Study on Shear Behaviours of Reinforced High-Performance SFRC Beams

This thesis mainly focused on developing high-performance SFRC (steel fibre reinforced concrete) with improved workability and enhanced mechanical properties. The bond behaviour between the high-performance SFRC and high-strength steel bar was investigated as the foundation of its application in structure members. SFRC beams, with steel reinforcement, under shear were then studied. The research is composed of three aspects: (1) The vibration process during the forming of fresh concrete alters the distribution patterns of the steel fibre in high-flowable SFRC. This directly affects the mechanical properties of SFRC, including compressive strength, flexural tensile strength and toughness, and energy absorption capability, etc. The effect of the forming process on the uniformity of SFRC matrix and mechanical properties of SFRC needs to be determined for further applications. A series of experiments were conducted with different parameters, including the slump of fresh SFRC of 80 - 200mm, the steel fibre volume fraction of 0–2.0%, the vibration time of 10s, 30s, 60s, and the vibration methods of using the standard vibration table and a plate vibrator on the top surface of specimens. Standard cubic, cylinder, and beam specimens were cast for testing. Some of the specimens were tested for compressive strength, splitting tensile strength, and flexural tensile strength, while others were cut into pieces for counting steel fibres dispersed in the cross-section. Data were collected and plotted into charts for analysing steel fibre distribution patterns and mechanical properties of SFRC with various flowability of fresh SFRC. Results indicate that steel fibre has a tendency to sink towards the bottom of specimens with the increase of flowability of fresh SFRC and vibration time. This led to the segregation of composites among the concrete matrix and worse mechanical properties of SFRC. Thus, the optimal vibration time for each flowable SFRC was determined to ensure the uniformity of the concrete matrix. Different compacting methods also have an impact on the performance of SFRC. To account for the influence of various compacting methods, a compaction impact factor has been introduced. Also, formulas for calculating splitting tensile strength and flexural tensile strength, considering the reinforcement of steel fibre and the effect of the compacting method, have been proposed. (2) The steel fibre among SFRC significantly controls the micro- and macro- crack growth during the steel bar pull-out test. This enhances the bond performance between high-performance SFRC and high-strength steel bars. Experiments of specimens for central pull-out test were conducted to study the bond strength and bond-slip with parameters, including steel fibre volume fraction of 0~ 2.0%, compressive strength of SFRC of 40 MPa and 50 MPa, diameter of steel bars of 14~ 20 mm, and yield strength of steel bars of 400 MPa and 500 MPa. The height and length of the specimen for the bond strength test are 13d (d = diameter of steel bar). The bond length is 5d. Charts and curves were plotted using the collected data. Results indicate that the addition of steel fibre significantly increases the bond strength and the bond-slip ductility. By conducting numerical analyses, formulae for calculating the bond strength and anchorage length were proposed. Bond stress-slip curves based on different codes and methods were plotted and compared. (3) Considering the different levels of controlling shear crack width, steel fibre is expected to be used to promote the shear capacity of reinforced high-flowable SFRC flexural members. This needs a design method for the high-flowable SFRC to be applied in engineering structures. The shear performances of reinforced SFRC T-section beams were experimentally studied at a shear-span-to-depth ratio of 2.5 under the four-point test. Twelve SFRC T-section beams reinforced with longitudinal reinforcement only were produced, each with a depth of 700 mm and span of 3.5 m. Additionally, twenty-four SFRC T-section beams reinforced with stirrups were manufactured, varying in depth from 400 mm to 1300 mm and spanning from 2.0 m to 6.5 m. The parameters of the experimental study are the steel fibre volume fraction of 0.4~2.0%, and the stirrups spacing set as 300 mm, 350 mm and 400 mm. The strains of SFRC at the shear-compression zone and diagonal section, the stirrups strain, the mid-span deflection, the shear crack width, the first shear-crack force, and the ultimate shear force were measured. Effects of these experimental parameters on the first shear-crack resistance, elongation and expansion of diagonal cracks, deformation and ultimate shear capacity were studied. The size effect of section depth and the possibility of using steel fibre to partially replace stirrups were evaluated as well. Experiment data from 512 SFRC slender rectangular-section beams reinforced with only longitudinal reinforcement, as well as beams with stirrups, were collected and plotted in diagrams. These diagrams were then used to compare the results with predictions obtained from well-known existing formulae. This comparison aims to provide further clarity on the effects of multiple factors on shear capacity. Based on the shear model for the critical shear crack considering multi-actions, various components contributing to shear resistance were evaluated, which include the shear resistances provided by the SFRC in uncracked shear-compression zone, the dowel action of longitudinal tensile steel bars, aggregate interlock along the sides of the critical shear crack, the steel fibres bridging critical shear crack, and the section depth size effect. Based on these evaluations, a semi-empirical synergetic formula was proposed for predicting the shear capacity of the SFRC slender rectangular-section beams. Considering the different limit levels of shear-crack width on the web of T-section beams and accounting for the influence of the flange of T-section on shear performance, a semi-empirical synergetic formula and two conservative formulae are proposed to predict the shear force subjected by SFRC for reinforced SFRC T-section beams.<p></p>