AGGREGATE CRUSHING VALUE TEST

Object of the Experiment

a) To determine crushing value of given aggregates. (Aggregate Crushing Value Test)

b) To assess suitability of aggregates for use in different types of concrete and pavements.

Theory and Scope

The road aggregate is generally manufactured to a specified grading and stock piled at a sight. Before being used in the pavement it undergoes certain operations as loading and transporting in trucks, tipping, spreading and compacting which may result in a change in grading and or the production of excessive and undesirable fines. Significant degradation may take place when the aggregate is weak. Thus, an aggregate complying with a specification at the production site may fail to do so when it is in the pavement.

The aggregate crushing value is a numerical index of the resistance of an aggregate to crushing under a gradually applied compressive load. It is expressed as the percentage by mass of the crushed (or finer) material obtained when the test sample is subjected to a specified load under standard conditions. Aggregates with lower crushing value indicate a lower crushed fraction under load or higher strength and would provide a longer service life to the structure. Weaker aggregates if used in road pavements would get crushed under traffic loads and produce smaller pieces not coated with binder and these would be easily loosened out resulting in loss of the surface or layer. The aggregate crushing test is conducted as per IS : 2386-1963 (Part IV)

Equipments

1. Compression Testing Machine

2. Test mould

3. Tamping rod

4. Balance

5. IS test sieves of sizes 12.5 mm and 2.36 mm

6. Cylindrical measure

7. Oven

Description of Apparatus

1. Compression Testing Machine of 500 kilo Newton capable of loading at uniform rate of 40 kilo Newton per minute.

2. Case Hardened Test Mould consists of a 152 mm diameter open-ended steel cylinder with square base plate plunger having a piston of diameter 150 mm, with a hole provided across the stem of the plunger so that a rod could be inserted for lifting or placing the plunger in the cylinder.

3. Tamping Rod of 16 mm in diameter, 450 to 600 mm long and rounded at one end.

4. Metal Cylindrical measure having internal diameter of 115 mm and height 180 mm.

5. Balance of capacity 5 kg, readable and accurate up to 0.1 gram.

ALSO READ

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• Sp. Gravity & Water Absorption Of Coarse Aggregate
• Sp. Gravity & Water Absorption of Fine Aggregates

Procedure

The material for the standard test consists of aggregates of size 10 mm to 12.5 mm. The aggregates should be in surface dry condition before testing. The aggregates may be dried by heating at 100 degree to 110 degree Celsius for more than 4 hours and cooled to room temperature before testing, if necessary.

Step-1 : Sieve the material through 12.5 mm and 10 mm IS sieves. The aggregates passing through 12.5 mm sieve and retained on 10 mm sieve comprise the standard test aggregate.

Step-2 : Take about 2 kg of aggregate prepared in Step-1 and fill the measuring cylinder with aggregate in three layers, each layer being tamped with 25 strokes of the rounded end of the tamping rod, distributing the strokes gently and evenly over the surface. The measuring cylinder is finally filled to overflow.

Step-3 : Strike off the surplus aggregate using tamping rod as straight edge.

Step-4 : Weigh the container full of aggregate.

Step-5 : Calculate the net mass of aggregate in the measuring cylinder to an accurate of 1 gram. This quantity of aggregate constitutes the test specimen.

Step-6 : Transfer the whole of this weighed specimen to the test mould by filling it in three layers as explained in Step-2. The total depth of the sample is then about 100 mm and the surface a little below the top of mould.

Step-7 : Level off the surface and place the plunger over it so that it rests horizontally on the surface of the aggregates.

Step-8 : Place this assembly on the pedestal of compression testing machine.

Step-9 : Apply the load at a uniform rate of 40 kilo Newton per minute until the total applied load is 400 Kilo Newton.

Step-10 : Release the load and take the crushed aggregate out of the mould.

Step-11 : Sieve the crushed aggregate through 2.36 mm IS sieve and weigh the fraction passing through it to an accuracy of 0.1 gram. This fraction is a measure of loss of material due to crushing. The mean value of two samples, rounded to nearest whole number is taken as the aggregate crushing value.

Observation and Calculation

Type condition of aggregate : Dry

Type of drying : Air dried / Oven dried

Size of aggregate (Standard size) : 12.5 mm

IS sieve used for separating fines : 2.36 mm

The Average Aggregate Crushing Value = …………………. per cent.

Discussions and Conclusions

The values of aggregate crushing test agree in general with that of aggregate impact test. The crushing strength test is most significant as far as the conditions to which the aggregates is likely to be subjected in practice are concerned. The suitability of aggregate in the pavement layers is dependent upon the type of road construction. The specified limits of aggregate crushing value for different types of road construction are given in the table below.

Limits of aggregate crushing value for different types of road construction

The Aggregate Crushing Value was found to be ……………………..