A fracture-mechanics model for debonding of external fibre reinforced polymer plates on reinforced concrete beams

A Fracture-mechanics model for debonding of external fibre reinforced polymer plates on reinforced concrete beams is presented. The conventional methods of concrete-FRP interface analysis use finite element models, which require details of unknown and unknowable interface characteristics. The present model assumes flaws in the vicinity of the interface and assesses whether sufficient energy can be released to cause these flaws to propagate. Energy released by an extension of an existing flaw depends on the change of recoverable energy stored in the system. This paper concentrates on the moment-curvature model for a cracked reinforced-concrete beam under a prestress caused by the force in a FRP plate. The use of the proposed model to determine the energy released from the system with the extension of an existing flaw is also presented. The energy required to create the associated new surfaces depends on interface fracture energy which is first reviewed and methods to determine is also discussed

Fracture mechanics of plate debonding: experimental validation

Premature plate debonding hampers the efficient use of externally bonded FRP plates for flexural strengthening of concrete beams. Existing research mostly concentrates on finite element (FE) modelling of the concrete–FRP interface but such analyses are of dubious validity because they require far more details than will ever be available for the interface. A fracture-mechanics-based plate debonding model has been developed by the authors; since detailed stress analysis of concrete is unattainable the model is based on the global energy balance of the system. Flaws will inevitability be present in the vicinity of the interface; the model investigates the energy balance when such a flaw propagates. The energy released when the crack extends (GR) is compared with the interface fracture energy required to create the new surfaces GF: If GR > GF the crack will extend causing debonding.Determination of both GR and GF associated with crack extension is not trivial because of the unknowable microstructure of concrete. The early work of the present study developed methods to find both parameters to accuracies sufficient for practical purposes. A modified version of Branson’s model, which takes account of the effects caused by the axial force in the FRP, has been developed for the moment–curvature and subsequent GR analyses, while GF has been determined according to the actual fracture mechanism that takes place in the interface.This paper presents comparisons with a variety of plate debonding test data (including steel plate bonded beams) reported in the literature and shows that the present model can correctly determine both the failure load and the debonding mode. Only simply-supported beams, without additional plate end anchors, under short-term monotonic loads are considered here, but the model could be extendedto analyse more complex practical problems

Fracture mechanics of plate debonding

The study has shown that the phenomena of plate debonding can be studied by means of a fracture-mechanics approach, which obviates the need for a finite element analysis which would have dubious validity in the presence of infinite stress concentrations.It has been necessary to produce a modified form of Branson’s method to allow the calculation of the beam stiffness when the section is partially cracked and when subjected to an axial load imposed by the FRP plate.Hutchinson’s interface breakdown model has proved to be a very useful tool for the study of the debonding of FRP plates from concrete structures. More work remains to be done to study the importance of the various parameters that influence the result. Comparisons with experimental data in the literature are being undertaken

Prestressing in Coventry Cathedral

Coventry Cathedral was completed in the early 1960s and has some prestressed elements to resist lateral thrust from the roof. Other prestressed structures of a similar age have had corrosion problems and this has drawn attention to the fact that there is little publicly available information about the structural system at Coventry. This paper addresses that issue and is in three sections. The first summarises the four different prestressing systems in the cathedral and estimates the amount of prestress and its purpose in each location. By placing the information in the public domain it will be useful for both historians of church architecture and engineers in future generations who may have to work on the building. Although there is no evidence of corrosion in the building at the moment, it is impossible to inspect the existing tendons, so the second section considers what might happen to the structure if corrosion of the tendons were to occur. It is concluded that very little warning of failure would be given, which would be especially important for the tendons over the Baptistry window and those in the Nave ties. The final section considers what could be monitored to give as much warning as possible about future problems. The effects of loss of an individual tendon, which would not by itself be sufficient to cause failure of the structure, would cause only very small strains that would be difficult to distinguish from the background strains caused by temperature change. Many of the principles discussed in the second and third sections would be applicable to many other prestressed concrete structures

Differences between FRP bond behavior in cracked and uncracked regions

Synopsis: Based on an analysis of the experimental results of a proposed bond test method, significant differences are shown to exist between the local FRP bond stress-slip relationships in the uncracked anchorage regions and in the regions between cracks. The proposed method simulates the bond behavior between the flexural cracks and anchorage regions of a flexurally FRP-strengthened RC beam. The boundary conditions, including the presence of cracks and steel, are shown to have significant effects on the local bond stress-slip models. The results showed that, at the same force, the bond stresses in the regions between cracks were lower than in regions outside the cracks, so the debonding formed in the anchorage regions. The local bond stress-slip models in the anchorage regions can be obtained from the conventional bond test methods but these do not mimic the conditions between the cracks

Retinal pigment epithelial cells mitigate the effects of complement attack by endocytosis of C5b-9

