| Self-compacting concrete(SCC)has excellent construction and filling performance,which can solve the problems of leakage vibration,over-vibration,and difficult vibration caused by dense reinforcement in traditional concrete construction.The design and control of SCC rheological properties are the key to achieving selfcompacting.However,due to the large number of raw materials of SCC and the complex interaction effect among the components,the relationship between the component properties and rheological properties still needs in-depth and systematic studies.At present,the effect of microscopic inter-particle forces and aggregate properties on the SCC rheological properties needs to be further developed.Therefore,in this paper,SCC was decomposed into solid-liquid two-phase bodies at three different scales:water-powder,paste-fine aggregate,and mortar-coarse aggregate.The interactions between each two-phase body and the correlation between two adjacent levels were studied.With the core of paste rheology performance,on the one hand,the influence mechanism of water reducer,water,and cementitious materials on the rheological performance was studied based on the inter-particle interaction force and related theories,and established the rheological model of paste based on microstructural parameters;on the other hand,through the interaction between paste and fine aggregate,mortar and coarse aggregate,the influence and mechanism of aggregate characteristics on the rheological performance of the mixer were studied,the rheological model of cement mortar/concrete was established based on the surplus slurry theory.The main contents and conclusions of this study are as follows:(1)A method for testing the inter-particle force of cementitious materials based on atomic force microscope(AFM)and a method for calculating the Hamaker constant are proposed.The force-distance curves of standard AFM probes-particle and particleparticle were tested in air,ultrapure water,and water-reducing agent solution,respectively.Typical values of the inter-particle Hamaker constant(effective Hamaker constant)for the cement and four commonly used mineral admixtures(fly ash,slag,limestone powder,silica fume)under different conditions were given,which provided theoretical basis for optimizing the storage and transportation and for exploring the mechanism of mineral admixture and water-reducer on the rheological behavior of cement paste.The results showed that the Hamaker constant between different particles in air ranges from 11.21 to 3 8.26×10-20 J.However,the interaction force and effective Hamaker constant between the particles in water are significantly reduced by more than 50%compared with those in air.When the water reducer was added,the repulsive force with 1.25-1.61 nN was generated because of electrostatic and steric hindrance of the water reducer molecules,which further significantly reduced the effective Hamaker constant.(2)The equivalent Hamaker constant of cement paste suspension system was proposed based on the effective Hamaker constant and the collision probability between particles.The influence of single and multiple mineral admixture on the flowability,yield stress and plastic viscosity of cement paste was revealed by combining with the accumulation and friction characteristics of powder materials in the dry state and the equivalent Hamaker constant.The results showed that mineral admixture had significant effects on the accumulation and friction properties of powder materials under dry conditions,which determined the rheological behavior of paste.The rheological properties,yield stress and plastic viscosity of the paste can be improved by reducing the specific surface area,bulk porosity,and friction coefficient of the powder material or by reducing the effective Hamaker constant of paste.(3)The equivalent Hamaker constant of cement-mineral admixture-water reducing agent system was calculated based on the effective Hamaker constant between particleparticle in water reducing agent solution.Based on the adsorption capacity of water reducer and Zeta potential on the particle surface,and the microstructure of the newly mixed paste,combined with the equivalent Hamaker constant,the internal mechanism of polycarboxylate acid high range water reducer changing the rheological properties of the freshly mixed paste was revealed.The results show that:the dispersion and water reducing effect of polycarboxylic acid high range water reducer on the paste mainly depends on its spatial site resistance.The size and number of flocs in the fresh paste decreased significantly after the addition of water reducer.When the dosage of water reducer was less than the saturated dosage,with the increase of water reducer dosage,the yield stress decreased and the rheological behavior index increased,and the paste gradually changed from shear thinning to shear thickening.There is a significant relationship between the rheological parameters of the paste and the equivalent Hamaker constant.(4)Based on digital image processing technology,the aggregate image measurement and analysis system AIMS was used to quantitatively characterize the geometric shape and surface texture of aggregate through the shape index,angular,texture,sphericity,flat&slender,flat or slender,angular texture and other parameters.Through the statistical analysis,the main aggregate parameters that affect the rheological properties of mixing were determined and the macro influence was established.Combined with the aggregate density,void ratio,specific surface area,friction characteristics and the aggregate shape and texture parameters,the influence and mechanism of aggregate characteristics on the rheological properties of mortar and SCC were revealed.The results show that reducing the shape index,angularity and specific surface area of fine aggregates or reducing the stacking void ratio by optimizing the fine aggregate grading can improve the flowability and reduce the yield stress and plastic viscosity of fresh mortar.Reducing the gradient angularity and texture of coarse aggregates or increasing their sphericity is beneficial to improve the rheology of SCC.CAAT integrally quantifies the geometry and surface texture of coarse aggregates,which show good linear relationships with the extension,T500,static yield stress and plastic viscosity of SCC.(5)For the cement paste,a yield stress model considering the particulate material volume fraction,particle size distribution,and equivalent Hamaker constant of the suspension system was established;In addition,a plastic viscosity model based on porosity,equivalent Hamaker constant and water-cement ratio was also established.According to the two-phase theory and surplus paste theory,the rheological models of cement mortar were established based on the significance rheological parameters of paste,fractal dimension of fine aggregate gradation,gradient angularity of fine aggregate and thickness of excess paste.Similarly,the rheological parameter models of SCC were established based on mortar rheological properties,coarse aggregate properties,thickness of excess mortar and time.The results of ANOVA and coefficient analysis of the rheological models showed that the established three-different-scale rheological models were statistically significant and the rheological parameters were significantly related to the selected multiple variables.The predicted values of the model were consistent with the experimental values,indicating that the established rheological models have high accuracy and precision and can provide guidance for the design and application of self-compacting concrete. |
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