Development of analytical flexural models for steel fibre-reinforced concrete beams with and without steel bars

[摘要] ENGLISH ABSTRACT: There is an increasing demand for the development and use of innovative materials withreduced cost of construction while offering improved structural properties. Steel fibrereinforced concrete (SFRC) can be used as a structural material to substitute the conventionalreinforcing bars partially or fully. However, there is little or no codified approach on thedesign procedures for SFRC members in the latest guidelines outlined in the draft 2010Model code.It is against this background that analytical methods are derived in this study for thedetermination of the flexural capacity of strain-softening, deflection-hardening SFRC withand without steel reinforcing bars. Models used for the determination of the flexural capacityof SFRC rectangular sections are based on equivalent stress blocks for both compression andtensile stresses. These are derived from an elastic-perfect plastic model for compression andeither an elastic-constant post-peak response or Rilem's multi-linear model for tension, inwhich strain compatibility and force equilibrium theories are used. By employing theequivalent stress blocks for both tensile and compressive stress states, parameters are definedby converting the actual stress-strain distribution to an equivalent stress block, depending onthe ratio of yield (or cracking) strain and post-yield (post-cracking) strains. Due to thesimplicity of a drop-down tensile model and a bilinear compression model, these materialmodels are used for the subsequent derivation of the flexural models for both SFRC with andwithout steel reinforcing bars.An experimental program is designed and executed for model verification. This includesmaterial characterisation experiments for the determination of material model inputparameters, and main beam flexural experiments for the determination of the beam bendingcapacity. An indirect tensile test is used for the characterisation of the tensile behaviour whilea four-point bending test is used for beam bending behaviour.Both flexural models for SFRC with and without reinforcing bars have been verified to fairlypredict the flexural capacity of the beams. However, the flexural model for SFRC with steelbars offers some challenges as to whether the synergetic effect of using both steel bars andsteel fibres should be incorporated at the low fibre volumes as used in the verificationexercise. Furthermore, the use of indirect methods to characterise tensile behaviour addedsome uncertainties in the material model parameters and hence may have affected thepredictability of the model. More research on the verification of the models is required toenable the use of a wider concrete strength spectrum for the verification and possiblemodification of the models. Studies on the model uncertainty may also help determine thereliable safety factor for the use of the model in predicting design strength of beam sections ata prescribed reliability index.

[发布机构] Stellenbosch University