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

Experimental fire behaviour of precast CFRP pretensioned HPSCC slabs

Optimized concrete elements have been developed using high-performance, self-consolidating, fibre-reinforced concrete (HPSCC) reinforced with high-strength, lightweight, non-corroding pre-stressed carbon fibre reinforced plastic tendons. This new type of thin-walled precast carbon FRP (CFRP) pretensioned HPSCC slab has been used in building façades and is under consideration for a range of other applications in buildings. However, it is well known that the bond strength between both steel and FRP reinforcements and concrete deteriorates at elevated temperature, and that HPSCC has a comparatively high risk of heat-induced concrete spalling. Reductions in bond strength, the occurrence of heat-induced concrete spalling, and their impacts on the load-bearing capacity of precast CFRP pretensioned HPSCC slabs during fire are not well understood. This paper gives insights into the fire behaviour of precast CFRP pretensioned HPSCC slabs by evaluating the influence of concrete mixture, slab thickness and the presence of local reinforcement (CFRP grids) in the prestress transfer zones. Selected results and analysis of a large scale fire resistance test on five HPSCC slabs are presented in this paper. It is shown that the occurrence of heat-induced concrete spalling resulted in sudden failure of the specimens, and that in cases where no spalling occurred failure was due to loss of bond strength

High temperature compatibility of CFRP versus steel reinforcement for concrete

Results are presented from a comprehensive experimental study to assess the occurrence of heat-induced longitudinal splitting cracks in concrete specimens reinforced with CFRP or steel when exposed to severe heating from one side, as would likely occur in a fire in a building. Tests were performed on large-and medium-scale precast CFRP reinforced or prestressed specimens. Large-scale specimens were tested in a standard fire resistance test, while medium-scale specimens were tested using a novel Heat-Transfer Rate Inducing System (H-TRIS) which controls thermal exposure by imposing a time-history of incident heat flux at a specimen's exposed surface. The formation of thermally-induced longitudinal splitting cracks and failure of the concrete cover to provide sufficient confining action, and thus sufficient bond strength, is shown to be more likely for FRP reinforced or prestressed concrete elements than for those reinforced or prestressed with steel. This appears to be at least partly due to thermo-mechanical incompatibility between CFRP reinforcement and concrete; formation of heat-induced longitudinal splitting cracks is related to rapid thermal expansion of CFRP tendons relative to the surrounding concrete. Many aspects of bond performance at elevated temperature remain poorly understood, and these require additional investigation before FRP reinforced or prestressed elements can be used in fire-rated applications with confidence

Fretting fatigue behaviour of pin-loaded thermoset carbon-fibre-reinforced polymer (CFRP) straps

This paper focuses on the fretting fatigue behaviour of pin-loaded carbon-fibre-reinforced polymer (CFRP) straps studied as models for rigging systems in sailing yachts, for suspenders of arch bridges and for pendent cables in cranes. Eight straps were subjected to an ultimate tensile strength test. In total, 26 straps were subjected to a fretting fatigue test, of which ten did not fail. An S–N curve was generated for a load ratio R of 0.1 and a frequency f of 10 Hz, showing a fatigue limit stress of the straps around the matrix fatigue limit, corresponding to 46% of the straps’ ultimate tensile strength (σUTS). The fatigue limit was defined as 3 million load cycles (N = 3 × 106), but tests were even conducted up to N = 11.09 × 106. Catastrophic failure of the straps was initiated in their vertex areas. Investigations on the residual strength and stiffness properties of straps tested around the fatigue limit stress (for N ≥ 1 × 106) showed little influence of the fatigue loading on these properties. Quasi-static finite element analyses (FEA) were conducted. The results obtained from the FEA are in good agreement with the experiments and demonstrate a fibre parallel stress concentration in the vertex area of factor 1.3, under the realistic assumption of a coefficient of friction (cof) between pin and strap of 0.5

