Basic Concrete

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Concrete

Concrete is a product obtained artificially by hardening of the mixture of cement, sand, gravel and water in predetermined proportions.

Concrete has enough strength in compression but has little strength in tension. Due to this, concrete is weak in bending, shear, and torsion. Hence the use of plain concrete is limited applications where great compressive strength and weight are the principal requirements and where tensile stresses are either totally absent or are extremely low.

concrete, cement, wall

Properties of Concrete

Weight

The unit weights of plain concrete and reinforced concrete made with sand, gravel of crushed natural stone aggregate may be taken as 24 KN/m3 and 25 KN/m3 respectively.

Compressive Strength

The compressive strength of concrete depends primarily on age, cement content and the water cement ratio. The strength of concrete is defined at 28th day.

Increase in strength with age

Due to hydration of concrete the strength increases with the age of concrete.

Tensile Strength of concrete

There is very low tensile strength in concrete. When the designer wishes to use an estimate of the tensile strength from compressive strength, the following formula can be used

Flexural strength, fcr=0.7√fck N/mm2

Elastic Deformation

The modulus of elasticity is normally related to the compressive characteristic strength of concrete

Ec=5000√fck N/mm2

Where, Ec= the short-term static modulus of elasticity in N/mm2

fck=characteristic cube strength of concrete in N/mm2

Shrinkage of Concrete

Shrinkage is the time dependent deformation, generally compressive in nature. The constituents of concrete, size of the member and environmental conditions are the factors on which the total shrinkage of concrete depends. However, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing for a given humidity and temperature.

Creep of Concrete

Creep is another time dependent deformation of concrete by which it continues to deform, usually under compressive stress. The creep strains recover partly when the stresses are released.

Thermal Expansion of Concrete

concrete will be having its effect of high temperature during fire.

The coefficient of thermal expansion depends on the nature of cement, aggregate, cement content, relative humidity and size of the section.

Workability

The workability of a concrete mix gives a measure of the ease with which fresh concrete can be placed and compacted. The concrete should flow readily into the form and go around and cover the reinforcement, the mix should retain its consistency and the aggregates should not segregate.

Durability

A durable concrete performs satisfactorily in the working environment during its anticipated exposure conditions during service.

Test on Concrete

Workability Test

Slump Test

Vee-Bee Consistometer Test

Compaction Factor Test

Air Content

Pressure Air Method

Setting Time

Initial Setting Time is defined as the period elapsing between the time when water is added to the cement and the time at which the needle of 1 mm square section fails to pierce the test block to a depth of about 5 mm from the bottom of the mould.

Final Setting Time is defined as the period elapsing between the time when water is added to cement and the time at which the needle of 1 mm square section with 5 mm diameter attachment makes an impression on the test block.

Other Tests

Segregation Resistance

Unit Weight

Wet Analysis

Temperature

Heat Generation

Bleeding

Test on Hardened Concrete

Compressive Strength

Compressive strength can be derived by casting cubes of size 150mm*150mm*150mm and cylinder of 300mm*150mm.

Compression Testing Machine (CTM)

Tensile Strength

Split tensile strength test

Flexural Test

In-situ test on Concrete

Rebound Hammer, Concrete Pull-out break-off, cones

Other Test

  1. Modulus of rupture
  2. Density
  3. Shrinkage
  4. Creep
  5. Freeze/thaw resistance
  6. Resistance to aggressive chemicals
  7. Resistance to abrasion
  8. Bond to reinforcement
  9. Absorption
  10. Permeability
  11. Modulus of Elasticity

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