AGGREGATE CRUSHING VALUE TEST

AGGREGATE CRUSHING VALUE TEST

AIM

1. To determine crushing value of given road aggregates;
2. To assess suitability of aggregates for use in different type of road pavements.

APPARATUS

The apparatus as per IS:2386-1963 (part IV) consists of the following:

1. Te test mould- a 15.2 cm diameter open ended steel cylinder with squqre base plate ; plunger having a piston of diameter 15cm, from a hole provided across the stem of the plunger so that a rod could be inserted for lifting the plunger in the cylinder.
2. A straight metal tamping road of circular cross-sectin 16mm in dimeter and 45 to 60 cm long, rounded At one end.

(iii) A balance of capacity 5 kg, readable and accurate up to 1 gm.

4. IS sieve of sizes 25mm, 20mm, 4.75mm.
5. A compression testing machine capable of applying load up to 40 tonnes at a uniform rate of 4 tonnes per minute.
6. A cylindrical measure having internal diameter 11.5cm and height 18cm.
   
   
   

THEORY

The “aggregate crushing value” give a relative measure of the resistance of an aggregate to crushing under agradualy applied compressive load. It s the percentage by weight of the crushed(finer) material obtained when the test aggregates are subjected to a specified load under standardized conditions, and is a numerical index of the strength of te aggregate used in road construction. Aggregates with lower crushing value indicate a lower crushed fraction under load and would give a longer service life to the load and hence a more economical performance.Weaker aggregates if used would get crushed under traffic loads, would produce smaller pieces not coated with binder and these would be easily loosend out reasulting in loss of the surface. In short, the aggregates used in road construction must be strong enough to withstand crushing under traffic.

PROCEDURE

The aggregate sample: The material for the standard test consists of aggregates sized 20.0mm to 25.5mm. The aggregates may be dried at heating at 100⁰-110⁰ C for not moe than 4 hours and cooled at room temperature before testing,if necessary.

1. Sieve the materials through 25 mm and 20 mm IS sieves. The aggregates passing through 25 mm sieve and retained on 20.0 mm sieve comprise the test material.
2. Take about 3.25 kg of this material.

3. Pour the aggregates to fill about just more than 1/3^rd depth of measuring cylinder.
4. Compact the material by giving 25 gentle blows with rounded end tamping rod.
5. Add 2 more layers in similar manner, such that the cylinder is full.
6. Remove the excess material with a straight edge.
7. Empty the cylinder and weigh the aggregates, accurate up to 1gm.
8. Transfer the whole of this weighed quantity to the test mould by filling it in 3 layers.
9. Level off the surface andplace the plunger horizontally over it.
10. Place the assembly on the pedestal of the compression testing machine.
11. Apply the load at auniform rate of 4 tonnes per minute until the total applied load is 40 tones.
12. Release the load.
13. Take the aggregate out off the cylinder and sieve through 4.75mm IS sieve.
14. Weigh the fraction passing through it to an accuracy of 0.1 gm.

OBSERVATION

	Sample 1	Sample 2
Total weight of the dry sample (W1 gm)
Weight of portion pasiing 4.75 mm sieve(W2 gm)
Aggregate crushing Value (percent)=w2/w1*100

WORKED OUT EXAMPLE

The sample of the aggregate was collected from KERALA, INDIA

	Sample 1	Sample 2
Total weight of the dry sample (W1 gm)	2913
Weight of portion pasiing 4.75 mm sieve(W2 gm)	771
Aggregate crushing Value (percent)=w2/w1*100	26.43

            Aggregate crushing value = W2/W1 * 100

                                                   = 771/2917 * 100

                                                   = 26.43%

RECOMMENDED VALUES

The suitability of aggregate is adjudged, depending on its proposed use inthe pavement layers. Specified limits of percent aggregate crushing value for different types of road construction s given bellow.

Sl No	Type of road construction	Aggregate crushing value not more than
1	Flexible Pavements
a)	Soiling	50
b)	Water bound macadam	40
c)	Bituminous macadam	40
d)	Bituminous surface dressing or thin premix carpet	30
e)	Dense mix carpet	30
2.	Rigid Pavements
a)	Other than wearing course	45
b)	Surface or wearing course	30