Fracture Energy of Steel Fiber-Reinforced Concrete
Steel fiber-reinforced concrete (SFRC) is a cementitious material reinforced with discrete fibers. The energy absorption capacity is the main material property benefited by fiber reinforcement. Closed-loop servo-controlled equipment should be used to evaluate this property. The tests should be carried out using displacement control in order to obtain the postpeak force- displacement relationship (tensile strain-softening branch). To assess the fracture energy of SFRC, three-point bending tests were carried out using displacement control. Series of notched beams reinforced with 30, 60, and 90 kg/m3 of hooked-end steel fibers were tested. Besides the energy dissipated in fracturing the concrete, the energy determined from the force-displacement relationship can also include the energy absorbed during nonlinear behavior of concrete in compression. Ductile materials, such as concrete reinforced with a high content of fibers, develop large deflections before exhausting their energy absorption capacity. In these cases, the "fixed" points of the bar supporting the displacement transducer may not remain fixed, adding an extra deflection into the control displacement transducer and thus leading to incorrect evaluation of the fracture energy. These factors are analyzed in the present work in order to assess the suitability of the specimen dimensions and the test procedures for evaluating the fracture energy of SFRC.
Tensile behaviour of glass fibre reinforced concrete
From the results of the research carried out in the last years on fibre reinforced cement based materials, it can be pointed out that, for the fibre contents usually employed in practice, the post-peak tensile behaviour is the most improved material characteristic. However, difficulties in carrying out valid direct tensile tests have limited the research in this field. The scarcity of investigation on the tensile behaviour of glass fibre reinforced concrete (GFRC) is also probably due to the ageing problems of GFRC systems. In order to contribute to a better knowledge of the uniaxial tensile behaviour of GFRC, deformation-controlled uniaxial tensile tests were carried out at Stevin Laboratory (NL). Polymer-modified glass fibre reinforced cement (PGFRC) specimens manufactured by spray up and premix techniques, and GFRC specimens are tested at the age of 28 days. The experimental response of the tested specimens is illustrated and the results are used to validate a computational code developed for the analysis of fibre reinforced concrete (FRC) structures, wherein the most recent concepts of fracture mechanics of brittle materials are included. 1 BACKGROUND INFORMATION 1.1 Glass fibre reinforced concrete Glass fibre reinforced concrete manufactured by spray or premix process has proven to be an attractive material for a wide range of applications (PCI 1981, Majumdar 1991). Several factors have contributed for this, namely: manufacture facilities in terms of shape, size and finishing; economy in transport and handling equipment; decrease of superimposed loads on structural components due to the relative low weight of GFRC systems; satisfactory mechanical properties and good physical performances. However, soon it was recognized the long-term decline of the properties of these composites (Majumdar 1974). The chemical attack on glass by the alkaline cement environment and the growth of hydration products, mainly calcium hidroxides, in between the filaments in the strand have been pointed out as the responsible by the durability of the GFRC mechanical properties (Bijen 1983, Leonard 1984, Bentur 1986, Shah 1988, Li 1991). The ways of improving the retention of the GFRC properties with age have been directed to the following main procedures: reduce the chemical aggressiveness of the cementitious matrix by employing low alkali cements (Akihama 1987); protect the fibres from the corrosive environment through a polymer or admixtures mix addition, which has the capacity to envelop the fibres by a protective film (Bijen 1990); interfer in the fibre chemical composition and fibre surface treatment (Fyles 1986). The submicron particles of the acrylic polymer applied in the Forton-PGFRC surround the strand filaments, obstructing the fibre engulfment by the cement hydration products and protecting the fibres against a chemical attack. A significative improvement in the durability of these composites under weathering conditions has been claimed (Bijen 1990). 1.2 Tensile behaviour The deformation-controlled uniaxial tensile test is probably the most suitable procedure to study the brittle behaviour of cement-based materials (Gopalaratnam 1985, Wang 1990, Hordijk 1991). The tensile load-deformation curve schematically represented in Fig. 1 can be regarded as a typical curve of a bar of PGFRC under deformation controlled uniaxial tensile test, as long as the fracture behaviour is governed by the fibre pull-out mechanism. Comparatively to plain concrete (PC), fibre reinforced concrete (FRC) has a less steep softening branch (branch III), a larger ultimate deformation and a more enlarged nonlinear branch before peak load (branch II). The initial part of the pre-peak curve (branch I) appears to be a straight line, indicating that FRC behaves linearly in this region. In this first branch, load does not create new microcracks until the stress level represented by the bending over point (BOP) is reached. Between BOP and peak load, the response is usually described by a nonlinear branch (branch II). This branch is more enlarged when a high percentage of fibres is employed. In this stage a substantial amount of energy is required to propagate the microcracks (process zone). The localization of the process zone is usually associated to a stress concentration which is due to a material heterogeneity or to a structural descontinuity (such as a notch). If the process zone is developed within the measuring length, whose deformation is used as test control parameter, a loaddeformation relationship, as indicated by branch III in Fig. 1, can be obtained and used to get the stresscrack opening relationship (Hillerborg 1976). The load that can be transferred decreases with an increasing deformation of the process zone. As a result of this load reduction, the composite material outside the process zone unloads (branch IV). The pull-out mechanism of fibres bridging the fracture surfaces in the process zone is responsible for the energy increase of the FRC as compared with PC.
Additional concrete fracture energy tests performed by 6 laboratories according to a draft RILEM recommendation
Svensk rapportering med sammanstallning av forsoksresultat fran Bologna, Lausanne, La Plata, Karlsruhe, Delft och Tohoku i Japan.