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...

Strain Hardening Cementitious Composite (SHCC) is an innovative type of fibre-reinforced cement-based composite that has superior tensile properties. Because of this, it holds the potential to enhance the shear capacity of reinforced concrete (RC) beams, if applied properly.

Experimental research was thus carried out with the purpose of investigating the shear behaviour of reinforced concrete beams enhanced with thin SHCC laminates (10 mm in thickness) in their webs (henceforth referred to as hybrid SHCC-concrete beams). This research distinguishes itself from other studies by the fact that the hybrid beams were manufactured by casting conventional concrete inside pre-cast SHCC laminates, consequently, forming an interface between concrete and SHCC. Moreover, two different types of SHCC-concrete interface designs have been used in the hybrid beams, namely smooth and profiled ones. Furthermore, beams with and without transverse reinforcement (stirrups) were both prepared in order to investigate the effect of two specific methods of shear reinforcing (i.e., SHCC and stirrups) on each other. On top of that, one of the hybrid beams with stirrups has been supported only at its normal concrete core to uncover any irregularities in the shear behaviour compared to a beam supported at its full-width. Conventional RC beams (without SHCC laminates) were also prepared as references. All beams were tested in a three-point bonding set-up while monitored by two separate systems: Digital Image Correlation (DIC) and Linear Variable Data Transformers (LVDTs). The camera images were analysed by the software package, GOM Correlate 2019. During the casting of the beams, samples of all materials were taken and tested to establish their mechanical properties for quality control purposes.

Results show that the hybrid beams have obtained higher shear capacity than the control group. Only the hybrid beams with a minimum amount of shear reinforcement were capable to activate SHCC web laminates to their full extent. In the case of hybrid beams without transverse reinforcement (TR), only half of SHCC laminate potential was utilised approximately.

This study proves that, by applying advanced material (i.e., SHCC) properly, an efficient way of improving the shear capacity of an RC beam can be achieved, so long there is minimum TR provided. If a minimum transverse reinforcement is not present, the benefits of shear enhancement by SHCC become less effective due to the low Young’s modulus of SHCC.