Transportation aggregates 2

• Aggregate is the major component used in road making. It is used in granular bases and subbases, bituminous courses and in cement concrete pavements.
• Aggregates


  • A study of the types of aggregates, their properties and tests is of great importance to a highway engineer
• Types of Aggregates


  • Igneous Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks
• Igneous Rocks


  Formed by the cooling, solidification and crystallization of molten rock on the earth's crust at different depths
• Igneous Rocks


  • The minerals, their proportions and the rate of cooling of the magma have a bearing on the strength characteristics of the rock
  • Igneous rocks are, in general, stronger than the other two types
  • Granite, diorite and gabbro are intrusive rocks which form at deep layers in the earth's crust
  • Basalt (or trap), andesite, rhyolite and dolerite are extrusive rocks which from at the top layers of the earth's crust
• Sedimentary Rocks


  Fine material or rock fragments and particles transported by water or wind and deposited in layers, get hardened in course of time to form sedimentary rocks
• Sedimentary Rocks


  • Sandstone
  • Argillite
  • Limestone
  • Dolomite
• Metamorphic Rocks


  Formed by the modification and re-crystallisation of igneous rocks and sedimentary rocks by geological and natural agents such as temperature, pressure, moisture, humidity, and movement of rock beds
• Desirable Properties of Aggregates

  • Strength
  • Hardness
  • Toughness
  • Durability
  • Soundness
  • Cementation
  • Appropriate Shape
  • Adhesion to Bitumen (Bitumen Affinity)
  • Attrition
  • Texture
  • Low porosity
  • Low Absorption rates
  • Particle Size Distribution/ Gradation
• Strength


  Resistance to crushing which the aggregates used in road construction, especially in the top layers and wearing course, have to withstand the stresses due to wheel loads of the traffic in addition to wear and tear
• Hardness


  Resistance to abrasion of the aggregate at the surface. The constant rubbing or abrading action between the tyres of moving vehicles and the exposed aggregate at the road surface should be resisted adequately
• Toughness


  Resistance to impact due to moving traffic. Heavily loaded trucks and other vehicles cause heavy impact loads on the road surface while moving at high speeds, and while accelerating and decelerating
• Durability


  Resistance to the process of disintegration due to the weathering action of the forces of nature
• Soundness


  The property by virtue of which the aggregate withstands weathering
• Cementation


  The ability of the aggregate to form its own binding material under traffic, providing resistance to lateral displacement
• Appropriate Shape


  Aggregates maybe either rounded, cubical, angular, flaky, or elongated. Each shape is appropriate for a certain use. Too flaky and too elongated aggregates have less strength and durability; so they are not preferred in road construction. Rounded aggregates are good for cement concrete because of the workability such aggregates provide. Cubical or angular aggregates have good interlocking properties; since flexible pavements derive their stability due to interlocking, such aggregates are the preferred type for construction
• Adhesion to Bitumen (Bitumen Affinity)


  The aggregates used in bituminous pavements should have less affinity to water than to bitumen; otherwise, the bituminous coating on the surface of the aggregate will get stripped off in the presence of water
• Attrition


  Mutual rubbing of aggregates under traffic; adequate resistance to attrition is a desirable property
• Texture


  A measure of the degree of fineness or smoothness of the surface of the aggregate
• Low porosity


  Porosity is the ratio of the space in a rock mass not occupied by the solid in relation to the total volume of the rock mass. Highly or more porous aggregates have high permeability, and they are weak if used in bituminous mixes it will take in some bitumen to fill the voids in the rock thus requiring more bitumen
• Low Absorption rates


  Hydrophilic aggregates are not suitable for road construction because they will absorb water and volume will increase thus causing unnecessary problems
• Particle Size Distribution/ Gradation

  The aggregate mix should have appropriate particle size distribution consisting of various sizes which gives a stable mix. A stable mix will result in low segregation and will give you a high bearing strength

  
• Tests on Aggregates


  • Aggregate Crushing Test
  • Ten Percent Fines Value
  • Aggregate Impact Test
  • Los Angeles Abrasion Test
  • Soundness of Aggregates by use of Sodium Sulphate
  • Relative Density and Water Absorption Test
  • Flakiness Index (FI)
  • Elongation Index (EI)
  • Grading

  
• Aggregate Crushing Value (ACV)


  Gives a relative measure of the resistance of an aggregate to crushing under a gradually applied load. The Aggregate Crushing Value (ACV) is determined by measuring the material passing a specified sieve after crushing under a load of 400 kN over a period of 10 minutes
• Ten Percent Fines Value (TFV)


  Gives a relative measure of the resistance of an aggregate to crushing under a gradually applied load. The 10% Fines Value (TFV) is determined by measuring the load required to crush a prepared aggregate sample to give 10% material passing a specified sieve after crushing
• Aggregate Impact Value (AIV)


  Gives a relative measure of the resistance of an aggregate to sudden shock or impact
• Los Angeles Abrasion Test (LAA)

  Assesses the hardness of coarse aggregates used in pavement construction. Due to the movement of traffic, the road stones used in the surface course are subjected to wearing action at the top. Resistance to wear or hardness is hence an essential property for road aggregates, especially when used in wearing course
• Aggregate Impact Value (AIV)


  Relative measure of the resistance of an aggregate to sudden shock or impact
• Aggregate Impact Value test

  1. Can be performed in either a dry condition or in a soaked condition

  2. Applicable to a standard fraction aggregates passing a 14 mm sieve and retained on a 10 mm sieve
• Los Angeles Abrasion Test (LAA)


  Assesses the hardness of coarse aggregates used in pavement construction
• Los Angeles Abrasion Test (LAA)


  1. Measures degradation of mineral aggregates of standard gradings resulting from a combination of actions including abrasion, impact and grinding in a rotating steel drum containing a specified number of steel spheres

  2. After the prescribed number of revolutions, the contents are removed from the drum and the aggregate portion is sieved to measure the degradation as a percent loss
• Los Angeles Testing Machine

  • Hollow rotating steel cylinder, closed at both ends, with an inside diameter of 711 mm and an inside length of 508 mm
  • Steel shelf extending the full length of the cylinder and projecting inward 89mm
  • Driven and counterbalanced to maintain a substantially uniform peripheral speed
• Soundness of Aggregates by use of Sodium Sulphate


  Provides a procedure for making a preliminary estimate of the soundness of aggregates subject to weathering action for use in concrete and road pavements
• Soundness of Aggregates by use of Sodium Sulphate


  1. Involves repeated immersion in saturated solutions of sodium sulphate followed by oven drying to partially or completely dehydrate the salt precipitated in permeable pore spaces

  2. The internal expansive force, derived from the rehydration of the salt upon re-immersion, simulates the expansion of water on freezing
• Sodium sulphate solution


  Made up of 225g of Sodium sulphate and 1 litre of water, with a specific gravity of 1.151 – 1.174 when ready for use
• Magnesium sulphate solution


  Made up of 350g of Magnesium sulphate and 1 litre of water, with a specific gravity of 1.295 – 1.308 when ready for use
• Relative Density and Water Absorption Test

  Used for pavement materials to be tested for mechanical properties and for calculations in concrete mix design
• Flakiness Index (FI)


  One of the tests used to classify aggregates and stones, with specific requirements in Pavement Design
• Flaky particles


  Particles whose thicknesses are less than 0.6 of their mean size

  
• Flakiness Index (FI) test

  1. Involves separating the flaky particles and expressing their mass as a percentage of the mass of the sample

  2. Applicable to material passing a 63 mm sieve and retained on a 6.3 mm sieve