International Concrete Abstracts Portal

Pretensioned precast concrete girders are mainly designed to resist high bending moments. Although limited shear forces act on these girders, a minimal amount of web reinforcement should be added in order to meet design code requirements. Since the cutting, bending and placing of stirrups is a labour-intensive job, a possible alternative can be the replacement of stirrups by adding fibres to the concrete. By using steel-fibre-reinforced concrete (SFRC), the production process of precast elements can be shortened and costs reduced significantly. In order to investigate the shear capacity of full-scale prestressed fibre-reinforced concrete elements, an experimental programme was carried out on 20 m span precast girders. Additionally, standard prisms were cast from the same concrete batch to derive the postcracking behaviour of the SFRC. The ultimate shear capacity of all the girders is evaluated with respect to the current shear design provisions for SFRC according to the fib Model Code for Concrete Structures 2010.

Experimental and numerical studies on steel-fibre-reinforced concrete (SFRC) over the last five decades, or so, have indicated that the post cracking strength of concrete can be improved by providing suitably arranged, closely spaced, wire reinforcement. While the database of experimental and numerical shear tests of SFRC members is extensive, the pool of test data and numerical models, alike, of SFRC beams containing conventional transverse shear reinforcement (stirrups) are limited. The behaviour of full scale steel-fibre-reinforced reinforced concrete (SFR-RC) beams are analysed herein using a smeared crack model provided by ATENA 2D integrated with a constitutive law derived after an inverse analysis from prism bending tests. The numerical model is validated against experimental results obtained from four large scale SFR-RC beams and is shown to reasonably model the experimental responses. The model allows a better understanding of SFR-RC structures failing in shear and can be used as a basis for developing new design procedures for such structures.

This paper reports the recent findings of an experimental investigation on the influence of steel fibres in RC blocks under quasi static direct tensile loading. Structural blocks were designed with rebar reinforcement ratios of 0.40, 0.63 and 1.00%. A structural direct tensile testing system was developed at the COPPE laboratories resulting in a state-of-the-art in house apparatus. The RC blocks were reinforced with 1.25% volume fraction of steel fibres and without any type of fibre reinforcement and then tested until a strain level of approximately 0.0015mm/mm. The results show that the steel fibres improved the stress transfer efficiency between the rebars and the concrete matrix. By partially replacing the rebars by steel fibres the ductility of the concrete block was augmented and the post-crack stiffness increased. These results and possible mechanisms are discussed on the basis of the observed crack patterns, deformation measured on the steel rebars, computed deformation of the concrete matrix and on the overall mechanical behaviour of the composite concrete block.

Fibre-reinforced self-compacting concrete uses the flowability of fresh concrete to improve fibre orientation, thereby enhancing toughness and energy absorption capacity. In the past few years there has been a boost in the development of concretes with macro-synthetic fibres added. In this paper, the mechanical properties of self-compacting concrete with low, medium and high fibre contents of macro polyolefin fibres are studied. Their fracture behaviour is compared with plain self-compacting concrete. It is possible to fit this behaviour within the existing standards requirements. Dispersion obtained for mean fracture values among the different amounts was analysed using a fracture surface analysis and the amount and distribution of fibres.

Plastic shrinkage cracking due to a high rate of evaporation detrimentally affects durability and serviceability of concrete structures. The effect of different types of fibres to control these cracks, including steel, glass, polypropylene, and polyolefin fibres on the moisture loss and evaporation rates is investigated by performing ASTM C1579 tests. Using a dual stage methodology of constant drying rate period (stage I) and falling drying rate period (stage II), results are analysed. Moisture diffusivities are computed which in turn can be used for modelling the drying shrinkage and cracking under different environmental conditions. The formation of microcracks is documented using digital photography and processed by image analysis. The results show that moisture diffusivities at stage I drying are similar to each other between FRC and control samples. These magnitudes are approximately 50 times higher than diffusivities at stage II drying. The main difference is observed in stage II drying where the diffusivities in FRC are lower compared to plain concrete. Image analysis results indicate significant effects of fibres on controlling plastic shrinkage cracks.