A highway PAVEMENT DESIGN may be a structure consisting of superimposed layers of processed materials higher than the natural soil sub-grade, whose primary perform is to distribute the applied vehicle masses to the sub-grade. The pavement structure ought to be able to give a surface of acceptable riding quality, adequate skid resistance, favorable light-weight reflective characteristics, and low noise pollution. the ultimate aim is to confirm that the transmitted stresses because of wheel load are sufficiently reduced, in order that they’ll not exceed the bearing capability of the sub-grade. two types of pavements are usually recognized as serving this purpose, specifically flexible pavements, and rigid pavements. This article provides an outline of pavement types, layers, and their functions, and pavement failures. Improper design of pavements ends up in early failure of pavements affecting the riding quality.
Requirements of a pavement
An ideal pavement should meet the subsequent requirements:
- Sufficient thickness to distribute the wheel load stresses to a secure worth on the sub-grade soil,
- Structurally sturdy to face up to all sorts of stresses obligatory upon it,
- The adequate constant of friction to stop skidding of vehicles,
- Smooth surface to supply comfort to road users even at high speed,
- Produce the smallest amount of noise from moving vehicles,
- Dustproof surface in order that traffic safety isn’t impaired by reducing visibility,
- Impervious surface, in order that sub-grade soil is well protected, and
- Long design life with low maintenance cost.
Based on structural behavior pavements are generally classified into two categories: Flexible & Rigid
- It consists of a series of layers (Soil subgrade, sub-base, base course, surface course) with the highest quality materials at or near the surface.
- It reflects the deformation of subgrade and subsequent layers on the surface.
- It functions by way of load distribution through the component layers, transmits the vertical stresses to the lower layers by grain to grain transfer.
- Design is based on IRC:37-2001
Flexible pavements can transmit wheel load stresses to the lower layers by grain-to-grain transfer through the points of contact within the granular structure wheel load working on the pavement are going to be distributed to a wider area, and therefore the stress decreases with the depth. Taking advantage of this stress distribution characteristic, flexible pavements usually has several layers. Hence, the design of flexible pavement uses the construct of a layered system. based on this, flexible pavement is also constructed during a number of layers and the prime layer needs to be of the highest quality to sustain most compressive stress, additionally to wear and tear. The lower layers will expertise a lesser magnitude of stress and inferiority material will be used. versatile pavements are created exploitation bituminous materials. These can be either within the kind of surface treatments (such as bituminous surface treatments usually found on low volume roads) or, asphalt concrete surface courses (generally used on high volume roads similar to national highways). versatile pavement layers replicate the deformation of the lower layers onto the surface layer (e.g., if there’s any undulation in sub-grade then it’ll be transferred to the surface layer). within the case of flexible pavement, the look is based on the overall performance of flexible pavement, and also the stresses created ought to be kept well below the allowable stresses of every pavement layer.
|Conventional flexible pavements||Full – depth asphalt pavements||Contained rock asphalt mats|
|This Conventional flexible pavement is layered systems with high quality expensive materials are placed in the top where stresses are high, and low-quality cheap materials are placed in lower layers.||This Full – depth asphalt pavements are constructed by placing bituminous layers directly on the soil sub-grade. This is more suitable when there is high traffic and local materials are not available.||These Contained rock asphalt mats are constructed by placing dense/open-graded aggregate layers in between two asphalt layers. Modified dense graded asphalt concrete is placed above the subgrade will significantly reduce the vertical compressive strain on soil sub-grade and protect from surface water.|
Failure of flexible pavements
The major flexible pavement failures are fatigue cracking, rutting, and thermal cracking. The fatigue cracking of flexible pavement is because of horizontal tensile strain at the rock bottom of the mineral concrete. The failure criterion relates an allowable number of load repetitions to tensile strain and this relation will be determined within the laboratory fatigue test on asphaltic concrete specimens. Rutting happens solely on flexible pavements as indicated by permanent deformation or rut depth on the wheel load path. two design methods have been wont to control rutting: one to limit the vertical compressive strain on the highest of the subgrade and therefore the alternative to limit rutting to a tolerable quantity (12 millimeters normally). Thermal cracking includes both low-temperature cracking and thermal fatigue cracking.
- It consists of the main concrete slab (with base course and soil subgrade) of relatively high bending resistance
- Transfer the load through slab action but not grain to grain as in the case of flexible pavement
- Its structural capacity is supplied by the pavement slab itself by beam action
- It distributes the load over a wide area of subgrade
Rigid pavements have sufficient flexural strength to transmit the wheel load stresses to a wider space below. A typical cross-section of the rigid pavement Compared to the flexible pavement, rigid pavements are placed either directly on the ready sub-grade or on one layer of granular or stable material. Since there’s only 1 layer of material between the concrete and also the sub-grade, this layer may be known as a base or sub-base course.
In rigid pavement, the load is distributed by the block action, and the pavement behaves like an elastic plate resting on a viscous medium Rigid pavements are created by Portland cement concrete (PCC) and may be analyzed by plate theory rather than layer theory, forward an elastic plate resting on the viscous foundation. Plate theory may be a simplified version of layer theory that assumes the concrete block as a medium thick plate that is plane before loading and to stay plane when loading. Bending of the slab thanks to wheel load and temperature variation and also the ensuing tensile and flexural stress.
|Jointed Plain Concrete Pavement||Jointed Reinforced Concrete Pavement||Continuous Reinforced Concrete Pavement|
|JPCP are plain cement concrete pavements constructed with closely spaced contraction joints. Dowel bars or aggregate interlocks are normally used for load transfer across joints. They normally has a joint spacing of 5 to 10m.||Although reinforcements do not improve the structural capacity significantly, they can drastically increase the joint spacing to 10 to 30m. Dowel bars are required for load transfer. Reinforcements help to keep the slab together even after cracks.||Complete elimination of joints are achieved by reinforcement.|
Failure criteria of rigid pavements
Traditionally fatigue cracking has been thought of because the major, or sole criterion for rigid pavement design. The allowable variety of load repetitions to cause fatigue cracking depends on the stress ratio between flexural tensile stress and concrete modulus of rupture. Of late, pumping is known as a very important failure criterion. Pumping is that the ejection of soil suspension through the joints and cracks of cement concrete pavement caused throughout the downward movement of the block beneath the significant wheel loads. different major types of distress in rigid pavements embody faulting, spalling, and deterioration.