Thursday, 28 February 2019

Aggregates Impact Value Test

Aggregates Impact Value Test (IS 2386 Part 4)

• Toughness is the property of a material to resist the impact. Due to traffic loads, the road stones are subjected to the pounding action or impact and there is possibility of stones breaking into smaller pieces. The road stones should therefore be tough enough to resist fracture under impact. A test designed to evaluate the toughness of stones i.e., the resistance of the stones to fracture under repeated impacts may be called an impact test for road stones.

Apparatus

 Impact testing machine: The machine consists of a metal base with a plane lower surface supported well on a firm floor, A cylindrical metal measure of 75mm dia. and 50mm depth,  tamping rod of 10mm circular cross section and 230mm length, rounded at one end, Sieves of sizes - 12.5mm, 10mm and 2.36mm, Oven: A thermostatically controlled drying oven capable of maintaining constant temperature between 100° c and 110°c.

Aggregate impact testing machine Theory: Aggregate impact value gives a relative measure of the resistance of an aggregate to sudden shock or impact, which in some aggregates differs from its resistance to a slow compressive load. Aggregate impact value shall be less than 45% for aggregate used in concrete for concrete other than wearing surface and 30% for concrete used in wearing surface.  
Impact Testing Machine

Sampling

  1. Take aggregates passing through 12.5mm IS Sieve 100% • Retention on 10mm IS Sieve 100% 
  2. The sample should be oven-dried for 4hrs. at a temperature of 100 to 110oC and cooled.
  3. The measure should be about one-third full with the prepared aggregates and tamped with 25 strokes of the tamping rod.

Procedure

  1. • The cup of the impact testing machine should be fixed firmly in position on the base of the machine.
  2. Whole of the test sample is to be placed in it and compacted by 25 strokes of the tamping rod.
  3. Weigh the sample with cup to nearest gram.Let it be W1. Also weigh the empty cup, let it be W2.
  4.  The hammer should be raised to 380mm above the upper surface of the aggregates in the cup and allowed to fall freely onto the aggregates.
  5. The test sample should be subjected to a total of 15 such blows, each being delivered at an interval of not less than one second.
  6. Lock thee hammer at the top and remove the cup from the base of the machine.
  7. Sieve the crushed sample through the sieve of 2.36 mm. Weigh this fraction and say it is W3

 Calculation 


 Aggregate impact value = W3/(W1-W2) x100

W1=Weight of sample with cylinder
W2=Weight of cylinder
W3= Weight of fraction passing through 2.36 mm sieve.



Impact value vs Strength


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