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Why should you Test Aggregate?
Aggregate is the most essential construction material and plays an important role in construction. Aggregate is an inert material which contains 70 to 80 % volume of the concrete. Hence, if you want to know about the concrete, it’s essential to know about aggregate which constitutes a major volume of it. Also, the aggregate is the prime material of the pavement construction because the major portion of the pavement also contains aggregate.
Aggregates influence, to a great extent, the load transfer capability of pavements. Therefore, it is necessary that they should be completely tested before using for construction. Not only the aggregates should be strong and durable but should also have proper shape and size to make the element slab/pavement act monolithically. Hence, it’s important to perform the test on aggregate to ensure the quality of aggregates and check the properties of aggregate.
The aggregates are tested for strength, toughness, hardness, shape, and water absorption. Different types of test on aggregate like impact test, abrasion test, crushing value test, soundness test, etc. are carried out to decide the suitability of aggregate.
Aggregate Testing Apparatus
The Important Properties of Aggregates
- Strength: The coarse aggregates to be used in construction should be sufficiently strong to withstand the stresses. because it’s prime construction material in the building as well as pavement construction.
- Hardness: When the aggregates used in the surface course, its subjected to constant rubbing or abrasion due to moving traffic on the pavement. So, they should be hard enough to resist the wear and tear due to the abrasive action of traffic load. Not much but some amount of abrasion may be induced due to the friction of particle of aggregate in concrete or pavements.
- Toughness: Aggregates used in the construction of buildings and pavements are also subjected to impact due to sudden load in buildings and moving wheel loads on pavements. Hence the resistance to impact is the desirable property of aggregates.
- Durability: The aggregate used in the construction of buildings and pavements should be durable i.e. it should have long life and should resist disintegration due to the weather action. The property of aggregate to withstand the adverse action of weather may be called soundness. Hence aggregate should be sound enough to withstand the weathering action.
- Shape of Aggregates: Shape of aggregate is also the essential property of aggregate because it also plays a significant role in the strength, grading of aggregate and interlocking with each other, which is ultimately responsible for strength.
- Adhesion with Bitumen: The aggregates used in bituminous pavements should have less affection with water as compared with bituminous material; otherwise the bituminous coating of the aggregates will be stripped off in presence of water.
- Alkali-Aggregate Reaction: Generally, aggregates are the chemically inert material but if the active silica is present in the aggregate, it will be reacting with the alkalis present in the cement, causing expansion and cracking over a period of many years.
- Specific Gravity: Specific gravity is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. It’s a quality measure and determine the strength.
Different Types of Test on Aggregate
In order to decide the suitability of the aggregate for use in concrete/pavement construction, following tests are carried out:
01. Crushing Test on Aggregate
The crushing test on aggregate gives a relative measure of the resistance of an aggregate crushing under gradually applied compressive load. If the aggregate crushing value is less than 10, it signifies an exceptionally strong aggregate. While crushing value above 35 would normally be considered as weak aggregates.
- Use of Crushing Test:
The aggregate crushing value test indicates the strength of aggregate which is the most essential property of coarse aggregate. Lower crushing value means higher strength, it’s recommended for roads and pavements as it indicates a lower crushed fraction under load and would give a longer service life and a more economical performance.
- Recommended Result of Crushing Test:
After performing the test, crushed material is separated and expressed as percentage of the original weight taken in the mould. This percentage is known as a crushing value of aggregate. It should not be more than 30% of the weight of aggregate taken for the test. The aggregate crushing value test result should not be more than 30 % for the concrete used for roads and pavement sand 45 % may be permitted for other structures
- Standard Guideline of Crushing Test:
Followings are the various guidelines for the crushing test on aggregate:
IS 2386(Part 4):1963, BS 812–110:1990.
Crushing Mould for Aggregates Test
Apparatus of Crushing Test:
- Compressive Testing Machine
- Crushing Mould – A steel cylinder 15 cm diameter with plunger and base plate.
- Weighting machine,
- Sieve – Size of 12.5 mm, 10 mm and 2.36 mm
02. Abrasion Test on Aggregate
To determine the abrasion value of the coarse aggregate, this test is carried out. The principle of Los Angeles abrasion test is to find the percentage wear due to relative rubbing action between the aggregate and steel balls used as an abrasive charge.
- Use of Abrasion Test:
Abrasion test on aggregate indicates the hardness of aggregate. It shows the ability of aggregate to resist the wear and tear.
- Recommended Result of Abrasion Test:
Abrasion value of aggregate should not be more than 30 per cent for aggregate used for surface wearing course and should not be more than 45 per cent for surface other than wearing course.
- Standard Guideline of Abrasion Test:
Followings are the various guidelines for the abrasion test on aggregate:
IS 2386(Part 4):1963, BS 812-113:1990
Apparatus of Abrasion Test:
- Los Angles Abrasion Machine
- Steel balls-11no.
- Weighing balance
- Sieves: 20, 12.5, 10, 1.7mm.
Also Read: Los Angeles Abrasion Test on Aggregate
03. Impact Test on Aggregate
Aggregate may be subjected to the sudden shock or impact during and after the construction. These loads are different from the gradually applied compressive load. Because due to such kind of load, aggregate breaks down into smaller pieces. Aggregate impact value gives the idea about how much impact load can be resisted by aggregate. Thus, aggregate should be sufficiently tough to resist their disintegration due to impact. This distinctive property is measured by impact value test.
- Use of Impact Test:
Impact test on aggregate indicates the toughness of aggregate. It shows the ability of aggregate to resist the sudden shock or impact.
