Thermo-mechanical compatibility of CFRP versus steel reinforcement for concrete at high temperature

Optimization of the design of concrete structures has become a driver for the use of nonconventional reinforcing materials. One example of this is the emerging use of non-corrosive, highstrength, and lightweight carbon fibre reinforced polymer (CFRP) prestressing tendons. It is widely known that the bond between FRP reinforcing tendons and concrete deteriorates at elevated temperature due to a combination of factors. Lateral thermal expansion of FRP reinforcing tendons at elevated temperature has been shown to have consequences for the bond performance of these systems. This paper presents the results of an experimental study carried out to assess the occurrence of heat-induced longitudinal splitting cracks in concrete specimens reinforced with CFRP or steel prestressing tendons. A novel testing methodology, namely a Heat-Transfer Rate Inducing System (H-TRIS), is used to subject specimens to thermal loading which replicates that experienced by equivalent specimens in a standard fire resistance test. A comparison between CFRP and steel tendons is made, and the occurrence of longitudinal splitting cracks is evaluated in terms of the time to occurrence and thermal gradient within the concrete. Results are compared against an available analytical model

Effects of polypropylene fibre type and dose on the propensity for heat-induced concrete spalling

The term high-performance concrete (HPC) is typically used to describe concrete mixes with high workability, strength, and/or durability. While HPC outperforms normal strength concrete in nearly all performance criteria, it also displays a higher propensity for heat-induced concrete spalling when exposed to severe heating or fire. Such spalling presents a serious concern in the context of the historical approach to fire safe design of concrete structures, where structural engineers typically rely on concrete's inherent fire safety characteristics (e.g. non-combustibility, non-flammability, high thermal inertia). It has been widely shown that the inclusion of polypropylene (PP) fibres in concrete mixes reduces the propensity for heat-induced concrete spalling, although considerable disagreement exists around the mechanisms behind the fibres’ effectiveness. This paper presents an experimental study on the effects of PP fibre type and dose on the propensity for heat-induced spalling of concrete. A novel testing method and apparatus, the Heat-Transfer Rate Inducing System (H-TRIS) is used to test medium-scale concrete specimens under simulated standard fire exposures. Results show (1) that although the dose of PP fibres (mass of PP per m of fresh concrete) is currently the sole parameter prescribed by available design guidelines, both the PP fibre cross-section and individual fibre length may have considerable influences on the effectiveness of PP fibres at reducing the propensity for heat-induced concrete spalling; and (2) that current guidance for spalling mitigation with PP fibres is insufficient to prevent spalling for the HPC mixes tested

Fretting Fatigue Performance of Unidirectional, Laminated Carbon Fibre Reinforced Polymer Straps at Elevated Service Temperature

The fretting fatigue performance of laminated, unidirectional (UD), pin-loaded, carbon fibre-reinforced polymer (CFRP) straps that can be used as bridge hanger cables was investigated at a sustained service temperature of 60 °C. The aim of this paper is to elucidate the influence of the slightly elevated service temperature on the tensile fatigue performance of CFRP straps. First, steady state thermal tests at ambient temperature and at 60 °C are presented, in order to establish the behaviour of the straps at these temperatures. These results indicated that the static tensile performance of the straps is not affected by the increase in temperature. Subsequently, nine upper stress levels (USLs) between 650 and 1400 MPa were chosen in order to establish the S–N curve at 60 °C (frequency 10 Hz; R = 0.1) and a comparison with an existing S–N curve at ambient temperature was made. In general, the straps fatigue limit was slightly decreased by temperature, up to 750 MPa USL, while, for the higher USLs, the straps performed slightly better as compared with the S–N curve at ambient temperature

Carbon nanotube (CNT)-epoxy nanocomposites: a systematic investigation of CNT dispersion

A systematic investigation of the dispersion of carbon nanotubes (CNTs), 1-6nm in diameter and a few microns in length, in a bisphenol F-based epoxy resin has been presented. Several dispersing techniques including high-speed dissolver, ultrasonic bath/horn, 3-roll mill, etc. have been employed. Optical microscopy has been extensively used to systematically characterise the state of CNT dispersion in the epoxy resin during the entire processing cycle from mixing CNT with resin to adding and curing with hardener. Complimentary viscosity measurements were also performed at various stages of nanocomposite processing. A method to produce a good CNT dispersion in resin was established, but the state of CNT dispersion was found to be extremely sensitive to its physical and chemical environments. The cured nanocomposites were further tested for their thermo-mechanical properties by dynamic mechanical thermal analysis (DMTA), and for flexural and compressive mechanical properties. The measured properties of various nanocomposite plates were then discussed in view of the corresponding CNT dispersio