CFRP-Strengthening and Long-Term Performance of Fatigue Critical Welds of a Steel Box Girder

Empa’s research efforts in the 1990s provided evidence that a considerable increase of the fatigue strength of welded aluminum beams can be achieved by externally bonding pultruded carbon fiber reinforced polymer (CFRP) laminates using rubber-toughened epoxies over the fatigue-weak welding zone on their tensile flange. The reinforcing effect obtained is determined by the stiffness of the unidirectional CFRP laminate which has twice the elastic modulus of aluminum. One can therefore easily follow that an unstressed CFRP laminate reinforcement of welded beams made of steel will not lead to a substantial increase in fatigue strength of the steel structure. This consideration led to the idea of prestressing an external reinforcement of the welded zone. The present investigation describes experimental studies to identify the adhesive system suitable for achieving high creep and fatigue strength of the prestressed CFRP patch. Experimental results (Wöhler-fields) of shear-lap-specimens and welded steel beams reinforced with prestressed CFRP laminates are presented. The paper concludes by presenting a field application, the reinforcement of a steel pendulum by adhesively bonded prestressed CFRP laminates to the tensile flanges of the welded box girder. Inspections carried out periodically on this structure revealed neither prestress losses nor crack initiation after nine years of service

Prestressing low clinker structural concrete elements by ultra-high modulus carbon fibre reinforced polymer tendons

The combination of low clinker high-performance concrete (LCHPC) and ultra-high modulus (UHM) carbon fibre reinforced polymer (CFRP) tendons was recently proposed for prestressed structural elements. The 70% reduction in cement content resulting in limited creep and shrinkage of the LCHPC in comparison to a conventional high-performance concrete (HPC) and the very high UHM-CFRP tendon stiffness (> 509 GPa) were expected to impact the mechanical behaviour of such structures. This study focuses on the behaviour of 3 m-long beam specimens during prestressing, concrete hardening and in 4 point-bending experiments. Fibre optic sensors were implemented inside the CFRP tendons to measure strain during those stages and a digital image correlation system was employed to monitor the 4-point-bending tests. After 28 days, the LCHPC recipe, despite a 70% cement reduction and much smaller environmental footprint, did not show measurable differences in the prestress loss behaviour in comparison to a conventional HPC. The UHM-CFRP prestressing tendons, because of their stiffness, showed both higher prestress losses of around 40% and on average a nearly doubled prestress transfer length. However, they increased the beam`s maximum load-bearing capacity by 21% and showed 47% less deflection at failure in comparison to beams prestressed with the standard modulus (UTS)-CFRP tendons.ISSN:1359-5997ISSN:0025-5432ISSN:1871-687

Fire experiments of thin-walled CFRP pretensioned high strength concrete slabs under service load

Sustainable precast concrete elements are emerging utilizing high-performance, self-consolidating, fibre-reinforced concrete (HPSCC) reinforced with high-strength, lightweight, and non-corroding prestressed carbon fibre reinforced polymer tendons. One example of this is a new type of precast carbon FRP pretensioned HPSCC panel intended as load-bearing panels for glass concrete building facades. It is known that the bond strength between both steel and FRP reinforcing tendons and concrete deteriorates at elevated temperature and that high strength concrete tends to an explosive spalling failure mode when subjected to a fire. The bond strength reductions in fire, their impacts on the load-bearing capacity of prestressed concrete structures, and the spalling behaviour of high-strength concrete remain poorly understood. This paper gives insight in the fire behaviour of filigree CFRP prestressed HPSCC slabs and presents selected results and analysis of an experimental fire test series on 45 mm and 60 mm thin-walled slabs with and without an additional external mineral fire protection coating. The main findings are that the fire resistance of the slabs is determined by spalling of the HPSCC or - if spalling can be avoided by the use of 5 kg/m PP microfibers in the concrete - by the thermal splitting-crack induced bond failure of the CFRP tendons in their prestress transfer zone