Research On Evaluation Methods Of Concrete Rheology And Homogeneity Under Vibration

Vibration is widely used in concrete works to facilitate the filling of fresh concrete in formworks and around of steel bars,and to expel air voids to ensure the mechanical properties and durability of concrete components.However,conventional vibration process relies on empirical judgment,and improper vibration can lead to segregation,bleeding,and the ‘voids and pits’ phenomenon,which seriously harm the mechanical properties and design working life of concrete components.Currently,existing segregation test methods still have deficiencies,and it is difficult to achieve a rapid,accurate,in situ,and real-time monitoring of fresh concrete segregation.In addition,rheology is a key indicator to evaluate the stability of fresh concrete,but conventional rheometers have significant limitations when used under vibration.Therefore,it is of great theoretical and practical significance to study the evaluation method of rheology and homogeneity of fresh concrete under vibration to improve the concrete construction quality and ensure the structure safety and durability.Based on Stokes’ law,a new technique – Pulling Ball Method(PBM)-for measuring the rheology of fresh concrete under vibration was developed.In PBM,the viscous drag applied to the steel ball was monitored at different pulling velocities.We established the quantitative relation between viscous drag and pulling velocity to determine the rheological parameters.Also,we compared the results from our new method with those from conventional rheometer.The results confirm that,the plastic viscosity and yield stress measured by PBM have a good linear correlation with those measured by rheometer.The effects of air entrained agent(AEA)content,molding temperature,vibrating modes,and CA volume fraction under vibration were systematically studied.We found that,far away from the poker vibrator,the vibration effect was improved by increasing AEA content,amplitude,vibration frequency,CA volume fraction,or by decreasing molding temperature;near the poker vibrator,however,the above conditions showed less impact on vibration effect.Moreover,we found that vibration can decrease the viscous drag of mortar by more than 90% within 200 mm from the poker vibrator,which effectively expelled large air bubbles(diameter > 500 μm)and keep tiny bubbles(diameter < 200 μm),and decreased the spacing factor.A theoretical model of non-conducting spheres in conductive medium was developed.Based on this model,we proposed a new method to calculate the electrical resistivity,and established quantitative relation between fresh concrete electrical resistivity and CA volume fraction.We verified the theoretical model by using nonconducting glass and plastic spheres as CA in experiments.The mean relative error between the calculations and the measurements was less than 3.1%.Moreover,the model predicted that CA size distribution,maximal diameter,settlement and surface morphology had little influence on the electrical resistivity.This was also verified by our experiments.Based on this method,we also evaluated the segregation index by measuring the electrical resistivity of fresh concrete at various heights.The results were compared with the sieving-washing method,and good linear relation was found.Our model and experiments laid the foundation for applying electrical resistivity method to segregation measurement.The effects of vibration on hardened concrete homogeneity were investigated by characterizing its surface air permeability,CA distribution,air void structure and microstructure characteristics of the interfacial transition zone(ITZ).The results demonstrate that,the air permeability coefficient and depth of the lower part is significantly lower than those of the upper part.The vibration results in severe segregation in the part near the poker vibrator.The proportion of tiny bubbles in the part near the poker vibrator increases significantly,while the proportion of large bubbles decreases.Under vibration,near the poker vibrator,the ITZ average elastic modulus of the lower part is increased by 54.4% compared with that of the upper part,and the ITZ porosity decreases obviously.Based on Lattice Boltzmann Method(LBM),we numerically simulated the settling process of CA in static segregation,and established the relation between Bingham number(Bn)and settling velocity.The results show that,the MultiRelaxation-Time model can be used to optimize the relaxation process by adjusting the relaxation time to meet the stability requirements.The settling velocity decreases with the increase of Bn,shortening the particle wake flow and decreasing the area of yield zone in fluids.