Combined Flexural and Shear Strengthening of RC T-Beams with FRP and TRM: Experimental Study and Parametric Finite Element Analyses

Due to inadequacies of reinforcement design in older structures and changes in building codes, but also the change of building use in existing structures, reinforced concrete (RC) beams often require upgrading during building renovation. The combined shear and flexural strengthening with composite materials, fibre-reinforced polymer sheets (FRP) and textile reinforced mortars (TRM), is assessed in this study. An experimental campaign on twelve half-scale retrofitted RC beams is presented, looking at various parameters of interest, including the effect of the steel reinforcement ratio on the retrofit effectiveness, the amount of composite material used for strengthening and the effect of the shear span, as well as the difference in effectiveness of FRP and TRM in strengthening RC beams. Significant effects on the shear capacity of composite retrofitted beams are observed for all studied parameters. The experimental study is used as a basis for developing a detailed finite element (FE) model for RC beams strengthened with FRP. The results of the FE model are compared to the experimental results and used to design a parametric study to further study the effect of the investigated parameters on the retrofit effectiveness

Realistic FRP seismic strengthening schemes for interior reinforced concrete beam-column joints

The observation, in recent earthquakes, of brittle collapses of reinforced concrete (RC) structures built before the introduction of detailed seismic design codes (pre-1970’s), underlines the need for significant upgrades to the existing RC building stock. In particular, weak-column/strong-beam mechanisms and shear failures have potentially catastrophic impacts that could be addressed by repair and retrofit solutions. In recent years, retrofits with fibre reinforced polymers (FRP) are becoming increasingly popular due to the benefits of corrosion resistance, high strength-to-weight ratio and reduced labour time. Experimental evidence for the efficiency of such schemes for joint strengthening can be found in the literature. An analysis of all available literature shows that the reduced scale of most tested specimens, as well as the omission of slabs and transverse beams in many studies, may lead to an unrealistic assessment of FRP retrofit schemes. In this study, pre-1970’s full-scale interior beam-column joints with slab and transverse beams are hence tested under realistic conditions in order to propose and assess new and practical FRP retrofit solutions for seismic actions. Three carbon FRP (CFRP) retrofit schemes with selective retrofit objectives are designed using outcomes from the literature and from calibrated finite-element models. The retrofit schemes are composed of a combination of FRP strengthening and selective weakening components to ensure failure of inadequately reinforced RC beam-column joints according to capacity design principles. The objectives of the schemes include the enhancement in lateral capacity and ductility, as well as changing the failure mechanism of the joint. Results from full-scale cyclic tests on the CFRP retrofitted specimens are compared to the behaviour of a deficient specimen and a specimen designed to modern guidelines (EC8), highlighting the successful achievement of the respective retrofit objectives. To evaluate the effect of the realistic set-up, the results are also compared to specimens without slab and transverse beams, highlighting their importance. Finally, new design equations, to be used in conjunction with existing guidelines, are formulated to ease the practical adoption of the proposed retrofit scheme

A combined FRP and selective weakening retrofit for realistic pre-1970's RC structures

A large proportion of existing reinforced concrete (RC) buildings is vulnerable to brittle failure mechanisms in earthquakes due to inadequate seismic detailing. Efficient and practical retrofit solutions are required to prevent future losses. To date, most experimental studies in the field of seismic retrofit with fibre - reinforced polymer (FRP) have concentrated on simplified beam - column joint specimens. These often ignore the real practical challenges incurr ed by the presence of a floor slab and transverse beams in retrofitting deficient specimens . In this study, the results from four cyclic tests on realistic full - scale typical pre - 1970’s beam - column joints with s labs and transverse beams are presented . Two realistic retrofit scheme s using carbon CFRP sheets are proposed. The first scheme aims to improve the duc tility of the deficient joint by providing continuity of the column flexural strengthening through the joint. The second scheme includes selective weakening of the slab and aims to relocat e the failure mechanism to the beams . The significance of the presence of the slab on the global beha viour of the sub - assembly is confirmed by the experimental findings. The res ults highlight that the proposed combined retrofit and selective weakening scheme can successfully increase the ductility of the joint by activating deformation in the beams

Effect of slab and transverse beam on the FRP retrofit effectiveness for existing reinforced concrete structures under seismic loading

