To ascertain the promise of functionally graded TRC as a route for CO2 reduction, a multi-dimensional optimisation strategy that was pursued throughout the project that ranged from (a) investigations into the early age properties of layered concrete cast wet on wet, (b) quantification of the mechanical performance of functionally graded concrete, (c) fabrication techniques to combine concrete layers and TRC, (d) parametric design studies where a stepwise grading of concrete was implemented within a structure and (e) numerical modelling of functionally graded TRC beams to gain a better understanding of the structural behaviour.
Different mixes were developed throughout the project, with a wide spectrum of compressive strength and cement consumption, that allowed fur a stepwise grading of the concrete in TRC structures. The uniqueness of the approach followed here is, that the same raw materials were used where only the quantities varied, which facilitates the manufacturing process and reduces logistical challenges. The placement of multiple layers of concrete enables to grade the concrete according to the requirements within a structure, however, it also increases complexity. Results from the project demonstrate the high variability of stresses due to restraint shrinkage in dependency of different boundary conditions. These different influences were studied so that optimal settings, e.g. delay time of casting, were determined. The potential of a functional grading to increase performance was demonstrated on bond tests, where it was shown that when the reinforcement is fully enclosed by a high-strength mix, an increase in splitting resistance can be expected.
The functional grading in TRC components was subsequently realised on a structural level. Starting from an initial design of a lightweight TRC structure that was taken from literature, a stepwise refinement of the concrete grade was implemented, so that all requirements for the structure were still fulfilled. It was shown that such a functional grading would result in a reduction of 73.8 % of the cement mass compared to the original design. Moreover, inspired by an experimental campaign from members of Cambridges Concrete Infrastructure Research Group, comparative numerical calculations showed, that through the judicious replacement of concrete in an RC beam with a novel low-strength TRC shear panel, not only the ultimate capacity of the beam could be increased but also the structural stiffness could be retained at a high level. Finally, methods for the fabrication of functionally graded TRC beams were introduced, showcasing how functionally graded TRC beams can be produced.
The work cumulated in a tailored TRC prototype, where it was proven that a significant amount of material, and in particular cement, can be saved when designing concrete structures, while simultaneously achieving a high load-bearing capacity.
