Tests on Aggregates

What are Tests on Aggregates?

Aggregates are crucial in the construction industry and can be utilized for a variety of projects. Aggregates are a key component in the production of concrete and provide various advantages. Their principal application is to reinforce concrete, hence strengthening its structure and reducing cracking.

As a result, before using aggregates for construction, they must be examined and evaluated for quality. Many types of aggregate studies show that aggregate quality has a significant impact on the quality of concrete and, eventually, the building.

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Types of Tests on Aggregates 

In the case of aggregates, 9 key tests have been identified that define aggregate quality. These tests on aggregates are listed below –

• Aggregate Crushing Test
• Aggregate Impact Test
• Aggregate Abrasion Test
• Flakiness Index Test
• Elongation Index Test
• Angularity Test
• Soundness Test
• Specific Gravity and Water Absorption Test
• Stripping Value Test 

Aggregate Crushing Test

The strength of aggregate is defined as the resistance of the aggregate against gradual loading. The strength of aggregate is determined by the Crushing Value Test on aggregates. The aggregates passing through a 12.5 mm IS sieve and retained on a 10 mm IS sieve are taken. These aggregates are subjected to gradual loading of 40 tonnes with the help of a plunger.

The crushed aggregates are then passed through a 2.36 mm sieve. The weight of the aggregates passing through the 2.36 mm sieve, expressed as the percentage of the total weight of aggregates, is referred to as Aggregate Crushing Value (ACV). Lesser is the ACV, more will be the strength of aggregate. ACV less than 10 indicates exceptionally strong aggregate whereas, ACV greater than 35 indicates weak aggregate.

Aggregate Crushing Value (ACV) =weight of material passing through 2.36 mm sieve/ weight of total aggregate

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Aggregate Impact Test

The toughness of the aggregate is defined as the ability to resist impact loading. The toughness of the aggregate is determined by Impact Value Test on aggregates. The aggregates passing through a 12.5 mm IS sieve and retained on a 10 mm IS sieve are taken. This sample of aggregate is subjected to 15 blows with the help of a metallic hammer having a mass of 13.5-14 kg, free-falling from a height of 38 cm.

The aggregates after impact are passed through the sieve of size 2.36 mm. The weight of aggregates passing through the 2.36 mm sieve, expressed as the percentage of the total weight of aggregates is referred to as Aggregate Impact Value (AIV). Lesser is the AIV, more will be the toughness of the aggregate. The AIV of aggregate should not exceed 30% for wearing course, 35% for bituminous macadam and 40% for water-bound macadam.

Aggregate Impact Value (AIV)=weight of material passing through 2.36 mm sieve/weight of total aggregate

Aggregate Abrasion Test

Hardness is the property of aggregate that allows it to withstand wear and tear (abrasion). The hardness of the aggregates can be determined by Deval Abrasion Test, Dorry’s Abrasion Test and Los Angeles Abrasion Test. The Los Angeles Abrasion Test on aggregates is the standardized method for determining the hardness of aggregates in India. In this test, aggregates passing through a 12.5 mm sieve and retained on a 10 mm sieve are placed in a cylinder having steel balls in it. The sample is subjected to abrasion by rotating the cylinder 500 times at the speed of 30 to 33 rpm.

The aggregates after the test are passed through a 1.7 mm sieve and the weight of the aggregates passing through the sieve is noted, which when expressed in terms of percentage of the total weight of aggregates is referred to as Aggregate Abrasion Value. Lesser abrasion value means more hardness of aggregate. Abrasion value should not exceed 35% for bituminous macadam and 40% for WBM base course.

Los Angeles Abrasion Value =weight of material passing through 1.7 mm sieve/weight of total aggregate

Flakiness Index Test

The flakiness index of the aggregates is defined as the percentage by weight of the aggregates in the sample whose least dimension is less than 0.6 times the mean dimension. The flakiness index test on aggregates is performed using Thickness Gauge.

