Deterioration Mechanism Of Recycled Concrete Beams Induced By Steel Corrosion Under Load

Recycled aggregate concrete can achieve the recycling of waste concrete,reduce the exploitation of natural resources such as sand and stone and landfill of construction waste,solve some environmental protection problems,and play a significant role in the sustainable development of construction resources.Recycled aggregate concrete has broad engineering application prospects.Before being applied to actual engineering structures,a large number of experimental studies on its structural properties are required.In this paper,a combination of experimental research and theoretical analysis is used to systematically study the degradation mechanism of recycled concrete beams induced by steel corrosion under load.Based on the accelerated corrosion test of recycled concrete beams,the effects of load and recycled coarse aggregate replacement ratio on the mass loss rates of longitudinal tensile steel bars and stirrups are studied.The relationship between mass loss rate of longitudinal tensile reinforcement and mass loss rate of stirrup is established.The distribution of bending failure cracks,load and mid-span deflection curve,and ductility of corroded recycled concrete beams under different load levels and different recycled coarse aggregate replacement ratios are analyzed,and the relationships between the yield moment and the ultimate moment of corroded beams and the maximum mass loss rate of longitudinal tensile reinforcement are established.The calculation method of instantaneous chloride diffusion coefficient is proposed,and the influence of the coupled effects of chloride ion erosion time and concrete stress on the instantaneous chloride diffusion coefficient is analyzed.A chloride ion concentration prediction model considering the coupled effects of time and stress is established,and the initial corrosion time of longitudinal tensile reinforcement in recycled concrete beams is predicted by the model.The results show that the proposed method for calculating the instantaneous chloride diffusion coefficient is simple and feasible,and the computed values of the proposed chloride ion concentration prediction model are in good agreement with the experimental values,which can be used for predicting the initial corrosion time of steel bars embedded in concrete.The calculation model of stress influence coefficient considering the effect of carbonation time and the prediction model of carbonation depth considering the influence of coupled effects of tensile stress and carbonation time are established.Based on the proposed prediction model of carbonation depth,the corresponding relationship between theaccelerated carbonation time and the natural carbonation time is discussed,and the initial corrosion time of longitudinal tensile reinforcement in recycled concrete beams is predicted by using the established model.The results show that both the stress influence coefficient calculation model and the carbonation depth prediction model have high prediction accuracy and can be used for predicting the initial corrosion time of steel bars embedded in concrete.Considering the effects of external load and rust filling layer on the corrosion of steel bars,a theoretical prediction model of the corrosion-induced cracking time of concrete cover surface is derived,and the rationality of the model is verified by using experimental data.The results show that during normal service stage,the external load has a significant effect on the critical corrosion depth of steel bars when the interface between the steel bar and the concrete is cracked,but has little effect on the critical corrosion depth of steel bars when the surface of the concrete cover is cracked.The larger the external load is,the smaller the critical corrosion depth of the reinforcement is,and the influence degree of the external load on the critical corrosion depth is related to the thickness of rust filling layer.The influence degree of the external load on the corrosion-induced cracking time of concrete cover surface is affected by the actual rust volume expansion rate and the rust filling layer.The distribution of corrosion depth and the degree of local uneven corrosion of longitudinal tensile steel bars are investigated by using the methods of deterministic analysis and probabilistic analysis.The influence laws of the load level and the replacement ratio of recycled coarse aggregate on the distribution of corrosion depth and the local uneven corrosion of longitudinal tensile steel bars are revealed.The effects of corrosion time,replacement ratio of recycled coarse aggregate and applied load level on the development law of corrosion-induced cracks in the tension zone of beams are studied,and the distribution law of the width of longitudinal corrosion-induced cracks is analyzed by probabilistic method.The calculation formula of the corrosion depth of steel bars in the current code is revised,and the results show that the calculated values of the revised formula of corrosion depth of steel bars are in good agreement with the test values,which is suitable for the prediction of the corrosion depth of steel bars in recycled concrete beams under load.The calculation formula of cracking moment of reinforced concrete beams in the current code is revised using existing test data,and the results show that the modified calculation formula is applicable to recycled concrete beams without corrosion and with the average mass loss rate of longitudinal tensile reinforcement less than 3.5%.Based on the accelerated corrosion test of recycled concrete beams,the calculation method of short-term stiffness of corroded reinforced concrete beams in the current code have been revised to make it suitablefor the corroded recycled concrete beams under load.The relationship between the strength utilization coefficient of longitudinal tensile reinforcement and the maximum width of longitudinal corrosion-induced cracks on the surface of concrete cover in the tension zone of beams and the relationship between the strain inconsistency coefficient and the average mass loss rate of the longitudinal tensile reinforcement when the corroded beam yields are established,which can be used to calculate the flexural capacity and yield capacity of corroded recycled concrete beams.