The seismic behaviour of reinforced concrete (RC) structures is critically influenced by the complex mechanical interactions at beam-column joints. To ensure the desired hierarchy of failure is achieved when retrofitting existing structures, numerical and experimental assessments need to represent realistic structures. A review of published literature indicates that most experimental work on the seismic behaviour pre-1970′s RC beam-column connections considers sub-assemblies without slabs or transverse beams, which are unrepresentative of reality. To evaluate the effect of these elements on the failure mechanism, retrofit need and retrofit effectiveness, experiments on four full-scale beam-column joints are carried out. Two specimens with and without slab and transverse beams, are tested in their as-built and FRP strengthened configurations. As expected, the experimental results demonstrate that the progression of damage and failure mechanisms differ significantly when slabs and transverse beams are present, confirming previous numerical and experimental evidence on the strong contribution of these elements on the overall joint behaviour. Moreover, a significantly higher retrofit effectiveness is observed for the specimen without slab and transverse beam. This implies that experiments on retrofitted joints without slab and transverse beam can lead to a focus on joint shear strengthening alone as they inadequately represent the hierarchy of strengths of the framing members. They can also lead to an overestimation of retrofit effectiveness. These observations have implications when considering common simplifying assumptions made in the numerical modelling of RC moment resisting frames when assessing their seismic performance

A Novel FRP Retrofit Solution for Improved Local and Global Seismic Performance of RC Buildings: Development of Fragility Curves and Comparative Cost-Benefit Analyses

This paper evaluates the effectiveness of a novel capacity-design based retrofit for improving both the local and global behaviour of existing reinforced concrete buildings using fibre-reinforced polymers. The approach was previously tested on beam-column joints and is here extended to the building level. A new fibre-element model for the global retrofit is proposed and validated against full-scale experiments. Non-linear push-over analyses confirm the adequacy of the retrofit and fragility analyses are used to compare it to a simpler local retrofit. Finally, cost-benefit analyses for three levels of seismicity highlight the cost-effectiveness of the global retrofit for moderate to high seismicity

A realistic full CFRP retrofit of RC beam-column joints compared to seismically designed specimen

The brittle collapse of reinforced concrete (RC) structures built before the introduction of detailed seismic design codes (pre- 1970’s) in recent earthquakes, has underlined the need for significant upgrades to the existing RC building stock. In particular, the observation of weak-column/strong-beam mechanisms has potentially catastrophic impacts that could be addressed by retrofit solutions. Retrofits with fibre reinforced polymers (FRP) have become increasingly popular and experimental evidence for their effectiveness can be found in the literature. The lack of tests on full-scale specimens with slabs and transverse beams in many studies may however lead to an unrealistic assessment of FRP retrofit schemes. In this study, three realistic full-scale interior beam-column joints with slab and transverse beams are tested under cyclic loading in order to propose and assess practical FRP retrofit solutions. A complete Carbon FRP (CFRP) retrofit strategy aiming to obtain a similar performance to a specimen designed to modern European design guidelines (Eurocode 8) is presented. The retrofit scheme is composed of selective strengthening and weakening components to ensure ductile failure of the specimen according to capacity design principles. Results from full-scale cyclic tests on the CFRP retrofitted specimen are compared to the behaviour of a deficient, pre-1970’s design specimen and a specimen designed to modern guidelines. The observed failure mechanisms and global lateral capacities for the Eurocode 8 and retrofitted specimen show that the CFRP retrofit is effective in improving seismic behaviour. By means of a combined selective weakening and strengthening scheme, a change in hierarchy of strengths can be achieved, which leads to an improved ductile behaviour with significant strength enhancement. The results suggest that a CFRP retrofit scheme can be devised for realistic, significantly under-designed structures in order to achieve a similar performance to modern RC structures designed to sophisticated seismic guidelines

Integrated seismic and energy retrofitting of existing buildings: A state-of-the-art review

