Research On Crack Development And Normal Section Design Method Of Unreinforced Steel Fiber Concrete Segments

The problem of subway segment cracking has a significant impact on the loadbearing capacity and durability of the segment.Steel fiber concrete has good prospects for use in subway segments due to its excellent crack resistance and toughness.At present,in domestic tunnel segments,steel fiber concrete is basically used in conjunction with steel bars,and the use and research of unreinforced steel fiber concrete segments are rare.In view of this,this article relies on the second phase project of Chongqing Rail Transit Line 4,using literature research,basic mechanical tests,fullscale segment bending tests,numerical simulations and other methods to carry out crack development and crack control of unreinforced steel fiber concrete segments.Research on cross-section design methods,the main research work and conclusions are as follows:(1)Cube compressive test and bending toughness test were carried out for two different mixing conditions.The results show that the increase of steel fiber content has limited improvement in concrete compressive performance,and has little effect on bending and tensile strength,but for residual bending The tensile strength has been greatly improved.(2)The reinforcing nature of steel fiber to concrete lies in the bridging effect of fiber,which is related to fiber distribution and fiber drawing characteristics.The bridging effect of hook-shaped steel fiber is divided into straight section and hook section.After a certain simplification of the masterpiece,combined with the distribution function,the bridging stress model of the steel fiber is obtained.Comparing the applicability of various softening curves to steel fiber concrete,a softening curve suitable for this hook-shaped steel fiber is obtained.(3)Combining fiber reinforcement theory,bridging stress model and unreinforced steel fiber concrete segment compression and bending test results,statistical analysis of crack characteristic values was carried out,and the development law of width and height was summarized,combined with composite material theory and fracture mechanics Theoretically,the calculation method of crack width and spacing is obtained.Using the ABAQUS plastic damage model,the damage factor is introduced to simulate the crack development of unreinforced steel fiber concrete segments.The damage area overlaps with the crack distribution area in the actual test.It shows that this method is suitable for unreinforced steel fiber concrete segment cracking height and Calculation of range.(4)Analyze the applicability of the equivalent method for the tensile strength of steel fiber concrete in the specification,and propose a method for calculating the equivalent tensile strength based on the softening curve.From the perspective of fracture mechanics,combined with the softening curve and the compression stress distribution law in the compression zone,a design method for the normal section of unreinforced steel fiber concrete segments is proposed.Bring in full-scale segment loading test load calculation,the ratio of segment test bending moment to calculated bending moment is between 1.0 and 1.5,indicating that the design method is safe and reliable;combined with the softening curve and the above-mentioned normal section design method,no ribs are determined The limit state crack width index of the steel fiber concrete segment is 0.2mm,and the limit state crack width index of the bearing capacity is 1.6mm.