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     Micro-cracking of Concrete /
Behavior under Multiaxial Loading
                                        

Strength properties of concrete
o Concrete is a complex material consisting of coarse aggregate, sand,
   cement gel, unhydrated cement particles, capillary and gel pores, pore
   water, air voids etc.
o Concrete is multi-phased at several levels:
       Micro-level: multi-element
       Meso-level: multi-phase
       Macro-level: homogeneous
o A mortar-aggregate system is considered to be at meso-level.
o Cracking stages:
                        σ
                               ≅ 0.3
   1. Bond cracks at
                         f c'
                             σ
                                   ≅ 0.75 − 0.80
   2. Mortar cracks at
                              f c'
   3. Unstable crack propagation
   4. Failure
   where f c' = reference strength, which is usually obtained from
   uniaxial testing (compression) of standard size cylinders.
Stress-strain behavior of concrete
o Stress-strain ( σ − ε ) relation is used to determine material property,
   such as the modulus of elasticity (Young’s modulus), E, strength, and
   failure strain as a basis for analysis and design purposes.