Pavement

Pavement

 INTRODUCTION

Pavement is a durable surfacing of a road, airstrip, or similar area. The primary function of a pavement is to transmit loads to the sub-base and underlying soil. Modern flexible pavements contain sand and gravel or crushed stone compacted with a binder of bituminous material, such as asphalt, tar, or asphaltic oil. Such a pavement has enough plasticity to absorb shock. Rigid pavements are made of concrete, composed of coarse and fine aggregate and portland cement, and usually reinforced with steel rod or mesh.

FLEXIBLE PAVEMENT

Flexible pavement can be defined as the one consisting of a mixture of asphaltic or bituminous material and aggregates placed on a bed of compacted granular material of appropriate quality in layers over the subgrade. Water bound macadam roads and stabilized soil roads with or without asphaltic toppings are examples of flexible pavements.

The design of flexible pavement is based on the principle that for a load of any magnitude, the intensity of a load diminishes as the load is transmitted downwards from the surface by virtue of spreading over an increasingly larger area, by carrying it deep enough into the ground through successive layers of granular material.

Fig: Flexible Pavement Cross-section

Thus for flexible pavement, there can be grading in the quality of materials used, the materials with high degree of strength is used at or near the surface. Thus the strength of subgrade primarily influences the thickness of the flexible pavement.

 Distribution of load to avoid permanent deformation in a flexible pavement

Base: higher strength material than subbase, often a cementing material is used. Cementing material can be portland cement or asphaltic cement, or other material.

Open-Graded: An open-graded HMA mixture is designed to be water permeable (dense-graded and SMA mixes usually are not permeable). Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands. There are two types of open-graded mixes typically used in the U.S.: Open-graded friction course (OGFC).

Dense-Graded: A dense-graded mix is a well-graded HMA mixture intended for general use. When properly designed and constructed, a dense-graded mix is relatively impermeable. Dense-graded mixes are generally referred to by their nominal maximum aggregate size. They can further be classified as either fine-graded or coarse-graded.

Full-Depth Reclamation : 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.

1. DIFFERENCES BETWEEN THE CONSTRUCTION OF FLEXIBLE AND RIGID PAVEMENT

The fundamental difference between a flexible, semi-rigid, and rigid pavement is the load distribution over the subgrade. The semi-rigid pavement has a higher composite modulus of elasticity than a flexible pavement and begins to resemble the rigid structure in terms of how the traffic loads are distributed over the subgrade. The elements contributing to the higher modulus may be:

1. Increased thickness in asphalt concrete
2. chemical stabilization of the base, subbase, and/or subgrade layers
3. Asphalt stabilization of the base course.

Below are the differences between the flexible pavement construction and rigid pavement construction.

Difference between Flexible Pavements and Rigid Pavements:

1. IDENTIFY THE FUNCTION OF THE FOLLOWING

– SUB-BASE: Sub-Base course is the layer (or layers) under the base layer. A sub-base is not always needed. A proper sub-base consists of various sizes of crushed stone aggregate, commonly known as crusher run. Depending on the sub soils on your site you may need 8-12 inches of various sizes of sub-base. With well drained sub soils, without movement, added sub-base materials may be sufficient, along with proper pitch & grade.

Function of Sub-base course of flexible pavement

It functions primarily as structural support but it can also help:

1. Minimize the intrusion of fines from the sub-grade into the pavement structure.
2. Improve drainage.
3. Minimize frost action damage.
4. Provide a working platform for construction.
5. The sub-base generally consists of lower quality materials than the base course but better than the sub-grade soils.

• SUB-GRADE: The “sub-grade” is the in situ material upon which the pavement structure is placed. Although there is a tendency to look at pavement performance in terms of pavement structure and mix design alone, the sub-grade can often be the overriding factor in pavement performance. It is essential that at no time soil sub-grade is overstressed It should be compacted to the desirable density, near the optimum moisture content
• BASE COURSE: The base course is the layer of material immediately beneath the surface of binder course and it provides additional load distribution and contributes to the sub-surface drainage It may be composed of crushed stone, crushed slag, and other untreated or stabilized materials.

• ROAD – BASE: Road-base is the most important structural layer in bituminous pavement. It is designed to take up the function of distributing the traffic loads so as not to exceed the bearing capacity of subgrade. In addition, it helps to provide sufficient resistance to fatigue under cyclic loads and to offer a higher stiffness for the pavement structure.
• However, the basecourse is normally provided to give a well-prepared and even surface for the laying on wearing course. Regarding the load distribution function, it also helps to spread traffic loads to roadbase but this is not the major function of basecourse.
• SUB-GRADE: The top soil or sub-grade is a layer of natural soil prepared to receive the stresses from the layers above.
• WEARING COURSE: The wearing course is the upper layer in roadway, airfield, and dockyard construction. The term ‘surface course’ is sometimes used, however this term is slightly different as it can be used to describe very thin surface layers such as chip seal. In rigid pavements the upper layer is a portland cement concrete In flexible pavements, the upper layer consists of asphalt concrete, that is a construction aggregate with a bituminous binder. The wearing course is typically placed on the base course, which is normally placed on the subbase, which rests on the subgrade.

REFERENCE

Holtz, R.D., Christopher, B.R., and Berg, R.R., 2008. Geosynthetic Design and Construction Guidelines, Participant Notebook, FHWA Publication No. FHWA HI -07- 092, Federal Highway Administration, Washington, D.C., 592 p.

Christopher, B.R. And Perkins, S.W., 2008. “Full Scale Testing of Geogrids to Evaluate Junction Strength Requirements for Reinforced Roadway Base Design,” Proceedings of the Fourth European   Conference, Edinburgh, United Kingdom, International Geosynthetics Society.

Cuelho, E.G. and Perkins, S.W., 2009. Field Investigation of Geosynthetics used for subgrade Stabilization, Summary Report 8193, Montana Department of Transportation, 4 p. (http://www.mdt.mt.gov/ research/projects/geotech/subgrade.shtml).

Lay, M. G. (2009). Handbook of Road Technology (4 ed.). Taylor & Francis. ISBN 0203892534.

Phatak, D. R.; Gite, H. K. Highway Engineering. Nirali Prakashan. ISBN 8185790922.