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The initial, almost linear part whichcorresponds to an elastic behaviour.

The non linearity comes when stress levelexceeds about one-third to one-half of themaximum

Adjacent bond cracks at the interface of

mortar and aggregates, caused by thedifferent stiffness of the two materials, startto bridge in the form of mortar cracks, due tostress concentrations at the tips of bondcracks.

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At 75 to 80% of the ultimate stress - Crackspropagate rapidly in both the matrixcement paste) and the transition zone.

Failure occurs when the cracks join togetherand become continuous.

The descending part, along which strain

increases while stress decreases(descending branch). This phenomenon,called strain softening is attributed to theunstable propagation of the internal cracksparallel to the loading.

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For Higher strength concretes, the compressivestresses drop faster than those of lowerstrength concretes after passing the peak

strengths. Lower strength concretes have more ductility

than higher strength concretes.

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The Youngs modulus of elasticity is aconstant, defined as the ratio, within the linear

elastic range, of axial stress to axial strain,under uniaxial loading.

Static Modulus Secant modulus is mostly used in civil

engineering It is obtained by calculating theslope of the line linking the initial point (theorigin) and the point considered (usually atabout 1/3rdof the maximum compressivestress).

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Tangent Modulus - The slope of a line drawntangent to the stress-strain curve at any

point on the curve. Dynamic Modulus - The modulus of elasticity

corresponding to a small instantaneousstrain. It can be approximated by the tangent

modulus drawn at the origin. The Code (Cl. 6.2.3.1) gives the following

empirical expression for the static modulus

Ec (in MPa units) in terms of the characteristic

cube strength fck (in MPa or N/mm2units):

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Effects of moisture condition

Dry condition elastic modulus but, compressive strength .

Effects of Aggregate properties

Porosity - An aggregates with a low porosity has a highmodulus of elasticity. The elastic modulus of concrete isaffected by the volume fraction of the aggregate as well as theelastic modulus of the aggregate.

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ffects of cement matrix The lower the porosity of the cement paste,

the higher the elastic modulus of thecement paste.

The higher the elastic modulus of thecement paste, the higher the elasticmodulus of the concrete.

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Effects of transition zone The void spaces and the micro-cracks in the

transition play a major role in affecting thestress-strain behaviour of concrete.

The transition zone characteristics affect theelastic modulus more than it affects thecompressive strength of concrete.