Basalt Fibre Reinforced Polymer Strengthening of Normal Strength Reinforced Concrete Floor Slabs Subject to Arching Effects

With ever increasing demands to strengthen existing reinforced concrete structures to facilitate higher loading due to change of use and to extend service lifetime, the use of fibre reinforced polymers (FRPs) in structural retrofitting offers an opportunity to achieve these aims. To date, most research in this area has focussed on the use of glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP), with relatively little on the use of basalt fibre reinforced polymer (BFRP) as a suitable strengthening material. In addition, most previous research has been carried out using simply supported elements, which have not considered the beneficial influence of in-plane lateral restraint, as experienced within a framed building structure. Furthermore, by installing FRPs using the near surface mounted (NSM) technique, disturbance to the existing structure can be minimised.
This paper outlines BFRP NSM strengthening of one third scale laterally restrained floor slabs which reflect the inherent insitu compressive membrane action (CMA) in such slabs. The span-to-depth ratios of the test slabs were 20 and 15 and all were constructed with normal strength concrete (~40N/mm2) and 0.15% steel reinforcement. 0.10% BFRP was used in the retrofitted samples, which were compared with unretrofitted control samples. In addition, the bond strength of BFRP bars bonded into concrete was investigated over a range of bond lengths with two different adhesive thicknesses. This involved using an articulated beam arrangement in order to establish optimum bond characteristics for use in strengthening slab samples.

Mitigation of concrete cover separation in frpstrengthened RC beams using FRP U-jackets

Concrete cover separation is a common failure mode of reinforced concrete (RC) beams strengthened with a fibre-reinforced polymer (FRP) plate bonded to the tension face (FRP-plated RC beams). Plate-end FRP U-jackets have previously been explored as a mitigation measure to delay or suppress concrete cover separation, although its effectiveness needs further clarification. The paper presents the first systemic experimental study on the use of FRP U-jackets of different forms for mitigating the concrete cover separation failure. A total of ten full-scale FRP-plated RC beams were tested. The test results show that both the ultimate load and the ductility of the beams were enhanced by the U-jackets. Among the forms of U-jackets explored, those inclined at 45o are the most effective.

Strengthening an in-service reinforcement concrete bridge with prestressed CFRP bar

Carbon fiber reinforced polymer (CFRP) bars were prestressed for the structural strengthening of 8 T-shaped reinforced concrete (RC) beams of a 21-year-old bridge in China. The ultimate bearing capacity of the existing bridge after retrofit was discussed on the basis of concrete structures theory. The flexural strengths of RC beams strengthened with CFRP bars were controlled by the failure of concrete in compression and a prestressing method was applied in the retrofit. The field construction processes of strengthening with CFRP bars—including grouting cracks, cutting groove, grouting epoxy and embedding CFRP bars, surface treating, banding with the U-type CFRP sheets, releasing external prestressed steel tendons—were introduced in detail. In order to evaluate the effectiveness of this strengthening method, field tests using vehicles as live load were applied before and after the retrofit. The test results of deflection and concrete strain of the T-shaped beams with and without strengthening show that the capacity of the repaired bridge, including the bending strength and stiffness, is enhanced. The measurements of crack width also indicate that this strengthening method can enhance the durability of bridges. Therefore, the proposed strengthening technology is feasible and effective.

Investigation of Ultimate Strength of Deck Slabs in Steel-Concrete Bridges

In the last 50 years, many bridges have been built as composite structures with decks of reinforced concrete that are supported by longitudinal steel beams. The presence of the longitudinal steel beams and the unloaded area of concrete slab cause the loaded deck slabs to be restrained against lateral expansion. As a result, a compressive membrane thrust is developed. In experimental tests, the authors built a series of one-third scale steel-concrete composite bridge models with several varying structural parameters, including concrete compressive strength, reinforcement percentage, and the size of steel supporting beams. After comparing the results of different models, the influence of these structural parameters on the amount of compressive membrane action in the deck slab was evaluated. Furthermore, the improvement of an existing theoretical model provided accurate predictions for the loading-carrying capacities.

