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Purdue University School of Civil Engineering West West Lafayette, Indiana. Autogenous Shrinkage, Residual Stress, and Cracking In Cementitious Composites: Influence of Internal and External Restraint Jae-Heum Moon, Farshad Rajabipour, Brad Pease, and Jason Weiss - PowerPoint PPT Presentation

Text of Purdue University School of Civil Engineering West West Lafayette, Indiana

  • Purdue University School of Civil EngineeringWest West Lafayette, Indiana

    Autogenous Shrinkage, Residual Stress, and Cracking In Cementitious Composites: Influence of Internal and External Restraint

    Jae-Heum Moon, Farshad Rajabipour, Brad Pease, and Jason Weiss

    4th International Seminar on Self-Desiccation and Its Importance in Concrete Technology

  • IntroductionWe Typically use Effective Properties

  • Equivalent Strain (eComposite)Equivalent Strain as determined using Picketts Approach from 1956 Picketts equation has an awkward computation for nHere results of simulations (hex cell)

  • Equivalent Elastic Modulus (EComposite)T.C. Hansen developed an approach to estimate the elastic modulus using a similar approach to those described by Pickett (an aggregate sphere in a paste cell).Here we see hexagonal unit cell simulations which compare well

  • Equivalent Residual Stress (sComposite)sComposite = EComposite eCompositeEPaste= 20 GPa, EAgg= 40 ~ 200 GPaeSH-Paste -100 meIf we neglect creep, we could simulate the effect of restraint (using Picketts and Hansens estimates) as we increase the volume of the aggregateHere we can see that as the volume of aggregate increases the stresses decrease This would imply that the residual stress would decrease

  • Scope of this Research and ObjectivesDoes the presence of aggregate would result in local internal stresses that are different than the stresses obtained from the equivalent property approach? To evaluate the role of aggregate on the residual stress development as it is influenced by both internal and external restraintTo investigate how external restraint changes the shape of the stress field around the aggregateTo begin to try to incorporate microcracking and cracking in the composite systems

  • Introduction to the Idea of Residual Stress in a Homogenous SystemResidual stress development: (For now we will assume no creep effects to keep the problem somewhat straightforward)ExternallyUnrestrainedHomogenous PasteNo stress(epaste)ExternallyRestrainedStress(spaste=Epasteepaste)LPaste

  • Residual Stress in a Heterogenous SystemResidual stress development: (For now we will assume no creep effects to keep the problem somewhat straightforward)Internal StresssInternal ?LStress ( s ?)Under External +Internal Restraint?Agg.sdExternallyUnrestrainedExternallyRestrainedHeterogeneous

  • A Model to Investigate the Residual Stress FieldsANSYS FEA ModelQuadratic rectangular eight-node elements plane-stress Autogenous shrinkage applied using a temperature substitution analogyPaste - assumed to have a modulus of 20 GPa and a Poissons ratio of 0.20 Perfect-bond between aggregate and cement paste is assumed Length (5) to Width (1)EPaste=20 GPa, nPaste=0.2, EAgg=200 GPa, nAgg=0.3eSH-Paste =-100 me

  • Single Aggregate Prism Model - Externally Unrestrained Sample - Internal Stress( s1: MPa )Externally unrestrained sample is nearly axi-symmetric

  • Single Aggregate Prism Model - Externally Unrestrained Sample - Internal Stress( s1: MPa )Externally unrestrained sample has stress fields which are nearly axi-symmetric

  • Single Aggregate Prism Model - Externally Restrained Sample - ( s1: MPa )Externally restrained sample exhibits different behavior

  • Single Aggregate Prism Model - Externally Restrained Sample - ( s1: MPa )Externally restrained sample exhibits different behavior

  • Comparing Single Aggregate Prism Models We can see the stresses perpendicular to the B-Axis in the unrestrained specimen are higher than the other direction

  • Single Aggregate Prism Model(Bond Condition)Externally RestrainedPerfectlyBondedPerfectlyUnbondedExternally UnrestrainedPerfectly Bonded/Unbonded(Vertical Direction)Stress LocalizationVoid NoStressExternally Restrained

