Tensile and Flexural Properties of Discrete Innovative Hooked-End Steel Fibre Reinforced Self-Compacting Concrete
: A Comprehensive Experimental Study

  • Clifford Alexander Okeh

Student thesis: Doctoral Thesis


The use of steel fibres to reinforce plain concrete has shown remarkable improvement in the tensile and flexural properties with respect to ductility and strength in tension through crack control, with optimum performance reported at 0.75% steel fibre content in bending and effect on the serviceability limit state design. Additional benefits in the mechanical properties have been reported when used in self-compacting concrete because of the alignment it provides to the steel fibre to control crack amongst other (environmental, health, and safety) benefits it offers. The use of different types of steel fibres have contributed to the improved performance of concrete over the decades, yet the hooked-end steel fibre still provides the most significant enhancement in the material properties when compared to others, which accounts for the 67% of the steel fibres sold. Moreover, up till date, there persists variability in the scatter of the tensile and flexural properties of steel fibre reinforced concrete, which accounts for higher material safety factors in current design practice, making it difficult to define an acceptable design standard for the material behaviour in tension. This study aims to use the new innovative S1, M1, and M2 hooked-end steel fibres to reduce the large scatter of the material properties in a bid to lower design safety allowance for future SFRC applications. The study adopts the experimental research design due to limited empirical data, and it comprises the use of eight experimental test methods which include the uniaxial direct tension test, bending test methods, pull-out test, amongst others. The research variables include the effect of the S1, M1, and M2 macro hooked-end steel fibre types; effect of 0%, 0.25%, 0.5%, 0.75%, and 1% steel fibre content; and steel fibre hybridisation (macro: micro steel fibres) on the material properties of self-compacting concrete. The effect of S1, M1, and M2 steel fibre types at 10mm, 20mm, and 30mm embedded depths on the pull-out parameters is also studied as well as the influence of the M2 steel fibre type on the flexural properties and resistance in a large-scale beam test. The results show that the shape and geometrical characteristics of the hooked-end steel fibres influence the workability of the concrete mix with a maximum reduction of up to 9%, and they have a significant effect on the material properties in the pull-out test, direct tension test, bending test, and additional benefits seen in steel fibre hybridisation with compressive strength increasing from 64MPa to 88MPa. The use of the M1 and M2 hooked-end steel fibres with double hooks modifies the softening behaviour during stress degradation in a pull-out test because of the influence on the residual stresses. Maximum values of up to 120% in the pull-out load and 150% in the pull-out dissipated energy are recorded for M1 and M2 respectively when compared to S1 as the embedded depth increases from 10mm to 30mm. The ultimate bond strengths derived are 9.86MPa for S1, 20.53MPa for M1, and 26.74MPa for M2. In the tension test, maximum increments of 4% for S1, 9% for M1, and 17% for M2 are observed in the mean tensile strength to the mean ultimate tensile strength only at high steel fibre content (1%), which depicts hardening behaviour. However, in the bending test, the maximum mean ultimate flexural strengths recorded are 15% for S1, 11% for M1, and 15% for M2 when compared to the mean flexural strength. Optimum performance in bending varies—S1 at 0.75% fibre content, M1 at 0.25%, and M2 at 1%. However, in tension, optimum is at 1% fibre content for S1, M1, and M2. Steel hybridisation shows a significant increase of up to 40% in both tensile and flexural strengths in M2 condition. Meanwhile, in S1 condition, 77% is seen in tensile strength while 13% is observed in the flexural strength. Optimum performance in the flexural strength in both conditions is observed at 75% Macro + 25% Micro. However, in tension, optimum tensile strength is seen at 25% Macro + 75% Macro. The constant parameters (K) and (C) which define the relationship between the tensile and bending stress as well as the tensile and bending strain at initial crack propagation for different fibre contents and hybridisation ratios are derived. The low residual stress values observed in the tensile and bending tests allow for a bi-linear relationship to describe the material behaviour in tension, which the proposed tensile material model from this study is based on. The proposed model has been calibrated from existing material models, and it uses key design parameters alpha (α) and eigen (ξ) ratios to define critical characteristic points which describe the material’s ductile nature and energy absorption. The study shows that, at the structural level, steel fibre inclusion in self-compacting concrete produces higher first crack load and ultimate load of 13% when compared to a similar beam without steel fibres and a reduction of up to 40% in the corresponding deflection. There is a 55% reduction in the crack spacing. The failure strain is also reduced by 56%, which results in lower energy absorption, reduced toughness, and ductility, because of the change in the failure behaviour, making the beam more catastrophic if used in earthquake resistant structures. The study concludes that there is the potential to reduce the material safety factor of SFRC structures and the associated design cost because of the reduction in the large scatter of the tensile and flexural material properties offered by the M1 and M2 hooked-end steel fibres, when compared to S1, although variability still exists in the scatter. A maximum of up to 42% reduction in the scatter is observed in the material properties associated with the pull- out test, 26% with the tensile test, 21% with the bending test, 75% with steel hybridisation, and 97% at the structural level.
Date of Award21 Jul 2023
Original languageEnglish
SupervisorDavid Begg (Supervisor), Stephanie Barnett (Supervisor) & Nikos Nanos (Supervisor)

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