Effects Of Blast Furnace Slag On Mechanical Properties And Damage Mechanism Of Recycled Aggregate Concrete

Due to the acceleration of urbanization,the construction industry has developed rapidly.But concrete buildings must be demolished when they reach their useful life,leading to a sharp increase in discarded concrete.Waste concrete is composed of a certain proportion of fine aggregate,coarse aggregate,water and cement.In the production process,a large number of raw materials will be consumed,which brings great damage to the ecological environment.Moreover,the traditional way to dispose of these waste concrete is only stacking and landfilling,which poses a greater threat to the scarce land resources.Recycled aggregate concrete（RAC）is produced by converting dismantled waste concrete into recycled coarse aggregate（RCA）as a partial or total substitute for natural coarse aggregate（NCA）.This treatment can not only improve the recycling rate of waste concrete,but also reduce the consumption of natural resources,which is consistent with the requirements of sustainable development.Due to the degradation of RAC properties caused by the defects of RCA,blast furnace slag（BFS）was used to replace cement in equal amounts to improve the mechanical properties of recycled concrete.Through uniaxial compression tests with different slag replacement amount（B）,different curing ages（T）and different dynamic strain rates under different curing ages,the influence on the mechanical properties of each test block was discussed.The microstructure,microcrack propagation process,reaction products and porosity were analyzed by acoustic emission（AE）,scanning electron microscopy（SEM）and nuclear magnetic resonance（NMR）.Based on the statistical damage constitutive model,the meso-damage mechanism of blast-furnace slag concrete during uniaxial compression under different conditions is analyzed,and the internal relationship between the microscopic failure mechanism and the macroscopic nonlinear mechanical properties of concrete is revealed.The main contents are as follows:（1）Uniaxial compression tests of natural aggregate concrete（NAC）and RAC under different BFS replacement amount,curing age and dynamic strain rate under different curing age were carried out.The influences of the above conditions on the mechanical properties of concrete are analyzed.The results show that the mechanical properties of NAC first increase and then decrease with the increase of displacement amount,while the mechanical properties of RAC gradually decrease.Based on the literature and experimental data,the optimal replacement amount of BFS is determined to be 35%from the aspects of economic and mechanical properties.The mechanical properties of the concrete test blocks are improved with the increase of curing age.After curing for 56 days,the compressive strength of the concrete with BFS is higher than that without BFS.The pore size and pore size of the specimen were analyzed.（2）AE tests were carried out on NAC and RAC with 0%and 35%BFS replacement.The corresponding curves of stress-strain and AE ringing count were obtained,and the relationship between them was observed.NMR tests were carried out for NAC and RAC with different BFS displacement amounts,0%and 35%BFS displacement amounts under different curing ages.SEM tests were carried out on BFS replacement 0%and 35%RAC specimens with curing age of 7d,28d and 150d.Observe the internal microstructure of the specimen.The results show that:The peak value of ringing count curve lags behind the peak value of stress and is located in the descending section of stress-strain curve.With the increase of BFS replacement,the NAC pore decreases first and then increases,and the RAC increases.With the increase of curing age,the porosity of the test block decreases significantly.After 56 days,the porosity of the concrete containing BFS is smaller than that of ordinary concrete,and the internal structure is more compact.（3）Statistical damage constitutive model was used to analyze the effects of curing age on NAC and RAC,and the effects of different dynamic strain rates on meso-damage evolution of RAC under different curing ages.Considering the fracture and yield damage modes,the probability density function is assumed to follow the triangular distribution,which can be characterized by E₀、_a、_h、_band H.The results show that the five characteristic parameters show obvious regular changes,which are consistent with the macroscopic behavior,establishing the relationship between the macroscopic nonlinear mechanical behavior and the microscopic damage mechanism.