Crack-width control of concrete structures, a serviceability limit state (SLS) criteria, could be governing in design calculations over ultimate limit state (ULS). To ensure adequate crack-width control, additional steel reinforcement is often used, which, while essential for SLS, is redundant for ULS purposes, thereby increasing the environmental impact due to excessive steel use. In efforts to reduce the amount of steel reinforcement required in RC beams, a series of MSc thesis studies at TU Delft have explored the potential of incorporating a layer of Strain-Hardening Cementitious Composites (SHCC) in the tension zone of beams, creating what are known as hybrid Reinforced concrete/SHCC (R/SHCC) beams. SHCC is a composite material that, through its specific composition and fiber incorporation, exhibits the ability to form multiple fine cracks when subjected to tensile stresses, offering an effective solution for enhancing crack-width control. Huang [1] demonstrated the effectiveness of a 70mm thick SHCC layer in a 200mm high hybrid R/SHCC beam. Singh [2] investigated the role of interface preparation on the crack-width control performance and found that both smooth and grooved SHCC-concrete interfaces provide similar flexural crack-width control in a 200mm high hybrid R/SHCC beam with a 70mm thick SHCC layer. Bezemer [3] explored the effectiveness of a 70mm thick SHCC layer in hybrid R/SHCC beams of more practical heights of 300mm and 400mm, revealing an anticipated decline in the layer’s flexural crack-width control efficiency as beam height increased.

The current study investigates the effect of three key parameters on crack width control: (1) the SHCC-concrete interface, ranging from a smooth interface to a profiled interface coated with Vaseline; (2) the rebar-SHCC bond, weakened by changing from ribbed to smooth rebars; and (3) the SHCC type, varying from PVA-based SHCC to PE-based SHCC. Additionally, the effect of curing time is examined. The beams, with a total height of 400mm and a 70mm thick SHCC layer, are tested experimentally in a four-point bending set-up to generate a constant bending moment region (CBMR), allowing for the study of flexural cracks. Digital Image Correlation (DIC) is performed to evaluate the crack patterns and the crack-widths. The performance of the beams is assessed by checking its capability to restrict crack-widths below the 0.2mm and 0.3mm crack-width limits.

The experimental results demonstrate that the use of a Vaseline-coated profiled SHCC-conventional concrete (SHCC-CC) interface enhances the crack-width controlling ability of hybrid R/SHCC beams compared to a smooth interface. The beam with this profiled interface demonstrates a significant increase in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded of 37.6% and 22.7%, respectively, compared to the beam with a smooth interface. This improvement is attributed to the interface’s ability to show controlled delamination (as a result of the mechanical interlock provided by the shear keys) over the full length of the region of interest (as a result of the chemical debond facilitated by the Vaseline coating).

The use of smooth rebars significantly compromises the crack-width controlling ability of hybrid R/SHCC beams, as the beam with smooth rebars experiences a decrease in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded of 51.0% and 35.7%, respectively, compared to the beam with ribbed rebars. This reduction in performance can be attributed to the weakening of the reinforcement-SHCC bond, which relies solely on chemical adhesion and friction with smooth rebars. This limitation hinders the activation of SHCC and leads to rapid localization of cracks within the SHCC. Despite the application of a Vaseline-coated profiled SHCC-concrete interface in both beams, the benefits of the Vaseline-coated profiled interface observed in the beam with ribbed rebars were not realized in the beam with smooth rebars, indicating that a sufficient bond strength is a prerequisite for effective crack-width control.

PE-SHCC improves crack-width control in hybrid R/SHCC beams compared to PVA-SHCC when used in combination with smooth rebars. The beam with PE-SHCC exhibits an increase in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded of 6.1% and 14.9%, respectively, compared to the beam with PVA-SHCC. This improvement can likely be attributed to the larger ductility of PE-SHCC and the superior reinforcement-SHCC bond strength in PE-SHCC, which facilitates greater activation of the SHCC. However, it remains uncertain whether these findings can be extended to the use of ribbed rebars, as the superior reinforcement-SHCC bond strength in PE-SHCC may lead to a excessively high bond strength which could hinder strain redistribution.

The experimental results also demonstrate that curing time significantly influences the crack-width controlling ability of hybrid R/SHCC beams, with the beam tested at a later age (85 days of SHCC age) showing superior crack-width control compared to the beam tested by Bezemer [3] at 55 days of SHCC age. Specifically, there is an increase in the load, expressed as a percentage of the yield load, at which the 0.2mm and 0.3mm crack-width limits are exceeded in the beam tested at 85 days of SHCC age of 10.6% and 5.5%, respectively, compared to the beam tested at 55 days of SHCC age. This improvement is attributed to two factors: (1) the tensile properties of SHCC degrade over time, delaying crack localization, and (2) the SHCC-CC interface bond strengthens over time, resulting in less pronounced delamination. Consequently, while less SHCC is activated, the ability of SHCC to serve as effective reinforcement is enhanced.

In conclusion, the current study demonstrates that crack-width control in hybrid R/SHCC beams can be significantly improved by employing a roughened interface and utilizing ribbed rebars instead of smooth rebars when working with PVA-SHCC. Additionally, when using smooth rebars, opting for PE-SHCC enhances crack-width control. The current study also shows that hybrid R/SHCC beams of more practical height can effectively control crack-widths beyond reinforcement yielding when implementing the proposed design adjustments.