PROPERTIES AND TESTING OF AGGREGATES FOR PAVEMENT WORKS

🕑 Reading time: 1 minute

Aggregates plays vital role in the construction of pavement. They have great capability of load transfer to the Subgrade soil. Aggregates have different properties which are tested individually with different types of tests for the construction of pavement. Aggregate should qualify all the tests conducted to give better results after construction. The properties of aggregate and their respective tests are given below.

Properties and Tests of Aggregates for Pavement Works

Crushing Strength Test on Aggregates

Aggregate crushing value gives the Crushing strength of aggregate up to which it can bear the load without fail. To conduct crushing strength test we need compression testing machine, cylindrical measure, plunger and Isa sieves. First sieve the sample aggregate, aggregate passing 12.5mm sieve and retaining 10mm sieve is oven dries at 100-110^oC for 3-4 hrs. The cylinder is filled with aggregate in 3 layers, 25 strokes of tampering for each later. Note down its weight and insert the plunger and placed it on compression testing machine. Apply the load at uniform rate of 40 tonnes load in 10 minutes. Then stop the machine and crushed aggregate is sieved through 2.36mm sieve and aggregate passing 2.36mm sieve is weighed. Aggregate crushing value can be obtained from below formula:

Aggregate crushing value = (W2/W1) *100 %

Abrasion Test on Aggregates

Hardness property of aggregate is determined by conducting abrasion test. Los Angeles abrasion testing machine is used to conduct this test. For this test, the sample taken should be clean and dried. The sample is weighed W1 and placed in Los Angeles testing machine and the machine is operated. Machine should be rotated at a speed of 20-33 revolutions per minute. After 1000 revolutions the sample is taken out and sieved through 1.7mm sieve. Sample retained on 1.7mm is washed and dried and note down its weight W2.

Aggregate abrasion value = {(W1-W2)/W2} x 100%

Impact Test on Aggregates

Impact value of aggregate will give aggregate capability against sudden loads or forces. For this test also aggregate passing through 12.5mm and retained on 10mm sieve is taken and oven dried. Fill the cylinder with aggregate in 3 layers, 25 strokes of tamping for each layer. Weight w1 noted. The cylinder is placed in impact testing machine which consist a hammer. After placing the cylinder, hammer is raised to 380mm and release freely. Then it will blow the aggregates. Repeat it for 15 such blows. After that take down the sample and aggregate passing through 2.36mm sieve is weighed as w2.

Aggregate impact value = (W2/W1) *100 %

Soundness Test on Aggregates

To determine the weathering resistance of aggregate soundness test is conducted. If the resistance against weathering is good for aggregate, then it will have high durability. For soundness test we need some chemical solutions namely sodium sulphate or magnesium sulphate. The sample of aggregate passing through 10mm sieve and retained on 300 micron sieve is taken. Dry and weigh the sample and immerse them in the chemical solution for about 18 hours. After that, Take the sample and dried it in oven at 100 -110^oc. repeat this procedure 5 times for one sample, and weigh the aggregate finally and note down the difference in weight loss. The weight loss should be below 12% if sodium sulphate is used, below 18% if magnesium sulphate is used.

Shape test on Aggregates

Shape of aggregate is also important consideration for the construction of pavement. Aggregate should not contain flaky and elongated particles in it. If they contain this type of particles, they will affect the stability of mix. The percentage by weight of aggregates whose least dimension is less the 3/5th of its mean dimension is called as flakiness index. The percentage by weight of aggregate particles whose greatest dimension is 1.8th times their mean dimension is called as elongation index. In this test shape test gauges are taken and minimum of 200 pieces containing sample is passed through respective gauges. Material retained on Thickness gauge and material retained on length gauge is weighed to an accuracy of 0.1%.

Bitumen Adhesion test on Aggregates

Bitumen adhesion test will give the stripping of bitumen from the aggregate. In case of bitumen pavement, the bitumen should be in pure contact with aggregate. To attain this aggregate should be clean and dry. To determine the stripping value of bitumen static immersion test is conducted on aggregates. In this test the aggregates are coated with bitumen and dried. After drying they are immersed in water at 40^oc for about 24 hours. Stripping value of aggregate should not exceed 5%.

Specific gravity test on Aggregates

Specific gravity of an aggregate is the ratio of its mass to that of an equal volume of distilled water at specific temperature. The specific gravity of aggregate is of two types. Bulk specific gravity, in which total volume of aggregates along with their void space is considered. Apparent specific gravity, in which the volume of aggregates without considering void spaces is taken into account.

Bulk specific gravity G = {dry weight of aggregate/total volume of aggregate}/ density of water

Apparent specific gravity G= {dry weight of aggregate/volume of aggregate without void space}/ density of water

Water absorption test on Aggregates

This test helps to determine the water absorption value of aggregate. To perform this test minimum 2 kg sample should be used. The sample should be cleaned and dried. Place the sample in wire basket and dip the basket in distilled water bath. To release the air between aggregates just lift and dip the basket for about 25 times in 25 seconds. Leave the basket for 24 hours and after that allowed it to drain for few minutes. Aggregates should be taken on dry cloth and exposed them to atmosphere sunlight. After drying, weigh the aggregates W1. Then place the aggregate in oven at 100-110^oc for 24 hrs. After oven drying again weight the aggregate W2.

Water absorption of aggregates = {(W1-W2)/W2} x 100%