Study On Bond Behavior And Degradation Mechanism Of Composite Limestone Powder Concrete And Reinforcement Bar Under Sulfate Freeze-thaw Environment

Sulphate freeze-thaw damage often occurs in reinforced concrete structures in cold regions.Sulphate freeze-thaw not only degrades the frost resistance of concrete,but also destroys the bond performance between concrete and reinforcement.With the gradual application of limestone powder,fly ash,slag and other admixtures to replace cement in concrete,it is of great significance to study the bonding performance between composite limestone powder concrete and steel bar under sulfate freeze-thaw environment.This paper adopts physical test,theoretical analysis and model establishment method.C30 ordinary concrete,limestone powder concrete,limestone powder concrete,limestone powder-fly ash concrete,limestone powder-slag concrete and limestone powder-fly ash-slag concrete with water-binder ratio of 0.49 were taken as research objects.The bond properties between concrete and steel bars in conventional environment,freeze-thaw resistance?apparent morphology,mass loss,relative dynamic modulus of elasticity,compressive strength and splitting tensile strength?of concrete and bond properties?ultimate bond strength,peak slip and bond slip curves?between concrete and steel bars after 25,50,75 and 100 cycles of rapid freeze-thaw cycles in pure water and Na₂SO₄solution with 5%mass fraction,as well as microstructures?SEM,XRF and XRD?were studied.The main conclusions and innovative results were as follows:?1?The degradation law of sulfate freeze-thaw resistance of composite limestone powder concrete and the mechanism of sulfate freeze-thaw damage are revealed.Limestone powder aggravates the deterioration of freezing resistance of water-freezing cycle and salt-freezing cycle.Fly ash and slag slow down the deterioration of freezing resistance of water-freezing cycle.Slag also slows down the deterioration of freezing resistance of salt-freezing cycle,making the freezing resistance of salt-freezing cycle better than that of ordinary concrete.?2?The distribution of bond stress between composite limestone powder concrete and steel bar under conventional environment is obtained.The bond stress has a bimodal distribution on the anchorage length of 3d,5d and 7d.The maximum peak value of most specimens is at L/4^L/2 from the loading end,and the other peak value is at 3L/4^L from the loading end.?3?Obtained the change rule and influence mechanism of bond performance between composite limestone powder concrete and steel bar in different anchorage lengths in conventional environment with different admixtures and dosages.Limestone powder and fly ash reduce the ultimate bond strength and increase the peak slip of splitting failure;slag increases the ultimate bond strength,but reduces the peak slip of splitting failure.The bond performance between composite limestone powder concrete and steel bar decreases with the increase of anchorage length.?4?The degradation law and mechanism of bond property between composite limestone powder concrete and steel bar in sulfate freeze-thaw environment are revealed.Limestone powder aggravates the deterioration of bonding properties between water-freezing cycle and salt-freezing cycle.Slag and fly ash slow down the degradation of the bonding properties of the water-freezing cycle.At the same time,slag slows down the reduction of the ultimate bonding strength of the salt-freezing cycle,but the peak slip of the later period of the salt-freezing cycle is lower and the damage is more brittle.?5?Constitutive relationship of??s,d/L?in conventional environment and?(s,f_ts)in sulfate freeze-thaw environment is established.Correction coefficient of critical anchorage length in sulfate freeze-thaw environment is deduced,and the influence of limestone powder,fly ash and slag on critical anchorage length of structure is analyzed.?6?The adverse effects of rapid freeze-thaw cycles in 5%Na₂SO₄ solution on the frost resistance and bonding behaviors of composite limestone powder concrete with steel bars are generally greater than those in pure water.