This test does not apply to aggregates with a size of less than 6.3 mm. An aggregate sample having a minimum of 200 pieces is considered and each aggregate is passed through respective gauges of the thickness gauge. The weight of the aggregates passing through various gauges is noted which when expressed as a percentage of the total weight of aggregate sample is referred to as Flakiness Index.

Flakiness Index=Weight of material passing the various gauges/Total weight of the sample

Elongation Index Test

The elongation index of the aggregates is defined as the percentage by weight of the aggregates present in the sample having their greatest size greater than 1.8 times of their mean size. Elongation index test on aggregates is performed using Length Gauge.

This test does not apply to aggregates with a size of less than 6.3 mm. An aggregate sample having a minimum of 200 pieces is considered and each aggregate is passed through the respective gauges of the length gauge. The weight of aggregates retained over various gauges is noted which when expressed in terms of the total weight of the aggregates is referred to as Elongation Index.

Elongation Index=Weight of material retained on various gauges/Total weight of the sample

Angularity Test

The angularity test on aggregates determines the angularity of aggregates in a sample. The angularity of the aggregates is measured in terms of Angularity Number which is defined as the amount by which the percentage of voids in it, exceeds 33. For this test, a quantity of single-sized aggregate is filled into a metal cylinder of known capacity and compacted. The percentage of void in the aggregate sample is determined. Angularity number is calculated using the following equation –

Angularity Number=67-100W/C×G

Where,

• W = weight of the aggregate required to fill the cylinder
• C = weight of water needed to fill the cylinder
• G = specific gravity of aggregate

For well compacted, single-sized rounded aggregate percentage air void is 33% i.e., angularity number is zero. Angularity number will be expressed as the nearest whole number. The angularity number value varies in the range of 0 to 11. A higher value of angularity number indicates that aggregates are more angular.

Soundness Test

Soundness of the aggregate can be defined as its resistance against weathering action. Repeated cycles of freezing and thawing can cause the aggregates to disintegrate. The soundness test on aggregates is meant to investigate the resistance of aggregates against weathering action by subjecting them to accelerated weathering test cycles.

Aggregates of a particular size are wetted in a saturated solution of sodium sulphate or magnesium sulphate for 16 – 18 hours, then dried to a constant weight in an oven at 105 – 110°C. This cycle is repeated 5 times and the weight loss of aggregates is determined after removing all undersized particles. Loss in weight should not exceed 12% in test with Sodium sulphate and 18% in test with Magnesium sulphate.

Specific Gravity and Water Absorption Test

Specific Gravity and Water Absorption Test are two important tests on aggregate as these parameters are important in the design of concrete and bituminous mixes. The ratio of an aggregate’s mass to that of an equal volume of distilled water at a specified temperature is known as specific gravity. There are two types of specific gravity. Bulk specific gravity in which total volume of aggregate along with volume occupied by voids are considered and Apparent specific gravity in which only volume of aggregate is considered without accounting for the volume of voids.

Bulk specific gravity G_bulk=dry mass of aggregate or volume of aggregate/Density of water

Apparent specific gravity G_app=dry mass of aggregate or volume of aggregate without voids/Density of water

Water absorption of aggregate can be determined by taking the weight of dry aggregates (W₁) and the weight of aggregate in saturated-surface dry conditions (W₂). Water absorption can then be calculated by –

Water absorption of aggregates=[(W2-W1)/W1]×100

The specific gravity of aggregates normally used in pavement construction ranges from 2.6 to 2.9. Water absorption value greater than 0.6% is unsatisfactory. Water absorption from 0.1% to about 2% is normally used in road surfacing.

Stripping Value Test

Stripping Value Test on aggregates is performed for pavement construction. The adhesion of aggregates to bitumen is an important factor in determining the quality of bituminous mix and the performance of the pavement. The presence of water can cause the bituminous binder to strip off from the coated aggregates. A static immersion test is recommended by IRC to measure the Stripping Value.

In this test, aggregates fully coated with the bituminous binder are kept in water maintained at 40°C temperature for 24 hours. The level of stripping is visually measured after 24 hours. The upper limit for the stripping value of aggregates is 5%.

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