6 PROPERTIES OF CONCRETE USED BY DESIGNERS
Flexural Strength Test Arrangement
The following are the
properties of concrete in its hardened state, used by the designer during
design process of reinforced concrete structure.
1. COMPRESSIVE
STRENGTH
The characteristics
strength is defined as the strength of concrete below which not more than 5% of
the test results are expected to fall. As per IS: 456 concrete mix always
designed for the target strength computed as,
Target strength: Characteristics
strength + (1.65 x standard deviation)
M20 is the minimum
grade of concrete for use in RCC work.
2. TENSILE
STRENGTH
Flexural strength is
one measure of the Tensile strength of concrete. In concrete structure one set
of visible cracks occur under flexure to compute load factor against cracking. According
to IS: 456 the tensile strength of concrete can be computed from the
compressive strength using empirical relation given by:
Flexural
strength: fcr=0.7√fck N/mm2
3. MODULUS
ELASTICITY
Modulus of elasticity
of concrete which is significantly influenced by the following factors.
§ Type of the aggregates used,
§ Type of cement and
§ Mix proportions
This property is
required for the computations of deflections of structural concrete members
which forms an important limit state in the design of concrete members. In the
absence of test data, the modulus of elasticity of concrete is normally related
to the compressive strength and is computed by the empirical relation
recommended by IS: 456-2000 code and is expressed as,
Ec=5000√fck
Where Ec is
the short-term static modulus of elasticity of concrete expressed in N/mm2
fck is
the characteristic compressive strength of concrete expressed in N/mm2.
4. SHRINKAGE
OF CONCRETE
The ingredients of
concrete and environmental conditions like temperature and humidity influence
the total shrinkage of concrete. Water content in concrete significantly
affects the shrinkage. The IS: 456-2000 recommends the total shrinkage strain
as 0.0003 in the absence of test data. Drying shrinkage in
plain concrete may result in surface cracks. Shrinkage of concrete also
influences the deflections of reinforced concrete members.
5. CREEP
OF CONCRETE
The inelastic time
dependent strain developed in a concrete ember under sustained loading is referred
to as creep of concrete. Creep of concrete is influenced by following factors.
§ Cement content,
§ W/C ratio,
§ Temperature and humidity,
§ Size of structural element,
§ Type of loading and period of loading.
In the absence of
reliable experimental data, the creep coefficient is expressed as the ratio of
ultimate creep strain/elastic strain at various ages of loading as recommended
by IS: 456-2000 are given below.
|
Age at Loading
|
Creep Coefficient
|
|
7 days
|
2.2
|
|
28 days
|
1.6
|
|
1 year
|
1.1
|
Creep of concrete
significantly affects the deflections of reinforced concrete flexural members.
Higher creep coefficient results in large deflections. The value of creep
coefficient is useful in the computation of time dependent deflections in
reinforced concrete members.
6. COEFFICIENT
OF THERMAL EXPANSION
The coefficient of
thermal expansion of concrete, influenced mainly by the type of aggregate used
in concrete is required for the design of structures like chimneys, water
tanks, silos etc. the values recommended in IS:456-2000 are compiled below.
|
Type of Aggregate
|
Coefficient of Thermal Expansion for
Concrete
|
|
Quartzite
|
1.2 to 1.3 x 10-5
|
|
Sandstone
|
0.9 to 1.2 x 10-5
|
|
Granite
|
0.7 to 0.95 x 10-5
|
|
Basalt
|
0.8 to 0.95 x 10-5
|
|
Lime stone
|
0.6 to 0.9 x 10-5
|
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