Study On Flexural Fatigue Performance Of Ultra High Performance Concrete With Coarse Aggregate

With the rapid development of China's economy,the demand for large span bridges,highways and other buildings is increasing year by year.The performance of ordinary concrete has been unable to meet,and it is urgent to popularize concrete materials with better comprehensive properties.Ultra high performance concrete containing coarse aggregate(UHPC(CA))is the cement base material with high compressive strength,high flexural strength and excellent durability,besides,because of its incorporation of coarse aggregate,it has lower cost,lower requirement for molding and curing process,less energy consumption and higher elastic modulus and shrinkage cracking resistance relative to RPC material.However,adding coarse aggregate will not only bring the coarse aggregate-matrix interfacial transition zone,but also influence the distribution of steel fiber,it brings certain risks to the flexural performance and fatigue performance of concrete,so we need to study the influence and mechanism of coarse aggregate content on flexural and fatigue properties of ultra-high performance concrete,and on this basis,the fiber content and fiber morphology are adjusted to improve the fatigue performance.The effect of coarse aggregate volume fraction(0 kg/m~3,280 kg/m~3,400 kg/m~3,480 kg/m~3,560 kg/m~3),fiber content(2.0%,2.5%)and fiber morphology(flat type,end hook type)on compressive strength,elastic modulus,four point bending strength and flexural fatigue properties are studied in this experimental.The performance of coarse aggregate-matrix interface transition zone and three-dimensional distribution of fiber in matrix are tested combined with the SEM micro analysis technology,nano indentation technique and X-CT scanning technology.Furthermore,the micro mechanism of the effect of coarse aggregate and steel fiber on ultra high performance concrete is revealed.The results are as follows:(1)The coarse aggregate has a negative effect on the flexural strength of UHPC(CA),and the negative effect is enlarged with the increase of the amount of coarse aggregate.When the content is 560 kg/m~3,the bending strength is reduced by 21.2%;The effect of coarse aggregate on the compressive strength is little(0.4%-4.5%);The effect of coarse aggregate on modulus of elasticity is significant,the maximum can be raised by 7.8%compared with the base group.Increasing the fiber content or adjusting the fiber morphology can improve the flexural strength of UHPC(CA),and the addition of end hook type fiber has more obvious effect on flexural strength,flexural strength increased by 14%.The effect of the amount and morphology of steel fiber on modulus of elasticity and compressive strength is not obvious;(2)The trend of average fatigue life with the amount of coarse aggregate is related to the stress level.When S=0.9,the average fatigue life decreases with the increase of coarse aggregate content.When S=0.7 and 0.8,the average fatigue life presents a trend of first increase and then decrease with the increase of coarse aggregate content,and average fatigue life reaches the maximum value when the coarse aggregate content is 400 kg/m~3.At all levels of stress,increasing the amount of steel fiber can increase the fatigue life.The influence of fiber morphology on fatigue life is related to the maximum stress level.When S=0.7,the end hook type fiber can improve fatigue life.When S=0.8 or 0.9,the end hook type fiber has little effect on fatigue life;(3)The damage of concrete is characterized by micro crack initiation and expansion on the meso,and is characterized by the unrecoverable residual plastic deformation after unloading on the macroscopic.With the decrease of the stress level,the number of fatigue micro cracks increases,which is characterized by multiple cracks.With the development of fatigue,fatigue damage accumulation,and the accumulation rate is faster and faster until the specimen is destroyed.There is a good linear relationship between the residual deflection value and the degree of fatigue damage,when the residual deflection reaches about0.07mm,the damage variable D=1,that is to say,the material begins to have a macro damage;(4)The correlation coefficient of S-N curve of UHPC(CA)is around 0.9,and this indicate that the S-N curve is in accordance with the linear relationship.The effect of the amount of coarse aggregate on the S-N curve is mainly reflected on the slope of the curve,and the slope of the S-N curve becomes larger with the increase of the volume of coarse aggregate.When S=0.7,the fatigue life is reduced with the increase of the amount of coarse aggregate,and the opposite trend is present when S=0.9;(5)The equivalent fatigue life of N_P under the two parameter distribution is basically linear and the linear relationship of the single logarithmic fatigue equation is basically established.The equivalent fatigue life of UHPC(CA)can obey two parameter distribution well.Moreover,the correlation coefficient r is related to the guarantee probability P.The greater the guarantee probability,the lower the correlation coefficient;(6)The coarse aggregate has a negative effect on orientation coefficient of steel fiber.The increase of the amount of coarse aggregate leads to the disorderly distribution of steel fibers,the smaller distribution probability along the long axis,and the smaller effective length of the fiber.The volume of coarse aggregate has little influence on the interface transition zone,and the volume of coarse aggregate increases,the thickness of the interface and the modulus of elasticity are almost invariable.The change trend of fatigue life with the effective fiber length and orientation coefficient is related to the level of stress level,when S=0.9,the fatigue life of concrete increases with the increases of effective length of the fiber;When S=0.7 and 0.8,the fatigue life of concrete increases first and then decreases slightly with the increase of the effective length and orientation coefficient of the fiber.