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Plastic cracking of concrete and the effect of depth

dc.contributor.advisorCombrinck, Riaanen_ZA
dc.contributor.authorSteyl, Lourensen_ZA
dc.contributor.otherStellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.en_ZA
dc.date.accessioned2016-12-22T13:45:37Z
dc.date.available2016-12-22T13:45:37Z
dc.date.issued2016-12
dc.identifier.urihttp://hdl.handle.net/10019.1/100369
dc.descriptionThesis (MEngSci)--Stellenbosch University, 2016.en_ZA
dc.description.abstractENGLISH ABSTRACT: Plastic settlement and shrinkage cracking dominate cracking in the early life of conventional concrete. The mechanisms responsible for these cracking types include differential settlement and restrained shrinkage. The driving force behind differential settlement and restrained shrinkage is settlement and capillary pressure build-up (shrinkage). These plastic cracks are largely a problem for slab-like elements with large exposed surfaces, where the cracks act as pathways for degrading agents to penetrate the concrete. These premature detrimental defects can decrease the service life of the structure. Plastic cracks can be prevented by using effective construction techniques and/or applying the correct finishing operations and curing techniques at the appropriate time. Nevertheless, these cracks still occur due to ignorance towards effective construction techniques as well as towards concrete mix designs, and due to the lack of knowledge of the interaction between these cracking types. This is especially true as a result of the limited test methods that exist to isolate plastic shrinkage from plastic settlement cracking. The study aims to isolate and understand plastic shrinkage cracking as well as describing the interaction between settlement cracking and shrinkage cracking. This was done by designing a mould capable of inducing a single isolated plastic shrinkage crack, or settlement induced plastic shrinkage crack. Finally, the framework for a plastic cracking risk model is introduced, which can aid engineers and contractors in appropriate cracking risk estimations. The test results aided in the understanding of both cracking types for different mixes and climates, as well as identifying and describing their cracking mechanisms. Three dominant cracking phases in plastic concrete were identified along with three distinct cracking types. The three cracking types are plastic settlement cracking, plastic settlement induced plastic shrinkage cracking, and pure plastic shrinkage cracking. Phase 1 starts once the concrete is placed, and ends once the capillary pressure starts to rise. Pure settlement cracking occurs only in this phase and can be accompanied by the start of settlement induced plastic shrinkage cracking. Phase 2 starts once the capillary pressure starts to rise, and ends once settlement reaches its maximum. Pure plastic shrinkage cracking can start in this phase, as well as further widening of a settlement induced plastic shrinkage crack. Phase 3 starts once the settlement stops, and ends once the concrete hardens and reaches the final setting time. This end of Phase 3 is characterised by the stabilisation of plastic cracking. Furthermore, the investigation of the effect of depth and finishing operations on plastic cracking, proved invaluable in describing the different cracking phases and their respective mechanisms. For deeper sections the severity of pure plastic shrinkage cracking decreases while increasing the severity of plastic settlement cracking. If these two cracking types interact in deeper sections, the cracking severity drastically increases. Finally, the plastic cracking model presented, showed great potential in predicting the risk, and especially the interaction risk, between the plastic cracking types. The empiric model makes use of dominant influencing factors of plastic cracking to ultimately calculate the risk estimation. This model should be improved by future studies, which can further estimate the effect of the influencing factors, especially using large scale tests.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Plastiese versakking en krimpkrake oorheers kraakvorming in die vroeë lewe van konvensionele beton. Die meganismes wat verantwoordelik is vir hierdie kraak tipes sluit in differensiële versakking en opgehoude inkrimping. Die dryfkrag agter differensiële versakking en krimpkrake is versakking en die opbou van kapillêre druk. Hierdie plastiese krake is hoofsaaklik 'n probleem by bladagtige elemente met groot blootgestelde oppervlaktes waar die krake optree as deurgange vir afbrekende middele in die beton. Hierdie vroeë nadelige defekte kan die diensleeftyd van die struktuur verminder. Plastiese krake kan voorkom word deur die gebruik van effektiewe konstruksie tegnieke en/of die uitvoering van die korrekte afwerkings metodes en kuurtegnieke op die regte tyd. Nietemin ontstaan hierdie krake steeds as gevolg van onkunde oor effektiewe konstruksietegnieke en betonmengselontwerp sowel as die gebrek aan kennis oor die wisselwerking tussen die verskillende krake. Dit is veral weens die gebrek aan toetsmetodes om plastiese krimpkrake te isoleer van plastiese versakkings krake. Die studie beoog om plastiese krimpkrake te isoleer en te bestudeer en gevolglik die interaksie tussen versakkings krake en krimpkrake te beskryf. Dit is gedoen deur die ontwerp van 'n vorm wat of ‘n enkele plastiese krimpkraak kon isoleer. Laastens is die raamwerk van ‘n plastiese kraak risiko model bekend gestel wat ingenieurs en kontrakteurs kan help met toepaslike skattings vir kraakrisiko. Die toetsresultate het gehelp om beide kraak tipes en mengsel eienskappe te verstaan sowel as om kraak meganismes te beskryf. Daar bestaan drie dominante plastiese kraak fases in beton asook drie kraak tipes: Plastiese versakkings krake, versakking geïnduseerde plastiese krimp krake en suiwer plastiese krimp krake. Fase 1 begin sodra die beton geplaas is en dit eindig sodra die kapillêre druk begin styg. Suiwer versakkings krake kom slegs in hierdie fase voor en kan vergesel word deur die begin van versakking geïnduseerde plastiese krimpkrake. Fase 2 begin sodra die kapillêre druk begin styg en dit eindig sodra die versakking ‘n maksimum bereik het. Suiwer plastiese krimpkrake begin in hierdie fase, sowel as by die begin of verdere vergroting van die versakking geïnduseerde plastiese krimpkrake. Fase 3 begin sodra die beton se versakking ophou en dit eindig sodra die beton verhard en die finale set tyd bereik. Die einde van hierdie fase word gekenmerk deur die stabilisering van plastiese krake. Verder was die ondersoek oor die invloed van diepte sowel as oor afwerkings praktyke op plastiese krake, van onskatbare waarde vir die beskrywing van die verskillende kraak fases en hul onderskeie meganismes. Suiwer plastiese krimp krake verklein in dieper beton snitte terwyl plastiese versakkings krake vergroot. Die interaksie tussen die twee kraak tipes lei tot groter krake in dieper beton snitte. Laastens het die plastiese kraak model wat bekend gestel is, groot potensiaal getoon vir die voorspelling van die kraakrisiko en veral vir die interaksie risiko tussen die verskillende kraak tipes. Die model maak gebruik van dominante faktore wat plastiese krake beïnvloed. Hierdie model kan verbeter word deur toekomstige studies wat die effek van die verskillende invloeds faktore kan bestudeer deur veral grootskaalse toetse te gebruik.af_ZA
dc.format.extent99 pages : illustrationsen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.subjectUCTDen_ZA
dc.subjectPlastic concrete -- Shrinkage -- Contractionen_ZA
dc.subjectPlastic concrete -- Shrinkage -- Preventionen_ZA
dc.titlePlastic cracking of concrete and the effect of depthen_ZA
dc.typeThesisen_ZA
dc.rights.holderStellenbosch Universityen_ZA


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