2.1 Highway materials Properties of sub grade | unit 2 highway materials

Unit - 2

Highway materials

2.1 Highway materials- Properties of sub grade

1. Properties of sub grade:

Soil is an accumulation or deposit of earth material, derived naturally from the disintegration of rocks or decay of vegetation that can be excavated readily with power equipment in the field or disintegrated by gentle mechanical means in the laboratory.

The supporting soil beneath pavement and its special under courses is called sub grade.

Undisturbed soil beneath the pavement is called natural sub grade.

Compacted sub grade is the soil compacted by controlled movement of heavy compactors.

Desirable Properties of Sub grade Soil

Stability

Incompressibility

Permanency of strength

Minimum changes in volume and stability under adverse conditions of weather and groundwater

Superior drainage

Ease of compaction

Soil Types

Gravel	Sand			Silt			Clay
	Coarse	Medium	Fine	Coarse	Medium	Fine	Coarse	Medium	Fine

	0.6mm 0.2mm			0.02mm 0.006mm			0.0006mm 0.0002mm
2mm	0.06mm			0.002mm

Gravel: These are coarse materials with particle size under 2.36 mm with little or no fines contributing to cohesion of materials.

Moorum: These are products of decomposition and weathering of the pavement rock. Visually these are similar to gravel except presence of higher content of fines.

Silts: These are finer than sand, brighter in color as compared to clay, and exhibit little cohesion. When a lump of silty soil mixed with water, alternately squeezed and tapped a shiny surface makes its appearance, thus dilatancy is a specific property of such soil.

Clays: These are finer than silts. Clayey soils exhibit stickiness, high strength when dry, and show no militancy. Black cotton soil and other expansive clays exhibit swelling and shrinkage properties. Paste of clay with water when rubbed in between fingers leaves stain, which is not observed for silts.

2.2 Sub base

In highway engineering, subbase is the layer of aggregate material laid on the subgrade, on which the base course layer is located. It may be omitted when there will be only foot traffic on the pavement, but it is necessary for surfaces used by vehicles.

Subbase is often the main load-bearing layer of the pavement. Its role is to spread the load evenly over the subgrade. The materials used may be either unbound granular, or cement-bound. The quality of subbase is very important for the useful life of the road and can outlive the life of the surface, which can be scrapped off and after checking that the subbase is still in good condition, a new layer can be applied.

Unbound granular materials are usually crushed stone, crushed slag or concrete, or slate.

2.3 Base course and surface course materials

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.

Surface course is the layer directly in contact with traffic loads and generally contains superior quality materials. They are usually constructed with dense graded asphalt concrete (AC). The functions and requirements of this layer are: It provides characteristics such as friction, smoothness, drainage, etc.

2.4 Test on sub grade soil

Test on sub grade soil:

Sub grade soil is an integral part of the road pavement structure as it provides the support to the pavement from beneath.

The main function of the sub grade is to give adequate support to the pavement and for this the sub grade should possess sufficient stability under adverse climatic and loading conditions. Therefore, it is very essential to evaluate the sub grade by conducting tests.

The tests used to evaluate the strength properties of soils may be broadly divided into three groups:

Shear Tests
Bearing Tests
Penetration Tests

Shear Tests: are usually carried out on relatively small soil samples in the laboratory.

In order to find out the strength properties of soil, a number of representative samples from different locations are tested.

Some of the commonly known shear tests are direct shear test, tri axial compression test, and unconfined compression test.

2. Bearing Tests: are loading tests carried out on sub grade soils in-situ with a load bearing area.

The results of the bearing tests are influenced by variations in the soil properties within the stressed soil mass underneath and hence the overall stability of the part of the soil mass stressed could be studied.

3. Penetration Tests: may be considered as small-scale bearing tests in which the size of the loaded area is relatively much smaller and ratio of the penetration to the size of the loaded area is much greater than the ratios in bearing tests.

The penetration tests are carried out in the field or in the laboratory.

Key Takeaways:

Undisturbed Soil: - An undisturbed sample is one where the condition of the soil in the sample is close enough to the conditions of the soil in-situ to allow tests of structural properties of the soil to be used to approximate the properties of the soil in-situ.

Disturbed Soil: - Soil that has been changed from its natural condition by excavation or other means.

2.5 Aggregates and Bituminous materials

1. Aggregates

Aggregates form the major portion of the pavement structure, bear stresses occurring on the roads and have to resist wear due to abrasive action of traffic.

Aggregates are also used in flexible as well as in rigid pavements. Therefore, the properties of aggregates are of considerable importance to highway.

The aggregates are specified based on their grain size, shape, texture and gradation.

