Elongation Index is defined as the percentage of the weight of retained aggregate on different sets (sets which represent 7 gaps of different sizes) on elongation or length gauge to the total weight of aggregate sample for test. Elongation index is generally considered for the shape and size of aggregate. In Elongation index, we generally check for the length of aggregate which is suitable for any construction giving proper workability, durability, and desirable strength.
Elongation index or Elongated aggregate is the particle whose largest dimension is more than 1.8 or 9/5 times of their mean dimension. This test is not applicable for the aggregate size of less than 6.3 mm. Elongation index is more than 15% then the aggregate is undesirable for any construction work.
There are some apparatus which is used for Elongation Index Test
The apparatus shall be consisting of following:-
- Weight Balance (Digital balance):- Used for weighing the aggregate sample and It shall have an accuracy of 0.1 percent of the weight of the test sample.
- Elongation or Length gauge:- Used for passing different sizes of aggregate between different size ranges gaps on the gauge.
- Sieves sizes:- 6.3mm,10mm,12.5mm,16mm,20mm,25mm,40mm,50mm,63mm.
- Aggregate Sample.
Procedure of performing Elongation Index Test
- Firstly, take 1000 grams of aggregate sample and weigh it w1.
- Arrange the sieves in a systematic manner (in order to smaller size opening sieve is on the bottom side and bigger size opening sieve is on the top side). Example:- (6.3mm size sieve in bottom and 63mm size sieve in top)
- After that, just we need to perform a sieve analysis test and shake all arranged sieves with help of a vibrating shaker and manually.
- After shaking, aggregate should be distributed on different sizes of sieves (in systematic arranges order) and weigh all the sieves one by one on which aggregate is retained.
- Now, the aggregate is selected on each size of sieves in a specific number and passed the aggregate (lengthwise) through the different sizes sets of a gap on elongation or length gauge. ( In such a manner that the selected aggregate which is collected from different sets or sizes of sieve should be passed through the same size of a gap on the elongation gauge. (Gap size is mentioned on elongation index apparatus).
- The aggregate which is retained on the elongation gauge or not passed through the elongation gauge should be taken for weight w2.
- Then, we put the weight values w1 and w2 in the formula given below, to get the elongation index percentage.
In other words, the Total weight of aggregate retained on the elongation gauge is divided by the total weight of the aggregate sample and then multiplied by 100.
Purpose of Elongation Index
Purpose of the elongation index is to check the shape and size of the aggregate and whether it is suitable for work or not. It tells me about the aggregate whose larger dimension is more than 1.8 times of their mean dimension.
Factors Affecting the Elongation Index of an Aggregate
There is some factor that affects the elongation index of aggregate
Affect on workability when used for work or construction:-
- If the shape of the aggregate is round, in that case, the surface area of the aggregate is less, and the workability of that aggregate is more.
- If the shape of the aggregate is flaky and elongated, then the surface area of that aggregate is increased due to one dimension being more than the other two-dimension, so the result of that type of aggregate has less workability. (this concept is applicable when the elongation index is more than 15%)
- If the size of the aggregate is smaller then the surface area of aggregates is increased due to more number of the total number of voids volume, because of this the workability of that type of aggregate is less.
- If the size of the aggregate is bigger, in that case, the total number of voids volume is less. then the type of aggregate results reduction in surface area of aggregate and an increment in workability.
- If the texture of the aggregate is rough, then the surface area of that aggregate is more. Hence workability will be more.
- If the texture of the aggregate is smooth then the surface area of that aggregate is decreased hence workability will be reduced.
- If the aggregate is well graded, then the voids are minimum. Hence the workability will be more.
- If the aggregate is poorly graded or uniformly graded, then the voids are maximum in that type of aggregate workability will be less.
Affect on strength and durability when used for construction work:
- For cement concrete structure, if used aggregate elongation index percent is more than its permissible limit then it is undesirable for any type of construction work. It has a possibility of breaking down the aggregate under heavy load. Hence, the strength and durability of aggregate are affected.
- If the aggregate is using having more water absorption capacity than the 5-10% then it will reduce the strength and durability of any structure.
- The maximum permissible limit of the elongation index is not more than 15%.
- And for mixed aggregate(having flaking and elongated aggregate) then its permissible limit is 30%.
Frequently Asked Questions
IS code for flakiness and elongation index
IS 2386-1963 part 1 are used for perform and having maximum limits of flakiness and elongation index of aggregate.
What do you infer from elongation index of an aggregate
The elongation index just tells us if the aggregate having a larger dimension is more than 1.8 times. its mean dimension then such type of aggregate known as elongated aggregate and elongated index is calculated by the percentage of the total weight which is retained on elongated or length gauge to the total weight of aggregate used for the sample.