The elongation index is a metric used to evaluate the size and shape of aggregate particles. It is defined as the weight percentage of particles whose greatest dimension exceeds 1.8 times their mean dimension. To determine the elongation index, particles are first passed through a mesh screen with a size of 63mm, and then retained on a mesh screen with a size of 6.3mm.
In other words, the elongation index measures the amount of elongated or flattened particles in an aggregate sample. These types of particles can affect the workability and strength of concrete mixes. By knowing the elongation index, engineers and construction professionals can select aggregates with the desired particle shape and size distribution to achieve optimal concrete performance.
Necessity of Elongation Index Test
The presence of elongated aggregates in a mixture can be problematic as it can interfere with the proper packing of particles, leading to the creation of more empty spaces. Additionally, elongated particles have a high surface area to volume ratio, which can reduce the workability of concrete. This reduced workability can make it challenging to create a concrete mixture that is easy to manipulate and shape.
When it comes to constructing a pavement base course, using elongated particles can lead to significant problems. Under heavy loads, these particles may break down, causing damage to the pavement. As such, it is essential to understand the elongation index of a given aggregate mix. This information can help engineers and construction professionals make informed decisions about which materials to use in their pavement base course construction projects. By avoiding elongated particles, they can create a more durable and long-lasting pavement that can withstand the heavy loads that it will be subjected to over time.
Fig 1 : Shape of Elongated Aggregate Particles
Apparatus required
To determine the Elongation index of coarse aggregates, certain equipment is necessary. This includes an aggregate sample, a digital balance, and an Elongation gauge or length gauge. Additionally, test sieves of varying sizes are required, which include sizes of 63mm, 50mm, 40mm, 31.5mm, 25mm, 20mm, 16mm, 10mm, and 6.3mm. These tools are essential for accurately determining the Elongation index of the coarse aggregates, which is an important measurement in the field of construction and civil engineering. By using this apparatus, engineers and researchers can gather valuable information about the properties of coarse aggregates, which can ultimately inform decisions related to material selection and construction practices.
Fig 2 : Elongation Gauge or Length Gauge
Test Procedure
The elongation index test involves several steps. Firstly, an aggregate sample with a minimum of 200 pieces of any fraction is taken. This sample is then sieved through a range of sieves, from 63mm to 6.3mm in size. It is important to note that the aggregate that is retained on the 63mm sieve and passed through the 6.3mm sieve should not be included in the test.
The aggregate that is passed through the 63mm sieve and retained on the 6.3mm sieve, along with any sieves above it, is taken and weighed. This weight is noted down as “W1”. Each particle from this aggregate sample, in each size range, is then taken and passed through the corresponding gauge of a length gauge or elongation gauge, with its longest side facing the gauge.
The weight of the aggregate retained on each sieve is recorded, and the total weight is noted down as “W1”. The elongation gauge is used to measure the particle’s elongation index, which is the ratio of the length of the particle to its maximum thickness. This test is essential in determining the aggregate’s ability to resist deformation when subjected to traffic loads, as well as its suitability for use in road construction.
Fig 3 : Conducting Elongation Index Test
Calculations:
I understand that you would like me to rewrite a given context in paragraphs related to the calculation of the elongation index of aggregate in percent. Here is my attempt:
The elongation index of aggregate is a measure of the amount of particle elongation in a sample of aggregate. It is expressed as a percentage and is calculated by measuring the length of particles along their longest axis and comparing it to the length of the same particles along their shortest axis. The elongation index is an important parameter used in determining the quality of aggregates used in construction materials, such as concrete and asphalt.
To calculate the elongation index, a representative sample of aggregate is obtained and the individual particles are measured along their longest and shortest axes using a caliper or other measuring device. The length measurements are then used to calculate the elongation index as a percentage by dividing the difference between the longest and shortest axis lengths by the shortest axis length and multiplying by 100.
The elongation index is used to evaluate the angularity and surface texture of the particles in the aggregate sample, which are important factors in determining the performance of the aggregate in construction materials. Higher elongation indices indicate a greater degree of particle elongation and angularity, which can lead to increased friction between particles and improved stability in the final material. Conversely, lower elongation indices indicate more rounded and smoother particles, which may not provide the necessary friction and stability in construction materials.
The given context provides a formula for calculating the weight of particles retained in a length gauge during a test. The formula involves two variables: W1 and W2.
W1 represents the weight of particles that are retained in the length gauge, measured in grams. This weight is determined as part of the test process and is used in the calculation of the test results.
W2 represents the weight of the total sample taken for the test, also measured in grams. This weight includes all of the particles that are present in the sample, whether or not they are retained in the length gauge.
To use this formula, both W1 and W2 must be measured and recorded. Once these values are obtained, the formula can be applied to calculate the weight of particles retained in the length gauge. This information can be used to help evaluate the properties and characteristics of the sample being tested.
Elongation Index Limits
The requirement for pavements, whether they are made of bituminous or non-bituminous materials, is that the elongation index of the coarse aggregate should not exceed 15%. The elongation index is a measure of the ability of an aggregate particle to resist breaking under compressive loads, and it is expressed as a percentage of the particle’s original length. In other words, if an aggregate particle has an elongation index of 15%, it means that it can stretch up to 15% of its original length before it breaks.
It is essential to ensure that the elongation index of the coarse aggregate used in pavements is within the acceptable limit to avoid any potential damage or failure of the pavement structure. Therefore, it is necessary to measure the elongation index of the coarse aggregate and report it as a percentage rounded off to the nearest whole number. This will help ensure that the pavement meets the required specifications and performs satisfactorily over its intended service life.