Road making materials may be defined as the materials that are used for constructing and / or supporting the road strictures.
These material are:
- All types of soils
- Tar and Asphalt
- Cement, sand and aggregates
- Miscellaneous materials like Bricks wood, rubber, metal, glass etc.
Out of the above stated materials, only the material which are generally used for road construction, repairs and supporting road structures will be explained.
- All the types of soils
- Tar and Asphalt
A soil is defined as a mixture of earthy material, with particles differing physically in size shape and structure and varying in chemical composition. It forms the top layer of ground. The undisturbed soils lying below the natural top soil is known as “sub-soil” the common soils used in road making are
Common Soils used in Road Making
- Mixture of these soils if it is composite soil
Clay is a fine-grained hygroscopic silicate of alumna mixed usually with impurities. It becomes plastic when moist.
Silt is a mud consisting of fine and barely visible grains of specified size, with little or no plasticity and no cohesion, it is usually deposited as a sediment in water.
Sand consists of granular, gritty loose grains of siliceous origin. The grain size being 0.002 to 2.00 millimetres.
It is free from appreciable quantities of clay and silt and hence it is non-plastic and cohesion less.
A loam is a soil having a relatively even mixture of different grades of sand and of silt and clay.
It has somewhat gritty feel and is yet fairly smooth and slightly plastic. Some loamy soils posses considerable properties of organic matter.
Marl is an early mixture of minerals consisting of quartz, clay, calcite and sometimes glauconitic sands. It is quite often found in swamps and lakes.
Peat is a dark fibrous spongy soil showing evidence of vegetable origin.
Shale is a compressed and laminated clay not so hard as slate. It may be with or without organic matter.
8. Black Cotton Soil
Black cotton soil is a composite soil formed by the decomposition of rocks like trap, basalt etc, after long and continued weathering. The clay content in it is from 40 to 50 percent.
It is black or blue-black in colour and is found in tropical countries. This soil contains 4 to 8 percent humidity. It has great affinity for water and attracts water, from all directions by capillarity. It is extremely sticky when wet and has a high coefficient of shrinkage. It has a better bearing capacity when dry but becomes almost a fluid when wet. It is generally suitable for cotton crop and hence it is called “black cotton soil”.
It is very unsuitable as a sub-grade for road structure and other engineering structures because of its great change in volume when it is allowed to dry after first being wetted.
If used as sub-grade, it must be properly treated before putting the road structure on it.
We have also composite soils like sandy-clay, clayey-sand etc, having sand and clay in certain preparations. In sandy-clay, the clay content is more but in clayey-sand, the sand content is more.
In fact, in nature, we have mostly the composite soils.
All road structures are supported by the soil of sub-grade. Knowledge of the soil is therefore of great importance to a Road Engineer.
The properties and the characteristics of soils are affected by the size of their constituent particles and by the relative method of classifying soil particles by their grain size. This is known as “Mechanical Analysis of Soil”.
A satisfactory soil for earth road will be one in which the main property of cohesion, plasticity and internal friction are present in the required proportions. The ideal surface layer of earth road should consists of following.
- Clay 9 to 18%
- Silt 5 to 15%
- Sand 65 to 80%
It will be sufficient to have 70% of sand and 30% of silt and clay together.
The characteristics of soil may be classified as
a. Physical Constant of Soil like
- Internal friction
- Cohesion is internal molecular attraction, which opposes the rupture of soil
- Compressibility and permeability
b. Physical Characteristics of Soil
These characteristics are shown by soil on the addition of water to the dry soil or, on drying a wet soil. For determining these characteristics, following tests are performed in soil mechanics Laboratory.
- Shrinkage limit test
- Liquid limit test
- Plastic limit test
- Plastic index
- Field Moisture equivalent test
- Shrinkage ratio test
- Centrifuge moisture equivalent test
- Volume change or volume shrinkage test
- Linear shrinkage test
- The first four tests are known as “Altenburg Limit Test”
c. Textural Characteristics
These characteristics show the size and grading of the individual particles of soil.
It consists of small stones of irregular shape and size, which are usually water worn and are sometimes mixed with finer materials. It occurs in riverbeds or open beaches and is also called shingle.
It is a gritty siliceous material with big size lumps or, stone not exceeding 3-4 inches (about 18 mm) in size and with a natural mixture of clay of the calcareous or laetrile origin usually of former origin.
