Pavement material testing comprises all the methods used to test the pavement materials for their specification. It consists of two parts: test on aggregates and bitumen. In this blog, we will see in brief the methods used for testing the aggregates.
Test on Aggregates
Aggregate is used in layers of the flexible pavement. Therefore, testing of aggregates becomes important. Properties like strength, toughness, etc. are all checked as explained below. As such aggregate testing could be classified into two broad categories as mentioned below.
Test for Strength of Aggregate
Test for Shape of Aggregate
Test for Strength of Aggregate
Crushing Test of Aggregates
This test is done to find the strength of aggregates. The test procedure involves the following steps.
The aggregate specimen is weighed (W1).
The specimen is filled in a cylindrical mold of diameter 15.2cm in three layers with each layer being subjected to 25 blows.
A plunger of diameter 15cm is placed on the top of the mold.
The entire setup is placed in a compression testing machine and is subjected to a load rating of 4 tons/min until it reaches a load of 40ton.
After attaining the max load, the load is released and is allowed to settle down.
The specimen is taken and sieved in a 2.36mm sieve. The weight of material retained in the sieve is found (W2).
Crushing value = ((W1 - W2 ) / W1) *100
For a good quality aggregate, the crushing value should be as low as possible. IRC recommends a crushing strength of 30% for the wearing course and 45% for the base course.
Impact Test of Aggregates
This test is done to find the toughness characteristics of aggregates. The test procedure involves the following steps.
The aggregate specimen is weighed (W1).
The specimen is filled in a cylindrical mold in three layers with each layer being subjected to 25 blows.
A hammer weighing 13.5 to 14kg is dropped on the specimen from a height of 38cm 15 times.
The specimen is taken and sieved in a 2.36mm sieve. The weight of material passing the sieve is found (W2).
Impact value = (W2 / W1) *100
For a good quality aggregate, the impact value should be as low as possible. IRC recommends an impact value of 30% for the wearing course and 45% for the base course.
Abrasion Test of Aggregates
Abrasion test is done to find the wearing and tearing characteristics of the aggregate. Los Angels abrasion test is usually performed. The test procedure involves the following steps.
A specimen of aggregate (5 - 10kgs) is taken and weighed (W1)
Los Angel's apparatus consists of a hollow drum with a diameter of 70cm and length of 50cm. The cylinder is placed in a horizontal position.
The specimen is loaded up in the cylinder and the abrasion charges are placed. (Abrasion charges are spheres made of cast iron having a diameter of 48mm each and a weight of 390-450gm each). The drum is rotated at 30-33 rpm for a total revolution of 500-1000.
The specimen is taken and sieved in a 1.7mm sieve and passing material weight is obtained (W2).
Abrasion Value = (W2/W1) * 100
The abrasion value shall not exceed 30% for the wearing course and 45-50% for the base course.
Soundness Test of Aggregates
This test is done to check the durability characteristics of the aggregate. The test procedure involves the following steps.
A sample of aggregate is taken (W1)
It is immersed in a solution of sodium sulfate or magnesium sulfate for 16 to 18 hours
After immersion, the sample is taken out from the solution and it is dried at a temperature of 105 to 110°C
The dried aggregate is immersed once again in the magnesium sulfate or sodium sulfate solution and the process is repeated 10 times
After 10 cycles the weight of the specimen of aggregate is obtained (W2)
Soundness Value = ((W1-W2) / W1) * 100
The soundness value shall not exceed 12% in the case of sodium sulfate solution and 18% in the case of magnesium sulfate solution.
Test for Shape of Aggregate
Three types of tests are available to determine the shape of aggregates. Below mentioned are the tests available.
Flakiness Test
Elongation Test
Angularity Test
Flakiness Index Test
A flakiness test is done to find the thickness of the aggregate i.e., the least lateral dimension. The thickness of the aggregate or least lateral dimension should not be less than 0.6 times the mean size of the aggregate i.e., 0.6(x+y/2), where, x smaller size of the aggregate in the sample, y is the larger size of the aggregate.
The flakiness index is the ratio of the weight of particles/aggregates smaller than 0.6 times the mean size to the weight of the total sample of aggregate. In other words, % by weight of particles smaller than 0.6 times the mean size.
Flakiness index = (W2/W1) * 100
where,
W2 - the weight of aggregate passing through a slot having size 0.6 times the mean size of the aggregate
W1 - Total weight of the aggregate specimen
The flakiness index shall not exceed 15%
Elongation Index Test
Elongation represents the greatest dimension of aggregate which shall not exceed 1.8 times the mean size of the aggregate.
The elongation index is defined as the % of the weight of aggregate having size more than 1.8 times the mean size.
ELongation index = (W2/W1) * 100
where,
W2 - the weight of aggregate retained on slot having size 1.8 times the mean size of aggregate
W1 - Total weight of the aggregate sample
The elongation index shall not exceed 15%. The combined value of flakiness and elongation index shall not exceed 30%
Angularity Number Test
Angularity represents the opposite of roundness. If the aggregate is angular beyond a certain limit more voids will be present which is not preferable. According to Indian Standard, the maximum percentage of solids in aggregate is 67%. This means the standard voids in the aggregate is 33%. If the void percentage is greater than 33% then it means that the aggregate is angular.
The angularity number is defined as the percentage of void in excess of 33% or the percentage of solid lower than 67%.
Angularity Number = 67% - ((Vs/V)*100)
where,
Vs - the volume of solids
V - total volume of aggregates
Angularity Number = 67% - ((Ws/Gs*V)*100)
where,
Ws - the weight of solids
Gs - specific gravity of solids
The angularity number shall be in a range of 0 to 11%.
Practice Problem
With this, we hope we had covered all the important topics related to the test on pavement materials. The test on bitumen will be covered in our next blog in this series. Or do you want us to cover a topic of your interest in a simple manner? Let us know in the comments below.