What Is Aggregate Impact Value?
It is the ability of aggregates that resist sudden impact or shock load on it. Also, it can be defined as the resistance of aggregate to failure by impact load is known as the Impact Value of Aggregate.
The characteristic of any material to resist sudden impact is known as toughness. When we use aggregate in roads and pavement on which vehicular movement is done and there is a possibility of impact load on aggregate.
This impact load can break aggregate into smaller pieces and which results in the failure of roads and pavements.
Therefore, aggregate used for such propose should have sufficient toughness to resist their disintegration due to the impact. This important property of aggregate is determined by the aggregate impact value test.
This resistance of aggregate may differ from resistance to gradual load which we measure in the crushing value test.
Importance of Impact Test on Aggregates
The impact test on aggregate is carried out to know the response of aggregates to different kinds of loads that the aggregates will be subjected to during their service life.
The need for impact value test is used to measure the toughness of aggregates which is nothing but the ability of aggregates to resist the sudden loading or impact loading.
Find out the impact resistance of aggregate is important because, during service life, the aggregates will be actually subjected to this type of load. For example, aggregates will be subjected to impact after speed breaker, or when the vehicle is moving on the undulating road, etc.
So, by AIV Test, we can find out the response of aggregates to sudden loading. If the result is satisfactory, we use those aggregates in pavement. If not, aggregates are rejected.
Objectives of Impact Value Test
This method of test used to measure the impact value of aggregate use in the construction of Highway pavement and concrete structure.
Reference:- Aggregate impact value test IS Code is 2386 (Part 4) – 1963 PDF
1. Impact testing machine
2. The total weight of the machine used in the test is not more than 60kg or not less than 45 kg.
3. A steel cup shape as a cylinder usually internal dimension:
Diameter = 102mm
Depth = 50mm
Thickness = 6.3mm
4. 13.5 to 14 kg hammer with the end of cylindrical shape is used for the impact load:-
Diameter = 100mm
Length = 50mm
Chamfer at lower edge = 2mm
5. Sieves:- The following IS sieves are used
The cylinder made with the steel used.
The following internal dimensions are:
Diameter = 75mm
Depth = 50mm
7. Tamping rod
A straight tamping rod made with the metal of circular c/s is used to tamping the aggregate. Rounded at the end.
The following are the dimension of the rod:
Diameter = 10mm
Length = 230mm
A balanced used for the weight measure is not less than 500 gm, or it readable or its accuracy is up to 0.1gm.
The oven used for drying the aggregates is well ventilated or it thermostatically controlled to maintain the temperature in the oven 100 to 110 °C.
Preparation of Aggregate Sample
- The aggregate which uses for the sample is passed through the 12.5mm IS sieve and retain on the 10mm IS sieve.
- The aggregate which is retained on the 10mm sieve is dried in an oven for a time period of 4 hours at the oven temperature of 100 to 110º C.
- The cylindrical measure use for the collection sample filled about 1/3 full with the aggregates and it tamped 25 stokes with help of the rounded end of the tamping rod. Further similar 1/3 quantity of aggregates are filled in it and again 25 stokes are given. Than measure finally filled to overflowing than apply 25 stokes again and the extra aggregates are removing form the measure with the help of the straight portion of the tamping rod.
The Procedure of Impact Test of Aggregate
- The impact machine is rest on the horizontal flat surface like the level plate, block, or floor without any packing. So, the application of a hammer is straight and vertical.
- The cup is fixed in a proper position on the base of the impact machine.
- Transfer aggregate sample from cylindrical measure to cut fitted in an impact test machine and apply 25 strokes of the rod to compact it.
- Then the hammer is lifted 380mm and falls on the upper surface of aggregate which fills in the cup.
- A total of 15 blows are applied on the cup each delivered at an interval of not less than 1 second.
- The crushed aggregates are removed from the cup and the crushed sample is sieve from the 2.36mm IS sieve.
- Weight the fraction of sample passing through 2.36mm IS sieve accuracy up to 0.1 gm. (W1)
- Weight the fraction of sample retains on the 2.36mm IS sieve. ( weight W2)
- The total weight of the sample (W1 + W2) is less than the initial weight (weight W) by more than one gram than the result discarded.
- Aggregate impact value is the ratio of the weight of the fraction passing through 2.36 mm (weight W2) by the total weight of the sample (weight W1 + W2).
Aggregate Impact Value = (W2/ (W1+W2)) * 100
= 80 gm / 300 gm
= 26.67 (Impact Value)
W1= Weight of the fraction passing through 2.36 mm IS sieve.(220 gm)
W2= Weight of sample retained on 2.36 mm IS sieves. (80 gm)
W1+W2 = Total Weight of Sample (220 + 80 = 300 gm)
Result: The Aggregate Impact Value is _________.
Recommended Aggregate Impact Test Values
|Aggregate Impact value||Nature|
|20-30%||Satisfy for surface course|
|>35%||Weak for surface course|
Maximum Allowable Impact Value of Aggregate for Different Types of Pavements
|Sr. No.||Types of Pavements||Aggregate Impact Values (Max.)|
|1||Water Bound Macadam (WBM) Sub- Base Course||50|
|2||Cement Concrete, Base Course||45|
|3||i) WBM base course with bitumen surfacing||40|
|ii) Built up- Spray grout, base course|
|4||Bituminous macadam, base course||35|
|5|| i) WBM, surface Course |
ii) Buit-up spray grout, surface course
iii) Bituminous penetration macadam
iv) Bituminous macadam, binder Course
v) Bitumen surface dressing
vi) Bitumen carpet
vii) Bituminous / Asphaltic concrete
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