Research On The Failure Behavior Of Concrete Based On The Theory Of Damage And Fracture

Concrete is a heterogeneous mixture of coarse aggregate, fine aggregate and cement slurry, its mechanical properties is closely related to its internal composition structure rather than simple superposition of composing materials’mechanical properties. Concrete has different structure forms at micro-level, meso-level and macro-level. Meanwhile, initial imperfections such as bubbles, inclusions and micro-cracks inevitably produced during molding process. All the factors above make the mechanical properties of concrete become extremely complex. The previous researches on concrete are mostly based on macro-level, and take concrete as a single homogeneous material. Without a doubt, the homogenization approach is effective in the research of concrete’s macroscopic mechanical properties. However, in order to reveal the internal mechanism of concrete’s macroeconomic performance, and study its working mechanism deeply, researchers should start with the microscopic composition of concrete, and seize the heterogeneity characteristics of concrete material. In this paper, numerical model of the concrete at meso-level was established at first, the damage and failure mechanism was then analyzed. Based on these, analysis model of damage and fracture theory was proposed at macro-level. The fracture behavior of concrete was researched between meso-level and macro-level with the relationship and complement of them.Concrete was considered as a three-phase composite material composing of aggregates, mortar and bonding material at meso-level. A3-D mesoscopic numerical model with random aggregate and mechanical parameter was proposed. This model can simulate the random distribution of aggregates. Furthermore, according to a random distribution, the mechanical parameters of the meso-phase elements in this model can be assigned. Thus the nonhomogeneiry simulation of concrete has realized. Six groups of concrete cube numerical model were established, the parameters which influence mesoscopic numerical model have been analyzed. The results show that the random distributing of aggregate location has influence on the generating location and expanding paths of crack, while its influence on the carrying capacity of specimens can be neglected. The elastic modulus of interface has greater influence on carrying capacity of concrete than that of mortar. The carrying capacity of concrete descends when taking the non-uniformity of the meso-phase materials into account. The residual strain coefficient in damage constitutive model of interface and mortar has a significant effect on the development speed of damage and the carrying capacity of the specimens. If the residual strain coefficient of a certain meso-phase material increases, the damage speed of elements with this phase will reduces, and the carrying capacity of concrete will increases slightly. According to the above model and analysis results, utilizing the method of displacement loading, the loads were imposed on the concrete beam and the prism axis tensile specimen gradually untill they became damaged, and the status of the beam and specimen all through the process were simulated numerically. The process of crack appearance and growth were obtained and the quasi-brittle failure characteristic of concrete was also revealed.Based on the mesoscopic model for concrete, steel bars and bar-mortar interface were used to establish a reinforced concrete numerical model. Pullout test of crescent-ribbed bar specimens and screw thread bar specimens were simulated respectively, the influence of rib outline on concrete damage was analyzed. Results show that the overall failure patterns of these two specimens are similar to each other. During the pullout process, stronger squeezing action appears at the load-end, moreover, the stress of concrete around steel bar is stronger than that away from it. The damage of concrete presents a gradually decay trend from the circumambience of steel bar to specimen outsides. Damage and failure occurs first in the steel bar-mortar interface of the load-end and extends gradually to the free end. With the increase of the pulling force, the failure elements of steel bar-mortar interface and mortar gradually impenetrate, the occlusion gears of concrete among steel bar ribs destruct in turn, and the longitudinal sliding cracks come into being. The damage elements of crescent-ribbed bar specimen distribute a taper pattern, the stress concentration in concrete around steel bar is low. As a contrast, the damage elements of screw thread bar specimen distribute a shuttle pattern, the stress of concrete is obviously concentrated, and the damage of concrete is more severe.At the macro level, the concrete fracture process zone was assumed to be consist of micro-crack generating zone and developing zone according to its characteristics. Combining the damage theory and fracture mechanics, a damage-fracture model named D-K model for concrete was put forward. This model takes the initial fracture damage threshold, DIG as the incipient cracking criterion and unstable fracture toughness, KIc as the crack unstable growth criterion. Based on this method, the whole damage and fracture process from initial damage, macro-crack forming to crack unstably propagating can be analyzed. It is known that the shortcoming of existing fracture models is they cannot analyze specimens without original cracks. However, in this model, the damage incipient cracking criterion is innovative used, which not only has the definite physical meaning but also can overcome the shortcoming above. Furthermore, according to the elastic stress field distribution of type I crack tip, several micro-crack generating zone outline expression has been derived under different yield criteria. The influence of concrete tension and compression strength ratio, Poisson’s ratio and the intermediate principal stress on outline was analyzed. Based on the softening curve of pullout specimens from mesoscopic numerical simulation and physical tests, the author tried to propose a segmental power function damage model and determine the distribution rule of cohesive stress of equivalent crack according to the model. The expressions of concrete initial fracture damage threshold, as well as the ultimate length of the fracture process zone were derived. The crack growth process of three-point bending concrete beams with and without original cracks was simulated using numerical methods. And then the feasibility and validity of numerical solving method with D-K model were verified. Finally, the influence of initial ao/D on unstable fracture toughness, KIc was discussed.It is a homogenization process for concrete from meso multiphase structure to macroscopic uniform material, the quantitative relationship between the meso composition and macroscopic properties can be reflected by homogenization. Based on inclusion theory of composite materials, a mixture inclusion model for concrete was put forward, the analytical solution for the model was derived with generalized self-consistent and Mori-Tanaka method. The elastic modulus of concrete was predicted using this model, the main influencing factor such as aggregate volume percentage, interface thickness, elastic modulus, content of void and micro cracks were analyzed.