Key Engineering Materials Vols. 400-402

Abstract: The rust distribution and the crack expansion in the deteriorated concrete were studied through the macroscopic and microcosmic method. The results show that the corrosion of steel bar in the concrete is non-uniform even though by Galvanostatic Method. The crack appears earlier and expands quicker at the thin concrete cover than others side and the short crack filled by the rust is found near by the steel bar and converges the main crack. The distribution, composition and the developing of the rust at steel-concrete interface were studied by the microscopic methods of BSE, EDAX and Raman Microscope. By these methods, the rust distributes inhomogeneous and the ion element area was divided into zones evidently. From concrete to steel bar, the rust in turn is: initial rust zone, the mixture area of the rust and mortar about 20-50 μm in width and main containing the goethite (FeOOH); the secondary rust zone, high density area of ion element and about 10-20μm in width and being the mixture of hematite（Fe2O3）and magnetite(Fe3O4); the final rust zone, similar as the initial rust and being the mixture of hematite(Fe2O3)and goethite(FeOOH). According to the electrochemistry principle, the forming process of rust does not follow the iron oxidation procedure entirely but form middle production under the special situation of the condition interior concrete and the electric field outside. The extrusion action of concrete around steel bar makes the rust compressed and metamorphic and the secondary rust forming. So the process and the final state of this special corrosion production are the key parameters leading to the deterioration of concrete.

Abstract: Electrochemical chloride extraction (ECE) is used for the rehabilitation of chloride-contaminated concrete. High current densities of steel surface are applied between the steel and a temporary external anode which is placed on the concrete surface. Anions are pushed away from the cathode (steel reinforcement), and cations are accumulated at the cathode. In this study chloride ions were found in the electrolyte during ECE. The extraction efficiency of chlorides was obvious. After treatment, the amount of chlorides around the reinforcement was greatly reduced. ECE had insignificant effect on the compressive strength. Steel potentials in treated specimens showed a strong shift towards more positive values, while potentials in control specimens remained strongly negative.

Abstract: Corrosion products which occupy much greater volume accumulate, and generate expansive pressures on the surrounding concrete. The pressure builds up and eventually leads to the cover cracking of the structures. The cracking accelerates further corrosion and leads to the loss of the bond strength and the load carrying capacity. Corrosion cracking would reveal the reducing of the residual service life of the corrosion-affected structures. In this paper, an analytical model is proposed to predict the critical corrosion penetration at cover cracking in RC structures based on the crack process. An attempt has been made to develop the model by considering material properties of the surrounding concrete and expansive corrosion products. The problem is established as a boundary-value problem and the governing equations are expressed in terms of the radial displacement. The analytical predictions of the proposed model have also been in agreement with the available experimental data.

Abstract: The emphasis of this paper is to experimentally study the fracture behaviors of dam and wet-screening concrete such as softening curve, uniaxial tensile strength , maximum crack width and fracture energy . The direct tension tests were performed on prismatic specimens with no initial notch include dam concrete specimens which size is 250mm×250mm×500mm and wet-screening concrete specimens which size is 150mm×150mm×300mm by employing a servo-hydraulic closed-loop testing machine with large stiffness. The uniaxial tensile load-strain curves, uniaxial tensile stress-strain curves, uniaxial tensile stress-deformation curves were acquired. On the basis of principle of fracture mechanics of concrete, the stress-crack width curves and the relative stress-relative crack width curves were obtained. Finally, the expressions of softening curves and fracture parameters such as , and of dam and wet-screening concrete were achieved. The relationships of the foregoing fracture parameters between dam concrete and wet-screening concrete were discussed in this paper. The achieved softening curves and fracture parameters can provide input data for the fracture simulating analysis of dam and wet-screening concrete.

