Vistasp M. Karbhari

On the durability of composite rehabilitation schemes for concrete: use of a peel test

Composites present a potentially cost-efficient and more durable alternative to the use of externally bonded steel plates for the rehabilitation and strengthening of concrete members, due to their high stiffness-to-weight and strength-to-weight ratios, corrosion resistance and overall ease of application in the field. Although a number of demonstration projects have shown the initial viability of such schemes, a number of critical questions still remain as related to short- and long-term durability as well as damage and failure mechanisms. In this paper a modified peel test is used to investigate the durability of the bond between concrete and composites under five different environmental exposure regimes. Two different epoxies were used with E-glass and carbon fibre reinforced composites. Differences in peel force and interfacial fracture energies based on material and environmental influences are discussed and modes of failure are presented.

Use of composites for 21st century civil infrastructure

Abstract Due to their light weight, high stiffness-to-weight and strength-to-weight ratios, and potentially high resistance to environmental degradation, resulting in lower life-cycle costs, polymer composites are increasingly being considered for use in civil infrastructure applications ranging from the retrofit and rehabilitation of buildings and bridges to the construction of new structural systems. This paper outlines the need for such materials, describing a variety of applications ranging from seismic retrofit of columns to the fabrication of new light-weight bridge decks for renewal. Aspects related to the development of novel hybrid bridge structural systems are described that would lead to a new genre of bridge structures that efficiently combine functionality and aesthetics in a way previously not possible with conventional materials. Challenges and recent developments related to materials and processing methods, as well as issues related to life-time durability related to this cost-sensitive environment are discussed, and it is shown that solutions derived for this area will provide significant advances in other application areas.

Durability of composites for civil structural applications

Introduction: The use of composites in civil structural applications. Part 1 Aspects of composite durability: Fabrication, quality and service life issues for composites in civil engineering Durability of composites in aqueous environments Durability of composites in freeze-thaw conditions Durability of composites exposed to ultraviolet radiation Durability of composites exposed to elevated temperature and fire Durability of composites under fatigue loads Creep and time-dependent response of composites Durability of composites due to wear and erosion. Part 2 Applications and monitoring of composites in civil engineering: Fibre-reinforced polymer composite structures and structural component Current applications and durability issues Reinforcement of concrete using fibre-reinforced polymer composites External strengthening of structures using fibre-reinforced polymer composites Rehabilitation of concrete structures using fibre-reinforced polymer composites: Identifying potential defects Structural health monitoring and field evaluation of composite durability.

STRUCTURAL CHARACTERIZATION OF FIBER-REINFORCED COMPOSITE SHORT- AND MEDIUM-SPAN BRIDGE SYSTEMS

The paper describes the development of a new structural system for short and medium span bridges wherein use is made of both advanced composites and conventional materials such as concrete. The concept uses prefabricated composite tubes as girders which are then filled with concrete, after which a conventional precast or cast-in-place, or advanced composite, deck system is integrated to form the bridge superstructure. The paper presents experimental results of large-scale tests aimed towards the structural characterization of the girders, anchorages, and girder-deck assemblies for both serviceability and ultimate limit states.

Fiber Reinforced Composites – Advanced Materials for the Renewal of Civil Infrastructure

Fiber reinforced polymer matrix composite materials hitherto used predominantly in aerospace and marine applications are increasingly being considered for use in the renewal of civil infrastructure ranging from the seismic retrofit of bridge columns and the strengthening of parking garage floor slabs to their use in replacement bridge decks and in new bridge structures. Their corrosion resistance, potentially high overall durability, light weight, tailorability and high specific performance attributes enable their use in areas in which the use of conventional materials might be constrained due to durability, weight or lack of design flexibility. This paper provides an overview of the use of composites in the renewal of civil structures with particular emphasis on bridges and pipelines. Examples of large scale testing for the validation of structural effectiveness are given and future design and research advances are presented.

