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
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