Elaborate Simulations And Analysis Of Mechanical Behavior Of Ultra High Performance Concrete

Ultra high performance concrete(UHPC)is a new type of concrete,which is mainly composed of cement,admixtures,fibers,natural fine sand,water and so on.It has ultra-high mechanical properties and excellent durability.At the macroscale,UHPC can be regarded as isotropic material or homogeneous material.At the mesoscale,UHPC is often considered to consist of four substances,namely aggregate,fiber,matrix and Interfacial Transition Zone(ITZ).Fiber is one of the core components of UHPC.The addition of fiber changes the failure mode of UHPC from brittle fracture to ductile one.Based on experimental data,this paper systematically studied the model generation method,elastic modulus prediction,fiber pull-out mechanical behavior,size effect of UHPC and its static and dynamic flexural properties through numerical simulation and/or analytical model.In terms of performance,the mechanism of fiber reinforcement on strength and toughness of UHPC was analyzed.The relevant content and conclusions are as follows:(1)A practical and applicable method was proposed for generating the mesostructure of concrete in this study.In this method,cell fracture algorithm was used to obtain arbitrary-shaped aggregates,Surface subdivision(Catmull–Clark subdivision algorithm),Displacement mapping and Laplace smoothing algorithm were used to constructed smooth or rough surface of realstic aggregates.The geometric characteristics of fibers were obtained by Euclidean geometry.The interactions between aggregates and fibers were detected and solved by collision algorithm.The distribution of fibers can be obtained by controlling the active and passive states of the object.The influence of shape,size and volume fractions of aggregate,together with fiber's orientation on the structure and properties of concrete were studied.It was found that the volume fraction of ITZ decreased with the increase of aggregate gradation and particle size,but increased with the increase of aggregate specific surface area fiber volume fraction.This method,combined with analytical models and/or theoretical empirical formulas,can well predict the volume fraction of ITZ and the elastic modulus of UHPC.Combined with other numerical methods,such as finite element method and discrete element method,etc.,the mechanical properties of UHPC can be further studied.(2)In order to further study the elastic modulus of UHPC and establish the relationship between the elastic modulus and compressive strength of UHPC,seven different machine learning methods,namely linear regression,artificial neural network,support Vector regression,decision tree,random forest,extreme gradient boosting,and k-nearest neighbors,are used to predict the elastic modulus of UHPC.It was found that XGBoost performed the best when using large training data sets,followed by KNN.When the training data set was small,the prediction accuracy of decision tree was the best,followed by XGBoost.The prediction accuracy of linear regression was the lowest in both cases.(3)Based on the pull-out experiments of straight,hooked and corrugated fiber in UHPC with different curing ages,numerical models and three analytical models were established respectively to predict the fiber-matrix bond behaviors.In this paper,a modified interfacial friction law(MIFL)considering reinforcement factor was proposed.In this law,the enhancement factor increased with the increase of the curing age of UHPC.The growth rate was fast at early stage,and then tends to be stable.The reinforcement factor of straight fiber was the largest,followed by corrugated fiber,and the smallest was hooked fiber.In the analytical models,frictional force following a power function was considered.For the hooked and corrugated fiber,the pull-out energy at the plastic hinge was calculated by the elastic-plastic moment,and the rotation force was calculated by the law of conservation of energy.Predicted results from those numerical and analytical models were compared with the existing models and corresponding experimental results.The comparisons indicated that the enhancement factor employed in the MIFL efficiently reflected the change in fibermatrix bond properties with age.The predicted pullout load-slip relationships and energies from those proposed models showed higher accuracy than those from the existing models.(4)To further investigate the static and dynamic flexural properties of UHPC,three 2D homogenization finite element models with different sizes of 75 × 75 × 300,100 × 100 × 400 and 150 × 150 × 550 mm were established.Based on commercial finite element software ABAQUS,the size effect of UHPC with short straight,short hooked fiber and long hooked fiber was analyzed.It was found that the static flexural performance of UHPC decreased with the increase of beam size.The static flexural performance of UHPC with 0.5% short straight and 1.5% long hooked fiber was the best,and the worst was UHPC with 2% straight fiber.In addition,according to the proposed method(2),a two-dimensional mesoscale model and a three-dimensional multi-scale model of UHPC with mono and hybrid short straight and long hooked fiber were generated.The model size was 250 × 40 × 10 mm.The dynamic flexural properties of UHPC with mono and hybrid steel fibers were studied in national supercomputing center in Changsha.ANSYS Workbench software was used to generate finite element mesh,and Autodyn was used to analyze the impact resistance of UHPC.It was found that the geometry and content of fiber had a significant impact on the dynamic flexural properties of UHPC.Short straight fibers bridged microcracks,while long hooked fibers played an important role in preventing the propagation of main cracks.Fibers under the hammer head dissipated energy through deformation(including bending,twisting,deflection and pulling out,etc.).UHPC with 1% long hooked and 1% short straight fiber had the best dynamic flexural performance,and the worst was the plain UHPC without fiber.This study provides a simulation framework for the impact resistance of UHPC based on three-dimensional multi-scale model,which can obtain the real-time stress-strain and deformation characteristics of fibers in UHPC,and provide reference for further research on UHPC dynamic behaviors.