The aim of aggregate abrasion test is to determine the measure of the resistance of aggregate to the surface wear by abrasion and also to assess the durability of the pavement surfacing when tranversed by many repetitions of truck wheel load.
Los Angeles abrasion test on aggregate is the measure of aggregate toughness and abrasion resistance such as crushing, degradation and disintegration. This test is carried out by AASHTO T 96 or ASTM C 131: Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine.
The aggregate used in surface course of the highway pavements are subjected to wearing due to movement of traffic.
When vehicles move on the road, the soil particles present between the pneumatic tyres and road surface cause abrasion of road aggregates. The steel rimmed wheels of animal driven vehicles also cause considerable abrasion of the road surface.
Therefore, the road aggregates should be hard enough to resist abrasion. Resistance to abrasion of aggregate is determined in laboratory by Los Angeles test machine.
The principal of Los Angeles abrasion test is to produce abrasive action by use of standard steel balls which when mixed with aggregates and rotated in a drum for specific number of revolutions also causes impact on aggregates.
The percentage wear of the aggregates due to rubbing with steel balls is determined and is known as Los Angeles value
Recommended Los Angeles Test Values for Pavements
Los Angeles test is commonly used to evaluate the hardness of aggregates. The test has more acceptability because the resistance to abrasion and impact is determined simultaneously.
Depending upon the value, the suitability of aggregates for different road constructions can be judged as per IRC specifications as given:
Different types of pavement are listed below with their maximum permissible abrasion value in %
• Water bound macadam sub base course – 60%
• WBM base course with bituminous surfacing – 50%
• Bituminous bound macadam – 50%
• WBM surfacing course – 40%
• Bituminous penetration macadam – 40%
• Bituminous surface dressing, cement concrete surface course – 35%
• Bituminous concrete surface course – 30%
To carry out the aggregate abrasion value test, the following are the procedures used:
1. The cover of the machine is remove and the sample x & y is pour above into the machine.
2. The abrasion charges is place on the sample and the cover is fix back firmly.
3. The machine is switch on and it is allow to undergo 500 revolutions at 28 – 30rpm.
4. The machine is switch having completed the stipulated number of revs/min., and the content is empty into a tray at the base of the machine.
5. The sample is sieve to separate the fines using 1.70mm sieve until no significant amount could pass through in 1 minute.
6. The fraction retained (A) on sieve 1.70mm and the fraction passing (B) through the sieve 1.70mm is weigh respectively to an accuracy of 1gm. The result is then discard and a fresh sample is made.
7. The whole procedure is repeated for a fresh sample.
The following are the apparatus used during the execution of the test:
• Abrasion machine
• BS test sieves of aperture size SS 20mm, 14mm, 10mm, 1.70mm.
• Abrasion charges
• A weighing balance of capacity of not less than 1kg.
• Fine and stiff brushes
DATA AND CALCULATION
The ratio of the mass of fines formed to the total mass in each test of three repetitions was calculated using the expression as a percentage i.e.
% finess = B/(x+y) X 100
Where (x+y) = mass of sample used (g).
B = mass of fraction passing the sieve no. 2.36mm (g).
1. Ensure that the weighing balance is working perfectly before commencing the experiment.
2. Ensure that the sample is remove and wash over a No. 12 (1.70mm) and place in an oven to dry.
3. Ensure that the sample is specifically sized Aggregate and the abrasion charges is place in Las abrasion machine.
4. Ensure that the abrasion machine is being rotate at 30 – 33rpm.
• Natural Error: This include uneven distribution of air and humidity in the practical lab.
• Systematic Error: This can occur due to old age and inefficiency of the apparatus being used
• Human Error: This may be in form of parallax or as a result of the inexperience of the group members.
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