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Aggregates
By: Haseeb Jamal
Aggregates
Definition:
Aggregates are coarse particulate
rock-like material consisting of a collection of particles ranging in size from
< 0.1 mm to > 50 mm.
It includes gravel, crushed rock,
sand, recycled concrete, slag, and synthetic aggregate.
Aggregate is a granular
material, such as sand, gravel, crushed stone, crushed hydraulic-cement concrete, or iron blast-furnace
slag, used with a hydraulic cementing medium to produce either concrete or
mortar.
Types of aggregates include Coarse
aggregate and fine aggregate.
The aggregate of each type is further
sub-divided into many types and classification based on its size.
The technique of Sieve
Analysis is used for gradation of aggregate for use in
concrete and for other applications.
Aggregate is called bound material
when it is mixed with cement or binding materials and referred to as unbound
material when used without cement or binding materials.
Aggregate Origin
and Geology
Aggregates are commonly obtained by
crushing naturally occurring rock.
The properties of aggregates depend
on the parent rock which can be igneous, sedimentary, or metamorphic.
Aggregates are evaluated through
tests to determine their suitability for various applications.
Mineralogy, grain size and texture,
and petrographic description of rock samples are also used to evaluate
suitability.
Types of Aggregates
Coarse Aggregate
Coarse-grained aggregates will not
pass through a sieve with 4.75 mm openings (No. 4).
Those particles that are
predominantly retained on the 4.75 mm (No. 4) sieve and will pass through 3 inch
screen, are called coarse aggregate.
The coarser the aggregate, the more
economical the mix.
Larger pieces offer less surface area
of the particles than an equivalent volume of small pieces.
Use of the largest permissible
maximum size of coarse aggregate permits a reduction in cement and water
requirements.
Using aggregates larger than the
maximum size of coarse aggregates permitted can result in interlock and form
arches or obstructions within a concrete form.
That allows the area below to become
a void, or at best, to become filled with finer particles of sand and cement
only and results in a weakened area.
For Coarse
Aggregates in Roads following properties are desirable:
1.
Strength
2.
Hardness
3.
Toughness
4.
Durability
5.
Shape of aggregates
6.
Adhesion with bitumen
Fine Aggregate
The other type of aggregates are
those particles passing the 9.5 mm (3/8 in.) sieve, almost entirely passing the
4.75 mm (No. 4) sieve, and predominantly retained on the 75 µm (No. 200) sieve
are called fine aggregate.
For increased workability and for economy as reflected
by use of less cement, the fine aggregate should have a rounded shape.
The purpose of the fine aggregate is
to fill the voids in the coarse aggregate and to act as a workability agent.
Properties of Aggregates also has
effect on resulting concrete. e.g. variation in size, grading, texture, shape
and strength of aggregates means variation in the properties of resulting
concrete.
Purpose & Uses of Aggregates
In concrete, an aggregate is used for
its economy factor, to reduce any cracks and most importantly to provide
strength to the structure.
1.
Aggregates are used as the base, subbase, and/or surface of
roads in several forms
2.
In roads and railway ballast, it is used to help distribute the
load and assist in ground water running off the road.
3.
Increases the volume of concrete, thus reduces the cost.
Aggregates account for 60-75% of the volume of concrete and 79-85% weight of
PCC
4.
Provide dimensional stability
5.
Influence hardness, abrasion resistance, elastic modulus and
other properties of concrete to make it more durable, strong and cheaper.
6. Other uses include fills, backfills, and drainage and filtration applications.
Haseeb Jamal
I am a Civil Engineer, graduated from University
of Engineering and Technology, Peshawar, Pakistan in 2010. I also
have a PG-Diploma in Disaster Management and
MS in Urban
Infrastructure Engineering (In Progress). My expertise include civil
related softwares like AutoCAD,
SAP2000, MS Project, Primavera, MS Office and GIS. My technical skills include
project management, monitoring and evaluation, structural assessment, disaster
risk management, Quantity survey, land survey, material testing, site
management and technical writing. I am trained in writing project progress
reports as well as proposals and concept papers. I have also received advanced
training on surveying, proposal writing, Monitoring and Evaluation of projects
as well as organizations.
I have worked as Project Engineer at
National Research and Development Foundation, Peshawar and CENCON Associates. I
also worked with Spectra Engineering Solutions as Senior Civil Engineer in
monitoring of World Bank and UNDP funded projects all over Khyber Pakhtunkhwa
and FATA. Currently, I am working as Deputy Manager Development at
NayaTel, Peshawar.
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