Research On Design,performance And Mechanism Of High Performance Concrete Based On Manufactured Sand Morphology

In recent years,with the rapid development of infrastructure in China,the amount of river sand used has been huge,and the reserves have sharply decreased.High quality natural sand resources have been almost depleted.In order to protect the ecological environment,national and local governments have successively issued bans on natural sand mining,resulting in many regions no longer having natural sand available.The use of manufactured sand instead of natural sand to prepare high-performance concrete is an inevitable trend in future development.Due to the complex morphology,multiple edges,rough surface,and high content of stone powder of manufactured sand,the accumulation density of aggregates is small,the fluidity of concrete is poor,and the development of pore structure is poor.After hardening,the density of concrete decreases,resulting in poor mechanical properties.The morphology of manufactured sand directly determines the stacking state of particles,the flowability of fresh concrete slurry,the interface transition zone between aggregate and matrix,and the mechanical properties of hardened concrete.It is the key core element that determines the mechanical properties of manufactured sand concrete.However,the current research on the morphology of manufactured sand is not in-depth enough,especially in the precise three-dimensional morphology characterization.The study on the influence of morphology on the micro mechanism,rheological properties,and mechanical properties of concrete is not yet systematic,and there is a lack of corresponding prediction models.Therefore,using modern advanced analysis and testing techniques to accurately characterize the morphology of manufactured sand and studying the influence mechanism of the characteristic parameters of manufactured sand morphology on the stacking,flow,and mechanical properties of concrete is a necessary condition for achieving high-performance manufactured sand concrete.This article first studies the characterization parameters of mechanical sand morphology through scientific methods,and independently develops a method for adjusting the morphology parameters based on accurately characterizing the morphology parameters.Secondly,the influence of morphology parameters on the stacking performance of manufactured sand particles was studied.The compactness of manufactured sand stacking determines the thickness of the excess slurry layer in concrete.Based on the theory of excess slurry,a series of manufactured sand concrete mix proportions with different strength levels were designed.Using the designed mix ratio,the performance evolution mechanism of sand concrete with different morphology parameters was first studied from a microscopic perspective,and then the rheological properties of mortar with different morphology parameters were studied.Finally,based on the rheological test results,the mechanical constitutive relationship of mechanical sand concrete based on flowability optimization was studied.Finally,the main innovative achievements achieved are as follows:(1)A Method for Characterizing and Adjusting the Morphological Parameters of Mechanized Sand Based on CT Scanning TechnologyBased on CT scanning and image processing technology,the 3D particle shapes of various particle sizes of machine sand and river sand were accurately reconstructed,and four dimensional parameters for quantitatively characterizing the morphology of sand particles were proposed: sphericity,angularity,roughness,and specific surface area.It was found that the sphericity of each particle size fraction of manufactured sand is smaller than that of river sand,and the angularity and roughness of each particle size fraction of manufactured sand are greater than those of river sand.At the same time,it was found that as the particle size increased,the sphericity of manufactured sand first increased and then decreased,while the sphericity of river sand gradually decreased.The angle,roughness,and sphericity of each particle level showed a reverse change pattern with the particle size.At the same time,it was found that there is a certain relationship between the four dimensional morphology parameters and a more accurate mathematical expression was found.Design and manufacture a square hole drum screen for grading adjustment,while a strip drum screen is used for particle shape adjustment.First,use a square hole drum screen to classify the manufactured sand according to its particle size,and then use the corresponding strip drum screen to optimize the morphology.After screening all the sheet-like particles of single particle size manufactured sand with a size of 4.75-10 mm,2.36-4.75 mm,and 1.18-2.36 mm,the sphericity of the optimized morphology manufactured sand increased by 3.14%,2.98%,and 1.90%,respectively.(2)Establishment of PFC3 D Particle Simulation Stacking Model and Optimization of CPM Stacking ModelA typical database of PFC3 D based on the true morphology of mechanism sand was established based on CT scanning mechanism sand morphology data.The database contains nine typical mechanism sand particles with sphericity ranging from 0.375,0.425,0.475,...,and 0.775 morphology ranging from poor to excellent.According to statistics,it was found that the sphericity of each particle size mechanism sand follows the Weibull distribution law.Based on shape parameters and scale parameters,the generation ratio of typical particles was selected to ensure that the simulated data particles follow the same distribution.Firstly,conduct actual single particle mechanical sand stacking tests,followed by numerical simulation of single particle mechanical sand stacking tests.The average relative errors of loose and tight stacking compactness simulated by single particle mechanical sand are 2.39% and 0.55%,respectively.Secondly,conduct stacking tests on actual full size manufactured sand,and then conduct numerical simulation tests on the stacking of full size manufactured sand.The relative errors of loose and tightly packed compactness for continuous level manufactured sand simulation are 0.54% and 0.46%,respectively.Finally,based on the results of single particle stacking and the improvement of the CPM model by morphology parameters,the packing density of full particle sized manufactured sand can be predicted.By optimizing the particle shape of the CPM model,the prediction accuracy of the CPM model for virtual compactness can be improved by 6 times,and the relative error of different grading predictions can be reduced to 3%.