| Ultra-high Performance Concrete(UHPC)is prepared based on the Maximum Density Theory and added with discrete short and thin steel fibers,which has ultra-high compressive strength,high tensile strength,good toughness and excellent durability.Thus,it has a good application prospect in civil engineering.When UHPC is applied to flexural members such as beams and plates,the bending properties of UHPC become one of its basic mechanical properties.Moreover,even in central region of China,the temperature of the structure may also approach-20℃ during the period of winter.Under this low temperature environment,the porosity of UHPC will be reduced due to the freezing of liquid water,and the properties of the micro steel fibers that determine the tensile properties of UHPC will also be changed,which may affect the mechanical properties of UHPC.Otherwise,UHPC can effectively reduce the dead load.When a structure made of UHPC material is subjected to cyclical load,the stress amplitude of live load is greatly increased,which increases the risk of fatigue damage.Therefore,the research on the anti-fatigue performance of UHPC has become one of the major issues that should be paid attention to.To sum up,the bending properties and fatigue properties of UHPC under low temperature environment are worthy of attention.However,there are few studies on the bending properties of UHPC at low temperature environment,especially the flexural fatigue properties.Therefore,in order to clarify the UHPC bending performance under low temperature,the bending properties and fatigue properities of UHPC at-20℃ and 20℃ were experimentally studied.the main research conclusions are as follows:(1)The compressive and split tensile tests of UHPC under-20 ℃ and 20 ℃were completed,and the results showed that: the compressive strength,split tensile strength and elastic modulus increased with the decrease of temperature.Compared with the corresponding values at 20℃,the cube compressive strength,the prismatic compressive strength,the splitting tensile strength and axial compressive elastic modulus of UHPC at-20℃ increased by 7.3% 8.0%,10.8%,and 4.1% respectively.(2)Four point bending tests were carried out on two groups of 6 UHPC prismatic specimens with rectangular cross-section under the temperature of-20℃and 20℃.The results showed that: All the bending specimens with rectangular section had bending failure in the pure bending section,and formed an obvious main crack in the pure bending section.The main crack of the specimen was slightly serrated at20℃,while the main crack of the specimen was relatively straight at-20℃.With the decrease of temperature,the strength of UHPC increases,but the deformation and ductility properties of UHPC decreases.Compared with the corresponding value at20℃,the flexural crack strength and bending strength of UHPC at-20℃ increased by13.7% and 9.0%,respectively.Compared with the corresponding value at 20℃,due to the flexural cracking strength of UHPC increased more significantly than its elastic modulus,so the initial crack strain of the flexural specimen increased by 6.3%.But the peak strain,ultimate strain,residual strain and ductility coefficient were reduced by 2.9%,8.2%,5.5% and 6.4%,respectively.(3)Four point bending tests were carried out on two groups of 6 I-section UHPC specimens under the temperature of-20℃ and 20℃.The results showed that: All i-shaped sections have bending failure in the pure bending section.With the decrease of temperature,the strength of the specimen increases,but its deformation performance is generally declining.Compared with the corresponding value at 20℃,the flexural cracking strength and bending strength of the I-shaped section specimens increased by 20.7% and 9.1% respectively at-20℃.Compared with the corresponding value at 20℃,the initial cracking strain of the I-section at-20℃ increased by 3.5%,but the peak strain,ultimate strain and residual strain were reduced by 13.5%,21.7%and 25.2%,respectively.When the temperature were constant,the two section forms have less influence on the cracking strength of UHPC,while they have greater influence on the bending strength of UHPC and the average longitudinal tensile strain within the base gauge.(4)Four point bending fatigue tests were carried out on UHPC prismatic specimens under the temperature of-20℃,-10℃ and 20℃,respectively.The results showed that: the UHPC flexural specimens show approximate brittle fracture fatigue failure in the pure bending section.As the temperature decreases,the fatigue life and fatigue strength of UHPC decrease.According to the fatigue life regression curve of UHPC bending specimens at different temperatures,the fatigue strength at-20℃,-10℃ and 20℃ is 17.43 MPa,16.98 MPa and 20.24 MPa respectively,and the corresponding stress amplitude is 0.51,0.49 and 0.64 respectively.The fatigue strength of the specimen without fatigue failure differs little from that of the specimen under static load,indicating that UHPC has good fatigue resistance and its residual secant modulus drops to about 55% of the initial secant modulus.(5)The variation of mid-span deflection,fatigue strain of the bottom and top surfaces of the pure bending section,crack width and fatigue deformation modulus with the number of cycles were studied.The results showed that: the mid-span deflection,fatigue strain and fatigue crack width increase with the increase of cycle number at the same stress level,and the increase at high stress level is greater than that at low stress level.When the specimen fatigue damage occurs,UHPC midspan deflection,strain fatigue and fatigue crack width with cyclic number broadly in line with the change rule of three phase change law: deflection,strain and crack width rapid development,steady growth of deflection,strain and crack width,deflection,strain and crack width rapid development stage.each stage respectively were 15%,75% and 10% of the total fatigue life;Under the action of fatigue load,the influence of temperature on mid-span deflection and crack width was not significant.With the decrease of temperature,the tensile strain at the bottom,the compressive strain at the top and the residual strain of the specimen in the pure bending section decreased.Once the fatigue deformation modulus of UHPC cracks,it will decrease significantly.Under the same stress level,the fatigue deformation modulus will decrease with the increase of cycle-index,and the decline will be greater at high stress level.The increase of deflection after fatigue is 1.2～1.8 times of static load.The increase of strain is 3～6times that of static load,and the increase of strain at -20℃ is generally greater than 20℃. |
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