Ageing of the building stock is an issue affecting many regions in the world. This means a large proportion of existing buildings being considered energy inefficient, with associated high energy use for heating and cooling. Through renovation, it is possible to improve their energy-efficiency, hence reducing their significant impact on the total energy household and associated greenhouse gas emissions. In seismic regions, additionally, recent earthquakes have caused significant economic losses, largely due to the vulnerability of older buildings not designed to modern standards. Addressing seismic and energy performance by separate interventions is the common approach currently taken, however to achieve better cost-effectiveness, safety and efficiency, a novel holistic approach to building renovation is an emerging topic in the scientific literature. Proposed solutions range from integrated exoskeleton solutions, over strengthening and insulation solutions for the existing building envelope or their replacement with better materials, to integrated interventions on horizontal elements like roof and floor slabs. To identify pathways to combined seismic and energy retrofitting of buildings, a state-of-the-art review of all materials and solutions investigated to date is presented. This is followed by a critical analysis of their effectiveness, invasiveness, building use disruption as well as their impact on the environment. The assessment of current combined retrofitting research highlights a great potential for their application, with a potential to provide cost-effective renovation solutions for regions with moderate to high seismic risk. Still, to-date there is a lack of experimental research in this field, a need for further work on truly integrated technologies and their validation through applications on existing large-scale buildings. Moreover, there is a need for adequate design methods, regulations and incentives that further the implementation of integrated retrofitting approaches

Experimental Comparison of Novel CFRP Retrofit Schemes for Realistic Full-Scale RC Beam-Column Joints

Existing RC moment-resisting frames (MRF) built with inadequate detailing or before the introduction of detailed seismic design codes (pre-1970s) are highly vulnerable to brittle failure mechanisms under earthquake loading. To prevent potentially catastrophic failures and consequent human and economic losses in future earthquakes, efficient and practical retrofit solutions are required for these buildings. This paper presents an experimental study focused on the development of retrofit solutions that adopt carbon fiber–reinforced polymers (CFRP) to improve the seismic performance of existing RC MRF at their beam–column connections. It is highlighted that to date, most experimental studies in this field used simplified test specimens that have ignored the presence of slabs and secondary beams at beam–column connections. This may lead to an unrealistic assessment of FRP retrofit schemes. Hence, in this study, results from six full-scale cyclic tests on typical pre-1970s interior beam–column joints with slab and transverse beams are presented. The tests are used to assess three proposed CFRP schemes composed of a combination of FRP strengthening methods and selective slab weakening. Each scheme is designed to meet a distinct retrofit objective: (1) enhancement of the lateral-load capacity, (2) enhancement of ductility, and (3) enhancement of the lateral-load capacity and ductility, as well as changing the dominant failure mode of the joint from a column hinging mechanism to one where the plasticity is mainly concentrated in the beams. A comparison of the retrofitted specimens with the behavior of a deficient specimen and a specimen designed to modern guidelines highlights the successful achievement of the respective retrofit objectives and the necessity to weaken the slab to achieve a favorable failure mechanism that will allow compliance to be achieved with current retrofit codes

Integrated seismic and energy retrofitting of existing buildings: A state-of-the-art review

Ageing of the building stock is an issue affecting many regions in the world. This means a large proportion of existing buildings being considered energy inefficient, with associated high energy use for heating and cooling. Through renovation, it is possible to improve their energy-efficiency, hence reducing their significant impact on the total energy household and associated greenhouse gas emissions. In seismic regions, additionally, recent earthquakes have caused significant eco-nomic losses, largely due to the vulnerability of older buildings not designed to modern standards. Addressing seismic and energy performance by separate interventions is the common approach currently taken, however to achieve better cost-effectiveness, safety and efficiency, a novel ho-listic approach to building renovation is an emerging topic in the scientific literature. Proposed solutions range from integrated exoskeleton solutions, over strengthening and insulation solu-tions for the existing building envelope or their replacement with better materials, to integrated interventions on horizontal elements like roof and floor slabs. To identify pathways to combined seismic and energy retrofitting of buildings, a state-of-the-art review of all materials and solutions investigated to date is presented. This is followed by a critical analysis of their effectiveness, invasiveness, building use disruption as well as their impact on the environment. The assessment of current combined retrofitting research highlights a great potential for their application, with a potential to provide cost-effective renovation solutions for regions with moderate to high seismic risk. Still, to-date there is a lack of experimental research in this field, a need for further work on truly integrated technologies and their validation through applications on existing large-scale buildings. Moreover, there is a need for adequate design methods, regulations and incentives that further the implementation of integrated retrofitting approaches