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Outline_05 - 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 / 11 Massachusetts Institute of Technology 1.054/1.541 Mechanics and Design of Concrete Structures (3-0-9) Outline 5 Creep and Shrinkage Deformation ± Creep of concrete o Concrete under stress undergoes a gradual increase of strain with time. The final creep strain may be several times as large as the initial elastic strain. o Creep is the property of materials by which they continue deforming over considerable length of time under sustained stress. o Relaxation is the loss of stress by time with constant strain. o In concrete, creep deformations are generally larger than elastic deformation and thus creep represents an important factor affecting the deformation behavior. o Concrete under constant axial compressive stress ε load removed (unloading) inst (elastic recovery) creep recovery permanent or residual deformation elastic deformation = inst creep loaded constantly Time ² Experiments indicate that for working stress range – i.e. stresses not exceeding 0.5 – creep strains are directly proportional to stress that is a linear relationship between ' c f σ and cr .
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1.054/1.541 Mechanics and Design of Concrete Structures Spring 2004 Prof. Oral Buyukozturk Outline 5 ± Microcracking effect on creep in high stresses. ² Mechanisms of concrete creep o Two phenomena are involved: 1. Time dependent deformation that occurs when concrete is loaded in a sealed condition so that moisture cannot escape. Æ basic creep 2. Creep of the material when moisture exchange is permitted. Æ drying creep o Basic creep is primarily influenced by the material properties only, while drying creep and shrinkage also depend on the environment and the size of the specimen. o The real situation might be the combination of the two phenomena, sometimes, one being the dominating factor. o Creep deformation contains three regions: 1. Primary creep Æ initial increase in deformation 2. Secondary creep Æ relatively a steady deformation region 3. Tertiary creep Æ leads to creep fracture ² Specific creep o c sp ε σ = , ' 0.5 c f < where c = a function of time. Æ Stress levels above ' 0.8 c f creep produces failure in time. o Relationship between ' c f and sp : ' c f (psi) sp ( 6 10 per psi) Final strain ( per psi) 6 10 3000 1.0 3.1 4000 0.80 2.9 6000 0.55 2.4 8000 0.40 2.0 2 / 11
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1.054/1.541 Mechanics and Design of Concrete Structures Spring 2004 Prof. Oral Buyukozturk Outline 5 ± Creep coefficient o c t inst C ε = where inst = instantaneous (initial) strain. ± Factors influencing creep: o Internal factors (composition) Creep Creep Creep ² Aggregate (concentration + stiffness) ² Water/cement ratio ² Aggregate permeability ² Aggregate creep Creep Creep ² Aggregate stiffness ² Aggregate grading and distribution ² Cement o External factors (environment, time history) ² Size ² Shape Creep ² Cross-section ² Environmental factors (ambient humidity, temperature) Creep ² Stress intensity ² Time (age of loading) Æ
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Outline_05 - 1.054/1.541 Mechanics and Design of Concrete...

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