Definition: Concrete is a construction material composed of cement, fine aggregates (sand), and coarse aggregates mixed with water which hardens with time. Portland cement is the commonly used type of cement for the production of concrete. Concrete technology deals with the study of properties of concrete and its practical applications.
In building construction, concrete is used for the construction of foundations, columns, beams, slabs, and other load-bearing elements. There are different types of binding material is used other than cement such as lime for lime concrete and bitumen for asphalt concrete which is used for road construction.
Properties of Concrete: The properties of concrete in construction are listed as follows:
1.Strength
Perhaps most widely known
property of concrete in its hardened state is its strength, which is split into
four types:
·
Compressive
·
Tensile
·
Flexural
·
Shear
Compressive: The compressive strength of concrete is
considered one of the most important structural properties it has. As an
industrial standard, concrete is classified by different grades. These grades
are based on the compressive strength of the material when a specimen of it is
made into a cube or a cylinder. According to the British Standard, the strength
of the cylinder specimen should be equal to three-quarters of the strength of the
cube.
Tensile: Tensile strength in ordinary concrete is
weak, ranging from between seven and ten percent of the compressive strength.
It is measured by the units of Force per Cross-Sectional area.
Flexural:
The
flexural strength in ordinary concrete almost entirely depends on the tensile
strength. It is the measure of an unreinforced concrete beam or slab’s
resistance to failure in bending.
Shear: This is the real factor that determines the compressive strength of short columns. The average strength of concrete mixtures in shear vary from about half the compressive strength if the mixture is strong, to 0.8 of the compressive strength if the mixture is considered lean.
2. Durability
The durability of concrete
is the property that determines whether or not it can withstand the condition
it was designed to endure, without breaking down, over a number of years.
Concrete can lack durability owing to factors caused by external agents, such
as the environment, or internal agents, such as faults within the concrete.
These causes can be physical, chemical, or mechanical.
3. Workability
How strong a concrete mix
is can be determined quite highly by its degree of compaction. As such, it is
extremely important that the consistency of the mix used means that the
concrete can be supplied, installed, and finished quickly and easily, without
segregation. Concrete mixes which can do this are called “workable” mixes, and
the factors which affect this status are:
·
Mix
proportions
·
Size
and shape of aggregates
·
Grading
and surface texture of aggregates
·
Water
content
·
Use
of admixtures
·
Use
of supplementary materials
·
Temperature
·
Time
To determine the
workability as a property of concrete, a slump test will often be carried out.
This measures the ease at which a mixture flows and can indicate when a batch
has been improperly mixed.
4. Elastic
Properties
The elastic properties of
concrete will vary with the richness of the mix used, the age of the concrete
and the intensity of the stress.
5. Impermeability
Ensuring that a concrete
mix is impermeable is vital, as permeable concrete can be left open to
penetration from materials that will affect its durability. This becomes
especially important for the property in reinforced concrete, as water, frost
and air can all result in the corrosion of the steel reinforcing fibres mixed
into the material. As the steel expands with the corrosion, this can result in
the concrete becoming cracked and damaged.
6. Segregation
The separation of coarse
aggregate from the rest of the concrete mass is referred to as segregation.
This happens and increases when the concrete mix is both too lean and too wet,
or when aggregate which is large and has a rough texture is used. You can avoid
this problem when mixing concrete by:
·
Adding
little air-entraining agents into the mix
·
Restricting
how much water is poured into the mix
·
Ensuring
handling, placing and consolidation is conducted carefully – the concrete
especially must not be allowed to fall from a large height.
7. Bleeding
After the concrete has been
laid, it is common for water to rise to the surface in a process known as
bleeding. When it does ths, it will often carry particles of sand and cement
with it, forming a scum layer called laitance. This problem can be reduced or
avoided by:
·
Adding
more cement to your mix
Using a more finely ground type of cement
Using the minimum quantity of water needed in a properly designed mix
·
Using
more fine aggregate
Using little air entraining agent
8. Fatigue
·
When
subjected to flexure, ordinary concrete will exhibit fatigue. The ability of
concrete to resist flexure is indicated by an already-established endurance
limit. The value of this limit is dependent upon the number of repetitions and
strength.