Mechanical Properties Of Basic Magnesium Sulfate Cement Concrete Members Under Natural Exposure Conditions

As a new type of concrete material,basic magnesium sulfate cement concrete(BMSCC)has the advantages of early strength,high strength,good toughness and good crack resistance.Its mechanical properties have many unique advantages compared with ordinary concrete.Beamcolumn members also have characteristics different from ordinary concrete members.However,the attenuation of the mechanical properties of BMSCC components exposed to natural environment for several years is not known.In order to apply this new concrete material to practical engineering,the study of the decay law of mechanical properties in the natural environment appears.Especially necessary.In this paper,three different strength grades of BMSCC were designed,and 24 BMSCC beamcolumn members were prepared.The mechanical properties of the BMSCC were tested in the natural environment of Shenyang for two years.At the same time,the influence of different steel corrosion rate on the physical and mechanical properties such as yield strength,ultimate strength and elongation at break of BMSCC environment was studied.Considering the influence of steel corrosion and the attenuation of concrete strength on the mechanical properties of the members,the calculation model of ultimate bearing capacity and deformation of BMSCC beam-column members is established.The main contents and conclusions of the full text are as follows:(1)The dry and wet cycle test of BMSCC simulated solution was carried out in three test groups of 9 ?10HPB300 steel bars,and 24 different degrees of rusted HRB300(secondary)in BMSCC beam-column members exposed for two years in the natural environment were intercepted.Reinforcement with HRB400(secondary).Through the tensile test research,the calculation formula of the relationship between the yield strength,ultimate strength,elongation at break and elastic modulus of the steel bar and the maximum section corrosion rate of the steel bar was established.The results show that when the average quality corrosion rate of steel bars is below 5%,the corrosion of steel bars is evenly rusted,and there is no pit corrosion.When the average quality corrosion rate of steel bars is 5% to 10%,the corrosion of steel bars is local pit erosion.With the increasing corrosion rate of steel bars,the yield strength,ultimate strength,elongation at break and elastic modulus of steel bars have different degrees of attenuation,and the relative change rate is linear with the corrosion rate of steel bars.(2)The ultrasonic propagation velocity(abbreviated as wave velocity)and surface rebound value of BMSCC specimens after two years of natural exposure were measured,and the special strength calculation formula of ultrasonic rebound synthetic method and rebound method for estimating BMSCC intensity was established.The calculation formula of the ultrasonic rebound comprehensive method with lower estimation accuracy was used,and the concrete strength of 24 BMSCC beam-column members was estimated.The results showed that BMSCC had a certain degree of attenuation of compressive strength after 2 years of natural exposure.(3)Using BTS two-way loading test bench to test the bending resistance test and shear performance of 12 BMSCC beam members(size 150mm×200mm×1500mm,reinforcement ratio 1.72%)of three concrete strength grades,Mechanical properties of crack development,failure morphology,deformation and bearing capacity of BMSCC beams.The results show that with the increase of concrete strength grade,the cracking load and ultimate load of BMSCC flexural beam and shear beam increase gradually,and the mid-span deflection decreases gradually.The number of cracks after cracking increases continuously but the extension is slow or even extended.The total number of cracks in the whole process is about 10;the maximum crack width before reaching the ultimate load is basically between 0.02mm-0.1mm.The maximum deflection of BMSCC before bending failure is between 5-8mm;the maximum deflection of BMSCC before shearing is 3-5mm.(4)Using the 200 T two-way loading test bench,the 12 members of the three concrete strength grades(240mm × 200 mm × 1500 mm,reinforcement ratio 2.68%)were subjected to large eccentricity and small eccentric compression test.Through the whole process of force observation,the mechanical properties such as crack propagation,failure morphology,deformation and bearing capacity of BMSCC column were studied.The results show that the number of cracks in the whole process of large eccentricity and small eccentric compression is very small,about 5-8,and the crack development speed is extremely slow;the maximum crack width before the maximum eccentric compression BMSCC column reaches the ultimate load is basically 0.02 mm.Between-2mm,the BMSCC column with small eccentric compression is basically between 0.02 mm and 0.08 mm.With the increase of concrete strength,the cracking load and ultimate load of BMSCC column increase gradually,and the axial displacement and lateral displacement are gradually increased.Reduced.The displacement in the mid-size and eccentric compression is basically between 4 and 7 mm,and the lateral displacement is between 3 and 6 mm.(5)Considering the influence of steel corrosion and natural attenuation of BMSCC strength on the ultimate bearing capacity and deformation of BMSCC beam-column members,the ultimate bearing capacity calculation model and stiffness calculation model of BMSCC beam members under bending and shear conditions are established.The ultimate bearing capacity and stiffness calculation model of BMSCC column members under large eccentricity and small eccentric compression.The established model is used to study the degradation law of the bearing capacity of BMSCC members.It is found that the ultimate bearing capacity of BMSCC beams is degraded by about 12%,the bending stiffness is reduced by about 17%,and the ultimate bearing capacity of BMSCC beams is not degraded.BMSCC column eccentricity The ultimate bearing capacity of the compression is degraded by 6%-7%,and the stiffness is degraded by 6%-13%.