The influence of rheology on the cracking of plastic concrete

Kolawole, John Temitope (2020-03)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Plastic cracking occurs during concrete’s early hours and impairs its serviceability and durability. The early hours refer to the time of mixing to the time around the final set. Concrete possesses rheological properties during the plastic phase while settlement and shrinkage leading to plastic cracking also occur during this phase. Therefore, there is bound to be an interaction between the concurrent concrete’s rheological behaviour and cracking behaviour. During this same plastic phase, concrete possesses pronounced rheo‐viscoelastic properties that influence the plastic cracking behaviour. The heterogeneous nature of concrete coupled with its time‐dependent behaviour (due to hydration) at early hours makes investigations into the above‐identified properties in relation to plastic cracking complicated and lacking in the literature. With this in mind, the goal of this study was to establish links between the rheo‐related properties of concrete and its plastic cracking. Experimental investigations started with concrete mixes designed for varied rheological properties but similar hardened properties. Rotational and dynamic shear rheometries were employed to characterise the plastic phase of the concrete in order to establish the shear rheo‐physical and rheo‐viscoelastic properties. The plastic cracking behaviour of the concrete mixes were also investigated. Analytical methods were, thereafter, used to predict the occurrence of plastic cracking of the concrete mixes. Results show that concrete can both be thixotropic and rheopectic due to rheology modifiers and condition pre‐history. Various thixotropy evaluation methods adopted for the study showed similar trends except for the hysteresis loop area that was dependent on the concrete’s initial set condition and was, therefore, pegged as merely suitable for qualitative measurement. Plastic concrete’s viscoelastic behaviour is majorly influenced by hydration, coarse solid volume fraction and constituent materials such as rheology modifiers. Furthermore, it was discovered that plastic concrete possesses pseudo‐strain hardening ability under the application of linear (microscopic) shear strain. This was similar to the physical (macroscopic) response earlier detected as shear thickening and rheopexy. The study further shows that self‐settlement is the major dominating process during the early plastic phase of concrete’s cracking behaviour and takes the form of a gravitational shearing process. This self‐settlement is directly linked to the yield stress and thixotropy of the concrete. This same process influences the plastic shrinkage and rate of capillary pressure during the period of the self‐settlement, thereby, linking the plastic shrinkage and rate of capillary pressure to the yield stress and thixotropy. Plastic concrete generally possesses the inherent ability to relax early restrained cracking stress, but its ability for strain dissipation heavily depends on the material constituents such as rheology modifiers. The analytical methods for prediction revealed that the plastic concrete damage tends to be strain‐oriented with a pressure‐insensitive form of ductile failure. In addition, the cracking strains during the self‐settlement period are mainly shear‐related. Finally, it was proposed that it is possible to control the concrete mix design to avoid damage/microcracking associated with the plastic phase.

AFRIKAANSE OPSOMMING: Plastiese kake kom gedurende die vroeë ure van beton voor en verswak die diensbaarheid en duursaamheid daarvan. Die vroeë ure verwys na die tyd van meng tot die tyd rondom die finale set. Beton besit reologiese eienskappe gedurende die plastiese fase, terwyl versakking en krimping ook gedurende hierdie fase plaasvind. Daarom sal daar interaksie tussen die reologiese gedrag van die beton en plastiese krake wees. Gedurende dieselfde plastiese fase besit beton duidelike reovisko‐elastiese eienskappe wat die plastiese kraakgedrag beïnvloed. Die heterogene aard van beton, gekoppel met die tyd‐afhanklike gedrag daarvan (as gevolg van hidrasie) in die vroeë ure, maak ondersoeke na die bogenoemde eienskappe met betrekking tot plastiese krake ingewikkeld, en dit ontbreek in die literatuur. Hiermee in gedagte, was die doel van hierdie studie om die skakels tussen die reo‐verwante eienskappe van beton en plastiese krake te bepaal. Eksperimentele ondersoeke het begin met betonmengsel ontwerp vir verskillende reologiese eienskappe maar soortgelyke verharde eienskappe. Rotasie en dinamiese skuif reometrie is gebruik om die plastiese fase van die beton te karakteriseer om die skuif reofisiese en reovisko‐elastiese eienskappe te bepaal. Die plastiese kraakgedrag van die betonmengsels is ook ondersoek. Daarna is ontledingsmetodes gebruik om die ontstaan van plastiese krake van die beton te voorspel. Resultate toon dat beton beide tiksotropies en reopekties kan wees as gevolg van reologie‐modifiseerders en voorgeskiedkundige toestand. Die verskillende tiksotropiese evaluasiemetodes gebruik vir die studie het soortgelyke tendense getoon, behalwe vir die histerese lus‐area wat van die beton se aanvanklike versakkingstoestand afgehang het en daarom slegs geskik is vir kwalitatiewe meeting. Die visko‐elastiese gedrag van beton word grootliks deur hidrasie, growwe soliede volume‐fraksie en samestellende materiale soos reologie‐modifiseerders beïnvloed. Verder is gevind dat plastiese beton pseudo‐vervormingsverhardingsvermoë onder die aanwending van liniêre (mikroskopiese) skuifspanning besit. Dit was soortgelyk aan die fisiese (makroskopiese) gedrag wat vroeër as skuifverdikking en reopeksie waargeneem is. Die studie toon verder dat self‐versakking die hoof dominerende proses gedurende die vroeë plastiese kraakgedrag van beton is en die vorm van 'n gravitasie skuifproses aanneem. Hierdie self‐versakking word direk verbind met die skuifvloeispanning en tiksotropie van die beton. Dieselfde proses beïnvloed die plastiese krimping en die tempo van kapillêre druk gedurende die tydperk van die self‐versakking, wat die plastise krimping en tempo van kapillêre druk met die skuifvloeispanning en tiksotropie verbind. In die algemeen besit plastiese beton die inherente vermoë om vroeë plastiese kraakvorming‐spannings te ontspan, maar die vermoë is grootliks afhanklik van die materiaal‐bestanddele soos reologie‐modifiseerders. Die ontledingsmetodes vir voorspelling het onthul dat die plastiese beton‐skade geneig is om spanningsgeoriënteerd te wees met 'n druk‐onsensitiewe vorm van duktiele faling. Bykomend is die kraakvorming‐spannings gedurende die self‐versakkingstydperk hoofsaaklik skuif verwant. Laastens is voorgestel dat dit moontlik is om betonmengsels te ontwerp om beskadiging/mikrokrake wat met die plastiese fase geassosieer word, te beheer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107774
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