Experimental characterization of permeability and fibre wetting for liquid moulding

Liquid moulding processes are unique in that resin is infused into a dry fibre preform. Appropriate wet-out of the reinforcing fibres is thus a necessity for the achievement of good composite properties. For this class of manufacturing methods, both macroscopic flow, as related to Darcys Law and characterized by permeability, and microscopic flow, as related to fibre wet-out, are important. The current research investigates factors affecting permeability and fibre wet-out as related to liquid moulding. Specifically, it is shown that fabric permeability is dependent on the type of test fluid used. Surface tension and contact angle measurements indicate that interactions at the microscopic level between fibre and test fluid account for these differences in permeability. The investigation illustrates the competing nature of macroscopic and microscopic flow in liquid moulding.

Experimental dynamic characterization of an FRP composite bridge superstructure assembly

A system comprised of concrete filled, filament wound, circular carbon/epoxy girders and an E-glass/polyester deck, representative of a bridge section in the positive moment region was tested at large scale to assess the overall and component structural response. The system was characterized for stiffness and overall response under monotonic and cyclic fatigue loads. Forced vibration testing was conducted as part of a level I non-destructive evaluation (NDE) procedure at each of the test stages, including after failure. Experimental results from the tests were seen to correspond well with analytical results for mode shapes and frequencies obtained through an eigenvalue analysis of a plane-grillage finite element model. The test method was shown to be effective in indicating changes in response as a function of load level and damage accumulation, and is expected to have significant potential for eventual routine health monitoring and damage detection of such structural systems in the field.

Issues related to composite plating and environmental exposure effects on composite-concrete interface in external strengthening

The effect of short-term environmental exposure on the response of in situ formed externally composite strengthened concrete is investigated. Measured response from static loading is described in terms of environmental degradation of composite and composite-concrete interface. Results indicate that degradation occurs primarily at the level of the resin in contact with the concrete, and that due care should be taken of changes in composite stiffness due to moisture exposure and consequent resin plasticization, as well as due to stiffness increases under cold conditions.

E-Glass/Vinylester Composites in Aqueous Environments – I: Experimental Results

Abstract2- and 4-layered specimens of E-Glass/Vinylester fabricated from uniaxial, biaxial, and triaxial, non-woven fabrics processed using the resin infusion process are immersed in deionized water at 23 °C (73 °F) and 60 °C (140 °F), and a potassium based pH 10 buffer at 73 °F, for a period of 57 weeks in order to investigate durability in aqueous environments. It is shown that the coefficients of apparent diffusion and levels of moisture gain are the highest for the deionized water immersed samples at 60 °C (140 °F), and this results in the highest levels of tensile strength and modulus degradation. Tensile tests show the presence of an aqueous medium based post-cure that competes with the conventionally recognized mechanisms of deterioration in the resin, at the level of the fiber-matrix interface, and in the fiber, resulting in a retardation of absolute level of effects. It is also shown that effects of the immersion are different in the warp and fill directions and can in fact be affected by intricacies of the fabric architecture and thickness. It is shown that damage takes place through interface debonding and degradation as well as fiber pitting, and cracking, each of which serve as the means for renewed absorption of water resulting in moisture uptake at levels above the initial plateau. Effects of immersion on short-beam-shear strength and glass transition temperature are also elucidated.

Notes on the Modeling of Preform Compaction: I -Micromechanics at the Fiber Bundle Level

Resin Transfer Molding (RTM) can be described as a two step process: fiber preforming followed by resin infusion and cure. In the first stage, the fabric form making up the preform element is placed in the tool and is compacted due to the pressure from tool closure. During this process, the microstructure, and the resultant properties, change considerably since compaction flattents the weft yarn bundles from the initial configuration into flat ellipsoids of higher aspect ratio, while simultaneously reorienting and flattening the bundles in the warp direction. The movement also results in nesting and inter-layer packing yielding a higher localized fiber volume fraction. An aspect unique to RTM and other allied resin infusion processes is the compaction of fibers in the dry (unlubricated) form. This gives rise to rather unique behavior at the fiber tow level, which in turn affects the compaction of the fabric form itself. It is thus essential that any model constructed for the specific purpose of investigating the compaction stage in RTM (or allied processes that involve the compaction of a dry preform) be able to reflect that the tow is dry, usually untwisted and when compressed in bulk it has frictional resistance to the shear mode of deformation. In this paper we focus on the possible tow behavior, rather than the behavior of the fabric structure, with the motivation being that of developing a simple, yet comprehensive model at that level which can then be intregrated into a fabric level model at a later stage.