Carbon reduction and strength enhancement in functionally graded reinforced concrete beams

The production of cement alone is responsible for 5-8% of global CO2 emissions. This high environmental footprint can be partly attributed to inefficiencies at the material and structural level in concrete structures. Although the performance requirements vary spatially within a structural element, a single concrete with the same properties is typically used ubiquitously, instead of being tailored to the requirements. This leads to inefficiencies in concrete utilisation and embodied carbon. This research addresses these inefficiencies and investigates a novel approach to the enhancement of the environmental and mechanical performance of reinforced concrete structures. Shear-critical elements with a shear span-to-depth ratio of 2.5, a longitudinal reinforcement ratio below 1.3% and no transverse reinforcement were tested to failure. Functionally graded concrete and voids were used to engage a preferential internal resisting mechanism reliant on the internal arch action. Volumes that were superfluous or detrimental were either cast with a lightweight concrete and with lower embodied carbon, or left unfilled. Novel geometries were developed through an innovative fabrication methodology with removable inserts, achieving wet-on-wet interfaces between concrete mixes. Net increases in performance were achieved in all specimens compared to conventional prismatic reference beams. In the specimen with the highest performance, the overall resistance almost doubled whilst the calculated embodied carbon was reduced by approximately a third. The strategic use of functional grading and voids altered the cracking pattern and the development of critical shear cracks in particular. The initiation, evolution and propagation of cracks was engineered to preserve the preferential resisting mechanism. Specimens were consequently allowed to deform and divert damage away from the internal arch. As a result, brittle shear failures were delayed or prevented. In the most beneficial cases, the full plastic flexural capacity of the specimens was developed through yielding of the steel reinforcement. These findings lead to a better fundamental understanding of damage, cracking and shear behaviour in reinforced concrete. They demonstrate the potential for a new generation of more sustainable reinforced concrete structures with higher mechanical resistance and lower environmental impact.