Lifetime performance prediction of reinforced concrete structures in multi-threat areas

Reinforced concrete (RC) structures are usually subjected to various natural hazards and environmental stressors during their lifetime. Over the time, structures are continuously aging and rapidly deteriorating in their lifecycle, becoming increasingly vulnerable to catastrophic failures after natural or manmade hazards. Corrosion of steel reinforcement has been identified as one of the major causes of deterioration in reinforced concrete structures. Chloride ingress is the dominant mechanism for initiation of deterioration in coastal regions or areas with high exposure to deicing salts. The chloride-induced crack initiation stage in deterioration process, which defines the end of functional service life for corroded RC structures has been investigated in this study. Crack initiation is governed by the expansion of corrosion products. It was found that crack initiation time is significantly affected by rate of corrosion, thickness of interfacial transmission zone (ITZ), composition of corrosion products, and mechanism of corrosion. Other factors which can also influence crack initiation time are the structure’s geometrical parameters such as concrete cover depth, rebar diameter and spacing, as well as the material parameter concrete tensile strength. Different reinforced concrete structural components have been simulated using nonlinear 3-D finite element (FE) models in order to study their lifetime performance under corrosion. The developed FE models are validated with the available experimental tests. All of the corrosion effects on structural behavior of RC structures, such as reduction of steel cross sectional area, change of steel and concrete properties, as well as deterioration of bond has been implemented into the 3-D FE models. The structural performance of corroded RC beams is obtained through FE analysis. The results show that corrosion influences the strength and ductility of a structure at ultimate condition, and may also cause excessive cracking and deflection, which leads to serviceability failure. Moreover, a large number of RC structures that suffer from corrosion mechanisms are located in high seismic risk areas, which leads to the necessity of investigating the combined effects of corrosion and earthquake in order to provide a more reliable prediction for the lifetime performance of RC structures in both corrosive and high seismic risk areas. Therefore, a comprehensive FE framework has been developed to study the structural response of RC columns under earthquake hazards while they are constantly exposed to chloride attack. This framework is capable of including all of the degrading effects due to chloride-induced corrosion and has been validated by a previous set of experimental test results. The extent of structural degradation has been updated as a function of time. Equivalent static analysis and nonlinear time history analysis have been conducted to evaluate the seismic performance of corroded columns at multiple time periods as well as under various hazard levels. The region, type and extent of damage have been identified and damage states has also been defined. Full details of hysteretic loops, frequency changes, as well as cover crack propagation patterns have been obtained through FE analysis. Furthermore, this study has also considered multiple seismic events occurring during the lifetime of RC structures. Detailed FE models that are able to transfer residual damage from previous earthquake to the next earthquake have been proposed. The extent of damage after each earthquake has been quantified. The result of this study shows that the corrosion can dramatically reduce the strength and stiffness of the column. Under severe earthquake, extensive corrosion may result in a brittle failure of the column without the development of concrete cracks. When a critical section of the column experiences a much higher corrosion risk, the seismic performance can be greatly compromised. Such columns could perform much worse than a column undergoing a consistent corrosion rate at a much older age, thus engineers must be alerted to draw special attention to those columns to prevent catastrophic failure during seismic events. The outcome of this research will provide more reliable predictions for the lifetime performance of RC structures, thus help engineers and inspectors improve their designs, identify necessary test regions and define comprehensive inspection plans, as well as optimize rehabilitation strategies for RC structures under multi-threat areas

Infrared-Improved Soft-wall AdS/QCD Model for Mesons

We construct and investigate an infrared-improved soft-wall AdS/QCD model for mesons. Both linear confinement and chiral symmetry breaking of low energy QCD are well characterized in such an infrared-improved soft-wall AdS/QCD model. The model enables us to obtain a more consistent numerical prediction for the mass spectra of resonance scalar, pseudoscalar, vector and axial-vector mesons. In particular, the predicted mass for the lightest ground state scalar meson shows a good agreement with the experimental data. The model also provides a remarkable check for the Gell-Mann-Oakes-Renner relation and a sensible result for the space-like pion form factor.Comment: 15 pages, 4 figures, 7 tables, published versio