Shear Strength Model for FRP-Strengthened RC Beams with Adverse FRP-Steel Interaction

RC beams shear strengthened with externally bonded fiber-reinforced polymer (FRP) U strips or side strips usually fail owing to debonding of the bonded FRP shear reinforcement. Because such debonding usually occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups intersected by the critical shear crack may not have reached yielding at beam shear failure. Consequently, the yield stress of internal steel stirrups in such a strengthened RC beam cannot be fully utilized. This adverse shear interaction between the internal steel shear reinforcement and the external FRP shear reinforcement may significantly reduce the benefit of the shear strengthening FRP but has not been considered explicitly by any of the shear strength models in the existing design guidelines. This paper presents a new shear strength model considering this adverse shear interaction through the introduction of a shear interaction factor. A comprehensive evaluation of the proposed model, as well as three other shear strength models, is conducted using a large test database. It is shown that the proposed shear strength model performs the best among the models compared, and the performance of the other shear strength models can be significantly improved by including the proposed shear interaction factor. Finally, a design recommendation is presented.

Multibody modelling of gabion beams for impact applications

Gabions are stone-filled wire containers which are frequently used as retaining walls. However, due to their high mass, relatively low cost and visual appeal, a row of single gabion blocks, joined at the ends, has the potential to be used as a roadside impact absorption device where traditional steel or concrete devices may not be suitable. To evaluate such application, the shear and bending deformation of gabions under vehicle impact need to be investigated. In this paper, the shear response of a single gabion block is analytically modelled and a gabion beam multibody model is developed using a discretisation method to capture the deformability of the gabion structure. The material properties of the gabion beam are adopted from experimental values available in the literature and the modelling is statically validated over a three-point bending test and a distributed loading test. The results show that the discretised multibody modelling can be effectively used to describe the static deformation behaviour of gabion blocks.

Monitoring the development of microcracks in reinforced concrete caused by sustained loading and chloride induced corrosion

Chloride-induced corrosion of steel in reinforced concrete structures is one of the main problems affecting their durability and it has been studied for decades, but most of them have focused on concrete without cracking or not subjected to any structural load. In fact, concrete structures are subjected to various types of loads, which lead to cracking when the tensile stress in concrete exceeds its tensile strength. Cracking could increase transport properties of concrete and accelerate the ingress of harmful substances (Cl ^-, O₂, H₂ O, CO₂). This could initiate and accelerate different types of deterioration processes in concrete, including corrosion of steel reinforcement. The expansive products generated by the deterioration processes themselves can initiate cracking. The success of concrete patch repairs can also influence microcracking at the interface as well as the patch repair itself. Therefore, monitoring the development of microcracking in reinforced concrete members is extremely useful to assess the defects and deterioration in concrete structures. In this paper, concrete beams made using 4 different mixes were subjected to three levels of sustained lateral loading (0%, 50% and 100% of the load that can induce a crack with width of 0.1mmon the tension surface of beams - F _0.1) and weekly cycles of wetting (1 day)/drying (6 days) with chloride solution. The development of microcracking on the surface of concrete was monitored using the Autoclam Permeability System at every two weeks for 60 weeks. The ultrasonic pulse velocity of the concrete was also measured along the beam by using the indirect method during the test period. The results indicated that the Autoclam Permeability System was able to detect the development of microcracks caused by both sustained loading and chloride induced corrosion of steel in concrete. However, this was not the case with the ultrasonic method used in the work (indirect method applied along the beam); it was sensitive to microcracking caused by sustained loading but not due to corrosion. © 2014 Taylor & Francis Group.

