Transport Property Of Fiber Reinforced Cementitious Composites Under External Load

The scale of infrastructure construction in China is unprecedented in recent years.Many major projects have been under construction,including bridges,tunnels,high speed railways,highways,hydroelectric dams,etc.However,the cracking in concrete constantly occurs,resulting in premature failure and sharp reduction in their service life of concrete structures.The service characteristics of these structures would be greatly compromised and it costs enormous expenses for the repair work.Therefore,improving the durability and service life of concrete structures has attracted great attention in the field of concrete science and engineering at home and aboard.Fiber reinforced cementitious composites(FRCC)is an important and effective way to favorably modify the properties of concrete.The tensile strength,toughness and durability of concrete can be greatly improved by adding fibers.Steel fiber reinforced concrete(SFRC)had been applied in various fields due to its excellent mechanical properties.The application of SFRC in major constructions significantly improves their service characteristics.Moreover,a new type of FRCC,which is called high ductility cementitious composites(HDCC),exhibits extremely high toughness and excellent ability in crack control.The application of HDCC as a repair material can restrain its crack width and provide better protection for the parent concrete structure.Surprisingly,the applications of SFRC and HDCC are not wide enough as expected in real constructions.The purpose of this research is to promote the application of FRCC in real constructions.In order to prove the advantages of FRCC quantificationally over normal concrete,the dissertation focused on the transport properties of SFRC and HDCC under external load.The raw materials and mix proportions were optimized and the mechanical properties of SFRC and HDCC were tested.The chloride transport property of SFRC under applied bending load and the water permeability of HDCC under applied compressive stress were studied.The microstructure of SFRC paste and HDCC paste were simulated with HYMOSTRUC model.The transport properties of paste and FRCC were calculated and the effect of load was considered in the model.SFRC specimens with a w/b ratio of 0.35 were prepared.Cement in SFRC was partially replaced with fly ash(FA).The compressive strength,splitting tensile strength and flexural strength were tested.The influences of curing age and fiber volume fraction on the mechanical properties were investigated.The results showed that steel fibers could slightly improve the compressive strength of concrete,although the increase was generally small.The splitting tensile strength and flexural strength were greatly improved by steel fibers.The load-deflection curve indicated that the toughness of SFRC was much higher than that of plain concrete.The raw materials and mix proportions of HDCC were optimized to reduce the materials cost and improve its greenness.Surface-oiled polyvinyl alcohol(PVA)fibers were replaced with regular PVA fibers,and microsilica sand was replaced with natural sand.The FA content was increased and silica fume(SF)was added in the matrix as well.The HDCC prepared exhibited strain-hardening and multiple cracking behavior.The compressive strength and uniaxial tensile property of HDCC were tested and the influences of matrix composition,fiber volume fraction and loading rate were investigated.The results showed that as the content of FA increased,the compressive strength and tensile strength decreased,but the ductility of HDCC was improved and the tensile strain capacity was increased.The effect of w/b ratio on mechanical properties was similar with FA.The addition of SF could improve the compressive strength and tensile strength.The fiber/matrix interface was improved by SF,resulting in an increase in strain capacity of HDCC.The sand content had an insignificant effect on compressive strength,while the tensile strength and strain capacity would be reduced by increasing sand content.Fiber type and volume fraction didn’t affect the compressive strength significantly.Reducing the volume fraction of PVA fibers would greatly reduce the strain capacity of HDCC.Cementitious composites with Polyesters(PET)fibers didn’t exhibit strain-hardening or multiple cracking behavior.However,composites with hybrid fibers(PVA and PET fibers)attained strain-hardening behavior.As the curing age increased,the strain capacity increased firstly,then started to decrease.The tensile strength of HDCC increased remarkably under a higher loading rate,while the tensile strain capacity decreased greatly.The effect of load on chloride transport property of SFRC was investigated through bulk chloride diffusion test.SFRC specimens was subjected with bending load at a stress level of 0.5 through loading device.Based on the Fick’s second law,the apparent chloride diffusion coefficient was calculated with the concentration distribution of free chloride in SFRC.The time-dependence of chloride diffusion coefficient was taken into consideration.The corrosion initiation of SFRC under bending load was predicted.The results indicated that the apparent chloride diffusion coefficient decreased as the immersion time increased.For unstressed specimens,the addition of steel fiber had insignificant effect on chloride diffusion coefficient.For specimens under bending load,the chloride diffusion coefficient of SFRC under tension was much lower than that of plain concrete.The application of SFRC could prolong the initiation period of concrete structures to 2.2~3.6 times of that for plain concrete.Water permeability of HDCC was measured with an adapted permeability test setup under water equilibrium condition.The permeability could be monitored continuously,and the response of permeability to both loading and unloading could be evaluated.The permeability evolution of HDCC and control mortar specimens at early ages was recorded,and the influence of compressive stress at different stress levels was investigated.The results showed that the permeability of unstressed HDCC and mortar specimens decreased over time.Permeability of HDCC was slightly lower than mortar.Compressive stress at low stress levels had no obvious influence on permeability of both HDCC and mortar specimens.When the stress level exceeded a critical level(referred as critical stress level),permeability increased as the compressive stress was applied to the specimen,and it decreased to some extent when the specimen was unloaded.Beyond the critical stress level,the permeability increased remarkably as the stress level increased.The critical stress level was 0.5~0.6 for HDCC,and 0.4~0.5 for control mortar.When compressive stress at stress level of 0.7 was applied to the specimens,the increase in permeability of mortar due to loading was two orders of magnitude higher than that of HDCC.The microstructures of SFRC paste and HDCC paste were simulated with HYMOSTRUC hydration model.The chloride diffusion coefficient of SFRC paste and water permeability coefficient of HDCC paste were calculated with finite element method.The influences of grain size distribution and interfacial transition zone on transport property of concrete were taken into consideration with general self-consistent scheme.The transport property of FRCC was predicted accordingly and the effect of load was expressed with a stress factor.The chloride diffusion coefficient of SFRC from calculation agreed well with results from tests.The calculated water permeability coefficient of HDCC was lower than the measured value,while the error was acceptable.