19. Introduction to pavement design

A highway pavement is a structure consisting of superimposed layers of processed materials above the natural soil sub-grade, whose primary function is to distribute the applied vehicle loads to the sub-grade. The pavement structure should be able to provide a surface of acceptable riding quality, adequate skid resistance, favorable light reflecting characteristics, and low noise pollution. The ultimate aim is to ensure that the transmitted stresses due to wheel load are sufficiently reduced, so that they will not exceed the bearing capacity of the sub-grade. Two types of pavements are generally recognized serving this purpose, namely flexible pavements and rigid pavements. This chapter gives an overview of pavement types, layers and their functions, and pavement failures. Improper design of pavements leads to early failure of pavements affecting the riding quality also.

In this overview, we learn that a highway pavement is a multi-layered structure built above the natural soil. Its main role is to support vehicle loads and distribute them to the ground below safely. Key features of good pavement include ride comfort, skid resistance, proper light reflection, and minimal noise. Pavements must also minimize stress so that the underlying soil (the sub-grade) is not harmed. There are two main types of pavements: flexible and rigid. This section warns that poor design can lead to early failures, impacting the smoothness and quality of driving.

Think of a highway pavement like a cushioned floor for your home. Just as a well-cushioned floor distributes your weight and makes walking comfortable, a pavement distributes the weight of vehicles for a smooth ride. If the floor is poorly made, it might crack or be uneven, just like a bad pavement can lead to bumps and holes affecting safety and ride quality.

The pavement should meet the following requirements:

• Sufficient thickness to distribute the wheel load stresses to a safe value on the sub-grade soil.
• Structurally strong to withstand all types of stresses imposed upon it.
• Adequate coefficient of friction to prevent skidding of vehicles.
• Smooth surface to provide comfort to road users even at high speed.
• Produce least noise from moving vehicles.
• Dust proof surface so that traffic safety is not impaired by reducing visibility.
• Impervious surface, so that sub-grade soil is well protected.
• Long design life with low maintenance cost.

This section outlines crucial requirements that a pavement must fulfill to function effectively. Firstly, it should be thick and strong enough to safely transfer vehicle loads to the sub-grade soil. Adequate friction is essential for vehicle safety, and the pavement must provide a smooth surface for comfortable and safe driving. Reduced noise levels are important for the environment, and the surface should be dust-proof, ensuring visibility is not compromised. Additionally, it should protect the underlying soil and have a long lifespan with minimal maintenance.

Imagine driving on two different types of roads: one is smooth and well-maintained, while the other is rough and full of potholes. The smooth road is like a well-designed pavement that meets all requirements and provides a comfortable ride. Just as you'd prefer to drive on the smooth road to avoid discomfort or accidents, pavements need to have specific features to ensure safety and durability.

The pavements can be classified based on the structural performance into two, flexible pavements and rigid pavements. In flexible pavements, wheel loads are transferred by grain-to-grain contact of the aggregate through the granular structure. The flexible pavement, having less flexural strength, acts like a flexible sheet (e.g., bituminous road). On the contrary, in rigid pavements, wheel loads are transferred to sub-grade soil by flexural strength of the pavement, and the pavement acts like a rigid plate (e.g., cement concrete roads). In addition to these, composite pavements are also available. A thin layer of flexible pavement over rigid pavement is an ideal pavement with most desirable characteristics.

This section introduces two main types of pavements based on how they perform structurally: flexible and rigid. Flexible pavements work by allowing load to pass through the aggregate layers, resembling a flexible sheet. Rigid pavements, however, are designed to act more like a stiff plate that spreads loads over a larger area. Composite pavements, which combine both types, are ideal for specific situations but are less common due to costs and complexity.

Think about a yoga mat (flexible pavement) versus a piece of strong plywood (rigid pavement). When you place your weight on the yoga mat, it flexes, spreading your weight but remains soft. The plywood, however, supports your weight firmly without bending, thus spreading the load evenly. The best option might be to place the yoga mat on the plywood for an ideal balance of comfort and support, just like composite pavements do.

Flexible pavements will transmit wheel load stresses to the lower layers by grain-to-grain transfer through the points of contact in the granular structure. The wheel load acting on the pavement will be distributed to a wider area, and the stress decreases with the depth. Taking advantage of this stress distribution characteristic, flexible pavements normally has many layers. Hence, the design of flexible pavement uses the concept of layered system. Based on this, flexible pavement may be constructed in a number of layers and the top layer has to be of best quality to sustain maximum compressive stress, in addition to wear and tear.

Flexible pavements function by spreading the weight of vehicles across many layers of materials, which means that stresses decrease the deeper you go. They typically consist of multiple layers designed to provide support, with the highest quality materials used at the top where the most wear occurs. This layered design helps ensure the pavement remains durable and effective.

Imagine a multi-layer cake. The top layer needs to be the best quality because it is what everyone sees and takes the first bite of, just like the top layer of a flexible pavement bears the most load. The layers below support it but don’t need to be quite as perfect, similar to how the cake layers support the top layer.

The major flexible pavement failures are fatigue cracking, rutting, and thermal cracking. The fatigue cracking of flexible pavement is due to horizontal tensile strain at the bottom of the asphaltic concrete. The failure criterion relates allowable number of load repetitions to tensile strain and this relation can be determined in the laboratory fatigue test on asphaltic concrete specimens. Rutting occurs only on flexible pavements as indicated by permanent deformation or rut depth along the wheel load path. Two design methods have been used to control rutting: one to limit the vertical compressive strain on top of the subgrade and the other to limit it to a tolerable amount (12 mm normally). Thermal cracking includes both low-temperature cracking and thermal fatigue cracking.

This section discusses typical failures that can occur in flexible pavements. Fatigue cracking is a result of repeated stress and strain, often found at the bottom of the pavement layer due to traffic loads. Rutting, which is common in flexible pavements, refers to the permanent deformation along pathways where vehicles frequently drive. To manage rutting, engineers limit the amount of strain that the underlying subgrade can take. Additionally, thermal cracking can occur due to temperature changes affecting the pavement's structure.

Think of flexible pavements as a piece of clothing. If you wear a shirt repeatedly, it may develop cracks and wear, especially in high-stress areas like the elbows or knees. Just like how a shirt can wear out due to frequent use in specific areas, pavements can develop fatigue cracking or rutting due to consistent vehicle pressure on certain paths.

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