Pavement types | Flexible pavements | Rigid pavements

 Road pavement

In highway traffic, paving is the most basic supporting framework. Each layer of pavement has a variety of functions that must be taken into account throughout the design process. Depending on the traffic requirements, many types of pavements might be used. Pavement design flaws result in early pavement failure, which has an impact on ride quality.

Types of pavements

Flexible and stiff pavements are the two types of pavements based on design concerns. The way loads are distributed to the subgrade determines the difference between flexible and stiff pavements. Before we can distinguish between flexible and rigid pavements, we must first understand what they are.

The structural performance of the pavements can be divided into two categories:

·         Rigid pavements

·         flexible pavements

Wheel loads are transferred via the granular structure of flexible pavements through grain-to-grain contact of the aggregate. Because of its lower flexural strength, the flexible pavement works like a flexible sheet (e.g. bituminous road).

In rigid pavements, on the other hand, the flexural strength of the pavement transfers wheel stresses to the sub-grade soil, and the pavement acts like a rigid plate (e.g. cement concrete roads).

Composite pavements are also available in addition to these options. A thin layer of flexible pavement on top of stiff pavement is an ideal pavement with the best properties. However, due of the high cost and complicated study required, such pavements are rarely employed in new construction.

1.     Flexible pavement

Grain-to-grain transmission through the points of contact in the granular structure will convey wheel load strains to the lower layers in flexible pavements.

The design of flexible pavement is founded on the idea that as a load is transmitted downwards from the surface by virtue of spreading over an ever greater area and transporting it deep enough into the ground through successive layers of granular material, the intensity of the load reduces.

The wheel force on the pavement is distributed across a larger area, and the stress lessens as the depth increases. Flexible pavements typically have numerous layers to take use of these stress distribution characteristics. As a result, the concept of a layered system is used in the design of flexible pavement. Below is a cross section of a typical flexible pavement.

 

Flexible pavement can be built in a number of layers based on this, with the top layer having to be of the highest quality to withstand maximal compressive stress as well as wear and tear.

Lower layers will be subjected to less stress, allowing for the use of lower-quality materials. Bituminous materials are used to make flexible pavements. Surface treatments (such as bituminous surface treatments, which are commonly encountered on low-volume roads) or asphalt concrete surface courses are two options (generally used on high volume roads such as national highways). The deformation of the lower layers is reflected on the surface layer by flexible pavement layers (e.g., if there is any undulation in sub-grade then it will be transferred to the surface layer). The design of flexible pavement is based on its overall performance, and the stresses generated should be maintained considerably below the allowed stresses of each pavement layer.

TYPES OF FLEXIBLE PAVEMENTS

In flexible pavement, the following forms of construction have been used:

1.       Conventional layered flexible pavement,

2.       Full – depth asphalt pavement, and

3.       Contained rock asphalt mat (CRAM).

Conventional flexible pavements are layered systems with high-quality, expensive materials at the top where stresses are greatest, and low-quality, inexpensive materials at the bottom.

Bituminous layers are laid directly on the soil subgrade to create full-depth asphalt pavements.

When there is a lot of traffic and local materials aren't available, this is a better option.

Contained rock asphalt mats are made up of dense/open graded aggregate layers sandwiched between two layers of asphalt. When modified thick graded asphalt concrete is laid above the sub-grade, the vertical compressive strain on the soil sub-grade is greatly reduced, and surface water is protected.

CAUSES OF FLEXIBLE PAVEMENT FAILURE

Fatigue cracking, rutting, and heat cracking are the most common flexible pavement failures.

The horizontal tensile strain at the bottom of the asphaltic concrete causes fatigue cracking in flexible pavement. The permissible number of load repetitions is related to tensile strain in the failure criterion, which can be measured in a laboratory fatigue test on asphaltic concrete specimens.

Only flexible pavements rust, as evidenced by permanent deformation or rut depth along the wheel load path. To manage rutting, two design strategies were used: one to restrict vertical compressive strain on the top of the subgrade, and the other to limit rutting to a bearable quantity (12 mm normally).

Low-temperature cracking and thermal fatigue cracking are both types of thermal cracking.

RIGID PAVEMENTS

Rigid pavements have enough flexural strength to disperse wheel load strains across a larger region. Below is a cross section of a typical stiff pavement.

 

Cement concrete or reinforced concrete slabs make up a rigid pavement. Grouted concrete roads belong to the semi-rigid pavements category. The design of rigid pavement is centred on constructing a structural cement concrete slab that is strong enough to withstand traffic loads. The stiff pavement has a high modulus of elasticity and is rigid, allowing the load to be distributed across a large area of soil.

Rigid pavements are laid directly on the prepared sub-grade or on a single layer of granular or stabilised material, as opposed to flexible pavement. This layer can be referred to as the base or sub-base course because there is only one layer of material between the concrete and the sub-grade.

The slab action distributes force in rigid pavement, and the pavement behaves like an elastic plate sitting on a viscous medium.

Portland cement concrete (PCC) rigid pavements should be studied using plate theory rather than layer theory, assuming an elastic plate sitting on a viscous basis. Plate theory is a simplified variant of layer theory in which the concrete slab is assumed to be a medium-thick plate that is plane before loading and will remain so after loading. The bending of the slab as a result of wheel load and temperature variations, as well as the tensile and flexural stress that results.

RIGID PAVEMENTS TYPES

Rigid pavements are divided into four categories:

Pre-stressed concrete pavement (PSCP), Jointed plain concrete pavement (JPCP), Jointed reinforced concrete pavement (JRCP), and Continuous reinforced concrete pavement (CRCP) (PCP).

Plain cement concrete pavements with closely spaced contraction joints are known as joined plain concrete pavements. Load transfer across joints is usually accomplished with dowel bars or aggregate interlocks. Joint spacing is usually between 5 and 10 metres.

Jointed Reinforced Concrete Pavement: While reinforcements do not greatly improve structural capacity, they can significantly increase joint spacing to 10 to 30 metres. Load transfer necessitates the use of dowel bars. Even after cracks, reinforcement helps to keep the slab together.

Continuous Reinforced Concrete Pavement: Reinforcement eliminates all joints completely.

CAUSES OF RIGID PAVEMENT FAILURE

Fatigue cracking has traditionally been regarded as the primary or exclusive requirement for rigid pavement design. The stress ratio between flexural tensile stress and concrete modulus of rupture determines the maximum number of load repetitions that might cause fatigue cracking.

Pumping has just been discovered as a significant failure criterion. Pumping is the ejection of soil slurry via the joints and fissures of a cement concrete pavement as the slab moves downhill under the weight of heavy wheels.

Faulting, spalling, and degradation are other important types of discomfort in stiff pavements.