Cracks that develop in concrete due to tensile stresses can lead to the corrosion of embedded reinforcing steel, which is a primary cause of concrete deterioration. It's common practice to introduce additional reinforcement to limit crack width in concrete. Another potential solution involves partially substituting concrete with alternative materials. Strain-hardening cementitious composites (SHCC) display ductile behavior, forming multiple fine cracks under tension, making them a potential candidate for use in conjunction with traditional concrete. This study aims to explore the flexural behavior of SHCC-RC hybrid beams and the shear behavior of SHCC-RC hybrid beams without transverse reinforcement. This investigation encompasses load-bearing capacity, crack patterns, crack width control, and post-cracking shear ductility.

The SHCC-RC hybrid beams consist of a SHCC U-shaped formwork with lower-quality concrete cast inside. The feasibility of 3D printing the stay-in-place formwork is also under examination. Two types of experiments were conducted to examine the structural response of the SHCC-RC hybrid members: a four-point bending test to assess flexural behavior and a three-point bending test to study shear behavior. Digital image correlation (DIC) was used to evaluate cracking patterns and crack widths, and these measurements were corroborated with linear variable differential transformers (LVDTs).

Four specimens underwent bending tests: a control beam, a hybrid beam with a transverse profiled interface, a hybrid beam with both transverse and longitudinal profiled interfaces, and a hybrid beam with a 3D printed SHCC formwork. The experimental results of the bending tests reveal that SHCC-RC hybrid beams exhibit significantly higher bending capacities compared to the control beam. Specifically, the control beam reached a capacity of 98.3 kN, while the hybrid beams with transverse, transverse and longitudinal interfaces, and a printed SHCC formwork had capacities of 145.1 kN, 159.1 kN, and 152.4 kN, respectively. The interface properties between SHCC and concrete were robust enough to prevent complete delamination of the SHCC formwork. Additionally, the hybrid beams demonstrated superior crack width control. The hybrid beams with precast SHCC U-shaped formwork, with or without a longitudinal profiled interface, exceeded a maximum crack width of 0.3 mm at 85.9% and 86.3% of their capacity after the reinforcing steel yielded, respectively. In the case of a printed SHCC formwork, the maximum crack width exceeded 0.3 mm at 78.1% of the capacity, while the control beam exhibited this at 49% of its capacity, well before the reinforcement's yield point.

Furthermore, four specimens without transverse reinforcement were subjected to shear tests: a control beam, a hybrid beam with a transverse profiled interface, a hybrid beam with both transverse and longitudinal profiled interfaces, and a hybrid beam with a 3D printed SHCC formwork. The shear test results suggest that a stay-in-place formwork can marginally enhance the shear capacity of an RC beam. The control beam and the hybrid beam with a transverse profiled interface exhibited capacities of 103.7 kN, while the hybrid beam with both transverse and longitudinal profiled interfaces had a capacity of 113.5 kN. The composite beam with a printed SHCC lost formwork achieved a capacity of 124.5 kN, possibly due to unintended increases in SHCC formwork thickness. All hybrid beams displayed greater energy absorption capacity and superior post-cracking shear ductility compared to the control beam. Specifically, the energy absorption capacity for hybrid beams without a longitudinal profiled interface, with a longitudinal profiled interface, and with 3D printed formwork was 26.7%, 106.5%, and 160.5%, respectively...