Chloride Transport Property And Service Life Prediction Of UHPFRCC Under Flexural Load

Ultra-high performance fiber reinforced cementitious composite (UHPFRCC) is a novel type of cement-based material with superior mechanical properties and durability, which makes UHPFRCC have very wide application prospects. It has been used in North America, Europe, Japan, Australia and etc., and positive feedbacks have been received. But it is not very extensively used wordwide. Besides the price, the main reason that impedes the increasing applications of UHPFRCC is the lack of standards for the design of UHPFRCC materials and UHPFRCC structures.UHPFRCC has very dense microstructure and great resistance to chloride, so one of the potential applications of UHPFRCC is the marine structures. The chloride induced corrosion is the main reason for the failure of concrete structures in marine areas. To construct marine structures with UHPFRCC could increase the service life of marine structures, and then reduce the maintenances and reconstruction cost. Normally, concrete structures are working with loads and cracks, and the most common reason for the cracking of concrete structures is the flexrual load. So, in this thesis, the chloride transport process in UHPFRCC structure under coupled effect of flexural load and chloride ingress was studied, and the service life prediction model for UHPFRCC structure under coupled effects was proposed. The results of this research could be used as basis for the durability design of UHPFRCC materials and UHPFRCC structures.The goal of this research is to predict the service life of UHPFRCC structures under coupled effects of chloride ingress and flexural load. In order to achieve this goal, UHPFRCC was prepared and its mechanical properties and durability were tested; The micro, meso and macro structures of UHPFRCC were simulated with HYMOSTRUC model, Anm model and Matlab program respectively; Based on Fick’s law on diffusion and finite element method, the chloride transport property of UHPFRCC was predicted with a multi-scale scheme; The flexural response of UHPFRCC was simulated with Lattice fracture model; The chloride transport process in cracked UHPFRCC structures were simulated based on Fick’s law on diffusion and finite difference method; In the end, the service life of cracked UHPFRCC in chloride environments were predicted, and the influencing factors were discussed. Details on the scope, methodology and results of this research are as follows:1) The mechanical properties and durability of UHPFRCC were tested, and the durability of UHPFRCC under flexural load was the main concernUHPFRCC with a compressive strength of 150 MPa was prepared, and the mechanical properties of UHPFRCC, such as compressive strength, flexural strength and toughness, were tested. The influences of curing age, fiber content and curing conditions on the mechanical properties of UHPFRCC were studied. The dry shrinkage, chloride resistance, froze resistance and carbonation resistance of UHPFRCC were tested, and the influences of immerging age, fiber content, load and curing conditions on the chloride resistance of UHPFRCC were investigated, as well as the influences of load, fiber content and curing conditions on the frost resistance of UHPFRCC. The results showed that, UHPFRCC owned excellent mechanical properties. With standard curing, the strength of UHPFRCC grew rapidly in the early age and slightly grew in the late age. The heat curing and steam curing could make UHPFRCC exhibit superior mechanical properties in a very short age. The addition of steel fibers could increase the flexural strength and toughness of UHPFRCC. UHPFRCC also showed great durability. The dry shrinkage of UHPFRCC was relatively low, and the resistances of UHPFRCC to chloride, frost and carbonation were outstanding. When the flexural load was exerted, the resistances of UHPFRCC to chloride and frost were impaired.2) The micro, meso and macro structures of UHPFRCC were simulated with structure models at different length scalesAccording to materials science, the macro properties of UHPFRCC are determined by the structures of UHPFRCC at different length scales. The structure of UHPFRCC was studied at three different length scales:micro (microns), meso (millimeters) and macro (centimeters). At micro scale, the UHPFRCC paste was studied. The microstructure of UHPFRCC paste was simulated with extended HYMOSTRUC model, which could simulate the microstructure of UHPFRCC paste containing fly ash and silica fume. Both the filling effect and pozzolanic effect of fly ash and silica fume were taken into account in the model. At meso scale, the matrix of UHPFRCC was studied. The matrix consists of paste and aggregates, and the mesostructure was simulated with Anm model. The advantage of Anm model is that it could simulate