............................................................................................................................................................
Aggregates
Coarse Aggregate, Fine Aggregate
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.
No comments:
Post a Comment