PROPERTIES OF HARDENED CONCRETE

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: f_cr=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,

E_c=5000√fck

Where E_c is the short term static modulus of elasticity of concrete expressed in N/mm²

f_ck 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 emeber under sustained loading is refered 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