A.M. Thorne

AN EXPERIMENTAL STUDY OF THE ANCHORAGE LENGTH OF CARBON FIBRE COMPOSITE PLATES USED TO STRENGTHEN REINFORCED CONCRETE BEAMS

Abstract Concrete structures deteriorate for various reasons and upgrading has been achieved for over 20 years by bonding steel plates using epoxy resins. Disadvantages of this method include transporting, handling and installing heavy plates and corrosion of the plates. The use of composite materials overcomes these problems and provides equally satisfactory solutions. The rehabilitation of concrete structures represents a large demand for efficient strengthening methods and composite materials are well suited to this application. It is now necessary to understand more about the factors that are important in the design of composite plate bonding, such as the influence of the plate anchorage length and additional plate end anchorage. Four point bending and cantilever loading are used to show that the ultimate capacities and failure modes of reinforced concrete beams, externally strengthened with bonded composite plates, are dependent on the shear span/depth ratio. Strengthened members are found to fail by separation of the concrete cover from the internal rebars throughout the whole of one shear span under low shear span/depth ratios, while a thinner concrete layer is separated at higher ratios. Peeling of the plate end is thought to occur under low shear span/depth ratios. The end of the bonded plate in the failed shear span experiences strain increases relatively soon after yield of the internal steel when the shear span/depth ratio is low, but the plate ends are less sensitive to yield under higher ratios, indicating the lower shear transferred into the concrete via the adhesive at the plate ends. Ultimate bending moments are found to reach an upper limit with increasing shear span/depth ratio in the range of ratios studied. Adhesive shear stresses are of greatest magnitude at the ends of the plate under low shear span/depth ratios.

The strengthening and deformation behaviour of reinforced concrete beams upgraded using prestressed composite plates

Externally bonded steel plates have been used world-wide for over twenty years to strengthen concrete members, but the disadvantages include the transportation and installation of heavy plates and steel corrosion. Polymeric composites avoid these disadvantages and provide an equally effective method of strengthening. The advantages of composites are exploited further by prestressing them before bonding to the concrete. This paper is concerned with the response of such prestressed members to applied load.After strengthening with externally bonded carbon fibre reinforced polymer (CFRP) plates, a number of beams of 1.0 m and 4.5 m lengths were tested in four point bending. The plates were bonded without prestress and with prestress levels ranging from 25% to 50% of the plate strength. The ultimate capacities of the non-prestressed beams were significantly higher than those of the unplated members and plate prestress brought about yet further strengthening. Prestressed plates are utilised more efficiently than non-prestressed plates, since a given plate strain is associated with a lower structural deformation in a prestressed member. The internal steel rebars yielded at a higher proportion of the ultimate capacity in the prestressed beams. Prestressing lowers the position of the neutral axis so more of the concrete section is loaded in compression, making more efficient use of the concrete.RésuméLe renforcement des éléments en béton par placage de plaques d’acier est pratiqué partout dans le monde depuis plus de vingt ans; cependant cette méthode présente les inconvénients du transport et de l’installation des plaques lourdes d’acier, ainsi que de la corrosion de l’acier. Des matériaux composites-polymère évitent ces inconvénients tout en fournissant une méthode aussi efficace de renforcement. Les avantages présentés par les composites peuvent être davantage améliorés par leur précontrainte avant leur placage au béton. Ce article traite de la réponse de tels éléments précontraints sous mise en change.Après renforcement par placage de plaques en polymère renforcé de fibres de carbone, des poutres ayant une longueur de 1,0 m et de 4,5 m ont été soumises à des essais de flexion-4 points. Les plaques étaient appliquées sans précontrainte et avec des niveaux de précontrainte allant de 25 à 50% de leur résistance. Les capacités ultimes des poutres non-précontraintes se sont montrées significativement plus hautes que celles des éléments non-renforcés par des plaques, et la précontrainte des plaques a apporté une résistance encore meilleure. Les plaques précontraintes sont plus efficaces que les plaques non-précontraintes, la contrainte sur une plaque donnée étant associée avec une déformation structurelle moindre dans l’élément précontraint. Dans les élément précontraints, les armatures métalliques internes ont cédé à une plus haute proportion de la capacité ultime. La précontrainte diminue la position de l’axe neutre et une plus grande partie de la section en béton est donc chargée en compression, résultant en une utilisation plus efficace du béton.

The use of advanced polymer composites to form an economic structural unit

Abstract An advanced polymer/continuous fibre composite (APC) and concrete system is an example in which two very dissimilar materials can be joined to form a composite structure. In the current investigation a duplex beam will be analysed where the high-compressive strength concrete is placed above, and the high strength and stiffness fibre/polymer composite is placed below the neutral axis; in this form the two materials will be used to their best advantage. When this structural duplex Tee beam is under load, two failure criteria of the system have been identified. These are the failure of the shear bond between the APC permanent shuttering and the concrete, and failure by buckling of the composite web of the beam. The experimental test results show that the shear bond is not critical. Buckling occurs in the specified panels at 20% of the ultimate load. On increasing the load, the beam continues to perform conventionally and fails in the concrete by crushing. In addition, the paper shows that the performance of the APCs are not impaired by long-term creep and fatigue loading whilst the concrete element shows a typical reduction in stiffness for these types of loading regimes.