Based on the strength property, the coarse aggregates maybe divided as hard aggregates or soft aggregates (Moorum, kankar, laterite, brick aggregates).

Desirable Properties of Road Aggregates

1) Strength:

The aggregates to be used in road construction, particularly the aggregates used in the wearing course of the pavement should be sufficiently strong/ resistant to crushing to withstand the high stresses induced due to heavy traffic wheel loads.

2) Hardness:

The aggregates used in the surface course are subjected to constant rubbing or abrasion due to moving traffic.

Abrasive action may be increased due to the presence of abrasion material like sand between the tires of vehicle and the aggregates exposed to the top surface. Thus, they should be hard enough to resist the wear due to abrasive action of traffic.

3) Toughness:

Aggregates in the pavement are also subjected to impact due to moving wheel loads.

The magnitude of impact increase with roughness of road and speed of vehicle

Severe impact is common when heavily loaded steel-tired vehicles move on WBM. The resistance to impact or toughness is thus another desirable property of aggregates.

4) Durability

The aggregates used in roads are subjected to physical and chemical actions of rains and ground water, the impurities in them and that of atmosphere.

Thus, it is desirable that the road stones used in the construction should be sound enough to withstand the weathering action.

The property of aggregates to withstand the adverse actions of weather may be called soundness.

5) Shape of Aggregates

Road aggregates may be rounded, angular, flaky or elongated.

Flaky and elongated particles have less strength than rounded and cubical particles. Thus, too flaky and too much elongated particles should be avoided.

6) Adhesion with Bitumen

The aggregates in bituminous pavements should have less affinity with water when compared with bitumen otherwise the bituminous coating on the aggregates will be stripped off in presence of water.

2. Bitumen

Bitumen refers to the viscous liquid or solid consisting essentially of hydrocarbons and their derivatives.

Bitumen is soluble in Carbon Disulphide C2S

Substantially non volatile

Softens when heated

Black or brown in color

Petroleum Bitumen: Obtained by refining process of petroleum

Natural Bitumen: Occurring as natural deposits

Straight Run Bitumen: Petroleum bitumen whose viscosity has not been adjusted by blending or by softening with cutbacks or other methods.

Blown Bitumen: Straight run bitumen further treated by blowing air through it while it is in hot condition

Composition of Bitumen:

Complex chemical mixture of molecules that are predominantly hydrocarbons

Carbon 82-88%

Hydrogen 8-11%

Sulphur 0-6%

Oxygen 0-1.5%

Nitrogen 0-1%

1) Traces of metal:

Vanadium, Nickel, Iron, Calcium, Magnesium

Bitumen constituents are broadly classified as Asphaltenes, Resins and oils

2) Asphaltenes:

Dark brown friable solids

Have high polarity and interact or associate more actively

Mainly responsible for viscosity of bitumen

Higher Asphaltenes content result in harder, viscous bitumen with low penetration and high softening point

3) Resins:

Dark, Semi solid to solid

Fluid when heated and brittle when cold

Disperses the Asphaltenes throughout the oils to provide a homogenous liquid

Yields Asphaltenes on oxidation

4) Oils:

Colorless liquids

Yields Asphaltenes and resins on oxidation

Bitumen is considered to be a colloidal system consisting of Asphaltenes, resins and oils.

Key Takeaways:

Elongation Index: - Elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than one and four-fifth times (1.8 times or 9/5 times) their mean dimension. It is measured on particles passing through mesh size of 63mm and retained on mesh size of 6.3mm.

Flakiness Index: - The flakiness index is defined as the percentage (by mass) of stones in an aggregate having an ALD of less than 0.6 times their average dimension.

Higher Asphaltenes content result in harder, viscous bitumen with low penetration and high softening point

2.6 Traffic Engineering- Definition

Traffic engineering is a branch of civil engineering that uses engineering techniques to achieve the safe and efficient movement of people and goods on roadways. It focuses mainly on research for safe and efficient traffic flow, such as road geometry, sidewalks and crosswalks, cycling infrastructure, traffic signs, road surface markings and traffic lights. Traffic engineering deals with the functional part of transportation system, except the infrastructures provided.

2.7 Fundamentals of traffic flow

Traffic Flow is the study of the movement of individual drivers and vehicles between two points and the interactions they make with one another. Unfortunately, studying traffic flow is difficult because driver behavior cannot be predicted with one-hundred percent certainty. Fortunately, drivers tend to behave within a reasonably consistent range; thus, traffic streams tend to have some reasonable consistency and can be roughly represented mathematically. To better represent traffic flow, relationships have been established between the three main characteristics: (1) flow, (2) density, and (3) velocity. These relationships help in planning, design, and operations of roadway facilities.

