The study presents an analytical and experimental investigation on the combined torsion and flexural behavior of bare and fiber-reinforced polymer (FRP) strengthened RC members. A refined model, COMBINED-SMM-FRP, is proposed to simulate the pre-crack, post-crack, and post-peak response of bare and FRP-strengthened members subjected to a constant torsion to flexure ratio, where each of the membrane elements is subjected to different boundary conditions and responses. The model predictions are compared with experimental results from the literature covering different FRP types, FRP spacing, wrapping methods, and torsion-to-flexure ratios. Consistent agreement is obtained, and the model can be used to predict the behaviour of bare and FRP-strengthened RC beams. In addition to the analytical model, an experimental program is presented to examine whether externally bonded FRPs can fully suppress premature concrete cover spalling under torsional action. Specimens were strengthened by externally bonded FRP and tested under the combined action of torsion and flexure. The provision of externally bonded FRP is observed to substantially enhance both the cracking and ultimate capacity of members that are susceptible to premature concrete cover spalling. However, it was found that FRPs did not fully suppress premature cover spalling. By incorporating a limit for the spalling of cover, the proposed model can also be used to explore the behavior of RC members with a thick concrete cover where premature spalling occurs.

Inverted T precast girders with a cast-in-situ topping layer, recognized as precast composite girders, are commonly used in Dutch bridge construction. Notably, the bridges built before 1974 often lacked sufficient shear reinforcement, raising concerns about their shear capacity under increasing traffic loads. However, how to assess these composite girders under the scope of the second-generation Eurocode remains challenging, as the shear formulations were originally developed for monolithic structural members. Consequently, their direct applicability to precast composite systems, due to the distinctive stress distribution in the web of the composite structural members, lacks theoretical substantiation and experimental validation. This study first presents the three alternative failure criteria equations based on the same theory, and after that, an experimental investigation of the shear behaviour of precast composite girders through two full-scale tests is discussed. The test data is later used to compare the alternative failure criteria.

Experimental data on RC members subjected to pure torsion is compiled from published studies so far. Interestingly, the past studies have concentrated largely on members with small to moderate concrete covers, with handful data on members with thick concrete cover. Widely accepted torsion models including the diagonal compression field theory (CFT), softened truss model (STM) and softened membrane model for torsion (SMMT) are verified on the domains of experimental data missing-in specimens with thick concrete cover. With increasing demand for durability design of RC structures use of thick concrete cover, say up to 75 mm, is being introduced in practice. Recent experimental investigation by the authors demonstrated that spalling of concrete cover, particularly in cases of thick covers, significantly impacts the torsional behavior of RC members. The present study uses these set of experiments to look at the response prediction capability of the existing advanced models and assess the reliability of existing concrete cover spalling theories. For cases with thick cover, the existing models either did not adequately capture the ultimate capacity or erroneously predicted the torsional behavior of the members due to the different mechanics between RC beams with thin and thick covers. Similarly, the spalling theories failed to fully explain the physically observed spalling behavior. Guided by the recent experimental observations and the apparent gaps, the study provides a rational theory for the spalling of concrete cover. The initiation and gradual evolution of concrete cover spalling is traced by observing the acting spalling moment and spalling resistance formulated in the proposed spalling model. The proposed spalling model inherently assumes members susceptibility to spalling of cover concrete and is governed by, the thickness of the concrete cover, tensile strength of concrete, presence of rebar cage and their location, and size of the member. The theory is unified and can explain the spalling of cover due to torsion as well as shear. Its capability is examined by integrating the approach into existing truss model. When compared with state-of-the-art models, the proposed method provides consistent prediction with a relatively smaller scatter, for cases with thick concrete cover as well.