Investigation On The Chloride Transmission Mechanism At The Joints Of Precast Monolithic RC Frame Structures

Precast monolithic reinforced concrete structures（PC）have been accepted by more and more new constructed buildings as a result of its simple construction process,short construction period and less amount of environment pollution;however,there are some natural discontinuous joint faces inside this kind of concrete structures,i.e.the joint between precast concrete and cast-in-situ concrete,which provides convenient channels for chlorides to reach the surface of the steel bar,and the chloride-induced steel bar corrosion will significantly reduce the performance of concrete structures in severe cases,leading to the collapse of structures and posing a serious threat to people’s lives and property.This research designed 24 sets of composite beam joint specimens and 6 sets of precast column bottom joint specimens,after considering the reasonable simplification of the joint forms suggested in the specifications.The effects of water-cement ratio（W/C）,strain state,crack width and erosion time on the chloride diffusion at joints were considered in this research.A theoretical model of chloride ion transmission mechanisms for describing the PC components was proposed,and the chloride ion transmission at the joints was simulated and analyzed using COMSOL Multiphysics.The test specimens of the composite beam joints were experimentally studied by Na Cl soaking method,and the effects of W/C,crack width,strain state and erosion time on the chloride ion transmission at the composite beam joints were considered.The test results showed that when the W/C was large,the chloride concentration at the joints was significantly higher than that of the precast and cast-in-place parts,and the chloride concentration profile within 10 mm on the left and right sides of the joint,respectively,presented an asymmetric inverted "V"-shaped distribution mode;however,as the W/C decreased,the chloride concentration at the joint was comparable to the precast and cast-in-situ parts.The closer to the surface of the specimen,the greater the effect of strain on the chloride transmission at the composite beam joints,and the variation of chloride concentration with strain presented a linear relationship.When the crack width on the upper surface of the tensile side of the joint was 0-0.13 mm,it had an almost negligible effect on the chloride transmission along the depth of the joint.When the crack width exceeded 0.13 mm,the chloride concentration increased obviously at the same erosion depth.As the crack width increased,the influence width of the joint on the transmission of chloride also increased significantly.The chloride concentration at the joints showed a gradually increasing trend with increasing of soaking time,which presented more obviously in the period of 60-90 d than that of 30-60 d.The experiment on the chloride transmission at the joints of the precast column mainly considered the influence of the grout and the crack width on the chloride ion transmission.The test results showed that the grout made that the chloride ion along the joint at the bottom of the column was significantly greater than that of the ordinary concrete.Within the erosion depth of 15 mm,the chloride concentration distribution in the range of 10 mm on both sides of the joint showed an asymmetric inverted "V" shape distribution,and when the erosion depth was greater than 15 mm,the chloride concentration at the same erosion depth of the precast column bottom joint specimen did not show a significant difference.When there is grout at the joint,the prefabricated part was affected by the crack in the range of 10-20 mm,and the cast-in-situ part was influenced by the crack in the range of 5-10 mm,along the direction perpendicular to the joint.When the crack width at the precast column bottom joint was between 0.08-0.11 mm,the internal chloride concentration is between 0-1%,and with the increase of the crack width,the concentration value did not change significantly.Based on the existing theoretical model of chloride diffusivity,the numerical method of chloride diffusivity was first proposed based on Python,and then the chloride transmission influence zone model in the PC was derived based on the influence of multiple factors.The chloride diffusivity at joints was deduced based on the composite materials,and the theoretical formula for the influence of the load on the chloride transmission at the joint was deduced based on the strain damage theory.The theoretical analysis showed that when the theoretical W/C was 0.20,the existence of joints had no effect on the transmission of chloride ions.The engineering W/C of the precast monolithic RC frame structure proposed in this paper should be below 0.4.In addition,the theoretical calculation results of the proposed model of the influence zone were in good agreement with the experimental results,and can be effectively used to describe the chloride transmission mechanism at the joints of the PC under the action of multiple factors.The COMSOL software was secondary developed by the Matlab programming language,and a random aggregate packing algorithm suitable for describing the mesoscopic model of PC was proposed.The COMSOL analysis of the chloride transmission at the joints was carried out and the simulation results showed that the erosion time would significantly increase the breadth and depth of the impact of the joint;on the meso and microscopic point of view,the chloride concentration of the aggregate and cement mortar interface（ITZ）was significantly higher than that of the cement mortar at the same erosion depth,and a local chloride accumulation of the concentration appeared at the joint closing to the eroded surface.The simulation results of the joint specimens at the bottom of the precast column showed that the chloride concentration distribution on the side of the grout did not exceed 10 mm along the longitudinal direction of the specimen.The joint geometric model considering the damage zone and the equivalent rectangular crack can well simulate the chloride transmission law at the joint under the actual cracking state.Numerical simulation results showed that with increasing of erosion time and crack width,the depth of erosion and the width affected by the cracks along the specimen also increased significantly.The chloride concentration at the joints near the surface had obvious accumulation phenomenon,and the chloride concentration at the cracked joints was obviously higher than that at the uncracked joints.The numerical simulation results were in good agreement with the experimental results,which demonstrated the high reliability of the numerical and theoretical models.This master thesis contains 109 figures,12 tables and 117 references.