AN INVESTIGATION OF THE COMPOSITE ACTION OF AN FRP/CONCRETE PRISMATIC BEAM

Abstract A novel composite/concrete beam has been fabricated to use two component materials, fibre-reinforced polymer composite and concrete, to their best advantage. The concrete and composite are wholly under compressive and tensile strains, respectively, together with any shear strain components, except where under direct compression at the supports. The shear transfer at the interface between these two components requires investigation and this paper describes six techniques used to develop this connection to give the most efficient shear transfer in order for the whole beam to act compositely. The six methods are used independently of one another and include the use of indents in the vertical walls of the permanent composite shuttering retaining the concrete, bonding the concrete to the permanent shuttering, forming the shear connection by using bolts through both the concrete and permanent shuttering, two methods using a resin injection to fill a pre-formed gap between the concrete and permanent shuttering and finally using an adhesive compatible with freshly made concrete. It was concluded that the most appropriate method of ensuring complete composite action between the two component materials was to bond the cured concrete to the permanent shuttering along the two vertical sides. However, this is not a practical option as it assumes that the concrete is removed from the mould after it has cured and then bonded back into the permanent shuttering. It is suggested that the best practical method is to use the adhesive compatible with freshly made concrete which still gave an adequate level of composite action.

Prediction of the reflected spectra from chirped fibre Bragg gratings embedded within cracked crossply laminates

Matrix cracking damage is a generic type of damage that develops under load in the off-axis plies of laminated composites and is generally the precursor of more serious damage mechanisms, particularly delamination. Hence, it is important to identify and if possible locate this type of damage. Chirped fibre Bragg grating sensors have been embedded in a transparent glass fibre reinforced plastic crossply laminate and changes to the reflected spectra as a consequence of crack development have been studied. An approximately sinusoidal variation of the intensity of the reflected spectrum occurs at the position of the crack, enabling both crack development and crack position to be identified. A simulation of a reflected spectrum, incorporating a stress transfer model to predict the strains and an optical model to predict the reflected spectrum, is in reasonable agreement with the experimental results.

Interaction between optical fibre sensors and matrix cracks in cross-ply GRP laminates—part 1: passive optical fibres

Abstract The obtrusivity of a passive optical fibre embedded in the 0° ply of a model cross-ply GRP laminate has been investigated experimentally and theoretically. The experimental results show that for quasi-static loading and cyclic loads with a peak strain greater than the quasi-static cracking threshold, both the initiation and development of matrix cracks are insensitive to the presence of the optical fibre. However, for cyclic loading with a peak strain just below the cracking threshold, the matrix crack-growth rate is reduced significantly in the vicinity of the optical fibre. The reduction in the crack-growth rate can be understood in terms of a reduction in the strain-energy release rate for cracking as a consequence of the local stiffening of the 0° ply due to the presence of the optical fibre.

The use of an embedded chirped fibre Bragg grating sensor to monitor disbond initiation and growth in adhesively bonded composite/metal single lap joints

Chirped fibre Bragg grating (CFBG) sensors embedded within glass fibre reinforced plastic (GFRP) adherends have been used to monitor disbond initiation and growth in an adhesively bonded GFRP/aluminium alloy single lap joint. The elevated temperature curing of the adhesive used in the manufacture of the joint leads to thermal strains being generated within the GFRP and aluminium adherends. Disbond initiation and growth between the adherends during fatigue cycling causes a relaxation of the residual thermal strains within the composite adherend and perturbations (peaks or dips) in the reflection spectra from the CFBG sensor. These perturbations can be used, when the joint is unloaded, to monitor both the initiation of a disbond in the joint and to monitor the growth of the disbond with fatigue cycling.

THE STRUCTURAL CHARACTERISTIC OF A POLYMER COMPOSITE CELLULAR BOX BEAM IN BENDING

Abstract Pultruded glass fibre reinforced polyester composite systems are currently being considered for large scale structures such as footbridges, sign gantries over motorways and lightly trafficked road bridges. This paper discusses the experimental and numerical analyses of the short term loading performance up to failure of the cellular box beam under a four-point loading configuration. The non-linear numerical analyses utilize, as input data, the mechanical property values of coupon specimens of the material; the values are not given in the paper. It is shown that there is satisfactory agreement between the two methods of analysis. The beam displayed non-linear behaviour above a moment value of 860 kN m. The weakest part of the structure was the connection at the support end.

Experimental results and finite element modelling of an embedded polarimetric sensor

‘Smart structures’ or ‘smart skins’ will require structurally integrated sensing systems that can operate in practical situations. Optical sensing techniques are receiving considerable attention for the monitoring of such systems. In this work an optical-fibre sensor has been embedded, without removing its acrylate protective coating, in a glass fibre-reinforced polymer. A single-ended polarimetric sensor was utilized with a large dynamic range for strain measurements up to 2000 μe, at which value the sensor failed. The optical measurements show good agreement with conventional electrical resistance strain gauge measurements. Finite element modelling was used to investigate the stress/strain distributions within the composite material and the embedded optical fibre. The modelling results show excellent agreement with the experimental results and suggest that the soft acrylate coating is debonding, thus reducing the sensors dynamic range.

Detection of defects in as manufactured GFRP–GFRP and CFRP–CFRP composite bonded joints using chirped fibre Bragg grating sensors

Abstract Chirped fibre Bragg grating (CFBG) sensors have been embedded in composite coupons which have been used to form one half of single lap bonded joints. The bonded joints have been made with deliberately included defects, consisting of either a PTFE insert or an air gap, and the sensors have been used to detect the presence and location of the defects. The experimental results, and the modelling, show that defects in both GFRP–GFRP joints and CFRP–CFRP joints can be detected by the embedded CFBG sensors, though it is easier to detect defects in the lower stiffness (GFRP) joints.