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Rheology of cement mortar and concrete as influenced by the geometric features of manufactured sand

Boek - Dissertatie

Manufactured sand (MS) has attracted increasing attention and been graduallyused to replace natural sand as aggregate of mortar and concrete, due to the decreasingavailability of natural sand and environmental impacts caused by over exploitation.The crushing process provides MS with irregular particle shape and rough surfacetexture. These features make the properties of mortar and concrete with MS differentfrom those with natural sand, especially in terms of rheological properties. Evaluationof the rheology of mortar and concrete with MS is not only a key step to ensuringproper flow and successful casting in formworks, but also provides guidance for mixdesign of concrete mixtures and production of MS.This thesis addresses a study about the effects of geometric features of MS on therheology of mortar and concrete mixtures. The geometric features of MS particleswere analyzed in terms of overall shape, angularity and roughness from randomsections of particles. Various MS particles with different geometric features andparticle sizes were used to prepare cement mortar mixtures, of which the rheologicalproperties were measured by means of a rheometer. The relationship between therheology of mortar mixture and the characteristics of MS was analyzed from theviewpoints of packing properties of MS, paste film thickness (PFT) and relative pastefilm thickness (R_PFT) of mixtures. A new method for measuring the specific surfacearea (SSA) was proposed for the purpose of calculation of PFT and R_PFT. Thisrelationship at mortar level was then extended to concrete level to predict the rheologyof concrete based on a multi-level biphasic approach.A 2D image-based methodology was proposed to measure the SSA of irregularaggregate by random sectioning. Factors that may influence SSA result were analyzed.Conventional methods including spherical assumption, BET and computedtomography (CT) tests were used and compared in this study. Results show thatspherical assumption provides the lowest SSA among these methods since the featureof anisotropy in dimensions is not considered. The BET method yields SSA valuesone order of magnitude higher than others, which is attributed to the fact that the BETmethod measures each position of particles that nitrogen molecule can be adsorbedon during the applied relative pressure, based on the U+2018pixelU+2019 of nitrogen molecule. Theproposed random sectioning method presents very similar SSA result compared to CTmethod, indicating that it can be considered as a reliable method. To improve thereliability of SSA by random sectioning method, the number of samples should behigh enough to reach a constant result and the thresholding algorithm should beadequate. Besides, a higher pixel resolution provides a higher SSA value. Thecomparison among these methods demonstrates that it is necessary to determine thescale at which features of the surface are supposed to be captured before selecting theoptimal testing method.The geometric features including overall shape, angularity and surface roughnessof MS particles were analyzed by aspect ratio, convexity area ratio and convexityperimeter ratio respectively. Circularity was also employed to comprehensivelyevaluate the geometric features of MS particles by measuring the similarity betweenthe particle and the perfect sphere. The number of tested particles for steady geometricparameters was analyzed and it is found 600 particles are sufficient for themeasurement of geometric features of particles used in this study. All MS particles inthis study show similar angularity results. The roughness of each MS increases as theparticle size increases while the circularity decreases with the increase of particle size.For aspect ratio, different sands show diverse trends with particle size.The plastic viscosity and yield stress of mortar mixtures was calculated based onthe Bingham model. In terms of relative plastic viscosity, the Robinson model showsoptimal fittings for all mixtures and is thus used to determine the packing fraction ofMS under shearing. From the particle packing viewpoint, shear-induced orientationincreases the packing fraction of non-spherical MS particles from the random loosepacking fraction. This influence is increasingly prominent with the decrease ofcircularity. The relative volume fraction is an important parameter influencing therelative plastic viscosity of mortar mixtures with MS whereas the relative paste filmthickness (R_PFT) is found as the dominating factor. The dependence of plasticviscosity of cement mortar on geometric features and particle size of MS can beattributed to their influences on the packing fraction and surface area of MS particles.Similarly, the relative yield stress of mixtures is dominated by R_PFT as well. Whilecomparing the fitted parameters from the relative plastic viscosity and relative yieldstress, it is found that the mixture with spherical glass beads (GB) has a higher relativeplastic viscosity than its relative yield stress while the mixtures with non-sphericalMS particles have slightly higher relative yield stress than relative plastic viscosity.Moreover, the perceived effect of MS on the plastic viscosity of mortar mixtures isinfluenced by water to cement (W/C) ratio. Increased W/C ratio slightly intensifiesthe influence of R_PFT on relative plastic viscosity. Nevertheless, the dependence ofrelative yield stress of mortar mixtures on R_PFT is independent of W/C ratio.The relationship between the relative rheology of mortar mixtures and R_PFT isextended to concrete level. The relative rheology of concrete is considered dependenton the relative mortar film thickness (R_MFT). Then the rheology of concrete withMS is predicted based on a multi-level biphasic approach. Concrete is considered asa two-phase suspension of coarse aggregates in the suspending matrix of cementmortar. The relative rheology of concrete to mortar mixtures is predicted fromR_MFT. At mortar level, the mixture is taken as a suspension with MS particles asinclusions and cement paste as the matrix. The relative rheology of mortar mixture tocement paste is predicted based on R_PFT.According to these findings, one may predict the rheology of mortar and concretewith MS. Besides, this approach can also be used for the mix proportioning ofconcrete, especially self-compacting concrete while rheology needs to be specificallyconcerned. In terms of optimization and manufacturing of aggregate, sphericalgranules with smooth surface are preferred from the rheological viewpoint.
Pagina's: XII, 194 p.
Jaar van publicatie:2021