International Concrete Abstracts Portal

In recent years important efforts have been devoted to develop new types of polypropylene (PP) macro fibres able to provide significant toughness and ductility to concrete. These PP fibres, which are now widely available in the market, present a series of advantages: a significantly number of fibres per unit volume that allows less variability of experimental results (hence, higher characteristic values for specific mean values); a higher number of fibres intercepting cracks and controlling their propagation (of outmost importance for early-age shrinkage cracking); last but not least, no corrosion stains at the concrete surface. However, even if several experiments available in literature showed that fibres, if provided in sufficient amount and with an adequate toughness, are significantly effective as shear reinforcement, just few of them focused on the shear behaviour of elements made of polypropylene-fibrereinforced concrete (PFRC). In this context, structural applicability of PP macro fibres, adopted as shear reinforcement, is investigated in this paper. Experimental results of full scale tests on fourteen wide-shallow beams (WSBs) and nineteen deep beams in reinforced concrete (RC) and PFRC are presented. These results show that, in both beam typologies, the addition of PP fibres significantly enhances both the shear bearing capacity and the ductility. PP fibres can completely replace and reduce the conventional shear reinforcement in WSBs and deep beams, respectively.

In a research project funded by the Austrian Research Foundation (FFG), ultra-highperformance fibre-reinforced concrete (UHPFRC) mixtures were developed and optimized. Compression tests, uniaxial tension tests and several small-scale four-point bending tests were performed and the full constitutive law was derived. A series of 10 large-scale beams with a span of 3 m and an I-shaped cross-section was subsequently produced and tested in a fourpoint setup, where the reinforcement amount and the fibre content were varied. The tensile strength of the longitudinal reinforcement was 800-1100 MPa. Several circular openings were foreseen in order to facilitate potential installation crossings. In the tests with flexural failure at ultimate load level, a pronounced ductile behaviour was observed. Cracking and crack widths were controlled in a satisfying way by the longitudinal reinforcing bars and the fibres as well. Apart from the mostly observed bending failure mode, two beams with increased longitudinal reinforcement and 2% fibres exhibited diagonal tension failure in shear. Regarding the ultimate load, in the tests with shear failure the fibre contribution was much more significant than in the tests with bending failure.

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.