Abstract: Fatigue fracture is one kind of common destruction form in structure which bearing dynamic load frequently. Reinforce concrete structure’s fatigue fracture behaved commonly as the reinforcing steel bar’s fatigue fracture. The fatigue fracture destruction process of the reinforcing steel bar in the reinforced concrete is divided into three phases as follow: before the crack propagation stage; stable crack growth stage and instable propagation of crack phases, and the corresponding damage state for initial damage state; stable development state and structural damage fracture state. Describing every state using Markov stochastic process, and has carried out a probability analysis to the structural destruction process. The example analysis indicated that this method is accurate and convenient, while corresponding action for structural maintenance according to the above adopts can effectively avoids the accident occurrence.

Abstract: : In order to analyze the cause of water leakage in highway tunnels, the damage property of waterproof board in composite tunnel lining with concrete matrix defects under hydraulic pressure was studied using Galerkin method for plates with large deflection. The deflection, stress and strain of waterproof boards were calculated in the conditions of rectangular and elliptic boundary due to concrete matrix defects. First strength theory was used to determine the critical load of the board and the critical depth of the defect. Investigation results showed that the maximum principal stress generates in the middle of longer side in the rectangular boundary condition and in the center in the elliptic boundary condition, the failure of the waterproof board under certain hydraulic pressure is related with the dimensions of concrete matrix defects and the dimension of the concrete matrix defect needs to be controlled in the design and construction of waterproof layer for tunnels.

Abstract: The present work concentrates on the model of concrete under dynamic loading. The stochastic damage constitutive model for concrete under static loading developed by the authors’ research group is firstly reviewed in this paper. The strain rate effect is considered as viscous effect so that the dynamic generalization of the static model could be developed by analogy with viscous-plastic theory. Combined with static damage expressions, the frame work of dynamic stochastic damage constitutive relationship for concrete is established. The analytical expression of dynamic increase factor (DIF) of peak stresses under tension and compression are derived according to the present dynamic damage model. Several simulation results of concrete under static as well as dynamic loading are provided to demonstrate its capacity of reproducing the salient features experimentally observed.

Abstract: According to the spatial distribution theory of damping energy dissipation, the calculation formula of damping energy dissipation and damping ratio of a cantilever beam in one cycle are derived. It is shown that the damping characteristic of a member is affected by the material damping characteristic, stiffness distribution, mode of vibration and the sectional shape of the member, etc. The damping energy dissipation coefficient J for reinforced concrete materials is calculated by using the proposed formula considering different concrete strength, elastic modulus and reinforcement ratios based on test data available.

Abstract: Magnesium Oxychloride Cement (MOC) is an air-hardened material with chemical reaction between magnesium oxide and magnesium chloride solution. It has many properties superior to Portland cement. It does not need wet curing, has very good fire resistance, low thermal conductivity. In this study, wood composites were developed by using MOC as binder and wood powders as filler. It was demonstrated that MOC was suitable for binding wood particles in large quantities. In order to ensure smooth extrusion process, additives and rheological modifiers were used to modify the rheology behaviors and properties of the mixture and the rheology behaviors of fresh materials were also investigated with upsetting and relaxation tests simultaneously. Then MOC-based panels with different cross-sections were produced by using extrusion technology and mechanical properties of them were investigated.

Abstract: Several parameters and corresponding performance of reinforced concrete column cross-sections of different shapes (square, rectangular, circular, T-shape, I-shape, cross-shape, L-shape and C-shape) under various loading conditions have been studied in order to determine the suitable and optimum cross-sections for strength and ductility. In each cross-section shape, parameters include compressive strength of concrete (f’c), tensile strength of steel (fy), steel ratio (As/Ag), and angle of bending. In order to demonstrate the behavior and performance of the sections in terms of strength and ductility, CSISectionBuilder software was used to define the stress-strain curve for concrete and steel and then compute the moment-curvature relationship for each section. Considering different sections, the number of parameters in every section and various loading conditions, a total of around 1,800 sections were analyzed. The comparison procedures started within each section shape, and then across different sections in order to determine the most suitable cross-section for strength and ductility. Results of the study are deemed very useful in the system selection and preliminary design of important structures such as buildings with complicated geometry and high architectural demand including bridge piers and hydraulic structures.