COMPRESSIVE STRENGTH:-
Concrete is a
structural material which consists of Portland cement, aggregate (sand and
rock), and water (to make the chemical reaction called hydration occur.)
Concrete can sometimes contain other substances, such as fly ash from
industrial smoke stacks, which can change its properties.
Concrete is a
very strong material when it is placed in compression. It is, however,
extremely weak in tension. It is for this reason that we use reinforcement in
concrete structures. The reinforcement, which is usually steel, takes up the
slack for the weakness of the concrete in tension.
The
main measure of the structural quality of concrete is its compressive strength.
This property of concrete is commonly considered in structural design.
Depending on the mix (especially the water-cement ratio) and time and quality
of the curing, compressive strength of concrete can be obtained up to 14,000
psi or more. Commercial production of concrete with ordinary aggregate is
usually in the 3,000 to 12,000 psi range with the most common ranges for
cast-in-place buildings from 3,000 to 6,000 psi. On the other hand, precast and
prestressed applications often expect strengths of 4,000-8,000 psi.
The compressive strength of structural
concrete begins at 17 MPa (2500 psi) and can be produced commercially at 138
MPa (20,000 psi) or more. With such a wide range of strengths to choose from,
the following guidance is provided to assist in making an appropriate choice
for specific projects:
- Residential
and light commercial building projects typically use concrete strengths
ranging from 17 MPa (2500 psi) to 34 MPa (5000 psi). Keep in mind that the
lower strength concrete is only appropriate for mild environmental
exposures, and interior concrete protected from the elements. Severe
environmental exposures (freezing and thawing cycles and deicer chemical
exposure) require a minimum strength of 4000 psi to assure durability.
Local codes commonly provide guidance for the minimum requirements, but in
many cases do not address long term durability issues.
Heavy commercial
and special structures (high rise buildings, long span bridges, slabs exposed
to heavy abrasion, etc.) typically require concrete strengths of 28 MPa (4000
psi) or more. The actual required strength may be controlled by the structural
loading, durability requirements, special property requirements (low
permeability, high abrasion resistance, etc.) or a combination of these
factors. Concrete design professionals should always be consulted for guidance
regarding these important structures.
Test for Compressive
Strength:-
There
are many ways to test the strength of a batch of concrete. The tests used can
be categorized as destructive and nondestructive tests.
Generally, tests for this property are made on cylindrical specimens of height
equal to twice the diameter; usually 6x12 in. impervious moulds of this shape
are filled with concrete during the operation of placement as specified by ASTM C172, “Standard Method of
Sampling Fresh Concrete”, and ASTM C31
“Standard Method for Making and Curing Concrete Test Specimens in The Field”.
The cylinders are moist-cured at about 70 degree F, generally for 28 days, and
then tested in the laboratory by compression until they are crushed at a specified rate of loading. The compressive strength
obtained from such tests is known as the cylinder strength fc' and this term is
mainly used in design purposes.
Once
the concrete has been placed for a particular structure, there is a
nondestructive test which can be performed to estimate the strength of the
concrete. This method uses a Schmidt hammer (also called a Swiss hammer). This
method of testing is based on the inertia of a ball inside the Schmidt hammer
testing apparatus that is "bounced off" of the concrete.
