The significance of shape tests for coarse aggregates, such as the flakiness index and elongation index, in concrete construction is a crucial topic of discussion. These tests aid in evaluating the shape and size of the coarse aggregates, which can affect the properties of concrete. Determining the flakiness index helps identify the amount of flat, elongated particles in the aggregates, which can impact the workability and strength of the concrete mix. Meanwhile, the elongation index determines the amount of elongated particles in the coarse aggregates and can affect the mix’s stability and durability.
Various methods are available for determining the flakiness and elongation indexes. The most common methods for determining the flakiness index include measuring the dimensions of the individual particles, sieving them through different gauges, and calculating the ratio of the thickness and the length. Meanwhile, the elongation index is determined by calculating the ratio of the length and width of each particle and classifying the particles based on this ratio. These shape tests for coarse aggregates play an essential role in ensuring the quality and durability of concrete structures.
Shape Tests on Coarse Aggregates:
Coarse aggregates undergo shape tests to determine their elongation and flakiness index. These tests are crucial as they assess the angularity, texture, and shape of the aggregates, which are important factors in determining their suitability for construction purposes.
The elongation index test involves measuring the length of the aggregate particles along their longest axis and comparing it with their average width. This helps to determine the extent to which the aggregates are elongated and helps to identify particles that are elongated beyond the specified limit.
Similarly, the flakiness index test involves measuring the thickness of the aggregate particles along their thinnest dimension and comparing it with their average width. This test helps to identify particles that are flat and thin beyond the specified limit.
To conduct these tests, the apparatus includes a standard thickness gauge and a standard length gauge, along with IS sieves of various sizes ranging from 6.3mm to 63mm. The apparatus also includes a balance with a capacity of 5kg, which is readable and accurate up to 1 gm. The accuracy of the apparatus is essential to ensure precise measurements and accurate results.
Theory of Flakiness Index and Elongation Index Tests
Aggregate particle shape is an important factor to consider in the construction of bituminous and cement concrete types, as well as for base courses. The presence of flaky and elongated particles in aggregates is considered undesirable, as they can cause inherent weakness that may result in breakdown under heavy loads. Therefore, it is necessary to evaluate the shape of particles, specifically with reference to flakiness and elongation.
The Flakiness index is a measure of the percentage by weight of particles in an aggregate whose least dimension (thickness) is less than three-fifths (0.6 times) of their mean dimension. It should be noted that this test cannot be applied to sizes smaller than 6.3mm.
On the other hand, the Elongation index is a measure of the percentage by weight of particles in an aggregate whose greatest dimension (length) is greater than nine-fifths (1.8 times) their mean dimension. It is important to note that this test cannot be applied to sizes smaller than 6.3mm.
By conducting these tests, the particle shape of aggregates can be evaluated and assessed for suitability in construction projects. This is crucial to ensure that the aggregates used do not have flaky or elongated particles that could compromise the strength and durability of the concrete or base course under heavy loads.
Procedure of Shape Tests on Coarse Aggregates
The task at hand involves sieving a sample through IS sieves as specified in the table. It is required to take a minimum of 200 pieces of each fraction to be tested and measure their weight. To gauge flaky materials, the thickness of each fraction needs to be measured using a thickness gauge. The slot width used in the measurement should be in accordance with the dimensions mentioned in column (4) of the table for the appropriate size of the material.
The weight of flaky material that passes through the gauge needs to be measured with an accuracy of at least 0.1% of the test sample. Additionally, elongated materials must also be separated by gauging each fraction for length using a length gauge. The width of the slot used for the measurement should be in accordance with the dimensions specified in column (6) of the table for the appropriate size of the material.
