Study On Particle Size Effect Of Concrete Aggregate Via Mechanical Experiments And Peridynamics

Concrete is a multiphase composite material consisting of aggregates,hardened cement paste and the interfacial transition zone between them.Aaggregates can account for more than 60%of the total volume of concrete.Aggregate properties,especially aggregate particle size,are crucial factors affecting the homogeneity and mechanical properties of concrete.Nevertheless,the mechanism of the interaction between the aggregate particle size and the hardened cement paste is still unclear,and the fracture of concrete under load needs to be studied urgently.Peridynamics can avoid the limitations of classical continuous mechanics and make accurate prediction of fracture behavior of materials.Therefore,based on the tests about mechanical properties of concrete and numerical simulations of Peridynamics,three strength grades of concrete(ultra-high strength concrete(120 MPa),high strength concrete(80 MPa)and normal concrete(30MPa))containing two kinds of aggregates(high strength aggregate and low strength lightaggregate)were designed for investigating the effect of the aggregate particle size on the mechanical properties of concrete.In addition,by establishing peridynamic numerical models,the compressive state and failure damage of numerical specimens under uniaxial compressive load were simulated.Furthermore,the mechanism of the effect of aggregate particle size on the mechanical properties of concrete was explained from the perspective of numerical simulation.Based on the above,the following conclusions can be drawn from this study.(1)The results of mechanical properties experiments for three strength grades of concrete(ultra-high strength concrete(120 MPa),high strength concrete(80 MPa)and normal concrete(30 MPa))showed that the effect of aggregate particle size on the mechanical properties of concrete tended to be similar,which means that the smaller the aggregate particle size,the better the mechanical properties of concrete.In this study,the compressive strength of concrete prepared by the minimum particle size(0.075-0.6mm)of low strength lightaggregate and ultra-high strength matrix unexpectedly reached 96.3 MPa.(2)The effect of aggregate particle size on the compressive strength was more obvious than the flexural strength of concrete prepared from different strength grades,different types of aggregates and concrete at different curing ages.The compressive strength of ultra-high strength concrete was most affected by aggregate particle size,followed by high strength concrete,and the least effect on normal concrete.In addition,the compressive strength of concrete at 28 days curing age was more affected by aggregate particle size than that at 1 day curing age.Furthermore,the effect of aggregate particle size was more significant in low strength light aggregates than in high strength aggregates.(3)The effect of aggregate particle size on the compressive strength of concrete prepared by low strength lightaggregate concrete was significant and the compressive strength was significantly enhanced when the aggregate particle size was below 0.6 mm.Particularly,the compressive strength of concrete prepared by the minimum particle size(0.075-0.6mm)of low strength lightaggregate and ultra-high strength matrix reached96.3 MPa.At the same time,the density of the concrete prepared with low strength lightaggregate was less than 1950 kg/m~3,meeting the requirements for lightaggregate concrete,which indicates that the goal of preparing high strength concrete with lightaggregate can be realized by reducing the particle size of lightaggregate.(4)Numerical models of three strength grades of concrete(ultra-high strength concrete,high strength concrete and normal concrete)were established based on the Peridynamics theory.Afterwards,several numerical models damage processes and stress-strain curves under uniaxial compressive loading were investigated respectively.The simulation results showed the same trend as experimental results,indicating that the reduction of aggregate particle size contributes to the improvement of the mechanical properties of concrete.There were four stages during the development of cracks in each numerical specimen:initiation,extension,penetration and damage,which was consistent with the typical damage process of concrete and verified the rationality of the numerical model of concrete established in this study.(5)Combined with mesomechanics and numerical simulations of Peridynamics,this study provided a rational explanation of the mechanism about the effect of aggregate partical size affects the mechanical properties of concrete(mainly referred to compressive strength).The simulation results showed that as the aggregate partical size decreases,the concentrated weak zone(interface transition zone or low strength light aggregate)transforms into dispersed weak zone,which hinders the expansion of cracks under load.In particular,transgranular fracture occured in the damage process of lightweight aggregate under compression,and the reduction of aggregate particle size further discretized the stress transfer pattern.Therefore,the mechanical properties of lightweight aggregate concrete were significantly improved.This study provided theoretical support for the preparation of high-strength lightweight concrete and also laid a good foundation for further broadening the application of lightaggregate concrete in practical engineering.