Study On Behaviors Of Partially Prestressed Concrete Beams With 600MPa Non-prestressed Steel Bars

High strength steel bars can improve the ultimate bearing capacity of concrete structure effectively,yet whose normal service performance can not meet the requirements frequently.The combination of high strength non-prestressed steel bars and prestressed technology can enhance the performance of concrete structures.At present,there are few researches on partially prestressed concrete structure with 600 MPa steel bars domestically.Combining of test studies,theoretical analyses,and numerical calculations,this paper presents a study on flexural behaviors of partially prestressed concrete beams with 600 MPa non-prestressed steel bars under the static load and fatigue load.The main researches are summarized as follows:(1)The flexural tests of bonded and unbonded partially prestressed concrete beams with 600 MPa non-prestressed steel bars under the static load were carried out to analyze the stress characteristics,damage patterns,and development laws of crack and deflection.The results indicate that the loading process of beams falls into three stages,and the beams keep high plastic deformation capacity until ductile failure.600 MPa steel bars and prestressed strands maintain good synergetic tensile behavior,with bringing the strength and ductility of 600 MPa steel bars into full play.(2)Based on the static loading tests,the crack,deformation,ductility,cracking load,flexural capacity,etc were analyzed.The results indicate that the formulas in Code for Design of Concrete Structures can be used to calculate the prestressed loss,crack spacing and width,bending stiffness,cracking load and flexural capacity of partially prestressed concrete beams with 600 MPa non-prestressed steel bars,and the formulas of calculating the maximum crack-width and post-cracking bending stiffness should be modified by introducing the coefficients.The maximum crack-width of specimens under the permanent or frequent combination of load can meet the limit basically,but the maximum crack-width under the standard combination of load exceeds the limit.(3)The flexural behaviors of bonded and unbonded partially prestressed concrete beams with 600 MPa non-prestressed steel bars were studied comparatively.The results indicate that the cracks of specimens distribute evenly and the deflections develop stably.About the normal service performance,the bonded specimens are better than the unbounded ones,because of smaller crack-width and deflection.With regard to the flexural capacity,it is very effective to improve the ultimate flexural capacity by applying 600 MPa steel bars or raising the reinforcement ratio of them,and the capacity of the bonded specimens are 3.20% higher than that of the unbounded ones.As for the ductility,the displacement and curvature ductility coefficients of the unbonded specimens are 7.34% and 10.02% higher than that of the bonded ones.(4)Based on the theoretical analysis and linear regression,the formula of calculating ultimate stress increment of the unbonded prestressed reinforcement,adapted to unbonded partially prestressed concrete beams with 600 MPa non-prestressed steel bars,is proposed,which has the reasonable scope and high accuracy.Based on the theoretical analysis,the simplified formula of calculating flexural capacity,adapted to partially prestressed concrete beams with 600 MPa non-prestressed steel bars with the mixed reinforcement index as a single variable,is suggested.The strength design value of 600 MPa steel bars is put forward.(5)The flexural tests of partially prestressed concrete beams with 600 MPa non-prestressed steel bars under the fatigue load were conducted to analyze the varying regularities of the concrete strain,600 MPa steel strain,deflection.The flexural behaviors of the fatigue-specimens were analyzed by comparing with the contrastive ones.The results indicate that there is only a slight growth of the 600 MPa steel strain and strand stress under the fatigue load.Comparing with the contrastive ones,the flexural capacity and 600 MPa steel strain of the fatigue-specimens remain changed slightly.The strain distribution at mid-span section still confirms to the plane-section assumption,yet the crack-width and deflection are larger.The development laws and calculation methods of fatigue stress amplitude of concrete and 600 MPa steel bars are discussed,and the stress amplitude limit of 600 MPa steel bars is proposed.