What are Aggregate in Construction Industry ?

In the construction industry, aggregate are used as filler material in the production of concrete and mortar. Aggregate occupy around 70% to 80% of the volume of the concrete, reduce shrinkage effects and minimize costs. The coarse aggregate form the main matrix of concrete and the fine aggregates form the filler matrix between the coarse aggregate.

Classification of Aggregate :

A. Based on Geological Origin :

On the basis of origin, aggregates can be classified into natural aggregates and artificial aggregates.

Natural Aggregate :

The aggregates which are obtained crushing igneous, sedimentary or metamorphic rocks are called natural aggregates. Aggregates obtained from igneous rocks have the best engineering properties and hence they are most widely used aggregates.

Artificial Aggregate :

The aggregates which are obtained from man made processes are called as artificial aggregates. Surkhi, Blast furnace slag aggregate and synthetic aggregates are some kind of examples of artificial aggregates.

B. Based on Size :

According to size, aggregates can be classified as coarse aggregates and fine aggregates.

Coarse Aggregate :

The aggregates which pass through the 80 mm sieve and are retained on the 4.75 mm sieve are called as coarse aggregates.

Fine Aggregate :

All the aggregates which pass through the 4.75 mm sieve are called as fine aggregates. On the basis of particle size distribution, the fine aggregates are classified into four zones – (Zone I to Zone IV.) The grading zones are progressively finer from grading Zone I to grading Zone IV.

C. Based on Shape :

Based on the shape of the aggregates, they are classified as rounded, irregular, angular and flaky.

Rounded Aggregate :

The aggregates which are obtained from river or sea shores are generally close to spherical in shape and are called as rounded aggregates. These aggregates have minimum surface area to the volume and have poor interlocking bond, making them unsuitable for the production of concrete.

Irregular Aggregate :

Theses aggregates are irregular in shape and require more cement paste compared to rounded aggregates. Because of the irregularity in shape, these aggregates form good interlocking bond and are suitable in the production of concrete.

Angular Aggregate :

These aggregates are sharp and they have angular shape and rough texture. Those aggregates are best suited for the preparation of high strength concrete.

Flaky Aggregates :

The aggregates whose thickness is less than 0.6 times the mean dimension are called as flaky aggregates.

Elongated Aggregates :

The aggregates whose length is 1.8 times the mean dimension are called as elongated aggregates.

Testing of Coarse Aggregate.

A. Shape and Size Test :

The shape and size of aggregates can be determined by flakiness index and elongation index tests.

Flakiness Index Test

Flakiness index is defined as the ratio of weight of aggregates whose least dimension is less than 0.6 times of their mean dimension to the weight of the sample taken.

A minimum of 200 pieces of aggregates are required for testing.

Elongation Index Test :

Elongation index test must be performed only on non-flaky aggregates. Elongation Index is defined as the ratio of weight of aggregates whose greatest dimension is greater than 1.8 times of their mean dimension to the weight of the sample taken.

E.I = ( Weight of aggregates whose greatest dimension is greater than 1.8 times of their mean dimension / weight of sample taken ) * 100

Both F.I and E.I must be less than or equal to 30%.

B. Strength Test :

The strength of aggregates is based on three parameters – Aggregate Crushing Value (ACV), Aggregate Impact Value (AIV) and Aggregate Abrasion Value (AAV).

Aggregate Crushing Value/Strength Test :

• This test gives the Aggregates Crushing Value (ACV), which is an index of crushing strength of aggregates.
• The apparatus used for this test is Compression Testing Machine / Universal Testing Machine.
• A sample of aggregates in surface dry condition, which pass through the 12.5 mm sieve and are retained on 10 mm sieve are taken for this test (W1).
• A load of 400 KN is applied gradually on the sample of aggregates for 10 minutes.
• The sample is now sieved on 2.36 mm sieve and the fraction passing through the sieve is weighed (W2).
• Aggregate Crushing Value (ACV) =

Aggregate Impact Value Test :

