PhD Oral Exam - Mostafa Rabea Attia Ibrahim, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Steel reinforcement has been used as internal reinforcement in reinforced concrete (RC) structures for many years. However, due to the corrosion problem, glass fibre-reinforced polymer (GFRP) reinforcement has been used as an alternative reinforcement in concrete structures subjected to harsh environments for decades. In many design cases, steel RC elements are controlled by ultimate limit state (ULS) requirements, resulting in over-designed sections for serviceability limit state (SLS). GFRP reinforcements have a lower modulus of elasticity than steel reinforcement. Thus, GFRP RC flexural members usually exhibit larger deflections and wider crack widths under flexural loads, which means the design of these elements is usually controlled by SLS checks, resulting in over-designed sections for the ULS. Therefore, the concept of combining steel reinforcement with GFRP reinforcement in RC structures appears to be a practical solution to overcome the serviceability problem of GFRP RC flexural members and benefits from the large ductility of steel reinforcement. This research investigates the serviceability and flexural behaviour of steel-GFRP hybrid RC flexural members through analytical investigations and experimental studies. It also provides design recommendations for designing hybrid RC flexural members and assesses the design equations of deflection and crack width in North America standards and guidelines.

This research started with analytical investigations to set a platform for designing steel GFRP hybrid RC sections. Based on fundamental theories of RC sections under flexure and the available literature, design considerations were suggested for designing steel GFRP hybrid RC flexural elements. The design considerations were recommended for both SLS and ULS, except deflection. Practical design charts were developed for the proposed design recommendation based on the fundamentals of section analysis. Finally, an extensive parametric study was undertaken to transform steel RC sections into alternative steel-GFRP hybrid RC sections with the same total number of bars.

The study then experimentally investigated the flexural and serviceability behaviour of steel-GFRP hybrid reinforced concrete (RC) beams through testing and analyzing 15 RC beams. Five concrete beams reinforced with steel reinforcement, eight reinforced with steel-GFRP hybrid reinforcement, and two reinforced with GFRP reinforcement were tested in flexure under a four-point load test setup. The fifteen RC beams were designed with different reinforcement ratios and steel and GFRP reinforcement configurations to investigate the effect of the ratio of GFRP to steel reinforcement and the influence of the effective reinforcement ratio on flexural behaviour. The experimental results of the beams were analyzed and compared in terms of the first cracking moment and yielding moment, failure modes, flexural capacity, concrete and rebar strain, mid-span deflection, crack width, and ductility. Based on the experimental results, steel-GFRP hybrid RC beams exhibited a greater ultimate strength than the steel RC beams but a lower ultimate strength than the GFRP RC beams. Moreover, the steel and hybrid RC beams exhibited higher ductility and energy absorption than the counterpart GFRP beams with the same reinforcement area. In addition, the results showed that the effective steel reinforcement ratio significantly affected the flexural stiffness of hybrid RC beams during the post-cracking stage until the yielding of steel reinforcements. Based on the cracked section analysis, a new equation was proposed to estimate the rebar strain for the post-yielding stage in the hybrid RC beams. The experimental results were also used to assess the bond-dependant coefficient, kb of the steel, GFRP, and hybrid RC beams.

The study then experimentally investigated the flexural behaviour of steel-GFRP RC one-way slabs reinforced with a combination of steel rebars and GFRP bars. Hybrid RC slabs can offer enhanced ductility, serviceability, and durability compared to those reinforced with the same total number of steel or GFRP rebars. Eight reinforced concrete slabs were tested under a four-point bending setup, including two reinforced with steel rebars and two with GFRP bars, and four steel-GFRP hybrid slabs were tested under a four-point bending setup. The main parameters investigated were the ratio of GFRP to steel reinforcement, bar diameter effect, and bar spacing. The experimental results of the slabs were analyzed and compared in terms of strain distribution, cracking behaviour, flexural response, and ductility. Based on the experimental results, steel-GFRP hybrid RC slabs exhibited a higher ultimate strength than the steel RC slabs but a lower ultimate strength than the GFRP RC slabs. Moreover, the deflection at the same load level increased by increasing the GFRP to steel reinforcement ratio in the cross-section. In addition, the results showed that the GFRP to steel reinforcement ratio significantly affects the flexural behaviour of hybrid RC slabs. The bond-dependent coefficient, kb, of the reinforcement was assessed using the experimental results. Finally, the cracking moment, the moment capacity, and the mid-span deflection of the slabs were calculated using existing design models.