Design Of UHPC Modified By Porous Powder And Optimization Mechanism Of Mesoscopic Packing Structure

The optimization of meso-scale stacking structure of ultra-high performance concrete(UHPC)is an effective way to improve the performance of concrete.At present,a large number of studies have used discrete element simulation to focus on the accumulation optimization of solid particles,and systematically explored the influence mechanism of key parameters such as particle morphology and particle size on the accumulation structure.However,the accumulation of UHPC is a typical suspension accumulation system involving liquid phase.The adsorption of water by particles directly affects the packing density,and the applicability of the law based on the solid phase accumulation system is doubtful.In this study,the molecular dynamics simulation was used to construct the interparticle shear model.Taking the inter-particle friction force as the main consideration index,the correlation mechanism between the particle surface morphology-water adsorption / lubrication effect and the packing density was explored,and the ’water reduction’ mechanism of the particle surface defects was revealed.The traditional cognition that the packing density increases with the increase of sphericity in the traditional packing was broken,and the optimization of the meso-stacking structure was realized.On this basis,the influence of the addition of this functional filler on the macroscopic properties,microstructure and hydration process of UHPC was systematically studied,which provided a new research idea of the meso-scale stacking structure of water-covered particles and the improvement of UHPC performance.The main research contents and results of this paper are as follows:(1)Based on molecular dynamics simulation,a model of shear action between cement hydration layer(C-S-H)and defect particles was constructed.The internal friction force was calculated by intermolecular force,and the lubrication effect between particles was evaluated by internal friction force.The simulation results show that the defect particles can reduce the contact area between the surface silicon hydroxyl and water molecules,increase the contact area with water molecules,effectively reduce the internal friction,improve the lubrication between particles,and optimize the stacking system.(2)Using the above porous powder particles,based on the MAA model,a dense accumulation system was constructed to guide the design of UHPC and verified by experiments.The rheological property experiment further proves the lubrication effect of the functional filler,and the wet packing density experiment also shows that the addition of the functional filler can also effectively construct the UHPC system with improved packing density.(3)The hydration process,microstructure and macroscopic properties of UHPC reinforced by the meso-stacked structure of gear-like fillers were systematically evaluated.The results showed that the addition of functional particles did not change the type of UHPC hydration products,but could effectively accelerate the hydration process.The designed UHPC has excellent strength and durability(the compressive strength of 28 d is153.9 MPa,and the penetration depth of 28 d is 0 mm).At the same time,it has the advantages of reducing hydration heat and reducing the risk of cracking caused by temperature and thermal stress.