the packing process of aggregates with irregular shapes. At macro scale, UHPFRCC was studied. It contains matrix and steel fibers. The macrostructure of UHPFRCC was simulated with a homemade Matlab program.3) The multi-scale method for the prediction of chloride transport property of UHPFRCC was proposed by solving the Fick’s law on diffusion with finite element methodThe chloride transport property of UHPFRCC is the key factor that influences the service life of UHPFRCC structure in chloride environments. By solving Fick’s law on diffusion, the chloride transport property of UHPFRCC paste and matrix were predicted, using the microstructure and mesostructure as inputs. At micro scale, the effects of water to binder ratio (w/b), fly ash content and silica fume content on the chloride diffusivity of UHPFRCC paste were studied. The results showed that the chloride diffusivity of UHPFRCC paste rose up when w/b or fly ash content went up and the silica content went down. At meso scale, the influences of the volume fraction and the shape of the aggregates on the chloride diffusivity of UHPFRCC matrix were studied. The results showed that the chloride diffusivity of UHPFRCC matrix ascended when the volume fraction of aggregates increased. The shape of aggregates did not show much effect on the chloride diffusivity of UHPFRCC matrix. At macro scale, the chloride diffusivity of UHPFRCC was predicted with a two-phase model. The chloride diffusivity of UHPFRCC was related to the steel fiber content.4) The behavior of UHPFRCC under flexural load was simulated with Lattice fracture model. The initiation and propagation of the crack in UHPFRC were detected, and a method that could deduce information on crack geometry from the results of Lattice fracture model was proposedUHPFRCC normally shows strain hardening and multi-cracking phenomenon under the effect of flexural load. With the Lattice fracture model, the load-deformation curve and the crack pattern in failed UHPFRCC could be obtained. The simulation results agreed with the experiment results quite well, so Lattice model could simulate the flexural response of UHPFRCC, including strain hardening and multi-cracking phenomenon. The effects of fiber content and orientation on the flexural behavior of UHPFRCC were studied with Lattice fracture model. The results showed that UHPFRCC with oriented fibers had better flexural property, and with the increase of fiber content, the flexural strength of UHPFRCC improved. The method that could deduce information on crack geometry from the results of Lattice fracture model was proposed. The information on crack geometry includes crack width, depth and etc.. This information is very important for the chloride transport property of cracked UHPFRCC.5) The chloride transport process in cracked concrete was simulated with finite element method. On this basis, the service life prediction method for cracked UHPFRCC structure in chloride environments was proposedBesides the chloride diffusivity of UHPFRCC, the chloride transport property in crack is also crutial to the chloride transport process in cracked UHPFRCC. In this thesis, the steady-state chloride diffusion test was simulated, the apparent chloride diffuvity in crack was calculted, and the effect of crack geometry on the chloride transport propery in crack was studied. The resutls showed the apparent chloride diffusivity in crack was not influenced by the crack width, but the tortuosity and the width and number of narrow points in the crack. Based on the electrochemistry, the chloride transprot property in crack solution was studied. The results showed the chloride transport speed in solution was influence by the chloride concentraion in the solution. The chloride transport process in cracked concrete was simulated with finite difference method, and an example was given. It can be seen from the results that the chlorides could transport into UHPFRCC through the crack walls, as well as the crack tip. The chloride penetration depth was the highest at the crack tip. Based on the simulated chloride transport process in sound and cracked UHPFRCC, the service life of sound and cracked UHPFRCC structure was predicted. The effect of chloride diffusivity of UHPFRCC, the surface chloride concentration and the crack geometry on the service life of UHPFRCC structures were investigated. The results showed that, when the chloride diffusivity of UHPFRCC went down, the service life of UHPFRCC structure increased; when the surface chloride diffusivity went up, the service life of UHPFRCC structure decreased; when the crack width and depth became bigger, the service life of UHPFRCC structure decreased; when the crack tortuosity and the number of narrow points in crack went up and the width of narrow points became smaller, the service life of UHPFRCC structure increased.