Experimental and numerical investigations on the ultimate flexural strength of concrete beams reinforced with grade 750 steel bars

High strength steel bars have recently been added to design code provisions for reinforced concrete structures. However, there is a gap in the current knowledge on the flexural strength of beams reinforced with ultra-high strength steel, with nominal yield strength exceeding 690 MPa. Grade 750 ultra-high strength steel bars offer a 50 % increase in yield strength over traditional grade 500 bars alongside other benefits such as reduced mass, diminished reinforcement congestion and lower carbon emissions. To address this knowledge gap and evaluate the applicability of current design codes, a series of experimental tests were conducted on concrete beams longitudinally reinforced with commercially manufactured grade 750 steel. Equivalent beam designs employing grade 500 steel bars were tested for comparative analysis. The outcomes of these experiments revealed that current standard guidelines tend to provide conservative predictions for the moment capacity of beams reinforced with grade 750 steel. For further investigations on the ultimate flexural capacity of concrete beams reinforced with ultra-high strength steel, a finite element model was developed and validated against the experimental results. This model incorporated combined damage plasticity for concrete, nonlinear damage for steel and a three-dimensional bond-slip mechanism capturing the steel-concrete interface. Furthermore, a detailed analytical method was presented based on the constitutive behaviour of grade 750 steel. The strength predictions generated by this model were compared to results obtained from experiments and finite element simulations. This model demonstrated increased accuracy in predicting the ultimate flexural strength of beams reinforced with grade 750 steel bars to 92 %.