summation of all these.

Every time a vehicle drives over a pavement, it causes a slight deformity. Each of these deformations may seem small and insignificant on their own, but when you add them all up over time, they can create substantial damage. This cumulative effect emphasizes the importance of considering how often and repeatedly loads are applied to pavements when designing them.

Imagine walking on a soft carpet with heavy shoes. Every step leaves a slight indentation. Initially, it looks the same, but after many steps, those minor indentations create a noticeable dip in the carpet. Similarly, pavements experience tiny changes with each vehicle load that accumulate into larger, more serious damage that requires repairs.

Although the pavement deformation due to single axle load is very small, the cumulative effect of number of load repetitions is significant.

While a single axle load causes minimal deformation, when you consider the total number of times that load, and similar loads, are repeated over the lifespan of the pavement, the total deformation can be considerable. This is why engineers focus on the total number of standard axle loads when designing pavements, to ensure they can withstand these repeated stresses.

This can be compared to filling up a bucket of water with a small drip. Each individual drop might not seem to make a difference, but eventually, after enough time, that small drip can fill the entire bucket. In terms of pavement design, each car that goes over contributes to the eventual 'fill' of wear and tear that leads to necessary maintenance or replacement.

Therefore, modern design is based on total number of standard axle load (usually 80 kN single axle).

Modern pavement design incorporates the understanding of load repetitions, meaning that engineers always calculate and plan for the total number of standard axle loads that will travel over a pavement. This calculation is crucial as it helps determine the thickness and materials needed to ensure the pavement lasts without premature failure, typically using a standard value like an 80 kN single axle load for consistent analysis.

Think of it like constructing a bridge. You wouldn't just consider one car’s weight; instead, you design for the maximum number of vehicles that might cross at once during rush hour. This ensures that the bridge remains safe and effective for all users. Similarly, pavement designs consider not just individual loads, but the expected total usage over time.

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