7 Lab Test on Aggregates that we Use in Concrete Construction!
Nowadays, with the rapid urbanization and growth in population, the construction industry is booming, and hence the demand for high-rise buildings has gone up. This development in building construction has led to an increase in demand for better performing building/construction materials such as high strength concrete and its components, i.e. cement, sand and aggregate. For long-lasting performance, each and every building material should be checked for their desired properties for durability of the structure. Here, in this article, we are discussing aggregate – its property and test on aggregates. Aggregate is one of the primary building materials and constitute of concrete. Aggregate material used in construction, includes sand, gravel, crushed stone, slag, recycled concrete etc.
The aggregate has to undergo several tests and has to pass satisfactory results before it is used in concrete. This article gives in detail information regarding seven different tests normally conducted on the aggregates.
According to ‘Prof. Dr. B. Bhattacharjee’ (Professor of Indian Institute of Technology, Delhi), aggregate is the second most important constitute which forms the skeleton for concrete. Aggregates can either be natural or manufactured. Natural aggregates are generally extracted from larger rock formations through an open excavation. Extracted rock later is typically reduced to usable sizes by mechanical crushing. These aggregates are required for manufacturing concrete.
Why is it Necessary to Test Aggregates?
Aggregate is the essential construction material and plays an important role in construction. Aggregate is an inert material which means, that it does not take part in any chemical reaction in the concrete. It contains 70 to 80 % volume of the concrete. Hence, if you want to know about the concrete, it is essential to know about aggregates, which constitutes major volume of the same. Also, the aggregate is the prime material of the pavement construction because the major portion of the pavement is made up of aggregate.
Aggregates influence, to a greater extent, the load transfer capability of pavements and slabs. Therefore, before using for construction, it is necessary that they should be completely tested. Not only the aggregates should be strong and durable, but should also have proper shape and size to make the element slab/pavement act monolithic ally. Hence, it is important to perform the test on aggregates to ensure the quality of aggregates and check the properties of aggregate.
The aggregates are tested to determine their properties like strength, toughness, hardness, shape, and water absorption. Different types of tests i.e. impact test, abrasion test, crushing value test, soundness test, etc. are conducted on it to ascertain the suitability of aggregate.
The Important Properties of Aggregates
01. Strength of Aggregate
The coarse aggregates to be used in construction should be sufficiently strong to withstand the loads because; it is the prime construction material in the building as well as pavement. According to ‘A. M. Neville’ (Honorary member of the American Concrete Institute and author of Properties of Concrete), the strength and elasticity of aggregate depend on its composition, density, texture and structure. For a high-quality pavement, the aggregate should possess high resistance to crushing, and to withstand the stresses due to traffic load.
02. Hardness of Aggregate
When the aggregates are used in the surface course, they are subjected to constant rubbing or abrasion due to the constant movements of traffic on the pavement. Hence, they must be hard enough to resist the wear and tear occurring due to the abrasive action of traffic load. The abrasive action is severe over the aggregate exposed on the top surface. Not much but some amount of abrasion may be induced due to the friction between particles of aggregate in concrete.
03. Toughness of Aggregate
According to ‘M.S. Shetty’ (Author of Concrete Technology Theory and Practice), toughness is the resistance of the aggregates to impact. 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.
04. Durability of Aggregate
The property of aggregate to withstand the adverse action of weather and other chemical reactions can be termed as soundness. 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 aggregates are subjected to various physical and chemical reactions of rain water, soil, water percolated in ground and many other impurities. Hence, aggregate must be sound enough to withstand such reactions as well as the weathering action.
05. Shape of Aggregate
According to ‘Zongjin Li’ (Author of Advanced Concrete Technology), shape of aggregate affects the workability of fresh concrete. Because the differences in the surface area caused by different shapes, will determine cement paste required to coat the aggregate, to provide lubrication and increase work ability. Apart from workability of concrete, shape of aggregate is also an essential property of aggregate because it also plays a significant role in the strengthen, grading and interlocking of aggregate with each other, which is ultimately responsible for toughness and firmness. Aggregates happen to fall in different shapes i.e. round, angular, cubical, flaky or elongated. It is evident that aggregates with round, angular, cubical shapes are more durable and sustainable as compared to flaky and elongated ones.
06. Alkali-Aggregate Reaction
As per the ‘Fournier, B & Berube, M’ (2011) (Published in: Alkali-aggregate reaction in concrete: A review of basic concepts and engineering implications), the alkali-aggregate reaction is the chemical reaction that occurs in the concrete which can produce the premature distress and loss in serviceability thereby, affecting concrete structures. In other words, alkali-aggregate reaction is the expansive reaction that takes place between alkali and elements within an aggregate which can result in pattern cracking, surface pop outs and break off into fragments if it is severe enough. There are two types of alkali- aggregate reaction occurring,
- Alkali-Silica Reaction (ASR)
- Alkali-Carbonate Reaction (ACR)
Generally, aggregates are the chemically inert material but if the active silica or carbonate is present in the aggregate, it reacts with the alkalis present in the cement paste, causing expansion and cracking over a period of time.
07. Specific Gravity of Aggregate
The specific gravity and water absorption of aggregates are important properties for designing concrete and bituminous mixes. Specific gravity of an aggregate is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. It is a quality measure and determines the strength of aggregate.
08. Adhesion with Bitumen
The aggregates used in bituminous pavements must have less affection for water as compared with bituminous material; otherwise the bituminous coating of the aggregates will get stripped off in presence of water. In absence of water or moisture, there is no problem in bituminous construction.
Different Types of Test on Aggregates
In order to decide the suitability of the aggregate for using in concrete/pavement construction, following tests are carried out:
01. Crushing Test on Aggregates
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. Whereas, if the crushing value is 35 or above, it would normally be considered as weak aggregates.
