In this article we will discuss about:- 1. Definition of Road Aggregates 2. Qualities of Aggregates 3. Common Road Aggregates.
Definition of Road Aggregates:
Those types of rocks or mineral fragments that can be used as an aggregate material in combination with other natural or artificial materials like cement and bitumen etc. for preparing an ideally suitable road surface may be termed as Road Materials.
The greater part of the body of a road is generally constituted by the aggregates, which are supposed to bear the main stress under all types of traffic without undergoing much surface abrasion. Thus, much importance is always attached to the selection of the right type of aggregates for ensuring stability and durability of a road. This requires a thorough understanding of the properties that a good road aggregate should always possess on the one hand and also a comprehensive knowledge of important rock types that are suitable for use as source of these aggregates.
Qualities of Aggregates:
Following are most important properties a good roadstone should possess:
(i) Sufficient hardness and toughness
(ii) Durability at the place of use
(iii) Cementation capacity
(iv) Non-swelling or hydrophobic property.
A brief account of these properties is as follows:
It defines the resistance of a stone to rubbing and grinding action under the wheel of the traffic as also due to mutual attrition. It is known that the aggregates of roads and pavements undergo repeated shaking movements under traffic load. This leads to mutual rubbing and grinding action among the aggregates.
If the rock is not sufficiently resistant to this type of action, it may wear out at a rapid rate causing losses in terms of time and money. The rubbing action of wheels of traffic has also to be given due consideration.
Two tests are performed in a civil engineering laboratory for this purpose. These are:
a. The attrition test and
b. The abrasion test.
a. The Attrition Test:
Definite weight of the sample, comprising fragments of 5 cm size and well dried up at temperatures of 100-110°C are placed in two closed cylinders. These cylinders are then rotated for about five hours at a speed of 30 rpm to complete 10,000 rotations.
As the fragments are constantly hurled up against each other repeatedly during this process, they lose weight due to wear and tear. This weight of the fine powder after the test is determined and loss in weight gives the measure of resistance of the aggregate to attrition. Greater the loss in weight, poorer is the resistance to abrasion.
b. The Abrasion Test:
This test is carried out to determine the capacity of a stone to resist the wear against the rubbing action of the wheels of the traffic.
In addition to the rubbing action under the wheels of the traffic, aggregates in the road surface are subjected to repeated pounding. This tends to break the road metal into small pieces, and hence the balance achieved by the stone with respect to stresses is disturbed. It is, therefore, desirable to test the road materials for their resistance against fracturing under the influence of impacts.
This may be achieved in the following manner:
i. Impact Test (For Single Stone):
The test specimen, a cylinder of 2.5 cm diameter and 2.5 cm height is placed on the anvil of a testing machine which consists of a 2 kg hammer falling freely between two vertical guides. The blows of the hammer are applied to the specimen through a small
steel plunger which is kept resting all the time on the specimen. The height of the fall of hammer is increased gradually till the stone breaks; this gives the measure of the toughness of stone.
ii. Impact Test (For Aggregates):
The aggregate is crushed to the size of 1-2 cms. About 5 kg of this is taken and subjected to 15 blows from a 15 kg hammer falling from a height of 40 cm. After these blows are given fines passing through BSS 7 are weighed and the result expressed in percentage terms of original weight. The more the amount of fines, the less is the resistance of the aggregate to pounding.
Another test using a Pendulum Tester involves imparting an impact from a standard pendulum on to a specimen held at an anvil. The pendulum is released for its full swing or from a smaller height.
The impact toughness of the specimen rock then can be calculated by using the relationship:
Impact Toughness = [P / (l.a)] kg/cm2
where P is the energy in the pendulum at the time of the impact, l is the specimen length and a is the cross-sectional area of the specimen.
(iii) Binding Properties:
Roadstones should possess sufficient affinity for binding materials to ensure stability. Stones with an ability to form their own binding material under traffic arc preferred.
These properties are tested by cementation test as described below:
Prepare a finely ground paste of the test specimen and make small briquettes out of this paste. Dry them; when dried, these briquettes are tested by using a standard hammer. The number of blows required to break the briquette gives approximate measure of the binding value of the roadstone.
The binding power is described as:
i. Low – if the briquette breaks for about 10 or less hammer blows;
ii. Fair – if it withstands between 10-25 blows
iii. Good – between 26-75 blows
iv. Very good – above 75 blows
(iv) Crushing Strength:
This is in fact the most important property of roadstones and aggregates because it is from the crushing strength tests that the ability of these materials to withstand heavy traffic loads can be estimated.
i. For Stones:
Test specimens of 5 cm side cubes or cylinders of 5 cm dia. and length are tested under Universal Testing Machines individually; the load at breaking points (average of 6 specimens) gives a measure of the crushing strength of the stones.
ii. For Aggregates:
The aggregates are first reduced to size range of 1 -2 cm and are then dried to a constant weight 100-110°C. About 3 kg of dried aggregates so prepared is placed in a steel cylinder of 25 cm dia. with closely fitting ram. A load of 40 tonnes is applied through compression testing machine. The percentage weight of fines passing through BSS 7 sieve gives a measure of the crushing strength of the aggregate.
