Tim Stratford

Temperature Effects in Adhesively Bonded FRP Strengthening Applied to Steel Beams: Experimental Observations

FRP plates can be used to strengthen a steel beam in flexure, but this method relies critically upon the adhesive used to bond the FRP plate to the existing steel member. When the temperature of the strengthened beam is increased, differential thermal expansion occurs between the steel and FRP. In addition, the glass transition temperature of a typical two-part ambient-cure epoxy adhesive is typically between about 50°C and 65°C, and the stiffness and strength of the adhesive will decrease at temperatures somewhat below the glass transition temperature. This paper reports tests conducted on steel beams strengthened with CFRP plates and ambient-cure epoxy adhesive. Load was applied to the beams in four-point bending, and the temperature of the strengthening was then increased until failure occurred. Slip deformations were directly observed across the adhesive joint, giving an indication of the performance of the strengthening at elevated temperatures. The consequences of this preliminary study upon the design of externally-bonded FRP strengthening for steel structures are discussed.

Thermal Propagation through Tensile Cracks in Reinforced Concrete

The thermal propagation through tensile cracks in reinforced concrete beams is examined experimentally. A comparison is made between the rate of thermal propagation through beams that are undamaged, beams that have minor cracking, and beams that have major cracking. The results show a small decrease in the thermal propagation of the cracked beams in comparison with undamaged beams during heating. It is determined that the differences observed are most likely attributable to small geometric, mechanical, and concrete compositional differences. Consequently, it is concluded that the effects of tensile cracking on the thermal propagation through concrete can be ignored in structural analyses. Significantly, this means that analyses of heated concrete structures that are cracked can be carried out with heat-transfer and mechanical analyses being conducted sequentially, as is currently normal, and fully coupled thermomechanical analyses are not required.

BONDED FIBRE REINFORCED POLYMER STRENGTHENING IN A REAL FIRE

FRP strengthening is critically dependent upon the bonding adhesive. The adhesive used is typically an ambient cure epoxy with a glass transition temperature as low as 60°C. This paper describes the performance of bonded FRP strengthening within real compartment fires (the Dalmarnock Fire Tests), one of which was allowed to grow past flashover. The aim of these real fire tests was to complement the laboratory-based fire tests on FRP strengthened members that are currently being undertaken at various research centres. In this study, externally bonded plate and near-surface-mounted FRP strengthening were applied to the ceiling of a concrete structure. The FRP was protected using either an intumescent coating or gypsum boards, alongside FRP that was left unprotected. The tests demonstrated the vulnerability of FRP strengthening during a real compartment fire. The glass transition temperature was rapidly exceeded in the bonding adhesive for all samples. The near-surface mounted strengthening and the gypsum board protected strengthening was in a visibly better condition after the fire.

Effect of Warm Temperatures on Externally Bonded FRP Strengthening

AbstractFiber-reinforced polymer (FRP) plate strengthening relies critically upon the adhesive that is used to bond it to the existing structure. A typical two-part ambient-cure epoxy adhesive for structural strengthening has a glass transition temperature of approximately 40°C–70°C, but the stiffness and strength of the adhesive typically decrease at temperatures somewhat below this characteristic temperature. This paper investigates the implications of the changes in adhesive properties at warm temperatures (<100°C) for FRP-strengthened beams, through short-term experimental and analytical work. Tests were conducted on FRP-strengthened steel beams subjected to sustained load and increasing temperature; the results, however, are also relevant to strengthened concrete beams. Digital image correlation was used to measure the slip between the strengthening plate and beam, and hence to observe the behavior of the adhesive joint. A bond analysis was also developed to predict the slip across the adhesive joint...

The Ultimate Condition of FRP Confined Concrete Columns: New Experimental Observations and Insights

A large body of research is available on FRP confinement of concrete. Many hundreds of tests have been performed and dozens of empirical models are available. However, some of the key mechanics of FRP wrapped concrete are still not understood. Research is needed to understand, quantify, and rationally account for the hoop strain variation in FRP wraps at failure; since failure is fundamentally defined by hoop rupture of the FRP in tension. In this paper, a digital image analysis technique is used to quantify the variation of axial and hoop strain over the surface of FRP wrapped concrete cylinders. Tests on FRP wrapped cylinders of varying aspect ratio are presented to study factors influencing strain variability. The first ever quantified statistical description of hoop strain variability is provided, and the consequences of this variability are discussed.

FRP strengthening of web panels of steel plate girders against shear buckling Part-I: Static series of tests

Abstract The result of an experimental programme investigating a novel technique to strengthen web plates against breathing fatigue is presented in this paper; the programme was divided into five phases, including: (1) the development of a novel preformed corrugated FRP panel for strengthening thin-walled steel plate girder webs against buckling, (2) selecting the adequate adhesive and epoxy using double-lap shear and tension specimens, (3) producing the FRP panel, and (4, 5) testing its performance in two main experimental series; the initial (static) series and the final (cyclic) series. Only the initial series which involved tests on 13 steel plates strengthened with the proposed preformed corrugated FRP panel and subjected to in-plane shear will be reported in this paper. This series investigated the performance of different forms of strengthening under static load, in preparation for a subsequent series of cyclic tests to investigate their fatigue performance. Test results showed the efficiency of the technique at increasing the stiffness of the strengthened specimens in comparison to the unstrengthened ones and reducing the critical stresses which will serve as a precursor for the anticipated increase in the fatigue life of the girders.

Modelling the Response of Simply-Supported Two-Way Reinforced Concrete Slabs Exposed to Fire

The historically good performance of concrete structures in real fires, and the lack of urgent drivers for the concrete industry to support research on the fire performance of concrete structures, means that research on the full frame response of concrete buildings to fires has received much less attention than for steel-framed structures. However, a credible understanding of, and ability to model, the response of concrete structures under fire exposure is crucial to make further progress in the field of structural fire engineering, and to make best use of the flexibility enabled by performance-based fire codes. This paper presents a computational study on the structural behaviour of reinforced concrete slabs during fire tests undertaken by Zhang et al.[16]. The distribution of stresses in the slabs is discussed, as is the need for further research to better understand structural response during fire. Amongst other factors, the assumed tensile strength of the concrete is crucial to accurately predict response. The results corroborate the existing consensus that concrete slabs in real buildings can, in some cases, withstand fires for longer than expected; this is due to mobilisation of membrane actions, amongst other factors.