Research For Mechanical Performance Of Recycled Concrete Beams Under Load Coupled With Chloride Ion Erosion

Recycled concrete technology is an important measure to achieve sustainable development of buildings,resources and the environment.Due to the characteristics of recycled coarse aggregates,the corrosion of steel bar in recycled concrete and the mechanical behavior of its reinforcing structures are clearly different from ordinary concrete.In order to study the durability of recycled concrete beams better.In this paper,the research on mechanical performance of recycled concrete beams under loading coupled with chloride ion erosion is carried out.The main contents and results of the study are as follows:1.According to different influencing factors,15 recycled concrete beams with a size of 120mm×180mm×1700mm were produced.The corrosion of steel bars in recycled concrete beam was accelerated by the method of electrified corrosion and a continuous load was applied to beams to simulate the actual service state.Considering the effects of recycled coarse aggregate(0%,50%,70%and 100%),steel corrosion rate(0%,3%,5%,7%and 10%),mineral admixture(fly ash and slag)and the strengthening methods of recycled coarse aggregate(silica solution immersion and carbon dioxide strengthening in reaction kettle),the mechanical properties of recycled concrete beams under load coupled with chloride ion erosion were studied.2.The immersion test of recycled concrete beams Uunder continuous load was carried out in the solution of sodium chloride.The research shows that the stiffness of the beam decreases as the corrosion rate of the steel bars increases.The stiffness of the beam significantly decreases with the increase of the replacement ratio of recycled coarse aggregates.The stiffness of the beams made of strengthening recycled coarse aggregate is better than that of the ordinary recycled concrete beam.And the stiffness of beams made of recycled coarse aggregate strengthened by nano-silica soaking is better than that of carbonized reinforced beams;The ability of beam to resist deformation is improved after adding mineral admixture into recycled concrete.While the stiffness of recycled concrete beams with fly ash and slag compounded is better than that of single mixed fly ash or slag.3.After the steel bar reaches the target corrosion rate,the bending bearing capacity test of the recycled concrete beam was tested.The crack development and failure mode of the beams were observed under different loading conditions.The variation regularity of the cracking load,the ultimate load,the deflection,the cross section strain and displacement ductility of the concrete beams were analyzed.The results show that the bearing capacity and ductility of the beam gradually decrease with the increase of the corrosion rate of the steel bar.The greater the replacement rate of the recycled aggregate,the lower the bearing capacity and ductility of the beam.After strengthening the recycled aggregate,the bearing capacity and ductility of the corroded reinforced recycled concrete beams are improved.Under the same conditions,the bearing capacity and ductility of the beams made of recycled coarse aggregates strengthened by nano silica soaking are superior than that of carbon dioxide-reinforced beams;The bearing capacity of recycled concrete beams with fly ash and slag compounded is better than that of single mixed fly ash or slag.By comparing the optimum mix ratio of the mineral-containing admixtures and the strengthening recycled coarse aggregate method.,it was found that under the combined action of continuous loading and NaCl soaking,the ultimate bearing capacity of the beams made of recycled aggregate strengthened by nano silica solution is quite similar to that of ordinary concrete beam.What’s more,recycled concrete beam with fly ash and slag has better bearing capacity than ordinary concrete beams.4.After crushing the loaded beam,the corrosion rate of longitudinal steel bars and stirrup took from the beam was measured and tensile test of corroded longitudinal steel bars was carried out.The research shows that the actual corrosion rate of the steel bar in the test beam is lower than the theoretical corrosion rate.Under the condition that the replacement rate of the recycled coarse aggregate is 100%,compared with the actual corrosion rate,the error of the corrosion rate of steel bars with a theoretical corrosion rate from 5%to7%is within 8%.Specially,the error is minimum when the corrosion rate of the steel bar is 5%.The corrosion rate of steel bars increases and the mechanical properties of steel bars decrease with the increase of the replacement ratio of recycled coarse aggregates.The corrosion rate of steel bars in the beam made of recycled aggregate strengthened by nano silica solution or carbon dioxide decreases significantly and the mechanical properties of steel bars are improved significantly.Fly ash and slag can greatly reduce the corrosion rate of steel bars in recycled concrete beams and the recombination effect is optimal.