What is Pavement?
- Pavement or carriageway is that part of the road or highway which supports the wheel loads imposed on it from traffic moving over it.
- It ought to be strong enough to resist the stresses that are being developed as a result of traffic.
- Pavement should also be thick enough to distribute the external loads on the earthen sub-grade so that sub-grade can safely bear these external loads.
- The pavement consists of a few layers of pavement materials over a prepared soil sub-grade
- Pavement consists of one or more layers. A simple classification of layers as per Indian practice is shown in as per below fig. no-1
Sub Grade Fig No-1
- The surfacing is the topmost layer, which should be smooth, abrasion-resistant, dustproof, and strong. The base course immediately below the surfacing is a medium to distribute the stress evenly.
- The sub-base course gives additional help in distributing the stress. Sub-grade is the compacted earth below the sub-base course. Pavement should be even. It should have light-reflecting materials for night visibility.
- The layer surfacing is the topmost layer, which should be smooth, abrasion-resistant, dustproof, and strong. The base course immediately below the surfacing is the medium to distribute the stress evenly.
- Sub-base the base course that gives additional help in distributing the stress. Sub-grade is the compacted earth below the sub-base course. Pavement should be even.
- It should have light-reflecting materials for night visibility.
- Based on location, materials of construction, a fund available, necessity, use, traffic volume, roads are classified into different types of roads like earth road, gravel road, water-bound macadam (WBM) road, bituminous or blacktop road, and cement concrete road.
Also, read: Core Cutter Method | What is Compaction of Soil
Type of Pavement.
This is classified from the point of structural behavior as:
- Flexible Pavement
- Rigid Pavement
- Semi-rigid Pavement
- Composite Pavement
- Flexible pavements are Such, which on the whole have Minimal or Low flexural strength and Therefore are rather flexible at their structural action under the loads
- The flexible pavement layers reflect this deformation of these lower layers on-to that the top layer of the layer. Therefore, if the lower layer of the pavement or soil subgrade is undulated.
- The flexible pavement surface also gets undulated. Flexible pavement consists of four components :
- Subgrade (Prepared Road Bed).
- Subbase Course.
- Base Course.
- Surface Course.
Schematic of a Flexible Pavement
- The flexible pavement layers transmit this vertical or compressive stresses to the lower layers by grain transfer through the points of contact at the granular structure.
- A well-compacted granular structure consisting of strong graded aggregate (interlocked aggregate structure with or without binder materials) can move the compressive stresses through a wider area, and so forms a good flexible pavement layer This load spreading ability of this layer, therefore, depends on the type of the mix design and the materials factors.
- Bituminous concrete is among the best flexible pavement layer materials Other materials that fall under the group arc, All granular materials without a bituminous binder, granular base and sub-base course materials such as the Water Bound Macadam, crushed gravel, aggregate, soil-aggregate mixes, etc.
- The vertical compressive Pressure is equal to the pavement surface directly under the wheel load and also is equivalent to the contact pressure under the wheel because of the ability to distribute the pressures to a larger area from the shape. Of a truncated cone, the pressures decreased at the lower layer
- Hence by taking full advantage of the Pressure distribution characteristics of the flexible pavement, the layer system concept has been developed,
- According to this. The flexible pavement might be constructed in many layers, and the top layer was the strongest as the highest compressive stresses should be sustained by this layer, along with the tear and wear as a result of the traffic.
- The lower layers have into take up only lesser magnitudes of stresses, and there’s not any direct wearing action because of traffic loads. Therefore inferior materials with lower cost can be used in the lower layers.
- The lowest layer is the prepared surface consisting of the local soil itself, called the subgrade.
- A typical cross-section of a flexible pavement structure is shown in as per below fig. no-1 ; this consists of a wearing surface on top, below which is the base course followed with the subbase course and the lowest layer consists of the soil subgrade that has the lowest stability one of the four typical flexible pavement components.
- Each of the flexible pavement layers above the subgrade, viz. sub-base, base course, and the surface course may consist of one or more layers of the same or slightly different materials and specifications.
Also, read: What Is Cement | Type of Cement
Rigid Pavement Fig no-2
- Rigid pavements are those that possess noteworthy flexural strength or flexural rigidity. The pressures aren’t transferred from grain with the lower layers, as in the ease of flexible pavement layers.
- The rigid pavements are made with Portland cement concrete-either plain, reinforced or prestressed concrete the plain cement concrete slabs are expected to take-up roughly 40 kg/sq. Cm. Flexural stress.
- The rigid pavement has the slab action and is capable of transmitting the wheel load stresses through a wider area below.
- The main point of difference in the structural behavior of rigid pavement compared to the flexible pavement is the critical condition of stress from the rigid pavement ist that the maximum flexural stress was occurring from the slab because of wheel load and the temperature changes where-as from the flexible pavement It’s the distribution of compressive stresses.
- Since the rigid pavement slab has tensile strength, tensile stresses are developed due to the bending of the slab under a wheel load and temperature variations.
- So the types of pressures developed and their distribution inside the concrete slab that is concrete are quite different.
- The rigid pavement doesn’t get deformed into the shape of the surface as it could bridge the minor variations of the lower layer.
- The cement concrete pavement slab may very well serve as a wearing surface in addition to an effective base course.
- Therefore generally, the rigid pavement structure is made up of cement slab that is concrete, below that the granular base or sub-base-course can be provided (see above fig. 2).
- Although the cement concrete slab may also be laid directly over the soil subgrade, this isn’t preferred, particularly when the subgrade, consists of fine-grained soil.
- Supplying a good base or sub-base course layer below the concrete slab that is concrete increases the pavement life considerably and thus works out more economical in the long run.
- The rigid pavements are often designed, along with the stresses are analyzed using the elastic theory and assuming the pavement within an elastic plate resting over a elastic or a viscous foundation.
- When bonded materials like the pozzolanic concrete, lean cement concrete, or soil-cement are used in the base course or sub-base course layer, the pavement layer has considerably higher flexural strength than the common flexible layers.
- However, these bonded materials do not possess as much flexural strength as the cement concrete pavements. Therefore when this intermediate class of materials is used in the base or sub-base course layer of the pavements are either designed on experience, or by using a new design approach.
- These semi-rigid pavement materials have a low resistance to impact and abrasion and therefore are usually provided with flexible pavement surface course.
Difference Between Rigid Pavement and Flexible Pavement
- Design of rigid pavement is based-scientific design stresses of concrete on whereas flexible pavement design is mostly empirical in nature.
- Life of rigid pavement is more than the flexible one.
- Maintenance of well deigned rigid pavement is practically very small, whereas the bituminous surface of flexible pavement needs frequent maintenance.
- The initial cost of rigid pavement is much more than the flexible one. Considering the stage of construction, flexible pavement is preferred to the rigid pavement. The selection of a type of pavement depends on the availability of materials of construction.
- Surface characteristics play another role in the selection of a type of pavement. A good cement surface is smooth, free from potholes and corrugations. Flexible pavement does not possess the above facilities.
- An impervious layer of the pavement is essential for the sub-grade. Preference will always be the rigid pavement as concrete is impervious.
- Traffic dislocation during construction for about a month takes place as concrete requires a minimum of 28 days for curing and setting. Inflexible pavement traffic is allowed to move on the pavement once it is rolled.