Modelling and Quantification of the Effect of Mineral Additions on the Rheology of Fresh Powder Type Self-Compacting Concrete

摘要

Powder type self-compacting concrete (SCC) mixtures are characterised by a higher powder content compared to traditionally vibrated concrete (TC) mixtures in order to increase the viscosity of the concrete mixture and thus achieving a sufficiently high resistance to segregation, while a third generation (polycarboxylate ether based, PCE) superplasticizer is used in order to increase the flowability of the SCC mixture. Besides cement, powder type SCC mixtures mostly incorporate readily available mineral additions like limestone powder, quartz powder, fly ash or silica fume in order to reduce heat generation during cement hydration.The objective of this work is to describe the workability of powder type SCC by means of a rheological approach. In this way, a fundamental description of the concrete flow behaviour during placement is obtained, which is indispensable in case of special concretes (such as SCC) and/or in cases of congested reinforcements.For fresh concrete, two major different rheological approaches can be used, depending on the scale of observation: the suspension approach (local scale) or the continuum/fluid approach (global scale). In this thesis, mainly the global flow behaviour of fresh SCC is treated and a continuum/fluid approach is used in order to describe the concrete flow behaviour.Due to the viscoplastic behaviour of the fresh concrete, it is generally agreed that, as a good first approximation, fresh concrete can be described by a linear flow curve, according to the Bingham model. However, test results on powder type SCC mixtures revealed that a non-linear flow curve (Herschel-Bulkley model) is often needed in order to describe more correctly the shear rate-shear stress relationship.The wide-gap Couette concentric cylinder rheometer used for the experiments, together with the theoretical derivation of the solution of the Couette inverse problem by means of the integration method for both a Bingham and a Herschel-Bulkley fluid are presented in this work.The description of the shear thickening flow behaviour of powder type SCC in terms of shear rate and shear stress can be used as input for the global scale (CFD) simulations of powder type SCC processing operations (e.g. casting, mixing or pumping) and can be considered as a small step into the promising direction of (fresh) concrete flow simulation/prediction tools.To illustrate the importance of predicting the thixotropic behaviour for a given (powder type) SCC mixture, two practical applications, i.e. formwork pressure and distinct-layer casting are discussed.It was found that the Herschel-Bulkley parameters are reliable, absolute rheometry parameters (at least for the shear rate region actually tested). The wide-gap concentric cylinder rheometer provides a reproducible test procedure, from which the parameters and their influencing factors can be derived.Fresh powder type SCC is susceptible to a shear thickening flow behaviour. This is due to its intrinsic mix design philosophy, i.e. the combination of a sufficient amount of small (powder)particles and a low amount of coarse aggregates (where the hydrodynamic forces dominate), which promotes cluster formation.Furthermore, the addition of a PCE superplasticizer will result in a more dispersed state of the smaller (powder)particles, so that a larger amount of these particles is available for shear thickening, while test results also indicated that the molecular design of the PCE superplasticizer has most-likely a non-negligible impact on the shear thickening effect.The intensity of the shear thickening effect can be modified by the nature and fineness of the mineral addition used. It was found that the limestone, quartz and fly ash addition used in this research project respectively increase, unalter and decrease the shear thickening intensity. Increasing the fineness of the limestone addition resulted in a higher shear thickening intensity.Microstructural interpretations of the rheological behaviour are proposed, based on the order-disorder transition theory and the cluster theory, complemented with local mechanical actions caused by the shear effect.