Lab 3:
Determination of aggregate impact value (AIV) of given aggregate sample
(BS-812)
Significance:
- Aggregates used in the construction of roads should be strong enough to resist crushing under traffic wheel loads.
- It gives relative measure of the resistance of an aggregate to sudden shock or impact.
- Increase in impact value will indicate reduction in strength of material.
Apparatus:
- Aggregates passing sieve ½” and retained on 3/8” from different sources.
- Aggregate impact value apparatus
- Weighing balance
- Tamping rod
- Sieve # 7 (opening 2.83 mm)
- Brush with stiff bristles
- Drying Oven
- Metal try of known mass
Specifications of impact value apparatus:
Cylindrical container having internal diameter of 75 mm and depth of 50mm
Weight of hammer = 13.5 – 14 kg
Height of fall of hammer = 15”
|
Impact Value Apparatus with its Parts |
Related theory:
Types of tests performed on aggregates:
- Impact value test
- Loss Angeles abrasion test
- Aggregate crushing value
- Soundness test
- Moisture content of aggregate
- Relative density and water absorption
- Sieve tests
- Flakiness index
Aggregate Impact value:
It is the % of fine material which is crushed after the application of load w.r.t. total weight of material.
Test specifications:
The test sample shall consist of aggregates which passes through ½ in B.S. test sieve and is retained on a 3/8 in B.S. test sieve.
The aggregate comprising the test sample should be dried in an oven for a period of four hours at a temperature of 105-110 0C and cooled.
Procedure: (Dry Condition)
- Rest the impact machine, without wedging or packing, upon the level plate, block or floor, so that it is rigid and hammer guide columns are vertical
- Take oven dried sample that passes through sieve ½” and is retained on sieve 3/8”
- Place the sample in the cylindrical container of aggregate impact value apparatus and weigh it. Suppose this value is W
- Remove the sample from container in clean tray.
- Place the sample in cylindrical container of aggregate impact value apparatus in 3 layers.
- Each layer must be given 25 no. of blows
- Each blow being delivered at an interval of not less than 1s
- The hammer should be fall freely for about 15”
- Remove the crushed aggregate by holding the cylinder over a clean tray and hammering on the outside with the rubber mallet until the particles are sufficiently disturbed to enable the mass of specimen to fall freely on tray.
- Transfer fine particles adhering to the inside of the cup and the underside of the hammer to the tray by means of the stiff bristle brush.
- Sieve the whole of specimen in the tray on the 2.36 mm test sieve until no further significand amount passes during a further period of 1 min.
- Weigh the mass that is passed as W2 and that is retained as W
- Now, if total mass differs from the initial mass W1 by more than 1 g, discard the result and test a further specimen.
- Repeat the same procedure for the other samples and note the readings.
- Find the aggregate impact value from the formula given below:
where, W1 is the total mass of sample
W2 is the mass that passes sieve # 7
Observations and calculations
| Sr. no. | Sample Source | Total weight of sample (W1) | Fraction passing sieve # 7 (W2) | Fraction retained on sieve # 7 (W3) | |
| gm | gm | gm | % | ||
| 1 | Margalla | 365 | 53 | 312 | 14.52 |
| 2 | Margalla | 353 | 50 | 303 | 14.16 |
| 3 | Sargodha | 341 | 37 | 304 | 10.85 |
| 4 | Sargodha | 372 | 45 | 327 | 12.10 |
Precautions:
- Care should be taken while giving blows with hammer.
- Aggregates should be oven dried.
- The mass should be noted nearest to 0.1 g.
- The time interval between the blows should not be less than 1s.
Comments:
The aggregates are classified on the basis of their impact value as
| Impact Value | Classification |
| <10% | Exceptionally strong |
| 10 – 20% | Strong |
| 20 – 30% | Satisfactory |
| >30 | Weak |
From the above classification, the given aggregate samples can be classified as Strong Aggregates because their respective impact values are in the range 10 to 20%