Aggregates form the major portion of pavement structure and they form the prime materials used in pavement construction.
Aggregates have to bear stresses occurring due to the wheel loads on the pavement and on the surface course they also have to resist wear due to abrasive action of traffic.
These are used in pavement construction in cement concrete, bituminous concrete and other bituminous constructions and also as granular base course underlying the superior pavement layers.
Therefore the properties of the aggregates are of considerable significance to the highway engineers.
Most of the road aggregates are prepared from:
- Natural rock
- Slags derived from metallurgical process (steel plants)
Gravel aggregates are small rounded stones of different sizes which are generally obtained as such from some river beds.
Sand is fine aggregate from weathering of rock. The properties of the rock, from which the aggregates are formed, depend on the properties of constituent materials and the nature of bond between them.
Based on the origin, natural rocks are classified as igneous, sedimentary and metamorphic rocks.
Predominantly crystalline and are formed by the cooling of molten rock magma.
Further igneous rocks are classified as below:
Classification based on grain size:
a) Course (>2mm)
b) Medium (2mm to 0.2mm)
c) Fine (<0.2mm)
Classification based on composition:
a) Acid rocks (>66% silica, light in color & specific gravity <2.75)
b) Intermediate rocks (55 to 66% silica)
c) Basic rocks (<55% silica, dark in colour & specific gravity >2.75)
Sedimentary rocks Formed either from the deposition of insoluble granular material resulting from the disintegration of pre-existing rocks, or form the inorganic remains of marine and animals which are deposited in great quantities on the sea floor.
Sedimentary rocks are deposited in layers, they have stratified or laminated structure.
Classification based on predominating mineral:
a) Calcareous rock (chalk, lime stone & dolomite)
b) Siliceous rock (sand stone, flint & chert)
c) Argillaceous (clay & shale)
These are sedimentary or igneous rocks that have been subjected to great heat or to great pressure or both, that has resulted in the formation of minerals and in textures different from those of the original rock.
Classification based on grain size:
a) Fine grain size (hornfels & schist)
b) Coarse grain size (gneiss & Granulite)
Based on the strength property, the coarse aggregates may be divided as hard aggregates and soft aggregates.
Generally for the bearing course of superior pavement types, hard aggregates are preferred to resist the abrading and crushing effects of heavy traffic loads and to resist adverse weather conditions.
In the case of low-cost road construction for use in lower layers of pavement structures, soft aggregates can also be used.
The soft aggregates include moorum, kankar, laterite, brick aggregates and slag.
A different set of test specifications are adopted for soft aggregates.
The aggregates are specified based on their grain size, shape, texture and its gradation and it is specified by various agencies like ASTM, BSI, ISI and IRC.
Based on strength property, the coarse aggregates are divided into hard aggregates and Soft ggregates.
- They must be crushed aggregate.
- They shall be clean, hard, durable and cubical in shape.
- They must be free from the dust, organic matter and other deleterious matter.
- They not be flakey or elongated.
- They must not consist of injurious or harmful materials such that they reduce the strength of structure.
- They should resist wear due to abrasive action of traffic on the surface course.
Properties of aggregates:
The aggregate have three properties
- Physical properties.
- Mechanical properties.
- Chemical properties.
Main Desirable Properties of Aggregate
The aggregates to be used in road construction should be sufficiently strong to withstand the stresses due to traffic wheel loads.
The aggregates which are to be used in top layer of the pavements, particularly in the wearing course have to be capable of withstanding high stresses in addition to wear and tear; hence they should possess sufficient strength and resistance to crushing.
The aggregates used in the surface course are subjected to constant rubbing or abrasion due to moving traffic.
It should be hard enough to resist the wear due to abrasive action of traffic.
Abrasive action may be increased due to the presence of abrasive material like sand between the tyres of moving vehicles and the aggregates exposed at the top surface.
This action may be severe in the case of steel tyred vehicles.
Heavy wheel loads can also cause deformations on some types of pavement resulting in relative movement of aggregates and rubbing of aggregates with each other within the pavement layer.
The mutual rubbing of stones is called attrition, which also may cause a little wear in the aggregates; however attrition will be negligible or absent in most of the pavement layers.
Aggregates in the pavements are also subjected to impact due to moving wheel loads.
Severe impact like hammering is quite common when heavily loaded steel tyred vehicles move on water bound macadam roads where stones protrude out especially after the monsoons.
Jumping of the steel tyred wheels from one stone to another at different levels causes severe impact on the stones.
The magnitude of impact would increase with the roughness of the road surface, the speed of the vehicle and other vehicular characteristics.
The resistance to impact or toughness is hence another desirable property of aggregates.
The stone used in the pavement construction should be durable and should resist disintegration due to the action of weather.
The property of the stones to withstand the adverse action of weather may be called soundness.
The aggregates are subjected to the physical and chemical action of rain and ground water, the impurities there-in and that of atmosphere.
Hence it is desirable that the road stones used in the construction should be sound enough to withstand the weathering action.
5) Shape of aggregates
The size of the aggregates is first qualified by the size of square sieve opening through which an aggregate may pass, and not by shape.
Aggregates which happen to fall in a particular size range may have rounded, cubical, angular flaky or elongated shape of particles.
It is evident that the flaky and elongated particles will have less strength and durability when compared with cubical, angular or rounded particles of the same stone.
Hence too flaky and too much elongated aggregates should be avoided as far as possible.
The voids present in a compacted mix of coarse aggregates depend on the shape factors.
Highly angular, flaky and elongated aggregates have more voids in comparison with rounded aggregates.
Based on the shape of the aggregate particle, stones may be classified as rounded, angular, flaky and elongated.
Angular particles possess well-defined edges formed at the intersection of roughly plane faces and are commonly found in aggregates prepared by crushing of rocks.
Flaky aggregates have lesser thickness when compared to the length and width.
Elongated aggregates have one of the dimensions or the length higher than the width and thickness.
The shape factors of aggregates depend on the source, properties of the rock and the type and condition of the crushers.
The shape of aggregates is generally described in terms of its shape factors such as flakiness index, elongation index and angularity number.
Several researchers have indicated that in pavement construction flaky and elongated aggregates are to be avoided, particularly in surface course.
If flaky and elongated aggregates are present in appreciable proportions, the strength of the pavement layer would be adversely affected due to possibility of breaking down during compaction and under loads.
6) Adhesion with Bitumen
The aggregates used in bituminous pavements should have less affinity with water when compared with bituminous material; otherwise the bituminous coating on the aggregates will be stripped off in presence of water.