Concrete compressive strength:7,28 days

The characteristic compressive strength of concrete is determined by performing cube tests and they are performed utilising concrete specimens of a cubical mould of size 15 cm x 15 cm x 15 cm and the cylindrical form of dimension 15 cm in diameter x 30 cm in height.

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The compressive strength test of the concrete cylindrical specimen is 0.8 times the strength of cube.

Concrete compression test of specimens is done after 28 days of curing.

With the increase in age, the strength of concrete will increase.

concrete compressive strength at any age = Strength at 28 days x age factor.

 Age factors  at different age of concrete

S.No	Minimum age	Age factor
1	7 days	0.7
2	28 days	1.0
3	3 months	1.10
4	6 months	1.15
5	12 months	1.2

For example,

concrete Compressive strength of M 25 grade of concrete at 28 days in 25 N/mm2.

So concrete compressive strength of M 25 grade concrete at 7 days = 0.7 x 25= 17.5 N/mm2.

Note: By using High Alumina cement concrete, the strength of concrete will not increase with time.

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Apart from the concrete compressive strength, given below other properties of reinforcement concrete which are related to the compressive strength of concrete.

1. Tensile strength of concrete
2. Elastic deformation
3. Shrinkage
4. Creep of concrete
5. Workability
6. Durability of concrete

1. Tensile strength of concrete

Tensile strength of concrete is determined by using the characteristic compressive strength of concrete.

Formula :

ft = 0.7 x square root of fck.

The modulus of elasticity is principally controlled by the elastic properties of the aggregate and to a more secondary by mix proportions, the conditions of curing, the type of cement, age of the concrete.

Modulus of elasticity is reliant on the concrete compressive strength.

Concrete modulus of elasticity can be determined utilising the formula

Ec = 5000 x square roof of fck  (new IS 2000),Ec is the Static modulus of elasticity

The complete shrinkage of concrete depends on the environmental conditions, size of the member, constituents of concrete.

Total shrinkage of concrete is influenced by the total amount of water or moisture present in concrete.

Shrinkage strain in concrete = 0.0003.

Under suspended loading, if there is a continuous deformation of concrete with time then it is termed as the creep of concrete.

The coefficient of Creep   = Ultimate creep strain / elastic strain at the age of loading

S.No	Age of loading	Creep coefficient
1	7 days	2.2
2	28 days	1.6
3	1 year	1.1

Workability of concrete is the ease with which concrete can be placed and compacted.

Based on slump cone test values, degree of workability given below

S.No	Slump(mm)	Vee-Bee time (sec)	Compaction factor	Degree of workability
1	0 – 25	25 – 10	0.75 – 0.8	Very low
2	25 – 50	10 – 5	0.8 – 0.85	Low
3	50 – 100	5 – 2	0.85 – 0.93	Medium
4	100 – 175	–	> 0.92	High

6. Durability of concrete

i.For ductile material like mild steel

ii.For brittle material like concrete

The factor of safety for RCC is more than Steel.