(v) Hydrophobic Property:
When road material is used with bitumen as binding material, the shearing strength of the mix will depend mainly on the degree of cohesion achieved by bitumen layer held in between the aggregate particles. In some aggregates this cohesion is lost in situations where they are constantly exposed to water.
The reason for this loss of cohesion is to be sought in the affinity of aggregates for water. When an aggregate has greater affinity or love for water, the bitumen layer around it is replaced by a film of water thereby destroying the bond between the aggregate and the bitumen. Such an aggregate is called hydrophilic (water-loving).
Quartz, quartzites and sandstones are typical hydrophilic aggregates. In some other aggregates, however, there is a marked resistance against this property of stripping-off of the bitumen layer in the presence of water. These aggregates are termed as hydrophobic aggregates. Limestones form a typical example. Naturally, such aggregates will be preferred to others where the road surface is liable to be exposed to water for prolong periods.
No adequate explanation has so far been given for such contrasting properties of aggregates towards water. The view that electrolytic character of the aggregate is responsible for such an action is considered convincing. Common bitumen is generally anionic (+) in character. Any aggregate having (+) electrolytic property will behave as hydrophobic or water repelling and vice versa. This holds well in aggregates mentioned above as typical examples.
Common Road Aggregates:
Natural rocks are the raw materials from which the road aggregates are obtained. All the major rock groups contain such rock types that can be successfully used as aggregates. It is a matter of practical importance that some rock types are better suited and frequently used than the other. The mineralogical composition and texture of the rocks are important factors that are also considered in the selection of road aggregates.
Rocks commonly used as sources of roadstones are:
These are dark coloured basic igneous rocks that have been extensively used as road materials in areas of their occurrence. The rock is generally characterised with a crushing strength between 2000-3000 kg/cm2 and also with great toughness. Their specific gravity varies between 2.8-3.
Basalt is one of the most common rocks of India. Lava flows of basaltic composition cover more than four lakhs square kilometers in large parts of southern and central India and are known as Deccan Traps.
All varieties of granites are eminently suitable as building as well as road materials by virtue of their properties. The darker varieties are used more as road materials.
Their average values with respect to aggregate tests are:
Coefficient of hardness – 18
Cementation value – 17 (fair)
Attrition loss – 2.6%
Besides basalts and granites, all the other igneous rocks like syenites, diorites, gabbros, andesites and lamprophyres are also used as roadstones where they are locally available.
These are sedimentary rocks covering large parts of the globe. These form the most commonly used type of road aggregates. Many varieties are known to exist and almost all of them are generally found suitable as roadstones. The crushing strength of sandstones varies from 600 kg/cm2 for calcareous varieties to as much as 2500 kg/cm2 for quartzitic types. Quartzites are widely used as roadstones because of their very hard nature, high crushing strength, compact character, and good weathering resistant properties.
These are also very common sedimentary rocks forming many hills and sub-mountainous and mountainous areas of the world. They vary considerably in their strength characteristics depending upon their texture and structure. Hard and compact varieties are sufficiently tough to withstand normal traffic loads. Clayey varieties, however, may be too soft.
Rounded and smooth pebbles and rock fragments of any composition above 2 mm diameter are collectively termed as gravels. Their common size range is between 2-7 cm. These are found as deposits and also along river bed and sea beaches.
Some common types of gravels are:
(i) Stream Bed Gravels:
These are found along the banks and dried river bottoms. The streambed gravels are characterised by rounded, smooth outlines and roughly uniform grain size. They are very commonly used as concrete aggregates and road aggregates.
(ii) River Terrace Gravels:
These occur in the form of layers deposited along stream terraces and generally possess all the requisite qualities of good road aggregates. Sometimes these gravels are covered with clayey, siliceous, calcareous or gypsum coatings which may render the material useless in original form; these may be used after thorough cleaning and washing.
(iii) Flood Plain Gravels:
These occur mostly as unassorted deposits along flood plains and as such are mostly heterogeneous. They require special sorting and cleaning treatment before use.
(iv) Beach Gravels:
These gravels are found strewn widely over beaches where they are deposited by waves. Beach gravels are well-rounded, compact and hard materials. However, they often show salt and clay coatings and require thorough washing before use in any construction.
(v) Glacial Gravels:
These are found at the terminal points of glaciers in hilly regions where they are mostly deposited by the melt waters from glaciers. They are also unassorted and may not be as rounded as the other varieties but can be used as aggregates when and where found in abundance.