Finally, the weight of elongated material retained on the gauge must be measured with an accuracy of at least 0.1% of the test sample. Overall, the process involves careful measurement of different properties of the sample to obtain accurate and reliable data.
| Size of aggregates | Weight of fraction consisting of at least 200 pieces,g | Thickness gauge size, mm | Weight of aggregates in each fraction passing thickness gauge,mm | Length gauge size, mm | Weight of aggregates in each fraction retained on length gauge,mm | |
| Passing through IS Sieve, mm | Retained on IS Sieve, mm | |||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 63 | 50 | W1 | 23.90 | X1 | – | – |
| 50 | 40 | W2 | 27.00 | X2 | 81.00 | Y1 |
| 40 | 31.5 | W3 | 19.50 | X3 | 58.00 | Y2 |
| 31.5 | 25 | W4 | 16.95 | X4 | – | – |
| 25 | 20 | W5 | 13.50 | X5 | 40.5 | Y3 |
| 20 | 16 | W6 | 10.80 | X6 | 32.4 | Y4 |
| 16 | 12.5 | W7 | 8.55 | X7 | 25.5 | Y5 |
| 12.5 | 10 | W8 | 6.75 | X8 | 20.2 | Y6 |
| 10 | 6.3 | W9 | 4.89 | X9 | 14.7 | Y7 |
| Total | W = | X = | Y = | |||
Record of Shape Test
The given context includes two equations that calculate different indices based on certain variables. The Flakiness Index is calculated by adding up all the values of X (X1, X2, etc.) and dividing it by the sum of all the values of W (W1, W2, etc.), and then multiplying the result by 100. The Elongation Index is calculated by adding up all the values of Y (Y1, Y2, etc.) and dividing it by the sum of all the values of W (W1, W2, etc.), and then multiplying the result by 100.
Both of these indices are useful in different contexts. The Flakiness Index is often used in the construction industry to measure the ratio of flat or elongated particles in aggregate materials, such as gravel or sand. A high Flakiness Index indicates a larger proportion of flat or elongated particles, which can have negative effects on the strength and durability of the resulting concrete or asphalt. Therefore, a lower Flakiness Index is generally considered more desirable.
The Elongation Index, on the other hand, is often used in the textile industry to measure the flexibility and stretchiness of fibers or fabrics. A high Elongation Index indicates that the material is more stretchy and can be pulled further before breaking. This can be desirable in certain contexts, such as athletic wear or medical textiles. However, a very high Elongation Index may also indicate that the material is weaker overall, so it is important to consider other factors such as tensile strength and durability as well.
Results:
- Flakiness Index =
- Elongation Index =
Recommended Values of Flakiness Index and Elongation Index
Aggregates play a crucial role in pavement construction, and it is essential to ensure their quality before using them. To determine the relative shapes of aggregates, shape tests are conducted. However, it is important to note that the shape tests provide only a rough idea of the relative shapes of aggregates. In particular, flaky and elongated particles should be avoided in surface courses, as they can adversely affect the strength of the pavement layer by breaking under loads.
The presence of flaky and elongated particles in appreciable proportions can be detrimental to the strength of the pavement layer. Therefore, it is recommended to adhere to the maximum limits of flakiness index as specified by the IRC. These limits have been put in place to ensure that the strength of the pavement layer is not compromised due to the presence of such particles.
It is also worth noting that the use of flaky and elongated particles can reduce the workability of cement concrete. Therefore, it is crucial to ensure that the aggregates used in pavement construction are of good quality and do not contain a significant proportion of flaky and elongated particles. By adhering to these guidelines, the quality and strength of the pavement layer can be maintained, and the risk of damage due to breaking under loads can be minimized.
| Sl No: | Type of pavement | Maximum limits of flakiness index, % |
| 1 | Bituminous carpet | 30 |
| 2 (i) | Bituminous / Asphaltic concrete | 25 |
| (ii) | Bituminous Penetration macadam | |
| (iii) | Bituminous surface dressing (single coat, two coats & precoated) | |
| (iv) | Built up spray grout | |
| 3 (i) | Bituminous macadam | 15 |
| (ii) | WBM base course and surface course |