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Outline_02 - 1.054/1.541 Mechanics and Design of Concrete...

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1.054/1.541 Mechanics and Design of Concrete Structures Spring 2004 Prof. Oral Buyukozturk 1 / 14 Massachusetts Institute of Technology 1.054/1.541 Mechanics and Design of Concrete Structures (3-0-9) Outline 2 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: ² Micro-level: multi-element ² Meso-level: multi-phase ² Macro-level: homogeneous o A mortar-aggregate system is considered to be at meso-level. o Cracking stages: 1. Bond cracks at ' 0.3 c f σ 2. Mortar cracks at ' 0.75 0.80 c f ≅− 3. Unstable crack propagation 4. Failure where = reference strength, which is usually obtained from uniaxial testing (compression) of standard size cylinders. ' c f ± Stress-strain behavior of concrete o Stress-strain ( 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.
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1.054/1.541 Mechanics and Design of Concrete Structures Spring 2004 Prof. Oral Buyukozturk Outline 2 o For concrete, since it is mainly used in compression, the modulus of elasticity, , is determined from compressive stress-strain curves that are generally obtained from uniaxial compressive tests. c E ± Deflection and failure behavior as affected by microcracking o Model study: Deformation and fracture of a concrete model: Shear-compression tension mortar inclusion Interface element o Interface elements are used to study the influence of matrix-inclusion that is bond effect and particle-interaction phenomena in modeling. o Precise analysis should account for the influence of nonhomogeneity, models of interfacial behavior, and include recognition of interface debonding and resulting progressive fracture. o Sophisticated analysis shows that bond failure initiates due to compression-shear and that the failure behavior is controlled by microcracking due to direct tensile stresses and splitting of concrete. o Two debonding modes: 1. Compression-shear (including pure shear) failure at interface 2. Tensile-shear (including pure tension) debonding ± Compression-shear strength 2 / 14
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1.054/1.541 Mechanics and Design of Concrete Structures Spring 2004 Prof. Oral Buyukozturk Outline 2 o The compression-shear strength of concrete is based on: 1. Chemical bond which is an attraction between the mortar and aggregate constituents. 2. Mechanical bond which results from the interlocking of the mortar and the aggregate in the irregularities of the aggregate surfaces. ± Uniaxial compression o Inelastic volume increase Æ “dilatancy” o The behavior is related to microcracking.
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Outline_02 - 1.054/1.541 Mechanics and Design of Concrete...

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