POST-TENSIONING OF TIMBER BEAMS WITH BASALT FIBRE REINFORCED POLYMER

Improvements in the structural performance of glulam timber beams by the inclusion of reinforcing materials can improve both the service performance and ultimate capacity. In recent years research focusing on the addition of fibre reinforced polymers to strengthen members has yielded positive results. However, the FRP material is still a relatively expensive material and its full potential has not been realised in combination with structural timber. This paper describes a series of four-point bending tests that were conducted, under service and ultimate loads, on post-tensioned glulam timber beams where the reinforcing tendon used was 12 mm diameter Basalt Fibre Reinforced Polymer (BFRP). The research was designed to evaluate the additional benefits of including an active type of reinforcement, by post-tensioning the BFRP tendon, as opposed to the passive approach of simply reinforcing the timber beam.
From the laboratory investigations, it was established that there was a 16% increase in load carrying capacity, in addition to a 14% reduction in deflection under service loads when members containing the post-tensioned BFRP composite are compared with control timber specimens. Additionally a more favourable ductile failure mode was witnessed compared to the brittle failure of an unreinforced timber beam. The results support the assumption that by initially stressing the embedded FRP tendon the structural benefits experienced by the timber member increase in a number of ways, indicating that there is significant scope for this approach in practical applications.

Strength model for end cover separation failure in RC beams strengthened with near-surface mounted (NSM) FRP strips

As an alternative to externally bonded FRP reinforcement, near-surface mounted (NSM) FRP reinforcement can be used to effectively improve the flexural performance of RC beams. In such FRP strengthened RC beams, end cover separation failure is one of the common failure modes. This failuremode involves the detachment of the NSM FRP reinforcement together with the concrete cover along the level of the tension steel reinforcement. This paper presents a new strength model for end cover separation failure in RC beams strengthened in flexure with NSM FRP strips (i.e. rectangular FRP bars with asectional height-to-thickness ratio not less than 5), which was formulated on the basis of extensive numerical results from a parametric study undertaken using an efficient finite element approach. The proposed strength model consists of an approximate equation for the debonding strain of the FRP reinforcement at the critical cracked section and a conventional section analysis to relate this debondingstrain to the moment acting on the same section (i.e. the debonding strain). Once the debonding strain is known, the load level at end cover separation of an FRP-strengthened RC beam can be easily determined for a given load distribution. Predictions from the proposed strength model are compared with those of two existing strength models of the same type and available test results, which shows that the proposed strength model is in close agreement with test results and is far more accurate than the existing strength models.

Finite element modeling of debonding failures in FRP-strengthened RC beams: A dynamic approach

This paper is concerned with the finite element simulation of debonding failures in FRP-strengthened concrete beams. A key challenge for such simulations is that common solution techniques such as the Newton-Raphson method and the arc-length method often fail to converge. This paper examines the effectiveness of using a dynamic analysis approach in such FE simulations, in which debonding failure is treated as a dynamic problem and solved using an appropriate time integration method. Numerical results are presented to show that an appropriate dynamic approach effectively overcomes the convergence problem and provides accurate predictions of test results.

Analytical model for bar-end cover separation failure in RC beams strengthened with near-surface mounted FRP

The flexural performance of RC beams can be improved using the near-surface mounted (NSM) FRP strengthening technique. A likely failure mode of such FRP-strengthened RC beams is bar-end cover separation which involves the detachment of the NSM FRP reinforcement together with the concrete cover along the level of the steel tension reinforcement. This paper presents a new analytical strength model for this failure mode. The proposed strength model and two existing strength models for this failure mode are compared with test results to demonstrate the superior performance of the new analytical model.

Improved shear strength model for FRP-strengthened RC beams accounting for adverse FRP-steel interaction

RC beams shear-strengthened with externally-bonded FRP side strips or U-strips usually fail by debonding. As such debonding occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups may not have reached yielding at beam shear failure. Consequently, the internal steel stirrups cannot be fully utilized. This adverse shear interaction between internal steel stirrups and external FRP strips may significantly reduce the benefit of shear-strengthening FRP but has not been considered by any of the existing FRP strengthening design guidelines. In this paper, an improved shear strength model capable of accounting for the effect of the above shear interaction is first presented, in which the unfavorable effect of shear interaction is reflected through a reduction factor (i.e. shear interaction factor). Using a large test database established in the present study, the performance of the proposed model as well as that of three other shear strength models is then assessed. This assessment shows that the proposed shear strength model performs better than the three existing models. The assessment also shows that the inclusion of the proposed shear interaction factor in the existing models can significantly improve their performance.