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.

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.

Improved model for interfacial stresses accounting for the effect of shear deformation in plated beams

A significant increase in strength and performance of reinforced concrete, timber and metal beams may be achieved by adhesively bonding a fibre reinforced polymer composite, or metallic such as steel plate to the tension face of a beam. One of the major failure modes in these plated beams is the debonding of the plate from the original beam in a brittle manner. This is commonly attributed to the interfacial stresses between the adherends whose quantification has led to the development of many analytical solutions over the last two decades. The adherends are subjected to axial, bending and shear deformations. However, most analytical solutions have neglected the effect of shear deformation in adherends. Few solutions consider this effect approximately but are limited to one or two specific loading conditions. This paper presents a more rigorous solution for interfacial stresses in plated beams under an arbitrary loading with the shear deformation of the adherends duly considered in closed form using Timoshenko’s beam theory. The solution is general to linear elastic analysis of prismatic beams of arbitrary cross section under arbitrary loading with a plate of any thickness bonded either symmetrically or asymmetrically with respect to the span of the beam.

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.

Arching action in high strength concrete slabs

The corrosion of reinforcement in bridge deck slabs has been the cause of major deterioration and high costs in repair and maintenance.This problem could be overcome by reducing the amount of reinforcement and/or altering the location.This is possible because, in addition to the strength provided by the reinforcement, bridge deck slabs have an inherent strength due to the in-plane arching forces set up as a result of restraint provided by the slab boundary conditions. This is known as arching action or Compressive Membrane Action (CMA). It has been recognised for some time that laterally restrained slabs exhibit strengths far in excess of those predicted by most design codes but the phenomenon has not been recognised by the majority of bridge design engineers. This paper presents the results of laboratory tests on fifteen reinforced concrete slab strips typical of a bridge deck slab and compares them to predicted strengths using the current codes and CMA theory. The tests showed that the strength of laterally restrained slabs is sensitive to both the degree of external lateral restraint and the concrete compressive strength.The tests particularly highlighted the benefits in strength obtained from very high strength concrete slabs. The theory extends the existing knowledge of CMA in slabs with concrete compressive strengths up to 100 N/mm[2] and promotes more economical and durable bridge deck construction by utilising the benefits of high strength concrete.

Pullout test correlations and in-place strength assessment - the European Concrete Frame Building Project

A full-scale, seven-story, reinforced concrete building frame was constructed in-place at the Building Research Establishment's Cardington Laboratory, which encompassed a range of different concrete mixtures and advanced construction techniques. This provided an opportunity to assess in-place nondestructive test methods, namely the pullout test, and more specifically the Danish version, which has been known as the Lok test, on a systematic basis during the construction of the building. It was used in conjunction with both standard and temperature-matched cube specimens to assess its practicality and accuracy under site conditions. Strength correlations were determined using linear and power function regression analysis. Strength predictions from these were found to be in very good agreement with the compressive strengths of temperature-matched cube specimens. When a general correlation is used, however, estimates for compressive strength are likely to have 95% confidence limits of around '20% of the mean value of four results.

Analysis of compressive membrane action in concrete slabs

This paper summarises the results obtained from non-linear finite-element analysis (NLFEA) of a series of reinforced-concrete one-way slabs with various boundary conditions representative of a bridge deck slab strip in which compressive membrane action governs the structural behaviour. The application of NLFEA for the optimum analysis and design of in-plane restrained concrete slabs is explored. An accurate material model and various equation solution methods were assessed to find a suitable finite-element method for the analysis of concrete slabs in which arching action occurs. Finally, the results from the NLFEA are compared and validated with those from various experimental test data. Significantly, the numerical analysis was able to model the arching action that occurred as a result of external in-plane restraint at the supports and which enhanced the ultimate strength of the slab. The NLFEA gave excellent predictions for the ultimate load-carrying capacity and far more accurate predictions than those obtained using standard flexural or elastic theory.