Properties Of Stone as a Highway Material

Properties Of Stone as a Highway Material

Following are the materials to deal with in highway materials. Soil Characteristics, Stone Characteristics, Bituminous Characteristics.
Desirable Properties Of Stone as a Highway Material

a) Strength- They should be strong enough to withstand stresses due to traffic wheel load
b) Hardness- They should be hard enough to resist wear and tear due to abrasive action. The mutual rubbing of stones is called attrition
c) Toughness- Resistance to impact due to moving wheel load
d) Durability – The property to withstand the adverse action of weather may be called soundness.
e) Shape of Aggregates- Flaky and elongated particles have less strength than cubical and angular or rounded particles. Rounded particles are not preferred in granular base courses. Whereas for WBM & Bituminous construction angular particles provide better stability.

TESTS FOR AGGREGATES

A) Crushing Test –

  • Dry aggregates passing 12.5 IS sieve and retained on 10mm sieve is filled in a cylindrical vessel filled in 3 equal layers
  • A crushing load of 40 tones is applied at a rate of 4 tones/min and the crushed aggregate is sieved on 2.3 mm IS Sieve
  • Aggregate crushing value =(W2/W1)*100
  • Where W2 is crushed material passed 2.36 mm sieve and W1 sample material

Note- The Crushing value for the surface course should be less than 30% and should not exceed 45% for the base course

B) Abrasion Test-
The test is carried out to find the hardness of stones. Tests are following

i) Los Angeles Abrasion test

  • It is based on the rubbing action between the aggregate and steel balls
  • The machine is rotated at 30-33 rpm for 500 to 1000 revolutions
  • The abrasion value for good aggregates for high-quality pavement materials should be less than 30%. However, for the base course in WBM, it may be 50%
  • The abrasion value is expressed as percentage loss in weight due to abrasion

C) Impact Test

  • Used to evaluate the toughness of stone or resistance to fracture under repeated impacts
  • A metal hammer of weight 13.5 to 14 kg having a free fall from a height of 38 cm is dropped through vertical guides
  • Hammer is given free fall of 15 blows and crushed aggregate is sieved on a 2.36mm sieve
  • Aggregate should have a size between 12.5 mm to 10 mm and are filled in 3 layers
  • Value is expressed as a percentage of the fine form in terms of the total weight of the sample.
  • The aggregate value should not usually exceed 30% for aggregate to be used in wearing course of pavements, which is restricted to 35% for bituminous macadam and 40% for WBM base

D) Soundness Test

  • The test is intended to study the resistance of aggregates to weathering action, by conducting accelerated weathering test cycles.
  • The resistance to the disintegration of aggregate is determined by using a saturated solution of sodium sulphate and magnesium sulphate. Dry aggregate is immersed in the above solution for 16-18 hours. The specimen is dried in an oven at 105-110 degree temp. Thus making one cycle of immersion and drying.
  • The average loss in weight of aggregates to be used in pavement construction after 10 cycles should not exceed 12% when tested with sodium sulphate and 18% when tested with magnesium sulphate

D) Shape Tests

a) Flakiness index
The flakiness index of aggregate is the percentage by weight of the aggregate particle whose least dimension /thickness is less than 3/5 or 0.6 of their mean dimension. The test is applicable to a size larger than 6.3mm F.I=w/W*100 %, where w is flaky material passing the appropriate slot from each size range of test aggregate are added up and W is the Total weight of sample taken. It is desirable that the flakiness index of aggregate used in road construction is less than the 15% and normally does not exceed 25

b) Elongation Index

  • The elongation index of an aggregate is the percentage by weight of particle whose greatest dimension or their length is greater than one and 4/5 or 1.8 times their mean dimension.
  • The test is not applicable for a size smaller than 6.3mm
  • Value more than 15% is undesirable

c) Angularity Number

  • It represents the degree of packing or measure the void excess of 33%
  • 67 represents the volume of solid (in %) of most rounded gravel in a well-compacted state, which would have 33% voids.
  • Higher the angularity number more angular is the aggregate
  • The range of A.N. for aggregates used for construction varies between 0-11
  •  A.N=67\frac{W}{C.G_{a}}\times100
    Where W is the weight of aggregate in a cylinder C is the weight of water in the same cylinder Ga is the specific gravity of aggregate
Properties Of Stone as a Highway Material

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