Lecture-2: What is Aggregate? classification of aggregates? Concrete technology lecture by Dr K Mohan.
Dr K Mohan is an internationally renowned and an acclaimed cement scientist. He is a former Director General of National Council for Cement and Building Materials (NCCBM).
He holds a Ph.D. degree in Cement Chemistry from University of Aberdeen, Scotland, U.K.
• #Aggregates are the inert or chemically inactive materials which form the bulk of cement #concrete.
• The aggregate is used primarily for the purpose of providing bulk to the concrete.
• They occupy 70 – 75 % of total volume of concrete and hence its influence is extremely important.
• Aggregates are generally cheaper than cement and impact greater volume stability and durability to concrete.
• To increase the density of the resulting mix the aggregate is frequently used in two or more sizes and well graded.
• The most important function of the fine aggregate is to assist in producing in workability and uniformity in mixture.
• Physical, chemical and thermal properties of aggregates influence the performance of concrete.
Characteristics of Good Aggregates:
• It should be hard, strong and durable.
• It should be chemically inert.
• It should be clean i.e. it should be free from coatings of clay and silt
and organic materials.
• It should not be soft and porous.
• It should be angular or cubical in shape.
Question 1 by Mr. J. K. Singh: Sir, can you please enlighten us about the classification of aggregates?
Very good question Mr. Singh. I will tell you about classification of aggregates.
Classification of Aggregates:
According to Size
• Fine Aggregate
• Coarse Aggregate
Fine Aggregate
• Aggregates passing through a 4.75 mm IS sieve are defined as fine aggregate.
• Sand is generally considered to have a lower size limit of about 0.07 mm.
• Material between 0.06 mm. and 0.002 mm. is classified as silt.
• Smaller than 0.002 mm. particles are called clay.
The fine aggregate may be one of the following types:
Natural sand – deposited by rivers
Crushed stone sand – obtained by crushing stones
Crushed gravel sand – obtained by crushing natural gravel
• According to size, fine aggregate may be described as coarse, medium and fine sands.
• Coarse sand - 2 mm to 4.75 mm, medium sand - 0.425 mm to 2 mm and fine sand - 0.075 mm to 0.425 mm.
• Depending upon the particle size distribution, IS:383 – 1970 has divided the fine aggregate into four grading zones – Zone I to Zone IV.
• The grading zones become progressively finer from grading Zone I to grading Zone IV.
Coarse Aggregate
• Aggregates retained on 4.75 mm sieve are termed as coarse aggregate.
The coarse aggregate may be one of the following types:
Crushed gravel or stone – obtained by crushing gravel or hard stone
Uncrushed gravel or stone – resulting from the natural disintegration of rock
Partially crushed gravel or stone – obtained as a product of blending of the above two types
• 80 mm size is the maximum size that could be conveniently used for concrete making.
• The graded coarse aggregate is described by its nominal size I.e. 40 mm, 20 mm, 16 mm and 12.5 mm. For example, a graded aggregate of nominal size 12.5 mm means an aggregate most of which passes the 12.5 mm Is sieve.
According to Source (Origin)
• #NaturalAggregate
• #ArtificialAggregate
#classificationofaggregates
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