Characteration and Simulation of Non-uniform Stress/strain and Its Influence on Formation and Development of Cracks in Concrete under Drying
Supervisor:wei jiang xiong
Concrete is the most widely used building material,which has been applied in different fields of civil engineering,including buildings,highways and dams,etc.During the service process,complicated environmental factors and external loads will lead to the deformation of concretes,which will cause cracks if concretes are restrained.Accordingly,the durability and safety of concrete structures will be significantly influenced.Hence,it is important to investigate the cracking of concretes.As a kind of heterogenous material,concrete has a multiscale structure including microscale,mesoscale and macroscale,etc.However,to investigate the cracking of concretes,many studies focused on macroscale and considered concrete as homogenous material.As a result,the evolution of mesoscale structure on the performance of concrete was neglected.To understand the foundamental cracking mechanism of concrete,it is important to establish the methodologies to determine and simulate the non-uniform stress/strain of concrete based on microstructure and mesostructure of concrete.This study focuses on determining and simulating the non-uniform stress/strain and the induced microcracks of concretes at the mesoscale and investigates the influence of non-uniform stress/strain and induced microcracks on the mechanical properties and cracking process of concretes.The obtained results are helpful to reveal the mechanism of the initiation and propagation of cracks.The research sinificances are described in detail as follows:Firstly,three-dimensional Digital Image Correlation(3D-DIC)was used to determine the non-uniform deformation of concretes dried under different relative humidity(RH).The first principal strain and the second principal strain were used to reflect the local deformation of concretes.With the 3D-DIC measurement,the influence of coarse aggregate size,gradation and volume on the non-uniform strain distribution of concretes under drying conditions was investigated.The results indicate that:(1)The first principal strain of concretes included both shrinkage strain and tensile strain.The second principal strain of concretes was mainly shrinkage strain and showed a non-uniform distribution.The first principal stain distribution of concretes consisted of one Gaussian distribution,and the second principal strain distribution of concretes consisted of two Gaussian distributions,including the areas around mortar phase and coarse aggregate phase.(2)With increasing the drying time,the overall shrinkage increased,the characteristic peak of the first principal strain distribution moved towards tensile strain with wider distribution range and became more non-uniform,the characteristic peak of the second principal strain distribution moved towards shrinkage strain with wider distribution range and became more non-uniform.(3)With decreasing the relative humidity,the first principal strain distribution moved towards tensile strain with more non-uniform,and the second principal strain distribution moved towards shrinkage strain with more non-uniform.(4)In comparison with the reference mortar,the concrete made of 5~10mm aggregates had larger tensile strain and wider distribution range in the first principal strain,and smaller shrinkage strain and wider distribution range in the second principal strain.The concretes made of aggregates larger than 10 mm showed the first principal strain moved towards tensile strain with wider distribution range,and the second principal strain moved towards shrinkage strain with wider distribution range.With increasing the coarse aggregate volume fraction from 0.25 to 0.45,the first principal strain of concretes moved towards tensile strain with smaller distribution range,and the second principal strain of concretes moved towards smaller shrinkage with smaller distribution range.With decreasing the volume of larger size coarse aggregates,the first principal strain of concretes decreased and showed wider distribution range,and the second principal strain increased and showed smaller distribution range.Secondly,based on the mesostructure of concretes obtained with 3D-DIC,the Lattice model was used to simulate the non-uniform stress/strain and induced microcracks of concretes dried under different RH.The simulation of the non-uniform strain and microcrack distribution in concretes was validated with the results of the 3D-DIC measurement and the fluorescent resin measurement,respectively.Both the distributions of non-uniform strain and microcracks in concretes obtained with simulation and experiments indicate that:(1)The shrinkage strain was smaller around coarse aggregates.(2)With increasing the coarse aggregate size,the bulk drying shrinkage of concretes became smaller,the mortar phase and interfacial phase were apt to crack.(3)With increasing the coarse aggregate volume,although the bulk drying shrinkage of concretes was reduced,the number of microcracks in concretes was increased.(4)The continuous gradation aggregates not only reduced the bulk drying shrinkage of concretes,but also reduced the cracking risk of concretes.The simulated distribution of non-uniform stress in concretes indicate that: under drying condition,the residual stress concentrated on the interfical zone between mortar phase and aggregates.Larger residual compressive stress and tensile stress are distributed at the interfacial zone,smaller residual compressive stress and tensile stress are loacted at the crack,and some of the smaller residual stress are located in the area rich of mortar.With the amount of larger coarse aggregate in the concrete increases,the residual tensile stress decreases.With the volume of coarse aggregate in concrete increases,the residual tensile stress decreases.At last,the influence of residual non-uniform stress/strain and initial microcracks on the mechanical properties and propagation of cracks in concretes during tensile process was investigated with the simulation of Lattice model.The simulation results indicate that:(1)During tensile process,the microcracks of the concrete without initial microcracks or residual stress/strain formed and propagated in interfacial zone,because the tensile strength of interfacial zone was the lowest.For the concrete only contains initial microcracks,during the tensile process,which significantly decreased the maximum tensile stress and the displacement reaching the maximum tensile stress of concrete,and reduced the fracture energy accordingly;the initial microcracks became the propagation points and changed the location of the macroscopic crack.(2)In comparison with the concrete without residual non-uniform stress/strain,the concrete with residual non-uniform stress/strain showed the increased maximum tensile stress(22%),the increased displacement reaching the maximum tensile stress(1.25 times),and the increased fracture energy(10%).(3)In addition,the influence of aggregate gradation and volume on the mechanical properties and propagation of cracks in concretes during the tensile process was investigated in consideration of residual non-uniform stress/strain and initial microcracks of concretes.The results indicate that increasing the aggregate volume appropriately can increase the maximum tensile stress,the displacement reaching the maximum tensile stress and fracture energy of concretes.