(3)Design Theory and Preparation Technology of High Performance Concrete Mix Proportion with Mechanized SandA high-performance mechanism sand mix design theory based on morphology was proposed using the morphology parameters calculated in this article.Firstly,based on the theory of maximum density and rheological principles,a ternary composite cementitious system suitable for high-performance concrete made of manufactured sand is prepared using fly ash and slag with different proportions,taking the maximum flow performance and strongest mechanical performance as criteria.This system can fully stimulate the potential effectiveness of various cementitious materials,leverage the complementary promoting effect between them,and set a prerequisite for the addition of manufactured coarse and fine aggregates;Secondly,based on the morphology characteristics of mechanism sand,the maximum compact packing design theory and surplus slurry theory are used to study the influence of aggregate morphology and sand content on the porosity of sand and gravel aggregates,in order to achieve the principle of minimizing the porosity of sand and gravel aggregates and select a reasonable sand content.Finally,based on the verification of flowability and strength tests,determine the design method for the mix proportions of different strength grades C30,C40,C50,C60,C70,and C80 series of manufactured sand concrete.(4)Evolution law of microstructure and performance of high-performance concrete with manufactured sand.According to the self-developed square hole drum screen and strip drum screen,the flaky particles in the machine sand are screened out,and the machine sand concrete with optimized morphology is prepared according to the designed C40 concrete mix ratio.A systematic study was conducted on the microstructure evolution laws of chemical composition,interface transition zone,pore structure,and other microscopic properties of optimized morphology mechanism sand concrete and undisturbed mechanism sand concrete at different stages of 3,7,and 28 days.The influence of mechanism sand morphology on chemical reactions and physical changes during the entire hydration process was deeply analyzed.The C-S-H and CH gels generated in the interface transition zone of the manufactured sand concrete with optimized morphology are more thick and plump.The pore structure is more dense,with a decrease of 13.99% in porosity and 4.14% in average pore size,respectively.The calcium silicon ratio of concrete near and far sand points decreased by 36.8% and 13.04%,respectively,while the calcium aluminum iron content of near and far sand points decreased by 33.96% and 19.51%,respectively.(5)Rheological Model of Mechanized Sand High Performance Concrete SlurryBased on the mix design results of mechanism sand concrete,the designed C40 concrete mix is formed by removing coarse aggregate to form a concrete slurry mix,which is also known as the mortar mix.Five types of sand samples with different morphologies were used,each with four dimensional morphology characterization parameters.After assigning different weights to each morphology parameter through entropy method,it was transformed into a comprehensive morphology parameter,so that a sand sample was only characterized by one comprehensive morphology parameter.The shear rate shear stress curves of mortar with different sand samples and different sand volume fractions were tested using a mortar rheometer.The classic Chateau rheological model was modified by comprehensive morphology parameters.This modification further improved the accuracy of Chateau model in predicting concrete rheological properties.The correlation coefficient of single particle mortar fitting increased by 0.54% compared to the original model,and the correlation coefficient of full particle mortar fitting increased by 1.39% compared to the original model.(6)Mechanical Constitutive Model of Mechanized Sand High Performance ConcreteBased on the research results of the morphology of mechanism sand on the rheological properties of concrete slurry,the mechanical constitutive behavior of granite mechanism sand with the greatest impact of morphology on the rheological properties of slurry is studied.According to the designed concrete mix proportions for each strength level,prepare granite manufactured sand as the basic mix proportion and optimize the mix proportion(with an additional 1.65 times the amount of water reducing agent added).Using a concrete pore structure analyzer to monitor the air content of each mix proportion,it was found that the average slump values of the optimized group were 8.56% and 13.93% higher than the reference group,respectively.The peak stress and elastic modulus of the optimized group’s mechanism sand concrete were higher than those of the reference group,and the average values of peak stress and elastic modulus increased by 11.37% and 3.84%,respectively.On the contrary,the average value of peak strain decreased by 6.35%,and the correlation coefficients predicted by the proposed mechanical sand concrete damage constitutive model were all above 0.995.