(a) Use of Crushing Test:
The aggregate crushing value test indicates the strength of aggregate which is the most essential property of coarse aggregate. The strength of coarse aggregates is assessed by aggregates crushing test. The aggregate crushing value provides a relative measure of resistance to crushing under a gradually applied compressive load. For achieving high quality of pavement, aggregate of low crushing value should be preferred. Therefore, lower crushing value means higher strength as it indicates a lower crushed fraction under load. This gives a longer service life and a more economical performance.
(b) Recommended Result of Crushing Test:
After performing the test, crushed material is separated and calculated in terms of percentage of the original weight taken in the mould. This percentage is known as a crushing value of aggregate. This percentage should not exceed 30% of the total 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 pavements and, in case of other structures; it may be permitted up to 45%.
(c) Standard Guideline for Crushing Test:
Followings are the various guidelines for the crushing test on aggregates:
- IS 2386(Part 4):1963
- BS 812–110:1990
Crushing Mould for Aggregates Test Image
(d) 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 Aggregates
This test is carried out to determine the abrasion value of the coarse aggregate. 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.
Followings are the various guidelines for the abrasion test on aggregates:
- IS 2386(Part 4):1963
- BS 812-113:1990
03. Impact Test on Aggregates
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. Due to such kind of load, aggregate breaks down into smaller pieces. In order to evaluate the resistance to impact of aggregate, the aggregate impact test is conducted. 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.
(a) Use of Impact Test:
Impact test on aggregates indicates the toughness of aggregate. It shows the ability of aggregate to resist the sudden shock or impact.
(b) Recommended Result of Impact Test:
Aggregate impact value is the same as the abrasion test result. Impact value of aggregate should not exceed 30 percent for aggregate used for surface wearing course and should not exceed 45 per cent for surface other than wearing course.
(c) Standard Guideline for Impact Test:
Followings are the important guidelines for the impact test on aggregate:
- IS 2386(Part 4):1963
Impact Test Apparatus Image
(d) Apparatus of Impact Test:
04. Soundness Test on Aggregates
Soundness test on aggregate is conducted to understand the resistance of aggregates to weathering action. 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 and substandard for the durability of construction. The property of aggregates to withstand the adverse action of weather is known as soundness of aggregate.
(a) Use of Soundness Test:
Soundness test on aggregates 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, variation in temperature in salt water. When aggregate do not resist such things, then it said to be unsound.
(b) 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.
(c) Standard Guideline for Soundness Test:
Followings are the various guidelines for the soundness test on aggregates:
- IS 2386 (Part 5):1963
- BS 812-121:1989
(d) Apparatus of Soundness Test:
05. Shape Test on Aggregates
Aggregates are available in various size and shape, i.e. rounded, cubical, and angular flaky or elongated. It is proved that the flaky and elongated particles have less strength, less bond, 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.
(a) 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.
(b) Recommended Result of Shape Test:
Recommended result of shape test on aggregates should range in 15% to 30 % according to the area of use.
(c) Standard Guideline for Shape Test:
Followings are the various guidelines for the shape test on aggregates:
- IS 2386(Part 1):1963
- BS 812-105.1:1989
Shape/Flakiness Index Apparatus for Aggregates Test Image
(d) Apparatus of Shape Test:
06. Specific Gravity and Water Absorption Test on Aggregates
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 capacity, they are called porous in nature. Such aggregates are unsuitable and generally not preferred for the construction.
(a) Use of Test:
The specific gravity and water absorption test on aggregates measures a strength or quality of the aggregate. This property helps in a general identification of aggregates.
(b) Recommended Result of Impact 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 %
(c) Recommended Result of Test:
Followings are the various guidelines for the specific gravity and water absorption test on aggregates:
- IS 2386(Part 3):1963
- BS 812-2:1995
(d) Apparatus of Test:
- Density basket
- Weighing balance
- Water tank
- IS sieves- 10mm and 20mm
07. Bitumen Adhesion Test on Aggregates
Bitumen gets well attached to all the normal types of road aggregates provided, they are dry and free from dust. In the absence of water, there is no adhesion problem in 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 determined as the ratio of the uncovered area observed visually to the total area of aggregates which is ultimately expressed in percentage.
Bitumen adhesion test on aggregates determines the adhesion of bituminous binder to aggregate in the presence of water.
Recommended result (stripping value) of bitumen adhesion test should not exceed 5 percent for aggregates to be used in bituminous construction.
Followings are the various guidelines for the bitumen adhesion test on aggregates:
- IS:6241- 1971
Reasons for the Failure of Tests on Aggregate (All Tests Mentioned Above) & Some Suggestions to Deal with them
If aggregate tests results are not in range of recommendations, there can be many reasons like weak aggregates, error in testing procedure, error in test appliance etc. If any of tests on aggregates fail, first of all, ensure that, there is no error in test appliance or the procedure. If that seems to be the likely reason, retest then. However, even if the test appliances are in order, test procedures are followed religiously and still the test result have failed; it is advisable not to use those aggregate for structural as well as for non-structural purpose.
Thus, we hope, the information that we have provided regarding properties of aggregate and test on aggregates should help you in making a wise and rational selection of aggregates. This article highlights on the various quality of aggregates, thereby helping you to make a choice of your own and instill confidence in you about the longevity of your house. This article further provides information as to why and when to conduct test on aggregates, which tests you need to perform and why? It emphasizes as to under which circumstances the aggregates should be rejected and some suggestions in order to overcome the problem. The best recommendation we can come up with is – Aggregates are important for the strength and longevity of your structure, hence, do not ignore these tests.