Hard Mao rum is broken to 3 inches (about 72mm) in size, it is used as soling under water bound macadam road and also as the wearing surface on shoulders of the traffic way.
Soft moorum is used as blind doge on most of the medaled road surfaces and as a casing for the side slopes and for the top of banks made of black cotton soil.
Various types of stones are used for road construction. Following are some of the common types of stones used for this purpose.
- Literate (hard Variety)
- Limestone (hard variety)
- Sand stone
It is a blended or foliated rock of metamorphic origin. It has the mineral composition of granite. The main minerals contained in it are quartz and feldspar with black or white mica. It yields good road metal.
Granite is a coarse grained polycrystalline, plutonic rock which is composed of quartz, orthoclase, sub ordinate plagioclase, ferromagnesian mineral and accessories. It yields good road metal.
Laterite is a mixture of red and yellow residual soils or surface products that have originated in site from the atmospheric weathering of rock and is the result of infiltrating compounds.
Tough variety should be used for roadwork.
Limestone is a stone, which is formed by consolidation of sand, predominantly of quartz. It forms dust when traffic goes over it.
Basalt is a dark colored, fine-grained rock composed essentially of elastic plagioclase, Augite and is on ores usually from divine. It yields good road metal.
Kankar is a nodular form of carbonate of lime and its bajri makes very good binder for water bound macadam roads.
Its hard variety in used for road surfacing, when so used, it is broken in pieces of bigger size.
Trap is a mixture of feldspar and hornblende. It is often compact and fine grained. It is fairly hard and tough and makes good road stone.
In most situations, road stone is used in road construction in the form of crushed or below 3 inches such broken stone of size, 1,2.5,2.5 or 3 inches is called ‘road metal’.
It is desirable that in metric system, this size may be approximately shown as 24 mm, 36mm, and 72mm respectively. Stone is also used in the form of small stone chips, boulders, dressed blocks and sets.
Stone is quarried from quarries by various methods and it is then broken to the required size. This may be done by hand hammers or, mechanically by stone crushing machines.
In places where the road stone is not available, pieces of over burnt bricks are used as road metal in case of unimportant roads.
Desirable Qualities of Road Stone
The road should have the following desirable qualities.
- Hardness to resist rubbing due to moving traffic.
- Toughness to resist fracture under impact of traffic load.
- Resistance to attrition and wear, is resistance to mutual rubbing or grinding of stone pieces under the action of traffic.
- Good binding or cementing value.
- Low absorption of water or less pore space.
- Hardness to resist crushing under the load of traffic.
- Durability against weather.
A road stone should be able to resist the crushing and abrasion caused by traffic, it should be able to take up traffic and it should be able to take up the below or impact from the wheels of traffic without breaking. It should have good binding value.
Granite is very hard, but granite dust or powder has no good binding value. Lime stone is not a hard stone but its dust has very good cementing property.
Tar is a viscous product which results from the destructive distillation of organic materials and it has adhesive properties.
The word ‘tar’ should be preceded by the name of the material form which it is produced e.g., crude coal tar shows that the tar is produced from coal.
Crude coal tar is dehydrated and then put to fractional distillation at a maximum temperature of 400o to 450oF 9 (204 to 232oC).
Various light oils, like phenol etc are given out, leaving “refined tar ‘which is useful for road work and is therefore known as ‘road tar’.
A non-crystalline solid or a viscous material (having adhesive properties) which is derived from crude petroleum either by natural or artificial distillation process and which is substantially soluble in Cs2 is called “bitumen”.
Bitumen is a mixture of hydrocarbons and is black or brown in colour. It is viscous fluid or semi solid with a capacity for adhering to rock surfaces.
It can be produced in a variety of grades in respect of surfaces, volatility, etc to suite particular requirements in various types of road surfacing.
Though it may occur naturally, it is usually obtained artificially as the last residual and highly viscous product from the distillation of, or an extract form, crude petroleum oil. This product is known as “petroleum Asphalt or Asphaltic bitumen” and its grade can be varied by varying the temperature during processing.
When as phallic bitumen is of such consistency that it can be directly used for making bituminous roads, it is called Asphalt cement.
The word asphalt should always be qualified by the indication of its origin or nature, thus we have:
- Lake Asphalt
- Mastic Asphalt
- Road Asphalt
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