2.8 Traffic management

Traffic Management refers to the combination of measures that serve to preserve traffic capacity and improve the security, safety and reliability of the overall road transport system. These measures make use of ITS systems, services and projects in day-to-day operations that impact on road network performance.

2.9 Prevention of road accidents

Drive in the prescribed speed limits on the various roads.

Always put on helmets, seat belts and other safety equipments before driving a bicycle/ motor cycle/vehicle.

Do not drink and drive.

Never use mobile phones or ear phones while driving.

Know the traffic signs, signals, lights and traffic safety rules before you hit the road. Always remember that “Road safety rules are best tools to avoid accidents”.

Do not drive for long hours in a stretch. Have a proper beak after every 2 hours of continuous driving. Always remember that “Man is a man and not a machine”.

2.10 Elements of transport planning, highway drainage

Highway drainage consists of removing or controlling surface water and subsurface water away from the road surface and the subgrade supporting it. Part of the rain water flows on the ground or road surface, while the other part percolates into the ground and reaches the ground water table, raising its level. The subgrade soil above the ground water table may raise through the soil pores due to the phenomenon of capillarity.

The continued presence of water on the road surface weakens the pavement causing pot holes and ruts; similarly, the presence of water in the subgrade reduces its bearing power and load dispersion capacity. Loss of subgrade support leads to the failure of the road pavement under traffic loads. Hence efficient drainage is an imperative need.

Lack of drainage or inadequate drainage has been the primary reason for the failure of highway pavements. The importance of drainage in the successful maintenance and operation of highways is reflected in the adage- “There are just three factors necessary for a good road: drainage, drainage and more drainage.”

Road builders of ancient era like the Romans and Aryans themselves understood the importance of drainage; pioneers of the modern era like MacAdam, Telford and Tresaguet also appreciated the need of good drainage for the success of a roadway.

In fact, constructing an efficient drainage system for the road is considered to be a cheaper, yet effective method to enhance its life than the current practice of the designing pavements for soaked subgrade conditions, which leads to the formation of thicker road sections.

The following geometric design aspects of highways are fully or partly governed by the requirements of drainage:

1. Longitudinal gradient

2. Cross-fall or camber

3. Vertical curves – summits and sags

4. Shoulders

5. Medians

6. Embankments

7. Intersections

8. Rotaries

9. Fly-over

References:

Khanna S. K., Justo C.E.G, & Veeraragavan, A. “Highway Engineering”, Nem Chandand Bros., Roorkee- 247 667.

2. Khanna S. K., Justo C.E.G, & Veeraragavan A., “Highway Materials and Pavement Testing”, Nemchand and Bros., Roorkee- 247 667.

3. LR Kadiyali, Transportation Engineering, Khanna Publication.

4. L.R. Kadiyali, Transportation Engineering, Khanna Publishing House

5. Saxena, Subhash C, A Textbook of Highway and Traffic Engineering, CBS Publishers &Distributers, New Delhi

6. Kumar, R Srinivasa, “A Text book of Highway Engineering”, Universities Press, Hyderabad.

7. Kumar, R Srinivasa, “Pavement Design”, Universities Press, Hyderabad.

8. Chakraborty Partha & Das Animesh, “Principles of Transportation Engineering”, Prentice Hall (India), New Delhi,

9. IRC: 37- Latest revision, “Tentative Guidelines for the design of Flexible Pavements” Indian Roads Congress, New Delhi

10. IRC:58-2015 Guidelines for the Design of Plain Jointed Rigid Pavements for Highways (Fourth Revision) (with CD)

11. IRC:65-2017 Guidelines for Planning and Design of Roundabouts (First Revision)

12. IRC:73-1980 Geometric Design Standards for Rural (Non-Urban) Highways

13. IRC:106-1990 Guidelines for Capacity of Urban Roads in Plain Areas

14. IRC: 93-1985 Guidelines on Design and Installation of Road Traffic Signals.

15. IRC:92-2017 Guidelines for Design of Interchanges in Urban Areas (First Revision)

16. IRC: SP: 68-2005, “Guidelines for Construction of Roller Compacted Concrete Pavements”, Indian Roads Congress, New Delhi.

17. IRC: 15-2002, “Standard Specifications and Code of Practice for construction of Concrete Roads” Indian Roads Congress, New Delhi.

18. MORTH, “Specifications for Road and Bridge Works”, Ministry of Shipping, Road Transport & Highways, Published by Indian Roads Congress, New Delhi.

Sign Up

Index

Notes

Highlighted

Underlined

Browse by Topics

Notes

Highlighted

Underlined