• This test gives the Aggregate Impact Value (AIV), which is an index of the resistance of
  aggregate to sudden shock or impact.
• The apparatus used for this test is called Impact Testing Machine.
• A sample of aggregates in surface dry condition, which pass through 12.5 mm sieve and are retained on 10 mm sieve are taken for this test (W1).
• The whole sample is compacted in three layers with 25 blows each time.
• Now the hammer of Impact Testing Machine is raised until its lower face is 380 mm above the upper surface of the aggregate sample and allowed to fall freely.
• The test sample is subjected to total of 15 blows, each being delivered at an interval of not less than one second.
• The sample is now sieved on a 2.36 mm sieve and the fraction passing through the sieve is weighed (W2).
• Aggregate Impact Value (AIV) =

Aggregate Abrasion Value Test :

The Aggregate Abrasion Value of coarse aggregates may be determined by either Deval Machine or by Los Angeles Machine.

Using Deval Machine :

• The test specimen of aggregates is weighed before transferring it to the machine along with Abrasive charge.
• Now this sample is placed in the drum of the machine inclined at 30° to the horizontal and is allowed to rotate @ 2000 revolutions per hour for 5 hours.
• After 5 hours the aggregates are sieved on a 1.70 mm IS sieve (As per IS:2386 Part IV) and the retained aggregates are weighed (W2).
• Loss in weight as percentage indicates the percentage of wear.
• Aggregate Abrasion Value (AAV) =

Using Los Angeles Abrasion Testing Machine :

• The test specimen of aggregates is weighed before transferring it to the machine along with Abrasive charge.
• This sample is now placed in the cylinder of the machine which is allowed to rotate at 20 to 33 rpm for a total of 500 revolutions.
• After 500 revolutions, the aggregates are sieved on a 1.70 mm IS sieve and the retained aggregates are weighed.
• Loss in weight as percentage indicates percentage of wear.
• Aggregate Abrasion Value (AAV) =

Soundness Test :

• This test is used to assess the durability of aggregates.
• It is a cycle procedure of alternative wetting and drying.
• Sodium Sulphate solution or magnesium sulphate solution is used for wetting.
• After 10 cycles of alternate wetting and drying, % loss of weight is observed.
• It should be less than 12%, if wettinbg is done using sodium sulphate solution.
• It should not be less than 18%, if wetting is done using magnesium sulphate solution.

Testing of Fine Aggregate :

Fineness Modulus Test :

Fineness Modulus is an index of fineness of aggregates. Sieve analysis is performed on the given fine aggregates. Fineness Modulus = ( Sum cumulative % retained on each sieve / 100 )

Bulking Test :

• The increase in volume of sand due to the presence of free moisture in it is called as bulking of sand.
• This free moisture forms a thin film over the sand particles which keeps the neighbouring particles away due to the effects of surface tension. Thus resulting in bulking of the volume of sand.
• The significance of surface tension forces and consequently how for the sand particles are pushed away will depend on the percentage of moisture present and also the size of fine aggregate particles.
• Generally, this phenomenon occurs in all sizes of aggregates, but it is significant mostly in fine sands and coarse silts.
• It is also to be noted that the effects of bulking increases with increase in moisture upto a certain limit and further increase in moisture content leads to decrease in the bulking effects.
• No bulking can be observed when the sand is completely saturated.
• To estimate the extent of bulking of sand a sample of moist fine aggregate is filled into a measuring cylinder. The level of the fine aggregate is noted as ‘h1’ Now water is poured into the measuring cylinder till the fine aggregate is completely inundated.
• Since the fine aggregate is completely saturated, there are no bulking effects.
• Now its level is noted down as ‘h2’ and the percentage bulking can be calculates using the following formula.

What is Alkali-Aggregate Reaction ?

Alkali-aggregate reaction is a reaction which occurs over time in concrete between the highly alkaline cement paste and non-crystalline silicon dioxide which is found in many common aggregates. This reaction can cause expansion leading to spalling and loss of strength of the concrete.

What is Grading of Aggregate.

• Grading is process of determining particle size distribution.
• One of the most important factors for producing workable concrete is good grading of aggregates.
• Continuous graded aggregate is the grading pattern of aggregate in which all particle sizes of certain proportions are present.
• Gap graded aggregate is the grading pattern of aggregate in which there are combinations of certain large size aggregates and certain small size aggregates.