  • Consider Models with More than One AggregateUp to now we discussed about the residual stress development in single aggregate systems We have also been studying hexagonal unit cell models to get a better idea of what is happening in the overall system

    These hexagonal cell models were shown to be similar to the case of restrained ring elements in some earlier studies

  • Unit Cell Composite Models(Finite Element Analysis)Unit Cell Composite Model( s: MPa )Externally UnrestrainedExternally Restrained

  • Unit Cell Composite Model- Externally Unrestrained - Results indicate that residual stress increases with an increase inAggregate VolumeElastic Modulus of the AggregateResidual stresses can be high even though the specimen is externally unrestrainedThis is consistent with the measurement of acoustic activity which may correspond to microcracking

  • Unit Cell Composite Model- Externally Restrained - Results indicate that residual stress is similar withAgg. VolumeElastic Modulus of the AggregateThis may suggest that while the stiffness and volume of the aggregate are important for free shrinkage they may be less critical for cases of restrained shrinkage

  • Comparing the Heterogenous Stress and the Homogenous StressThe maximum homogenous stress significantly varies with aggregate volume and stiffness The maximum heterogenous stress does not vary significantly with elastic modulus or aggregate volumeThis suggests that external restraint in a heterogenous system requires further study

  • The Need to Include Stable Crack Development at the AggregateUp to now we discussed about the residual stress development

    It has become clear from both experimental and numerical simulations that microcracking and cracking behavior in a heterogenous composite system are important and would substantially impact modeling

    We will discuss preliminary model results though substantially more experimental and numerical studies are underway

  • Preliminary ObservationBOND CONDITION MICROCRACKING (Key issue)MicrocrackingCracking(Example: Restrained Boundary Condition)

  • NIST - OOF Simulation ProcedurePolished SurfacephenolphthaleinDefine phasesImage AnalysisSurface TreatmentMeshMaterial PropertiesMeshed imageConcreteConcrete SpecimenSaw CutPolishing

  • NIST - OOF Simulation (2-Phase: Agg. & Paste)Apply boundary condition, shrinkage strain onto cement paste phase Before crackingAfter crackingStress Analysis s10 MPa25 MPa12 MPaStrain Analysis e1- 435 me467 me0 meCracked imageAfter Cracking(Example: Externally restrained B.C.)

  • NIST - OOF Simulation (3-Phase: Agg., Paste, Interface)Interface Bond Condition3-Phase Strain Analysis e11000 me- 435 me2-Phase Analysis0 me3-Phase AnalysisPasteAggregateInterface

  • ConclusionsThe Existence of Aggregate Provides Internal Restraint Higher Internal Stress Development (sMax-Internal > sComposite)

    The Bond Condition Between Aggregate and Cement Paste - Externally Unrestrained Little role - Externally Restrained Critical

    Role of Aggregate on the Internal Stress Development - Externally Unrestrained: Higher VAgg, EAgg Higher sMax.-Internal - Externally Restrained: Not Clear (But, small changes when EAgg/Epaste > 2)

  • ConclusionsEquivalent Stress vs. Maximum Internal Stress 1) sMax-Internal > sComposite 2) The increase of VAgg : sComposite Decreases sMax-Internal Does not vary significantly It is possible to underestimate the microcracking and cracking potential of concrete if estimation is performed only using equivalent parametersFurther Information http://bridge.ecn.purdue.edu/~wjweiss

    My presentation is about the autogenous shrinkage induced residual stress development and cracking in cement composites regarding the influence ofThe difference between the equivalent stress and the maximum internal stressProvide a modeling approach for the observations of..All these preliminary observations were performed by finite element analysisTo clearly observe the mechanical behavior of a composite system,To clearly observe the mechanical behavior of a composite system,Moon here you are showing s1 and I think you are after sx and syTransfer stressSuggestion of the simulation modeling approachSuggestion of the simulation modeling approach