Aggregate Impact Value Test (AIV)

The road aggregates are subjected to impact or pounding action during the construction process of pavement layers, particularly compaction by heavy rollers and also due to movement of heavy wheel loads of traffic, and there is a possibility that some stones would break into smaller pieces.

Therefore, the stone aggregates needs to be strong enough to withstand breakage under impact loads. This characteristic may not be the same as the aggregate’s resistance to crushing under gradually increasing compressive stress.

The aggregate impact Value test is used to assess how resistant aggregates are to impact to fracture under repeated impacts. Based on impact value, determine their suitability for road construction.

Apparatus

1. The total weight of the machine must neither exceed 60 kg nor fall below 45 kg.
2. The machine must be supported on a level and plane concrete or stone block or floor at least 45 cm thick, with a metal base weighing between 22 and 30 kg and a plane lower surface of not less than 30 cm diameter. The machine must be secured to the block or floor or supported on a level and flat metal plate cast into the surface of the block or floor to avoid wobbling.
3. IS Sieves of sizes 12.5, 10 and 2.36 mm
4. A cylindrical steel cup having internal dimensions of 102 mm diameter, 50 mm depth, and not less than 6.3 mm thickness that may be permanently fastened at the center of the base and readily removed for emptying.
5. A case hardened metal hammer or tup weighing 13.5 to 14.0 kg, with the bottom end cylindrical in form, 50 mm long, 100.0 mm in diameter, and a 2 mm chamfer at the lower edge. The hammer should be free to move between the vertical guidelines and concentric with the cup. Hammer free fall should be within 380±5 mm.
6. A cylindrical metal measure with an internal diameter of 75 mm and a depth of 50 mm used to measure aggregates.
7. Tamping rod with a diameter of 10 mm and 230 mm long, rounded at one end.
8. A balance with a minimum capacity of 500 g that is readable and accurate to 0.1 g.
9. A well-ventilated oven with a thermostatically regulated temperature range of 100 to 110°C.

Preparation of Sample

1. The test sample must be aggregate that passes a 12.5 mm IS Sieve and is retained on an 10 mm IS Sieve. The aggregate comprising the test sample must be dried and cooled in an oven for four hours at a temperature of 100 to 110°C.
2. Fill the measure roughly one-third full with aggregate and tamp with 25 strokes of the rounded end of the tamping rod.
3. Repeat the process with two additional layers until the cylinder is filled.
4. Remove the excess aggregates using the tamping rod as a straight-edge.
5. Calculate the aggregates’ net weight to the nearest gram (Weight A).

Procedure

1. The impact machine must be rigidly supported by the level plate, block, or floor, with no wedging or packing, and the hammer guide columns must be vertical.
2. The cup must be fastened to the machine’s base, and the entire test sample must be placed in it and compacted with a single tamping stroke of 25 strokes of the tamping rod.
3. The hammer should be raised until its lower face is 380 mm above the upper surface of the aggregate in the cup, then dropped freely on it. The test sample should be exposed to a total of 15 such strikes, each conducted at a one-second interval.
4. The crushed aggregate is then taken from the cup and sieved through the 2.36 mm IS Sieve until no more substantial amount passes in one minute. The percentage that passes through the sieve must be weighed to an accuracy of 0.1 g (Weight B). The percentage retained on the sieve must also be weighed (Weight C), and the result must be rejected if the overall weight (B+C) is less than the initial weight (Weight A) by more than one gram.
5. The preceding test is performed with another specimen of the same aggregate sample and the same weight as in the first test.

Based on the test results, the toughness property of the aggregate may be reported as given below.

Observations

In each test, the total weight of the sample (weight A) divided by the weight of the fraction passing through 2.36 mm (weight B) gives the aggregate impact value. The weight of fines generated to total sample weight must be expressed as a percentage.

The mean of the two test results is reported as the specimen’s Aggregate Impact Value (AIV), rounded to the nearest whole number.

In general, larger aggregate sizes provide a higher aggregate crushing value, and vice versa, however the relationship between the values obtained with different sizes differs from aggregate to aggregate. With larger aggregate sizes, extra care must be taken to ensure that the plunger does not jam in the cylinder.

The main advantage of aggregate impact testing is that the test equipment and procedure are quite simple; the test can be conducted in a short period of time even on a construction site or in a stone quarry because the apparatus is portable.

Another benefit is that, in addition to measuring toughness, the test is considered to provide an indirect indicator of strength characteristics.

For aggregate to be utilised in the wearing course of pavements, the aggregate impact value should not generally exceed 30%. The maximum allowable for bituminous macadam case courses is 35% and 40% for water bound macadam case courses.

The AIV of coarse aggregates used in Dense Bituminous Macadam (DBM) binder course and semi-dense Bituminous Concrete (SDBC) surfacing should not exceed 27 percent, and that used in Bituminous Concrete (BC) surface source should not exceed 24 percent, according to the Ministry of Road Transport and Highways (MoRT&H), Government of India.

Precautions

1. When sieving aggregates using a 2.36 mm IS sieve, the total weight of the fractions that are retained and pass the sieve shouldn’t vary from the specimen’s initial weight by more than 1 gram.
2. To ensure that the entire weight is imparted on the aggregates, position the plunger in the center of the cylinder so that it falls directly on the aggregate sample and does not contact the cylinder wall.
3. Tamping should be done properly by gently lowering the tamping rod, not by striking it with a hammer. Additionally, the aggregate surface should be uniformly tamped while taking care to prevent repeated strikes of the tamping rod on the mold’s wall.

Reference

1. IS : 2386 (Part IV) – 1963 – Methods of Test for aggregates for Concrete – Part IV : Mechanical Properties
2. IS : 5640 – 1970 – Method of test for Determining Aggregates Impact Value of Soft Coarse Aggregates

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