Workability of Concrete

• Workability of Concrete referes to how easily freshly mixed concrete can be placed, consolidated and finished with minimum loss or homogenity.
• The important facts in connection with workability are:
  (i) If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability.
  (ii) If the strength of concrete is not to be affected, the degree of workability can be obtained:
  → By slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too wet; and
  → By adding a small quantity of water cement paste in the proportion of original mix, in case the concrete mixture is too dry.
  (iii) The workability of concrete is also affected by the maximum size of the coarse aggregates to be used in the mixture.
  (iv) The workability of concrete is affected mainly by water content, water-cement ratio and aggregate cement ratio.

STRENGTH TEST ON CONCRETE

• The compressive strength of concrete is much greater than its tensile strength i.e. the tensile strength is about 15% of its compressive strength.
• Quality or grade of concrete is designated in terms of a numbers, which denotes it characteristic compressive
  strength (of 150 mm cubes at 28 days), expressed in MPa (or equivalently N/mm²).
• Number is usually preceded by the letter M, which refers to mix. For example M20 grade concrete denotes a
  concrete whose mix is so designed as to generate a characteristics strength of 20 MPa.
• But it is observed that tensile and bending strength of concrete are of the order of 10 and 15 per cent, respectively of the compressive strength and shear is approximately 20 per cent of per cent, respectively of the compressive strength and shear is approximately 20 per cent of the uniaxial compressive strength.
• Characteristic compressive strength is defined as strength of material below which not more than 5% of test results are expected to fall than 5% of test results are expected to fall.
• It means that accordingly, the mean strength of the concrete f_m (as obtained from 28-day compression tests) has to be significantly greater than the characteristic strength f_ck that is specified by the designer.

RHEOLOGICAL BEHAVIOUR OF CONCRETE

• Rheology is science of flow and deformation of matter and describes the interrelation between force, deformation and time.
• Rheology of fresh concrete like workability includes the parameters of stability, mobility and compatibility.
• Mechanical behaviour of hardened cement paste, which exhibits both elastic and inelastic deformations can be expressed in rheological terms.
• The rheology deals with rate of shear and shear stress of concrete.

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