Retinal pigment epithelial (RPE) cell death is a hallmark of age-related macular degeneration. The alternative pathway of complement activation is strongly implicated in RPE cell dysfunction and loss in age-related macular degeneration; therefore, it is critical that RPE cells use molecular strategies to mitigate the potentially harmful effects of complement attack. We show that the terminal complement complex C5b-9 assembles rapidly on the basal surface of cultured primary porcine RPE cells but disappears over 48 h without any discernable adverse effects on the cells. However, in the presence of the dynamin inhibitor dynasore, C5b-9 was almost completely retained at the cell surface, suggesting that, under normal circumstances, it is eliminated via the endocytic pathway. In support of this idea, we observed that C5b-9 colocalizes with the early endosome marker EEA1 and that, in the presence of protease inhibitors, it can be detected in lysosomes. Preventing the endocytosis of C5b-9 by RPE cells led to structural defects in mitochondrial morphology consistent with cell stress. We conclude that RPE cells use the endocytic pathway to prevent the accumulation of C5b-9 on the cell surface and that processing and destruction of C5b-9 by this route are essential for RPE cell survival

Chronically shortened rod outer segments accompany photoreceptor cell death in Choroideremia

X-linked choroideremia (CHM) is a disease characterized by gradual retinal degeneration caused by loss of the Rab Escort Protein, REP1. Despite partial compensation by REP2 the disease is characterized by prenylation defects in multiple members of the Rab protein family that are master regulators of membrane traffic. Remarkably, the eye is the only organ affected in CHM patients, possibly because of the huge membrane traffic burden of the post mitotic photoreceptors, which synthesise outer segments, and the adjacent retinal pigment epithelium that degrades the spent portions each day. In this study, we aimed to identify defects in membrane traffic that might lead to photoreceptor cell death in CHM. In a heterozygous null female mouse model of CHM (Chmnull/WT), degeneration of the photoreceptor layer was clearly evident from increased numbers of TUNEL positive cells compared to age matched controls, small numbers of cells exhibiting signs of mitochondrial stress and greatly increased microglial infiltration. However, most rod photoreceptors exhibited remarkably normal morphology with well-formed outer segments and no discernible accumulation of transport vesicles in the inner segment. The major evidence of membrane trafficking defects was a shortening of rod outer segments that was evident at 2 months of age but remained constant over the period during which the cells die. A decrease in rhodopsin density found in the outer segment may underlie the outer segment shortening but does not lead to rhodopsin accumulation in the inner segment. Our data argue against defects in rhodopsin transport or outer segment renewal as triggers of cell death in CHM.publishersversionpublishe

Symmetric arrangement of mitochondria:plasma membrane contacts between adjacent photoreceptor cells regulated by Opa1

Mitochondria are known to play an essential role in photoreceptor function and survival that enables normal vision. Within photoreceptors, mitochondria are elongated and extend most of the inner-segment length, where they supply energy for protein synthesis and the phototransduction machinery in the outer segment, as well as acting as a calcium store. Here, we examined the arrangement of the mitochondria within the inner segment in detail using three-dimensional (3D) electron microscopy techniques and show they are tethered to the plasma membrane in a highly specialized arrangement. Remarkably, mitochondria and their cristae openings align with those of neighboring inner segments. The pathway by which photoreceptors meet their high energy demands is not fully understood. We propose this to be a mechanism to share metabolites and assist in maintaining homeostasis across the photoreceptor cell layer. In the extracellular space between photoreceptors, Müller glial processes were identified. Due to the often close proximity to the inner-segment mitochondria, they may, too, play a role in the inner-segment mitochondrial arrangement as well as metabolite shuttling. OPA1 is an important factor in mitochondrial homeostasis, including cristae remodeling; therefore, we examined the photoreceptors of a heterozygous Opa1 knockout mouse model. The cristae structure in the Opa1+/− photoreceptors was not greatly affected, but the mitochondria were enlarged and had reduced alignment to neighboring inner-segment mitochondria. This indicates the importance of key regulators in maintaining this specialized photoreceptor mitochondrial arrangement

Tests on FRP-Concrete Bond Behaviour in the presence of Steel

ABSTRACT The bond behaviour between FRP and concrete is a key factor in composite members. The condition of the substrate material to which FRP bonds is crucial in this behaviour but is overlooked in most conventional bond tests. Moreover, in such tests the boundary conditions differ from the actual state where stresses develop between two flexural cracks. These bond tests also neglect the effects from the presence of steel bars. This article compares the distribution of the strain in the FRP and the slip relative to the substrate material both in conventional shear bond pull-out tests and at the tension face of a reinforced concrete beam strengthened with FRP; the two cases are not identical. A test method is proposed to consider the steel effects (pre-/post-yielding) and to comply with the actual boundary conditions. The specimens are designed as strengthened reinforced concrete ties subjected to pure tension. The preliminary test results show that the presence of steel in a section affects the shape of the FRP bond stress-slip relationship