Early Shrinkage Deformation And Crack Development Patterns In Ultra-High Performance Concrete Research

Ultra-high performance concrete(UHPC)is a new type of cementitious composite material with ultra-high strength,high toughness and ultra-low porosity,which has been used in the repair and reinforcement of bridges and structures for its excellent mechanical properties and high durability,and has produced good social and economic benefits.However,UHPC has been used in construction projects.However,UHPC has not been widely used in the construction field,mainly due to its low water-cement ratio and the large amount of cementitious materials in its components without coarse aggregates,which aggravates the early shrinkage deformation of UHPC and increases the risk of cracking of UHPC under constrained conditions.Based on the above analysis and combined with the Jilin Provincial Education Department’s science and technology project "R&D and application of mixed fibre low shrinkage steam-free ultra-high performance concrete",and in order to promote the application of UHPC in more engineering fields,this paper investigates the mechanical properties,early internal humidity,early shrinkage deformation performance and early age of ultra-high performance concrete with different steel fibre admixtures.In order to promote the application of UHPC in more engineering fields,this paper conducts an experimental study on the mechanical properties,early internal humidity,early shrinkage deformation performance and early age constrained shrinkage crack development of UHPC with different steel fibre admixtures.The main research contents and conclusions are as follows:(1)The compressive strength,splitting tensile strength,flexural strength and bending toughness of UHPC were tested under different steel fibre admixture levels according to the latest domestic and international standards and evaluation methods.The results show that the mechanical properties of UHPC are significantly enhanced by the incorporation of steel fibres with a length-to-diameter ratio of 65.At 3% steel fibre incorporation,the increase in compressive strength of UHPC was relatively small at 16.4%,but significantly increased the splitting tensile strength and flexural strength by 193.9% and 147.0% respectively.The optimum dose of steel fibres for a length-to-diameter ratio of 65 is 3%,taking into account economic factors and the increase in mechanical properties of steel fibres.(2)Through the early internal relative humidity test and early shrinkage deformation test,the study on the internal relative humidity and early shrinkage deformation of ultra-high performance concrete under different admixture levels of steel fibres with an aspect ratio of 65 was carried out,and the results showed that: the increase in the admixture level of steel fibres would reduce the early internal relative humidity;the prediction model of the influence of existing cementitious materials on the internal humidity was analysed,and the influence of the water-cement ratio and the action time was taken into account,and it was proposed that The new prediction model for the effect of cementitious materials on internal humidity was analysed and the reliability of the model was verified;the early shrinkage and deformation of UHPC shows a fast and then slow trend,which is the same as the decreasing trend of internal relative humidity,indicating that the internal relative humidity is the driving force for the early shrinkage and deformation of UHPC.The increase of steel fibre doping has an obvious inhibiting effect on the early shrinkage deformation of UHPC.The shrinkage reduction effect decreases when the steel fibre admixture is too high,and when the steel fibre admixture increases from 3% to 3.3%,the shrinkage value increases by 17.3% instead.(3)The effectiveness of steel fibres in limiting shrinkage cracks in UHPC at early age was investigated through early age crack resistance comparison tests,with the variation parameters of steel fibre L/D ratio and dosing amount.The results show that for steel fibres with an L/D ratio of 65,the crack limiting effect on UHPC is better with the increase of fibre dosing,and the best crack limiting effect is obtained at 3.3% of L/D ratio of 65 steel fibres;the crack limiting effect of 80 steel fibres on UHPC is better than that of 65 steel fibres,and the optimum crack limiting effect is achieved at 2% of dosing,while too high dosing will weaken the crack limiting effect of fibres.The high dosing rate will reduce the crack limiting effect of the fibres.