- Recommended Result of Impact Test:
Aggregate impact value is the same as the abrasion test result. Impact value of aggregate should not be more than 30 percent for aggregate used for surface wearing course and should not be more than 45 per cent for surface other than wearing course.
- Standard Guideline of Impact Test:
Followings are the various guidelines for the impact test on aggregate:
IS 2386(Part 4):1963
Impact Test Apparatus
Apparatus of Impact Test:
- Aggregate impact apparatus,
- IS sieves (12.5 mm, 10.0 mm and 2.36 mm),
- Cylindrical measure and cylindrical cup,
- Weighing balance,
- Tamping rod.
Also Read: Impact Test on Aggregates
04. Soundness Test on Aggregate
Aggregates are subjected to the physical and chemical action of rain-water, groundwater and the other impurities of the atmosphere. Due to which aggregate changes its volume which is not good for the prospect of the durability of construction. The property of aggregates to withstand the adverse action of weather is known as soundness of aggregate.
- Use of Soundness Test:
Soundness test on aggregate indicates the durability of aggregate. It shows the ability of aggregate to resist the adverse effect of weather.The soundness test is carried out to learn the resistance of aggregates to weathering actions like thawing, freezing, alternate wetting and drying in normal condition and in salt water, variation in temperature. When aggregate does not resist such things, then it said to be unsound.
- Recommended Result of Soundness Test:
Recommended result of soundness test should not more than 12 % and 18 % after 10 cycles when tested with sodium sulphate and magnesium sulphate respectively.
- Standard Guideline of Soundness Test:
Followings are the various guidelines for the soundness test on aggregate:
IS 2386(Part 5):1963, BS 812-121:1989,
Apparatus of Soundness Test:
- Sodium Sulphate or Magnesium Sulphate Solution,
- Weighing Balance,
- IS Sieves,
- Weight Balance.
Also Read: Soundness Test on Aggregate
05. Shape Test on Aggregate
Aggregates are available in various size and shape, i.e. rounded, cubical, angular flaky or elongated. It is proved that the flaky and elongated particles have less strength, less bond, as less interlocking and durability compared with cubical, angular or rounded particles of the same size aggregate. You can say that aggregates are flaky when aggregates are too thin compared to their average size (mean dimension). And elongated aggregate means aggregates are too long compared to their average value (mean dimension). Flakiness index test and elongation index test are performed to find flaky and elongated aggregate.
- Use of Shape Test:
Flakiness index test and elongation index test are used to determine the shape of the aggregate. It is defined in percentage by weight of aggregate particles to the mean dimension of thickness and length of it.
- Recommended Result of Shape Test:
Recommended result of shape test on aggregate should range in 15% to 30 % according to the area of use.
- Standard Guideline of Shape Test:
Followings are the various guidelines for the shape test on aggregate:
IS 2386(Part 1):1963, BS 812-105.1:1989,
Shape/Flakiness Index Apparatus for Aggregates Test
Apparatus of Shape Test:
- Thickness gauge,
- Length gauge,
- Weighing balance,
- IS Sieves
Also Read: Flakiness Index Test on Aggregates for Shape
06. Specific Gravity and Water Absorption Test on Aggregate
Specific Gravity of aggregate is defined as the ratio of the weight of aggregate to the Weight of equal volume of water. It shows the strength of aggregate. Aggregates having low specific gravity are generally weaker than those with having high specific gravity.
Water absorption of aggregate gives an idea about the internal structure of aggregate. If aggregates have more absorption, they are called porous in nature and generally not preferred and unsuitable for the construction.
- Use of Test:
The specific gravity and water absorption test on aggregate measures a strength or quality of the aggregate. This property helps in a general identification of aggregates.
- Recommended Result of Test:
The specific gravity of aggregates generally used in construction ranges from about 2.5 to 3.0 and water absorption ranges from about 0.1 % to 2.0 %
- Standard Guideline of Test:
Followings are the various guidelines for the specific gravity and water absorption test on aggregate:
IS 2386(Part 3):1963, BS 812-2:1995
Apparatus of Test:
- Density basket,
- Weighing balance,
- Water tank, Tray,
- IS sieves- 10mm and 20mm.
07. Bitumen Adhesion Test on Aggregate
Bitumen gets well attached to all normal types of road aggregates provided they are dry and free from dust. In the absence of water, there is no adhesion problem of bituminous construction. Adhesion problem may occur when the aggregate is wet and cold. This problem can be dealt with by removing moisture from the aggregate by drying and increasing the mixing temperature.
For finding bitumen adhesion, the stripping value test is conducted. Stripping value of aggregates is expressed as a percentage and determined as the ratio of the uncovered area observed visually to the total area of aggregates.
- Use of Bitumen Adhesion Test:
Bitumen adhesion test on aggregate determines the adhesion of bituminous binder to aggregate in presence of water.
- Recommended Result of Bitumen Adhesion Test:
Recommended result (stripping value) of bitumen adhesion test should not more than 5 percent for aggregates to be used in bituminous construction.
- Standard Guideline of Bitumen Adhesion Test:
Followings are the various guidelines for the bitumen adhesion test on aggregate:
Apparatus of Bitumen Adhesion Test:
- Water Bath,
- Mixer to mix aggregate and bitumen.
Why Tests on Aggregate (all tests mentioned above) Fails? & What if Test Fails?
If aggregate tests result is not in range of recommendations, there can be many reasons like weak aggregate, error in testing procedure, error in test appliance etc. If any of tests on aggregate fail, first of all, confirmed that, there is not any error in test appliance or procedure, if it is the reason then retest or if the all is good and tests result is failed, do not use those aggregate in structural as well as non-structural purpose.